Freeze-thaw Damage Model And Service Life Prediction Of Concrete Under Multi-factor Coupling

In fact, concrete in service is usually subject to complex environmental factors.Its deterioration is a process with coupling action including physics, chemistry,mechanics and so on. The damage caused by these coupling factors is far greater thanthat caused by a sum of each action. To guide the durability design of concrete inpractical projects, the research tendency on concrete durability and service lifeprediction system is developing from single factor to multiple factors. But there aresome problems on freeze-thaw durability research of concrete under multi-factorcoupling. Firstly, the durability research of concrete under multi-factor coupling alluses the test parameters under single factor. In other words, the evaluation method islagging behind the actual durability research. Secondly, the present damage modelsand service life prediction system of concrete are mostly established under singlefactor. Finally, lack of summary for the present research results of concrete durabilityunder multiple factors leads to that it is not easy to perfect the research system andrealize the accuracy and continuity of service life prediction system.To solve the problem above, a database system of concrete freeze-thawdurability as one of the premises to establish service life prediction models wasdeveloped to summarize the previous test results and reference models. Then strainwas used to analyze the damage and deterioration in concrete under multi-factorcoupling as another premise to establish service life prediction models. Finally, twofreeze-thaw damage models were established and used to evaluate and predict theservice life of practical engineering combined with freeze-thaw comparing relationbetween inside and outside, providing a new approach to predict the service life andguidance for durability design of concrete which is required for frost resistance. Thefollowing research results were obtained.1) A database system of concrete freeze-thaw durability was developed. With theaid of RILEM TC246-TDC as the platform, the durability research results, thefreeze-thaw damage and service life prediction models of concrete under multi-factorcoupling were collected from34famous scholars who are from15countries. Theexperimental data and models in the database were filtered strictly. These materialsscience results were combined with the computer technology, creating an integrateddata management platform. The database system has the feature of goodacross-platform, powerful expansibility, etc and the function of query and search.2) Strain was used to characterize the concrete durability under multi-factorcoupling. Strain may have certain inherent advantages when compared to other testparameters, such as real-time nondestructive monitor, more accurate and continuouswith little error that is caused by manual intervention, etc. Under single factor offreeze-thaw cycles (FTC), the produced residual strain proves that the damage of concrete matrix is a continued accumulated and irreversible deterioration process.This process is directly related to the continued initiation and propagation of cracks inmatrix. Under freeze-thaw cycles coupled with chloride attack (FTC-Cl), the residualstrain produced in3.5wt.%NaCl solution is larger than in water, showing that theNaCl solution accelerates the freeze-thaw damage of concrete. Under freeze-thawcycles, chloride attack coupled with bending stress (FTC-Cl-BS), the strain andresidual strain of concrete tensile region are both larger with the larger constantbending stress ratio, showing that the applied stress accelerates the damage ofconcrete.3) The residual strain freeze-thaw damage evolution equation was established.Under FTC, FTC-Cl and FTC-Cl-BS, the residual strain variation all conforms to asingle-segment variation mode or a double-segment variation mode. Thesingle-segment variation mode applies to the concrete that has better frost resistance,and the double-segment variation mode applies to the concrete that has poorer frostresistance. Furthermore, the validation of residual strain freeze-thaw damageevolution equation shows that there is good agreement between computed data andmeasured data.4) The freeze-thaw damage numerical model of concrete subject to FTC-Cl wasestablished based on residual strain. This damage numerical model applies to both thesingle factor of FTC and the two factors of FTC-Cl. The correlation coefficients of thedamage degree variable curve of concrete fitted by the model are all greater than0.95.The model can also predict the final number of rapid freeze-thaw cycles of concreteunder certain conditions, and then can predict the safe service life of concrete inservice under certain environmental factors combined with service life predictionmethod.5) Converting the microscopic action mechanism to a macroscopic mechanicaleffect, the damage mechanical model of concrete subject to FTC-Cl-BS wasestablished based on strain. This damage mechanical model applies to both twofactors of FTC-BS and three factors of FTC-Cl-BS, and establishes a relationshipbetween strain and number of rapid freeze-thaw cycles. The model computed data ofthe final number of rapid freeze-thaw cycles is very close to the measured data. Themodel can predict the safe service life of concrete in service under certainenvironmental factors combined with service life prediction method.6) Associating the established damage models with service life prediction ofpractical engineering, the service life of practical engineering was predicted. Bycomputing, the predicted service life of concrete of the airport runway and the tarmacused in Changbaishan airport are43years and52years respectively, meeting therequirement of the designed minimum service life of30years. The predicted servicelife of concrete of the bearing platform and the box girder used in Qingdao BayBridge are144years and110～123years respectively, also meeting the requirement of the designed service life of100years.