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5 PROPORTIONING <br /> It is the intent of this specification to secure, for every part of the work, concrete <br /> of homogeneous structure which, when hardened, shall have the required strength, <br /> impermeability and resistance to chemicals and weathering. <br /> 5.1 CONCRETE STRENGTH <br /> The concrete shall be proportioned to achieve a minimum twenty-eight (28) <br /> day compressive strength of 4,000 psi. <br /> 5.2 PROPORTIONING UMITATIONS <br /> 5.2.1 Water-cement ratio, as evidenced by the slump test, will not be <br /> greater than that required for workability, as determined by the <br /> Engineer. In no case shall the water-cement ratio exceed 0.45. <br /> Workability of concrete mixture shall be such that it may be <br /> readily worked into corners and angles of forms and around <br /> reinforcement without permitting materials to segregate, or free <br /> water to collect on the surface. <br /> 5.2.2 Concrete shall contain not less than 611 pounds of cement per <br /> cubic yard unless written authority is obtained in advance from <br /> the Engineer. <br /> 5.3 CHANGES IN PROPORTIONING <br /> 5.3.1 CHANGES BY ENGINEER <br /> Should it be impossible to obtain concrete of the required strength <br /> and workability, changes in proportions, materials or both, may be <br /> made by the Engineer. <br /> 5.3.2 CHANGES BY CONTRACTOR <br /> Changes by the Contractor, during the progress of work, may be <br /> made in proportions or materials originally approved, due to <br /> deviation from the initial characteristics of the material or for other <br /> justifiable reasons. Evidence, satisfactory to the Engineer, shall <br /> be submitted showing that new materials or conditions proposed <br /> shall produce concrete meeting the specific requirements. <br /> SPEC. B - CONCRETE <br /> PAGE 8 <br />