Advance Construction Materials

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PRE STRESSED CONCRETE
Pre-stressed concrete is a method for overcoming concrete's natural weakness in tension. It
can be used to produce beams, floors or bridges with a longer span than is practical with ordinary
reinforced concrete. Pre-stressing tendons (generally of high tensile steel cable or rods) are used to
provide a clamping load which produces a compressive stress that balances the tensile stress that the
concrete compression member would otherwise experience due to a bending load. Traditional
reinforced concrete is based on the use of steel reinforcement bars, rebar, inside poured concrete.
Pre-stressing can be accomplished in three ways: pre-tensioned concrete, and bonded or unbonded
post-tensioned concrete.
Prestressed concrete refers to concrete that has applied stresses induced into the member.
Typically, wires or “tendons” are stretched and then blocked at the ends creating compressive
stresses throughout the member’s entire cross section. Most Prestressed concrete is precast in a plant.
Advantages of Prestressed concrete vs. non-Prestressed concrete:
• More efficient members (i.e., smaller members to carry same loads)
• Much less cracking since member is almost entirely in compression
• Precast members have very good quality control
• Precast members offer rapid field erection
Disadvantages of Prestressed concrete vs. non-Prestressed concrete:
• More expensive in materials, fabrication, delivery
• Heavy precast members require large cranes
• Somewhat limited design flexibility
• Small margin for error
• More complicated design
PRE-TENSIONED CONCRETE
Pre-tensioned concrete is cast around steel tendons—cables or bars—while they are under
tension. The concrete bonds to the tendons as it cures, and when the tension is released it is

2. transferred to the concrete as compression by static friction. Tension subsequently imposed on the
concrete is transferred directly to the tendons.
Pre-tensioning requires strong, stable anchoring points between which the tendons are to be
stretched. Thus, most pre-tensioned concrete elements are prefabricated and transported to the
construction site, which may limit their size. Pre-tensioned elements may be incorporated into
beams, balconies, lintels, floor slabs or piles. An innovative bridge design using pre-stressing is the
stressed ribbon bridge.
Pre-tensioned concrete is almost always done in a precast plant. A pretensioned
Prestressed concrete member is cast in a preformed casting bed. The BONDED wires (tendons) are
tensioned prior to the concrete hardening. After the concrete hardens to approximately 75% of the
specified compressive strength f’c, the tendons are released and axial compressive load is then
transmitted to the cross-section of the member.
Post-Tensioned Prestressed Concrete:
A post-tensioned member has UNCOATED tendons cast into concrete in draped patterns.
After the concrete hardens to about 75% f’c, the tendons are tensioned and try to straighten out. This

3. creates an upward camber of the member which offsets anticipated downward deflection due to
gravity loads. Post-tensioning can be accomplished on-site as necessary.
Post-tensioning is a method of reinforcing (strengthening) concrete or other materials with
high-strength steel strands or bars, typically referred to as tendons. Post-tensioning applications
include office and apartment buildings, parking structures, slabs-on-ground, bridges, sports stadiums,
rock and soil anchors, and water-tanks. In many cases, posttensioning allows construction that would
otherwise be impossible due to either site constraints or architectural requirements.
Although post-tensioning systems require specialized knowledge and expertise to fabricate,
assemble and install, the concept is easy to explain. Imagine a series of wooden blocks with holes
drilled through them, into which a rubber band is threaded. If one holds the ends of the rubber band,
the blocks will sag. Post-tensioning can be demonstrated by placing wing nuts on either end of the
rubber band and winding the rubber band so that the blocks are pushed tightly together. If one holds
the wing nuts after winding, the blocks will remain straight. The tightened rubber band is comparable
to a post-tensioning tendon that has been stretched by hydraulic jacks and is held in place by
wedge-type anchoring devices.
ADVANTAGES/APPLICATIONS
There are post-tensioning applications in almost all facets of construction. In building
construction, post-tensioning allows longer clear spans, thinner slabs, fewer beams and more slender,
dramatic elements. Thinner slabs mean less concrete is required. In addition, it means a lower overall
building height for the same floor-to-floor height. Posttensioning can thus allow a significant
reduction in building weight versus a conventional concrete building with the same number of floors.
This reduces the foundation load and can be a major advantage in seismic areas. A lower building
height can also translate to considerable savings in mechanical systems and façade costs. Another

4. advantage of post-tensioning is that beams and slabs can be continuous, i.e. a single beam can run
continuously from one end of the building to the other. Structurally, this is much more efficient than
having a beam that just goes from one column to the next.
Post-tensioning is the system of choice for parking structures since it allows a high degree of
flexibility in the column layout, span lengths and ramp configurations. Post-tensioned parking
garages can be either stand-alone structures or one or more floors in an office or residential building.
In areas where there are expansive clays or soils with low bearing capacity, post-tensioned slabs-on-
ground and mat foundations reduce problems with cracking and differential settlement.
Post-tensioning allows bridges to be built to very demanding geometry requirements,
including complex curves, variable super elevation and significant grade changes. Post-tensioning
also allows extremely long span bridges to be constructed without the use of temporary intermediate
supports. This minimizes the impact on the environment and avoids disruption to water or road
traffic below. In stadiums, post-tensioning allows long clear spans and very creative architecture.
Post-tensioned rock and soil anchors are used in tunneling and slope stabilization and as tie-backs for
excavations. Post-tensioning can also be used to produce virtually crack-free concrete for water-
tanks.
CRITICAL ELEMENTS
There are several critical elements in a post-tensioning system. In unbonded construction, the
plastic sheathing acts as a bond breaker between the concrete and the prestressing strands. It also
provides protection against damage by mechanical handling and serves as a barrier that prevents
moisture and chemicals from reaching the strand. The strand coating material reduces friction
between the strand and the sheathing and provides additional corrosion protection.
Anchorages are another critical element, particularly in unbonded systems. After the
concrete has cured and obtained the necessary strength, the wedges are inserted inside the anchor
casting and the strand is stressed. When the jack releases the strand, the strand retracts slightly and
pulls the wedges into the anchor. This creates a tight lock on the strand. The wedges thus maintain
the applied force in the tendon and transfer it to the surrounding concrete. In corrosive environments,
the anchorages and exposed strand tails are usually covered with a housing and cap for added
protection.
CONSTRUCTION

5. In building and slab-on-ground construction, unbonded tendons are typically prefabricated at
a plant and delivered to the construction site, ready to install. The tendons are laid out in the forms in
accordance with installation drawings that indicate how they are to be spaced, what their profile
(height
above the form) should be, and where they are to be stressed. After the concrete is placed and has
reached its required strength, usually between 3000 and 3500 psi (“pounds per square inch”), the
tendons are stressed and anchored. The tendons, like rubber bands, want to return to their original
length but are prevented from doing so by the anchorages.
The fact the tendons are kept in a permanently stressed (elongated) state causes a
compressive force to act on the concrete. The compression that results from the posttensioning
counteracts the tensile forces created by subsequent applied loading (cars, people, and the weight of
the beam itself when the shoring is removed). This significantly increases the load-carrying capacity
of the concrete. Since post-tensioned concrete is cast in place at the job site, there is almost no limit
to the shapes that can be formed. Curved facades, arches and complicated slab edge layouts are often
a trademark of post-tensioned concrete structures. Post-tensioning has been used to advantage in a
number of very aesthetically designed bridges.
BONDED POST-TENSIONED CONCRETE
Bonded post-tensioned concrete is the descriptive term for a method of applying compression
after pouring concrete and the curing process (in situ). The concrete is cast around a plastic, steel or
aluminium curved duct, to follow the area where otherwise tension would occur in the concrete
element.
A set of tendons are fished through the duct and the concrete is poured. Once the concrete has
hardened, the tendons are tensioned by hydraulic jacks that react (push) against the concrete member
itself.
When the tendons have stretched sufficiently, according to the design specifications (see
Hooke's law), they are wedged in position and maintain tension after the jacks are removed,
transferring pressure to the concrete. The duct is then grouted to protect the tendons from corrosion.
This method is commonly used to create monolithic slabs for house construction in locations where
expansive soils (such as adobe clay) create problems for the typical perimeter foundation.

6. All stresses from seasonal expansion and contraction of the underlying soil are taken into the
entire tensioned slab, which supports the building without significant flexure.
Post-tensioning is also used in the construction of various bridges, both after concrete is
cured after support by falsework and by the assembly of prefabricated sections, as in the segmental
bridge.
Among the advantages of this system over unbonded post-tensioning are:
 Large reduction in traditional reinforcement requirements as tendons cannot destress in
accidents.
 Tendons can be easily "woven" allowing a more efficient design approach.
 Higher ultimate strength due to bond generated between the strand and concrete.
 No long term issues with maintaining the integrity of the anchor/dead end.
History of problems with bonded post-tensioned bridges
 The popularity of this form of prestressing for bridge construction in Europe increased
significantly around the 1950s and 60s. However, a history of problems have been
encountered that has cast doubt over the long-term durability of such structures.
 Due to poor workmanship of quality control during construction, sometimes the ducts
containing the prestressing tendons are not fully filled, leaving voids in the grout where the
steel is not protected from corrosion. The situation is exacerbated if water and chloride (from
de-icing salts) from the highway are able to penetrate into these voids.
UNBONDED POST-TENSIONED CONCRETE
Unbonded post-tensioned concrete differs from bonded post-tensioning by providing each
individual cable permanent freedom of movement relative to the concrete. To achieve this, each
individual tendon is coated with a grease (generally lithium based) and covered by a plastic
sheathing formed in an extrusion process. The transfer of tension to the concrete is achieved by the
steel cable acting against steel anchors embedded in the perimeter of the slab. The main disadvantage
over bonded post-tensioning is the fact that a cable can destress itself and burst out of the slab if
damaged (such as during repair on the slab). The advantages of this system over bonded post-
tensioning are:
 The ability to individually adjust cables based on poor field conditions (For example: shifting
a group of 4 cables around an opening by placing 2 to either side).
 The procedure of post-stress grouting is eliminated.

