In recent years, a large number of cracks are not directly related to load, but caused by deformation, including temperature deformation (hydration heat of cement, temperature change and heat generated by environment), shrinkage deformation (plastic shrinkage, drying shrinkage and carbonation shrinkage) and uneven settlement (expansion) deformation of foundation. These cracks caused by deformation are collectively called "cracks caused by deformation" [2].
Engineering examples of cracks:
During the construction of high-grade C70 silica fume concrete under the sluice chamber of Xiaolangdi orifice (diversion) tunnel, open flow section of sand discharge tunnel, open flow tunnel and spillway, cracks appear in concrete to varying degrees. The crack width is generally 0.4-0.8mm, the width is 2mm, and the average crack length is 4 m. Adding 25%-40% fly ash can minimize the cement consumption and reduce the temperature rise of concrete hydration heat.
In Qiafuqihai Water Control Project, local 32.5 ordinary portland cement mixed with Grade I fly ash and high-quality admixture was used to prepare high-fluidity and high-performance concrete. Using local 42.5-grade portland cement, silica fume and fly ash, and adding expansive agent and superplasticizer, the shrinkage-compensating concrete with large fluidity, high strength and high density is prepared. In 2000, a 527-meter diversion tunnel and a 300-meter sand-washing tunnel were completed, and two 3-meter-long micro-cracks were found. Several micro-cracks were found in C60 high-strength concrete with wear-resistant layer on the bottom plate [4].
Concrete expansion occurred in five dams in France, causing problems such as dam deformation and internal stress, such as dam elevation, dam displacement from upstream to downstream or from downstream to upstream, and the formation of some typical cracks, which is characterized by slow and gradual formation process [5].
Research and application:
After the cracks appear, they need to be repaired, which not only increases the cost, but also affects the durability of the building. Therefore, the principles of rational use of materials, reduction of cement consumption and reduction of adiabatic temperature rise of concrete should be followed on the premise of ensuring the strength and durability requirements stipulated in the foundation engineering design and meeting the construction technology requirements. Another important measure to prevent concrete from cracking and reduce cracks is to add additives to concrete, which can improve the compactness of concrete, effectively improve the carbonation resistance of concrete and reduce carbonation shrinkage; In addition, it can increase the workability of concrete, form a micro-film on the surface, reduce water evaporation and reduce drying shrinkage. If water reducing and crack preventing agent is added, the cement content in concrete can be reduced by15% on the premise of ensuring strength; In addition, it can improve the consistency of cement slurry and reduce bleeding and subsidence deformation. After adding water reducer, the retarding time of concrete is prolonged, which effectively prevents the rapid hydration and heat release of cement and avoids the plastic shrinkage caused by long-term non-solidification of cement.
At present, some achievements have been made in strengthening the crack resistance of marine concrete at home and abroad, and they have been applied to engineering practice.
Fuhuade power plant and Guangdong LNG receiving station located on the east coast of Shenzhen are both marine projects, and some of them are even in splash zone. Fuhuade Power Plant also uses seawater as circulating cooling water for cooling towers, which is the first case in China. Therefore, it is necessary to prepare high-performance marine concrete. A large number of concrete mixed with fly ash and silica fume as admixture was applied to marine engineering. When more than 20,000 cubic meters of concrete was supplied, no concrete cracks appeared [6].
The marine concrete of "Hutan Letan Paradise Underwater World" in Dalian has stood the test of seawater immersion and ebb and flow for more than 10 years. The concrete surface does not fall off, crack, leak or leak, and its durability is very good. Combined with engineering examples, the effects of air entraining agent, expansion agent and retarding water reducer on the durability of concrete are studied. The results show that the concrete prepared with three kinds of additives has excellent freeze-thaw resistance, impermeability and crack resistance, which meets the durability requirements of concrete buildings, and provides a new way and application experience for the mix design and durability research of marine concrete [7].
The scientific and technical personnel of the project department of Hangzhou Bay Cross-sea Bridge ⅷ cooperated with the Bridge Science Research Institute of the company to develop C50 marine durable concrete, which made a major breakthrough in the research of crack resistance and corrosion resistance of concrete. The successful application of secondary tensioning method finally solved the crack problem of concrete box girder. As of July 15, no obvious cracks appeared in the precast 5170m box girder [8].
When concrete is exposed to chloride ions in seawater or antifreeze, steel bars will be corroded. According to Herholdt et al., the corrosion product can be six times that of the original steel, thus generating enough pressure to crack the concrete. Mehta and Gerwick studied the corrosion damage of the high performance concrete bridge across the san mateo Bridge in San Francisco Bay. After the bridge was exposed to seawater for 17 years, the beam after steam treatment must be repaired due to corrosion damage. This paper introduces the mix proportion design of concrete mixture, so that it can be steamed for 18 hours [9].
Adding propylene fiber to concrete has two effects on the corrosion of reinforced concrete exposed to chloride ions. First of all, by reducing concrete cracking and delaying the start time of corrosion, uniform concrete with good quality can be produced in a "typical" environment. The second advantage of using PFRC is the most important, that is, once the corrosion begins, it can control the corrosion rate or weaken the corrosion. Ordinary concrete is prone to cracks due to the internal tension of iron oxide. Similarly, the phenomenon of using PFRC to control cracks and crack propagation has also been confirmed. When the corrosion by-products produce tension in concrete, it is necessary to use concrete with this performance [10].
In order to prolong the service life of the interstate bridge deck, Ohio Department of Transportation has implemented a material evaluation plan. Steel fiber is used as a candidate material of fine aggregate and mixed with micro-silica powder to modify concrete, which can increase the superplastic density. It is found that adding dispersed steel fiber to concrete can significantly reduce the formation and diffusion of early cracks [1 1].
Patents:
On March 9th, 2003, Shanghai Institute of Building Research issued CN 1403400A special admixture for high-performance marine concrete. Guided by the superposition effect theory of multicomponent cementitious materials, the industrial active additives such as slag powder, fly ash and silica fume are mixed and ground in a proper proportion for the first time. The application of this special admixture in concrete shows that the mechanical properties of concrete mixed with admixture can be comparable to ordinary portland cement concrete with the same amount of cementing material, and its workability and durability have been greatly improved [12].
The concrete branch of Beijing Urban Construction Group Corporation1September, 9981KLOC-0/93662 discloses the preparation method and construction technology of high performance concrete. Portland cement or ordinary portland cement is used as the main component, and 8-30% calcium-aluminum-silicon composite admixture is added. The admixture is a composite water reducer or a composite antifreeze containing a plastic water-retaining agent. Adding 8- 15% expansion agent, the strength grade reaches C80-C 120, which has good compactness, can avoid cracks on the concrete surface and has good fluidity, and can be used for large-scale pumping construction. Compared with C40 strength concrete, the dead weight is reduced by 40% and the cost is reduced by 16%. It is especially suitable for beams and columns of long-span railways, highway bridges and high-rise buildings.
On August 5th, 2002, the United States published WO02/0627 19 high elastic concrete material, which consists of: an elastic polymer in an amount sufficient to make the final material elastic; Silicone ester increases the adhesion between elastic polymer and cement; Cement with low shrinkage and expansion coefficient; Preparation of filler and water treatment cement, as well as concrete materials. This material can also be used to repair cracks on the concrete surface [14].