United States Department of Transportation
i
Field Implementation of Super-Workable Fiber-Reinforced Concrete for Infrastructure Construction
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2019-01-01
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Edition:Final Report
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Abstract:A fiber-reinforced super-workable concrete (FR-SWC) made with 0.5% micro-macro steel fibers and 5% CaO-based expansive agent was used for the new deck slab of Bridge A8509. The selected FR-SWC had a targeted slump flow of 20 in. at the casting location. Multiple trial batches were performed, in collaboration with the concrete supplier, to adjust the mixture composition to meet the targeted performance criteria. This was followed up by casting the fibrous concrete in a mock-up slab measuring 10 × 10 ft that was prepared to simulate the tight rebar and the roadway crown slope in the transverse direction. The results indicated the necessity to lower the concrete slump from the intended value for FR-SWC to hold the 2% crown slope of the bridge deck in the transverse direction. The final mixture that was selected following the trial batches and mock-up placement had a slump consistency of 8 ± 2 in. (FRC). Six sensor towers were installed in the slab within 18 ft to the East and West sides of the intermediate bent to monitor in-situ properties of the concrete. Each tower had three humidity sensors, three thermocouples, and 12 concrete strain gauges. The slump values varied between 6 and 10 in. Slump values were around 8.5 in. The fresh air volume ranged from 4.4% to 5.8%, and the concrete temperature ranged from 85 to 97 °F. At 56 days, the compressive strength ranged from 7,020 to 8,360 psi and had a mean value of 7,770 psi. Data up to 260 days are reported at the time of the preparation of this report. The in-situ concrete temperature was shown to increase around 45 oF during the first day, reaching a maximal temperature of 140 oF. The temperature then dropped to ambient temperature of approximately 95 oF during the second day. It then varied on a daily basis with the ambient temperature. The relative humidity of concrete ranged between 90% and 100% initially, then decreased with time until reaching approximate values of 80% to 85%. The loss of humidity was higher in magnitude and rate near the top surface of the bridge deck compared to the middle and bottom of the slab. A 3D finite element model (FEM) was developed to predict the top and bottom structural strain values in the concrete deck that can be developed due to the weight of the bridge. The estimated strain values were compared to those recorded by the in-situ sensors in the longitudinal and transverse directions. In the longitudinal direction, the stresses were shown to reach the maximum positive values at the points of contact of the girder with the concrete diaphragm. The values decreased gradually along the length of the bridge to reach the maximum negative values approximately at the mid-span of the bridge deck. The area under consideration, where the towers are located, was in complete tension in the longitudinal and transverse directions. The highest tensile strain values reached 2100 micro-strain at the intersection of the intermediate bent with one of the pre-cast concrete girders. A strain model was proposed to evaluate the strain data collected from the embedded sensors. The model represents the total strain as a summation of strains due to thermal deformation, drying and autogenous shrinkage, and structural deformation. The model was used to evaluate strains and estimate values of the concrete shrinkage during the first 30-36 hours, which corresponded to the time of demolding of the shrinkage samples as well as the load distribution factor between the concrete slab and the steel corrugated sheet that varied with concrete age. Findings indicated that the load distribution factor increased with concrete age reaching a value of 0.98 at 260 days. The concrete shrinkage during the first 30-36 hours was then estimated to be 75 micro-strain.
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