What Should be Paid Attention to in Concrete Construction in Spring?

Because it is dry and windy in spring, the temperature difference between day and night is large, the temperature of concrete varies greatly in different time periods, and the temperature difference between inside and outside of concrete is also large, which is easy to form temperature shrinkage cracks. In this season, the air is dry, and the wind is stronger on the empty construction site floor, and the surface of freshly poured concrete loses water quickly, which is easy to form air-dried plastic shrinkage cracks. In order to ensure the construction quality, the following points should be paid attention to in spring concrete construction:

I. Preparation before construction

There will be some noise during the operation of concrete mixing station, which will have a certain impact on the surrounding environment and residents' lives. Therefore, concrete mixing stations should take effective measures to reduce the generation and spread of noise.

(1) Check whether the construction equipment is in good condition, such as mixing station and delivery pump.

(2) Check the formwork, steel bars, etc. to ensure that their sizes and positions meet the design requirements.

(3) Conduct technical disclosure and safety training for construction personnel.

II. The material control

Strengthen the inspection of raw materials, control the silt content of sand and gravel materials, increase the inspection frequency of admixture, and put an end to the use of unqualified raw materials.

III. The mixture ratio control

(1) Strictly control the water cement ratio, reduce the water consumption, control the dosage of admixture and admixture, and control the final setting time in 8 ~ 12h. When the setting time is too long, the surface maintenance should be strengthened under the guidance of technicians, and the pay-off and other processes should be postponed appropriately.

(2) Control the appropriate slump, and prevent the slump from being enlarged due to the convenience of construction or adding water at random.

IV. Storage and Use of Materials

(1) Raw materials such as cement and aggregate should be stored in a dry and ventilated place to avoid moisture.

(2) Admixtures should be properly stored according to their properties to avoid direct sunlight and high temperature.

(3) Before use, the quality of raw materials should be tested to ensure that they meet the construction requirements.

V. Influence of temperature change

The temperature in spring is changeable, and the temperature difference between day and night is large, which has a direct impact on the hardening and strength development of concrete. In order to ensure the construction quality, we should pay close attention to the weather forecast and choose the time when the temperature is relatively stable and the wind speed is low for concrete pouring.

VI. Pouring and maintenance

The construction and maintenance of concrete in spring plays a key role in strength and engineering quality. According to the characteristics of spring construction, please cooperate with the construction unit to do the following:

(1) Arrange the pouring and construction reasonably, so that the concrete can be poured into the mould in the shortest time, and control the construction speed, so that the construction requirements such as vibrating, plastering and curing can be completed in time after the concrete is put into the mould. If the waiting time of concrete is too long and the slump loss is large, the concrete admixture should be used for adjustment. It is forbidden to add water to concrete in any way during pouring and construction.

(2) Before concrete pouring, the formwork and foundation should be fully watered and wetted in advance to prevent the formwork and foundation from absorbing water in concrete, so that the concrete loses water to form plastic and shrinkage cracks, or even through cracks, and at the same time, pay attention to avoid a large amount of water accumulation in the formwork due to excessive water application.

(3) Under the condition of meeting the construction requirements, try to reduce the slump of concrete. The slump is small, which is beneficial to reduce the shrinkage of concrete. At the same time, attention should be paid to vibrating evenly during construction to avoid local over-vibration and form mortar concentration area, otherwise plastic shrinkage cracks and dry shrinkage cracks are easy to appear locally. It is forbidden to use internal vibrator to drive materials and to disturb the steel bar on the board surface with vibrator.

(4) Arrange the pouring sequence of concrete reasonably. In the process of pouring, first pour the parts with large concrete settlement, such as beams, walls and columns, and then pour the concrete slab after it has settled for a certain period of time and the internal concrete tends to be stable, so as to avoid settlement cracks on the concrete slab surface.

When pouring, we should speed up the layered pouring and continuous pouring to prevent cold joints. Control the appropriate slump and put an end to the diagram. It is convenient to enlarge the slump or add water at will.