7.  The ability to de-stress the tendons before attempting repair work.
 Picture number one (below) shows rolls of post-tensioning (PT) cables with the holding end anchors
displayed. The holding end anchors are fastened to rebar placed above and below the cable and buried
in the concrete locking that end.
 Pictures numbered two, three and four shows a series of black pulling end anchors from the rear along
the floor edge form. Rebar is placed above and below the cable both in front and behind the face of
the pulling end anchor. The above and below placement of the rebar can be seen in picture number
three and the placement of the rebar in front and behind can be seen in picture number four. The blue
cable seen in picture number four is electrical conduit.
 Picture number five shows the plastic sheathing stripped from the ends of the post-tensioning cables
before placement through the pulling end anchors.
 Picture number six shows the post-tensioning cables in place for concrete pouring. The plastic
sheathing has been removed from the end of the cable and the cable has been pushed through the
black pulling end anchor attached to the inside of the concrete floor side form. The greased cable can
be seen protruding from the concrete floor side form.
 Pictures seven and eight show the post-tensioning cables protruding from the poured concrete floor.
After the concrete floor has been poured and has set for about a week,the cable ends will be pulled
with a hydraulic jack.

8. APPLICATIONS
Prestressed concrete is the main material for floors in high-rise buildings and the entire
containment vessels of nuclear reactors.
Unbonded post-tensioning tendons are commonly used in parking garages as barrier cable.
Also, due to its ability to be stressed and then de-stressed, it can be used to temporarily repair a
damaged building by holding up a damaged wall or floor until permanent repairs can be made.
The advantages of prestressed concrete include crack control and lower construction costs;
thinner slabs - especially important in high rise buildings in which floor thickness savings can
translate into additional floors for the same (or lower) cost and fewer joints, since the distance
that can be spanned by post-tensioned slabs exceeds that of reinforced constructions with the
same thickness. Increasing span lengths increases the usable unencumbered floor space in
buildings; diminishing the number of joints leads to lower maintenance costs over the design life
of a building, since joints are the major focus of weakness in concrete buildings.
The first prestressed concrete bridge in North America was the Walnut Lane Memorial
Bridge in Philadelphia, Pennsylvania. It was completed and opened to traffic in 1951.
Prestressing can also be accomplished on circular concrete pipes used for water transmission.
High tensile strength steel wire is helically-wrapped around the outside of the pipe under
controlled tension and spacing which induces a circumferential compressive stress in the core
concrete. This enables the pipe to handle high internal pressures and the effects of external earth
and traffic loads.

9. RAPID HARDENING PORTLAND CEMENT
Portland cement (often referred to as OPC, from Ordinary Portland Cement) is the most
common type of cement in general use around the world, used as a basic ingredient of concrete,
mortar, stucco, and most non-specialty grout. It usually originates from limestone. It is a fine powder
produced by grinding Portland cement clinker (more than 90%), a limited amount of calcium sulfate
(which controls the set time) and up to 5% minor constituents as allowed by various standards such
as the European Standard EN 197-1:
Portland cement clinker is a hydraulic material which shall consist of at least two-thirds by
mass of calcium silicates (3 CaO·SiO2 and 2 CaO·SiO2), the remainder consisting of aluminium-
and iron-containing clinker phases and other compounds. The ratio of CaO to SiO2 shall not be less
than 2.0. The magnesium oxide content (MgO) shall not exceed 5.0% by mass.
Portland cement clinker is made by heating, in a kiln, a homogeneous mixture of raw
materials to a calcining temperature, which is about 1450°C for modern cements. The aluminium
oxide and iron oxide are present as a flux and contribute little to the strength. For special cements,
such as Low Heat (LH) and Sulfate Resistant (SR) types, it is necessary to limit the amount of
tricalcium aluminate (3 CaO·Al2O3) formed. The major raw material for the clinker-making is
usually limestone (CaCO3) mixed with a second material containing clay as source of alumino-
silicate. Normally, an impure limestone which contains clay or SiO2 is used. The CaCO3 content of
these limestones can be as low as 80%. Secondary raw materials (materials in the rawmix other than
limestone) depend on the purity of the limestone. Some of the materials used are clay, shale, sand,
iron ore, bauxite, fly ash, and slag. When a cement kiln is fired by coal, the ash of the coal acts as a
secondary raw material.
The most common use for Portland cement is in the production of concrete. Concrete is a
composite material consisting of aggregate (gravel and sand), cement, and water. As a construction
material, concrete can be cast in almost any shape desired, and once hardened, can become a
structural (load bearing) element. Users may be involved in the factory production of pre-cast units,
such as panels, beams, road furniture, or may make cast-in situ concrete such as building
superstructures, roads, and dams. These may be supplied with concrete mixed on site, or may be
provided with "ready-mixed" concrete made at permanent mixing sites. Portland cement is also used
in mortars (with sand and water only) for plasters and screeds, and in grouts (cement/water mixes
squeezed into gaps to consolidate foundations, road-beds, etc.).

10. When water is mixed with Portland cement, the product sets in a few hours and hardens over
a period of weeks. These processes can vary widely depending upon the mix used and the conditions
of curing of the product, but a typical concrete sets in about 6 hours and develops a compressive
strength of 8 MPa in 24 hours. The strength rises to 15 MPa at 3 days, 23 MPa at 1 week, 35 MPa at
4 weeks and 41 MPa at 3 months. In principle, the strength continues to rise slowly as long as water
is available for continued hydration, but concrete is usually allowed to dry out after a few weeks and
this causes strength growth to stop.
CEMENT PLANTS USED FOR WASTE DISPOSAL OR PROCESSING
Due to the high temperatures inside cement kilns, combined with the oxidizing (oxygen-rich)
atmosphere and long residence times, cement kilns are used as a processing option for various types
of waste streams: indeed, they efficiently destroy many hazardous organic compounds. The waste
streams also often contain combustible materials which allow the substitution of part of the fossil
fuel normally used in the process.
Waste materials used in cement kilns as a fuel supplement:
 Car and truck tires – steel belts are easily tolerated in the kilns
 Paint sludge from automobile industries
 Waste solvents and lubricants
 Meat and bone meal – slaughterhouse waste due to bovine spongiform
encephalopathy contamination concerns
 Waste plastics
 Sewage sludge
 Rice hulls
 Sugarcane waste
 Used wooden railroad ties (railway sleepers)
 Spent Cell Liner (SCL) from the aluminium smelting industry (also called Spent Pot
Liner or SPL)
Portland cement manufacture also has the potential to benefit from using industrial by-products from
the waste-stream.[20] These include in particular:
 Slag
 Fly ash (from power plants)
 Silica fume (from steel mills)
 Synthetic gypsum (from desulfurization

11. The old adage time is money certainly applies to the construction industry. Delays waiting for
materials to arrive or properly cure have been the headache of contractors for decades. Time is of the
essence on many concrete projects, but contractors can't sacrifice quality, durability, or cost savings
simply to reduce construction time. That is why many concrete contractors are turning to rapid-
hardening hydraulic cement to meet tight schedules. Rapid-hardening hydraulic cement is not only a
more durable alternative to Portland cement on many projects, but its rapid-setting properties make it
an ideal solution for today's schedule-and budget-driven projects.
Rapid Hardening Portland Cement (RHPC) is a type of cement that is used for special
purposes when a faster rate of early high strength is required. RHPC has a higher rate of strength
development than the Ordinary Portland Cement (OPC).
The Rapid Hardening Portland Cement's better strength performance is achieved by
increasing the refinement of the product. This is the reason that its use is increasing in India.
Rapid Hardening Portland Cement is manufactured by fusing together limestone (which has
been finely grounded) and shale, at extremely high temperatures to produce cement clinker. To this
cement clinker, gypsum is added in small quantities and then finely grounded to produce Rapid
Hardening Portland Cement. It is usually manufactured using the dry process technology.
Rapid Hardening Portland Cement is used in concrete masonry manufacture, repair work
which is urgent, concreting in cold weather, and in pre-cast production of concrete. Rapid Hardening
Portland Cement has proved to be a boon in the places where quick repairs are required such as
airfield and highway pavements, marine structures, and bridge decks.
The Rapid Hardening Portland Cement should be stored in a dry place, or else its quality
deteriorates due to premature carbonation and hydration. As the Indian cement industry produces
Rapid Hardening Portland Cement in large quantities, it is able to meet the domestic demand and
also export to other countries. The cement industry in India exports cement mainly to the West Asian
countries.
The raw materials required for the manufacture of Rapid Hardening Portland Cement are:
 Limestone
 Shale
 Gypsum
 Coke

12. The major companies producing Rapid Hardening Portland Cement in India are:
 ACC
 Gujarat Ambuja
 J K Cement
 Grasim Industries
 Indian Cement Ltd.
- See more at: http://business.mapsofindia.com/cement/types/rapid-hardening-
portland.html#sthash.PLgrWFOV.dpuf
BENEFITS OVER PORTLAND CEMENT
The need for a more durable cement drove the research and development that produced CTS
Cement Mfg. Corp. Rapid Set cement. Although Portland cement has been successfully used for
many years, it is not without limitations. Portland cement concrete is prone to drying shrinkage
cracking. It is susceptible to attack by sulphates and has an undesirable reaction with certain
aggregates (ASR). Generally, when one accelerates the gain in strength of Portland cement concrete
through finer grinding or chemical additives, there is a significant increase in drying shrinkage.
Rapid-hardening hydraulic cement offers reduced shrinkage and superior resistance to chemical
attack. It achieves strength much faster than Portland cement and many installations can be put into
service in as little time as one hour. Compared to Portland cement, rapid-hardening hydraulic cement
reaches typical compressive strengths in a few hours that an equivalent Portland cement mix would
require one month to achieve.
Rapid-hardening hydraulic cement has been used for both concrete repair and new
construction, wherever superior durability and rapid strength gain are required. It is blended and
packaged into a wide range of high-performance products including nonshrink grout, structural
repair mortar, concrete, exterior plaster, and other cementitious products.
PROPERTIES
Rapid-hardening hydraulic cement is manufactured with similar raw materials, equipment,
and processes used to make Portland cement, but that is where the similarities end. The chemistry of
rapid-hardening hydraulic cement, which differs from Portland cement, is composed primarily of
hydraulic tetra calcium trialuminate sulphate (CSA) and dicalcium silicate (C2S). C2S is the most

