about mixing of concrete in site,types and structural design for repairs in rcc

1. REINFORCED CEMENT CONCRETE

2. HARDWARE: THE MIXERS
There are two main categories of mixer: batch mixers and
continuous mixers. The first type of mixer produces concrete one
batch at a time, while the second type produces concrete at a
constant rate. The first type needs to be emptied completely after
each mixing cycle, cleaned (if possible), and reloaded with the
materials for the next batch of concrete. In the second type, the
con- stituents are continuously entered at one end as the fresh
concrete exits the other end
BATCH MIXERS
Two main types of batch mixer can be distinguished by the
orientation of the axis of rotation: horizontal or inclined (drum
mixers) or vertical (pan mixers). The drum mixers have a drum, with
fixed blades, rotating around its axis, while the pan mixers may have
either the blades or the pan rotating around the axis.

3. DRUM MIXERS
All the drum mixers have a container purpose The blades are attached to the
inside of the movable drum. Their main is to lift the materials as the drum
rotates. In each rotation, the lifted material drops back into the mixer at the
bottom of the drum and the cycle starts again. Parameters that can be
controlled are the rotation speed of the drum and, in certain mixers, the
angle of inclination of the rotation axis. There are three main types of drum
mixers:
• non-tilting drum; • reversing drum; • tilting drum.
The non-tilting drum mixer implies that the orientation of the drum is fixed.
The materials are added at one end and discharged at the other .
The reversing drum is similar to the non-tilting mixer except that the same
opening is used to add the constituents and to discharge concrete. The drum
rotates in one direction for mixing and in the opposite direction for
discharging the concrete. There are two types of blades attached to the inner
walls of the drum. One set drags the concrete upwards and toward the center
of the mixer when the drum rotates in one direction; the second set of blades
pushes the concrete toward the opening when the drum rotates in the other
direction. The blades have a spiral arrangement to obtain the desired effect
for discharge and mixing.

4. PAN MIXERS
All pan mixers work on basically the same principle [3]: a
cylindrical pan (fixed or rotating) contains the concrete to be
mixed, while one or two sets of blades rotate inside the pan to mix
the materials and a blade scrapes the wall of the pan. The shapes
of the blades and the axes of rotation vary. The other element of
the mixer is the scraper.
CONTINUOS MIXERS
The second category of mixers is continuous mixers . As the name
indicates, the materials are continuously fed into the mixer at the
same rate as the concrete is discharged. They are usually non-tilting
drums with screw-type blades rotating in the middle of the drum.
The drum is tilted downward toward the discharge opening. The
mixing time is determined by the slope of the drum (usually about
15 degrees).
These mixers are used for applications that require a short working
time, long unloading time, remote sites (not suitable for ready-mix)
and/or small deliveries. A major use of these types of mixers is for
low slump concrete

5. DRUM AND PAN MIXERS

6. PAN AND CONTINUOS MIXERS

7. PRESSURE GROUTING
Pressure grouting involves injecting a grout material into generally
isolated pore or void space of which neither the configuration or
volume are known, and is often referred to simply as grouting. The
grout may be a cementitious, resinous, or solution chemical mixture.
The greatest use of pressure grouting is to improve geomaterials (soil
and rock). The purpose of grouting can be either to strengthen or
reduce water flow through a formation. It is also used to correct
faults in concrete and masonry structures. Since first usage in the
19th century, grouting has been performed on the foundation of
virtually every one of the world’s large dams, in order to reduce the
amount of leakage through the rock, and sometimes to strengthen
the foundation to support the weight of the overlying structure, be it
of concrete, earth, or rock fill. Although very specialized, pressure
grouting is an essential construction procedure that is practiced by
specialist contractors and engineers around the world.