When it rains during pouring, immediately cover the concrete that has not solidified to prevent the rain from washing away the slurry. In case of concrete surface bleeding, remove its moisture and laitance in time to avoid cracks and uneven appearance. After the water is volatilized, plastering shall be carried out at the right time. If the surface laitance is too thick, the laitance shall be scraped off or spread on the bottom of the uncasted concrete, and the secondary vibrating plastering shall be carried out before the initial setting of the concrete.

(5) In the process of construction, the concrete should be fully vibrated, so as not to leak vibration, but not to vibrate. Excessive vibration of concrete causes delamination and surface laitance, which leads to uneven settlement and shrinkage of concrete and cracks at the junction of structural thickness. Leakage vibration is not conducive to concrete compaction, resulting in concrete honeycomb, pits and other defects. The vibrating time of concrete shall be determined according to the workability of concrete. Concrete should be vibrated fully, which is marked by the appearance of slurry on the surface of concrete, no air bubbles overflowing, and the concrete is full of trial formwork and does not sink significantly.

In the dry spring, after the concrete is mixed with high-efficiency retarding water reducer, the surface is air-dried, and the water evaporates. In fact, the concrete seems to have condensed, but the interior is far from the initial setting, and it can even flow. This situation is called the "crusting" phenomenon of concrete.

The secondary vibration can eliminate the internal stratification caused by plastic settlement, block the coherent channel left by bleeding, improve the interface structure of aggregate, and improve the strength and impermeability of concrete.

(6) The slump of commercial concrete is large, and the setting time is relatively long (generally, the initial setting time is 6 ~ 10h). The temperature in spring is changeable, the temperature difference between day and night is large, and the temperature is low at night, which is easy to prolong the setting time. Therefore, attention should be paid to heat preservation and maintenance, and the influence of temperature on the setting time of concrete.

(7) Immediately after the concrete cast-in-place slab is poured, it should be covered with plastic film to keep moisture, so as to avoid the formation of hard shell due to serious water loss on the concrete surface. In the project that needs to close the surface, the work of closing the surface should be done well, which can eliminate the internal stratification caused by plastic settlement, block the coherent channel left by bleeding, improve the interface structure of aggregate, and improve the strength and impermeability of concrete.

(8) After concrete pouring, immediately cover it with plastic film after the first screeding, so as not to let the moisture evaporate, and rely on the moisture of concrete itself for moisture maintenance. If the surface is not covered and watered in time, the surface moisture will evaporate quickly, and it is easy to produce shrinkage cracks. Especially in the case of low relative humidity and high wind speed, drying shrinkage is more likely to occur. Some data show that when the wind speed is 16m/s, the evaporation rate of water in concrete is four times as high as that in the absence of wind. In case of dry and windy climate, due to surface water loss, the rapid pouring is covered with plastic straw curtain. Cover and wrap the plastic film in time after the plates, floors and other structures are pressed, so that there is always condensed water in the plastic film, so that the concrete can be cured by its own condensed water, reducing plastic and shrinkage cracks on the surface and even forming through cracks. Please wrap the wall column and other structures with plastic film and hang the straw curtain immediately after the formwork removal, so that there is always condensed water in the plastic film or the surface is coated with curing agent for maintenance.

(9) Strictly control the form removal time. When the strength of concrete can ensure that its surface and edges and corners are not damaged by dismantling formwork, the side formwork can be dismantled. Immediately after the side formwork is removed, it should be watered and covered with plastic film for curing, or sprayed with curing agent for curing, and the curing time should not be less than 14 days. Demolition of bottom formwork must be carried out according to the requirements of specifications. The standard value of design strength grade of concrete for structures such as beams and slabs with a span of less than 8 meters must be greater than 75%. Structures such as beams and slabs with a span of more than 8 meters can only be dismantled when the concrete strength is greater than or equal to 100%, otherwise the safety of structures will be affected. Before the concrete strength reaches 1.2n/㎜, it is not allowed to trample on it or install templates and supports.