How to define the temperature crack of bulk concrete

Answers]: a, b, c, d

2021 Textbook P104 2020 Textbook P99 2019 Textbook P102

Large-volume concrete temperature cracks include microcracks, surface cracks, deep cracks and penetrating cracks.

The measures to prevent temperature cracks in the concrete are to bump into: make high-quality concrete, reduce voids, and use steel bars to connect the concrete.

1. To make high-quality concrete.

Concrete is made from the withdrawal of electric cement and kita filler, and cement is made up of minerals, and the quality of these minerals is different, so the quality of the cement is also different. High-quality cement has a more vertical hydration reaction, so when the concrete shrinks in a high-temperature environment, the amount of shrinkage can be effectively reduced.

2. Reduce voids.

When constructing concrete, voids should be minimized, so that the concrete construction is as uniform and compact as possible. Generally speaking, the smaller the voids of concrete, the higher its strength and the greater its resistance to temperature cracks.

3. Use steel bars to connect concrete.

Reinforcement can also be used to connect concrete, so that the concrete has a certain elasticity, provide convergence space for concrete, reduce shrinkage caused by temperature changes, and prevent temperature cracks.

How to deal with cracks in concrete:

1. If the cracks in the concrete are not particularly serious, but there is only a problem of water printing, you can adopt the caulking and plugging method, chisel a V-shaped groove along the gap, remove the debris inside, and then rinse it with water.

2. If the problem of cracking is more serious, it is necessary to adopt chemical grouting method, because this kind of crack is generally penetrating, and it is necessary to groove first to fill the cement mortar into the gap. It is then cleaned with a wire brush, rinsed with water, and then sprayed with polymer cement mortar. After curing, a water-soluble polyurethane solution is used for chemical grouting.

3. If there is always a shrinkage crack, it is because the surface is not covered in time and exposed to sunlight and wind, resulting in faster water loss, or the amount of cement is too large, and the water-cement ratio is too large, which will lead to cracks.

Causes of cracks in large-volume concrete foundations and measures to control temperature cracks:

1) Reduce the temperature of concrete into the mold.

When pouring large volume concrete, a more suitable temperature should be selected, and pouring in hot weather should be avoided as much as possible. Groundwater with low temperature can be used to mix concrete, or ice cubes can be added to the concrete mixing water, and at the same time, the aggregate can be shaded and protected, sprinkled with water to cool down, etc., so as to reduce the mold temperature of the concrete mixture, and add the corresponding retarding superplasticizer.

2) Strengthen temperature control during construction.

After the concrete is poured, the insulation and moisturizing maintenance of the concrete should be done to slowly cool the concrete, give full play to its creep characteristics, and reduce the temperature stress. It is necessary to resolutely avoid exposure, pay attention to temperature and humidity, take long-term maintenance, and determine a reasonable time for demoulding to delay the cooling speed, prolong the cooling time, and give full play to the "stress relaxation effect" of concrete; Strengthen temperature and temperature monitoring.

It can be monitored by a thermal thermometer or multi-point monitoring by a special person to grasp and control the temperature changes in the concrete at any time. The temperature difference between the inside and outside of the concrete should be controlled within 25, the temperature difference between the base surface and the bottom surface of the base and the bottom surface of the base is controlled within 20, and the insulation and maintenance measures should be adjusted in time to make the temperature gradient and humidity of the concrete not too large, so as to effectively control the appearance of harmful cracks.

3) Improve the tensile strength of concrete.

Control the mud content of aggregates, the mud content of sand and stone is too large, not only increase the shrinkage of concrete and reduce the tensile strength of concrete, it is very unfavorable to the crack resistance of concrete, so the mud content of sand and stone must be strictly controlled when mixing concrete, and the mud content of stone is controlled below 1%, and the mud content of medium sand is controlled below 2%, so as to reduce the adverse impact of concrete crack resistance due to the failure of sand and stone mud content.

Improve the concrete construction technology, strengthen the early curing, improve the tensile strength and elastic modulus of the early and corresponding age of concrete, and set the necessary temperature reinforcement on the surface and inside of the large-volume concrete foundation to improve the stress division and prevent the appearance of cracks.

Answers]: b, c, e

p56。The content of this question is examined: large-volume concrete engineering.

The control method of large-volume concrete crack comprises: (1) the slag cement of low heat of hydration is preferentially selected to mix concrete, and the retarder superplasticizer is appropriately used. (2) On the premise of ensuring the design strength grade of concrete, appropriately reduce the water-glue ratio and reduce the amount of cement.

3) Reduce the temperature of concrete into the mold, and control the temperature difference between the inside and outside of the concrete (when the design is not required, control within 25). For example, reduce the temperature of the mixing water (add ice chips to the mixing water or use groundwater); The aggregate is washed with water to cool down and avoid exposure to the sun. (4) Cover the concrete with thermal insulation and moisturizing materials in time.

5) The cooling water pipe can be embedded in the foundation, and the circulating water can be introduced to force the temperature generated by the heat of hydration of the concrete to be lowered. (6) when mixing concrete, can also be mixed with appropriate amount of micro-expansion agent or expansion cement, make concrete get compensated shrinkage, reduce the shrinkage deformation of concrete. (7) Set up the post-pouring joint.

When the large volume concrete plane size is too large, can be appropriately provided with post-pouring joint, to reduce external stress and temperature stress; At the same time, it is also conducive to heat dissipation and reduce the internal temperature of concrete. (8) The secondary plastering process can be used for large-volume concrete to reduce surface shrinkage cracks.