Cracking in concrete : crack prevention with air-cooling and crack distribution with steel fibre reinforcement

Abstract: This Licentiate Thesis consists of four parts. The first two parts, paper A and B, deal with air-cooling by means of embedded pipes in order to reduce thermal cracking in concrete structures. These two papers are submitted for publication to RILEM Journal; ‘Materials and Structures'. The next part, paper C, treats fracture mechanics properties of steel fibre reinforced concrete, SFRC. Paper C has been published in the proceedings of a conference on utilisation of high performance concrete in Paris 1996, ‘BHP-96'. The last part, paper D, is a compilation of results and evaluations of tests performed after 1994 on SFRC at the Division of Structural Engineering at Luleå University of Technology.Paper A: Hedlund H., Groth P., (1996), ‘Air-cooling of concrete with embedded pipes. - Part I: Laboratory tests of heat transfer coefficients', RILEM Jour-nal: Materials and Structures, submitted for publication. Embedded cooling pipes may be used in order to reduce the temperature rise in massive structures as a measure against thermal cracking. A necessary parameter in a calculation of the risk for thermal cracking is the heat transfer coefficient of the cooling-pipe. In this paper this coefficient have been determined for two different types of pipes and for various pipe flows and temperature levels.Paper B: Groth P., Hedlund H., (1996), ‘Air-cooling of concrete with embedded pipes. - Part II: Applications in design', RILEM Journal: Materials and Structures, submitted for publication. The experimentally determined heat transfer coefficients of air cooling pipes, Pa-per A, have been used and verified in comparisons with in situ measurements at the Igelsta Bridge, Sweden. The close agreement between measured and calculated temperatures of air-cooled sections justifies the use of heat transfer coefficients determined as in Paper A. Some exemplifying calculations are also given. The general behaviour of cooled structures is discussed. The principles of designing cooling systems for the general case are proposed. It is further concluded that it is possible to design the cooling system of prismatic structures such as a columns with existing models for tem-perature and crack risk calculations by using the measured heat transfer coeffi-cients. Paper C: Groth P., Noghabai K. (1996), ‘Fracture Mechanics properties of steel-fibre reinforced high performance concrete.', Proceedings of 4th Interna-tional Symposium on Utilization of High Strength Concrete (BHP-96), Paris, 1996. The fracture mechanics properties of plain and steel-fibre reinforced high perform-ance concrete in Mode I have been obtained by means of uniaxial tension tests on cylinders. Considerations were taken in designing the notches. Four-point bend tests on small beams were also carried out. In all cases the ascending part of the structural responses was successfully monitored. This is valid for all tested speci-mens. The material data acquired, was used in an existent analytical model for splitting of a thick-walled concrete ring, in order to map the structural behaviour of the composite material. Good correspondence with the test results was observed. It was confirmed that even a moderate amount of steel fibres, drastically enhanced the ductility of the concrete. Paper D: Groth P. (1996), Steel fibre reinforced concrete - tests and evaluations Properties of various SFRC mixes have been determined by three types of tests; single fibre pullout, four-point bending and ring tests. The pullout tests were performed under closed-loop control on single-sided speci-mens using various combinations of concrete mixes and fibre types. Three different models for the interfacial bond between fibre and matrix have been applied on the results. Neither of the existing models seems to be fully able to reproduce the pull-out curves. However, qualitative comparisons can be made regarding the efficiency of the different types and dimensions of steel fibres tested. The four-point bend test, as regulated in ASTM C1018, are probably the most common method to characterise SFRC. The flexural toughness properties obtained by this method, as well as by the method regulated in JSCE SF-4, are compared for the different SFRC mixes tested. Some correlation with results from single fibre pullout tests has been found. By use of the Inner Softening Band-model encourag-ing results of simulation of bend tests have been obtained. The case of cracking due to restrained shrinkage and the ability of SFRC to dis-tribute cracks have been studied by ring tests. SFRC or plain concrete is cast around the perimeter of a steel ring. The shrinkage is caused by either drying, self-desiccation shrinkage or a combination of both. Due to the restraint, caused by the ring, cracking will occur. The length and width of cracks are measured over time and the results are compared for the different SFRC and plain mixes. Comparisons with results found in literature are also made.

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