Cracking due to plastic and autogenous shrinkage - Investigation of early age deformation of self-compacting concrete - Experimental study

Report, 2005

Early-age shrinkage and cracking have become reoccurring problem in concrete construction. Conditions such as reduced maximum aggregate size, increased amount of fines, presence of retarding admixtures, increased binder content and deficient covering and curing all contribute to this problem. At early age, when the cement paste is young and has poorly developed mechanical properties, autogenous and evaporating shrinkage, both incorporated in the plastic shrinkage, are the two main driving forces for cracking. For modern concretes developing large autogenous shrinkage, as the high performance and self-compacting concrete (SCC), early-age cracking is a highly topical and important area. When the concrete dries out due to evaporation there will be formation of water menisci near the concrete surface. The distance between the particles then tends to be reduced and the concrete will contract. These contracting capillary forces are in reverse ratio to the meniscus radius. The capillary tension stresses will therefore increase with decreasing interparticle spaces. For a concrete where the evaporation is prevented, the negative capillary pressure will also start to develop as the concrete sets. As a solid skeleton starts to form the chemical shrinkage is not totally transformed into external volume change and, if the water supply is restricted, empty pores will be formed inside the paste and water meniscus occurs. In this work, early age (< 24 h) autogenous deformation and crack tendency due to plastic shrinkage was measured. For the autogenous deformation, a specially developed digital dilatometer was used with great satisfactory, generating accurately measures of the linear displacements of the concrete cast in a vapor proof flexible tube mould. The plastic shrinkage cracking tendency was evaluated, using a modified Nordtest method (NT BUILD 433), where the concrete sample was exposed to early drying conditions and where the restraining inner steel ring causes development of tangential stresses which if sufficiently high leads to cracking. As the underlying mechanisms are not quantified by the ring-test and as plastic shrinkage strongly are related to the negative capillary pressure in fresh concrete, the test was complimented with transducer recording the development of the pore pressure. In some cases, pore pressure development was also measured on sealed specimens, referring to the autogenous deformation tests. A large number of different SCC constituents and mix compositions have been investigated; e.g. w/c-ratio from 0.38 to 0.67, coarse aggregate content, silica fume, fly ash, cement type, extra water and different admixtures (accelerator, retarder, shrinkage reducer, air entraining agent, superplasticizer dosage). For comparison, tests with standard concrete were made. The crack tendency test was also performed on mixes with different fibers and curing compounds. The influence of different constituents and mixes on the autogenous deformation and plastic shrinkage crack tendency was significantly observed. The results indicated that: • High crack tendency was generated when: - large autogenous shrinkage (silica addition, low w/c, high fineness) - high water evaporation (extra water, high w/c, low fineness) - retardation (retarder, slow hardening cement, high superplastisizer dosage), - low content of coarse aggregate • Minimum crack tendency at w/c 0.55 IV • Reduced crack tendency by: - shrinkage reducing admixture (large positive effect on both autogenous shrinkage and evaporation, without influencing the time to initial- and final set) - acceleration (accelerator, rapid cement) - air entraining agent - fibers - wax membrane (effectively for concretes with high evaporation) Finally the ring-test method and the experimental results were clearly verified by field studies.