Research On Durability Of Frozen-thawed Concrete And Bond Behaviors Under Load Coupling

Frost damage of buildings in contact with water, bridge deck and pavement in thepresence of deicing salt often occurs in cold regions. The treatment will be ratherdifficult if concrete is damaged due to freeze-thaw cycles. Meanwhile, it will be aserious threat to the service life of structures because of combined action of freeze-thawcycles and other factors, such as carbonation and chloride erosion. Permeablewaterproof coating has been investigated extensively for its perfect protectionperformance and little influence on the permeability of concrete. However, it is rarelyused to improve the durability of frozen-thawed concrete. Actually, concrete structuresin service are suffered mechanical load in addition to the influence of environmentalfactors. Nevertheless, so far there haven’t been any reports on the bond behaviorbetween concrete and steel bar under the coupling action of freeze-thaw cycles andsustained load. In this thesis, the major researches are summarized in the following.（1）The deterioration mechanism of non-air-entrained concrete under the fourmanners of freeze-thaw action has been investigated. Then, the frost damagedmechanism of concrete was preliminary discussed. It has been shown that the manner offreeze-thaw action has an important effect on frost failure. The degree of the importantinfluence from large to small was ordinal: freeze-thawing under soaking, freeze-thawingunder soaking and air cycles, freeze-thawing for saturated concrete under air and freeze-thawing for non-saturated concrete under air. Based on the analysis of freeze-thawprogress, the failure mechanism of concrete was mainly icing pressure in some pores ofconcrete during freeze-thaw cycles. Reducing the degree of saturation in concreteduring freeze-thaw cycles can evidently improve frost resistance of concrete.（2）The effect of water-cement ratio, drying temperature, submerged depth,environmental temperature and relative humidity on sorption capacity in the concretehas been investigated by the capillary absortion test. Through the macroscopical and microcosmic experiment, the permeability of concrete after freeze-thaw cycles also hasbeen studied. After surface treatment with silane gel, the water absorption property,influence of surface erosion and mechanism of water penetration have beeninvestigated and analyzed for frozen-thawed concrete through water absorption testlastly. The results indicate that the coefficient of capillary suction increases remarkablywith the number of freeze-thaw cycles. Moreover, the increasing tendency is obviousfor concrete with high w/c ratio. However, water absorption capability of treatedconcrete reduces sharply compared with that of untreated concrete. If the surface isabraded, the thickness of the hydrophobic layer will decrease so that the efficiency ofwater repellent treatment will be reduced. But for the frozen-thawed concrete treatedwith above300g/m2of silane gel, it still had superior water repellency even if thesurface concrete is abraded off4mm. As for concrete with severe frost damage, thecompactness of the hydrophobic layer will become loose. This will lead to the changeof water migration mechanism from―vapor transmission‖into―water transfer‖. So itwill be the fundamental reason for the early failure of water repellent treatment ofconcrete under frost environment.（3） Experimental study on the performance of chloride diffusion offrozen-thawed concrete was carried out. The results show that the influence of frostdamage on the chloride diffusion is notable. The regression analysis result shows thatthe exponential function relationship between damage and coefficient of chloridediffusion can be well established：DF=D0·ek·ω. Based on Fick’s second law of diffusion,a new diffusion equation is deduced and a theoretical model of chloride ion in concreteis obtained. In this equation, the influence of damage, surface scaling and convectiondepth are taken into consideration. The theoretical model is verified on the basis ofdata from Old XIZHIMEN Cloverleaf Junction of Beijing and Jiaozhou Bay SubseaTunnel; and it meets the simple and practical requirements in the field applications to agreat extent.（4）Exposure test on carbonation resistance and chloride penetration resistanceof frozen-thawed concrete treated with silane gel has been carried out. Then singlesurface deicer-frost test of concrete treated with different amount of gel from100g/m2to400g/m2have been studied by a quick freezing method. Lastly protectivemechanism of the treatment was analyzed. The results indicate that water repellenttreatment has different effect on carbonation resistance of frozen-thawed concrete. Asfor chloride penetration and surface scaling, the treatment has a beneficial protective effect. Based on the test results, the coefficient of frost damage and permeablewaterproof coating are introduced to modify the prediction model of carbonation depth.The minimum amount of silane gel for frozen-thawed concrete is suggested to resistchloride penetration lastly.（5）Thirty-six pullout specimens of ordinary concrete and air-entrained concretehave been manufactured firstly. The method of coupling action of sustained load andfreeze-thaw was developed by special equipment. The effect on the bond behavior wascarried out through the pullout test. The unglued failure due to the difference of linearthermal expansion coefficient between concrete and steel bar during freeze thawcycleswas proved by calculation. On this basis, degradation mechanism of bond behaviorunder the coupling action of sustained load and frost was revealed. The results showthat the sustained load nearly has no effect on the bond behavior if the damage degreeof ordinary concrete is low. While the damage degree of ordinary concrete is great,such as the index of damage is0.5, a low level of sustained loading can cause sharpbond degradation. But for pullout specimens of air-entrained concrete, the couplingaction of sustained load and freeze–thaw has no apparent effect on the bond behavior.（6）The influence of repair materials, transverse reinforcement and freeze-thawcycles on the bond behaviour has been investigated by the pullout test. Somesuggestions were put forward on the basis of test results. As a new material, strainhardening cement-based composites (SHCC for short) has many advantages, such asgood frost resistance, crack control capacity, corrosion resistance and better bondproperty with old concrete. It can also improve bond strength and ductility of bondfailure. Meanwhile, the influence of freeze-thawcycles on bond behaviour is improvedgreatly. Thus, it is very helpful to prolong the remaining service life of concretestructures.