Importance of initial maintenance time in controlling early shrinkage
- for ordinary concrete without water-reducing agent, the early shrinkage is small, and the early sprinkling maintenance is mainly to make the cement fully hydrated to ensure the development of concrete strength. Therefore, China’s concrete structure construction quality acceptance of the provisions of the code, the beginning of concrete curing time is within 12 hours after pouring.
- for a large number of modern commercial concrete mixed with water-reducing agent, the test proved that the shrinkage increased sharply within 8 hours after initial setting, and in the actual project, the most large number of large volume of floor cracks mainly appeared in the concrete within a few hours after initial setting. 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 carry out sprinkling maintenance, we will lose the best time to control the 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 used to the traditional construction maintenance experience, not according to the characteristics of the present commercial concrete, that is, in the concrete just initial setting (before the surface water loss), on the adoption of reasonable maintenance measures, effectively reduce shrinkage, reduce cracking.
Commodity concrete needs timely and sufficient wet curing
- 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 grasp the timely curing, which is often determined according to the performance of the concrete mix and the climate, etc. (Generally speaking, the wet curing should be carried out when the concrete is first set), and if the surface has lost water when curing, the defects caused by water loss should be treated before curing to ensure the effect of curing.
- 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.
Effects of stages on curing during the 7-day wet curing period
- according to the test data and production experience, the 7d strength of commercial concrete is about 28d strength (65% ~ 85%), so the specification requires 7 days of wet curing is reasonable, it is best to keep 7 days without water loss.
- According to our production experience, in the 7 days, the more time ahead, the more likely to water loss of concrete, the more likely to form defects, to prevent water loss is also more important.
- 3d strength is about 45% ~ 60% of 28d strength, so the first 3 days to prevent water loss is particularly critical. If there is no water loss in the first 3 days, then continue watering and moisturizing until 7 days afterwards, the effect is good from the actual viewpoint of the project.
- 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.
- we found that some sites, the first day do not pay attention to maintenance, the next day before the start of water storage maintenance, after the end of maintenance, the board still has a lot of cracks, analysis of the reasons.
It is because cracks have already developed on the first day, which means that the lack of maintenance on the first day has led to the formation of coarser pores that are difficult to heal.
So, the key to 7 days of wet maintenance is the first 3 days, and most importantly, day 1. Therefore, we need to make sure that there is no water loss on the first day, regardless of how we maintain it.
The importance of secondary concrete troweling
- Smoothing at once
After the concrete is poured and compacted, in order to ensure the flatness of the surface, the surface must be immediately smoothed with a wooden trowel, known as “a smoothing”.
- Second trowel
Once smoothed, if not immediately after the concrete curing measures, then to the first set of concrete, must be at least once again wiped, this time not just smoothed, but also “pressure”, the concrete surface wiping and compacting (the concrete inside the water channels, capillary channels, wipe pressure, eliminate), called “Secondary smearing”.
Secondary smearing has three main functions.
- Eliminate surface defects, secretory channels (i.e., capillary channels) within concrete and early plastic cracks.
- Improve the compactness of the concrete surface layer.
- Improve the compactness of the surface layer, which slows down the migration and evaporation of water inside the concrete and improves the anti-cracking ability of the concrete.
- From primary smoothing to secondary smoothing, it is the process of gradual initial setting of concrete.
the formation of cracks. In a longer period of time, as long as the relative humidity of the environment is less than 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 aperture will be further deepened, the further expansion of cracks, cracking, severe cases will cause penetrating cracks, resulting in floor leakage.
- we found that in the case of wet curing is not timely and adequate (such as traditional wet curing), only one smoothing without secondary wiping, the concrete cracking will be serious.
With secondary smoothing, 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.
- the traditional trowel manual wiping.
In the secondary wiping process, now often used to the spatula manual wiping, not only low efficiency, strength is not enough, when the defects from the surface and inside the development of deeper, it will be difficult to eliminate.
- disk type wiping machine.
Second wiping is best to use disc type smear machine, eliminate surface defects (and concrete internal secretion channel) and dense surface than the role of the spatula good, high efficiency.
- immediately after the second trowel, the concrete must be fully wet curing, in order to avoid concrete water loss again. Only in this way can ensure good early hydration of concrete, improve the quality of hardened concrete, and lay an early quality foundation for the improvement of concrete durability.
Having a secondary trowel does not necessarily mean that there are no cracks.
Now the manual trowel can’t keep up with the speed of pouring and condensation. Mechanical wiping words early grinding floating slurry too much; waiting for the trowel can go up, if exposure to the wind, if the surface layer has been lost water channel.
