Crack Stability in the Fracture of Cementitious Materials
- PDF / 1,829,452 Bytes
- 10 Pages / 414.72 x 648 pts Page_size
- 5 Downloads / 185 Views
387
Mat. Res. Soc. Symp. Proc. Vol. 370 @1995 Materials Research Society
water reducing a ent* (88.5 ml) was added to one batch (-0.3 ft. 3) of the high strength concrete mix. Silica fumeT was used as a mineral admixture to strengthen the interface between the aggregate particles and the matrix. The mix ratio (by weight) of the mortar, was cement: sand: water = 1: 2: 0.6. Type I portland cement, P (pea-) type gravel and ASTM # 2 sand were used. The maximum aggregate size in the mixes of normal and high strength concrete was 9.5 mm. Single-edge notched beams were cast, with beam depth, d = 76 mm. For all the beam specimens, the length-to-depth ratio was equal to 8/3, the ratio of notch length, ao, to depth was 1/3, and specimen thickness was 38 mm. All the specimens were compacted by rodding and vibration. The specimens were left in the molds during the first 24 h and were removed and cured in water until the time of testing. The notches were cut with a diamond band saw and were 1.8 mm wide. Companion cylinders of 76 mm diameter and 152 nun length were cast. These cylinders were capped with a sulphur compound and cured under water with the notched specimens. The cylinders were tested in compression after 28 days of curing. Normal strength concrete cylinders failed at an average maximum compressive strength of 46.4 MPa, with a standard deviation of 5.4 percent. High strength concrete cylinders had an average compressive strength of 73.2 MPa, with a standard deviation of 3.3 percent. Bend specimens were loaded on a servo-electric mechanical testing machinet at a constant crosshead displacement rate of 2 gm/min. During each test, the load, crosshead displacement and crack mouth opening displacement (CMOD) were monitored at 1 Hz with a data acquisition system.** The crack opening displacement was measured using a clip gage.tt For the three-point bend tests, the outer span, 10,was 188 mm whereas for the four-point bend tests, the outer span was 188 mm and the inner span, 1i,was 94 mm. Four to ten specimens were tested for each loading condition (three- or four-point bend test), for each material. A camera was attached to a traveling microscope mounted on the base of the testing machine. At the end of the test, the motion of the crosshead was arrested and crack pictures were taken. The specimen was unloaded and its surfaces were examined under an optical microscope for evidence of any side cracking or any other difference in the crack profiles on the front (surface facing the camera) and back surfaces. 3. RESULTS Figure 1(a) shows the load versus CMOD plot of a normal strength concrete specimen tested in three-point bending. Crack growth in this specimen is stable in contrast to normal strength concrete specimen tested in four-point bending (Figure 1(b)). In this experiment a sudden load drop occurs in the softening branch of the load-CMOD curve and the crack opens by a few hundred microns followed by a stable crack growth with a slowly decreasing load carrying capacity. To explore the effect of the inner span on the s
Data Loading...