13. durable compound found in Portland cement. The CSA compound, often referred to as calcium
sulfoaluminate, hydrates to form ettringite-a strong needle-like crystal that develops quickly to give
rapid-hardening hydraulic cement its high performance. Another significant aspect of this product's
chemistry is the absence of tricalcium aluminate (C3A), which makes a cement susceptible to sulfate
attack. Because rapid-hardening hydraulic cement has little or no C3A, it is very durable in sulfate
environments.
ADVANTAGES
As a viable alternative to Portland cement, rapid-hardening hydraulic cement offers several
advantages including durability, versatility, speed, and ease-of-use, as well as cost benefits and
environmental considerations.
DURABILITY. The amount of mixing water in concrete is a major factor in its durability. With
Portland cement concrete, the mixing water needed to make it fluid enough for placement is in
excess of the water needed to hydrate the cement. This excess water, often called water of
convenience, evaporates over time leaving voids or pores in the concrete and causes drying
shrinkage. In a typical Portland cement concrete mix, the excess water of convenience amounts to
about 50% of the water in the mix.
In rapid-hardening hydraulic cement, the water required to hydrate the CSA compound is
several times greater than that required to hydrate typical Portland cement compounds. In a typical
rapid-hardening hydraulic concrete mix, nearly all of the water used in the mix is used up in the
hydration process resulting in a dense concrete with very low drying shrinkage.
Voids or pores in concrete, along with drying shrinkage cracks, provide routes of entry for
substances that attack the concrete and reinforcing steel. With fewer pores and less drying shrinkage,
rapid-hardening hydraulic concrete is more durable than Portland cement concrete.
VERSATILITY. Rapid-hardening hydraulic cement can be formulated for a wide range of
applications. Various properties, such as setting time, fluidity, air content, and color are adjusted
easily by using commercially available additives. Rapid-hardening hydraulic cement is highly
resistant to freezing and thawing, and can, due to its rapid hydration, be used in cold weather
conditions that are not possible with Portland cement.

14. SPEED. Today's schedule-driven projects require quick construction solutions. One such
example is the Hyperion Sewage Treatment Plant in Los Angeles. Serving more than four million
residents, the city's oldest and largest wastewater treatment facility has an average capacity of 450
million gallons per day. The general contractor for the project, Kiewit, Santa Fe Springs, Calif.,
opted to use the product to reduce the construction time and ensure high durability. Ten pallets of the
Mortar Mix were used for full-depth repairs on sewage containment tanks. The original
specifications allowed for 28 days for the repair process and the use of rapid-hardening hydraulic
cement enabled the project to be complete in a mere three days.
EASE-OF-USE. As much of the infrastructure in the U.S. is wearing out, repair work becomes a
necessity both for safety and aesthetic reasons. The higher durability of rapid-hardening hydraulic
cement offers a real solution for repair work, not just a temporary, short-lived Band-Aid.
Restoration projects are extremely complex as design and construction teams have the added
challenge of matching an older look. First opened in 1910, the historic Hotel Shattuck in downtown
Berkeley, Calif., is one of the premier structures in the area. Although they had never used the
product, BPR Properties, Palo Alto, Calif., chose to use Cement All for grouting the concrete repairs.
Eighteen different types of repairs had to be used on the project and the contractor was able to use
Cement All for all of the repairs, which allowed the project to be completed faster.
COST BENEFITS. Although Portland cement is less expensive on a first-cost basis than
rapid-hardening hydraulic cement, its durability, rapid cure time, reduced shrinkage, and resistance
to chemical attack outweigh the cost differentials, especially when the cost of time is added to the
equation. For a contractor or owner, the value of time for a repair delay that results in the late
opening of a parking garage, an airport runway, or a retail center often are much greater than the
premium paid for rapid-hardening hydraulic cement. On many jobs, bonuses offered for getting a job
done on time or a project completed earlier is money well spent.
Superior Wall Systems, Fullerton, Calif., selected WunderFixx-a durable, fast-setting, one-
component concrete patching material formulated with a premium-grade hydraulic cement, high-
performance polymers, and a finely ground aggregate-for a project at Sony Picture Studios. For this
project, it was critical that an ultra-smooth finish was achieved on the plaster system. The contractor
for the project learned about the product at a trade show and thought it would be the best application
for this job as opposed to an acrylic system. WunderFixx required fewer coats because it had a much
larger spread than the acrylic system, so the contractor was able to provide a tremendous cost savings
by using less material. A scratch coat, brown coat, two coats of base material, and then the rapid-set
product served as the final coat on the plaster. The owner liked the smooth and fine spreadability of

15. the product, the elimination of chatter marks, as well as the fact the product is engineered for
sandability.
Yet another example of the cost savings rapid-set hydraulic cement can provide is found on the
Highway 23 project in California. Security Paving, Sun Valley, Calif., the contractor for the project,
used 20 pallets of the DOT Repair Mix for road and bridge repair work. Some of the work performed
was vertical in nature but the majority was flatwork that was 1 to 8 inches in depth. They were able
to use one-third less material, which saved a great deal of money.
ENVIRONMENTAL RESPONSIBILITY. Rapid-hardening hydraulic cement has a
much smaller carbon footprint than Portland cement. During the production process, rapid-hardening
hydraulic cement reduces CO2 emissions by 32% to 36% over conventional Portland cement
manufacturing procedures. This is because rapid-hardening hydraulic cement is produced at lower
temperatures, so less fossil fuel is required. It also requires less limestone per ton, further reducing
CO2 emissions.
Rapid-hardening hydraulic concrete is much more durable than Portland cement, and has a
greater resistance to sulfate and other types of chemical attack. Due to chemical formulation, lower
porosity and subsequent internal self-desiccation, rapid-hardening hydraulic cement is extremely
impervious to carbonation, freeze/thaw susceptibility, and acid rain leaching. Rapid-hardening
hydraulic cement has a proven record of field performance that exceeds the normal useful lifespan of
Portland cement concrete.
A PROVEN TECHNOLOGY
Contractors across the country have realized the benefits of using rapid-hardening hydraulic
cement products on a diverse range of projects for nearly three decades. Compute the cost of
durability and time, and it is apparent that rapid-hardening hydraulic cement is a viable, cost-
effective alternative to Portland cement. No special equipment is needed to mix rapid-hardening
hydraulic cement, and it can be put into service in as little as one hour, which allows contractors to
complete work faster. Rapid-hardening hydraulic cement can obtain the same strength in just six
hours, as the 28-day strength of an equivalent Portland cement mix.
READY MIX CONCRETE
Ready-mix concrete is concrete that is manufactured in a factory or batching plant, according
to a set recipe, and then delivered to a work site, by truck mounted in–transit mixers. This results in a
precise mixture, allowing specialty concrete mixtures to be developed and implemented on

16. construction sites. The first ready-mix factory was built in the 1930s, but the industry did not begin
to expand significantly until the 1980s, and it has continued to grow since then.
Ready mix concrete is sometimes preferred over on-site concrete mixing because of the
precision of the mixture and reduced work site confusion. However, using a pre-determined concrete
mixture reduces flexibility, both in the supply chain and in the actual components of the concrete.
Ready Mixed Concrete is also referred as the customized concrete products for commercial purpose.
The Ready-mix Concrete Company offer different concrete according to user's mix design or
industrial standard.
The ready mixed concrete company is required to equip themselves with up-to-date
equipment, such as transit mixer, concrete pump, and Concrete Batching Plant, which needs
visualized production management software and also PLC controller.
Ready Mixed Concrete, or RMC as it is popularly called, refers to concrete that is specifically
manufactured for delivery to the customer's construction site in a freshly mixed and plastic or
unhardened state. Concrete itself is a mixture of Portland cement, water and aggregates comprising
sand and gravel or crushed stone. In traditional work sites, each of these materials is procured
separately and mixed in specified proportions at site to make concrete. Ready Mixed Concrete is
bought and sold by volume - usually expressed in cubic meters.
Ready Mixed Concrete is manufactured under controlled operations and transported and
placed at site using sophisticated equipment and methods. RMC does not assures its customers
numerous benefits.
As the name indicates, Ready Mixed Concrete (RMC) is the concrete which is delivered in
the ready-to-use manner. RMC is defined by the American Concrete Institute’s Committee 116R-90
as:
“Concrete that is manufactured for delivery to a purchaser in a plastic and unhardened state”.
The Indian Standard Specification IS 4926:2003 defines RMC as:
“Concrete mixed in a stationary mixer in a central batching and mixing plant or in a truck-
mixer and supplied in fresh condition to the purchaser either at the site or into the purchaser’s
vehicles”.

