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discharge,weighing the supplementary cementitious material cumulatively with the cement is permitted. Certification shall <br /> contain the following: <br /> A. Test results for 2 compressive strength test cylinders of concrete taken within the first one-third and 2 compressive <br /> strength test cylinders of concrete taken within the last one-third of the concrete discharged from a single batch from <br /> the stationary paving mixer. Strength tests and cylinder preparation will be in conformance with the provisions of <br /> Section 90-9,"Compressive Strength"; <br /> B. Calculations demonstrating that the difference in the averages of 2 compressive strengths taken in the first one-third <br /> is no greater than 7.5 percent different than the averages of 2 compressive strengths taken in the last one-third of the <br /> concrete discharged from a single batch from the stationary paving mixer. Strength tests and cylinder preparation <br /> will be in conformance with the provisions of Section 90-9,"Compressive Strength;"and <br /> C. The mixer rotation speed and time of mixing before discharge that are required to produce a mix that meets the <br /> requirements above. <br /> The discharge gate on the cement and supplementary cementitious material hoppers or the cement plus <br /> supplementary cementitious material hopper shall be designed to permit regulating the flow of cement, supplementary <br /> cementitious material,or cement plus supplementary cementitious material into the aggregate as directed by the Engineer. <br /> If separate weigh boxes are used for each size of aggregate, the discharge gates shall permit regulating the flow of <br /> each size of aggregate as directed by the Engineer. <br /> • Material discharged from the several bins shall be controlled by gates or by mechanical conveyors. The means of <br /> withdrawal from the several bins,and of discharge from the weigh box,shall be interlocked so that not more than one bin can <br /> discharge at a time,and so that the weigh box cannot be tripped until the required quantity from each of the several bins has <br /> been deposited therein. Should a separate weigh box be used for each size of aggregate, all may be operated and discharged <br /> simultaneously. <br /> • If the discharge from the several bins is controlled by gates, each gate shall be actuated automatically so that the <br /> required mass is discharged into the weigh box,after which the gate shall automatically close and lock. <br /> • The automatic weighing system shall be designed so that all proportions required may be set on the weighing <br /> controller at the same time. <br /> 90-6 MIXING AND TRANSPORTING <br /> 90-6.01 GENERAL <br /> • Concrete shall be mixed in mechanically operated mixers, except that when permitted by the Engineer, batches not <br /> exceeding 0.25-m3 may be mixed by hand methods in conformance with the provisions in Section 90-6.05, "Hand-Mixing." <br /> • Equipment having components made of aluminum or magnesium alloys that would have contact with plastic <br /> concrete during mixing,transporting,or pumping of portland cement concrete shall not be used. <br /> • Concrete shall be homogeneous and thoroughly mixed, and there shall be no lumps or evidence of undispersed <br /> cementitious material. <br /> • Uniformity of concrete mixtures will be determined by differences in penetration as determined by California <br /> Test 533, or slump as determined by ASTM Designation: C 143, and by variations in the proportion of coarse aggregate as <br /> determined by California Test 529. <br /> • When the mix design specifies a penetration value, the difference in penetration, determined by comparing <br /> penetration tests on 2 samples of mixed concrete from the same batch or truck mixer load, shall not exceed 10 mm. When <br /> the mix design specifies a slump value,the difference in slump,determined by comparing slump tests on 2 samples of mixed <br /> concrete from the same batch or truck mixer load, shall not exceed the values given in the table below. Variation in the <br /> proportion of coarse aggregate will be determined by comparing the results of tests of 2 samples of mixed concrete from the <br /> same batch or truck mixer load and the difference between the 2 results shall not exceed 100 kg per cubic meter of concrete. <br /> Average SIM2 Maximum Permissible Difference <br /> Less than 100-mm 25-mm <br /> 100-mm to 150-mm 38-mm <br /> Greater than 150-mm to 225-mm 50-mm <br /> • The Contractor shall furnish samples of the freshly mixed concrete and provide satisfactory facilities for obtaining <br /> the samples. <br /> Contract No. 10-OM7104 <br /> 81 <br />