Seven major problems common to concrete

Early shrinkage of concrete

   1. Self-shrinkage is caused by the drying of concrete inside the cement hydration process. The self-shrinkage increases as the water-to-binder ratio decreases. The difference in shrinkage of concrete with different water-to-binder ratio mainly occurred in the early stage (1 hour ago), and the growth rate of self-shrinkage gradually slowed down with the increase of age.

   2. Influencing factors of early contraction: curing temperature and humidity. When the humidity is the same, the shrinkage increases as the temperature rises. At the same temperature, when the humidity is high, the concrete shrinks less. The effects of wind speed, temperature and humidity on the initial shrinkage of concrete. The humidity has a great influence within 1 to 2 hours after molding, and the wind speed has a great influence from the third hour. The influence of temperature is not changed with time compared with humidity and wind speed; the concrete shrinkage of mountain sand is larger than that of river sand concrete, and The origin of the mountain sand is different, causing the shrinkage to be different. The more clay is contained in the mountain sand, the larger the concrete shrinks; the tester compares that the test piece using the naphthalene-based superplasticizer has a large shrinkage.

   Early cracking of concrete

   1. Abnormal condensation of cement. Cement-concrete concrete with abnormal setting time is uncoordinated due to plastic shrinkage and condensation, and is more likely to cause early cracking. In China's construction experience, it has been found that the use of cement with fast setting time or admixture with coagulant often leads to the occurrence of horizontal cracks on the concrete surface. The control method is to adjust the setting time to increase the water-cement ratio by adjusting the setting time.

   2. Mix the effects of impurities in the water. The construction experience proves that the salt and corrosive acid in the mixing water can strengthen the early cracking tendency.

   3. The influence of mountain sand. The construction site investigation found that the concrete mixed with the mountain sand was cracked when the weather was fine and windy, and the newly poured floor slabs were cracked; no cracks were observed with the washed mountain sand. The mountain sand has different origins, and its effects are different. It has different effects on concrete cracking. The control measures are clean river sand.

   4. Early maintenance. Temperature, humidity, wind speed and concrete temperature all affect the evaporation rate of water. Temporary windshield, sunshade, plastic sheet covering, spray curing agent and other measures should be taken in time to avoid excessive loss of moisture, which can effectively prevent early cracking.

   Concrete alkali aggregate reaction

   Some active minerals in the alkali aggregate react with the alkali solution in the micropores of the concrete, and the alkali aggregate reacts to produce an alkali-silicic acid gel, which absorbs water and expands, and the volume increases by 3-4 times, thereby causing the concrete to peel off and crack. , the strength is reduced, and even lead to damage. The measures for preventing the alkali aggregate reaction are to use low alkali cement or fly ash and other admixtures to reduce the alkalinity in the concrete and to control the aggregate containing the active ingredient.

   Concrete floor and pavement crack

   There are many reasons for cracks in concrete open pits and pavements. The most intuitive and easy to find are: the foundation is not enough, the surface and groundwater are not smooth; the settlement of the excavation and filling is inconsistent; the natural environment is frozen and thawed; the cement used is unstable and fluctuating The aggregate contains a large amount of mud, and the alkali aggregate reacts and corrodes; the aggregate has large particle size, improper proportion, and small sand ratio; the water-cement ratio is not controlled tightly, the mixing time is short and uneven, the vibration is not accurate, and the calendering is improper. The design intensity is low, the maintenance is not timely, the driving is too early; the environmental drought and the temperature difference affect. The problem of preventing cracks in concrete floor should be controlled: the less the content of cement C3A, the better; the water-cement ratio of concrete should be small, the water consumption should be small, the effect of I-grade fly ash is better, the water reducing agent and retarding The agent should be mixed with a lot of advantages and disadvantages; the stone should not be thick; the surface contains less mud, the better, the aggregate gradation is good; the concrete mold temperature is lowered, avoiding the high temperature construction time; the temperature is suddenly reduced to take protective measures; And protection; cutting and timely and accurate.

   White surface of concrete

   All of the whitening is almost calcium carbonate (CaCO3) which is not melted in water, and other alkalis are whitened. Since most of these salts are meltable, they will disappear under the influence of rain and snow. The initial whitening generally appears more evenly on the surface, and the frequency of the leeward backlight appears much smaller than the sunny windward surface, and gradually decreases with the use of time. The whitening produced by re-infiltration of concrete into the concrete is a secondary whitening. This whitening is related to the variety, amount, concrete density, water absorption and voids of the cement. The surface is rough and easy to accumulate water, and the internal loose water absorption rate is the most. It is easy to produce multiple whitenings. At present, the precautionary measures against the whitening phenomenon are: reducing the amount of construction mixing water on the premise of meeting the permission of construction pouring; increasing the air moisture content around the concrete when conditions permit, slowing down the drying evaporation rate; the strength of the pouring structure is not complete Before reaching the drying, the maintenance and covering should not be stopped prematurely. When it is necessary to move, it should be moved after gradually drying. The construction ratio is reasonable, especially the coarse and fine aggregates are appropriate, and the vibrating and timely internal compaction makes the external water unsuitable. Internal is the key to preventing whitening again.

   Crack in concrete floor

   The most common cracks in floor slabs are plastic shrinkage cracks and dry cracks. Plastic shrinkage crack: It is easy to appear during dry and hot wind blowing in summer. The characteristics of the seam are that the width of the middle width is tapered, and the length is not connected to each other. The main reason is that in the plastic state, the surface is dry and hot, the water evaporates too fast, and the volume shrinks sharply. The measures taken are covered immediately after pouring, and the curing time is advanced to within 4 hours after pouring; the water-cement ratio and sand ratio of the mixed concrete should not be too large, and the sealing water is used; dry cracks: mainly due to untimely maintenance The water is not sprinkled, and the surface water is dissipated too quickly by the wind and the sun, and the internal temperature changes little. When the surface is dry and contracted, it is restrained by the internal concrete and generates a large tensile stress. In addition, the components are wet when dry and the surface temperature changes drastically, and such cracks are likely to occur in open storage. Strengthening the early curing of concrete and extending the curing time can be solved.

   Concrete surface "pulling"

   The main reason for “powdering” is the loose surface structure and low strength of concrete. Although the lighter ones have little effect on the compressive strength grade, serious ones will damage the abrasion resistance, impermeability, aesthetics and long-term durability of the concrete pavement or floor, which is unfavorable to the quality of the project. There are three main factors: 1 the concrete mix ratio is unreasonable; 2 the concrete surface layer has a water-cement ratio greater than the concrete interior, the surface hydration products are not densely overlapped, and the void ratio is large; 3 the over-vibration or calendering of the concrete construction process is not Dense; improper maintenance, water dispersion in the early stage of construction is too fast, forming a large number of water holes, the surface of the cement can not get enough water for hydration.

   Measures: 1. The concrete mix ratio should be designed reasonably to prevent serious water bleeding and lead to excessive water-cement ratio in the concrete surface. 2, sand, stone aggregates should meet the national quality requirements, in particular, pay attention to the particle content of 0.315mm below the sand; the cement setting time should be appropriate. 3, the construction process should prevent excessive vibration caused by serious segregation and bleeding of concrete; when the concrete is close to final condensation, the concrete should be double-faced (or pressed surface) to make the concrete surface structure more dense. 4, after construction, pay attention to timely insulation and moisturizing maintenance for not less than 14 days. To prevent the surface of the concrete from being washed by rain before it is hardened, the water-cement ratio of the concrete surface is too large to reduce the strength.