Comparison of methods for early age strength testing of shotcrete

The durability of Fibre Reinforced Concrete (FRC) and Shotcrete (FRS) in the uncracked state has been demonstrated to be superior to that of conventionally reinforced concrete even when steel fibres are used as reinforcement. However, the ability of FRC/FRS to resist corrosion and maintain residual load capacity once cracks have occurred is unclear. The present investigation was initiated to study the influence of exposure on residual toughness of FRS reinforced with either steel or macro-synthetic fibres in both coastal and inland environments. The results indicate that maximum crack width appears to control the rate of performance deterioration but that narrow cracks can undergo autogenous healing and present only minor durability concerns for steel FRS. mens tested at the start of the trial and that of the exposed specimens. The specimens consisted of ASTM C-1550 panels that had been produced using a shotcrete mix reinforced with either hooked-end steel fibres or macro-synthetic fibres (see Table 1). The steel fibre reinforced mix included 50 kg/m of 35 mm long hooked-end cold-drawn wire fibres with an aspect ratio of 65. The macro-synthetic fibre reinforced mix included 9 kg/m of Barchip HT48 polypropylene fibres. The 28 day UCS of each mix was not measured but was estimated from previous trials with this mix to be 55 MPa. Table 1. Mix design for shotcrete used in both trials. Component Quantity (kg/m) Coarse aggregate (10/7 mm CRG) 620 Coarse sand (2 mm) 600 Fine sand 400 Cement 360 Fly ash 40 Silica fume 20 Water reducer (L/m) 0.42 All specimens were produced by manually spraying shotcrete into round steel and plywood forms that were propped at 45° against a rack. The specimens were moved to a flat surface immediately after spraying and screeded so as to achieve a flat surface and uniform thickness. They were then left outside under plastic sheeting to harden overnight before being stripped and transferred to curing tanks for 56 days of immersed curing. Set accelerator was not used. A total of 80 specimens were produced as part of the investigation. 2.2 Parameters investigated The objective of the investigation was to examine the deterioration in performance of FRS reinforced with steel and macro-synthetic fibres following exposure to inland and coastal environments, and determine how the extent of deterioration is controlled by crack width. The present specimens were therefore divided into two groups: those reinforced with steel fibres (identified as ‘H’ in Table 2) and those reinforced with macro-synthetic fibres (‘B’). Each of these groups was, in turn, then divided into equal numbers of specimens subject to inland exposure (‘I’) and coastal exposure (‘S’). Each of these groups were then divided into specimen sets with an average initial crack width of approximately 0.1 mm or 1.0 mm. Finally, three exposure periods were applied (‘X’, ‘Y’, ‘Z’). The identification of specimens is listed in Table 2. The specimens that were tested at 56 days have been identified either as ‘L’ type specimens subject to normal C-1550 tests to establish the average performance of each mix, or ‘W’ type control specimens that were subjected to an initial load cycle followed by a re-test to establish a benchmark against which the later specimens could be compared. Note that construction contractors inadvertently destroyed all the remaining specimens stored at the inland site several weeks before the 24 month tests were due. Performance data are therefore only available for 7 and 14 months of exposure at this site. Table 2. Identification of specimens according to fibre type, exposure site, initial crack width, and duration of exposure. Parameter Type (and abbreviation) Fibre type Hooked-end steel Barchip HT48 H B Exposure Site Coastal Inland S I Crack width Narrow 0.1 mm Wide 1.0 mm 01 10 Exposure period No exposure 7 months W X 14 months 24 months Y Z 2.3 Selection of exposure sites Chloride ions are known to play a significant role in accelerating the corrosion of steel bars in concrete (Neville, 1992). The two sites selected for the present investigation were therefore chosen to permit the influence of chloride ions on corrosion rate to be investigated. One site was a grassy field at the University of Western Sydney, which is located 60 km inland from the east coast of Australia. This site represented an environment with a very low concentration of chloride ions. The pre-cracked specimens were placed on the ground at this site with the cracked face upward so that crevice corrosion was encouraged. Grass grew around the specimens and decayed throughout the duration of the study, thus carbonic acid was likely to be present on the surface of these specimens (see Figure 1). Figure 1. Exposure site for inland specimens showing growth and decay of grass around specimens. The second site was the Royal Australian Navy diving school, adjacent to Sydney Harbour, Australia. This was selected because of its ready accessibility and low risk of vandalism. However, the specimens were exposed at a depth of 2-4 metres seawater due to a lack of space on the foreshore and relatively heavy surf at the waterline. The specimens at this site were therefore maintained in an environment with a high chlorine ion content but relatively low oxygen concentration. The specimens were manually placed into the water and sat on the sandy bottom throughout the exposure period, and were then retrieved by Navy clearance divers when required (see Figure 2). Every specimen was individually identified at each site so that preand post-exposure crack widths and performance could be correlated. Figure 2. Retrieval of specimens by Navy divers from coastal exposure site. 2.4 Performance evaluation The performance of ASTM C-1550 panels is normally evaluated on the basis of energy absorption between the point of first loading and a specified central deflection. In the present investigation, the majority of specimens were pre-cracked by applying a central point load under displacement control until cracks of a specified width had been generated, after which the direction of loading was reversed and the specimen unloaded. This process resulted in a loaddeflection curve of type ‘a’ described in Figure 3. The three radial cracks were measured at numerous locations to determine average crack width. The test machine available for this investigation did not permit the loads and displacements that occur during a reversal in loading to be logged, so the loaddeflection record for the retraction of the piston could not be recorded. This shortcoming resulted in a small error in the calculated energy absorption for each specimen because a portion of the energy absorbed in the initial cracking test was regained by the specimen during the retraction. Each specimen was thereafter taken to the allocated exposure site and left in place until the required duration of exposure had transpired. They were then retrieved, cleaned, and re-measured for crack width, and subjected to a second load test under a displacement-controlled central point load to obtain a load-deflection curve of type ‘b’ shown in Figure 3. The performance of each specimen was assessed on the basis of energy absorbed prior to exposure, energy absorbed after exposure up to a central deflection of 40 mm, and the sum of energy absorption up to a cumulative central deflection of 40 mm prior to and after exposure.