Summer has the climate characteristics of high temperature, much precipitation, large precipitation and fast drying, which are embodied in high external temperature, low air humidity, strong sunlight and fast water evaporation. Although the higher temperature in summer is beneficial to the early growth of concrete strength, there are also disadvantages such as high temperature, slump loss, rapid water evaporation after pouring, large concrete shrinkage and so on.

1. Problems needing attention in concrete production in summer

The external temperature has a great influence on the cement hydration rate in concrete. Generally speaking, the cement hydration rate increases with the increase of external temperature. For example, when the temperature is 20 ℃, the hydration rate of cement is about 3 times that when the temperature is 5 ℃. The acceleration of cement hydration speed will inevitably lead to the loss of concrete slump and the decrease of fluidity, which will have an adverse impact on the construction, but the slump loss caused by this situation is difficult to estimate accurately. It is generally believed that the hourly time loss of slump of concrete mixture increases by 10 ~ 40mm when the external temperature increases by 10 ℃. In addition, the increase of external temperature also increases the evaporation rate of mixing water in concrete, accelerates the thickening speed of concrete in contact with air, and reduces the fluidity of concrete to varying degrees.

1.1 pay attention to the cement mobilization temperature

The high ambient temperature in summer slows down the cooling rate of cement. The ex factory temperature of cement is often higher than that in winter, so it can not be cooled during transportation. The temperature of cement transported to the concrete company is high, and some even exceed 80 ℃, resulting in rapid hydration of cement itself. In Table 1, cement with different temperatures is used for slurry test. It is found that the higher the cement temperature, the smaller the initial and time-lapse fluidity, and the higher the temperature, the greater the impact. In addition, in the process of cement grinding, the increase of mill temperature makes part of dihydrate gypsum dehydrated into hemihydrate gypsum. Due to the difference of dissolution rate between the two, the hydration of C3A in cement can not be effectively controlled, and it is difficult to control the slump loss of concrete mixture. Sugar retarder and lignin in admixtures are easy to affect the solubility of anhydrite. When anhydrite is used as cement retarder, admixtures containing sugar substances should be avoided.

1.2 properly increase the content of mineral admixture

In summer, the temperature is high, the cement hydration rate is fast, and the cement hydration products increase with the increase of hydration rate, which provides favorable conditions for the secondary hydration reaction of mineral admixtures. At the same time, the increase of temperature also increases the activity of mineral admixture, which is conducive to the activity of mineral admixture. Therefore, under the condition of ensuring the strength of concrete, the content of mineral admixture can be appropriately increased, the amount of cement can be reduced, and the time loss of concrete slump can be improved.

1.3 select admixtures with good compatibility

In the construction of concrete engineering in summer, the selection of admixtures with good compatibility with concrete raw materials is a powerful means to ensure the construction performance of concrete. Generally, admixtures with good compatibility with concrete raw materials should meet: (1) the amount of admixtures is moderate and easy to control（ 2) The change of admixture content is about ± 0.1%, and the concrete state does not change significantly（ 3) When the water consumption changes by about 5kg, the concrete state does not change significantly（ 4) The slump loss of concrete is small over time, meeting the requirements of transportation and construction（ 5) Before and after pumping, the change of concrete state is small, that is, the pump loss is small.

1.4 reduce the temperature of concrete mixture as much as possible

The increase of concrete mixture temperature makes it difficult to cool down after pouring, increases the probability of temperature shrinkage cracks of concrete structure, and reduces the temperature of concrete mixture, which can effectively reduce temperature cracks, especially mass concrete with strict requirements for temperature rise. Generally speaking, reducing the temperature of concrete raw materials is an effective method to reduce the temperature of concrete mixture. Aggregate is the most used raw material in concrete. If the temperature of aggregate is reduced by 2 ℃, the temperature of concrete mixture will be reduced by 1 ℃. At present, the stockyards of most mixing plants are closed, so as to avoid direct sunlight on the aggregate. When using underground cold water spray to cool the aggregate, pay attention to the uniformity of the spray to avoid the difference of aggregate moisture content, that is, the difference of moisture content between the surface and the interior (bottom) and different parts of the aggregate. The specific heat of water is 4 ~ 5 times that of cement and aggregate. Generally, the concrete temperature can be reduced by 1 ℃ by reducing the water temperature by 4 ℃. Therefore, it is a feasible method to use underground cold water to mix concrete in summer with high temperature. It is also suggested to use ice water mixture to mix concrete, but it should be noted that the amount of crushed ice should be controlled within 25%, and ensure that the crushed ice can completely melt when mixing concrete, so as to avoid the slow melting of unmelted ice in the mixture, resulting in "large" concrete slump and difficult to control the workability of concrete mixture.

1.5 reasonably dispatch concrete transport vehicles

The high temperature weather in summer brings certain adverse factors to the concrete pouring work. The production dispatching personnel shall reasonably allocate vehicles according to the pouring situation on the construction site and the traffic situation on the transportation road, so as not to press the vehicle, but also not to break the vehicle on the construction site for a long time. Generally speaking, the truck pumping speed is 10 ~ 20min, and a truck (12m3) of concrete is poured. The dispatcher can reasonably arrange the vehicles according to the traffic conditions and distance. After the concrete transport vehicles arrive at the construction site, the construction unit shall timely arrange personnel for pouring to ensure that the concrete is poured to the project site when it has good workability and ensure the quality of concrete. The concrete pouring on the site shall be carried out at night as far as possible. The ambient temperature at night is slightly lower than that in the daytime, and the performance of concrete slump and other aspects can be maintained better.

1.6 regularly inspect the weighing system

The change of external temperature is easy to cause the transmission of sensor signal of concrete production equipment, and then affect the accuracy of measurement. Therefore, calibrate various weighing load cells from time to time to make the weighing value within the error range. In particular, the admixture load cells can prevent the occurrence of concrete retarding accident caused by excessive admixture.