A Brief Discussion on the Construction Temperature and Cracks of Concrete

Abstract: Through years of on-site observation and by consulting monographs on the internal stress of concrete, this paper elaborates on the causes of temperature cracks in concrete, the control of on-site concrete temperature, and measures to prevent cracks.

Key words: Concrete temperature, stress, crack control

Concrete plays an important role in modern engineering construction. Today, cracks in concrete are quite common, and they are almost everywhere in bridge engineering. Despite taking various measures and being cautious during the construction, cracks still occur from time to time. One of the reasons for this is that we did not pay enough attention to the changes in temperature stress of concrete.

In mass concrete, temperature stress and temperature control are of great significance. This is mainly due to two reasons. Firstly, during construction, concrete often experiences temperature cracks, which affect the integrity and durability of the structure. Secondly, during operation, temperature changes have a significant and non-negligible impact on the stress state of the structure. The main issues we encounter are temperature cracks during construction. Therefore, this article only discusses the causes and treatment measures of concrete cracks during construction.

1. Causes of cracks

There are various reasons for cracks in concrete, mainly including changes in temperature and humidity, brittleness and unevenness of concrete, unreasonable structure, substandard raw materials (such as alkali-aggregate reaction), formwork deformation, and uneven settlement of the foundation.

During the hardening of concrete, cement releases a large amount of heat of hydration, and the internal temperature keeps rising, causing tensile stress on the surface. During the cooling process in the later stage, tensile stress will occur inside the concrete again due to the constraints of the foundation or old concrete. The drop in temperature can also cause significant tensile stress on the surface of concrete. Cracks will occur when these tensile stresses exceed the crack resistance of the concrete. The internal humidity of many concretes changes very little or slowly, but the surface humidity may change significantly or undergo drastic changes. If the maintenance is not thorough and the surface is alternately dry and wet, the surface shrinkage deformation is constrained by the internal concrete, which often leads to cracks. Concrete is a brittle material, with tensile strength being about one-tenth of compressive strength. The ultimate tensile deformation under short-term loading is only (0.6-1.0) ×104, and even under long-term loading, it is only (1.2-2.0) ×104. Due to the uneven raw materials, unstable water-cement ratio, and segregation during transportation and pouring, the tensile strength of the same concrete block is also uneven. There are many weak areas with very low tensile capacity and prone to cracking. In reinforced concrete, tensile stress is mainly borne by the steel bars, while the concrete only bears compressive stress. If tensile stress occurs within the structure of plain concrete or at the edge of reinforced concrete, it must be borne by the concrete itself. In general design, it is required that no tensile stress or only a very small tensile stress occurs. However, during construction, when the concrete cools from its maximum temperature to the stable temperature during operation, it often causes considerable tensile stress inside the concrete. Sometimes, temperature stress can exceed the stress caused by other external loads. Therefore, mastering the variation law of temperature stress is extremely important for conducting reasonable structural design and construction.

2. Analysis of temperature stress

According to the formation process of temperature stress, it can be divided into the following three stages

(1) Early stage: From the start of concrete pouring until the heat release of the cement basically ends, it usually takes about 30 days. Two characteristics of this stage are: one is that cement releases a large amount of heat of hydration, and the other is the sharp change in the elastic modulus of concrete. Due to the change in elastic modulus, residual stress is formed within the concrete during this period.

(2) Mid-term: From the time when the heat release of cement basically comes to an end until the concrete cools to a stable temperature, during this period, the temperature stress is mainly caused by the cooling of the concrete and changes in external temperature. These stresses are superimposed with the residual stresses formed in the early stage. During this period, the elastic modulus of the concrete changes little.

(3) Late stage: The period of operation after the concrete has completely cooled down. Temperature stress is mainly caused by changes in external temperature, and these stresses are superimposed with the residual stresses of the first two types.

According to the causes of temperature stress, it can be divided into two categories:

(1) Self-generated stress: In a structure with no constraints or complete rest on the boundary, if the internal temperature is distributed nonlinearly, the temperature stress that occurs due to the mutual constraints within the structure itself. For instance, in the case of bridge piers, the structural dimensions are relatively large. When the concrete cools, the surface temperature is low while the internal temperature is high, resulting in tensile stress on the surface and compressive stress in the middle.

