Due to the disconnect between ready-mixed concrete production and construction, coupled with increased labor costs, there are often many deficiencies in the on-site maintenance of ready-mixed concrete. In many cases, there are surface quality defects, cracks, lack of strength and other engineering quality problems. Due to the wide geographical area and differences in climatic conditions in china, in summer, many regions are characterized by high temperature and rain, some regions are characterized by high temperature and drought, and some places are characterized by high winds and large temperature differences between day and night, it is a question worth thinking about how to develop effective maintenance measures to reduce the occurrence of concrete quality problems.
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Strengthening the maintenance of concrete is an important measure to ensure the growth of the mechanical properties of hardened concrete, prevent cracks and improve the durability of concrete. In fact, no matter when and where, no matter what type of concrete components for maintenance, and no matter what kind of maintenance methods, the direct effect is to achieve is to stabilize the concrete system, minimize water loss and dry shrinkage of the components, so as to reduce the shrinkage stress and effectively control the concrete cracks. In terms of reducing the dry shrinkage of concrete, the most fundamental is to solve the “Moisture” Problem; for bulk concrete, not only to “Moisture”, but also to “Temperature control”.
About how to maintain, the current national standard gb50666-2011 “Concrete structure construction specification” Article 8.5.1: “After the concrete is poured should be timely moisture conservation, moisture conservation can be taken to sprinkle, cover, spraying curing agent and other ways. Maintenance mode can be determined according to site conditions, environmental temperature and humidity, component characteristics, technical requirements, construction operations and other factors.”
The so-called “Moisture conservation”, can be understood as water conservation, that is, to keep the concrete in the mix of water as much as possible without loss of maintenance, the original chongqing university gan changcheng teacher to this no water loss, no water loss defects in the maintenance method called “Perfect wet maintenance”, he he pointed out that the concrete mix water has three major functions, in addition to meeting the needs of construction performance, the need for hydration of the cementitious materials, there is an important function that has not been fully understood by the industry in the past, that is, to maintain the balance of the concrete system, the need to maintain volume stability. As long as the mixing water is not lost, the concrete body will not cause water loss defects (such as connected capillary pores and visible and invisible cracks), eliminating the water loss defects, the elimination of the concrete system within the most important imbalance factors. Because, the water loss defects generated by the shrinkage stress, harmful media into the defects generated by the expansion stress, will destroy the system balance, so that the concrete volume instability. Mixing water is the important element to maintain the balance of the system, only mixing water is not lost, in order to make the concrete to achieve “Zero defects”, to achieve high impermeability, in order to make the concrete system stability, volume stability, improve the crack resistance.
Specifications emphasize that moisture conservation to “Timely” Is very necessary, but also very important. The so-called timely, is in the newly poured concrete surface wipe pressure (to eliminate water loss defects), immediately after the spray, cover or water storage maintenance. Only by doing so can maximize the concrete to keep the mix of water no longer lost. For concrete pouring surface, especially the flat structure, it is appropriate to use plastic film to cover the moisturizing while pouring and forming.
Here is not to say that “Moisture conservation” Should be limited to the length of time within the number of hours, but should not set the boundaries of time, but should emphasize the “Immediate or timely moisture conservation”. Usually in the summer in a dry, hot and windy environment in the process of pouring floor, roof panels, sometimes visible after still in the pouring plaster, the front has lost water whitening, cracking, only a few hours only.
As for the “Moisture conservation” Time, should not be less than 14d, especially to focus on the first 7d days, which is the golden maintenance period, and the key is the first three days, especially the first day, which is the key in the key. Because, the young age of concrete in this period is the most active hydration of cementitious materials, hydration products generated the most, the fastest growing strength of the time. However, if you do not pay attention to maintenance, is also the fastest loss of concrete mix water, the most likely to produce water loss defects of the time. Some builders do not understand this, in order to meet the deadline, not only not covered, even water is not poured, just the final condensation on the start of the line, followed by lifting reinforcement, formwork, scaffolding, which care about maintenance? The results are endless, so we have to spend more money, waste more time to deal with leakage, really laborious and costly, but also make the durability of concrete after hardening greatly reduced.