17. In India, concrete has traditionally been produced on site with the primitive equipment’s and
use of large labor force. Ready mixed concrete is an advanced technology, involving a high degree of
mechanization and automation. A typical RMC plant consists of silos and bins for the storage of
cement and aggregates respectively, weigh batchers for proportioning different ingredients of
concrete, high efficiency mixer for thorough mixing of ingredients, and a computerized system
controlling the entire production process. The quality of the resulting concrete is much superior to
site-mixed concrete.
SITE MIXED VERSUS READY MIXED CONCRETE
Technologically speaking, ready mixed concrete is certainly an advancement over the age-old
site mixed concrete. The benefits of RMC in terms of quality, speed, life-cycle cost and
environmental friendliness are overwhelmingly superior to those of site mixed concrete. Following
brief comparison illustrates this vividly:
QUALITY OF CONCRETE:
RMC-India uses sophisticated plant and equipment, which enables it to produce quality
concrete. The Company exercises strict control on the quality of all ingredients through rigorous
testing, applies stringent controls on process parameters, meticulously monitors key properties of
concrete in the fresh and hardened state and applies the well-known Cusum technique to quickly
detect any changes in the properties of concrete. All these efforts result in providing uniform and
assured quality of concrete to customers. In contrast, in a typical site-mixed concrete there is poor
control on the quality of input materials, batching of ingredients and mixing of concrete, thus the
resultant quality of concrete is poor, non-uniform and inconsistent.
SPEED OF CONSTRUCTION:

18. Mechanized operations at RMC-India’s plants ensure that construction activities are speeded
up. While the production output from a typical site-mixed concrete operation using 8/12 mixer is
around 4-5 m3/hour, the output form a 60-m3/hour RMC plant is around 45 m3/hour. Thus there is
nearly 10-fold increase in the output from RMC plant, which translates into direct savings to the
customer!
ELIMINATION OF MATERIAL PROCUREMENT REQUIREMENTS AND STORAGE
HASSLES:
With the use of RMC, customers are not required to procure and store cement, aggregates,
sand, water and admixtures at site. This not only drastically reduces the space requirements at
construction sites but also minimizes efforts on the part of customers to procure different materials,
ensure their proper storage and check their quality parameters from time to time.
SAVING IN LABOUR REQUIREMENT:
Site-mixed concrete is a labor-intensive operation and managing large labor force is a big
hassle for the customer. With the use of RMC the labor requirements are minimized considerably,
thus benefiting customers. Further, as RMC-India looks after the entire QA & QC needs, the
customer’s manpower requirement for QA & QC operations is minimized. This is a saving for the
customers.
REDUCTION IN WASTAGE:
In site-mixed concrete job, wastage occurs in handling of all materials, including cement. The
latter is generally of the order of about 2-3 kg per 50 kg bag of cement. All such wastages are
considerably minimized at RMC facility.
IMPROVED LIFE CYCLE COST:
Increased speed of construction coupled with reduction in labor cost and wastage results in
considerable savings to customers. Further, the improved quality of concrete translates into enhanced
long-term durability of concrete, thus minimizing the maintenance and repair costs. Overall, when
one considers the life cycle costs, the use of RMC become cost-effective in the long run. The
benefits directly accrue to the customers.
RMC IS ECO-FRIENDLY:
All plants of RMC-India pass the pollution control norms and are duly certified by the state
pollution control authorities. As mentioned earlier, wastages are reduced drastically with the use of

19. RMC. Further, RMC-India optimizes the mix proportions using the maximum possible potential
from each material ingredient. All these improve the environmental performance of concrete
produced by the Company. Finally, with the approval of customers/consultants, RMC-India uses a
variety of supplementary cementitious materials like fly ash, blast-furnace slag, silica fume, etc. in
concrete, thus conserving cement and helping in reduction in emissions of greenhouse gases like
CO2. Thus, concrete produced by RMC-India can certainly be considered to be eco-friendly.
OVERALL, THE USE OF RMC IS BENEFICIAL TO USERS FROM ALL ANGLES:
Better quality, higher speed, better durability, savings in labor, reduction in wastages,
reduction in life cycle cost, etc.
Q. What are the advantages of using ready-mixed concrete over site-mixed concrete?
How much is the difference between the cost of ready-mixed and site-mixed concrete?
A A detailed answer to the first part of the question is enumerated in the write-up on “What is ready-
mixed concrete?” under the heading Customer Support. Briefly, the two main advantages of
ready-mixed concrete are: vast improvement in the quality and uniformity of concrete and
enhanced speed of construction. Besides, there are many other advantages which include savings
in labor, reduction in wastage, elimination of material procurement requirements and storage
hassles, etc. All these advantages clearly establish the technical superiority of ready-mixed
concrete over site-mixed concrete. As regards cost, ready-mixed is slightly costlier than site-
mixed concrete. This increased cost is mainly on account of government taxation. It is indeed
unfortunate that a quality product like ready-mixed concrete is taxed in our country, whereas there
is no tax on site-mixed concrete! If tax component is removed, the cost of ready-mixed would be
quite close to that of the site-mixed concrete. However, if due consideration is given to the higher
speed of construction (which is possible due to use of ready-mixed concrete) the savings on
account of early completion of the project would far outweigh the increased cost of ready-mixed
concrete. Further, the improved quality of RMC would go a long way in enhancing the long-term
durability, thus reducing the maintenance and repair expenses and hence the life-cycle cost of
your structures. In addition, the savings in labor and wastage would also be available to you.
Thus, if you look at the overall advantages and savings, the increased cost of ready-mixed
concrete would appear to be a paltry sum.
STANDARD READY-MIX CONCRETE VS. SITE-MIX CONCRETE
 A centralized concrete batching plant can serve a wide area. Site-mix trucks can serve a
larger area including remote locations that standard trucks cannot.
 The plants are located in areas zoned for industrial use, and yet the delivery trucks can
service residential districts or inner cities. Site-mix trucks have the same capabilities.

20.  Better quality concrete is produced. Site mix can produce higher compression strength with
less water than standard batching methods.
DISADVANTAGES OF READY-MIX CONCRETE
 The materials are batched at a central plant, and the mixing begins at that plant, so the
traveling time from the plant to the site is critical over longer distances. Some sites are just
too far away, though this is usually a commercial rather than a technical issue.
 Generation of additional road traffic. Furthermore, access roads and site access have to be
able to carry the greater weight of the ready-mix truck plus load. (Green concrete is approx.
2.5 ton per m³.) This problem can be overcome by utilizing so-called 'minimix' companies
which use smaller 4m³ capacity mixers able to reach more-restricted sites.
 Concrete's limited timespan between mixing and going-off means that ready-mix should be
placed within 90 minutes of batching at the plant. Modern admixtures can modify that
timespan precisely, however, so the amount and type of admixture added to the mix is very
important.
Ready Mix Concrete is manufactured under computer-controlled operations and transported and placed at site
using sophisticated equipment and methods. RMX assures its customers numerous benefits:
 Uniform, consistent and assured quality of concrete
 Flexibility in concrete design mixes
 Easier addition of admixtures
 Faster and speedier construction
 Reduced inventories, material handling and storage of raw materials at sites
 Savings in labor requirements, labor costs and supervision of labor
 Reduced wastage of materials
The use of RMX is an environmental friendly practice that ensures a cleaner work place and causes minimal
disturbance to its surroundings. This makes its utility more significant in crowded cities and sensitive localities.
In contrast to this, conventional methods of making, transporting and placing concrete at most construction sites
are somewhat labor-intensive and suffer from practices which may be erratic and not very systematic.
Therefore the use of Ready Mix Concrete can prove to more cost effective in the longer term while ensuring that
structures are built faster and using concrete that comes with higher levels of quality assurance.

21. Light weight concrete
What is it ?
Light weight concrete - or foamed concrete - is a versatile material which consists primarily of a cement based
mortar mixed with at least 20% of volume air. The material is now being used in an ever increasing number of
applications, ranging from onestep house casting to low density void fills.
Foamed concrete has a surprisingly long history and was first patented in 1923, mainly for use as an insulation
material. Although there is evidence that the Romans used air entrainers to decrease density, this was not really a
true foamed concrete. Significant improvements over the past 20 years in production equipment and better quality
surfactants (foaming agents) has enabled the use of foamed concrete on a larger scale.
Lightweight and free flowing, it is a material suitable for a wide range of purposes such as, but not limited to,
panels and block production, floor and roof screeds, wall casting, complete house casting, sound barrier walls,
floating homes, void infills, slope protection, outdoor furniture and many more applications.
Not everyone knows that density and compressive strength can be controlled. In the light weight concrete this is
done by introducing air through the proprietary foam process which enables one to control density and strength
precisely.
Normal concrete has a density of 2,400 kg/m3 while densities range from 1,800, 1,700, 1,600 down to 300 kg/m3.
Compressive strengths range from up to 40 mpa down to almost zero for the really low den sities. Generally it has
more than excellent thermal and sound insulating properties, a good fire rating, is non combustible and features
cost savings through construction speed and ease of handling.
The technology is the result of over 20 years of R&D, fine tuning the product and researching the possible
applications. It is used in over 40 countries world wide today and has not reached the end of its possible uses.
Frequently asked questions
 How strong is it ?
Strength is a relative term. Concrete mixes should be designed based on end use. High compressive
strength is useful where deadload or abrasion are factors, but are unnecessary for roofs and non -structural
partitions. All concrete is deficient in tensile and shear strengths, however these are supple mented
through structural reinforcement. Compressive strength can be made up to 40 Mpa, exceeding most
structural requirements.
 What are the advantages of pre-formed foam ?
The pre-formed foam process offers excellent quality control and assurance of specified density.
Preformed foam, unlike gas-forming chemicals, assures a consistent three-dimensional distribution of the
engineered air cell system. Pre-formed foam produces a consistent matrix of relatively small air cells which
are more desirable than a disorganized matrix of different size bubbles often created with the gas method
of reactive admixtures.
 What are the disadvantages of lightweight concrete, compared to typical concrete ?
In the lower density ranges lightweight concrete does not develop the compressive strength of plain
concrete. While this may be a disadvantage in plain concrete applications, it is an advantage in a
lightweight concrete application. It should be considered that lightweight concrete and plain concrete are
typically used for different types of applications. Each form of concrete exhibits a unique family of
performance characteristics. Each should be utilized in the appropriate type of project. But a high strength
of 33 Mpa has been achieved with a high cement content mix.

