Introduction

v  Concrete is a mixture of cement, aggregates and water, with any other admixtures which may be added to modify the placing and curing processes or the ultimate physical properties.

v  Initially when mixed, concrete is a plastic material, which takes the shape of the mould or formwork. 

v  Freshly mixed concrete before set is known as wet or green concrete.

v  When hardened it may be a dense load-bearing material or a lightweight thermally insulating material, depending largely on the aggregates used.

v  It may be reinforced or pre-stressed by the incorporation of steel.

v  The major factors responsible for wide usage of cement-concrete are its versatility, mouldability, early hardening, high early compressive strength, development of desired properties with admixtures to be used in adverse situations, suitability for guniting, pumpability and durability.

 

 

Types of Concrete

Plain Concrete –

v  The term plain concrete is used to describe any concrete mass used without any strengthening materials.

v  The physical properties of plain concrete include the ability to withstand great pressure.

v  After mixing cement, coarse and fine aggregate with water, it hardens in place after being poured. The cement reacts with the water, creating a new chemical bond between the different materials in the concrete mix. As the water dries, the cement cured into a stone-like material.

v  The density of the plain concrete will vary between 2200 and 2500 Kg/m3. The compressive strength is 200 to 500 kg/cm2. 

v  These types of concrete are mainly used in the construction of the  pavements and the buildings, especially in areas where there is less demand of high tensile strength.

 

 

Reinforced Concrete –

v  Concrete is strong in compression, with crushing strengths typically in the range 20–50 MPa. However, the tensile strength of concrete is usually only 10% of the compressive strength. 

v  Steel is the universally accepted reinforcing material as it is strong in tension, forms a good bond and has a similar coefficient of thermal expansion to concrete.

v  The location of the steel within reinforced concrete is critical, as shown in figures, to ensure that the tensile and shear forces are transferred to the steel

v  The longitudinal bars carry the tensile forces while the links or stirrups cover the shear forces and also locate the steel during the casting of the concrete.

v  The steel reinforcement used in the concrete can be in the form of rods, bars or in the form of meshes. Whatever be the type of reinforcement used in concrete, it is very necessary to ensure proper bond between the concrete and the reinforcement. This bond will control the strength and the durability factors of the concrete. 

v  High-yield steel has a minimum yield stress of 460 MPa, roughly double that of mild steel at 250 MPa. Welded steel mesh reinforcement is used for slabs, roads and within sprayed concrete. To obtain the most efficient mechanical bond with concrete, the surface of the steel should be free of rust, loose scale and grease.

v  The use of hooked ends in round bars reduces the risk of the steel being pulled out under load. Spacers are used to ensure the correct separation between reinforcement and formwork.

 

In-situ concrete –

 

v  or cast-in-place concrete is the concrete which is deposited in the place where it is required to harden as part of the structure, 

v  The quality of in-situ visual concrete is heavily dependent upon the formwork as any defects in the formwork will be mirrored in the concrete surface.

v  The formwork must be strong enough to withstand, without distortion, the pressure of the fresh concrete, and the joints must be tight enough to prevent leakage, which can cause honeycombing of the surface.

v  A wide range of timber products, metals and plastics are used as formwork, depending upon the surface finish required.

 

 Pre-stressed Concrete

 

 

v  In this technique, bars or tendons used in the concrete are stressed before the actual service load application. During the mixing and the placing of the concrete, these tensioned bars placed firmly and held from each end of the structural unit. Once the concrete sets and harden, the structural unit will be put in compression

v  Pre-stressing make the lower section of the concrete member to be stronger against the tension. The process of pre-stressing will require heavy equipment and labour skill (jacks and equipment for tensioning). These are used in the application of bridges, heavy loaded structures, and roof with longer spans6.

v  Pre-stressing with steel wires or tendons ensures that the concrete component of the composite material always remains in compression when subjected to bending up to the maximum working load. The tensile forces within the steel tendons act upon the concrete putting it into compression, such that only under excessive loads would the concrete go into tension and crack.

v  Two distinct systems are employed: in pre-tensioning, the tendons are tensioned before the concrete is cured; and in post-tensioning the tendons are tensioned after the concrete is hardened.

v  Post-tensioning - In the post-tensioning system the tendons are located in the formwork within sheaths or ducts.

v  The concrete is placed, and when sufficiently strong, the tendons are stressed against the concrete and locked off with special anchor grips incorporated into the ends of the concrete. After tensioning the free space within the ducts is grouted up, this then limits the support on the anchorage fixing. Tendon ducts are typically manufactured from galvanized steel strip or high-density polythene.

 

Lightweight Concrete –

v  Concrete that have a density lesser than 1920 kg/m3 will be categorized as lightweight concrete. 

v  The light weight concrete is applied for the protection of the steel structures and also used for the construction of long span bridge decks.

v  Structural LWC has an in-place density (unit weight) on the order of 1440 to 1840 kg/m³ compared to normal weight concrete.

 

Precast concrete –

v  Precast concrete units may be cast vertically or horizontally, although most factory operations use the latter, as better quality control can be achieved by this method. Moulds are usually manufactured from plywood or steel. Whilst steel moulds are more durable for repeateduse, plywood moulds are used for the more complex forms; they can also be  more readily modified for non-standard units.

v  Moulds are designed to be dismantled for the removal of the cast unit and must be manufactured to tight tolerances to ensure quality control on the finished product. Fixing and lifting systems for transportation must be i
ncorporated into precast units,usually in conjunction with the steel reinforcement.