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| Author(s): |
Fábio Cabral de Oliveira
Total Authors: 1
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| Document type: | Master's Dissertation |
| Press: | São Paulo. |
| Institution: | Universidade de São Paulo (USP). Escola Politécnica (EP/BC) |
| Defense date: | 2020-12-03 |
| Examining board members: |
Sérgio Cirelli Angulo;
Wellington Longuini Repette;
Luis Marcelo Marques Tavares
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| Advisor: | Sérgio Cirelli Angulo |
| Abstract | |
Hydrated cement pastes are brittle materials and therefore their mechanical performance is governed by pre-existing defects in the microstructure. During production and molding, critical defects (large pores and cracks) may be introduced causing a drastic reduction of the mechanical strength. The occurrence and distribution of such defects are random, which intensify the dispersion of the mechanical properties. Knowing the variability is important to determine reliable characteristic values; nonetheless, little attention is given to the subject. In this context, the present work evaluates the variability of the tensile strength and elastic modulus of cement pastes, correlating it to the microstructure and occurrence of critical defects. Among the variables investigated are the water/solids ratio, the content of limestone filler (in replacement and addition to cement) and the content of superplasticizer. For each composition, at least 40 small cubic samples (~10 mm) were cast to ensure statistical representativeness. The mechanical properties were determined through a simple and practical test, based on the Point Load Test and the Hertz elastic contact theory. The behavior of tensile strength and elastic modulus were evaluated through probabilistic analyses using the Weibull and lognormal distributions, respectively. Mercury Intrusion Porosimetry (MIP) and X-ray Microtomography tests were performed to correlate the dispersion of the mechanical results with the microstructure of the samples. The results showed that the effect of limestone filler on the open porosity depends on the strategy used: in replacement to cement (constant w/s), the porosity increases; on the other hand, in addition to cement (constant w/c), the porosity decreases. The magnitude of the elastic modulus was only affected by the total porosity, in a negative way, and the data variability was low regardless of the composition of the paste. The increase in total porosity also tends to reduce the magnitude of the tensile strength; however, the presence of limestone filler can compensate this effect and improve the mechanical performance thanks to toughening mechanisms during fracture. Tensile strength variability is relatively high due to the random distribution of critical defects; however, it can be substantially reduced with the inclusion of limestone filler and with the increase of porosity, which is also due to the increase in toughness. The occurrence of bleeding/segregation after casting the samples significantly increased the variability of the mechanical results. The critical diameter determined by the MIP had a strong negative correlation with the mechanical strength, and the w/c ratio seems to be the main factor affecting its dimension. (AU) | |
| FAPESP's process: | 18/07623-7 - Mechanical properties of cement pastes: microstructure, variability and probabilistic analysis |
| Grantee: | Fábio Cabral de Oliveira |
| Support Opportunities: | Scholarships in Brazil - Master |