22.  Is segregation a problem ?
Unlike plain concrete there is little to segregate in lightweight concrete which makes segregation a moot
point. The lightweight concrete equivalent to segregation would be a collapse of the air cell system and a
volume reduction in material. To prevent this one should use the most stable liquid foam concentrates and
treat the mixed lightweight concrete with some care in placing. Fresh lightweight concrete is not fragile and
can be pumped for long distances.
 Is lightweight concrete chemically compatible with common additives ?
Lightweight concrete is compatible with common concrete construction additives; however, most common
admixtures are added to plain concrete to effect a change in the characteristics of the concrete that are
not applicable to lightweight concrete application performance. As an example, lightweight concrete needs
no air entrainment or finishing aids; however, colour admixtures and strength enhancing admixtures work
well if they are applicable to the project.
 What additives are common to cellular concrete ?
Fiber reinforcement, Heat-of-hydration reducers (iced water or chemicals), Compressive strength
enhancers, Colouring pigments or colour enhancing admixtures
 What is the correct water to cement ratio for the cement water slurry ?
Typically, a .5 water to cement ratio slurry consisting of two parts cement to one part water is typically
used as a base mixture for lightweight concrete. The water cement ratio varies according to specific
project requirements. Note that lightweight concrete obtains it's natural fluidity from the air bubble
structure, not from excess water content.
 Does lightweight concrete mix contain either fine or course aggregate ?
Lightweight concrete may also contain normal or lightweight, fine and/or coarse aggregates. The rigid
foam air cell system differs from conventional aggregate concrete in the methods of production and in the
more extensive range of end uses. Lightweight concrete may be either cast-in-place or pre-cast.
Lightweight concrete mix designs in general are designed to create a product with a low density and
resultant relatively lower compressive strength (when compared to plain concrete). When higher
compressive strengths are required, the addition of fine and/or course aggregate will result in a stronger
lightweight concrete with resultant higher densities. We should note that most lightweight concrete
applications call for a lightweight material. When considering the addition of course aggregate, one must
consider how appropriate this heavy aggregate will be to a project, which typically calls for lightweight
material. The inclusion of aggregate, particularly course aggregate may be counter productive to the
materials intended performance.
 What type of cement is appropriate for lightweight concrete ?

23. Lightweight concrete may be produce with any type of portland cement or portland cement & fly ash
mixture. The performance characteristics of type II, type III and specialty cements carries forward into the
performance of the lightweight concrete.
 Is it appropriate to add fly ash to the cement and water slurry for lightweight concrete ?
Fly ash added to the cement does not adversely affect the basic hardened state of lightweight concrete.
Infusing and supporting the lightweight concrete with the air cell system is a mechanical action and is not
problematic with fly ash or other additives. Note that some fly ash mixes may take longer to set than pure
portland cement applications. Mixes with large percentages of fly ash may take an very extended time to
set up. High carbon content fly ash such as typical "bottom ash" should be generally avoided in most
cellular or plain concrete mixes.
 Is it appropriate to reinforce cellular concrete with synthetic fibers ?
Synthetic fiber reinforcement is a mechanical process and does not have any effect on the chemistry of
concrete. It is therefore perfectly acceptable to design fiber reinforced lightweight concrete. Fiber
reinforced cellular concrete is becoming a standard material for roof decks and Insulated Concrete Form
(ICF) construction. Oil palm fibers are also successfully being added and it produces a very good design
mix of 900 kg density per meter cube most suitable for high rise buildings wall panels.
 Is it appropriate to reinforce cellular concrete with steel fibers ?
There is no chemical or mechanical reason not to reinforce lightweight concrete with steel fibers. However,
most lightweight concrete applications require a lightweight material. Most steel fiber concrete applications
require heavy, high compressive strength steel fiber reinforced concrete. It would seem somewhat unlikely
that an application would require steel fiber reinforce lightweight concrete, but there is no technical reason
not to design a steel fiber reinforced lightweight concrete.
 Do the bubbles in lightweight concrete collapse, reducing its volume ?
Not with well engineered liquid foam concentrates. The pre-formed foam lightweight concrete products
made from top quality liquid foam concentrates do not collaps e. Air cell stability is the mark of a superior
foam concentrate and foam generator combination. Which is not to say that all lightweight concrete
products are stable. Particular care should be taken to test foams from water pressure type foam
generators, and gas-off chemical products. The proposed pre-formed foam for an application should be
tested for stability or certified for stability before actual project placement.
 Densities and Strengths
One of the most useful features of a lightweight concrete sys tem is the system's ability to be manufactured
in a wide range of low densities and strengths. Application requirements for lightweight concrete range
from very light density low strength fill dirt replacement to higher strength structural lightweight concrete.
To accommodate this wide range of performance properties lightweight concrete has developed a mix
design chart, which will illustrate the basics of making this wide range of materials from just one
lightweight concrete concentrate. With a lightweight concrete foam generator and a single liquid foam

24. concentrate the contractor now has available to them a wide variety of cost effective, high performance,
lighter lightweight concrete products.
 What are the different densities and strengths available ?
Lightweight concrete exhibits a much lighter density than typical aggregate concrete. Typical plain
concrete has a density of 2400 kg/m3, lightweight concrete densities range from 300 kg/m3 to 1800 kg /
m3. Lightweight concrete is an insulator and can be used in a variety of applications which require an
insulating material that can also exhibit some integrity and strength. Lightweight concrete at its lightest
density is still more stable and strong than well compacted soil. When replacing soils, lightweight concrete
can be designed to provide whatever strengths and characteristics needed for the soil stabilization project.
Some soil engineers lightheartedly refer to lightweight concrete used in Geotechnical stabilization projects
as "designer dirt." They know that lightweight concrete can be specified to easily exceed whatever
compacted soil requirements are needed.
 How much does lightweight concrete cost ?
Cost effective lightweight concrete varies in price by geographical area and by application requirements
such as density and strength requirement. A typical concrete structure project will be much less expensive
cubic meter to cubic meter when compared to plain concrete due to labour savings, less cost of forming
works, less steelworks, eliminate brickworks, cement renderings work and the price savings is very
substantial when compare to conventional methods.
 Is lightweight concrete suitable for long-term use as a marine float device ?
At the lower densities, lightweight concrete will float, and in many cases float indefinitely. Because of its
limited impact and abrasion resistance, lightweight concrete used for marine flotation should be encased
and used for the fill of a float. For example, a marine float could be made with sealed drums filled with low -
density lightweight concrete.
 Where do I purchase lightweight concrete ?
Lightweight concrete is purchased through a licensing system. For Australia the master licensee is
LYNKFS Pty Ltd and can be contacted through its representatives.
 How to produce lightweight concrete ?
The pre-formed foam is added to the cement slurry and mixed in the concrete mixer or in a continuous
process. From that point, lightweight concrete is placed in any way that a fluid mix can be transported.
Pumping is the most common method of placement. Tailgate ready mix truck delivery, bucket cranes,
wheelbarrows, hand carried buckets and any other acceptable method of delivering a fluid mix works well.
 Can lightweight concrete be under mixed ?

25. The cement and water slurry should be mixed until there are no dry clumps or balls of cement. The pre-
formed foam mixture is then added into the mixture. The foam mixes quite rapidly into the slurry and only
requires modest mixing times depending upon the mixing equipment.
 Can cellular concrete be over mixed ?
Mixing until there is a reduction of volume of product is not recommended. Air cell stability is the mark of
our liquid foam concentrates and our Foam Generators. With typical mixing procedures, lightweight
concrete formulated with pre-formed foam is very stable even with modestly extended mixing times.
 How far can lightweight concrete be pumped ?
Lightweight concrete is a very easily pumped, highly fluid mixture. The bulk of lightweight concrete is
placed by pumping. Lightweight concrete typically will move through the pump lines using less pressure
than typical heavier grout mixes
 How do you finish lightweight concrete ?
Most lightweight concrete is left to self-seek a level and not surface "finished" in the traditional sense.
Much lightweight concrete is covered by another material. A floor overlayment type smoother tool can be
used simply to break the surface air cells and create a more uniform and polished look to the surface in
the rare case when a more uniform surface appearance is desired.
 How do I test lightweight concrete to determine it is performing to specs ?
Test procedures for lightweight concrete are beyond the scope of this FAQ document; however, lightweight
concrete representatives will be happy to assistyou in the actual testing or furnishing descriptions of
common tests. Properties commonly tested are for its compressive strength The majority of regular
concrete produced is in the density range of 2400 kg permeter cube. The last decade has seen great
strides in the realm of dense concrete and fantastic compressive strengths which mix designers have been
achieved. Yet regular concrete has some drawbacks. It is heavy, hard to work with, and after it sets, one
cannot be cut or nailed into it without some difficulty or use of special tools. Some complaints about it
include the perception that it is cold and damp. Still, it is a remarkable building material - fluid, strong,
relatively cheap, and environmentally innocuous and available in almost every part of the world.
Lightweight concrete begins in the density range of less than 300 kg/m3 to 1800 kg per/m3. It has
traditionally been made using such aggregates as expanded shale, clay, vermiculite, pumice, and scoria
among others. Each has their peculiarities in handling, especially the volcanic aggregates which need
careful moisture monitoring and are difficult to pump. Decreasing the weight and density produces
significant changes which improves many properties of concrete, both in placement and application.
Although this has been accomplished primarily through the use of lightweight aggregates, since 1960
various preformed foams have been added to mixes, further reducing weight. The very lightest mixes (from
300 kg /m3 to 800 kg / m3) are often made using only foam as the sand and aggregate are eliminated, and
are referred to as floating lightweight concrete. The entrapped air takes the form of small, macroscopic,
spherically shaped bubbles uniformly dispersed in the concrete mix. Today foams are available which have
a high degree of compatibility with many of the admixtures currently used in modern concrete mix designs.
Foam used with either lightweight aggregates and/or admixtures such as fly ash, silica fume, synthetic
fiber reinforcement, and high range water reducers (aka superplasticizers), has produced a new hybrid of
concrete called lightweight concrete materials. For the most part, implementation of Lightweight Composite

