lvssn reminds you of several errors in the use of cement
Date：2019-06-13 09:46 Source：未知Views:
(1) the more cement, the better
The larger the amount of cement, the concrete shrinkage deformation is relatively increased, volume stability is poor, plus the superposition of various shrinkage tensile stress, once exceeds the tensile strength of concrete, it will cause concrete cracking.
In addition, the more cement, the higher the heat of hydration, temperature rise is large (fast), the resulting temperature deformation is correspondingly greater, the probability of concrete cracking also increased.
Engineering practice has proved that when the temperature difference between the inside and outside of concrete is more than 25℃, the temperature difference between the surface and the atmosphere is more than 20℃, concrete is likely to crack.
The increase of cement content will increase the amount of concrete slurry and increase the thickness of slurry wrapped on the surface of aggregate, thus reducing the friction between aggregates, increasing the fluidity of concrete and improving the workability of concrete.
But too much cement will lead to too much slurry, easy to produce the phenomenon of running slurry, reduce the cohesion of concrete, thus affecting the workability of concrete mixture.
(2) the higher the early strength of cement, the better
Nowadays concrete engineering often requires high early strength of concrete.
In order to meet the requirements of early strength, cement fineness is generally fine, the content of tricalcium silicate and tricalcium aluminate increases.
Although these changes are beneficial to the improvement of concrete strength and early strength, they increase the temperature shrinkage and dry shrinkage of concrete, plus the self-shrinkage generated by lower water-cement ratio, concrete is easy to crack.
The improvement of concrete's early strength makes the elastic modulus of concrete increase rapidly in the early stage, but the tensile strength increases slowly, sometimes hardly, which greatly increases the tensile stress generated by concrete's shrinkage.
At the same time, the high early strength makes creep very small, unable to relieve the shrinkage stress, which makes the concrete crack.
Choose the cement with low 3d strength and high 28d strength. The hydration speed is not too fast in the early stage and the strength of cement increases greatly in the later stage. It is easy to operate in use.
(3) cement shall be selected according to specific conditions
Different engineering sites have different requirements for cement performance. Cement should be selected according to engineering sites and environmental conditions.
For example, pavement concrete chooses cement with good flexural performance, large-volume concrete chooses low-heat cement with lower hydration heat, and in winter, early strength cement is selected.
(4) rational use of mineral admixture
Some people think adding mineral admixture is to reduce the amount of cement, save costs.
In fact, the benefits of adding mineral admixtures go far beyond that.
Concrete mixes with large fluidity are prone to segregation and bleeding water, thus undermining the homogeneity of concrete, and forming drainage channels and settlement cracks in the interior. Adding mineral admixture can increase the cohesion of concrete mix, reduce segregation and bleeding water, and improve the workability of concrete.
In the production of concrete, adding mineral admixture can reduce the hydration heat of cement, reduce temperature rise, reduce temperature crack.
Of course, different mineral admixtures have different effects on concrete shrinkage.
The shrinkage of concrete is increased by silica powder and granulated blast furnace slag powder.
Therefore, mineral admixture should be used reasonably.
(5) pay attention to the adaptability of cement and additives
With the same variety, with the same amount of admixture, for different manufacturers of cement, its effect is very different, even the same manufacturer of cement production batch is different, the effect of admixture is very different.
Similarly, different manufacturers of the same kind of additives, for the same manufacturer of the same variety of cement, even with the same batch of cement, the effect is also very different, mainly in the setting time, slump loss, reducing the effect of water and the impact on the flow of different, and even cause the reduction of concrete strength and durability decline.
Therefore, it is necessary to pay attention to the adaptability test of admixtures and cement, pay attention to the changes of cement quality or admixtures, find out the problems, and take measures in time to avoid the occurrence of engineering accidents.
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