Ready mixed concrete has the disadvantages of large amount of cement, large amount of water, large sand rate and small amount of stone. Measures must be taken from various aspects, such as quality control of raw materials, optimization design of concrete mix proportion, construction and production control, etc. only by strengthening management and strict control can the early cracks of concrete be reduced to a lower level.
1、 Quality control of raw materials
(1) Cement varieties should be selected with low heat. Except for winter construction, "R" cement should not be selected. Cement with low water demand and low bleeding should be selected. Cement from different cement plants, varieties and grades should not be mixed.
(2) The addition of fly ash and ground slag powder meeting the quality grade requirements can effectively reduce the amount of cement, inhibit the shrinkage of concrete or prolong the time of cracks.
(3) The silt content of coarse and fine aggregates must be strictly controlled.
(4) The pumping agent with air entraining, retarding and plasticizing properties should be selected, and the variety and dosage should be determined through adaptability test. Generally, the air content of pumping agent should be 1.5% ～ 3%. The pumping agent must have good adaptability with cementitious materials. The slump loss value of concrete mixture should be no more than 30mm in 1H.
(5) The initial setting time of concrete construction in normal temperature season should be 10 ~ 12h, which can be advanced appropriately in winter.
2、 Optimization design of mix proportion
(1) The concrete mix proportion should be determined by test and managed dynamically.
(2) The total cementitious material of c10-c45 conventional pumping concrete should be controlled within 260-450kg / m3.
(3) On the premise of ensuring the strength and construction performance of concrete, less cement should be used and more admixtures should be added. The effect of compound mixing is better than that of single mixing. Compound mixing has complementary advantages and can produce superposition effect.
(4) The mix proportion of low sand rate, high stone content and low water consumption should be selected to reduce the early cracks of concrete and the sand ratio should be controlled between 39% and 42%.
(5) The greater the slump of concrete mixture is, the better. The slump of ordinary pumping concrete should not be greater than 220mm. The slump of 160-180mm is excellent. The slump of 140-160mm and 180-200mm is good. When the slump is higher than 220mm, the concrete is prone to segregation, bleeding and early shrinkage cracks.
3、 Construction production control
(1) The ready mixed concrete can not add water arbitrarily at the construction site. Ordinary pumping concrete is not self compacting concrete and needs to be vibrated, but excessive vibration, under vibration and missing vibration are not allowed, especially over vibration. Otherwise, 3-5cm floating slurry will appear on the surface of concrete, and cracks will easily appear after concrete hardening.
(2) When concrete is vibrated, it must be inserted quickly and pulled out slowly. It can be vibrated for 10-20s according to the effective radius of the vibrator. Before or near the initial setting of concrete, secondary vibration can be carried out to reduce the early cracks of concrete.
(3) For the structure with large plate surface or plane, the surface treatment should be done well. First, scrape with a 2m long scraper, rub with a wooden trowel or roll with an iron trowel. After the surface treatment, the moisture conservation maintenance should be carried out immediately, and the vibration, treatment and maintenance should be carried out at the same time. The key here is to master the setting time of concrete, neither advance nor delay.
(4) If there are dry shrinkage cracks on the concrete surface, riprap method can be used to restrain and close the cracks. 10-20 mm Single Size stones are scattered on the concrete surface, about 3 kg of riprap per m2. It is effective to eliminate the early cracks of concrete by using a plate vibrator. It is also effective to spread a layer of pea stone or coarse sand to restrain the occurrence of dry shrinkage cracks.
4、 Concrete curing
Relevant standards and specifications put forward specific requirements for concrete early curing, which is the key to ensure the quality of concrete and structural durability, and is also the fundamental to prevent concrete early shrinkage cracks. At present, the attention to the maintenance of ready mixed concrete is generally poor. The construction unit often only considers the construction period, and the maintenance work is not in place. As a result, the early cracks of concrete often occur, which has become a common quality problem.
1. Maintenance requirements
Curing temperature, humidity and age (time) constitute the three main elements of concrete curing. The performance of concrete changes faster in the early stage than in the later stage, so it is more important to strengthen the early curing.
(1) The curing temperature of concrete should be in the range of 5 ~ 35 ℃, and corresponding technical measures should be taken if the temperature exceeds this range.
(2) The strength of concrete formwork removal depends on the temperature. If it is removed early, the concrete can not meet the strength requirements, and cracks will occur. When the temperature is above 36h, it is about 15.2mh when the temperature is above 36h; when it reaches to 15h-25h, it is about 10h when it reaches to 15h-25h when it is removed; when it reaches 15h-25h, it is about 10h when it reaches to 20h.
(3) The starting time of curing is also very important. After the completion of concrete pouring or surface treatment, the earlier the better. Do not wait for the plastic shrinkage of concrete to be cured. It should be maintained before the final setting of concrete. For concrete structures with large slab surface or plane size, the earlier the curing is, the better. According to the relevant information, the shrinkage value of concrete after 14 days of curing can be reduced by about 20% compared with that of 3 days.
2. Maintenance method
According to different seasons and different concrete structures, the maintenance methods can be targeted, and the maintenance scheme and measures can be formulated.
(1) In normal temperature season, it is generally necessary to cover with moisturizing materials, and then water or brush curing liquid for moisturizing maintenance. When the temperature is lower than 5 ℃, according to the requirements of winter construction, heat storage, heat preservation, moisture preservation and comprehensive heat storage methods are generally adopted for maintenance.
(2) The beam and column concrete shall be covered immediately after the formwork is removed, and the plastic film shall be wrapped for moisturizing maintenance. The curing of shear wall can adopt the methods of spraying, hanging grass curtain, watering, etc.
(3) For slab concrete structure, a thin plastic film shall be used to cover the concrete surface immediately after the concrete is poured. After the concrete surface receives water, a layer of plastic film or straw bag will be covered for watering and moisture conservation. The first layer of plastic film should be close to the concrete surface, which will not cause early cracks of drying and shrinkage, and the concrete surface can be fully hydrated The phenomenon of pulverization and sanding occurred, which ensured the quality of concrete.
To sum up, the cracks of concrete are not difficult to control. As long as the management and quality control are strengthened, the early plastic shrinkage cracks or other cracks of concrete will not appear.