26. design and construction utilizes existing technology. Its uniqueness, however, is the novel combination
drawing from several fields at once: architecture, mix design chemistry, structural engineering, and
concrete placement.
Multi-storey office buildings
The dominance of steel in the multi-storey commercial sector is based on tangible client-related
benefits including the ability to providecolumn free floor spans, efficient circulation space,
integration of building services, and the influence of the site and local access conditions on the
construction process. For inner city projects, speed of construction and minimum storage of materials
on-site require a high level of pre-fabrication, which steel-framed systems can provide.
There is a strong demand for high quality office space, especially in city centres. Corporate
headquarters for banks and other high profile companies require that buildings are built to high
architectural and environmental standards. Investment ‘value’ is the main criterion for choice of the
building architecture, form and servicing strategy. Many buildings are curved or of complex
architectural form, and have highly glazed façades and atria.
In many large commercial buildings, a two stage construction process means that the tenant is
responsible for the servicing and fit-out, and so the building structure has to be
sufficiently flexible to cope with these differing requirements. Many smaller buildings are designed
for natural ventilation and with a high proportion of renewable energy technologies built into them.
Many solutions are possible using steel construction.
Attributes of steel construction
Main article: The case for steel, Service integration, Cost of structural steelwork, Cost planning
through design stages, Cost comparison study, Health and safety
The commercial sector demands buildings that are rapid to construct, of high quality, flexible and
adaptable in application, and energy efficient in use. Steel, and in particular, composite
construction has achieved over 70% market share in this sector in the UK where the benefits of long
spans: speed of construction; service integration; improved quality; and reduced environmental
impact are widely recognised.
The overall building economics are fundamental on the rationale for using steel construction in the
commercial building sector, where the market share for steel has been consistently 65 to 70% for the
last 20 years.
Value for money
Recent cost comparison studies show that the building superstructure generally accounts for only
10% to 15% of the total building cost and that the influence of the choice of structure on the
foundations, services and cladding costs is often more significant. For example, a reduction of 100
mm in the ceiling to floor zone can lead to a 2.5% saving in cladding cost (equivalent to 0.5% saving
in overall building cost).
Therefore, best practice building design requires a synthesis of architectural, structural, services,
logistics and constructional issues. Where this synthesis has been achieved, long-span steel
systems with provision for service integration dominate commercial building design.

27. The results of a recent independent cost comparison study of multi-storey commercial buildings can
be seen here.
Factor Improvement Economic benefit
Speed of
construction
20 to 30% reduction in
construction time relative to
site-intensive construction,
depending on the scale of the
project.
The economic benefit depends on the business
operation. In terms of overall building cost, a
saving of 1% in interest charges and 2% in early
rental or use of the space is predicted.
Site
management
costs
Site management costs are
reduced because of the
shorter construction period,
and the packaged nature of
the construction process.
Site management costs can be reduced by 20 to
30% which can lead to a 3 to 4% saving in terms of
overall building cost.
Service
integration
The integration of services in
the structural zone leads to
reduction of 100 to 300mm in
floor to floor zone and hence
to savings in cladding cost.
A 5% reduction in floor to floor height can lead to
one additional floor in 20, and to a similar
reduction in cladding cost, which is equivalent to
about 1% in total building cost.
Foundations
Steel construction is less than
half the weight of an
equivalent concrete structure,
which is equivalent to a 30%
reduction in overall
foundation loads.
Foundation costs depend on the sub-structure and
factors such as underground services and represent
5 to 15% of the building cost. A 30% reduction in
foundation loads can lead to a 2 to 3% overall
saving in terms of construction cost.
Column free
space
Long span steel construction
provides more flexible use of
space, which depends on the
function of the building and
its future uses.
A large column in the middle of the space leads to
a loss of space of approximately 1m2, which
represents about 1% of the floor area, and may lead
to an equivalent loss of rental income.
Summary of the economic benefits of steel construction in office buildings
Speed of construction
All steel construction uses pre-fabricated components that are rapidly installed on site. Short
construction periods leads to savings in site preliminaries, earlier return on investment and reduced
interest charges. Time related savings can easily amount to 3 to 5% of the overall project value,
reducing the client’s requirements for working capital and improving cash flow. In many inner city
projects, it is important to reduce disruption to nearby buildings and roads. Steel construction
dramatically reduces the impact of the construction operation on the locality.

28. [top]Flexibility and adaptability
Long spans allow the space to be arranged to suit open plan offices, different layouts of cellular
offices and variations in office layout throughout the height of the building. Where integrated beam
construction is used, the flat soffit gives complete flexibility of layout allowing all internal walls to
be relocated, leading to fully adaptable buildings.
Long spans, open plan commercial office space – Vulcan House, Sheffield
[top]Service integration
Complex service routing through and between steel beams
Steel and composite structures can be designed to reduce the overall depth of the floor zone
by integrating major services within the depth of the structure, and/or by achieving the minimum
structure depth. This is important in cases in which the building height is restricted for planning
reasons, or in renovation projects.
[top]Quality and safety
Off-site prefabrication improves quality by factory controlled production, and is less dependent on
site trades and the weather. Working in a controlled, manufacturing environment is
substantially safer than working on site. The use of pre-fabricated components reduces site activity

29. for frame construction by up to 75%, thereby substantially contributing to overall
construction safety.
[top]Sustainability
Many of the intrinsic properties of steel usage in construction have significant environmental
benefits. For example, the steel structure is 100% recyclable, repeatedly and without any
degradation, the speed of construction and reduced disruption of the site gives local environmental
benefits and the flexibility and adaptability of steel structures maximise the economic life of the
building as it can accommodate radical changes in use.
Modern fully glazed façade system in an office building in Spinningfields, Manchester
The structural efficiency of steel and composite constructionleads to resource efficiency. For
example, composite steel construction achieves the highest rating of A+ in the Green Guide to
Specification.
Recent research under the Target Zero programme has confirmed that steel-framed commercial
buildings can achieve low operational carbon targets and the highestBREEAM ratings cost
effectively.
Anatomy of commercial buildings
Main articles: Concept design
The anatomy of a commercial office building is function of its size and location, i.e. city centre tower
or two-storey science park office building at the two extremes, and client and planning requirements.
Some key aspects of building anatomy are described below. The common features that influence the
building design are:
 Open plan areas that can be configured to suit the client requirements

30.  Partitioned space for executive offices, conference room etc. Partitions should be moveable for
future re-configuration
 Communal space for toilets, kitchens, etc, which are often located near to service risers
 Access space for lifts, stairs and services maintenance, including means of escape in fire
 Featured space, such as the entrance lobby, atria, and penthouse
 Service plant areas which may be located on the roof or in the basement
 Below ground car parking in some cases.
[top]City centre commercial buildings
Palestra building, London produces some of its operational energy from renewable sources
Commercial buildings in city centres tend to be relatively tall (6 to 12 storeys is a typical city centre
project) because of the high cost of land and the confinement of adjacent buildings and utilities.
Planning requirements have a strong impact on the building form and its architecture, and in many
parts of the country, it is a planning objective that commercial buildings are required to generate a
proportion of their on-site energy use from renewable sources, e.g. photovoltaics, heat pumps, CHP,
CCHP, as in the Palestra building near Waterloo, London.
An important aspect of many modern commercial building developments is the need for retail space
at ground floor, office space above, and in many cases, below ground car parking. This can lead to
complexity in the alignment of planning grids from floor to floor. A common solution is to create a
transfer structure at ground or first floor levels to optimise the space use above and below.
The sub-structure of city centre projects tends to be complex because of the high loads that are
supported, the need to avoid affecting the foundations of neighbouring buildings, and to avoid
obstructions and services in the ground. Piled foundations below basement level are most commonly
used and the piles are placed in a group of typically 3 or 4 below a pile cap. There are various

31. techniques to form basements including temporary sheet pile walls supported by steel H sections and
contiguous bored pile walls.
Services also tend to be complex and some form of combined structure-services zone is considered
in the building design. Vertical services are routed at discrete points on plan and distributed
horizontally through the building. Long-span solutions are commonly used in this sector in order to
optimise the internal space use. The building facades and roof tend to be lightweight, such as unitised
curtain walling or infill walls supporting metallic or architectural façade systems.
[top]Tall commercial buildings
In London, a number of major towers have taken steel to new frontiers. Two of these are the Swiss
Re building and the Broadgate Tower.
London’s Swiss Re building by Normal Foster is now an iconic building, and consists of a diagrid
assembly of inclined members and welded nodes to form the complex curved shape in two
directions. The steel structure was therefore a key part of the architectural concept.
Broadgate Tower was unusual in that it was a 35 storey super-structure constructed over the railway
lines to London’s Liverpool Street station, and therefore the need to minimise the weight of the
structure on its inclined supports and on the foundations was important to the design solution. This
project was completed without disrupting the day to day operation of this major London railway hub.
Swiss Re building – an iconic building on London’s skyline Broadgate Tower London spans over railway lines
Tall steel-framed commerical buildings

32. The requirements for access to the upper levels of tall buildings and for overall stability mean that
the core area is a high proportion of the plan area and is generally located centrally on plan. The
office space wraps around the core, and from a functional point of view, this space should be as
flexible as possible. The main beams therefore radiate from the core and are supported on
perimeter columns. The provision of natural lighting tends to mean that the width of the office space
is limited to about 15m. Services emanate from the core and are distributed through openings in the
structure.
The nature of the construction is that the core is generally in slip formed reinforced concrete. The
core construction progresses a few floors above the steel construction, which is faster and so its
progress is limited by the construction of the core.
[top]Commercial buildings with atria
Larger commercial buildings are often designed around an atrium, which provides natural lighting
and circulation space for the offices around it. There are many examples of this form of construction,
such as in Mid-City Place, More London 7 and Tower Place in London. The plan form of More
London 7 is shown below.
Complex plan form of More London 7 showing its atrium
The area of the building on plan tends to be large (over 1,000m2 per floor) and the atrium is often
located centrally, or may form part of an extended entrance area. The atrium is designed as part of
the whole building energy and lighting strategy, and also provides the safe means of escape in fire;
therefore smoke control in the atrium is a crucial part of the design solution. For a building layout
point of view, the commercial space is typically 15 to 18m wide around the atrium and the cores are
located at positions dictated by means of escape in fire. Generally, a minimum of two cores, and
often as many as four separate cores are required on plan in buildings with atria. The simplified plan
form of an office building with a central atrium is shown.