(2) Constraint stress: The stress caused by the entire or part of the boundary of a structure being constrained by the external environment and unable to deform freely. Such as the top slab concrete of box girders and the concrete of guardrails.

These two types of temperature stresses often act in conjunction with the stress caused by the dry shrinkage of concrete.

Accurately analyzing the distribution and magnitude of temperature stress based on the known temperature is a rather complex task. In most cases, model tests or numerical calculations are needed. The creep of concrete causes considerable relaxation of temperature stress. When calculating temperature stress, the influence of creep must be taken into account.

3. Temperature control and measures to prevent cracks

To prevent cracks and reduce temperature stress, efforts can be made from two aspects: controlling temperature and improving constraint conditions.

The measures for controlling temperature are as follows:

(1) Measures such as improving aggregate gradation, using dry-hard concrete, adding admixtures, and adding air-entraining agents or plasticizers are adopted to reduce the amount of cement in concrete.

(2) When mixing concrete, add water or use water to cool the crushed stones to lower the pouring temperature of the concrete.

(3) When pouring concrete in hot weather, reduce the pouring thickness and utilize the pouring layer for heat dissipation.

(4) Bury water pipes in the concrete and pass cold water through them to cool down.

(5) Specify a reasonable formwork removal time and conduct surface insulation when the temperature drops sharply to prevent a sudden temperature gradient on the concrete surface.

(6) For the surface of concrete pouring blocks or thin-walled structures that are exposed for a long time during construction, insulation measures should be taken in cold seasons.

The measures to improve the constraints are:

(1) Divide the seams and blocks reasonably;

(2) Avoid excessive fluctuations in the foundation;

(3) Arrange the construction procedures reasonably to avoid excessive height differences and long-term exposure of the sides.

In addition, improving the performance of concrete, enhancing its crack resistance, strengthening maintenance, and preventing surface dry shrinkage, especially ensuring the quality of concrete, are of great significance for preventing cracks. Special attention should be paid to avoiding the occurrence of through cracks. Once they appear, it is very difficult to restore the integrity of the structure. Therefore, in construction, the focus should be on preventing the occurrence of through cracks.

In the construction of concrete, in order to increase the turnover rate of formwork, it is often required that the freshly poured concrete be removed from the formwork as soon as possible. When the temperature of the concrete is higher than the ambient temperature, the formwork removal time should be appropriately considered to avoid early cracks on the concrete surface. Early formwork removal after fresh pouring causes significant tensile stress on the surface, resulting in a "temperature shock" phenomenon. At the initial stage of concrete pouring, due to the dissipation of hydration heat, considerable tensile stress is generated on the surface. At this time, the surface temperature is also higher than the air temperature. When the formwork is removed at this time, the surface temperature drops sharply, which will inevitably cause a temperature gradient, thereby adding tensile stress to the surface. When combined with the hydration heat stress and the dry shrinkage of the concrete, the tensile stress on the surface reaches a very large value, posing a risk of cracking. However, if lightweight insulation materials such as foam and sponge are promptly covered on the surface after the formwork is removed, it will have a significant effect on preventing excessive tensile stress from occurring on the concrete surface.

Reinforcement has little effect on the temperature stress of mass concrete because the reinforcement ratio of mass concrete is extremely low. It only has an impact on ordinary reinforced concrete. Under conditions where the temperature is not too high and the stress is below the yield limit, the various properties of steel are stable and are independent of the stress state, time and temperature. The linear expansion coefficient of steel and that of concrete differ very little, and only a small internal stress occurs between the two when the temperature changes. As the elastic modulus of steel is 7 to 15 times that of concrete, when the internal concrete stress reaches the tensile strength and cracks, the stress of the reinforcing bars will not exceed 100 to 200kg/cm ². Therefore, it is very difficult to use steel bars in concrete to prevent the occurrence of fine cracks. However, after reinforcement, the cracks within the structure usually become numerous, with smaller spacing, and less width and depth. Moreover, when the diameter of the reinforcing bars is thin and the spacing is dense, the effect on improving the crack resistance of concrete is better. Fine and shallow cracks often occur on the surfaces of concrete and reinforced concrete structures, most of which are dry shrinkage cracks. Although such cracks are generally shallow, they still have a certain impact on the strength and durability of the structure.

