How To Make Concrete - Some Vital Information
The concrete is formed by mixing paste and aggregates, or rocks. The paste, made of portland cement and water, coats the surface of the fine (small) and coarse (larger) aggregates. With the help of a chemical reaction known as hydration, the paste solidifies and attains strength to develop the rock-like mass called concrete.
Concrete is plastic and compliant when mixed lately. It becomes strong and long-lasting when solidified.
Correct proportioning and mixing of the materials lead to produce a strong and stable concrete. A mixture without adequate paste to fill all the voids among the aggregates can't be set easily and consequently rough surfaces and permeable concrete will be formed. A mixture with additional cement paste can be set easily and form a smooth surface. The concrete produced is not economical and can simply crack.
A paste is developed with cement and water to coat each particle of stone and sand—the aggregates. With help of a chemical reaction known as hydration, the cement paste solidifies and attains strength.
The character of the concrete is dependent on the quality of the paste. Alternately, the strength of the paste is based on the ratio of water to cement. The water-cement ratio is defined as the weight of the mixing water divided by the weight of the cement. To produce superior quality concrete, the water-cement ratio is reduced as far as possible keeping the workability of fresh concrete unchanged so that the concrete can be perfectly set, compact, and cured.
A properly designed mixture contains the required workability for the fresh concrete as well as the necessary stability and strength for the hardened concrete. Normally, a mix comprises of about 10 to 15 percent cement, 60 to 75 percent aggregate and 15 to 20 percent water. Entrained air in many concrete mixes may also comprise another 5 to 8 percent.
Almost any drinkable natural water without any pronounced taste or smell can be utilized as mixing water for concrete. Additional filths in mixing water may impact setting time as well as stability of concrete and lead to efflorescence, staining, decay of reinforcement, volume instability, and strength reduction. Concrete mixture specifications normally set limits on chlorides, sulfates, alkalis, and solids in mixing water until tests are conducted to find out the impact of the impurity on the final concrete.
Aggregates contain 60 to 75 percent of the total volume of concrete. The type and size of aggregate are based on the thickness and objective of the final concrete product.
Considerably thin building sections are suitable for small coarse aggregate, though aggregates up to six inches in diameter can be applied in large dams. A constant gradation of particle sizes is ideal for proficient application of the paste. Besides, aggregates should be clean and do not contain any matter upon which the quality of the concrete is impacted.
Time of Hydration
Once the aggregates, water, and the cement are mixed, the mixture starts to solidify. All portland cements are hydraulic cements which set and solidify via a chemical reaction with water known as hydration. Throughout this reaction, a node develops on the surface of each cement particle. The node extends unless it connects with nodes from other cement particles or complies with adjoining aggregates.
As soon as the concrete is mixed properly and becomes applicable it should be set in forms prior to mixture gets too rigid.
Throughout placement, the concrete is amalgamated to compact it inside the forms as well as to get rid of possible faults like honeycombs and air pockets.
Curing starts when the uncovered surfaces of the concrete are solidified properly to combat marring. Curing leads to the constant hydration of the cement in order that concrete attains strength. Concrete surfaces are cured with sprinkling water fog, or moisture-retaining fabrics like burlap or cotton mats. Other curing processes resist evaporation of the water by sealing the surface with plastic or special sprays known as curing compounds.
In bitterly cold or hot weather, special methods are applied for curing concrete to safeguard the concrete. As long as the, concrete is wet, it gains more strength and becomes more stable. The rate of hardening is based on the composition and fineness of the cement, the mix proportions, and the moisture and temperature conditions. The strength of concrete is increased as it becomes older. Most of the hydration and strength gain happens within the first month of concrete's life cycle, but hydration persists at a slower rate for several years.