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A Latin word "concretus" meaning compact or condensed marks the origin of the word concrete. Concrete is a thick, composite, artificial stone-like material which is made by mixing cement (generally Portland cement) and various other substances such as sand, pebbles, shale and gravel with water. The mixture is then hardened by a process known as hydration. Since ancient times it has been used in constructing structures and today it tops the chart of the most used construction materials in the world.
The cement paste so created by adding water fastens the aggregate together by filling the voids within the mixture. This allows it to form a smooth mixture that can flow easily. Adding water in the mix should be dealt with care as it can affect directly the strength of the concrete so created. High water content will give a free-flowing concrete with a more fluid consistency and higher slump. Less water content in the cement mixture will give more strength and durability to the concrete. Contaminated water if added in the mix can cause premature failure of the structure.
The curing process determines the degree and rates at which hydration occurs and the resultant final strength of the concrete. The mixture has to contain water and the temperature maintained at the optimum levels for hydration to continue. The strength of concrete is dependent on crystal growth within the concrete matrix through the process of hydration. If water is not adequate, the crystals cannot grow, due to which the concrete is unable to attain the desired strength. The presence of water through proper curing of the concrete facilitates the crystal growth that encloses the gravel and sand mix, causing interweaving with each other.
Immersion of the precast material for concrete in curing tanks for a particular time period is a vital step in curing. The process of ponding, to cure slabs of concrete for pavements and roofing, require the slabs to be covered under water in miniature ponds. Water is sprayed to cure surfaces that have been plastered surfaces, columns of concrete and vertical walls. Also, in order to keep the concrete wet, it is wrapped with soggy material like wet gunny bags and hessian cloth. In case of horizontal surface curing, substances like saw dust, sand and earth is used to cover the concrete to prevent hydration. The diagram below summarizes the major criteria for using water-added curing methods.
Rebound Hammer: Under this method the modulus of elasticity of the near surface concrete are measured. This method is most commonly used to estimate curing for pavements, in which near surface zones are important. Testing can be done in-situ. According to studies, the rebound numbers of uncured concrete when exposed to moderately to severe drying, can reduce in seven days in comparison to concrete that has been cured for longer time periods.
In order to keep the slab warm hydronic heating pipes or electric heating blankets could be used. These are made to lie on top of the slab. In cases where it is too cold to even pour the concrete, the only alternative is to heat the air after enclosing the work, however, this poses another potential problem of carbonation (Allcrete Industries, 2008). This is due to the fact that carbon dioxide levels rise in the enclosure where the unvented or gas-powered equipment is being used. This in turn reacts with the concrete to give a chalky layer (carbonated) at the surface causing a major setback. To fix this problem it is advisable to use heaters that release their exhaust outside the enclosure or building before the warm air is blown. To maintain the concrete temperature above 40ÂÂ°F for at least four or five more days, it is necessary to the use insulation blankets or heated enclosures.
Also in cold weather conditions, concrete must be cured without adding more water as that will damage the concrete when ice is formed in the voids. A difference by 20ÂÂ°F can offset the setting process by taking double the usual time. It may be useful to use concrete mixes that include accelerating admixtures to protect from freezing. ASTM C494 Type C is a possible solution.
Concrete maintains its superiority when it comes to building sustainable structures. Although there are a few shortcomings as discussed above, they can be prevented if the issues are dealt with proper care from the initial stages. The Portland Cement Association and the Environmental Council of Concrete are organizations that provide good quality concrete which a very cost effective, efficient and durable building material. The natural characteristics of concrete make it a better choice over the rest of the conventional building materials. Although handling it in extreme hot and cold weathers sometimes become tedious but once all the requisite measures are undertaken, the resultant structure would be of the greatest quality.