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Title: Characterisation of normal and high-strength plain and fibre-reinforced concretes by means of strength, fracture and combined fracture/relaxation tests.
Author: Taylor, Mark.
ISNI:       0000 0001 3510 9127
Awarding Body: University of Wales.Cardiff
Current Institution: Cardiff University
Date of Award: 1997
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The main objective of this study was the application of strength, fracture and creep/relaxation tests to plain and fibre-reinforced high-strength concretes. Initially, five grades of concrete were developed and evaluated. Target 28 day compressive strengths were 40,60,80 100 and 120 N/mm2, the latter three being high-strength concretes (HSCs) containing a süperplasticiser and 10% silica fume. The others were normal-strength mixes used for comparison purposes. Each grade was made with 10 mm maximum-sized crushed limestone and gravel coarse aggregates making ten mixes in total. All were required to have sufficiently high workability and stability to accept reasonable amounts of fibre reinforcement. The data reported allows estimates of mix proportions for a range of HSC mixes to be determined. Various amounts of steel and polypropylene fibres were then added to the ten mixes to determine their optimum and maximum practical concentrations. The traditional type of toughness test based on un-notched beams in four-point loading was not employed. Instead, notched beams in three-point loading (equivalent to the RILEM work-of-fracture arrangement) and compact compression specimens were used. Both were tested under closed-loop conditions using crack-mouth opening displacement (CMOD) control. Post-cracking toughness was determined by means of the 15a nd 110to ughness indices given in ASTM C 1018. It was found that though fracture-based tests under CMOD control were an improvement on more traditional techniques, 15 and 110 were too insensitive to allow fibre type and volume to be differentiated. Next, work-of-fracture tests to measure the fracture energy, GF, were carried out on the plain concretes, initially under quasi-static loading. Both load/deflection and load/CMOD curves were recorded. GF showed little change with strength for a given aggregate type. Even though similar grades of crushed limestone and gravel HSCs had different GF values, the measure was still considered unsuitable for characterising the fracture properties of concrete. Similar experiments were then carried out on all ten mixes at five orders of magnitude of test duration (30 seconds to 2 days). GF appeared to be independent of strain rate. Both types of test highlighted the greater suitability of load/CMOD rather than load/deflection curves when evaluating GF. Finally, combined fracture and relaxation tests were undertaken in an attempt to obtain medium term fracture parameters. Though the CMOD was locked at 90,70 and 50% of the peak load in the strain-softening region, the deflection, when measured, showed a noticeable reduction over the seven days of each experiment, suggesting that significant cracking and stress redistribution within the fracture process zone was taking place. This finding has opened up a major area of important future research by confirming that the use of quasi-static fracture parameters in finite element studies is suspect.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Creep; Steel; Polypropylene fibres