Study On Restraint Shrinkage Cracking Peroperties Of HPC

Early-age cracking of concrete not only affects structural appearance,but also provides a additional transmission path for corrosive agent in environment to penetrate into the concrete,leading to an adverse effect on the durability and service life of concrete structure.Many factors contribute to the early-age cracking,among which the restraint surrounding structure members is the most important.At present,relevant investigations on the restrained shrinkage cracking of concrete are limited,especially the durability of concrete structure after experiencing shrinkage and cracking.In addition,the influence of potential restrains was unconsidered in structural durability design procedure.In view of this,in this dissertation,restrained slab tests were carried out on concrete with different restraints(unrestrained,restrained on two-side and restrained on four-side),water-binder ratios(0.25,0.30,0.35 and 0.40)and mineral admixture dosages(0%,20%,30%,40%and 50%).According to experimental results,effects of restraint conditions,water-binder ratios and mineral admixture contents on early shrinkage and post chloride ion permeability behavior after cracking and service life were investigated.The main contents and conclusions were presented as follows:(1)Restrained concrete slabs tests with different mix ratios were carried out with respect to three restraint conditions,i.e.unrestrained,restrained on two-side and restrained on four-side.The crack area per unit area,maximum crack width,cracking time and strain development were measured during experiment process.For the unrestrained shrinkage concrete slab,no obvious shrinkage cracking was observed.While for the restrained concrete slabs,the scenario is quite different.In detail,the concrete slab with four-side restraints has a higher degree of restraint compared with that with two-side restraints,which results in it’s a larger crack area per unit area and the maximum crack width.In addition,with the increase of amount of mineral admixture,crack area per unit area,the maximum crack width decrease and detected cracking time delays.Moreover,crack area per unit area and crack width of concrete show a monotonous increasing tendency with water-binder ratio.Based on the experimental results,lognormal probability distribution function was introduced to describe the shrinkage crack widths of concrete with different water-binder ratios,mineral admixtures and restrained conditions.Through statistical analysis,an effective method to determine the water-binder ratio and concrete admixtures was proposed,which can provide guidance for mix design of restrained concrete to meet the requirements of early crack control with different guarantee rates.(2)Shrinkage strain with two-side and four-side restrained concrete slabs was theoretically analyzed with consideration of concrete creep.The prediction formula for determining shrinkage cracking of concrete slabs was established.Research indicates,for concrete slab tests with both two-side restraint and four-side restraint,a good agreement was observed for the shrinkage strain variation trends between the calculation by the formula established in this dissertation and the measurement in experiment in Chapter 2.It should be noted that the calculated results of concrete slab with two-side restraint are better than those with four-side restraints.Based on the theoretical analysis of shrinkage and cracking of restrained concrete,the formula for predicting the shrinkage cracking of concrete members is derived.Considering the complex confinement conditions of concrete structures in actual engineering,the recommended value of confinement coefficient is given.(3)After continuous placing in environment for 28 days,core concrete specimens were drilled from the restrained shrinkage slabs to conduct electrical charge test.The effect of restrained shrinkage cracking on chloride penetration resistance of concrete was investigated.A prediction model of electric charge of restrained shrinkage concrete was proposed with taking consideration of water-binder ratios,mineral admixture contents and restraint conditions.Results show that,for the same concrete mixture,the electrical flux of cracked concrete is greater than that of uncracked concrete.In addition,the chloride ion permeation of concrete with shrinkage cracks is still dominated by the pore and crack of concrete material itself.And the existence of shrinkage cracks only improves the chloride ion permeability of the concrete to a certain extent.The electric flux prediction model proposed in this dissertation can effectively reflect the effects of restraint conditions,concrete water-binder ratios and mineral admixture contents on chloride penetration resistance of concrete.Considering complex restraints of concrete structures in practical engineering,the recommended values of restraint coefficients are provided.(4)A chloride ion penetration model was established considering the restraint conditions and shrinkage cracking of concrete in chloride salt environment.Based on the model,probabilistic analysis for the service life of reinforced concrete structures in chloride salt environment was carried out.Results indicate that structural restraint degree has a greater effect on the service life of reinforced concrete structure.In detail,the greater of the restraint degree,the earlier the time of steel corrosion caused by chloride ion penetration,concrete cracking and structural member losing functions.