In practical engineering, concrete cracks caused by shrinkage are the most common. In the concrete shrinkage category, plastic shrinkage and dry shrinkage are the main causes of concrete volume deformation, and spontaneous shrinkage and carbonization shrinkage. The main reason for this kind of crack is that concrete has not been effectively maintained after hardening and forming, such as moisture, heat preservation, and maintenance measures are not in place, concrete surface wind, and sun, resulting in surface moisture loss too fast, a large shrinkage.
Lightweight concrete has become the mainstream of modern concrete because of its good seismic performance, heat preservation performance, and fire resistance and its light quality. The shrinkage of lightweight concrete is prone to dry crack at the early stage, which will damage the concrete structure and shorten the service life of the concrete.
After many tests and years of experience in making lightweight concrete, it is found that adding polypropylene fiber (PP fiber) or polyvinyl alcohol fiber (PVA fiber) and expansion agent to lightweight concrete can reduce its shrinkage rate, and the addition of expansion agent has obvious shrinkage reduction effect. The optimal effect is the mutual combination of fiber and expansive agent, the combined effect of the two is better than the effect of the two alone, the mutual combination of fiber and the expansive agent shrinkage rate is smaller than ordinary concrete.
Fiber reduce the shrinkage of the main reason is that in the mixed into lightweight concrete, all of the fibre can be very complex three dimensional chaos system, and can hinder part of the loss of moisture, in addition, the expansion of the fiber can effectively block the microfracture, so as to avoid cracks continuously carried out so as to further reduce the shrinkage of concrete.
Expansion agent to reduce the shrinkage of the main reason is that in the mixed into lightweight concrete expansive agent can appear early anhydrous calcium aluminum sulfate reaction, in the medium term will produce alunite, gradually into the lightweight concrete expansive agent and with water after will generate more expansion joint crystalline compound, leading to the expansion of the concrete in a certain, with the free shrinkage of lightweight concrete cancel each other out.
To sum up, because of the difference in shrinkage foundation between fiber and expansive agent, the mutual combination of fiber and expansive agent can effectively improve the anti-cracking performance of lightweight concrete, so as to improve the use efficiency of high-strength lightweight concrete in engineering.