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Concrete In Australia : September 2013
Concrete in Australia Vol 39 No 3 39 either better hydration or lower slag content. Numerous microcracks run throughout the entire cementitious matrix in different directions and some of the cracks traverse a portion of the coarse aggregate boundaries. It would be expected that any strength reduction would be more evident in this concrete than that represented by Core 2036-2. 2.2.3 Compressive strength of Concrete e results of compressive strength tests conducted on cores are shown in Table 5. e concrete from the downstream wall has a higher strength than that of the upstream wall. e more negative potentials for the downstream wall probably arise because the concrete is denser and slower to dry out. Both concretes met the 40 MPa strength requirement of VicRoads Specification Section 610, although the concrete from the upstream retaining wall may have been under the 40 MPa target at the age of 28 days. Core C10/2036-2 which had very little microcracking is of the same type as C10/2036- 3, which had 50 MPa strength, and C10/2036-5 which exhibited more microcracking is similar to the other two cores from the upstream wall, which showed lower strength, indicating agreement between strength and microscopic features. 2.3 Volume of permeable voids (VPV) of concrete e values of volume of permeable voids (VPV) determined on various cores are presented in Table 6. e VPV values of the two types of concrete can be summarised as follows: • Lower slag concrete (blended slag cement): 14.5 ± 1.5%. • High slag concrete (geopolymer or alkali-activated slag): 19.3 ± 0.8%. e blended slag cement slag concrete meets the requirement of VicRoads Specification Section 610 for drilled core samples (max VPV of 16%), whereas the high slag concrete exceeded the specified value. e results of SEM/EDX (see later) indicated that the geopolymer concrete sampled from the downstream wall contained gel-like hydrous material phases, and it is likely that the high VPV values for this concrete resulted from the loss of water associated with the hydrous phases rather than from interconnected pores. e additional hydrous gel present in the concrete may indicate that excessive amounts of sodium silicate activator was used in the concrete, which resulted in excessive loss of water in the drying phase of the VPV test. In fact, as shown later for this concrete, the penetrability to chloride ions, as assessed by ASTM C1202, was classed as very low and the chloride diffusion coefficient, as determined by the NT Build 443 test method was 1.58 x 10-13 m2/s, which is also considered very low and desirable, and indicates low porosity in concrete. erefore, the apparently high VPV of the geopolymer concrete is not due to the presence of larger volume of voids, but due to the use of excessive amount of activator in the concrete examined. 2.4 Resistance to chloride penetration As it was originally expected that the two retaining walls would have the same concrete, only one core from the downstream wall was tested for chloride penetrability. Two test methods were used to assess the chloride penetrability, i.e. ASTM C1202 (Electrical Indication of Concrete Ability to Resist Chloride Ion Penetration) and NT Build 443 Method. e results are presented in Table 7 and Table 8, respectively. e ASTM C1202 method ranked the concrete as having very low chloride penetrability. is may have resulted from the high slag content of the concrete, which would reduce the amount of ionic charges in the pore solution of the concrete, and from low porosity of concrete, despite the apparently high, misleading VPV value. As shown in Table 8, the NT Build 443 test method resulted in a very low chloride diffusion coefficient (D = 1.58 x10-13 Table 6. Results of VPV determinations for various cores. Table 5. Concrete compressive strength. Specimen ID Location Specimen Strength (MPa) Av e rage Strength (MPa) C10/2036-3 Downstream W all 50.9 50.9 C10/2036-4 Upstream W all Upstream W all 44.9 44.3 C10/2036-6 43.7 Specimen ID Location Specimen VPV (%) Average VPV (%) C10/2036-2A Down Stream W all 19.0 18.8 C10/2036-2B 18.5 C10/2036-5A Up Stream W all 20.0 19.9 C10/2036-5B 19.9 C11/2259-1 Downstream wall; 13.8 13.8 C11/2259-2 Downstream wall; geopolymer concrete 18.9 18.9 C11/2259-3 Upstream wall 14.7 14.9 C11/2259-4 Upstream wall 15.0