It is very common to use mass concrete in bridge engineering, but the biggest disadvantage is that it is easy to produce cracks, which will affect the mechanical properties and durability of concrete and even lead to structural damage, greatly shortening the service life of buildings. Therefore, strengthening the control of mass concrete cracks is the key to ensure the quality and safe use of bridge structures, which must be highly valued. According to the characteristics of mass concrete, this paper discusses the construction technology and measures of crack control of mass concrete.
1 characteristics of mass concrete in bridge engineering
1) in bridge engineering, mass concrete mostly appears on the bottom plate and pier (abutment); 2) The thickness of mass concrete is generally in the range of 1 ~ 3m, and most of them are slab-wall structures; 3) The construction climate is generally good, the annual temperature is generally between 0 and 35℃, and there are few days below 0℃ and above 35℃; 4) In recent years, on-site concrete transportation mostly adopts pumping mode.
2 types and causes of common cracks in concrete
Floor, pier wall, etc. Bridge structure is usually the place where concrete cracks are easy to occur. There are many forms of concrete cracks, which can be roughly divided into three types according to their causes: temperature cracks, structural deformation constraint cracks and shrinkage cracks during concrete curing. It should be noted that the above three types of cracks can not be completely divided, especially the deformation constraint cracks and temperature cracks are sometimes interrelated, and the deformation constraint cracks formed along the shrinkage cracks have also been found in practice.
2. 1 temperature crack
During the curing process of concrete, due to the chemical reaction between cement and water, a large amount of hydration heat is generated, which leads to the rapid increase of the internal temperature of concrete. The author has organized the concrete temperature measurement during the construction period of related projects, and the temperature reaches the maximum in 2 ~ 3 days after concrete pouring, and the central temperature of components can generally reach 55℃. However, the concrete surface is easy to dissipate heat, and the temperature difference between inside and outside causes internal compressive stress and external tensile stress, which leads to cracks on the concrete surface. With the decrease of hydration heat, the internal temperature of concrete decreases, and the compressive stress is gradually transformed into tensile stress. As long as the tensile stress is slightly greater than the allowable tensile stress of concrete, due to stress concentration, cracks may continue to develop to the inside where external cracks have appeared. For plate structures, such as piers and walls, through joints are usually formed. The main feature of temperature cracks is that they occur in the early stage of construction, and they can usually be found during or shortly after formwork removal. In particular, sometimes the temperature cracks that have occurred may be too small to be found in time, and then the cracks expand due to the constraint of concrete shrinkage and deformation.
2.2 Structural deformation constraint cracks
The curing of concrete is a slow process. Ordinary concrete is not only a thermal expansion process due to the rising hydration heat at the initial curing stage, but also a shrinkage deformation process in the subsequent cooling process and long curing process. In addition, the change of external temperature will also lead to the thermal expansion and cold contraction deformation of concrete members. The pier wall of a bridge building is usually constructed by stages with a long interval from its bottom plate. When the concrete of pier wall shrinks and deforms, the upper part is free, and the lower part is constrained by the bottom plate whose deformation has been basically stable, resulting in tensile stress. When the tensile stress is greater than the allowable tensile stress of concrete, cracks may occur.
2.3 Shrinkage cracks
The curing process of concrete is also the volatilization process of water in concrete. If the water loss is too fast, drying shrinkage cracks may occur. In addition, if the water-cement ratio of concrete is too large, drying shrinkage cracks will usually appear. Shrinkage cracks appear on the surface of the structure, which are irregular, small in width and shallow in depth. However, due to the large water-cement ratio, the shrinkage cracks may be wider and deeper. Surface cracks without formwork may appear immediately after initial setting, and free surface cracks with formwork usually begin after formwork removal. In engineering practice, it is often found that surface cracks may still appear after long-term pouring of concrete. The author thinks that the main reason of surface cracking in the later period of concrete.
