Concrete performance changes have a great relationship with the surrounding environment. A good environment can promote the development of concrete performance and make it serve the structure better. A poor environment can also deteriorate the performance of concrete, affecting the quality of the entire project. With the vigorous development of commercial concrete, modern concrete is not a simple combination of sand, stone, cement in the past, in the production of concrete mostly mixed with mineral powder, fly ash and additives with a variety of functions, the addition of these materials are required to create a suitable environment for concrete, in order to make a combination of various materials to give the best performance.
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Therefore, after the concrete is placed, it is necessary to maintain the appropriate temperature and sufficient humidity for a certain period of time in order to meet the good hardening conditions of concrete. The measures taken to meet such hardening conditions are known as the maintenance of concrete. Temperature, humidity and maintenance of the continuation of the time is the three main elements of concrete maintenance.
High summer temperatures, concrete early strength growth, while the evaporation rate of water in the concrete is also fast, the evaporation of a large amount of water will affect the hydration of the cementitious materials, thus affecting the final strength of concrete and the development of various properties. A large amount of water is quickly lost, not only to produce a large dry shrinkage deformation, and in the case of concrete strength is not high, it is very easy to cause the cracking of concrete. Therefore, the summer concrete pouring construction should pay particular attention to early moisture conservation, do not make the concrete exposure to water loss, which will cause irreparable damage to the strength of concrete and various properties. According to different parts of the concrete should be covered with plastic film, brushing the curing solution and other moisturizing maintenance measures in a timely manner.
1, the importance of concrete maintenance work
Due to improper maintenance measures, can not ensure that the concrete from the casting molding to the initial stage of solidification and hardening gradually produce strength in a fully moistened state without water loss, the most easily caused by the quality of concrete is cracking and surface strength decline.
When it comes to the causes of crack formation, many experts and scholars will list a lot of them, such as: Sand, stone containing too much mud and other raw material reasons; sand rate, water-cement ratio is too large and other reasons for the ratio; concrete inside and outside the temperature difference is too large and other environmental reasons, etc.. These reasons will increase the shrinkage of concrete to a certain extent, but this is not the main reason for the formation of early cracks in concrete. I believe that the main reason for the formation of early shrinkage cracks during the condensation and hardening of various kinds of concrete is that the concrete does not get sufficient perfect wet maintenance. That is, even for the above listed factors to take a lot of measures to help reduce the shrinkage of concrete, but did not ensure that the concrete after the pouring of adequate wet maintenance, if the concrete may not appear in the wet climate conditions, but in a dry, high-temperature climate, that the concrete inevitably will have to appear cracks. On the contrary, the reasons listed above you can not well avoid, but to ensure that the concrete after casting and forming concrete condensation, hardening of the early stage of adequate wet curing, that the concrete early shrinkage cracks can also be avoided.
An example that has been commonly used and proven to be effective is the cracking of cast-in-place slabs. In the past, the problem of cracks in cast-in-place slabs has been a major problem for the engineering community. But now after the cast-in-place slab is poured and pounded, with the wiping pressure, with a layer of plastic film method of construction maintenance measures, the problem of cracks in the cast-in-place slab has been basically solved. Even if the sand content of mud, concrete water-cement ratio is too large, segregation, these factors will affect the strength of concrete and other properties, but if the concrete from beginning to end to take a perfect wet maintenance, so that the concrete in the initial hydration strength growth process without water loss, then the concrete will not appear early shrinkage cracks. Just in june this year we supplied the pouring of an underground garage roof project, the entire roof 7800m2, consumption of c30p6 concrete 3500m3, continuous pouring for 5 days and 4 nights, the middle did not stay after the belt, there is no construction joints, concrete production is not mixed with special materials. It was taken with the vibrating, with the wiping pressure, with the covering of a layer of plastic film, to continue sprinkling water storage maintenance 7 days after the final set of concrete. Now it has been poured for 2 months, there is no long line crack on the top surface of the garage visible to the naked eye, and the individual short and shallow cracks are caused by the plastic film covering is not strict. There is also the first high-rise high-volume concrete foundation project in zibo which we supplied and constructed in may last year, the core cylinder part is 8.9m thick, the surrounding raft is 3.1m thick, the total amount of concrete is 12000m3, and it was poured continuously for 3 days and 3 nights. After pouring, the maximum temperature of the concrete was 78℃ inside and 47℃ on the surface, with a temperature difference of 31℃. Took the vibrating and smearing finished, then covered a layer of plastic film, to continue sprinkling water after the final set of concrete (groundwater extracted from the site, cooler). The water was stored and maintained for 4 days, and no obvious cracks were found on the top surface of the concrete visible to the naked eye. Here i would like to clarify that the use of groundwater with a large temperature difference directly pouring concrete surface, in many experts believe that it should be prohibited (only by imagination), but it has proved to be feasible. If the concrete can not even withstand this stimulation, then this concrete is a waste product.
