Studies On Chemical Change Type Self-healing Material For Cracks And Its Influence On Durability Of Concrete

The durability of concrete is an important research subject in the international engineering industry. A lot of researches indicated that the impermeability is the key factor to affect the durability of concrete. Therefore, the important path to improve the durability of concrete is to enhance the impermeability of concrete and make it gain the excellent self-healing function of cracks, which is of great significance to improve the durability of concrete. Based on analyzing the mechanism of self-healing function for cracks, a novel kind of self-healing materials for the cracks of concrete was prepared, and researched its self-healing ability and working mechanism, the influence of self-healing material on the durability of concrete was studied systematically also. The following conclusions and innovation can be obtained:1. Large numbers of literature and datum were consulted and analyzed in this paper, the mechanism of chemical change self-healing reaction was proposed for the first time. Based on this theory, a novel kind of chemical change type self-healing materials (CCSM) for cracks of concrete with good performance was prepared.2. CCSM could improve dramatically the impermeability and compressive strength of the cement-based materials (mortar or concrete). The first impermeability of mortar could be raised from 0.4MPa to higher than 1.4MPa as well as the secondary impermeability could be raised from 0.1 MPa to 1.2MPa. Moreover, the compressive strength was increased from 20.41 MPa to 23.54MPa either.3. The researches on secondary and multiple impermeability of coated specimens with CCSM showed that CCSM could improve dramatically the self-healing ability. Further studies on the impermeability of specimens which the coating have been removed indicated that the active compound of CCSM could penetrate fast into the inner of concrete, and cause a chemical change reaction to increase the impermeability of concrete and endow the concrete with a self-healing function.4. In this paper, the microstructure of concrete is studied by means of XRD, SEM and MIP, and the permeate depth of active compound was measured by chemical analysis also. All the results showed that the active chemical material has very strong ability of pervasion, it could penetrate into the inner of concrete in shorter time. The active compound of CCSM could cause a chemical change reaction between Ca(OH)2 and unhydrated cement, which generate a great quantity non-soluble dendritic crystalline. These dendritic crystalline blocks up the pores and cracks, decreased clearly the porosityof concrete from 36.4% to 14.8%, and reducing the number of the worst pores (the aperture was greater than 200nm) and worse pores (the aperture was between lOOnm and 200nm) in great extent. That is to say, CCSM made the inside structure of the cement concrete more compacted, inhibiting the pervasion of water and aggressive mediator. Moreover, the active chemicals of CCSM can be recovered after the reaction so that it can work forever, and endow cement-based materials with a self-healing function.5. In order to display thoroughly the chemical change function of active compound, the content of cement should be more than 10% of the cement-based materials. There are many factors which affected the chemical change function of CCSM, for example, the dosage of CCSM, the concentration gradient of active compound, pervasion time, the pore structure of concrete, the content of cement and the humidity of Surrounding environment etc.6. CCSM have concrete possess ability for chemical resistant and freeze-thaw resistant. The compressive strength of specimens treated by CCSM improved more than 28% immersed in dilute hydrochloric acid or sodium hydroxide solution for 3 months, and the compressive strength of specimens decreased about 10% immersed in potassium sulfate solution yet, the impermeability of all specimens increased from 1.4MPa to higher than 1.5MPa. However, the compressive strength of uncoated specimens decreased respectively 20 %, 18% and 27%, its impermeability debased from 0.6MPa to 0.4MPa either. Moreover, its loss of compressive strength after 150 freeze-thaw cycles was 3.5% that was much better than that of uncoated specimens whose loss was about 15.6%. The mechanism may be that the active chemical materials could blocks up the pores and cracks through the chemical change reaction, reducing the number of the capillary porosity (the distribution of aperture was between lOOnm and lOOOnm) in great extent, and decrease the porosity of concrete, inhibiting the pervasion of water and aggressive mediator. Therefore, it evidently debased the freeze-thaw injury caused by the water of capillary porosity, and improved effectively the ability of concrete for chemical resistant and freeze-thaw resistant.7. CCSM could effectively inhibited Alkali—Aggregate Reaction(AAR) of concrete. When the dosage of CCSM was 3% of the cement, its expansion ratio of length is only 1/9 of the uncoated specimens for AAR. The mechanism for inhibiting AAR may owe to the chemical change reaction of active chemical materials which obstructed the diffusion of water and alkali, weakening the superficial hydroxylation of aggregate, obstructing the diffusion of Na\ K+f P Ca2+ to the surface of aggregate that benefit the creation of steady solid structure and avoid the occurring of expansion by sopping water.8. The results of water absorption and chloride permeability of cement mortar indicated that CCSM could debase the water absorption of concrete from 5.91wt% of the uncoated specimens to 0.23~0.56wt%, the chloride diffusion coefficient was usually decreased to 20 % of the uncoated specimens. The steel corrosion test showed that the state of steel was in the passive state when the specimens were treated by CCSM. It is clearly that CCSM could prohibit water, chloride ion and any soluble chemical solution from penetrating the concrete to cause corrosion, leaking and other problems, therefore, CCSM was beneficial to the protection of corrosion of concrete.9. In this paper, according to the diffusion theory, the equation of diffusion of CCSM in concrete was established, adopting the method of finite element analysis for the first time to simulate and predict the pervasion of active chemical material in concrete, the data of pervasion depth and velocity of active compound for different time was gained. The results of predicted model of active compound match the actual value. It is proved by the contrast with the measured value and simulated value that the method of finite element analysis could be used effectively for the pervasion prediction of active compound and confirmed the strong pervasion ability of CCSM by theory that is favorable to the application of CCSM in construction work.