3 technical measures to prevent cracks
3. 1 design measures
1) Pay attention to the foundation treatment. Mass concrete is generally a whole structure, which is thick and heavy, and the influence of foundation on foundation is very obvious. In the design, it is mainly to prevent uneven settlement of foundation and improve the constraint influence on foundation. When the foundation is soft soil, sand cushion or other reinforcement methods are usually used to prevent uneven settlement of the foundation. Sand cushion can not only improve the bearing capacity of foundation, but also set blind ditch for drainage during construction, which has obvious effect on reducing the influence of groundwater or surface water. Before construction, the sand cushion should pass the test to make it have the best water content and maximum compactness before backfilling. Sand cushion can also reduce the constraint on concrete foundation, so in uneven soft soil foundation, if it meets the conditions, more economical sand cushion reinforcement scheme should be given priority.
2) Reasonable splitting and blocking can not only reduce the constraint effect and narrow the constraint scope; At the same time, the surface of the pouring block can also be used to dissipate heat and reduce the temperature inside the concrete. Reasonable joints and blocks should be able to adjust the temperature change of the structure, ensure the free expansion space of concrete, and achieve the purpose of releasing temperature stress. The treatment of joints must meet the requirements of preventing water leakage. 3) The rational distribution of steel bars, the basis of steel bars and concrete work is the cohesive force between them. Because the elastic modulus of steel is about 7- 15 times that of concrete, when the internal stress of concrete reaches the tensile strength and cracks begin to appear, the stress of steel bar is very small. It is impossible to prevent concrete cracks with steel bars, but reasonable arrangement and distribution of steel bars can reduce the shrinkage of concrete and limit the development of cracks. 4) The design label of concrete should not be too high. In mass concrete structures, mechanical strength and safety reserve are usually high, but too high strength reserve will increase cement consumption and hydration heat. The internal temperature of concrete is too high, the temperature difference between inside and outside is too large, and the structure cracks from the surface.
3.2 Formwork and reinforcement
3.2. 1 template mass concrete construction, the template bears the lateral pressure of sheep concrete and the vibration force of vibrating concrete, so it is absolutely necessary to ensure the reliability of the template and its supporting system to prevent the template from generating excessive deformation. In recent years, due to the use of pumping concrete, the pouring time is short and fast, which increases the lateral pressure on the formwork. For example, when the rigidity of formwork and supporting system is not enough, the possibility of cracks in concrete is greater. Therefore, the formwork of mass concrete should be analyzed and calculated from the formwork, beams, columns, tie rods and all components of the support system according to the stress situation, and a sufficient safety factor should be taken.
3.2.2 The corrosion and expansion of steel bars in concrete will enlarge the volume, which is also one of the reasons for cracks in mass concrete. The main reasons for the corrosion of steel bars are as follows: ① Before using seriously corroded steel bars, the rust removal of steel bars must be done well, and seriously corroded steel bars should not be used in projects. (2) Due to the poor compactness and impermeability of concrete, the steel bars inside the concrete are easily corroded by the environment or groundwater, leading to concrete cracking. (3) Some chemicals in concrete, such as calcium chloride and sodium chloride, are also one of the reasons for the corrosion of steel bars, so the content of chlorine salt should be controlled in concrete engineering to ensure that it is lower than the specification requirements.
3.3 concrete raw materials
3.3. 1 The influence of cement on the cracks of mass concrete has been mentioned before, so I won't repeat it here. The variety and dosage of cement have influence on the cracks of mass concrete. On the volume stability of cement. If uneven volume changes occur after cement hardening, it will cause expansion cracks in the structure, affect the engineering quality and even cause serious accidents. Therefore, cement with poor volume stability should be treated as waste and not used in mass concrete engineering.
