What is concrete

Admixtures or additives such as pozzolans or superplasticizers are included in the mixture to improve the physical properties of the wet mix or the finished material. Various types of concrete are manufactured these days for construction of buildings and structures.

These have special properties and features which improve quality of construction as per requirement. Components of concrete are cement, sand, aggregates and water.

Mixture of Portland cement and water is called as paste. So, concrete can be called as a mixture of paste, sand and aggregates. Sometimes rocks are used instead of aggregates. The cement paste coats the surface of the fine and coarse aggregates when mixed thoroughly and binds them. Soon after mixing the components, hydration reaction starts which provides strength and a rock solid concrete is obtained. Grade of concrete denotes its strength required for construction. For example, M30 grade signifies that compressive strength required for construction is 30MPa.

Based on various lab tests, grade of concrete is presented in Mix Proportions. For example, for M30 grade, the mix proportion can be , where 1 is the ratio of cement, 1 is the ratio of sand and 2 is the ratio of coarse aggregate based on volume or weight of materials.

The strength is measured with concrete cube or cylinders by civil engineers at construction site. Cube or cylinders are made during casting of structural member and after hardening it is cured for 28 days. Then compressive strength test is conducted to find the strength. Regular grades of concrete are M15, M20, M25 etc.

For plain cement concrete works, generally M15 is used. For reinforced concrete construction minimum M20 grade of concrete are used. Concrete is manufactured or mixed in proportions w. There are two types of concrete mixes , i. Nominal mix is used for normal construction works such as small residential buildings. Most popular nominal mix are in the proportion of Design mixed concrete are those for which mix proportions are finalized based on various lab tests on cylinder or cube for its compressive strength.

This process is also called as mix design. These tests are conducted to find suitable mix based on locally available material to obtain strength required as per structural design. A design mixed offers economy on use of ingredients. Once suitable mix proportions are known, then its ingredients are mixed in the ratio as selected. Two methods are used for mixing, i. Hand Mixing or Machine Mixing. Based on quantity and quality required, the suitable method of mixing is selected.

In the hand mixing, each ingredients are placed on a flat surface and water is added and mixed with hand tools. Excessive bleedwater will delay the finishing operation and can cause serious problems with the surface of the concrete. Adding more sand to the mix, adding more entrained air, using less mix water, or adding cement or fly ash are possible cures. Make sure your ready mix supplier knows if you will be pumping concrete. Pumping mixes require a sufficient amount of fines and there are limits to the size of the aggregate in order for the mix to be pumpable.

Fly ash and air entrainment improve workability and pumpability. The mix may be cooled in hot weather by replacing part of the mixing water with ice, sprinkling water on the aggregate pile at the ready mix plant, or injecting liquid nitrogen into the batch. Normal concrete weighs approximately pounds per cubic foot and should be placed as near as possible to its final position. Excess handling can cause segregation of the course and fine aggregates. Wetting up the concrete so it can be raked or pushed into a location far from where it is discharged is not acceptable.

Concrete is poured directly from the chute of the ready mix truck, wheeled into place with a buggy, or pumped into place with a concrete boom pump see concrete pumping. Concrete is normally specified at a " slump. Industrial, commercial, and some residential projects require an inspector on concrete pours who monitors the concrete slump and takes slump measurements at the required intervals.

Short handled, square ended shovels are recommended for spreading concrete. A come-along a tool that looks like a hoe and has a long straight edged blade can also be used.

Do not use a round edge shovel for spreading concrete since it does not spread the concrete evenly. Any spreader used should be rigid enough to push and pull wet concrete without bending: Normal concrete weighs approximately pounds per cubic foot. Decorative concrete mix design Related Information:. Get Job Leads Today! About Us Free Newsletters. Concrete Defined Learn all the basics, from the components and desired properties of concrete to installation and reinforcement.

Find Decorative Concrete Contractors. In addition, aggregates should be clean and free from any matter that might affect the quality of the concrete. Soon after the aggregates, water, and the cement are combined, the mixture starts to harden. All portland cements are hydraulic cements that set and harden through a chemical reaction with water call hydration.

During this reaction, a node forms on the surface of each cement particle. The node grows and expands until it links up with nodes from other cement particles or adheres to adjacent aggregates.

Once the concrete is thoroughly mixed and workable it should be placed in forms before the mixture becomes too stiff. During placement, the concrete is consolidated to compact it within the forms and to eliminate potential flaws, such as honeycombs and air pockets. For slabs, concrete is left to stand until the surface moisture film disappears, then a wood or metal handfloat is used to smooth off the concrete.

Floating produces a relatively even, but slightly rough, texture that has good slip resistance and is frequently used as a final finish for exterior slabs. If a smooth, hard, dense surface is required, floating is followed by steel troweling. Curing begins after the exposed surfaces of the concrete have hardened sufficiently to resist marring. Curing ensures the continued hydration of the cement so that the concrete continues to gain strength. Concrete surfaces are cured by sprinkling with water fog, or by using moisture-retaining fabrics such as burlap or cotton mats.

Other curing methods prevent evaporation of the water by sealing the surface with plastic or special sprays called curing compounds. Special techniques are used for curing concrete during extremely cold or hot weather to protect the concrete. The longer the concrete is kept moist, the stronger and more durable it will become. The rate of hardening depends upon the composition and fineness of the cement, the mix proportions, and the moisture and temperature conditions.