9. CAUSES OF FAILURE IN RCC STRUCTURES

10. DEFECTS IN CONCRETE
abrasion
Alkali aggregate
reaction
overload
MECHANICAL
movement
explosion
vibration
CHEMICAL
Aggressive agents
Biological action

11. freeze/thaw
Thermal effects
Salt crystallization
PHYSICAL
shrinkage
erosion
wear
FIRE

12. REINFORCEMENT CORROSION
Cement content and
type
w/c ratio
carbonation
Stray currents
curing
rainfall
Temperature/humidity
Corrosive
contaminants
Chloride salts
Sea water
Road salt
Other
contaminents

13. REPAIRS OF CONCRETE STRUCTURES
Shotcrete
Shotcrete is, in effect, a version of a cast-in-place concrete wall. Rather than
placing concrete into forms, however, a fresh mix is sprayed onto wall panels that
have been erected in the shape of the building. A nozzleman applies concrete
from a pressurized hose to encompass the reinforcement and build up the wall
thickness, forming structural shapes that include walls, floors, roofs, and other
assemblies. This material has also been called “gunite” in reference to the nozzle
or “gun” used to shoot material at the form face. Any surface suitable for
accepting fresh concrete can be used: wood, steel, and polystyrene are common.
Finishes are often applied directly to the concrete while it is still wet.
Shotcrete is a method of applying a combination of sand and portland
cement which mixed pneumatically and conveyed in dry state to the nozzle of
a pressure gun, where water is mixed and hydration takes place just prior to
expulsion. The material bonds perfectly to properly prepared surface of
masonry and steel. In versatility of application to curved or irregular
surfaces, its high strength after application and good physical
characteristics, make for an ideal means to achieve added structural
capability in walls and other elements. There are some minor restrictions of
clearance, thickness, direction of application, etc.

14. SHOTCRETE

15. EPOXY RESINS
Epoxy resins are excellent binding agents with high tensile strength. There are
chemi- cal preparations the compositions of which can be changed as per
requirements. The epoxy components are mixed just prior to application. The
product is of low vis- cosity and can be injected in small cracks too.
The higher viscosity epoxy resin can be used for surface coating or filling larger
cracks or holes. The epoxy mixture strength is dependent upon the temperature of
cur- ing (lower strength for higher temperature) and method of application

16. Epoxy mortar
For larger void spaces, it is possible to com- bine epoxy resins of either
low viscosity or higher viscosity, with sand aggregate to form epoxy
mortar. Epoxy mortar mixture has higher compressive strength, higher
tensile strength and a lower modulus of elasticity than Portland cement
concrete. Thus the mortar is not a stiff material for replacing reinforced
concrete. It is also re- ported that epoxy is a combustible material.
Therefore it is not used alone. The sand aggregate mixed to form the
epoxy mortar provides a heat sink for heat generated and it provides
increased modulus of elasticity too.
Quick setting cement mortar
This material is patented and was originally developed for the use as a
repair material for reinforced concrete floors adjacent to steel blast
furnaces. It is a non-hydrous magnesium phosphate cement with two
components, a liquid and a dry, which can be mixed in a manner similar
to portland cement concrete.

17. STRENGHTHENING RC MEMBERS
The strengthening of reinforced concrete members is a task that should be
carried out by a structural engineer according to calculations. Here only a few
suggestions are included to illustrate the ways in which the strengthening
could be done.
(i) RC columns can best be strengthened by jacketing, and by providing
additional cage of longitudinal and lateral tie reinforcement around the
columns and casting a concrete ring, the desired strength and ductility can
thus be built-up.
(ii) Jacketing a reinforced concrete beam can also be done in the above
manner. For holding the stirrup in this case, holes will have to be drilled
through the slab.
(iii) Similar technique could tie used for strengthening RC shear walls.
(iv) Inadequate sections of RC column and beams can also be strengthened
by removing the cover to old steel, welding new steel to old steel and
replacing the cover.
(v) RC beams can also be strengthened by applying prestress to it so that opposite moments are caused to those applied. The wires will run on both sides
of the web outside and an- chored against the end of the beam through a
steel plate.