3.3.2 Aggregate 1) harmful impurities, and the aggregate for mixing concrete should be clean and free of impurities to ensure the quality of concrete. 2) Aggregate variety and particle size requirements, sand is divided into coarse sand, medium sand and fine sand according to its average particle size and fineness modulus. Coarse sand or medium sand is usually used for mass concrete. Stones are divided into crushed stones and pebbles according to their shapes. When the mixture ratio is the same, crushed stone concrete has higher strength and better crack resistance. When the compressive strength and slump of concrete are equal, the cement content of pebble concrete is less. For projects with special requirements for crack resistance, pebbles can also be broken manually without gravel. The larger the maximum particle size of stone, the better the compactness of concrete, which can save cement; However, in order to facilitate tamping, the maximum particle size of stone shall not exceed 1/4 of the minimum size of the structural section, nor shall it be greater than 3/4 of the minimum spacing of steel bars. When pumping concrete, the maximum particle size of stones should also meet the requirements to avoid blockage.
3.4 concrete mixture ratio
Concrete mix proportion refers to the quantitative proportion of various components in concrete. The task of concrete design is to determine the concrete mixture ratio that can meet the engineering requirements according to the construction conditions of raw materials, design the concrete mixture ratio reasonably, minimize the concrete slump and reduce the cement consumption. 1) variety and dosage of cement. Low or medium heat cement should be selected for mass concrete, and 32.5 or 42.5 grade slag cement, pozzolanic cement and fly ash cement are preferred. Reduce water consumption as much as possible within the scope permitted by construction conditions. Using cement as little as possible can reduce the total calorific value of cement, thus reducing the maximum temperature inside concrete, which is the primary problem to be considered in the mix design of mass concrete. 2) Adding fly ash to reduce the dosage of cement. The dosage of fly ash is generally about 15% of cement dosage, which will affect the durability of concrete if it is too large. 3) The quality of stone grading has a great relationship with saving cement and ensuring good workability of concrete. In practical engineering, if a single material can't meet the above grading requirements, different particle sizes can be used for blending test, and the reasonable blending ratio can be finally determined through repeated screening.
4) workability requirements, workability refers to the performance that concrete mixture can keep the uniformity of concrete components, avoid segregation, and be easy to operate in construction. Mass concrete is characterized by a large quantity and a wide range, requiring continuous operation of concrete and strict temperature control. Therefore, most of the concrete is supplied by the centralized mixing station. Due to the great influence of transportation, pouring, vibration and climate, some new requirements are put forward for the workability of concrete. (1) Effects of different standing time and climatic conditions on water loss and slump of concrete mixture; ② The influence of different standing time and climatic conditions on the heat (cold) loss of concrete mixture. 5) The application of additives, adding a small amount of additives to concrete or mortar can improve its performance, save cement consumption, reduce the project cost and shorten the construction period. It is a new material with convenient use and remarkable effect. In order to ensure the engineering quality, we should pay attention to the following problems when adding additives: we must adapt in advance, mix accurately, mix evenly, and pay attention to the mixing order.
3.5 Concrete pouring method
The mass concrete pouring scheme should be based on the requirements of overall continuous pouring to avoid long-term use of concrete after mixing, and the following three methods should be selected according to the specific conditions such as structural size, steel bar density and concrete supply conditions (vertical and horizontal transportation capacity). 3.5. 1 Full-layer concrete should be poured in the whole structure, so that when the first layer is poured and the second layer is poured, the first layer of concrete that has been constructed has not yet been initially set, and it should be gradually carried out until the pouring is completed. This scheme is suitable for projects with small structural plane size, and the construction should start from the short side and advance along the long side; Can also be divided into two sections, from the middle to both ends or from both ends to the middle at the same time. 3.5.2 Sectional layering is suitable for projects with small thickness but large area or length. During construction, concrete is poured from the bottom first, then the second layer is poured after a certain distance, and the other layers are poured forward in turn.
