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Concrete Information
WHAT IS CONCRETE?
Concrete is an Artificial Rock. It is a construction material composed of cement (commonly Portland cement) as well as other cementitious materials such as fly ash and slag cement, aggregate (generally a coarse aggregate such as gravel limestone or granite, plus a fine aggregate such as sand), water, and chemical admixtures. Concrete solidifies and hardens after mixing with water and placement due to a chemical process known as hydration. The water reacts with the cement, which bonds the other components together, eventually creating a stone-like material.
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HOW IS CEMENT MADE?
The basic ingredient of concrete is cement.."Portland cement and similar materials are made by heating limestone (a source of calcium) with clay, and grinding this product (called clinker) with a source of sulfate (most commonly gypsum)."
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Curing of Concrete:
After concrete is placed it is important to properly cure the concrete to achieve the best strength and hardness. A decrease in strength, shrinkage and cracks can occur if not cured properly. Water and temperature are the significant factors. The cement in the concrete mix requires moisture to gain strength and a consistent temperature. If the water dries out to quickly, there will be insufficient water available to hydrate the cement At the beginning, the longer the concrete is kept damp the more strength it will gain. The cement and water reaction goes on for years, but 95% of the strength is attained in the first 28 days ( 7 days for Type III).
Typically this is not a problem in precast concrete. The critical strength is stripping strength ( the removal of the concrete piece from the mold) . Adding heat accelerates the hydration and causes higher early strengths. This allows for an earlier removal of the concrete piece from the mold.
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Types of Cement:
- Type 1 - Normal Portland cement. Type 1 is general use cement.
- Type 2 - Moderate sulfate resistant Portland Cement. Type 2 is used for structures exposed to soil or ground waters with higher than normal sulphate concentrations. Modified Type 2 cement is used to generate heat slower than Type 1.
- Type 3 - High early strength. Used when high strength are desired at very early periods.
- Type 4 - Low heat Portland cement. Used where the amount and rate of heat generation must be kept to a minimum.
- Type 5 - High sulfate resistant Portland Cement. Used where the water or soil has high sulfate content.
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CHEMICAL ADMIXTURES:
Chemical Admixtures are materials that are added to the concrete, during mixing, to obtain different characteristics than plain concrete mixes. The most common are:
Water Reducers:
When added to the concrete mix reduce the demand for water by making concrete more fluid. Better flowing concrete can be placed more easily in the form.
Superplasticizers : Superplasticizers are added to the concrete mix to increase slump ( consistency of fresh concrete) and workability. Superplasticizers distribute evenly particles of cement throughout the concrete mix.
Air-entraining Agents : Air-entraining agents add and distribute air bubbles. This helps the workability of the concrete, but its primary function is to protect against freeze / thaw damage.
Accelerators: Accelerators speed up the hardening process of concrete. Do not use CaCl accelerator with rebar.
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SOME TERMS TO UNDERSTAND CONCRETE AND THE REINFORCING OF IT:
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Compression - squeezing together
- Tension - pulling apart
Concrete is excellent in compression yet poor in tension, a ratio of approximately 10 to 1. (i.e. 6,000 psi compression = 600 psi tension) Steel reinforcing bar or rebar is excellent in tension, yet poor in compression. Rebar is 60,000 psi in tension. Combining concrete and rebar you get the compressive strength of the concrete combined with the tension strength of the rebar making a very which is call reinforced concrete.
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PRESTRESSING CONCRETE:
Pre stressing concrete minimizes the effect of cracks in concrete pieces. In regular reinforced concrete, steel rods are used to overcome low tensile strength. They resist tensile forces and limit the width of the cracks which occur under design loadings. Pre stressing limits cracking in construction by placing the concrete in compression before adding service loads. The Pre-stressing strand is used to put concrete into compression so that when tension is applied, the net effect is no tension. Pre stressed concrete has the ability to carry greater loads and span greater distances. It also enables both structural elements and structures to be formed from a number of precast units, e.g. Segmented and PlantName Construction.
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