Research On The Influence Factors For The Shrinkage Of Reactive Powder Concrete

ABSTRACT:It is generally believe that the shrinkage of reactive powder concrete(RPC) is larger than that of ordinary concrete in early age, cause of its extreme low water binder ratio, coarse aggregate free, large amount of mineral admixtures and superplasticizers. The risk of shrinkage cracking is significant. In order to study the early-age shrinkage of RPC, and put forward measures to improve the early shrinkage,75℃steam curing is chosen as the basic curing condition for RPC. Theinfluence of different curing systems, mineral admixtures and chemical admixtures on the early shrinkage is studied.Meanwhile,based on chemical combined water test and thermo-gravimetric analysis,the hydrationdegree and hydration products of RPC paste is investigated. The early-ageshrinkageis further discussed.The main work and conclusion are as follows:(1)The shrinkage of RPC mainly occurs in the early stage,and its development can be divided into five phases. The shrinkage will increase greatly in curing period if the humidity improved.The curing temperature may also promote the shrinkage while curing,but it won’t be obvious when the temperature is higher than40℃.High temperature curing is beneficial to reduce the dry shrinkage,since curing temperature is the main factor for the drying shrinkage after curing.(2)Substituting fly ash or slag for silica reduces the early shrinkage of RPC effectively.The more amount of fly ash is higher,the more shrinkage reducing.A low content of slag substitute silica has obvious reduction effect.Compared with slag,fly ash is better to reduce the shrinkage in the early stage. The optimal dosage of shrinkage reducing agent is1%, it can significantly inhibit the shrinkage of RPC while curing.During the former and curing stage.Expansive agent is good compensation for shrinkage.And the content should not be greater than2.5%.(3)In the curing period,the hydration degree ofpaste specimens can significantly increase when the curing temperature and humidity are improved,accordingly it will reduce after curing. Fly ash can retard hydration process of cementing material efficiently in each age,especially during curing. Slag can also retard hydration process of cementing material during and after curing to different degrees.Shrinkage reducing agent can reduce the hydration degree of cementing material before and after curing.Different amount of expansive agent have different influence degree to the hydration of cementing material in each age. (4)A large number of Ca(OH)2generated before thecontrol paste being cured,and a lot of high density C-S-H generated during curing. The Ca(OH)2contentcan be increasedduring the whole life not only by lower curing temperature,but also by the replacement of alternative silica fume by fly ash and mineral powder. TheCa(OH)2content of group mixed with expansive agent is higher than the control group before curing, but it presented a falling trendafter that.It can be concluded that the expansion agent compensation shrinkage occurs mainly in the early, especially before curing,and shrinkagecompensation ability is no longer apparent during the late.