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10.4 PLACEMENT OF LAYERS <br /> Concrete layers shall not exceed two feet (21 in thickness. Each layer shall <br /> be thoroughly consolidated before the succeeding layer is placed. Timing <br /> of placement shall be such that each succeeding layer is placed before the <br /> preceding layer has reached its initial set. <br /> 10.5 CONSOLIDATION <br /> 10.5.1 During and immediately after placing, all concrete shall be <br /> consolidated and worked to provide thorough placement around <br /> all reinforcement, embedded fixtures and into corners of forms. <br /> Unless otherwise approved by the Engineer, the Contractor shall <br /> accomplish consolidation by the use of high-frequency internal <br /> vibrators. The type and operation of vibrators is subject to the <br /> Engineer's acceptance. <br /> 10.5.2 EXTENT OF VIBRATION <br /> Vibration shall extend through the entire depth of each new layer <br /> and several inches into the preceding layer. Vibration shall be <br /> applied at the point of deposit and uniformly throughout the <br /> freshly placed concrete. <br /> 10.5.3 DURATION OF VIBRATION <br /> Usually vibration should secure the desired results within five (5) <br /> to fifteen (15) seconds at points eighteen inches (18") to thirty <br /> inches (30")apart rather than for longer periods at wider intervals. <br /> In general, to prevent segregation of ingredients, vibration shall <br /> continue only long enough to accomplish thorough consolidation <br /> and complete embedment of the reinforcement and fixtures. <br /> 10.5.4 LIMITATIONS <br /> Vibrators shall not be used as a means of moving concrete to a <br /> desired placement area. This action shall be accomplished by <br /> correct initial placement. The Contractor shall provide a sufficient <br /> number of vibrators and operators so that consolidation is started <br /> SPEC. B- CONCRETE <br /> PAGE 15 <br />