33. Typical columns layout in an office building with an atrium
The optimum use of space means that there is benefit in designing the structure to span from the
atrium to the façade columns, which are located typically on a 6 to 8m grid around the perimeter of
the building. The service routes from the cores can be relatively long, which means that the duct
sizes can be large when distributed from the core. In this area, the use of shorter span beams with
large rectangular openings may be more practical.
Column layout in an office building with a central core

34. The steel elements used in the atrium are generally in the form of hollow sections and tension ties,
which are often designed architecturally to emphasise the high quality of the public space that is
created.
Tower Place in London combines a wide range of steel members, including hollow sections in the 6
storey high entrance atrium, as shown.
Braced frames
The majority of structural systems used in office construction are braced by one of two methods;
 Steel bracing, generally in the form of cross-flat plates or hollow sections that are located in the façade walls, or in
internal separating walls, or around service areas and stairs.
 Concrete or steel plated cores that enclose the stairs and lifts, service risers, toilets etc.
The choice of this system depends on the form and scale of the buildings. In most buildings up to 6 storeys
high, steelbracing is preferred, although its location is strongly influenced by the layout of the building. V or K
bracing using tubular sections is often preferred as it is more compact and can be arranged around windows
and doors in some cases. X flat bracing is preferred for use in brickwork as it can be located in the cavity
between the leaves of the brickwork.
For taller buildings, concrete cores are more efficient and they can either be constructed floor by floor using
conventional formwork, or slip-formed continuously. The relative economics is dictated by speed of
construction, and slip forming is often used on tall buildings (see Commercial buildings with atria). Steel plated
or composite cores are also used where there is need to minimise the space occupied by the core and where
it can be constructed in parallel with the steel framework.
The structural design of the steel frame is therefore based on the use of simple shear resisting connections for
both the beam to column and beam to beam connections.
Lateral stability system overview

35. Continuous frames
Internal view of the Palestra building during construction showing the use of pairs of continuous cellular beams
Continuous frames achieve continuity of the beams either by design of the steel structure so that they are multi-
span, or by use of moment-resisting connections. In the Palestra building, theprimary beams were arranged in
pairs either side of the tubular columns, and the beams were continuous across the building, being spliced only
at the quarter span positions from the internalcolumns where bending moment were low. In that way,
the beamsare stiffer due to their continuity than the equivalent simply supported beam and so that depth can be
reduced. A view of the building during construction is shown.
In buildings up to four storeys in height, it may be economic to design the steel structure as a sway frame to
resist lateral loads applied to the building. The connections between the beams and the columns are
made moment-resisting by use of extended end plate connections. The columns may be heavier than in simply
supported design, but the beams can be lighter, and bracing is eliminated. This may be advantageous in low-
rise buildings with highly glazed facades.

36. Composite construction
Services located below downstand composite beam
Composite construction consists of downstand I-section steelbeams with shearconnectors (studs)welded to the top
flange to enable the beam to act compositely with an in-situ composite floor slab.
The composite slab comprises profiled decking of various shapes that span 3m to 4m between secondary
beams. Floor slabs are typically 130mm to 150mm deep, depending on the deck height. The shear connectors
are normally site welded through the steel decking which then supports the wet weight of the concrete and
construction loading and later acts compositely with the concrete.
The secondary beams in the floor grid support the composite slab and are supported by primary beams.
These beams are usually designed as composite, and in the optimum floor grid the secondaries span around
50% longer than the primaries, so that they are of similar depth. Therefore, 6m x 9m and 7.5m x
12mcolumn grids are commonly used in composite construction.
Heating and ventilation units can be positioned between beams, but ducts will generally pass below downstand
beams. Typically, for a 7.5m x 6m floor grid, the overall floor zone is 1100mm to 1200mm allowing for a 150mm
raised floor and 400mm deep air conditioning ducts below the beams. This floor depth may reduce to 700mm
in the case without air conditioning services. A typical example of a composite beam with service routing is
shown.
Long span systems
Beams withweb openings
Long span composite beams are often designed with large web openings to facilitate integration of services, as
shown. In long-span construction, grids are generally arranged so that the long span secondary beams are
supported by shorter span primary beams. It may be economic to design long span primary beams that support
shorter span secondary beams, when considering the use ofcellular and fabricated sections.

37. The two options are:
 Long span secondary beams: 10m to 15m span at 3m to 4m spacing.
 Long span primary beams: 9m to 12m span at 6m to 9m spacing.
Service openings can be circular, elongated or rectangular in shape, and can be up to 70% of the beam depth.
They can have a length/depth ratio typically of up to 3.5. Web stiffeners may be required around large
openings.
Elongated or rectangular openings should be located in areas of low shear, e.g. in the middle third of the span
for uniformly loaded beams. Isolated openings can be reinforced by horizontal stiffeners, as shown, which
increases their resistance to shear by local bending around the openings (Vierendeel bending).
Stiffened large web opening in a steel beam Services located through web openings in thebeams
Cellular beams
Cellular beams are beams with openings regular spacing along their length. The beams are made by cutting
and re welding hot rolled steel sections. Openings, or ‘cells’, are normally circular, which are ideally suited to
circular ducts, but can be elongated, rectangular or hexagonal.
The full range of hot rolled steel section sizes is available from which to choose the sizes of the top and bottom
chords. For compositedesign, the top chord is generally chosen as a lighter section than the bottom chord.
Cellular beams are generally arranged as long span secondary beams, supporting the floor slab directly, as
shown.

38. Long span cellular beams with regular circular openings
Fabricated beams
Fabricated beams are made from three steel plates whose sizes can be selected for the particular loading case.
Openings for services can be cut into the web, and the sizes of the openings can be designed depending on
the forces acting at a point in the span. An example of fabricated beams with circular elongated-circular and
rectangular openings is shown.
One of the advantages of the use of fabricated beams is that they can de designed to support relatively heavy
loads when used as long spanning primary beams.
Fabricated beams with large web openings of various shapes
Other types of long span beams
Tapered beams can be designed so that the depth of the beam is tailored to broadly match the bending moment
applied to it. In this way, the depth of the tapered section is normally in the form of a single linear variation

39. from mid-span to the supports, and the minimum depth at the supports is sized only to provide the required
shear resistance. Relatively wide zones for services are provided near to the supports. An example of a pair
of tapered beams is shown.
Tapered fabricated beams provide for service zones next to the columns
Shallow floor beams
Shallow floor beams such as asymmetric slim floor beams (ASBs) may be used to support composite
slabs using deep decking in theSlimdek system. ASBs are hot rolled steel beams with a wider bottom flange than
top. The section has embossments rolled into the top flange and acts compositely with the concrete
encasement without the need for additional shear connectors. The deep deckingspans between the bottom
flanges of the beams and supports the loads during construction.
Span arrangements are normally based on a 6m to 9m grid, with a slab depth of 280 to 350mm. The deep
decking requires propping during the construction stage for spans of more than 6m. Reinforcing bars (16 to
25mm diameter) placed in the ribs of the slab give sufficient fire resistance to ensure that no protection to the
deck is necessary.
ASB sections are generally approximately 300mm deep. The sections may be rolled with relatively thick webs
(equal to or thicker than the flanges), which offer a fire resistance of 60 minutes without
additional protection (for normal office loading) when used as part ofSlimdek.
Services can be integrated by forming elongated openings in the webs of the beams, and by locating ducts
between the ribs of thedeep decking, as illustrated. An even higher level of service integration can be achieved
by placing chilled beams and lighting between the ribs of the deep decking to minimise the overall floor depth to
around 500mm.
Edge beams can be Slimflor beams utilising a rectangular hollow section, or downstand beams. Ties,
normally Tees with the leg cast in the slab, are used to restrain the columns internally in the direction at right
angles to the main beams.

