Integrated design of high performance fibre reinforced cementitious composites

Abstract

From the materials level to the structural level, this thesis presents an integrated design of high performance fibre reinforced cementitious composites (FRCC), covering a packing model and fibre factor identification, a new test method design, and investigation of bond and tensile behaviours.

The concrete mix design of FRCC is still undergoing a frustrating trial and error process, as adding rigid fibres would cause a significant decrease in workability in an unknown way. The loosening of aggregate packing which increases the void volume between aggregate particles to be filled with paste is definitely one contributing factor, yet limited research exists. Under dry conditions, experimental investigation revealed that the proportional decrease in packing density due to the addition of steel fibres increases linearly with fibre volume Vf, but at different rates for different fibre types and aggregate sizes. Based on test results, useful design guidelines and a packing model for FRCC were developed. Under wet and hardened conditions, the effects of the fibre characteristics on the fresh and hardened properties of FRCC have been evaluated to identify the governing fibre factor (FF). Correlations revealed the traditional FF is overly simplistic and that fibre number is more fundamental than Vf. Moreover, design equations have been derived for the mix design of FRCC.

On one hand, the reinforcing bar-concrete bond is crucial in the anchorage of reinforcing bars and crack control of reinforced concrete. Adding fibres to concrete would improve this bond, but theoretical predictions remain a difficult task. As known, the bond is generally measured by the pull out test, while different test methods yield different test results. Finite element analysis (FEA) unveiled the variations in stress distribution inside the specimen due to uneven contact pressure and friction at the concrete block-steel platen interface. To minimize the test errors induced, the test setup was redesigned and found to yield lower and less variable test results under systematic experimental evaluation. Using this newly-designed method, the effects of steel fibres on the bond stress-slip behaviour were studied. It was found that the bond strength is strongly correlated to and thus may be predicted from the FF; the slip at peak bond stress is not sensitive to fibre addition; and the secant bond stiffness increases with the FF with moderate correlation. These results may extend the design model in the Model Code 2010 for application to FRCC.

On the other hand, a new direct tension test method of applying tension to a dumbbell-shaped specimen through side-glued steel plates applicable for FRCC was developed. FEA of the test setup revealed substantially reduced stress concentration in the specimen. Applying this new method to test FRCC specimens, it was found that both the first cracking strength and post cracking strength increased with the Vf but at different rates for different fibre types. From the stress-strain curve, two different values of tensile strain capacity were obtained. Regression analysis yielded good correlations to the FF and empirical formulas for estimating the minimum FF for strain hardening and the tensile strain capacities.