Further Studies on Fracture Properties of Paste/Rock Interfaces
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evious Results Table 1 summarises previously measured 28 d KcB and Rc. (and E) values from tests on paste, rock and composite beams. The w/c ratio used was 0.3, and 3 rock types are represented, all used commercially for crushed rock aggregates in South Africa. The table indicates that paste values may be lower than interface values (at w/c = 0.3), that use of silica fume generally causes embrittlement, that rock values are very much higher than paste values, and (by comparison with 367 Mat. Res. Soc. Symp. Proc. Vol. 370 0 1995 Materials'Research Society
other investigators' ) that use of rough rock surfaces yields significantly higher fracture values than sawn or polished specimens. In fact, rock surface roughness is, in our opinion, probably the single most important factor in the values measured.
TABLE I - Values from previous fracture tests (28 d) Measured KcB (MN/m'.s
E (GPa) 24.1
Paste
OPC
0.54
(Jim2 ) 14.9
(w/c=0.3)
OPC+SF
0.50
9.1
24.4
Rock
andesite dolomite granite
3.49 2.23 2.22
147.1 59.9 162.6
99.7 112.6 75.2
Rock/paste interface
andesite/OPC andesite/OPC+SF dolomite/OPC dolomite/OPC+SF granite/OPC
0.80 0.71 0.57 0.64 0.63
24.6 14.5 7.5 9.9(65d) 25.2
45.6 38.2 46.8 41.7
OPC - Ordinary Portland Cement; SF - silica fume at 15% replacement
a0
D S 20
= = =
diameter of cylindrical specimen support span, 3.33D chevron angle, 90'
a. F AF
= =
chevron tip distance from bottom fibre load on specimen
=
load point displacement (LPD)
FIGURE 1 - Chevron bend specimen for ISRM Method 1 368
Previous results also indicate significant differences between the rock types :
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In general, andesite composites have superior properties. This rock type also gives substantial improvements in concrete mechanical properties. Dolomite produces composites with enhanced stiffness. Granite provides considerable post-cracking ductility.
Modes of Failure Previous studies5 have shown that, for composite specimens &
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The failure crack starts at the notch and either runs at the original interface (cleavage failure), or else enters the paste at an angle before turning and running parallel to the interface. In the cleavage failure mode, crack branching may occur on both the paste and the rock side of the failure crack. In "within-paste" failure, the crack produces branches that travel through the bonded paste layer toward the original interface. In both failure modes, crack branching out of the failure plane occurs extensively and constitutes an important energy absorbing mechanism.
In the case of within-paste failure, the ISRM test is strictly no longer a valid test of interface toughness; all that can be said is that the interface toughness is greater than the value measured in the test. PRESENT STUDY: EXPERIMENTAL DETAILS The work reported here is part of an expanded study to look at the influence of two important variables: w/c ratio of the paste, and age. Test details were as reviewed earlier in this paper. Paste specimens were cast in steel cylindrical moulds which were placed in a fog roo
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