Reason 10.2.2 Crack
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high-lime gels are thought to be innocuous and unable to expand. Pozzolanic materials cause rapid reactions with the alkalies and cause the alkalies to be contained in the innocuous (nonswelling) gels. Gel masses formed later, especially within the aggregate, are able to absorb water and expand. This gel ruptures the aggregate particles and collects in pockets, cracks, and crevices. The gel in such pockets may expand, open any microcracks, and create new cracks. Sometimes, the entire center of a highly reactive aggregate particle, such as chert, may be converted to an expansive gel that can create, ooze out into, and enlarge microcracks. When the concrete is broken or cut open, many large cracks may be seen radiating from reactive particles. Destructive cracks may also be found radiating from gel-filled pores and voids.
Five items should be considered, as listed in Table 10-3: (1) the crack pattern;(2) structural evidence of expansion; (3) rocks and minerals in the aggregate of the distressed placement; (4) exudations, coatings, and pore fillings; and (5) sufficient sampling of the HCC.
The pattern of cracking indicating expansion of the concrete can be very similar to the cracks generated in masses of mud when the top layer dries and shrinks, plastic shrinkage cracks (see Chapter 4), and the cracks in certain lava flows that occur when the surface cools rapidly and shrinks. Each portion of the surface pulls away from every other portion, generating an irregular honeycomb pattern. The size and regularity of the pattern depend on the cohesiveness, uniformity, and isotropy of the material and the speed of shrinkage (see Fig. 10-1).
This pattern is generated because there is a differential volume change between the exposed surface material and the attached massive substrate. In plastic shrinkage cracking, mud cracking, or cooling lavas, the surface has shrunk relative to the substrate. In expansive alkali-aggregate reactions, the pattern is generated by an increase in volume of the substrate relative to the volume of the overlying surface material. The surface becomes cracked because it is attached to the underlying material. The lower portion of the concrete is damper than the surface portion due to contact with ground water or water vapors rising from the water table and lack of drying by sun and wind. Chemical reactions can take place more readily in the sence of water than in dry zones; therefore, more expansion takes place in the lower portions of the concrete that is reacting by these mechanisms and the surface does not expand. In the early stages of this reaction, the concrete in the lower expanding portions of the placement is squeezed together by the reaction and may not show much cracking. The surface crack pattern has been called pattern cracking, map cracking, Isle-of-Man cracking (see Fig. 10-2), and crows-foot cracking.
In ordinary concrete that is free to expand equally in all directions parallel to the plane of the surface, the classical cracking pattern is usually very evident on the surface in all the stages of the deterioration until the concrete is reduced to a rubble during the last stage. In continuously reinforced HCC or in concrete units that are much longer than they are wide, the concrete is not free to expand equally in all directions. In the early stages of the deterioration, it can expand only at right angles to the length and the cracks will of necessity be at right angles to the direction of the expansion and therefore parallel to the long dimension (and the reinforcing steel when present). In the early stages, longitudinal cracking caused by the
The expansion of the placement may be very evident. Expansion joints may have closed, with the joint compound having been squeezed out. Guardrail sections that had been planned with a space between may be abutting and grinding together and destroying each other. Occasionally, the expansion may cause blowups, with slabs of the concrete appearing to jump upward because they no longer fit the space. The expansion can cause shearing of bolts and, occasionally, humping of nearby flexible paving on the shoulders (see Figs. 10-5 through 10-7).If elements of the placement are being forced together or the crack pattern indicates that the lower or damper portions of the concrete may have expanded relative to the upper drier portions or both, the concrete is probably deteriorating by internal expansion. The expansion is not always clearly shown and may be difficult to understand and document. The expansion is greatest in the areas that have the least optimum combination of factors, that is, the areas that combine the reactive aggregate in sufficient amounts; a sufficient amount of alkali in the cement; and, most variable of all, sufficient amounts of moisture and a sufficiently high permeability to permit the reaction and expansion to occur. Because the reaction cannot occur without moisture, the reaction and expansion are greatest at the depths in the concrete that very seldom become dry. The surface zone of the concrete is the driest portion, does not react as much, and does not expand as much. The surface portion is bonded to the concrete beneath and must move with it. Therefore, it cracks (see Fig. 10-8). An internal RH of 80% is all that is needed for any and all chemical reactions using water to proceed (alkali-silica reaction, cement hydration, etc.). Nearly all concrete will have an internal RH of more than 80% if one side is on the ground and the RH is measured at a depth of 2 in. from the exposed surface-even in the desert (B. Mather, personal communication, October 1991).
In the literature, two main types of experimental setups can be found to investigate the fracture behavior of mode I/II mixity: the first one is based on asymmetrical rectangular specimens, such as [6,7], the modified compact tension (CT) specimen [8] or the mixed-mode bending (MMB) test [9]. This latter method allows one to cover a spectrum of different mode ratios by varying the length of the loading lever. The second one exploits disc-like specimens, such as the Brazilian disc (BD) [10], the semicircular bend (SCB) [11] specimen, the asymmetric semicircular bend (ASCB) [12,13] specimen and the Inclined edge-cracked semicircular bend (IASCB) [14,15] specimen. In this study, the IASCB specimen subjected to three-point bend loading (the test configuration is described in detail in Section 2.2) was used. This setup permits to study of a large range of mode I/II mixity levels without requiring an elaborate loading fixture. The IASCB specimens were sintered with different crack-like notch angles and tested on different support spans. In three of them (for pure mode I), the crack was manually induced, with the scope of carrying out a comparison with the ones with the sintered crack-like notch.
Figure 7 shows the path around the crack tip considered for the J-integral computation. The points belonging to the zone within 0.3 mm from the crack axis were removed since, in that region, the data were particularly noisy. In this region, in fact, due to the massive plasticization and to the crack opening, the digital image correlation algorithm performs poorly. The value of 0.3 mm was empirically defined based on a visual evaluation of the measurement noise and the performance of the correlation algorithm in the proximity of the crack boundaries. An example in which a region near the crack tip is similarly removed can be found, for instance, in [30]. Figure 8 shows the x and y displacement and strain maps obtained by the DIC for all the different specimen configurations tested. It can be observed that the strain in y and in particular in x-direction increases ranging from mode I to mode II, which implies a higher strain energy work to make the crack propagate.
Concerning the notch induction by the conventional subtractive method (by razor blade incision) or by additive method (by directly printing it), some considerations can be made. The J-integral mean value of the notch induced by the AM method is 12.2% higher than the one obtained by the SM method, but it is within the experimental error. This can be attributed to a less sharp crack-like notch in the case of AM specimens, which leads to a lower stress concentration in the area near the crack tip, and therefore to a higher apparent fracture toughness value.
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