Concrete technology in a dry hot climate

The conditions of a dry hot climate are characterized by summer outdoor temperatures of 35 ... 40 ° C at a relative humidity of 10 ... 25%, intense solar radiation and frequent winds. The combination of the impact of these climatic factors leads to rapid dehydration (drying) of concrete, which slows down and even stops the processes of cement hydration. With fast drying of concrete, its strength decreases by almost 50% compared to concretes hardening in normal temperature and humidity conditions. Intensive early dehydration leads to the formation of capillaries directed towards the evaporating surface, which impairs the pore structure of concrete and, consequently, reduces its durability. Dehydration also leads to peeling of the outer layers of the concrete structure.

The required quality of concrete in a dry hot climate can be achieved through the use of such methods of preparation, transportation and maintenance of concrete, which would reduce to the minimum possible its dehydration.

When preparing a concrete mix, it is necessary to apply measures that ensure the preservation of the required consistency by the time of laying in the formwork. This can be achieved by lowering the temperature of the mixture during its preparation and taking measures to prevent dehydration during transportation, laying and curing the concrete.

It was established that when the air temperature is up to 40 ° C and the low relative humidity, the temperature of the concrete mixture can be reduced to 20 ... 25 ° C by wetting the aggregates with cooled water, blowing them with cold air when sold to the mixer. The same purpose can serve the addition of up to 50% ice in the mass of water.

Conservation of the consistency of the concrete mixture can be achieved by introducing into the concrete mixture during its preparation of surface-active additives. They not only reduce the dehydration of the mixture, but also plasticize it, reducing water demand.

The duration of mixing the concrete mix, in a dry and hot climate, is increased by 30-50%. At the same time in the mixer load the aggregate, as well as 2/3 of the calculated amount of water and stirred for 1-2 minutes. Then the cement, the rest of the water is added, additives are added and mixed again for 3-4 minutes.

The finished concrete mix is ​​transported in a closed container. For these purposes, most suitable concrete trucks and concrete mixers. It is necessary to avoid long-distance transport of the mixture, because in the process of transportation it is dehydrated and loses its mobility.

The following scheme for the use of a concrete mix meets the conditions for a dry and hot climate: loading the dry mix at a central concrete mixing plant in a concrete mixer, transporting it in a dry form to the place of installation, mixing in concrete mixers directly at the concreting site and immediately laying it in the structure.

Formwork should not have the smallest crevices to eliminate the loss of cement milk and moisture. Before laying the concrete mix moisten the formwork. The forming surface of the deck of their moisture-absorbing materials should be covered with special compositions or polymer films that prevent adhesion to concrete, as well as the absorption of water from it.

The concrete mixture should be supplied and distributed by methods excluding its repeated overload or rapid dehydration. For example, it is not recommended to feed the mixture from open conveyors, as well as along long trays and vibration gutters. The most expedient is to supply the mixture with concrete pumps or in large-capacity tubs with the help of cranes. The free fall of the mixture should not exceed 1.5-2 m. Concreting is desirable to conduct continuously. In the case of breaks, special attention should be paid to the quality of the training of working seams. Careful vibration compaction of the mixture should ensure a dense concrete structure and reduce water evaporation.

Particular attention should be paid to the care of concrete, for which the exposed surfaces of freshly laid concrete are covered with sacking, matting, tarpaulin, after laying the concrete every 3-4 hours is moistened systematically. In contrast to the wetting of concrete in the middle zone in hot and dry climates, it is watered more often, and the duration of watering is increased to 28 days. Concrete surfaces are also covered with sand or wet sawdust, followed by systematic moistening. Where conditions allow, flooded the concrete with water after 6-12 hours after laying.

With water deficiency, wetting of concrete is associated with significant costs, so it is advisable to apply the so-called moisture-free methods of care for concrete. These include curing the concrete under special air-tight film caps or covering the concrete surface with various compositions.

Immediately after concreting, structures of small sizes are covered with light portable caps, the frame of which is made of steel tubes or rods with a diameter of 16-20 mm, and the coating is made of a polyvinyl chloride film with a thickness of at least 0.2 mm. The fill factor of the chamber (the ratio of the volume of the concrete structure to the volume of the chamber) should be 0.70 ... 0.85. When ensuring tightness under the chamber, conditions are created that are close to a soft steaming mode. Dehydration of concrete can be minimized by reducing the time it is kept by intensifying the hardening process. For this purpose, highly active, but low-shrinking, chemicals, hardening accelerators, as well as methods of heat treatment are used. The method of heat treatment may be the most effective, as it allows not only to reduce the risk of dehydration, but also to obtain the necessary strength of concrete in the shortest possible time. It should be borne in mind that after acquiring concrete 70 ... 80% of project strength, it does not require any special care in dry and hot climates.