Due to improper maintenance measures, not in place easy to cause another quality problem of concrete is the concrete surface strength decline. Such as ground, pavement concrete after the completion of the pouring and plastering, such as failure to ensure adequate wet maintenance, that the concrete surface strength will be significantly reduced, resulting in the direct consequences of ash, sand. For example, the main project of concrete commonly reflected by the use of rebound meter detection and drilling core method of detection, the concrete strength difference is too large (the actual surface strength and internal strength difference is large). Previously, the use of on-site mixing production of concrete, the basic non-adulterated fly ash, admixtures (here do not consider the issue of sand rate), the strength of concrete after pouring grows faster, concrete strength after the internal water is not easy to dissipate, the lack of maintenance of concrete less impact. And now the use of commercial concrete, concrete mixed with mineral powder, fly ash and a variety of admixtures, concrete early strength growth slowly, and at this time if the concrete surface can not ensure adequate wet maintenance, resulting in water loss on the concrete surface, that the concrete surface strength will be greatly reduced, the depth of carbonation increases, while the internal strength is not affected. This is also an important reason for the use of rebound method of testing and drilling core method of testing the concrete solid strength deviation.
The above talk a lot, just to illustrate the importance of maintenance of concrete, especially the summer heat season should ensure that the early maintenance of concrete.
2、how to do a good job of summer maintenance
How to do a good job in the summer concrete maintenance work? I think we should take the corresponding maintenance methods according to different construction parts.
(1) for cast-in-place slab, ground, pavement, etc. After pouring a surface is exposed outside the formwork, such a pouring surface, after the completion of the pouring surface should be covered with a layer of plastic film maintenance, to continue to sprinkle water maintenance after the final set of concrete, at least 3 days to ensure full moisture conservation, the best 7 days.
(2) for the column, after removing the formwork, it should be wrapped with plastic film or painted with maintenance solution to ensure that the concrete surface water does not dissipate.
(3) for shear walls, continuous wall parts should avoid premature demoulding. Now many construction units in the summer after the concrete is poured, less than 12 hours to start removing the column, wall formwork. Some concrete is just final set, the concrete strength is very low, the formwork can stick down the concrete, it is hard to remove the formwork. And after the removal of the formwork and can not ensure adequate maintenance, the result of the wall cracks are inevitable. For the wall parts of the concrete itself is high strength, thickness, internal heat of hydration, not easy to maintain, the concrete surface water is easy to dissipate, so it is very easy to cause cracks in the wall. But if we can ensure the perfect wet curing of concrete like cast-in-place slab (the area should be larger than the wall), and no water loss at the early stage of concrete strength growth, then the cracking problem of the wall can be solved.
For the maintenance of the wall parts, remove the formwork when the wall has sufficient strength (48h after pouring in summer, at least 30h) and apply the maintenance solution as it is removed to ensure that the concrete surface water is not dissipated.
According to different pouring parts to take the above maintenance methods, to ensure the perfect full wet maintenance of concrete after pouring and forming, that can avoid or reduce the problem of concrete cracks and low surface strength caused by the lack of maintenance.
Summer is a season of dry temperature and frequent rainfall, the hot weather and frequent rainfall will have an important impact on the construction of concrete placement and the quality of concrete, so the mixing station and the construction unit need to pay great attention to this. To ensure the quality of ready-mixed concrete in summer construction, we are required to do the following.
