Plastic shrinkage cracking in conventional and low volume fibre reinforced concrete

[摘要] ENGLISH ABSTRACT: Plastic shrinkage cracking (PSC) is the cracking caused by the early age shrinkage of concretewithin the first few hours after the concrete has been cast. It results in unsightly surfacecracks that serve as pathways whereby corroding agents can penetrate the concrete whichshortens the expected service life of a structure. PSC is primarily a problem at large exposedconcrete surfaces for example bridge decks and slabs placed in environmental conditionswith high evaporation rates.Most precautionary measures for PSC are externally applied and aimed to reduce thewater loss through evaporation. The addition of a low dosage of polymeric fibres toconventional concrete is an internal preventative measure which has been shown to reducePSC. The mechanisms involved with PSC in conventional and low volume fibre reinforcedconcrete (LV-FRC) are however not clearly understood. This lack of knowledge and guidanceleads to neglect and ineffective use of preventative measures. The objective of this study isto provide the fundamental understanding of the phenomena of PSC. To achieve theobjective, an in depth background study and experiments were conducted on freshconventional concrete and LV-FRC.The three essential mechanisms required for PSC are: 1→ Capillary pressure build-upbetween the particles of the concrete is the source of shrinkage. 2→ Air entry into aconcrete initiates cracking. 3→ Restraint of the concrete is required for crack forming.The experiments showed the following significant findings for conventional andLV-FRC: PSC is only possible once all the bleeding water at the surface has evaporated andonce air entry has occurred. The critical period where the majority of the PSC occurs isbetween the initial and final set of concrete. Any preventative measure for PSC is mosteffective during this period. The bleeding characteristics of a mix have a significant influenceon PSC. Adding a low volume of polymeric fibres to concrete reduces PSC due to the addedresistance that fibres give to crack widening, which increases significantly from the start ofthe critical period.The fundamental knowledge gained from this study can be utilized to develop apractical model for the design and prevention of PSC in conventional concrete and LV-FRC.

[发布机构] Stellenbosch University