International Concrete Abstracts Portal

Experiments of three kinds of FRP-strengthened concrete beams under three-point bending are reviewed first. Two different cracking behaviors, with and without distributed crack in concrete, were observed. To analyze the different cracking behaviors affected by the FRP strengthening through interfacial bond, the FRP strengthened concrete beam is subdivided into a plain concrete beam, under three-point bending, and a FRP sheets bonded concrete prism, subjected to shearing load, to address the fracture mechanism. Nonlinear fracture mechanics is used to model the cohesive crack along the FRP-concrete bond interface and concrete cracking. Finite element simulation is also performed to demonstrate the applicability of the fracture mechanism. Based on both experimental observations and finite element results, it can be concluded that the FRP strengthening effect occurs after the first flexural crack. The bond strength and interfacial fracture energy of bond interface determine the ability of stress transfer. The occurrence of the new flexural cracks after the first one is governed by the relation between the concrete tensile strength and the maximum concrete stress obtained by combining effects of shear stress transfer and bending moment including the stress release due to flexural cracks. Further strengthening effect is archived by the formation of new cracks.

Behavior of polyester polymer concrete (PC) with and without notch and graphite fiber was investigated using nondestructive and destructive testing techniques. The flexural strength of polyester PC was 14 MPa (2,000 psi). The effect of up to 6% chopped graphite fibers on the elastic modulus, shear modulus. Poisson’s ratio, flexural strength and fracture parameters were investigated. Nondestructive methods such as impact resonance and pulse velocity were used to determine the effect of notch depth on the mechanical and damping properties of PC. Fracture parameters, critical stress intensity factor Ktc and critical Jtc-integral were determined using single edge notched beam loaded in four-point bending by varying the initial notch-to-depth ratio from 0.2 to 0.7. By measuring the crack mouth openin g displacement (CMOD) during loading, the crack extension in the test specimen was determined. The critical stress intensity factor and critical J-integral for the polyester PC were 1.3 MN/m’ and 0.27 kN/m respectively. The addition of 6 mm long 6% chopped graphite fibers to the polyester polymer concrete improved the tlexural strength by 20% and Ktc and Jtc by over 25% and 125% respectively. Impact resonance test results were sensitive to the notch-to-depth ratio in the test specimen.

The strength and fracture behavior of very high performance concretes (compressive strength about 150 MPa) were studied using interferometric measurements. At peak load, the development of the damage zone in terms of size and shape was observed for geometrically similar specimens of different sizes. The cement-based materials had an aggregate/binder ratio of 1 5, a microsilica/binder ratio of 0.1 and a water binder ratio of 0.22. Beams made without and with I-2% by volume steel reinforcing microfibers were considered It was shown that, when fibers were introduced in the brittle matrix, their influence on the control of crack bridging generated a reduction of the size effect on strength and structural brittleness.

The wedge splitting test method was used for determining the crack growth resistance curve (R-curve) behavior of cementitious materials. Two things are needed to calculate a crack resistance curve: 1) A sufficiently large fracture surface area to obtain size-independent fracture values; 2) Stable crack propagation until complete separation of the specimen. It is very difficult to fulfill both requirements simultaneously because relatively high testing loads are necessary to fracture large specimens, which often leads to unstable crack propagation. A wedge splitting test has been shown to achieve stable crack propagation for suitable specimen sizes for mode I failure. A splitting load with a slim wedge is used to develop a load-displacement curve for the stable cracking of cubic specimens. A method was established using only the load-displacement curve for the cal-culation of the crack resistance curve. The testing procedure as well as the method to cal-culate the crack resistance curve is described. The results are shown on five examples of plain concrete and fiber reinforced concrete. The fracture mechanical behavior of plain concrete and fiber reinforced concrete was analyzed using the methods discussed.

Many laboratory fracture toughness tests have been devised for quasi-brittle materials such as portland cement concrete (PCC). The objective of these tests has typically been to determine linear elastic fracture mechanics parameters such as fracture toughness, Kt,, and fracture energy, Gr. More recently, proposals have been developed to determine the parameters for a variety oftwo-parameter models. For example, the inelastic correction factor method requires parameters KI, and inelastic correction factor p, while the Jenq and Shah two-parameter model requires KI, and CTODc, and the Bazant size effect method requires Grand cr. All of these parameters can be obtained from the stress versus crack opening displacement (o-COD) relationship. The authors describe a test device that is reliable, simple to understand and analyze, relatively inexpensive. and can be used on standard 6-inch diameter by 12-inch concrete cylinders in a universal testing machine (UTM) under load control. This device, called the stiff tensile test (STT) apparatus, is used to obtain the complete versus COD relationship. From this o versus COD relationship, which is fundamental at the meso-scale, can be derived most of the commonly used fracture parameters of concrete.