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18INFLUENCE OF DRY AND HOT CLIMATE ON CONCRETE AND REINFORCED CONCRETE STRUCTURES
Bakhromjon Adhamovich Dilfuza Tillavoldiyevna Madinakhon Tolib qizi Otakulov Sobirova Yokubova
Fergana Polytechnic Institute
ABSTRACT
The article discusses the problems of maintenance of concrete in hot and cold periods of the year and its negative consequences.
Key words: relative humidity, macro porosity, hydration
In the summer, when the temperature reaches 40-500 C and the relative humidity is low (10-20%), the construction of buildings and structures, concrete and reinforced concrete structures is very inconvenient.
Currently, measures based on the long-term durability of concrete in dry climates are being studied, focusing on their durability. In the hot periods of the year, in conditions of low relative humidity, as soon as the concrete is laid, water escapes, which leads to the destruction of its structure.
When concrete freezes, the volume of water in the concrete pores increases, leading to the destruction of the structure in it, and the process of hydration of the cement is partially or completely stopped. As a result of the evaporation of water as a structural element, micro and even macro pores are formed in the concrete, and the structure remains defective. The hydration processes of cement are incomplete and the concrete does not have the appropriate physical and mechanical properties. In this regard, in the manufacture of prefabricated reinforced concrete structures have to address complex issues of water leakage of concrete, protection from the effects of solar radiation and the like. In order to improve the quality of construction in dry hot climates, it is necessary to set requirements for the building materials industry to increase the production of high-strength, fast-setting cements, coarse and fine aggregates.
Given the lack of high-quality natural coarse aggregates and the effectiveness of the use of porous aggregates in dry climates, it is necessary to expand their production.
Dry and hot weather greatly complicates the technology of concrete work; water consumption for concrete mix increases with increasing temperature; quickly loses its mobility when transporting the concrete mix or storing it until it is laid; cracks harden in hardened concrete; under the influence of solar radiation, an uneven area of motion is formed in the structures; the conditions of concrete work become more complicated, their cost increases, and other results occur.
It is known that the concrete mixture quickly loses its mobility as a result of acceleration of hydration and biting of cement in hot and dry weather. This is caused by high temperatures, evaporation of the solidification water. As a result, the mobility of the concrete mix is not ensured, the accepted conditions of transportation and laying, as well as the conditions of surface treatment of structures are violated. During the dry and hot weather of the year, the time from preparation of the concrete mix to the end of its laying should be as short as possible and should be 30-60 minutes for temperature t = 250 C, and 15-30 minutes for temperature t = 300 C, temperature t = 250 C. should not exceed 10-15 minutes for. The factor influencing the rapid change in the consistency of the mixture over time is the high temperature and the consequent acceleration of hydration and biting of the cement, while the escape of concrete water remains a secondary factor.
Among the above-mentioned negative consequences of dry and hot weather, a large amount of water leakage due to insufficient care of the newly laid concrete is a special case.The rapid evaporation of water from the concrete body depends on the composition of the concrete mix, the amount of hardening water, the ratio of water and cement, the type of cement and other factors. Under certain conditions, the concrete mix will collapse considerably, which will have negative consequences. When the evaporation rate is 0.7kg / m2, the maximum sinking is 3.5-3.6mm / m, 3.9-4.0 mm / m at 0.8 kg / m2, and 4.5mm / m at 0.85 kg / m2. Poorly maintained and poorly laid concrete loses 50-70% of its hardening water during the first night and day, with the bulk of it leaving the bulk of the concrete in the first 6-7 hours of hardening.
Such excessive water leakage results in the compaction of new structures, which reduces the ingress of moisture into the unhydrated portion of the cement grains, resulting in some or complete cessation of hydration of the cement in the hardened concrete, deterioration of the concrete and other properties.
Rapid evaporation of water from freshly laid concrete leads to an increase in settling, which is a process that leads to structural failure in dry and hot climates, significantly deteriorating the structure and physical and mechanical properties of concrete, and causing premature cracking of hardened concrete. The harmful effects of dry and hot climates not only make it difficult to carry out concrete work, but also adversely affect the use of concrete and reinforced concrete structures.
In such unfavorable conditions, measures are taken to retain moisture, for example, regular watering is carried out, preservatives such as moistened material, sawdust are applied.
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