In practice, a single detail change can lead to concrete problems, such as concrete secretion, segregation, abnormal setting, etc. This article will introduce five concrete problems and their solutions.
Secretion and segregation of concrete problems
When formulating fluid concrete, if the concrete cohesiveness and water retention are poor, the balance of the uniformity and stability of the composition of the materials will be broken. Concrete in its own gravity and other external forces produce separation, that is, concrete segregation. If the mixing water precipitation out of the surface, that is, water secretion. Usually, water secretion is the prelude to segregation. Dialysis is bound to lead to delamination, increasing the possibility of pumping. A small amount of secretion in the project is allowed, and it is beneficial to prevent the production of concrete surface cracks.
The causes of concrete secretion and segregation and their solutions.
1. Low sand ratio or low content of fine particles in the sand, making the concrete low water retention. Sand with high mud content is easy to produce a slurry settlement.
Solution: Increase the sand ratio and reduce the amount of mud in the sand. A reasonable sand ratio can ensure the workability and strength of concrete.
2. The total amount of cementitious material is small. The volume of slurry is less than 300L/m3.
Solution: Mix with fly ash. Especially for the preparation of low-strength grade of large fluidity concrete, the amount of fly ash admixture should be appropriately increased, so as to improve its water retention.
3. Poorly graded stones or single-size stones.
Solution: Adjust the stone gradation. It is necessary to increase the sand ratio by using single-size stones.
4. The water consumption is large so that the concrete mix cohesion is reduced.
Solution: Increase the water reduction rate of the admixture or increase the admixture mixture, and reduce the water consumption.
5. The admixture is too large, and the admixture contains the component of water secretion.
Solution: Reduce the admixture or add thickening components and air-entraining components in the admixture to improve the cohesiveness of concrete and prevent water secretion and segregation.
6. Due to excessive storage time, the clinker in the cement has been partially hydrated, making the cement have poor water retention.
Solution: Use compound thickening component and early strength component in the admixture.
7. Use slag powder or slag silicate cement, which has poor water retention and is easy to secrete water and segregate.
Solution: Increase the amount of cement or fly ash, reduce the amount of slag powder, or replace the cement species.
The abnormal setting of concrete problems
Concrete sets quickly after mixing. This phenomenon is rarely encountered in daily work. Generally, it is because the factory temperature of cement is too high, the gypsum in cement is seriously insufficient, the admixture is seriously incompatible with cement, or hot water is in direct contact with cement.
Too long setting time
This phenomenon is frequently encountered. It can be divided into two cases, overall serious slow setting and partial serious slow setting
Overall serious slow setting
This situation is mostly caused by the admixture. Due to the addition of inappropriate slow-setting components (there are many slow-setting components affected by temperature, its setting time changes significantly), or admixtures beyond the normal amount, resulting in excessive slow-setting of concrete.
Partial serious slow setting
In this situation, for example, the majority of the floor or wall concrete set normally, and partial concrete retardation, the reasons may be.
- The use of the post-mixing method to add admixtures, and concrete mixing is not uniform, resulting in partial enrichment of admixtures.
- Adding water on site, the concrete cohesion is reduced, and water secretion or segregation occurs. Pouring vibrates so that the local slurry concentration and water-cement ratio become large and the admixture is relatively excessive.
- The deposit with slow-setting components in the pool of admixtures is not easy to mix evenly, causing the concrete to be over-set locally.
The concrete slump on site is larger than that out of the machine
When formulating concrete with higher strength grade, sometimes there is a phenomenon that the slump on site is larger than the slump out of the machine. The reasons for this may be the following four points.
- the use of amino sulfonate or its admixtures with similar properties.
- too many slow-setting components in the admixture or the later reaction are more violent.
- inappropriate ratio (such as small sand ratio, too much admixture, etc.) resulting in water secretion.
- there is water in the concrete tank.
Solution: For the first three types of problems can be found and adjusted through test matching in the laboratory, such as doing a slump loss test and setting time test, etc. In actual production, the admixture and water consumption should be strictly controlled. Aminosulfonate admixtures are particularly sensitive to water.
For the fourth problem, the mixing tank can be reversed before loading, and the remaining water will be dumped clean.
Suddenly accelerated slump loss during concrete production
There are three possible causes as follows.
- Changes in the water-reducing component of the admixture.
- Fewer admixtures in the pool, mostly precipitated sodium sulfate and other early-strength components.
- Change in cement composition, etc. These problems can be solved by adjusting the admixture components or their dosing.
Using volcanic ash cement in dry environments
When volcanic ash cement is used in a dry environment, the internal water will evaporate quickly. The reaction of hydration to produce colloids will be stopped. The strong growth will also stop. Moreover, the formed hydrated calcium silicate gel will gradually dry out, generating large volume shrinkage and internal stresses, resulting in micro cracks.
On the surface, due to carbonation, the hydrated calcium silicate gel powder crystallizes into a powdery mixture of calcium carbonate and silicon oxide, making the surface of the hardened concrete a very fine crack.
A project used concrete formulated with volcanic ash cement. The structural strength was 10-30% lower compared with the specimens made under the same conditions. Moreover, the specimens had different degrees of “powder drop” inside after crushing, which fully illustrates that the hydration reaction of volcanic ash cement in a dry environment is very insufficient.