【正文】 the sand contains about 3 percent moisture, the dotted lines should be used. Specifications usually demand that freshly placed concrete shall be kept at a temperature of not less than 70176。F for 5 days after it is placed. Some proper method must be provided to keep the demanded temperature when the cold weather is estimated. 4Reinforcing steels for concrete Compared with concrete, steel is a high strength material. The useful strength of ordinary reinforcing steels in tension as well as pression, the yield strength, is about 15 times the pressive strength of mon structural concrete, and well over 100 times its tensile strength. On the other hand, steel is a highcost material pared with concrete. It follow that the two materials are the best used in bination if the. 中國地質(zhì)大學(xué)長城學(xué)院 2021 屆畢業(yè)論文 10 Concrete is made to resist the pressive stresses and the pressive force, longitudinal steel reinforcing bars are located close to the tension face to resist the tension force. and usually additional steel bars are so disposed that they resist the inclined tension stresses that are caused by the shear force in the beams. However, reinforcement is also used for resisting pressive forces primarily where it is desired to reduce the crosssectional dimensions of pression members, as in the lowerfloor columns of multistory buildings. Even if no such necessity exits , a minimum amount of reinforcement is placed in all pression members to safeguard them against the effects of small accidental bending moments that might crack and even fail an unreinforced member. For most effective reinforcing action, it is essential that steel and concrete deform together, that there be a sufficiently strong bond between the two materials to ensure that no relative movements of the steel bars and the surrounding concrete occur. This bond is provided by the relatively large chemical adhesion which develops at the steelconcrete interface, by the natural roughness of the mill scale of hotrolled reinforcing bars, and by the closely spaced ribshaped surface deformations with which reinforcing bars are furnished in order to provide a high degree of interlocking of the two materials. Steel is used in two different ways in concrete structures: as reinforcing steel and as prestressing steel .reinforcing steel is placed in the forms prior to casting of the concrete. Stresses in the steel, as in the hardened concrete, are caused only by the loads on the structure, except for possible parasitic stresses from shrinkage or similar causes. In contrast, in priestesses concrete structures large tension forces are applied to the reinforcement prior to letting it act jointly with the concrete in resisting external. The most mon type of reinforcing steel is in the form of round bars, sometimes called rebars, available in a large range of diameters, from 10 to 35 mm for ordinary applications and in two heavy bar sizes off 44 and 57 mm these bars are furnished with surface deformations for the purpose of increasing resistance to slip between steel and concrete minimum requirements for these deformations have been developed in experimental research. Different bar producers use different patterns, all of which satisfy these requirements. Welding of rebars in making splices, or for convenience in fabricating reinforcing cages for placement in the forms, may result in metallurgical changes that reduce both strength and ductility, and special restrictions must be placed both strength and ductility, and special restrictions must be placed both on the type of steel used and the welding procedures the provisions of ASTM A706 relate specifically to welding. In reinforced concrete a longtime trend is evident toward the use of higher strength materials, both steel and concrete. Reinforcing bars with 40ksi yield stress, almost standard 20 years ago, have largely