To ensure the quality of concrete engineering, prevent cracking and enhance the durability of concrete, the correct use of admixtures is also one of the measures to reduce cracking. For instance, when using water-reducing and crack-preventing agents, the author has summarized in practice that their main functions are:

(1) There are a large number of capillary channels in concrete. After water evaporates, capillary tension is generated in the capillaries, causing the concrete to shrink and deform. Increasing the diameter of capillary pores can reduce the surface tension of capillaries, but it will lower the strength of concrete. This surface tension theory was recognized internationally as early as the 1960s.

(2) The water-cement ratio is an important factor affecting the shrinkage of concrete. The use of water-reducing and crack-preventing agents can reduce the water consumption of concrete by 25%.

(3) The amount of cement used is also an important factor in the shrinkage rate of concrete. Concrete with water-reducing and crack-preventing agents added can reduce the amount of cement by 15% while maintaining its strength, and the volume can be supplemented by increasing the amount of aggregates.

(4) Water-reducing and crack-preventing agents can improve the consistency of cement slurry, reduce bleeding in concrete, and minimize expansion and contraction deformation.

(5) Enhance the bonding strength between cement slurry and aggregates, and improve the crack resistance of concrete.

(6) When concrete shrinks, it is constrained and tensile stress is generated. When the tensile stress exceeds the tensile strength of the concrete, cracks will occur. Water-reducing and crack-preventing agents can effectively enhance the tensile strength of concrete and significantly improve its crack resistance.

(7) The addition of admixtures can make concrete more compact, effectively enhance its resistance to carbonation, and reduce carbonation shrinkage.

(8) After adding the water-reducing and anti-cracking agent, the retarding time of the concrete is appropriate. On the basis of effectively preventing the rapid hydration and heat release of cement, it avoids the increase in plastic shrinkage caused by the long-term non-setting of cement.

(9) Concrete with admixtures has good workability, its surface is easy to level, and it can form a micro-film, reducing water evaporation and drying shrinkage.

Many admixtures have the functions of retarding, increasing workability and improving plasticity. In engineering practice, we should conduct more experimental comparisons and research in this regard. Compared with simply improving external conditions, it may be more straightforward and economical.

4. Early curing of concrete

Practical experience has shown that the common cracks in concrete are mostly surface cracks of different depths. The main reason is that the temperature gradient causes a sudden drop in temperature in cold regions, which is also prone to form cracks. Therefore, it can be said that the insulation of concrete is particularly important for preventing early surface cracks.

From the perspective of temperature stress, insulation should meet the following requirements:

1) Prevent the temperature difference between the inside and outside of the concrete and the surface gradient of the concrete, and prevent surface cracks.

2) To prevent concrete from getting too cold, efforts should be made to ensure that the minimum temperature during the construction period of concrete does not fall below the stable temperature during its service life.

3) Prevent the old concrete from getting too cold to reduce the constraints between the new and old concrete.

The early curing of concrete is mainly aimed at maintaining appropriate temperature and humidity conditions to achieve two effects. On the one hand, it protects the concrete from adverse temperature and humidity deformation and prevents harmful cold shrinkage and dry shrinkage. On the one hand, it ensures the smooth progress of cement hydration, with the aim of achieving the designed strength and crack resistance.

The suitable temperature and humidity conditions are interrelated. The insulation measures on concrete often have a moisture retention effect as well.

Theoretically, the moisture content in freshly poured concrete is more than sufficient to meet the hydration requirements of cement. However, due to reasons such as evaporation, water loss often occurs, thereby delaying or hindering the hydration of cement. Surface concrete is the most vulnerable and directly affected by this adverse effect. Therefore, the first few days after concrete pouring are a crucial period for curing, and they should be given due attention during construction.

5. Conclusion

The above has conducted a preliminary theoretical and practical discussion on the relationship between the construction temperature of concrete and cracks. Although there are different theories in the academic circle regarding the causes and calculation methods of concrete cracks, opinions on specific prevention and improvement measures are relatively unified. At the same time, the application effect in practice is also relatively good. In specific construction, we need to observe and compare more. After problems arise, by conducting more analysis and summary, and combining multiple preventive and treatment measures, cracks in concrete can be completely avoided.

References

Ye Jianshu, Principles of Structural Design, People's Communications Press, 2005.

2. Lu Shusheng, Modern Prestressed Concrete Theory and Application, China Railway Publishing House, 2003.

3. Shao Xudong, Bridge Engineering, People's Communications Press, 2004.