CHAPTER 7CONCRETE BATCH PLANT SUPERVISORConcrete is a mixture of cement and water that bindsaggregates, such as sand and gravel or crushed stone,into a rockdikc mass and when properly constructed, itis very rigid and has a long life. This is a result of thechemical reaction between the cement and water calledhydration. The Naval Construction Force (NCF) hasvarious equipment used to provide concrete forconstruction purposes. This equipment ranges from the11S mixer, transit mixer, mobile concrete mixer plant(crete mobile), and the concrete batch plant.Most concrete production operations are supportedby an experienced Builder (BU). However, EOs mustunderstand the basic principles of concrete mix designand the procedures performed to produce qualityconcrete.CONCRETE BATCH PLANTSUPERVISOR RESPONSIBILITIESWhen assigned as the supervisor of a concrete batchplant in the NCF, you supervise the production andtransport of concrete products used to constructsidewalks, roads, footers, foundations, walls, roofs,runways, rapid runway repairs, and so forth.PORRLAND CEMENTPortland cement is usually made of materials,such as limestone or marl and shale or clay. The rawmaterials are crushed, pulverized, and mixed inproper proportions for the correct chemicalcomposition. Then, the raw material is fed into arotary kiln and is calcined (burned) at a temperatureof approximately 2700°F. This process transforms thematerial into a clinker. The clinker is cooled andpulverized so fine that nearly all of the powder canpass through a No. 200 mesh sieve.Types of CementWhen powdered Portland cement and water arecombined, hydration occurs, The amount of water perunit weight of cement is called the water-cement rationormally given in terms of pounds of water per poundsof cement. Concrete with a low water cement ratio gainsmore strength than cement with a higher water cementratio.There are many types of cement, but only the mostcommon types are listed here:Type I (normal portland cement) is the mostwidely used cement for pavements, sidewalks,buildings, bridges, masonry units, and soil-cementmixtures. In general, it is used when the concrete willnot be subjected to special sulfate hazards or where theheat generated by the hydration of the cement does notcause an objectionable rise in temperature.Type II (modified portland cement) has a lowerheat of hydration than Type I. This lower heat gives thiscement an improved resistance to sulfate attack. Type IIcement is used in large structures where cement ofmoderate heat of hydration tends to minimize a rise intemperature. Examples are as follows: large piers, heavyabutments, heavy retaining walls, and when the concreteis placed in warm weather. Type 11 cement is also usedin drainage structures where the sulfate concentrationsare higher than normal.Type III (high-early-strength portland cement) isused when superior strength is required in a short time.It is used in cold-weather construction to reduce theperiod of protection against low temperatures. Type IIIis also used when forms have to be removedimmediately to allow the concrete to be put in serviceas quickly as possible. Type III cement requires lessprotection time from freezing and attains normal 3-daystrength in 1 day. The volume of heat during hydrationis also accelerated. Normally, this cement is not used inlarge-scale construction operations because it is veryexpensive.Storage of CementPortland cement is a moisture-sensitive materialthat must be protected from damp air or moisture.Cement not protected when in storage sets more slowlybecause hydration has already begun; therefore, it hasless strength than portland cement that is kept dry.Most types of portland cement are shipped in bulkby rail, truck, or barge. Pneumatic loading andunloading of the transport vehicles is the usual method7-1
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