Fatigue Analysis of JPCP With Transverse Surface Crack

1. Age Fracture toughness ( ) Peak-load stress (MPa) Split tensile stress (MPa) 3 days 25 1.8 3.0 7 days 41.4 2.1 3.2 Conclusions Fatigue Analysis of JPCP With Transverse Surface Crack Ya Wei, Will Hansen, Elin A. Jensen, University of Michigan Collaborating Faculty: Prof. S. P. Shah, Northwestern University Industrial Advisor: Scott Johnson, MTS Introduction It has been known that surface edge crack of JPCP (Joint Plain Concrete Pavement) is due to the built-in negative temperature gradient from hot weather construction, and being able to develop into partial-depth, full-width transverse surface crack under repetitive joint truck loading (Figure 1). Hence the fatigue failure position will shift from the bottom to the surface of the slab (W. Hansen). A representative fatigue analysis method for JPCP with transverse surface crack is based on fracture mechanics, in which fatigue life is controlled by the rate of crack propagation relating with fracture properties and behavior of pavement concrete. Figure 2 Test Setup Comparing effective crack length-Number of cycles relation determined from experimental test and predicted using Kolluru’s fatigue model, Kolluru’s model exaggerates the real fatigue life of this particular case. From experimental fatigue curve, crack growths rapidly from initial notch length 40mm to 64mm at very first 25000 cycles, only 2.5% of total fatigue life, but with a crack increase of 30% of total amount of crack increase. Then followed by a steady stage with almost no crack growth, which accounts for 80% of total fatigue life. After steady stage, there is a significant crack growth up to a sudden failure. This fatigue trend also can be illustrated by crack growth rate-effective crack length relation. Results and Discussion From Quasi-static four-point bending test, 7-days notched concrete beam gives quite larger value of fracture toughness than 3-days’ at close peak loading, which indicates that crack is much more inclined to initiate at very early age (3-days). According to E A. Jensen & W. Hansen, the resistance to crack propagation does not change significantly as the strength or curing time increases for the same course aggregate source. Hence fracture toughness which relates to crack initiation becomes the most important fracture parameter to quantify the fatigue life of concrete pavement with transverse surface crack. Figure 1. JPCP along I-94 (East Bound) in Van Buren County, Michigan with premature mid-slab cracking four years after construction. Scope • the fracture properties of typical concrete pavement materials were determined under quasi-static and cyclic loading at various ages in the lab. • fatigue prediction model by Kolluru et al was adopted to quantify the fatigue life of JPCP with pre-mature crack subject to joint loading. • notched plain concrete beams were tested in a four-point bending test configuration under flexural fatigue loading to investigate the relation between crack growth and number of flexural loading cycles. • fatigue life of JPCP from designed experimental test was compared with the predicted fatigue life, and the variation was considered. Therefore, fatigue life of concrete pavement with transverse surface crack can be divided into three stage: accelerated stage 1; steady stage 2; failure stage 3. The first stage(early-age concrete pavement) is the critical stage which determines at what effective crack length the steady stage is being able to start and thus how fast the pavement will fail. Experimental Design Notched concrete beams with dimension of 630mm (span), 100mm (width), 200mm (depth) were tested using closed-loop MTS in this research to obtain load-CMOD curve under four-point bending, Figure 2. Illustrates the four-point bending test setup. Three types of tests were conducted at different beam ages (3 days and 7 days): Quasi-static test determining the failure envelop and fracture toughness K1c of notched beam according to Shah’s Two-Parameter Model, fracture toughness is related with the crack initiation and calculated from the peak load and geometric function representing the specimen size and load configuration; Cyclic test determining effective crack length- compliance relationship, the compliance is defined as the value of crack mouth opening displacement per unit load and determined by unloading the specimen at different points in both pre-peak and post-peak parts of the quasi-static response; High-cycle fatigue test of notched beams were conducted at 7days age. For highway concrete pavement, the combined tensile stress on the mid-slab surface due to curling stress at negative temperature difference –10 centigrade and federal standard trucking loading at joint is 1.2MPa calculated form EverFE for 200mm slab thickness. Comparing with average peak load of Quasi-static response of notched concrete beams, the stress ratio was obtained as 0.6. • For typical concrete pavement, 7days’ slab have a quite larger value of fracture toughness relating to crack initiation than 3 days’ slab. It is necessary to take steps to avoid any factors that could prompt crack to initiate at very early-age (3 days). • Kolluru’s fatigue model gives a longer fatigue life than that determined from experimental test. • According to Kolluru’s fatigue model, concrete pavement with different coarse aggregate can have same fatigue behavior during decelerated stage, which is conflict with the finding made by E A. Jensen & W. Hansen, that strong coarse aggregates improve concrete’s resistance to crack propagation in terms of increased fracture energy. • Fatigue behavior of concrete pavement with transverse surface crack can be divided into three stages. Stage 1 should be the critical stage, this needs more experimental test to verify. Cyclic beam test determined unloading compliance-effective crack length relation at 3 &7 days, as expected that 7-days’ beam gave larger effective crack length than 3-days’ for same unloading compliance. This relation provides a critical approach to determine the effective crack length at certain number of cycles during fatigue test since it is hard to measure the effective crack length through experimental way. This approach is based on such a hypothesis that unloading compliance is uniquely related with the effective crack length.