If you cover the film after the second wiping, firstly, the water is not enough to adhere to the film, and secondly, the footprints are difficult to repair. After shaking and mopping, cover the film, after countless site tests and later continuous soaking water, see no penetration. Can not wait for the first set before covering.
The second wiping will always appear after the initial set. Manual wiping before initial setting will also appear sinking during initial setting. If there is no secondary spreading and the film is covered with a flattened surface after vibration, there will be less subsidence at the time of initial setting, and if there is, it will only shrink and sink, and will not crack or seep water.
Pouring with vertical pounding, then flattening and covering the film at the same time, three steps together can avoid the secondary wiping, no problem.
After the initial condensation of water is not enough film adsorption, after covering is also easy to wind up, early lift or cover time is not enough early fly up will be the same crack.
According to the specifications required to cover the film maintenance, not cover cracked mostly caused by early maintenance of insufficient moisture loss too fast.
In the process of initial set to final set, less water loss and less cracking.
Concrete from the completion of mixing to the completion of construction and pouring (or before the initial set) for the plastic stage. After concrete pouring, the sand and gravel aggregate sinks in the plastic stage, and water migrates upward (secretion). The obstruction of steel and stone can lead to plastic cracks. Plastic cracks can be repaired by secondary vibrating and secondary troweling.
Some concrete companies have proposed the lagging vibrating method to reduce the cracking of the slab surface, which means that the beam is vibrated densely, and the concrete slab is lagged after the artificial leveling, so that the concrete will settle naturally and secrete some water, and then drag the plate vibrator to vibrate when the concrete slump is about 10 (and strengthen the vibrating above the beam). This will improve the homogeneity of the concrete, after the completion of the vibrating surface in a timely manner and sprinkle water conservation or membrane maintenance. Do not let the slab surface lose water, can greatly reduce the slab surface cracking.
Note: Before pouring concrete, the formwork should be kept in a moist state.
Control the concrete plastic stage of pouring, vibrating, surface compaction, to reduce the defects and cracks caused by plastic settlement of concrete. Concrete into the second stage, concrete initial to final setting stage.
The time from initial to final setting of concrete is basically about 45 minutes. The cement begins to hydrate to form CSH gel to form strength, and due to the low early strength, the tensile strength is even lower. Ensure that the value of concrete shrinkage is as small as possible (the tensile strength of concrete is only 1/6 to 1/10 of the compressive strength). At this time, concrete shrinkage is mainly reflected in the shrinkage of the internal pore diameter of the concrete pores. As a result of water loss in the pores, water inside the pores from the flat liquid surface into a concave liquid, thus forming internal tension, pulling the capillary pore size smaller, a large number of capillary pore size smaller, resulting in the overall concrete shrinkage stress. When the shrinkage stress is greater than the concrete’s own tensile strength, the concrete will have cracks to release the shrinkage stress to achieve its own balance.
Now the high level of mechanized construction, a slab of concrete 1 to 2 hours after pouring, while the slab of concrete hardening area is large, the initial set to final set stage to control the migration of water evaporation within the pores is an important means of controlling the slab of concrete, no other. The so-called high-performance concrete control is not good as cracking. This stage of water conservation and moisture retention is the most critical.
Commercial concrete (ready-mixed concrete) is not just concrete ratio design, is the process of management, save energy do not save work. On earth, the law of conservation of energy is always observed.
In the process of initial set to final set, less water loss, less cracking.
Curing of large slabs
It is difficult to estimate the material and the force of nature when designing, and most of the designed reinforcement only considers the load. Plain concrete, like concrete on a road, can be poured and maintained for hundreds of meters without cracking even if the temperature rises for a few hours until it is calendered. This balance is broken at the peak temperature or when the outside temperature drops, so it has to be designed with shrinkage joints and cut to give it a crack.
Reinforcing concrete is designed to meet the load requirements, taking into account the area of the project site and its location, whether it is an open-air outdoor component or not, and then configured with structural steel to increase the balance.
As for the roof, it is mostly covered and should not be an outdoor component, but the construction process takes a long time, some of which last for two years.
For concrete maintenance, I think that the thickness of the roof and negative floor plate is relatively thick, and the degree of concrete maintenance is not enough, there will be more rubber material involved in the later stages of hydration, and each is not synchronous hydration to produce shrinkage.
Different shrinkage is produced, and the concave opening is the concentration point of shrinkage stress, which is prone to produce radial cracks. It is recommended that the large area of the board be maintained with a covering film, and that the film be kept from flying up by continuous water spraying, and that it be maintained for 14 days according to the specifications to reduce the hydration contraction caused by rainwater and water from renovation in the middle and late stages. And as soon as possible to do waterproof layer and cover the upper material, let it enter the constant temperature as soon as possible, reduce the temperature is the overall volume of the plane shrinkage.