3.5.3 Inclined plane layering is suitable for structures with a length exceeding 3 times the thickness. Vibrating work should start from the lower end of the pouring layer and move upward gradually. At this time, the gradient of forward pouring and paving should be less than 1: 3 to ensure the construction quality between layered concrete. 3.5.4 The following problems should be paid attention to when pouring concrete: 1) The thickness of each layer of concrete should not exceed 1.25 times of the vibrator length. When vibrating the upper layer, the next layer of concrete should be inserted about 5cm to eliminate the joint between the two layers; At the same time, when vibrating the upper concrete, it should be done before the initial setting of the lower concrete. In addition to the length and vibration force of the vibrator, the layered thickness of concrete also needs to consider the supply of concrete, the pouring speed and the size of steel bars. In mass concrete engineering, it can generally be set at 400 ~ 600 mm, and in a few concrete engineering, 250 ~ 350 mm is appropriate. 2) Prevent concrete segregation. Concrete shall fall freely from a height of more than 2m, and shall fall along a string of drums, chutes and chutes. Ensure that the concrete does not segregate. The arrangement of funnels and string tubes should be determined according to the pouring area, pouring speed and spreading ability of concrete, but the spacing should not be greater than 3m, and they should be staggered or arranged in determinant.
3.6 Control the temperature difference between inside and outside of concrete
Generally speaking, when the temperature difference between the inside and outside of mass concrete exceeds 25℃, concrete is prone to temperature cracks. As far as construction is concerned, it is difficult to control the temperature difference between inside and outside of concrete due to different environment and seasons. Strengthening concrete curing and increasing concrete curing time can effectively prevent concrete cracks. Moreover, the maintenance of mass concrete construction in winter and summer is particularly critical: 3.6. 1 Temperature difference control of mass concrete construction in winter. Due to the low construction temperature of mass concrete in winter, the highest temperature in concrete can generally reach 70℃, so the thermal insulation of concrete should be done well and the frequency of temperature detection should be increased. Once it is found that the temperature difference exceeds the specification requirements, measures such as heat preservation (surface steam curing, adding covering layer) should be taken immediately to ensure the temperature difference between the inside and outside of concrete.
3.6.2 The temperature difference between inside and outside of mass concrete construction in summer should be controlled at a high level. During mass concrete construction, firstly, ensure that the pouring temperature of concrete does not exceed 30℃ and the thickness of concrete pouring layer is about 30cm, and try to take measures that are beneficial to the heat dissipation of concrete (without affecting the quality of concrete construction). In summer, the following measures are mainly taken to ensure the internal temperature difference of mass concrete: ① heat preservation method: water storage method is adopted when conditions permit. After the completion of concrete pouring, the surface water is about 20 ~ 30cm. Once the internal temperature of concrete rises, the temperature of accumulated water on its surface also rises, so that the temperature difference between inside and outside can be effectively controlled. That is, controlling temperature stress and reducing the probability of concrete cracks; (2) Internal cooling method: the internal temperature of mass concrete is evaporated through embedded water pipes, so that the temperature difference between inside and outside meets the specification requirements.
3.7 Temperature control requirements
Temperature control is an important link in mass concrete construction and the core to prevent cracks in mass concrete. 3.7. 1 Prepare the construction plan and put forward practical and concrete measures. When making a plan, we should listen to the opinions of experienced workers and technicians. The reason analysis chart and countermeasure table for ensuring quality are listed side by side, so that the relevant measures of the construction scheme can be implemented and problems can be easily corrected. In the past, the requirement of temperature difference was generally emphasized, that is, the so-called "temperature difference control", but the temperature stress was rarely considered, or it was considered that the temperature difference was proportional to the temperature difference stress, which could not effectively prevent the occurrence of temperature cracks. It must be pointed out that only temperature difference control, due to the different specific conditions of the structure (including structural size, reinforcement, foundation constraints, etc. ), their temperature stress is different. This fully explains why the same temperature difference control method is adopted, some projects do not crack, and some projects crack.
More inquiries about the winning performance, integrity information and qualification conditions of construction enterprises, one-click instant query results, download Jianjiantong app.
For more information about project/service/procurement bidding, and to improve the winning rate, please click on the bottom of official website Customer Service for free consultation:/#/? source=bdzd