REINFORCED CEMENT CONCRETE

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.

BATCH MIXERS

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

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.

DRUM 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.

PAN 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

CONTINUOS MIXERS

DRUM AND PAN MIXERS

PAN AND CONTINUOS MIXERS

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.

PRESSURE GROUTING

CAUSES OF FAILURE IN RCC STRUCTURES

MECHANICAL

abrasion

overload

movement

explosion

vibration

CHEMICAL

Alkali aggregate reaction

Aggressive agents

Biological action

DEFECTS IN CONCRETE

FIRE

PHYSICAL

freeze/thaw

Thermal effects

Salt crystallization

shrinkage

erosion

wear

carbonation

Cement content and type

w/c ratio

curing

rainfall

Temperature/humidity

Corrosive contaminants

Chloride salts

Sea water

Road salt

Other contaminents

Stray currents

REINFORCEMENT CORROSION

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.

REPAIRS OF CONCRETE STRUCTURESShotcrete

SHOTCRETE

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

EPOXY RESINS

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.

Epoxy 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.

Quick setting cement mortar

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 op- posite 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.

STRENGHTHENING RC MEMBERS