Studies On Durability And Service Life Prediction Of Hydra Ulic Concrete In Northren China

As an important part of national economic infrastructure,water conservancy projects play an irreplaceable role in the safety of flood control,the rational utilization of water resources and the promotion of national economic development.Due to the complicated technology,long construction period and large investment of large-scale water conservancy projects,the durability requirements of the structure of water conservancy projects are particularly strict.The common problems of concrete structures in water conservancy projects are cracking,freezing-thawing damage,steel corrosion and alkali aggregate reaction and reezing-thawing damage of hydraulic mass concrete is particularly serious in cold regions of north China.The weakest part of the durability for hydraulic concrete is the water level change area which shows the real service state of this area is the action of the dry-wet and freeze-thaw cycle under stress.Due to the problems above,it is of great significance to study the durability and life prediction of hydraulic concrete in north China for the safety and long-term operation of hydraulic engineering.Due to the probolems of using alkali-active aggregates and shrinkage-induced fracture in the later stage of hydration of some hydraulic mass concrete,the paper has taken the hydraulic mass concrete in the nroth inland and coastal areas as the research object.The effects of three admixtures(fly ash,light burned magnesium oxide and polypropylene fiber)on compressive strength,dry shrinkage deformation,frost resistance and spontaneous volume deformation of concrete were studied.The capability of chloride corrosion resistance and anti-freezing of concrete under the action of coupling multiple factors were analyzed.A concrete life prediction model was established based on freeze-thaw damage theory,and the model was verified and applied through practical engineering cases.The main research contents and conclusions of this paper are as follows:(1)The light burned magnesium oxide,fly ash and polypropylene fiber are taken as admixtures.A orthogonal experiment was carried out to study the influence of admixture type and content on compressive strength,dry shrinkage deformation,frost resistance and spontaneous volume deformation of concrete.Adding the three kinds of admixtures at the same time with reasonable dosage can improve concrete compressive rresistance,compensate concrete shrinkage deformation and improve crack resistance.Corresponding to the materials used in this paper,the reasonable content of light burned magnesium oxide is 1%?5%,that of polypropylene fiber is 0.5kg/m3?1kg/m3,and that of fly ash is 20%?40%.(2)The types of concrete samples containing alkali-active aggregate:adding fly ash alone and adding light burned magnesium oxide and fly ash at the same time were prepared.Accelerated reactions of 28d to 4a were applied to the prepared concrete samples.The experiments of freezing-resistance and autogenous volume deformation for concrete containing alkali-active aggregate and different admixtures after different reaction times were carried out.The reasonable fly ash content can effectively inhibit the alkali-silicic acid reaction of alkali active aggregate,but it can not inhibit the hydration spontaneous volume expansion of light burned magnesium oxide,which can still compensate for concrete shrinkage and prevent the occurrence of concrete cracks effectively.(3)A coupling test method of dry-wet cycle and freeze-thaw cycle considering the regional environmental characteristics for concrete was designed.The chloride diffusion properties and freeze-thaw damage of concrete with different admixtures under the action of dry-wet cycle and coupling of dry-wet cycle with freeze-thaw cycle under chloride environment were studied.Concrete with different admixtures has different resistance to chloride ion erosion and frost resistance.The erosion of chloride ion under the dry-wet cycle results from the diffusion of concentration gradient.The micro-cracks caused by freezing-thawing damage under the alternating coupling of dry-wet cycle and freezing-thawing cycle damage provide permeation channels for chloride ions,which can reduce the ability of concrete to resist chloride ion erosion.The damage of concrete caused by chlorine salt erosion and dry-wet-freeze-thaw cycle can be reflected by the change of mass loss rate and relative dynamic elastic modulus.(4)A freeze-thaw experimental device considering compressive stress was developed.The effects of light burnt magnesia and polypropylene fibers on the freeze-thaw damage of concrete under compressive stress were studied.Polypropylene fiber can effectively improve the frost resistance of concrete,while light burnt magnesium oxide has no obvious effect on the relative dynamic elastic modulus and mass loss of concrete.When the compressive stress ratio is less than 0.2,the compressive stress is beneficial to the anti-freeze performance of concrete.However,when the compressive stress ratio is greater than 0.6,the internal cracks of the concrete develop rapidly,which has obvious adverse effects on the frost resistance of the concrete.The research results can provide reference for the future frost resistance design of hydraulic large volume concrete.(5)Based on the theory of freeze-thaw damage and the research on multi-field coupled freeze-thaw experiments,the Weibull distribution model was optimized and improved.The hydraulic concrete 1:1:4 cuboid standard antifreeze test piece was taken as the physical model,and the freeze-thaw damage model which is more in line with the rapid freeze-thaw test standard sample was obtained.A concrete life prediction method based on this model was proposed.In order to solve the problem of the initial uncertian damage of the coring concrete on site,an iterative approximation method was proposed.The life prediction and verification were carried out with reference to the 35-year and 45-year freeze-thaw test data of the Shimen Reservoir project.The verification results show that the model can better describe the damage process of concrete freeze-thaw,and then predict the life of Wu Niubao sluice gate concrete.