Date of Award
Master of Science in Civil Engineering - (M.S.)
Civil and Environmental Engineering
C.T. Thomas Hsu
Studies in fracture mechanics of concrete use a testing setup which tends to include unacceptably large errors in deflection measurements. Recent work reported by Kim  has proposed a new testing setup to eliminate these errors and consequently produce better determinations of fracture parameters.
The error which occurs most frequently is in the measurement of the load-line deflection. Until recently, the way this measurement has been made, using a dial gauge or LVDT (Linear Variable Differential Transformer) to measure the deformation between the center of the specimen and the frame, includes the crushing of the concrete surface at the supports and other extraneous deformations. The crushing overstates the deflection measurement which leads to an overestimate of the energy absorbed by the specimen and consequently, the predicted fracture toughness of concrete. Rather than the load-line deflection the CMOD (Crack Mouth Opening Displacement) is another measurement which can be used to determine the fracture energy the tested concrete specimen absorbed.
Since there is a bilinear relationship between CMOD and load-line deflection, it can be shown that the load vs. CMOD relationship yields the same fracture toughness value as that determined by the actual load-line deflection. Previous research studied only plain concrete with different specimen sizes and notch depths. To extend the idea, the same experiment has been done for fiber reinforced concrete beams.
The results show another kind of error which may be due to large beam deformations found when testing fiber reinforced concrete beams. This error is very small in plain concrete but more pronounced in fiber reinforced concrete. To correct this error, a new testing setup is recommended.
Test results based on CMOD measurements indicate that the bilinear relationship between the load vs. CMOD exists for cementitious composites. For fiber reinforced concrete, the initial slope (S1) is 3.6 with the second slope (S2) equals 0.986. These values are larger than those reported for plain concrete. Experiments show that fiber reinforced concrete beams can resist more load with increased fiber length and volume fraction. Different types of fiber did not give significantly different results, except for hooked end fibers which gave higher strength. Determination of fracture parameters based on CMOD seems to eliminate errors due to crushing at contact surface between concrete and supports which results in extraneous deformations. Fracture energy computed based on CMOD should be smaller than those calculated based on the erroneous deflection measured with reference to the frame since extraneous deformations are eliminated. Furthermore, research conducted using measurements of deflection with reference to machine base should be reconsidered. It is recommended that this testing procedure be used as the standard for toughness testing of cementitious composites.
Tindukasiri, Sira, "Fracture of cementitious composites" (1993). Theses. 1911.