into a bin should be kept to a minimum by keeping thebin as full as possible at all times.WATERThe primary function of water used in a concretemix is to start the hardening process of the concretethrough hydration of the cement. A secondary functionis to make the mix workable enough to satisfy therequirements of the job. However, too much water willcause a loss of strength by upsetting the water-cementratio. It will also cause “water gain” on the surface, acondition which leaves a surface layer of weak materialcalled LAITANCE. Also, an excessive amount of waterwill impair the water tightness of the concrete.Water used in mixing concrete must be clean andfree from oils, alkalis, acids, and organic materials. Mostspecifications recommend the mixing water be fit fordrinking. This is because any water fit for drinking isusually satisfactory for use in mixing concrete.Seawater may be used for unreinforced concrete.ADDITIVES USED INCONCRETEThere are several additives or admixtures used tochange the composition of concrete or to accelerate orretard its hardening. The three commonly used areair-entraining agents, retarders, and accelerators.Additives are not recommended if the end result can bereached more economically by altering the mixproportions.Air-entraining portland cement is a specialcement that can be used with good results for a varietyof conditions. It was developed to produce concrete thathas a resistance to freeze-thaw action and scaling causedby chemicals applied for severe frost and ice removal,Air-entraining agents are liquids derived from naturalwood resins, animal fats, vegetable fats, or variouswetting agents, such as alkali salts and water-solublesoaps. Agents are blended with the cement duringmanufacturing or added at the mixing site. If done at thesite, the agent is added to the water used in themix.Manufactured air-entraining cements are indicatedby the letter A in the type number (Types IA, IIA, IIIA,etc.). Concrete made with this cement contains billionsof extremely small, entirely separated air bubbles percubic foot of concrete. These bubbles provide space forwater to expand due to freezing without damage to theconcrete.Retarders are used to slow down the rate of settingof a concrete. High temperatures of fresh concrete(85°F - 90°F and higher) are often the cause of anincreased rate of hardening that makes placing andfinishing difficult. One practical way to reduce thetemperature of the concrete is by cooling the mixingwater or the aggregates. Retarders do not decrease theinitial temperature of the concrete. Retarders aresometimes used (o do the following:1. Offset the accelerating effect of hot weather onthe setting of concrete2. Delay the initial set of concrete when difficult orunusual conditions of placement occur, such asplacing concrete in large piers and foundations3. Delay the set for a special finishing process,such as an exposed aggregate surfaceSome of the materials used to retard the set of aconcrete mixture are lignin, borax, sugar, tartaric acid,and salt. These materials should be added to the mixingwater.CAUTIONIf 20 percent by volume of regarding agentis added to the mix, the effect is reversed and itthen acts as an accelerator.Accelerators are used to accelerate the strengthdevelopment of concrete at an early age. Calciumchloride is the material most commonly used inaccelerating admixtures; however, in addition toaccelerating strength gain, calcium chloride causes anincrease in drying shrinkage, potential reinforcementcorrosion, discoloration, and scaling potential. Calciumchloride should be added to the concrete mix in solutionform as part of the mixing water. If added to the concretein dry form, all of the dry particles may not becompletely dissolved during mixing. Undissolvedlumps in the mix can cause pop-outs of dark spots in thehardened concrete. The amount of accelerator usedshould not exceed 2 percent by weight of cement.COMPUTING CONCRETE VOLUMETo compute the volume of concrete required for aconcrete pad, multiply the length of the pad by its widthtimes its depth to get cubic feet (L x W x D); forexample, a concrete pad is 20 feet in length by 30 feetin width and has a depth of 3 inches. First, convert the3 inches into feet by dividing 3 by 12 to get 0.25 foot.Next, multiply the 20-foot length by the 30-foot width7-3
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