Concrete Production Facilities. Concrete may be mixed in portable mixers brought to the pile locations, but generally it will be ready-mixed. Ready-mix concrete may be (1) batched and mixed at a central plant and delivered to the pile locations in agitating or in agitating trucks (central-mixed), (2) batched at a central plant and mixed in a truck mixer in transit to or after reaching the jobsite (truck-mixed), or (3) partially centrally mixed with mixing completed in a truck mixer in route to the job or on the jobsite (shrink-mixed). The central plant may be located on the jobsite.
The concrete batch and mixing plant should be inspected for adequacy of storage facilities for materials, accuracy, and reliability of batching equipment, condition of mixing equipment, and proper operational procedures.
Storage Facilities. Cement must be kept dry whether it is stored in bulk containers or in bags.
To avoid contamination, stockpiles of aggregates that have been cleaned, graded, and prepared for batching should be on a hard, clean base with the area around the stockpiles spread with a bedding material of sand, gravel, or rock. Side slopes of stockpiles should not exceed 7 in/ft (583 mm or less per meter) to prevent segregation. Coarse aggregate should be separated by type and size gradation.
Overlapping of stockpiles should be prevented and suitable drainage should be provided.
All reasonable precautions should be taken to keep the moisture content of aggregates as nearly uniform as possible.
Batching Equipment. Concrete is usually batched by weight. Batching scales should have recent calibration and certificate of inspection and must be clean and free of interference by other objects. Separate weight-batching facilities should be provided for cement. Batch-weight recording and cutoff devices must operate accurately. The bottom of batch bins must be fully sloped in all directions. Water-metering devices, whether at a central mixing plant or mounted on a truck mixer, must be accurate and equipped with indicating dials and totalizers.
Mixing Equipment. All mixing equipment, whether stationary or truck-mounted, must be in
good operating condition. The interior of drums should be clean, and mixing blades should not show signs of wear in excess of 1 in (25.4 mm). Truck mixers must be equipped with a reliable revolution counter.
Operations. All materials must be accurately batched, and batching should be by weight. Admixtures, if required, must also be accurately measured. Mixing drums must be cleaned after each use to prevent an accumulation of hardened concrete on the blades. All washwater must be removed from the mixing drum prior to batching. Cement should be used on the basis of first in-first out. The free-water content of the aggregates should be included as part of the total mix water. Aggregates should be allowed sufficient time to drain, and it may be necessary to have a moisture meter in the sand batcher to monitor moisture content. Proper equipment and methods must be used for handling aggregates to avoid segregation and breakage. Segregation of coarse aggregate can be reduced by separating it into several size fractions and batching them separately. Finished screening of aggregates at the batcher is recommended to avoid problems of segregation and contamination.
Concrete Materials. Materials including cement, sand, coarse aggregate, and water should be inspected for compliance with specifications and accepted practice.
Cement. Cement must be of the type specified or permitted with the approval of the engineer.
Mill certificates should be furnished to show that cement conforms with the requirements of the specifications and ASTM C150, Standard Specifications for Portland Cement.Type IV cement should not be used for pile concrete. Type III, or high-early, cement may be permitted for cast-in-place concrete test piles to get a fast gain in strength. Type II or Type V cement may be specified for sulfate exposure.
Cement remaining in bulk storage for more than 6 months or cement stored in bags longer than
3months should be retested before use to ensure that it meets the requirements of ASTM C150.
Cement should not be used directly from the mill if it is still hot. The cement should be allowed to cool before using to reduce the possible occurrence of false sets.
Cement should be inspected for lumps caused by moisture. Cement bags should be inspected for rips, punctures, or other defects. If cement is to be batched by bag, the weights of bags should be spot-checked and should not vary by more than 3 percent.
Sand. Sand should be clean, sharp, well graded, and free of silt, clay, or organic material. The specific gravity and/or fineness modulus may be specified for special mixes such as reduced coarse aggregate concrete.
Coarse Aggregate. Specifications may permit gravel or crushed stone. The use of crushed rock aggregate requires more cement and sand for comparable workability. Air entrainment also improves workability. Lightweight aggregates are not recommended, and slag aggregates are not generally used. Alkali-reactive aggregates or aggregates from shales, friable sandstone, chert, and clayey or micaceous rock should not be permitted. Aggregates should be uncoated and free of silt, clay, organic material, and chemical salts. The specific gravity of the coarse aggregate may be specified. Aggregates should be well graded with a maximum size of ¾ in (19.05 mm) and with the amounts of aggregates less than
3/16 in (4.762 mm undersize) held uniform and within 3 percent.
Water. As a general rule, mix water should be potable. It should contain no impurities which would affect the quality of the concrete. It should not have a sweet, saline, or brackish taste or contain silt or suspended solids. Very hard water may contain high concentrations of sulfate. Well water from arid regions may contain harmful dissolved mineral salts. If questionable, the water can be chemically analyzed.The quality of the water can be checked by comparing the strength of concrete reached at various ages for a mix using the water of unknown quality with the results of similar age tests on a mix made with water which is known to be acceptable. Impurities in mix water may affect both the compressive strength of the concrete and its setting time.
Admixtures. The authorized or mandatory use of admixtures will be noted on the mix design report. Special admixtures such as retarders and fluidizers may be required for pumped concrete.
Cold-Weather Operations. The minimum temperature of fresh concrete as mixed should be about 45°F (7.2°C) for air temperatures above 30°F (-1.1°C), 50°F (10°C) for air temperatures from 0 to 30°F ([1]17.2 to -1.1°C), and 55°F (12.7°C) for air temperatures below 0°F (-17.2°C). Frozen aggregate or aggregates containing lumps of ice should be thawed before being used. It may be necessary to preheat the mix water and/or the aggregate. For air temperatures between 30 and 40°F (-1.1 and 7.2°C), it is usually necessary only to heat the water to a maximum of about 140°F (60°C).
For air temperatures below 30°F (-1.1°C), the water can be heated to 140 to 212°F (60 to 100°C) and the aggregate to about 45 to 55°F (7.2 to 12.7°C). Overheating should be avoided. If both the mix water and the aggregates are preheated, it is recommended that the water be mixed with the aggregates before adding the cement to avoid a flash set. The temperature of the water-aggregate mixture should not be higher than 80°F (26.6°C) and preferably about 60°F (15.5°C).
Hot-Weather Operations. If the temperature of the concrete during mixing is above 80°F
(26.6°C), it could result in increased water demand (slump loss) or an accelerated set. The easiest way to control and reduce the concrete temperature is by using cold mix water, which can be achieved by mechanical refrigeration or by using crushed ice as part or all of the mix water. Mixing time should be kept to a minimum, and mixing drums, water tanks, and pipe should be painted white.
>>> part 2
>>> part 2
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