When concrete was mixed on the building site and then transported to its final location by wheelbarrow, segregation of the concrete mix was not a serious problem. With modern transportation methods, however, which may involve pumping the concrete or sending it by gravity down a chute over long distances, it becomes a problem.

Segregation is a separation of the constituents of concrete so that their distribution ceases to be sufficiently uniform. It may be caused by a differential settlement of the aggregates in which the larger particles, or the heavier particles, travel faster down a pipe or slope, or settle faster in the concrete when they reach their final destination.?Another type of segregation, which occurs particularly in wet mixes, results in the separation of the cement grout ( that is, the cement and the water) from the aggregate and its formation in a layer on top of the?concrete.

Segregation does not occur if the concrete mix is cohesive, but a workable mix is not necessarily cohesive.?The slump test, the?compacting factor test, and the Kelly ball test all measure workability, but not cohesiveness. The flow test, however, does give an indication of cohesiveness. Air-entraining agents reduce the tendency towards segregation.

Evidently, great economies are achievable by the use of concrete pumps and slides, but these should be manipulated so that the?concrete does not have to be moved sideways after it is deposited. The placement of concrete should be designed for it to travel the shortest possible distance to its final destination. Concrete should not be placed in layers thicker than 0. 5 m (18 in. ) to ensure that the layer below is still soft and that the two layers can be integrated by vibration. The vibration should be limited to the minimum required for consolidation, as excessive amounts produce segregation.

Closely related to segregation is bleeding, which is the collection of mixing water on the surface of freshly placed concrete; this water may carry some of the cement with it. Bleeding is a form of subsidence and can be expressed quantitatively as the total settlement per unit depth of concrete. There is an ASTM(American Society of Testing Materials) standard test ( C 232-71) for bleeding and the rate of bleeding.

Bleeding is not necessarily harmful if the water that collects on the surface evaporates before the concrete surface is given its final finish with a float. However, if the bleeding water brings cement to the surface, then a layer of set cement is formed on the surface; called laitance, this produces a dusty surface and a plane of weakness. If further concrete is to be placed on top, then the laitance must be removed by brushing to ensure proper adhesion of the new concrete. Finishing with a wooden float, instead of a steel float, avoids overworking of the surface and bringing an excess of cement to?the top.

Bleeding water may also become trapped under large aggregate particles or under reinforcing bars, where it forms capillary voids on evaporation; these may have an adverse effect on durability, particularly on resistance to frost.

The tendency to bleeding can be reduced by the use of air-entraining agents, by the use of finer cement and a greater proportion?of very fine aggregate, by a decrease in the water content, and by a decrease in the water/ cement ratio ( by reducing water content or increasing cement content).

Both high and low temperatures adversely affect the hydration of the cement. Special precautions are necessary when placing concrete in hot weather. The temperature of new concrete should be kept below 32°C (90 F), and preferably below 30° (85F). This may be accomplished by mixing crushed ice with the mixing water and by keeping all accessory materials in the shade. The heat of hydration produced by the cement can be reduced by using a low-heat cement and by using special care in curing to prevent evaporation of the mixing water.

Special precautions are also necessary in cold weather, that is, when the temperature falls below 4°C (40 F). The concrete should be kept above a temperature of 15°C (59 F). This may be accomplished by heating the mixing water and the aggregates; however, the water should not be heated above 65°C (150 °F) to avoid a flash set of the concrete. A cement with a higher content of calcium aluminate and of tricalcium silicate should be used to generate a higher rate of heat development. Calcium chloride as an accelerator increases the rate of hydration and thus generates further heat; in addition, it turns the mixing water into a salt solution and thus lowers its freezing point below that of pure water. The concrete must be further protected against frost during curing.

Curing is the term given to the protection of the concrete during the early stages of hardening, when it requires additional moisture for the continuing hydration of the cement, and to its protection from cold during the night and from heat during the day. The concrete surface can be covered with sand or burlap, which is kept moist by watering?or spraying. Precast concrete units and concrete slabs may be covered with water.

Alternatively, the concrete can be covered with a curing membrane, which provides a physical barrier to the evaporation of its water. This is applied either as a sheet or as a liquid that dries within a few hours to a continuous, adhesive film; a dye is sometimes added to facilitate uniform coverage. Polyvinyl chloride, polyethylene ( about 0.2 mm or 0. 001 in. thick), or waterproof building paper provide suitable sheet materials. Liquid curing compounds include various wax and oil emulsions as well as various plastics.

Precast concrete units are sometimes steam-cured or autoclaved (cured with high-pressure steam), a treatment that increases their strength and makes it possible to handle and transport them much sooner, thus greatly reducing the amount of storage space required.

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