(1) to keep abreast of the weather conditions on the day of construction, the construction of hot weather should be avoided as far as possible (it is best to choose the construction time between 4:00 pm and 11:00 am the next day). Try to avoid the rainy day pouring, to prevent the rainfall and change the water-cement ratio of the original concrete, affecting the strength of the concrete or due to rainfall caused by the secretion of concrete pouring surface, which leads to the occurrence of concrete surface crusting, sanding phenomenon. If it rains in the middle, the concrete should be covered with colored scaffolding cloth or plastic film to prevent the rain from damaging the concrete on the surface.
(2) when the length of the structure is greater than 40m, set up a back cast strip in the middle of the floor, and when the span of the floor is greater than 3.9m, set up a double layer of two-way reinforcement, and the diameter of the reinforcement is not less than 8mm, the spacing is not more than 150mm. If the double layer of two-way reinforcement is not set up as required or set up a back cast strip, it is very easy to produce cracks, so please pay attention to it.
(3) before pouring concrete, the formwork should be fully moistened, and in the process of pouring, timely pounding, timely heaving (before the initial set), timely surface wiping (more than twice), in order to prevent the generation of plastic cracks.
(4) pumping concrete construction, by the pump truck or ground pump supply concrete should be poured from far to near, in order to fabric uniform construction speed, and can avoid the impact of the pipe affect the concrete set. Out of the pump slump is most appropriate to control the 160 ~ 180mm, should not be too large, otherwise it is easy to cause concrete segregation, water, plugging and other phenomena.
(5) after pouring, it is strictly prohibited to add water at will. Must be in the initial set before the start of maintenance and not less than 8 hours, in the concrete after the collection of slurry and the second wipe pressure, covered with plastic film and moisture conservation, to prevent the evaporation of surface water, to be hardened to the concrete can be on, remove the plastic film, spread on the sack for insulation and moisture conservation, such as no sack can be responsible for sprinkling water conservation, to ensure that the 7-day maintenance period, the admixture and fly ash concrete, watering maintenance time for 14 days to effectively inhibit the occurrence of defects such as dry shrinkage and hardening shrinkage cracks.
(6) summer construction pumping concrete process, forbid the conveying pipeline or has been hardened concrete lumps scattered into the concrete and the place where the concrete is about to be poured.
(7) when pumping construction is used, the construction should be organized according to the program to minimize the retention time of concrete at the site and prevent the concrete from cold joints due to unreasonable fabric; the pump pipe should be covered and sprayed in hot weather to reduce the temperature of entering the mold.
(8) reasonable deployment of vehicles, good timely pumping of concrete, reduce the backlog or stagnation of vehicles on site, require unloading of all concrete within 1.5 hours, concrete at the site shall not exceed 2.5 hours, so as to avoid excessive loss of concrete slump, affecting the pumping performance of concrete.
(9) in the concrete strength of 1.2mpa before it shall not be stepped on or installed on the formwork and bracket.
(10) for some high height beams and columns, inconvenient pouring structures, the maintenance of concrete must be coated with curing agent or tightly covered with plastic film to keep the concrete fully hydrated and ensure the quality of concrete.
(11) concrete shear walls should be the focus of maintenance, maintenance time of not less than 14 days, always keep the concrete moist, reduce concrete shrinkage, creep, reduce crack generation.
The purpose of concrete maintenance is to ensure that the ambient temperature and humidity of the concrete poured and formed to meet certain requirements, so that the concrete to the specified age has to meet the design requirements of the use of function. For the north china plain area, the summer temperature is high, the temperature difference between day and night is small, the ambient humidity is small, especially this year, most of north china drought and little rain, the construction of concrete is particularly unfavorable.
Concrete maintenance, from the following three principles: Prevention, resistance, release.
