Analysis of measures to reduce shrinkage cracking of concrete

(Analysis of measures to reduce shrinkage cracking of concrete)

The cracks caused by concrete shrinkage affect not only the bearing capacity, waterproof performance, and safety of concrete but also its durability.

The prevention of concrete shrinkage cracks is mainly from the external and internal aspects of concrete.

Starting from the outside of concrete, mainly through reinforcement, setting expansion joints or post-pouring belts and other means to achieve the restraint and prevention of concrete shrinkage cracking.

On the other hand, it is to avoid the potential factors that may lead to concrete shrinkage cracking as far as possible, such as optimizing the mix proportion of concrete, adding fiber, expansion agent, shrinkage reducing agent, shrinkage reducing polyanionic acid superplasticizer or internal curing, etc.

The main results are as follows:

1) adding fiber to concrete can disperse the internal stress distribution of concrete and homogenize the tensile stress to reduce the shrinkage cracks of concrete.

PP fiber is randomly supported inside the concrete, which helps to restrain the concrete and reduce the shrinkage of the concrete.

Adding fiber can reduce the shrinkage of concrete and improve the splitting tensile strength, flexural strength and toughness of concrete.

Steel fiber, glass fiber and polymer fiber are commonly used in concrete.

However, the addition of fiber can not fundamentally eliminate the existence of cracks.

Moreover, steel fiber is easily eroded by chloride; the organic fiber will age and fail under the action of heat, oxygen, and ultraviolet rays; the dispersion of the fiber also dramatically affects its ability to prevent cracking.

In addition, the fiber also has the problems of high cost and poor compatibility with concrete.

2) Adding an expansive agent to concrete can produce expansive stress to compensate for shrinkage and effectively reduce early shrinkage and deformation of concrete.

Chen Meng mixed expansive agents into concrete and tested the molded specimens under standard and natural curing conditions. The test results show that the best age of the expansive agent is (1-3) d, and coagulation will produce more considerable shrinkage deformation after (3-5) d.

In addition, the effect of an expansive agent is closely related to concrete curing conditions. If cured under natural conditions, the shrinkage strain of concrete with an expansive agent after 28 days of age is the same as that of concrete without an expansive agent.

To ensure the full effect of the expansion agent, the specimens mixed with the expansion agent must be maintained under humidity greater than 95%.

Although expansive agents can compensate for concrete shrinkage to some extent, many factors affect the effect of expansive agents.

Generally speaking, the amount of expansive agent is 10% / 12% of cement quality, and the amount is relatively large, so it is more expensive to add expansive agent to compensate for shrinkage.

The effect of an expansive agent is also challenging to control; on the one hand, it is difficult to control the amount of expansion; on the other hand, the effect of an expansive agent has a great relationship with the humidity of the curing environment, and the concrete mixed with an expansive agent must be wet cured.

In the case of a relatively low water binder, the hydration of the expansion agent will also be inhibited, affecting the expansion effect.

3) Shrinkage reducing admixture

To solve the problem of shrinkage cracking of concrete, the most commonly used solution is to use chemical admixture, which is called shrinkage reducing agent (Shrinkage reducing admixture, SRA).

Concrete shrinkage-reducing agents can effectively reduce the self-shrinkage and drying shrinkage of concrete.

The mechanism of SRA can be summarized as follows:

1) reduction of shrinkage stress caused by surface tension and capillary pressure.

2) due to the slow evaporation of water, the internal humidity of concrete can be better maintained:

3) the total porosity decreases, and the pore structure is refined.

4) the polarity of water decreases, so the solubility of ions decreases, hydration is delayed, and shrinkage is reduced.

5) the adsorption of SRA on pores or C-S-H hinders the accumulation of C-S-H, increases the cavitation pressure of water in pores, and releases part of the tensile stress induced by capillaries, thus reducing the drying shrinkage.

However, SRA will prolong the setting time of concrete, affect the strength, and even lead to a decline in durability.

When SRA is mixed with other admixtures, there is also a compatibility problem.

 

4. High-performance super absorbent resin

Because of its strong water absorption capacity, superabsorbent resin (Superabsorbent polymer, SAP) can be mixed into concrete materials to carry out internal curing of concrete through the water absorption-release effect of SAP and reduce concrete shrinkage.

However, SAP often hurts the mechanical properties of concrete.

It is generally believed that although the addition of SAP can reduce the self-shrinkage of concrete, the existence of SAP will increase the porosity of concrete, resulting in the decrease of compressive strength and elastic modulus of concrete.

Summary

The cracks caused by shrinkage of concrete affect the mechanical properties, waterproofing and durability of concrete to a great extent.

The cracking problem related to shrinkage is the biggest threat to concrete's durability and structural safety.

To limit the shrinkage of concrete and reduce the shrinkage cracks, the researchers put forward some external measures, such as reinforcement, setting expansion joints and post-pouring belts. Still, they can not eliminate the cracks fundamentally.

Effective measures can be taken from the point of view of materials to reduce concrete shrinkage, such as adding expansive agents, fiber, super absorbent resin, shrinkage-reducing agents, reducing polyanionic acid superplasticizer, etc.

However, these methods still have limitations.

However, the author believes that synthesizing reduced PCE with both water-reducing and shrinkage-reducing functions by introducing functional groups into PCE is a technical measure to kill two birds with one stone.

In the future, the preparation, properties and application properties of reduced PCE should be further studied and explored.

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(Analysis of measures to reduce shrinkage cracking of concrete)