The batching plant and equivalent layout must provide a smooth flow of continuous supply and transport of materials to the work.
Stockpiles are built up in layers of not more than one meter in thickness with each layer completely in place before beginning the next that should not be allowed to “cone” down over the next lower layer.
All washed aggregates and aggregate produced or handled by hydraulic methods are stockpiled or binned for draining at least twelve hours before being batched.
The mixer should be charged without loss of cement and batched material should be weighed for each material required within a tolerance of one percent for cement and two percent for aggregates. Water may be measured by volume or by weight and the accuracy of measuring water shall be within a range of not more than one percent error.
MIXING OF CONCRETE
The concrete may be mixed at the site, in a central plant of by truck mixers of approved type and capacity. Mixing time will be measured from the time when all the materials except water are already inside the drum.
1. Ready mix concrete shall be mixed and delivered in accordance with AASHTO M-157 requirements, except that the minimum required revolutions at the mixing speed for transit mixed concrete may be reduced to not less than that recommended by the mixer manufacturer. The number of revolutions recommended by the mixer manufacturer should be indicated on a serial plate attached to the mixer.
2. When mixing is done at the site or in a central mixing plant, the missing time should not be less than 50 seconds nor more than 90 seconds, unless mixing performance tests provide adequate mixing of the concrete in a shorter time period. Mixing time ends when the discharge chute of the mixer opens. The contents of the individual mixer drum shall be removed completely before a succeeding batch is loaded therein.
3. The volume of concrete mix per batch should not exceed the mixers nominal capacity in cubic meter, as indicated on the manufacturers standard rating plate attached on the mixer except that an overload up to 10% above the mixers normal capacity may be permitted provided that concrete test data for strength, segregation and uniform consistency are satisfied and no spoilage of concrete should take place.
4. The batches shall be charged into the drum with a portion of the mixing water enter in advance of the cement and aggregates. The flow of water should be uniform that all water shall be inside the drum by the end of the first 15 seconds of the mixing period.
5. The throat of the drum shall be kept free of concrete accumulation that may restrict the free flow of materials into the drum.
6. Mixed concrete from the central mixing plant shall be transported in truck mixers, truck agitators, or non-agitating trucks. The time elapsed from the time water is added to the mix until the concrete is deposited in place at the site shall not exceed 45 minutes when the concrete is hauled in non-agitating trucks, nor 90 minutes when hauled in truck mixers or truck agitators, except that in hot weather or under other conditions contributing to quick hardening of concrete, the maximum allowable time may be reduced by the supervising Engineer.
7. Re-tempering concrete by adding water or by any other means shall not permitted, except that when concrete is delivered in truck mixers, additional water may be added to the batch materials and additional mixing is perform increasing the slump to meet the requirements if permitted by the Engineer, provided that all these operations are performed within 45 minutes after the initial mixing operations and the water cement ration is not exceeded.
Limitation of Mixing
No concrete should be mixed, placed, or finished, when natural light is insufficient, unless an adequate and approved artificial lighting system is operated.
