Research On Control Of Shrinkage Cracks In Super-long Concrete Structure

At present, the cracks in concrete structures due to load or temperature variationhave been efficiently controlled, attributed to well-established design theories.However, cracks due to conrete shrinkage always arise in many engineering structures,which threaten the durability of the structures even the serviceability and safety.Concrete shrinkage is the most important inducement to cracks for the super-longstructures with thin-wall.Focusing on the concrete cracking due to shrinkage in super-long structures, freeshrinkage tests of concrete blended with fly ash and slag are conducted, analysis ofdrying shrinkage prediction models and finite elment simulation for specimens andthe super-long reinforced concrete strucrue are performed in this reseach. The detailedwork is stated as follows:According to the shrinkage tests of concrete blended with fly ash and groundgranulated blast furnace slag in natural environments, the effects of fly ash contentand slag content on concrete shrinkage were investigated. It is shown that theshrinkage decreases as the fly ash content heightens whereas the influence of slagcontent on shrinkage is unobvious from the results. Incorporating the expansiveadditive or the polypropylene fiber can also decrease the shrinkage. The effect ofbinary mixture on shrinkage reducing is the most efficient. The expansive additiveused individually reduces shrinkage more than the polypropylene fiber individually,but reduces little than those used in combination.The national and overseas prediction models have been applied to calculate thedrying shrinkage for the specimens without the expansive additive or thepolypropylene fiber. It is shown that none of the shrinkage prediction modelsadequately reproduces the observed shrinkage behaviour of concrete made with flyash and slag from the comparison between the tests and the calculation. According tothe short-term experimental data, a new prediction model contaning the effect of flyash and slag is established by calibrating and modifying the existing shrinkage models.The predicted shrinkage strains from the proposed model agree well with themeasured.On the basis of the nonlinear moisture diffusion theory, the finite elementmodeling of shrinkage tests and inverse analysis of related calculation parameters for the concrete without the expansive additive or the fiber are conducted by ANSYS. It isshown that the humidity gradient of outer region is more than that of inner region inconcrete during drying from the simulation results. However the relative humidity ofouter region is less than that of inner and they gradually decreases with drying timeeverywhere. The environmental humidity significantly influences the shrinkageevolution while the period of moisture curing influences the short-term shrinkage only.It is shown that the drying shrinkage is insensitive to the maximum of the diffusioncoefficient and the suggestion on the diffusion coefficient in the CEB-FIP (1990)Model Code is practicable and reliable from the inverse analysis results.The finite element modeling of the super-long wellhead structure is performedused the identified parameters values. The influences of the optimization of mixproportion as well as the setting of post-casting strip on the moisture field andshrinkage-induced cracks in the wellhead structure are discussed. The results showthat the cracking time can be significantly delayed by the optimization of mixproportion due to the slower moisture diffusion, but post-casting strip can slightlydelayed the cracking time or reduce the cracking area for it influences the moisturefield around the strip only. The effect of cracking resistance by the post-casting strip issmaller than by the optimization of mix proportion.