1. Auto-shrinkage is caused by the internal drying of the concrete caused by the hydration process of the cement. The self-shrinkage increases as the water-to-binder ratio decreases. The difference in shrinkage of concrete with different water-binder ratios mainly occurred in the early stage (1 hour ago), and the growth rate of auto-shrinkage gradually slowed down with the increase of age.
2. Factors affecting early shrinkage: 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 influence of wind speed, temperature and humidity on the initial shrinkage of concrete. Humidity has a great influence within 1 to 2 hours after molding. Wind speed has a great influence from the third hour. Compared with humidity and wind speed, the influence of temperature does not change much over time; the concrete with mountain sand shrinks more than concrete with river sand, and Different production areas of mountain sand cause different shrinkage. The more clay contained in mountain sand, the greater the shrinkage of concrete; the comparison of admixtures shows that the specimens using naphthalene superplasticizers shrink more.
Early cracking of concrete
1. Abnormal setting of cement. Cement-prepared concrete with abnormal setting time is more suitable to cause early cracking due to the inconsistent speed of plastic shrinkage and setting. In our country's construction experience, it has been found that the use of cement with fast setting time or admixtures mixed with accelerators often leads to the appearance of horizontal cracks on the concrete surface. The control method is to adjust the setting time by adding retarders to appropriately increase the water-cement ratio.
2. The influence of impurities in the mixing water. Construction experience has proved that the salt and corrosive acid in the mixing water can enhance the tendency of early cracking.
3. The influence of mountain sand. The construction site investigation found that all the newly poured floor slabs made of mountain sand when the weather was clear and windy had cracks; the washed mountain sand did not show any cracks. Mountain sand has different origins and has different effects on concrete cracking. The control measure is to choose clean river sand.
4. Early maintenance. Air temperature, humidity, wind speed and concrete temperature all affect the rate of water evaporation. Temporary measures such as windshield, sunshade, plastic sheet covering, and curing agent should be taken in time to avoid rapid loss of water, which can effectively prevent early cracking.
Concrete alkali aggregate reaction
Some active minerals in the alkali aggregate react chemically with the alkali solution in the concrete pores. The alkali aggregate reacts to produce alkali-silicic acid gel, which absorbs water and expands, increasing its volume by 3-4 times, causing concrete peeling and cracking , The strength is reduced, and even leads to damage. The measures to prevent alkali-aggregate reaction are to use low-alkali cement or admixtures such as fly ash to reduce the alkalinity in the concrete, and to control the aggregates containing active ingredients.
Concrete floor and pavement cracks
There are many reasons for the cracks in the concrete open field and pavement. The most intuitive and easy to find are: insufficient foundation compaction, poor drainage of surface and groundwater; inconsistent settlement at the contact between excavation and filling; freezing and thawing of the natural environment; unstable stability of the cement used and large fluctuations ; The aggregate has a large mud content, and the alkali aggregate reacts and corrodes; the aggregate size is large, the proportion is improper, and the sand rate is small; the water-cement ratio is not strictly controlled, the mixing time is short and uneven, the vibration is not firm, and the calendering is improper. ; The design strength is low, the maintenance is not timely, and the driving is too early; the environment is affected by drought and temperature difference. The problems that should be well controlled to prevent concrete floor cracks are: 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 mixing grade I fly ash is better, water reducing agent and retarding It is suitable to mix with the agent and has more advantages and less disadvantages; the stones should not be coarse; the less mud on the surface, the better, and the aggregate gradation is good; reduce the concrete entering mold temperature to avoid high temperature construction time; take protective measures when the temperature drops suddenly; strengthen post-construction maintenance And protection; timely and accurate cutting.
The outer surface of the concrete is white
Almost all whitening is calcium carbonate (CaCO3), which is insoluble in water, and there are also other alkali whitenings. Since most of these salts are meltable, they will flow and disappear under the action of rain and snow. The initial whitening generally appears more evenly on the surface, and the frequency of appearance at the leeward backlight is much smaller than that on the sunny side, and gradually decreases with the extension of the use time. The whitening produced by the re-infiltration of external moisture into the concrete is secondary whitening. This whitening is related to the type of cement, amount, concrete density, water absorption and voids. The rough surface is easy to accumulate water, and the parts with large internal loose water absorption are the most Prone to multiple whitening. The current preventive measures against the phenomenon of whitening are: to reduce the amount of construction mixing water under the premise of meeting the permission of construction pouring; when conditions permit, increase the moisture content of the air around the concrete to slow down the drying and evaporation rate; when the strength of the pouring structure is not complete Before reaching dryness, maintenance and covering should not be stopped prematurely, and when it is necessary to move, it should be moved after gradual drying; the construction mix ratio is reasonable, especially the coarse and fine aggregates are appropriate, and the interior is compacted in time for vibrating, so that external water is not suitable for entering Internally, it is the key to prevent whitening again.
Concrete floor crack
Floor cracks are generally plastic shrinkage cracks and dry cracks. Plastic shrinkage cracks: It is easy to appear in the summer when it is dry and hot and windy. The characteristic of the cracks is that the width of the middle is tapered at both ends, and the length is not connected with each other. The main reason is that in the plastic state, the surface is dry, hot and windy, the water evaporates too fast, and the volume shrinks sharply. The measures taken are to cover immediately after pouring, and the curing time is advanced to spray water within 4 hours after pouring; the water-cement ratio and sand ratio of the mixed concrete should not be too large, and seal water retention, etc.; dry cracks: mainly due to untimely maintenance , Water has not been sprayed, the surface water is dispersed too quickly by wind and sun, and the internal temperature changes little. When the surface shrinks and deforms, it is restricted by the internal concrete and produces a large tensile stress. In addition, the components are often dry and wet when stored, and the surface temperature changes drastically. Such cracks are prone to occur when stored in the open air. It can be solved by strengthening the early curing of concrete and extending the curing time.
"Powdering" on the concrete surface
The main reason for "fluffing" is that the surface structure of concrete is loose and the strength is low. Although the lighter ones have little effect on the compressive strength level, the severe ones will damage the abrasion resistance, impermeability, aesthetics and long-term durability of the concrete pavement or floor, which is detrimental to the quality of the project. There are mainly three factors: ①The design of the concrete mix ratio is unreasonable; ②The water-cement ratio of the concrete surface layer is greater than that of the inside of the concrete, and the overlap between the surface hydration products is not dense, and the porosity is large; ③The over-vibration or poor calendering during the concrete construction process Dense; improper maintenance, the early stage of construction, the loss of water dispersion is too fast, a large number of water holes are formed, and the cement on the surface cannot get enough water for hydration.
Measures: 1. The design of the concrete mix ratio should be reasonable to prevent serious bleeding from causing excessive water-cement ratio of the concrete surface. 2. Sand and stone aggregates must meet the national quality requirements, with particular attention to the content of particles below 0.315mm in the sand; the setting time of cement should be appropriate. 3. During the construction process, excessive vibrating should be prevented from causing serious segregation and bleeding of the concrete; when the concrete is close to the final setting, the concrete should be plastered (or pressed) twice to make the concrete surface structure more compact. 4. After construction, pay attention to timely heat preservation and moisturizing for no less than 14 days. It is necessary to prevent the concrete surface from being washed away by rain before the concrete surface is hardened, causing the concrete surface water-cement ratio to be too large and reducing the strength.