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Control measures of temperature crack of concrete construction

Author: ChenShuangQuan From: www.yourpaper.net Posted: 2010-06-24 03:11:43 Read:
Abstract: This paper describes the analysis of concrete crack causes and the construction of concrete temperature stress, put forward to control the temperature during construction and crack prevention measures.
Keywords: concrete crack; temperature stress; control; measures of
In the process of concrete, temperature stress and temperature control is important, which is mainly due to two reasons often concrete temperature cracks, affect the structural integrity and durability.Secondly, with the impact can not be ignored in the process of using the stress state.We encountered is mainly the temperature crack in the construction.
As to the causes of concrete cracks and treatment measures:
1, the reason of the crack of
There are many reasons for concrete cracks.The main changes in temperature and humidity, the brittleness of concrete and aggregate uneven and irrational structure, raw material defects problems (such as alkali aggregate reaction), based on the use of process template deformation, uneven settlement.Hardening of concrete during hydration of cement shed considerable heat, the internal temperature rise, causing the surface tensile stress, late in the cooling process due to the constraint or the original concrete, it will appear in the concrete tensile stress.Lower temperatures will cause large tensile stress in the concrete surface.When the tensile stress exceeds the capacity of concrete cracking, the cracks will appear.Many concrete internal humidity changes, but the larger surface moisture may change or drastic changes.If conservation not weeks, dry when wet, the surface shrinkage deformation by internal constraints, but also often lead to cracks.Concrete is a brittle material, the tensile strength is a compressive strength of about 1/10, ultimate tensile deformation only short-term loading (0.6 ~ 1) กม 104, long-term loading and the limit tensile deformation only (1.2 ~ 2) กม 104.Because the raw materials is not uniform, the water cement ratio is not stable, segregation and transport and construction process in the same block, concrete in its tensile strength is not uniform, there are many tensile ability is very low, easy to crack the weak parts in the reinforced concrete, the tensile stress is mainly made of steel bear, the concrete is under compressive stress.In concrete or reinforced concrete structure in the tensile stress, they rely on their own concrete commitment.Not only a drag stress or tensile stress is small general design requirements are.But in the construction of concrete by the highest temperature cooling to the temperature stability of operation period, often pull stress caused considerable in the concrete.Stress is sometimes more than others can temperature stress caused by load, so to grasp the change rule of temperature stress for the structure design and the reasonable construction is very important.
2, the temperature stress analysis of
According to the formation of the construction process temperature stress in the process can be divided into the following stages:
Early: since the pouring of concrete began to cement the basic end of the heat, usually about 30 days.The two feature of this stage, a large release of heat of hydration of cement, the two is the rapid change in the elastic modulus of concrete.Due to changes in the elastic modulus, the period of the formation of residual stresses in concrete.
The medium-term: the role of basic self-heat cement concrete until the end when cooled to the temperature stability, this period, mainly due to thermal stress and external cooling of the concrete caused by temperature changes, the stress and the early formation of the residual stress superposition changed little during the elastic modulus of concrete.
According to the reasons caused by temperature stress can be divided into two categories:
(1) in situ stress: without any constraints or completely static structure at the border, if the internal temperature distribution is nonlinear, because the structure itself bound by the temperature stress.
(2) restraint stress: all or part of the boundary structure by the external constraints, can not be free deformation caused by stress.If the roof box girder concrete.These two kinds of temperature and stress is often common concrete dry shrinkage caused by stress.According to an accurate analysis of the known temperature distribution, temperature stress is a complicated job.The creep of concrete temperature stress have considerable relaxation, where no detail.
3, the construction process of temperature control and crack prevention measures
In order to prevent the crack, reduce the temperature stress can be from two aspects to control the temperature and the constraint condition.
Temperature control measures are as follows:
(1) used to improve the aggregate gradation, mixing material, entraining agent and plasticizer and other measures to reduce the cement content in concrete;

(2) by adding water or water will be crushed stone cooling to reduce the temperature of the concrete pouring concrete;

