Research Of The Degeneration Of Bearing Capacity Of Prestressed Concrete Bridges

Pre-stressed concrete (RC) bridges have been faced the rapid degeneration ofperformance, the fast growth of road traffic and the overload problems and many ofthem have suffered the decline of durability and a degeneration of bearing capacity.Domestic and foreign scholars have explored the depth of the concrete carbonization,chloride penetration and steel corrosion in depth and have made many achievements,but less of RC structures. As the reason, it has a big significance to carry out studiesof the degeneration of bearing capacity of PC bridges.Combined with auxiliary bridge of South-to-North Water Diversion Projectbridge of Zhengzhou-Shaolin Expressway, by means of the comparison andanalysis of different concrete carbonization models and takes the characteristics ofPC bridges into account, we select a concrete carbonization model based on “watercement ratio and cement consumption” with the consideration of concrete curingcondition, the dosage of admixture and the influence of environmental conditions,and propose the diffusion coefficient model considering multiple factors, study andgive the corrosion model of steel and pre-stressed steel strand under the carbonationand chloride erosion environments respectively。At last, cross sections at mid-spanand1/4of the span are selected respectively to predict their bearing capacity in thewhole design phase under the carbonation and chloride erosion environments. Themain work done and conclusions obtained in this paper about the degeneration ofbearing capacity of PC bridges are as follows:1. The mechanism and main factors of concrete carbonization are elaboratedsystematically. Calculation models of the depth of the concrete carbonization areanalyzed. By the comparison with the values of rapid carbonization test andengineering carbonation depth, it is proved that Zhang Yu model and Niu Ditaomodel are superior to other similar models. Through integrated into account, ZhangYu model is modified and verified, which shows that the new model is more suitable for predicting the depth of the concrete carbonization of pre-stressed concretebridges in Henan province.2. The main influencing factors of chloride diffusion coefficient aresummarized. Taking the influence from water cement ratio and the kinds anddosage of admixture and time and temperature and the combination capacity ofconcrete to chloride ion and the degeneration of concrete and stress level accountintegrally, the diffusion coefficient model considering multiple factors are proposedand verified with engineering example.3. Corrosion models of steel under the concrete carbonation environment arecompared and suggestions of use are given, corrosion model under chloride erosionenvironment are proposed on the basis of research results. The comparation betweencalculation results shows that corrosion resulted from chloride ion erosion is muchmore serious than that resulted from concrete carbonation. Strength degradationcoefficient and coordination coefficient between concrete and reinforcing bars arecompared and analyzed and the applications of coordination coefficients are pointedout.4. In combination with pre-tensioning method and post-tensioning method theirown characteristics, calculation models of the corrosion of pre-stressed steel strandunder the two erosion environments of pre-tensioned and post-tensioned PC bridgesare given respectively. The calculation formula of effective cross-sectional area andthe reduction coefficient of strength of pre-stressed steel strand rusted andcollaborative work coefficient are selected. On the basis of the current specification,the calculation methods of carrying capacity of PC bridges rusted are established.5. The bearing capacity in the design phase of the auxiliary bridge under thetwo erosion environment are predicted respectively: without considering theconstruction factors and conditions resulted in the crack of the concrete such asoverload, under the concrete carbonation environment, flexural bearing capacity is athigh level all the time, corrosion and damage of the concrete section result adegradation of the shear bearing capacity but not dramatically, two kinds bearingcapacity meet the design requirements; under the deicing salt chloride erosion environment, degradation of flexural bearing capacity cannot be ignored especiallyafter cracking due to corrosion expansion of Pre-stressed steel strands, obliquesection shear bearing capacity has a certain degree of degradation with cracking dueto corrosion expansion of stirrups appears.