40. Components in the Slimdek system Services located in the Slimdek system
Floor systems
Deck profiles used in composite construction

41. The three generic forms of flooring systems which are most commonly used in steel framed office buildings are:
 Shallow composite slabs using steel deck profiles typically of 50 to 80mm depth.The slab depth is usually 130 to
160mm, depending on the deck depth and fire insulation requirement. Typical spans are 2.5 to 4.5m depending on
the deck spanning capabilities.
 Shallow floor construction with deep composite slabs using steeldecking of 210 to 225mm depth in which the
typical slab depth is 280 to 350mm. Deep composite slabs are mainly used in shallow floor construction for spans of
5 to 9m.
 Precast concrete slabs generally in the form of hollow-core units of 150 to 300mm depth with an in-situ concrete
slab of 60 to 100 mm depth.Typical spans are 5 to 10m depending on the depth of the hollow-core units.
Composite floors using shallow decking
The generic shallow deck profiles used in composite floors are illustrated. During the construction stage, and
prior to composite action of the decking and concrete being fully achieved in the normal stage, the decking
alone will need to support the load due to wet concrete and construction live loads. Pattern loading due to the
construction sequence should also be considered during the construction stage. During construction, propping
to the decking may allow longer spans to be achieved in the normal stage. Propping may be seen as
impacting on access and programme and to whether or not to prop the decking should be discussed early in
the design process. Props should be left in place until the concrete has reached its design strength.
Modern composite slabs contain re-entrant portions to facilitate attachment of wires for suspended services and
ceiling. Trapezoidal profiles are based on a 300mm rib spacing, and often the cross-section is highly stiffened
to improve its bending and composite properties.
Mesh reinforcement in the slab provides up to 120 minutes fire resistance, but for longer periods of fire
resistance (which are unusual in the UK), reinforcing bars may be placed in the deck ribs.
Steel decking is installed by craning onto the primary steelwork in bundles and usually man-handling into
position. A fall arrest systemis installed immediately after the steelwork is erected and before the decking is
placed.
Completed and decked floors may be used as a safe working platform for subsequent installation of steelwork.
For this reason, the upper floor in any group of floors (usually three floor levels) is often concreted first.
Composite floors using deep decking
The cross-section through a shallow floor slab using deep decking is shown; this is the Slimdek system.
Cross-section through a deep composite slab – Slimdek

42. The deep decking is supported on end diaphragms which provide stability to the web of the decking and also
prevents loss of concrete when it is poured on the decking. Reinforcing bars of 12 to 20mm are placed in the
deck ribs, mainly for fire resistance purposes, and mesh reinforcement is placed in the topping. Un-propped
spans of up to 6m can be achieved. Slimdek is used in many sectors and often in mixed-use buildings and in
basements or car parks to multi-storey buildings where minimising of the floor depth has economic value.
Deep decking can also be used with Slimflor.
Precast concrete slabs
Precast concrete slabs are proprietary products that are manufactured in standard depths and widths. They are
pre-stressed to increase their spanning capabilities and stiffness which also means that they have some
negative curvature when unloaded. Hollow-core slabs have regular circular openings and in some cases,
elongated openings along their length to reduce their weight. Precast slabs are widely used in smaller offices,
but less so in large building projects. They are also used in Slimflor construction.
Key issues in the design of commercial buildings
Procurement
Procurement in the commercial building sector is often different from other sectors, depending on whether the
building is a speculative development, or is intended for a single client.
In major city centre projects, the concept of ‘shell and core’ was established in the late-1980s. In this ‘fast
track’ approach to design and procurement, the main fabric of the building is let as the first stage contract, and
then a second stage fit-out contract is managed by the team acting on the tenant’s behalf. This two stage
process means that the hand-over between architects and contractors in the two stages has to be carefully
managed to avoid division of responsibilities. The second stage fit-out can involve as little as internal
partitioning and decoration, but can include complex installation of services and specialist IT systems.
This lead to the concept of ‘loose fit’ structures giving maximum flexibility in servicing and internal space use,
for which long span steel and composite construction is ideally suited.
For single clients, the procurement process is more straightforward in that the client brief will define the
functional and spatial requirements, and then the whole construction process falls under one contract. There
are three generic contractual systems that may be used for commercial buildings depending on their scale
and the type of client:
 Traditional contracts,in which the contractoris appointed by competitive tender based on detailed drawings and
specifications. The architect often takes a more formal role in project management on behalf of the client.
 Design and build contracts,in which the contractorbids for the project, based on a more general scope of wo rk, and
is involved in much of the detailed design work. The architect involved in the early stages of the design development
with the client is often ‘novated’to the design and build contractor.
 In management contracts,the management contractororganises the project as a series of relatively large ‘packages’
and is paid a fee by the client. In this process,the supply of the steel frames and floors is a key package, as is the
detailed design and supply of the façade system etc.

43. For large commercial projects, management contracts are efficient in that the management fee are offset by
the gains that are possible by competitive tendering of the various ‘packages’ of work. Smaller commercial
building projects are often procured by ‘design and build’.
A recent important innovation is that of Building Information Management (BIM) systems in which the design
team, contractor and specialist suppliers share in a common design and drawing system so that interface and
scheduling problems are minimised. The BIM system is generally managed and controlled by the main
contractor and requires an early involvement of specialist suppliers in the design process.
Circular Slab
• Uses of Circular Slabs:
1. Roof of a room or hall circular in plan
2. Floor of circular water tanks or towers
3. Roof of pump houses constructed above tube wells
4. Roof of a traffic control post at the intersection of roads
5. In circular slab, Bending takes place in distinctly two perpendicular directions along the two
spans.
6. Reinforcement is provided in the form of a mesh of bars having equal area of cross section in
both the directions, the area being equal to that required for the bigger of the radial and
circumferential moments.
7. However, if the stresses near the edge are not negligible, or if the edge is fixed, radial and
circumferential reinforcement near the edge becomes essential.
8. Circular slabs are more commonly used in the design of circular water tank containers with
flat bottom and raft foundations.
9. The analysis of stresses in these slabs is generally based on elastic theory. Under uniformly
distributed loads, these slabs deflect in the form of a saucer and develop radial and
circumferential stresses. Tensile stresses develop on the convex surface and compressive
stresses develop on the concave surface.
10. Tensile stresses must be provided in the radial and circumferential directions near the convex
surface.
11. Alternatively, reinforcing bars can be provided in two mutually perpendicular directions
instead of in the radial and circumferential directions.
12. Normally, near the Centre of the slab, reinforcement is provided in the form of mutually right
angle mesh; and near the edge of the slab, in the form of radial and circumferential bars.
Beams
A beam is a structural element that is capable of withstanding load primarily by resisting bending. The bending
force induced into the material of the beam as a result of the external loads, own weight and external reactions to
these loads is called a bending moment.
Beams generally carryvertical gravitationalforces but can also be used to carry horizontal loads (i.e., loads due to
an earthquake or wind). The loads carried by a beam are transferred to columns, walls, or girders, which then
transfer the force to adjacent structural compression members. In light frame construction the joists rest on the
beam.

44. Types of beams
Generally beams are of five types: that is given below:
1. Simply supported beam
A simple beam is supported by a pin support at one end and a roller support at the other end.
2. Fixed beam
A beam with a laterally and rotationally fixed support at both the ends is called a fixed beam.
3. Over hanging beam
A beam simply supported at two points and having one end or both ends extended beyond the supports is called
an overhanging beam.
4. Continuous beam
Continuously supported beams aresupported by three or more support points. They deflect less than simple
beams of the same span because the positive and negative bending cancels each other out. Generally a
continuous span is 20% more efficient than a simple span as it is able to span longer distances.
5. Cantilever beam
A beam with a laterally and rotationally fixed support at one end with no support at the other end is called a
cantilever beam.
Beams are characterizedby their profile (the shape of their cross-section), their length, and their material. In
contemporary construction, beams are typically made of steel, reinforced concrete, wood, composites, or cased
fluids (inflatable beams). One of the most common types of steel beam is the I-beam or wide-flange beam (also
known as a "universal beam" or, for stouter sections, a "universal column"). This is commonly used in steel-frame
buildings and bridges. Other common beam profiles arethe C-channel, the hollow structural section beam, the
pipe, and the angle.
Beams are also described by how they are supported. Supports restrict lateral and/or rotational movements so as
to satisfy stability conditions as well as to limit the deformations to a certain allowance.

45. Structural characteristics
Moment of inertia
The moment of inertia of an object about a given axis describes how difficult it is to change its angular motion
about that axis. Therefore, it encompasses not just how much mass the object has overall, but how far each bit of
mass is from the axis. The farther out the object's mass is, the more rotational inertia the object has, and the
more force is required to change its rotation rate.
Stress in beams
Internally, beams experience compressive, tensileand shearstresses as a result of the loads applied to them.
Typically, under gravity loads, the original length of the beam is slightly reduced to enclose a smaller radius arc at
the top of the beam, resulting in compression, while the same original beam length at the bottom of the beam is
slightly stretched to enclose a larger radius arc, and so is under tension. The same original length of the middle of
the beam, generally halfway between the top and bottom, is the same as the radial arc of bending, and so it is
under neither compression nor tension, and defines the neutral axis (dotted line in the beam figure). Above the
supports, the beam is exposed to shear stress. There aresome reinforced concrete beams in which the concrete is
entirely in compression with tensile forces taken by steel tendons. These beams are known as prestressed
concrete beams, and are fabricated to produce a compression more than the expected tension under loading
conditions. High strength steel tendons arestretched while the beam is cast over them. Then, when the concrete
has cured, the tendons are slowly released and the beam is immediately under eccentric axial loads. This
eccentric loading creates an internal moment, and, in turn, increases the moment carrying capacity of the beam.
They arecommonly used on highway bridges.
General shapes
Most beams in reinforced concrete buildings have rectangularcross sections, but a more efficient cross section
for a beam is an I or H section which is typically seen in steel construction. Because of the parallel axis theorem
and the fact that most of the material is away from the neutral axis, the second moment of area of the beam
increases, which in turn increases the stiffness.
An I-beam is only the most efficient shape in one direction of bending: up and down looking at the profile as an I.
If the beam is bent side to side, it functions as an H where it is less efficient. The most efficient shape for both
directions in 2D is a box (a square shell) however the most efficient shape for bending in any direction is a
cylindrical shell or tube. But, for unidirectional bending, the I or wide flange beam is superior.
Efficiency means that for the same cross sectional area (volume of beam per length) subjected to the same
loading conditions, the beam deflects less.
Other shapes, like L (angles), C (channels) or tubes, are also used in construction when there are special
requirements.