(3) reduce the thickness in hot weather when pouring concrete pouring, the level of
heat;
(4) provides reasonable time of stripping, surface heat air temperature pelter, in order to prevent the concrete surface temperature gradient sharp.
(5) the construction of long-term exposed concrete block surface or thin wall structures.Adopting the heat preservation measure in the cold season; the constraint condition measures are: A: reasonable parting;

Arrange the construction process of B: reasonable, avoid excessive height and side long exposure; in addition, improve concrete performance, improve crack resistance, strengthen maintenance, prevent surface shrinkage, especially is very important to ensure the quality of concrete is to prevent cracks, special attention should be paid to avoid crack, whole appears to restore its structure is very difficult, so construction should give priority to prevent the occurrence of penetrating cracks; the right to use water reducing admixture, crack are effective measures to decrease the cracking.
In concrete construction, often require new pouring the concrete form removal as soon as possible, should be appropriate to consider the removal time when concrete temperature is higher than the temperature, so as not to cause the early crack of concrete surface.New casting early form removal, caused by large tensile stress at the surface, "thermal shock" phenomenon.In the beginning of concrete placement, because the water heat, tensile stress considerable cause surface, the surface temperature is the temperature is high, the removal of the template, the surface temperature drop, must cause the temperature gradient, which stress on the pull additional surface, and the heat of hydration stress superposition, plus the shrinkage on the surface of concrete, the tensile stress reach large values, will lead to fracture risk, but if the template is removed in time after covering the surface with a light heat insulating material, such as foam, sponge, to prevent concrete surface appearance of tension stress, has the remarkable effect.
C: in order to guarantee the quality of concrete engineering, to prevent cracking, improve the durability of concrete, the correct action of additives is one of the measures to reduce cracking.
(1) the existence of a large number of pores in concrete, capillary tension of water evaporation in capillary, the concrete shrinkage deformation, increasing the capillary diameter can reduce the capillary surface tension, but will reduce the strength of concrete.The surface tension theory in the early sixty's has been recognized by the international.
(2) the amount of cement is also the important factors of concrete shrinkage, mixed with water reducing the crack resistance of concrete to maintain the strength of concrete conditions, the 15% can reduce the dosage of cement, the volume by increasing aggregate amount to supplement.
(3) consistency water reducing anti-cracking agent can improve cement slurry, reduce the bleeding of concrete, reduce the shrinkage deformation.
(4) improve the cement and aggregate bond strength, improve crack resistance of concrete.
(5) concrete is constrained by the tensile stress produced in contraction, when the stress fracture occurs more than the tensile strength of concrete, reduce the tensile strength increased water anti-cracking agent can effectively concrete, greatly improve the crack resistance of concrete.
(6) mixed with admixture of concrete can be good compactness, can effectively improve the carbonation resistance of concrete, reduce the carbonation shrinkage.
(7) adding water retarding crack resistance of concrete after proper time, effectively prevent the rapid hydration heat of cement foundation, long-term non-condensable cement to avoid the plastic shrinkage increases.
(8) admixture concrete workability, surface smoothing, forming a micro film, reduce evaporation, reduce drying shrinkage.Many retarding admixtures Duyou increase workability, improve the plasticity of function, we should experiment and research in this respect more in the project practice, than simply by improving the external conditions, may be more simple, economy.

4, early curing of concrete
From the beginning of the earliest possible time to prevent moisture from the surface of concrete damage.Not from the initial setting and final setting start problem here, only when to begin to question.For example, concrete pavement, bridge or ground construction, plastic shrinkage crack is a long-term problem.The past concrete secrete water
Large, generally uses two times of collecting, and then began to maintenance, prevent the plastic shrinkage crack.Modern high performance concrete basic no bleeding, if the wind is high or high temperature, moisture evaporation, the concrete surface soon cracks, must be in before the final setting again finishing closed fracture.In engineering practice, has been exploring how to start earlier maintenance, get a lot of successful experience.For example, before the initial setting, the concrete surface plaster finish:
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