For the maintenance of the basement thick base plate and exterior walls, shear walls and other structural parts, in addition to timely moisturizing, but also temperature control, to properly insulate (covered with plastic sheeting and sacks moisturizing insulation), to ensure that the concrete body center and surface temperature difference and surface and environmental temperature difference are not greater than 20 ℃, to slowly cool down, not more than 2 ℃ per day. Remember that in the case of high surface temperature of the concrete body, do not pour cool water to prevent rapid cooling, resulting in large temperature stress, resulting in concrete cracking.
During the construction in the high summer season, sand and gravel can be sprayed with water mist to cool down; it can also be calculated by replacing part of the mixing water with a suitable amount of ice xiao to mix concrete in equal amounts, which can reduce the temperature of concrete into the mold to below 30℃. The mixer drum should be sprayed with water at the right time to cool down, and the pump pipe conveying concrete should be covered with wet sack to prevent sunlight and heat dissipation. If conditions permit, concrete should be poured avoiding the high temperature period. After the concrete is poured, it should be moisturized and maintained in time, and if necessary, it can be covered with a sunshade to prevent wind and sunlight and reduce water evaporation.
Basement exterior wall formwork, preferably with better insulation effect, not absorbing water lacquered multi-layer wooden formwork, demoulding time should be not less than 7d, but can be loosened on the second day to fix the bolts of the formwork, and spray and cover wet geotextile at its upper seams. After the removal of the formwork of the wall façade, you can spray, hang sacks or geotextile, you can also spray maintenance solution or sticky plastic film maintenance, watering and moisturizing maintenance to 14d or more. Column surface can be sprayed after the water, wrapped tightly with plastic sheeting, maintenance time of not less than half a month.
The above maintenance methods, by repeated practice verification, the effect is very good.
The three necessary conditions for the development of concrete strength are: Temperature, humidity and age. And the characteristics of the summer concrete is: High temperature and rain, high temperature and drought, high winds and excessive temperature differences between day and night, and whether directly or indirectly on the strength and durability of concrete have caused a great impact. For summer concrete construction, how to maintain a certain temperature, certain humidity conditions and effective maintenance time, to ensure the normal development of concrete strength? This requires us to carry out effective maintenance according to the characteristics of summer and project parts.
Seaside city, the characteristics of summer are basically the daytime temperature is generally above 30℃, and the nighttime temperature is about 15℃; windy, the wind is generally around 5 levels; the humidity is higher near the sea. For the above characteristics, the main measures of concrete maintenance in summer in weihai area is to try to avoid high temperature, high wind and maintain humidity as the main goal. For different parts, different curing measures and curing time are taken.
For the beam, the surface area and the atmospheric contact surface is large, vulnerable to direct sunlight and wind, direct sunlight and wind make the free water of concrete evaporate at a rapid rate, if not take certain measures, the rapid evaporation of water produces a tensile stress greater than the tensile strength of concrete without condensation hardening, resulting in dry shrinkage cracking of concrete. Therefore, we should cover white (white reflective anti-heat) plastic film in a timely manner after the concrete is poured and vibrated and smoothed, and sprinkle water in time for maintenance after reaching the final set. If the plane vibrator is used, the surface can be smoothed in time and then covered with film sprinkling. Also note that the plastic film must be fixed, weihai windy, if not fixed well, by the wind scraped away, the concrete may also crack. The next day after the pouring of the builder on the plate when the line, it is strictly prohibited to uncover all the plastic, for the need to hit the spot, you can use the wallpaper knife to cut open to hit the spot, and then fixed.
For the maintenance of the column, some construction units may think that there is no need for maintenance, that the contact area between the column and the outside world is small, and also rarely see the phenomenon of cracking of the column. First of all, this is a wrong view. Columns generally one day to two days to demould, this time with the mold maintenance, may not appear problems, the problem is that the concrete strength development needs a period of time, strength testing age is 28d, according to the maintenance conditions are generally at least 7d. Especially modern concrete, mixing plant in order to save costs, basically are using double blended fly ash, slag powder, the demand for early maintenance conditions are higher. From another angle of analysis, due to the wind, especially in summer, the wind is basically blowing from one direction, so after the later carbonation, rebound determination, the same a column, the windward side and leeward side of the carbonation value and rebound value are very different. Therefore, the column should also be maintained in a self-sustaining way. The maintenance method is to wait for the formwork to be dismantled, sprinkle water and moisten it in time, wrap it with plastic film, and use the water inside the concrete for self-maintenance.