“Prevention” Is our first choice for any measures to achieve our requirements from the essence, and is the most economical measure. If prevention measures cannot be achieved, then resistance and release measures will be considered in the context of the established objectives. The first consideration from the preventive point of view is to prevent the concrete from getting too hot. From the use of raw materials, the choice of low heat of hydration cement, the use of efficient water reducing agent, reduce the amount of cement, increase the amount of admixtures, the aggregate into the awning, sprinkling water to reduce the temperature of the aggregate, mixing water with ice to reduce the temperature; from the construction time to choose the low temperature, high humidity night construction; for the walls, slabs and other structural parts do not use iron formwork. Furthermore, construction measures are taken from the way of resisting the evaporation of concrete water and humidity becoming smaller. The surface of the concrete poured and formed to reduce the time of contact with air, and timely airtight cover. For walls, beams and other structural parts of the surface that are not easy to cover moisture conservation, spraying curing agent and other measures to maintain the humidity of the concrete. Finally, for other measures can not solve the concrete temperature rise too fast, the temperature is too high volume concrete, take “Put” Measures, the heat to the outside of the concrete: Concrete internal precautionary water pipe, pass into the cooling water, reduce the internal temperature of concrete.
Many sites, construction workers are wrongly believed: Concrete summer maintenance is not necessary or not enough attention to maintenance, they think: After pouring a few hours to solidify, simply do the surface process, sprinkling water on the line. But now the commercial mix are used water-reducing agent, when the concrete just final set, only completely lost plasticity; at this time, the cement hydration has begun, and the reaction is intense, the accumulation of heat of hydration will make the concrete temperature rise, its internal contains a lot of water, if not timely maintenance, it will volatilize through the capillary pores, so that the formation of early cracks, the formation of concrete strength caused by adverse effects, resulting in a series of quality problems, so the summer concrete must do a good job of pounding, compaction, smearing, covering, watering, moisturizing and other maintenance work.
(1) do a good job of construction technology transfer while emphasizing the need for maintenance, in writing, in duplicate for technical transfer. Supply and demand of the two sides of the project leader, technical person in charge of the signature approved. Let the project management pay high attention to do advance prevention.
(2) summer staggered construction: Avoid the high temperature at noon (when the temperature is higher than 35 ℃, noon 11:00 ~ 15:00 should stop construction), avoid direct sunlight, because the concrete poured at this time, the surface quickly false condensation, internal water will quickly evaporate, the formation of cracks.
(3) road concrete should be selected in the morning and evening construction, avoiding the high temperature at noon, stagger wind, rain and other bad weather. After pouring, pounding, compaction, surface finish immediately covered with straw bags or sisal, felt and other things, 2 to 3h after the final set can be watered, let it remain wet state maintenance 14d. And cut joints within 24h hours. During the construction process, it is strictly forbidden to sprinkle water on the surface, sprinkle dry cement powder to collect the surface, otherwise it will cause skinning, ash, cracking and other phenomena.
(4) pouring beam slab: Water dissipation is faster in summer, so the formwork should be fully wetted before pouring, but there should be no water. Before the initial set should be in the concrete surface for the second pounding, before the final set of the second compaction, wooden roller smoothing not less than two to three times, if necessary, the use of iron roller back and forth crushing more than two times, should be immediately covered with film, grass bags or sisal, felt and other things, timely watering maintenance 14d.
(5) shear walls, columns: First with the mold curing 3 ~ 5d after the peak of the heat of hydration, the removal of the side mold, you can use plastic film wrapping method of self-curing, closed concrete surface and prevent water evaporation.
(6) bulk concrete: First of all, we must consider the quality control and maintenance from the ratio design, for example, improve the amount of mineral powder, fly ash and other admixtures, choose low-heat cement, reduce the amount of cement, choose clean aggregates, the use of slow-setting or super slow-setting water reducing agent. This can reduce the adiabatic temperature rise of concrete, reduce the peak temperature of heat of hydration, but also reduce the chances of cracks, conducive to maintenance. After the concrete is poured in time to cover with insulation material, (according to the temperature test requirements, to determine the thickness of the cover insulation layer. Can cover a layer of plastic film, and then lay a layer or two layers of felt, straw bags, and other things to store heat insulation), and timely watering and moisturizing, control the temperature difference between inside and outside, maintenance 14d; if necessary, the surface for water storage maintenance.
Commercial mix maintenance requirements to achieve timely and adequate, grasp the “Maintenance of the first seven days, the key is the first three days, the most critical first day”. If the first day too much water loss, the defects caused may be difficult to make up later, therefore, the first day of timely and adequate wet curing, the compressive strength of concrete, seepage resistance, crack resistance are very critical.