Uppkomst av plastiska krympsprickor: Ny försöksuppställning och inverkan av cementtyp och vindhastighet
Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
The cause of early cracking in newly poured concrete slabs is often the plastic shrinkage. Plastic shrinkage occurs during the first hours after casting when the concrete is exposed to premature drying. During this time the concrete still is plastic, and could be vibrated and surface finished. Thus, the actual hardening has not started. In cases when the shrinkage is subjected to resisting forces tensile stresses can arise in the material and cracks may occur. The plastic shrinkage is assumed to be due to two simultaneous water transport in the concrete mass, namely dewatering (water transport from the concrete slab’s interior to the surface) and evaporation (water leaving the surface). When the evaporation exceeds the dewatering, the surface will dry out and the risk of cracking increases. Factors affecting evaporation and dewatering also affect the risk of plastic shrinkage cracking formation. Previous research highlights, among other things, wind speed, air temperature, humidity, concrete consistency, cement, additives and variations of specimen thickness as risk parameters. To counter the plastic shrinkage cracking formation, some finishing work can to be done. The literature suggests, for example, water curing, membrane curing, application of plastic wrap and, if necessary, the establishment of windbreaks and sunshades.The master thesis includes eight laboratory tests in which the occurrence of plastic shrinkage cracks in concrete with vct 0,38 has been studied at different cement types and under varying wind speed. The cement types that have been compared are ordinary Portland cement (byggcement) and base-cement (bascement). The Portland cement is now commonplace in the workplaces while the base-cement is going to be introduced on the market in 2013 and 2014.In order to study the phenomenon of plastic shrinkage cracks an experimental setup was built in which a concrete slab with dimensions 1200x400x80 mm was cast. The model is based on a previously used equipment but with some modifications and a more sophisticated measuring equipment. By making use of a wind machine a wind over the plate surface could be simulated. In the experimental tests the evaporation of the specimen, the concrete temperature, the air temperature, the relative humidity and the wind speed was measured. The evaporation measurement was performed by continuously weighing the concrete slab.The results from the experiments indicate that the base-cement gives rise to fewer cracks than the Portland cement. Also the time until the first crack was observed was longer for the base-cement which would mean that the possibility of curing the surface would increase. In the study no significant temperature variation between the cement types was observed. Further, a higher wind speed gave rise to a higher evaporation and a shorter time until cracking was noticed. The master thesis also includes studies of the phenomenon itself and theoretical analysis of different models of plastic shrinkage cracking formation. Various formulas have been studied and the measured evaporation agreed well with the theoretically calculated evaporation during the time the concrete was in its plastic state.
Place, publisher, year, edition, pages
2013. , 98 p.
Teknik, Examensarbete, Plastisk krympning, Plastiska krympsprickor, Avdunstning, Laborationsförsök
IdentifiersURN: urn:nbn:se:ltu:diva-57828Local ID: e735f419-8ec1-4927-bdb7-6121365364fdOAI: oai:DiVA.org:ltu-57828DiVA: diva2:1031216
Subject / course
Student thesis, at least 30 credits
Civil Engineering, master's level
Validerat; 20130225 (global_studentproject_submitter)2016-10-042016-10-04Bibliographically approved