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Fig. 1 Beam end specimen
Deformed rebar of 12 and 16 mm diameter with cover of three times bar diameter were investigated. Duplicate sets of confined and unconfined specimens were tested. The confined specimens had three sets of 6 mm stainless steel stirrups equally spaced from the plastic tube, at 75 mm centres.
This represents four groups of specimens with a combination of different bar diameter and with/without confinement. The specimens were selected in order to investigate the influence of bar size, confinement and crack width on bond strength.
2.2 Materials
The mix design is shown, Table 1. The cement was Type I Portland cement, the aggregate was basalt with specific gravity 2.99. The coarse and fine aggregate were prepared in accordance with AS 1141-2000. Mixing was undertaken in accordance with AS 1012.2-1994. Specimens were cured for 28 days under wet hessian before testing.
Table 1 Concrete mix design
Material Cement w/c Sand 10 mm washed aggregate 7 mm washed aggregate Salt Slump
Quantity 381 kg/m3 0.49 517 kg/m3 463 kg/m3 463 kg/m3 18.84 kg/m3 140 ± 25 mm
In order to compare bond strength for the different concrete compressive strengths, Eq. 1 is used to normalize bond strength for non-corroded specimens as has been used by other researcher [8].
(1)
where is the bond strength for grade 40 concrete, τ exptl is the experimental bond strength and f c is the experimental compressive strength.
The tensile strength of the Φ12 and Φ16 mm steel bars was nominally 500 MPa, which equates to a failure load of 56.5 and 100.5 kN, respectively.
2.3 Experiment methodology
Accelerated corrosion has been used by a number of authors to replicate the corrosion of the reinforcing steel happening in the natural environment [2, 3, 5, 6, 10, 18, 20, 24, 27, 28, 30]. These have involved experiments using impressed currents or artificial weathering with wet/dry cycles and elevated temperatures to reduce the time until corrosion, while maintaining deterioration mechanisms representative of natural exposure. Studies using impressed currents have used current densities between 100 μA/cm2 and 500 mA/cm2 [20]. Research has suggested that current densities up to 200 μA/cm2 result in similar stresses during the early stages of corrosion when compared to 100 μA/cm2 [21]. As such an applied current density of 200 μA/cm2 was selected for this study—representative of the lower end of the spectrum of such current densities adopted in previous research. However, caution should be applied when accelerating the corrosion using impressed current as the acceleration process does not exactly replicate the mechanisms involved in actual structures. In accelerated tests the pits are not allowed to progress naturally, and there may be a more uniform corrosion on the surface. Also the rate of corrosion may impact on the corrosion products, such that different oxidation state products may be formed, which could impact on bond.
The steel bars served as the anode and four mild steel metal plates were fixed on the surface to serve as cathodes. Sponges (sprayed with salt water) were placed between the metal plates and concrete to provide an adequate contact, Fig. 2.
Fig. 2 Accelerated corrosion system
When the required crack width was achieved for a particular bar, the impressed current was discontinued for that bar. The specimen was removed for pullout testing when all four locations exhibited the target crack width. Average surface crack widths of 0.05, 0.5, 1 and 1.5 mm were adopted as the target crack widths. The surface crack width was measured at 20 mm intervals along the length of the bar, beginning 20 mm from the end of the (plastic tube) bond breaker using an optical microscope. The level of accuracy in the measurements was ±0.02 mm. Measurements of crack width were taken on the surface normal to the bar direction regardless of the actual crack orientation at that location.