The widespread use and promotion of commercial concrete, greatly improved the construction speed and building quality, and promote the vigorous development of commercial concrete technology.
However, the resulting early cracking of commercial concrete, but until the end has not been an effective solution to the quality of construction projects in recent years is the focus of controversy and complaints, has caused great concern to the industry and society.
First, the cause of cracks in commercial concrete is of course complex and multifaceted
The lack of understanding of the use of commercial soil and improper maintenance, is the main reason for the prevalence of cracks in commercial concrete, this paper on the cracking of commercial concrete and maintenance issues to discuss the views.
Second, the importance of the beginning of the maintenance time to control the early shrinkage of commercial concrete
1, for ordinary concrete not mixed with water-reducing agent, early shrinkage is small, early sprinkler maintenance is mainly to make the cement fully hydrated to ensure the development of concrete strength, therefore, China’s concrete structure construction quality acceptance specifications, the beginning of concrete maintenance time is within 12 hours after pouring.
2, for a large number of modern commercial concrete mixed with water-reducing agent, it is proved by relevant tests that the shrinkage increases sharply within 8 hours after initial setting, and in the actual project, the most large number of large volume of floor cracks mainly appear within a few hours after initial setting of concrete, therefore, a good grasp of the starting curing time is the key to control the early shrinkage of commercial concrete cracks.
If we continue to wait 12 hours after pouring and then sprinkle maintenance, we will lose the best time to control early shrinkage cracks, and in terms of early shrinkage crack control, we will lose any role. However, most of the current construction unit is still accustomed to the traditional construction maintenance experience, not according to the characteristics of the present commercial concrete, that is, in the concrete just initial set (before the surface water loss), on the adoption of reasonable maintenance measures, effectively reduce shrinkage, reduce cracking.
Third,The need for timely and adequate wetting of commercial concrete maintenance
1, The so-called timely curing is mainly: to curing the concrete before the surface does not lose water.
In the actual operation, it is difficult to master timely curing, but it should be determined according to the performance of the concrete mix and the climate, etc. (Generally, wet curing should be carried out when the concrete is first set), and if water loss has occurred on the surface during curing, the defects caused by water loss should be treated before curing to ensure the effect of curing.
2, the so-called full curing, mainly: to ensure that in the entire specified maintenance period, concrete can not lose water, no water loss is the full curing; Otherwise, water loss is insufficient curing.
Concrete curing the more adequate, then the existence of concrete defects is less; concrete curing is not adequate, then the existence of concrete defects more. However, most of the current construction unit of the concrete maintenance are in the pouring of the next day after the start of watering, 2-5 times a day, so that the maintenance is neither timely nor adequate, resulting in a lot of cracks.
Fourth, wet curing 7-day period, the impact of the various stages of commercial concrete curing
1, according to the test data and production experience, the 7d strength of commercial concrete is probably 28d strength (65% ~ 85%), so the specification requires wet curing 7 days is reasonable, it is best to keep 7 days without water loss.
According to our production experience, in these 7 days, the earlier the time, the easier it is for the concrete to lose water, the more likely to form defects, the more important it is to prevent water loss.
2, 3-d strength is about 28d strength (45% ~ 60%), so the first 3 days to prevent water loss is particularly critical. If the first 3 days do not lose water, then continue to water and moisturize to 7 days, from the project actually, the results are good.
3, the first day is very critical, if the first day too much water loss, the defects caused by the future may be difficult to repair.
4, we found that some sites, the first day do not pay attention to maintenance, the next day before the start of water conservation, maintenance is over, the board still has a lot of cracks, the analysis of its causes: because the first day has been a crack, which means that the first day of the non-maintenance of the relatively coarse pores have been formed, it is difficult to heal.
So, 7 days of wet maintenance, the key is the first 3 days, the most critical is the first day, so we have to make sure that the first day is not water loss, no matter what kind of maintenance we use.
Fifth, The importance of secondary concrete troweling
1、Smoothing in one go. Immediately smooth the surface with a wooden trowel, known as “primary smoothing”.
2、Second smoothing. Once smoothed, if not immediately after the concrete curing measures, then to the first set of concrete, must be smoothed at least once again, this time not only smoothed, but also “pressure”, the concrete surface smoothed and compacted (concrete internal secretion channels, capillary channels, wipe pressure, eliminate.), known as “secondary wiping”. This is called “secondary wiping”.
The main role of secondary wiping has 3 points.
(1) Eliminate surface defects, internal secretory channels (i.e., capillary channels) and early plastic cracks in concrete.
(2) Improve the compactness of the concrete surface layer.
(3) Improve the surface compaction, slow down the migration and evaporation of water in concrete, and improve the anti-cracking ability of concrete.
3、From the first smoothing to the second smoothing, it is the process of gradual initial setting of concrete.
4、The formation of cracks. In a long period of time, as long as the relative humidity of the environment is below 100%, the concrete surface will lose water, the formation of internal defects – capillary channels. These defects are not eliminated, in the case of continued water loss on the concrete surface, the pore will be further deepened, the further expansion of cracks, cracking, serious cases will cause penetrating cracks, resulting in floor leakage.
5, we found that in the wet curing is not timely and adequate (such as traditional wet curing), only a smoothing without secondary erasure, concrete cracking will be very serious.
With secondary troweling, the degree of cracking is greatly reduced. Therefore, unless the implementation of immediate curing, so that the concrete surface can not generate water loss defects, otherwise the secondary wiping process is necessary.
6, the traditional trowel manual trowelling. In the secondary wiping process, now often used to trowel manual wiping, not only inefficient, not strong enough, when the defect from the surface and inside the development of deeper, it will be difficult to eliminate.
7、Disc type power trowel: the best secondary wiping using disc type power trowel, the elimination of surface defects (and concrete internal secretion channels) and compacted surface than the trowel, the role of better, high efficiency.
There are two types of power trowel: walk behind power trowel and ride on trowel machine, which makes the secondary trowel more efficient and more accurate. Better finish the ground leveling.
8、After the secondary wiping, the concrete must be immediately cured with sufficient wetness to avoid the concrete losing water again, only in this way, can guarantee the early hydration of concrete, improve the quality of hardened concrete, and lay an early quality foundation for the improvement of concrete durability.
Sixth, early membrane curing is very necessary
Practice has shown that the use of membrane curing after concrete pouring is a simple and effective way of curing. Especially in the vast northern regions, high temperature, wind speed, dry air, water loss fast, the use of membrane curing, you can quickly seal the concrete moisture in the interior, and in the membrane and the concrete surface to form a long-term wet curing environment, after the final set concrete surface temperature cracks, survival neat and tidy.
The membrane curing construction needs to pay attention to the timeliness, and must complete the membrane lamination before the initial setting of concrete. Film and concrete surface should be closely bonded without bubbles, no gap between the membrane and the membrane cover, no large area overlap, and do a good job of later surface watering maintenance, keep the membrane surface moist.
In the aspect of film coating maintenance, Weinan Hongji Da did a good job in the construction of Purple Emperor Residence and other projects, and achieved the comprehensive film coating maintenance of concrete pouring, no structural concrete cracking phenomenon, which ensured the quality and safety of concrete structure and beautiful image.
To sum up, as long as the construction is carried out strictly according to the construction specification, the concrete surface is secondary smoothed, and timely and sufficient membrane moisture preservation is implemented, the cracks of concrete can be reduced to a large extent, so that the concrete cracks can be effectively controlled.