The most problematic and difficult to maintain for concrete structures are shear walls
Shear walls for vertical and atmospheric contact area of the structure, and the use of plastic film winding is unrealistic, can be used to maintain the conservation solution, now many construction units simply will not adopt the use of. At present, shear walls using the most maintenance methods or delayed removal of the formwork method, which saves money and trouble, but also very effective.
Summer construction of concrete maintenance work is very simple, but we have to go to the concrete business technicians to supervise the delivery, so that the construction unit to give attention. As long as the concrete this “Grass” In a certain temperature, humidity and a certain age to grow healthy and strong.
Concrete pouring in summer has the obvious characteristics of “High ambient temperature and more thunderstorms”. Therefore, the following measures should be taken when placing concrete in summer.
(1) in the production of concrete mixes above c30, we should try to reduce the temperature of concrete mixes out of the machine. First of all, control the incoming temperature of cement and mineral powder not more than 60℃, and also use groundwater spraying gravel to reduce the temperature of gravel. If the above two measures can not be implemented, you have to use the method of adding ice chips, you can add 30 ~ 80kg/m3 ice chips to the concrete mix according to the requirements of the outgoing temperature, the ultimate goal is to make the temperature of the concrete into the mold is not higher than the average temperature at that time, so that the concrete will not produce cracks.
(2) when pouring concrete beam slab, it should be rolled flat after compacting, then thatched plastic film, and lightly pressed the film with wooden rubbing board at the film joint to make it bond firmly with the concrete surface and prevent it from being scraped up by high wind. When the concrete needs to be placed after the final set, do not uncover all the film, available paper cutter to put the line to cut off the parts in the way, the film will continue to retain the concrete surface, conducive to the maintenance of concrete. The advantage of covering with film is to prevent the sudden heavy rain from damaging the freshly poured concrete slab surface.
Note: Do not use groundwater to diffuse the slab surface maintenance in summer, which will easily cause rapid contraction of the concrete slab surface and produce temperature shrinkage cracks. Can be used to pinch the flat pipe mouth sprinkling method of maintenance, the plate surface wet can be.
(3) for the concrete independent column, in the demoulding can be sprinkled with water once, followed by plastic film wrapped tightly until the decoration and then uncovered. This period does not require water maintenance.
(4) for concrete shear walls, can be extended to 7d after demoulding; if early demolition is required, can be immediately after demoulding with a brush of qualified maintenance solution. Can also be arranged to sprinkle water maintenance, every 2h sprinkle water once during the day, every 4h sprinkle water once at night.
(5) for the maintenance of bulk concrete there are two methods: The first is the film covering method, after the surface of the concrete rubbed flat immediately thatched film, and then according to the bulk concrete temperature measurement in the plastic film covered with cotton felt. If the difference between the center temperature of the concrete floor and the surface temperature is greater than 25 ℃, it should be covered with a layer of cotton felt on top of the film; if the temperature difference is greater than 25 ℃, it should be covered with another layer of cotton felt. When the temperature difference is not greater than 25℃, you can not cover the cotton felt. Note that the covered plastic film can not be blown up by high winds, otherwise, apply warm water to sprinkle once, and then re-cover the film well.
The second method is the water storage method
After the initial set of concrete on the bottom plate surface, then the edge of the bottom plate leakage with 1:2 cement mortar masonry 120 brick wall, two skins high, followed by mortar to wipe the brick joints solid; to wait for the final set of concrete on the bottom plate surface, can be flushed into the plate surface by the sun or in the water tank stored for more than a day of warm water (≥ 25 ℃). This method has the advantage of good maintenance effect, but it is not convenient to continue construction on the bottom slab.