2.1 Materials and Mixture Proportions
Thirty three different types of ternary and binary cementitious mixtures including the control mixture of 100 % ordinary portland cement with a water/cementitious materials ratio of 0.44 were designed to give a wide range of values for this experimental program. This water/cementitious materials ratio is typical of exposed bridge deck and substructure concrete. All mixtures contained 335 kg/m3 (564 lbs/yd3) of cementitious material with a coarse aggregate factor (CAF) of 0.67. Limestone coarse aggregate of size 19 mm (3/4 inch) meeting ASTM C33 No. 67 gradation and ASTM C33 silica sand were used. All the SCMs were replaced by mass. Tests were performed on mixtures using:
Due to sulfate attack problems in California, it is mandatory to use Type II-V cement instead of Type I cement. The selection of mixture design was based on concrete mixtures meeting basic technical properties and also representing a diverse range of solutions to long term durability. The basic mixture parameters were coded into the names of the mixtures with percentage of each cementitious material, e.g. 75TII-V/20F/5SF means 75 % Type II-V Cement, 20 % Class F fly ash and 5 % Silica Fume. A high-range water reducing admixture (Glenium 7500) and an air entraining agent (MBVR AE90) were used to meet better workability and other durability performance specifications. All the mixtures were cast according to ASTM C192 practice and four cylinders 100 × 200 mm (4 × 8 in.) were prepared for both bulk and surface electrical resistivity testing at all ages. The cylinders were demolded after 24 ± 2 h and they were continuously cured in lime water tank. Electrical resistivity was measured on 7, 14, 28, 56, 91 and 161 days.
2.2 Measurement of Surface Electrical Resistivity
Surface resistivity measurement was performed by commercially available 4 point Wenner probe surface resistivity (SR) meter, manufactured by Proceq. In this study, Florida testing method was used for electrical resistivity measurement on 7, 14, 28, 56, 91 and 161 days for 100 × 200 mm (4 × 8 in.) cylinders except the curing condition and the probe spacing. All the cylinders were cast and then demolded within 24 ± 2 h. After demolding, the cylinders were placed in lime water tank. A multiplier of 1.1 is used for electrical resistivity data as suggested by AASHTO TP-95 specification for lime water curing condition. All the cylinders were removed from lime water tank on the specified testing days and tested in saturated surface dry (SSD) condition at 23 ± 2 °C by Resipod Wenner Probe meter. Readings were taken two times with 0, 90, 180 and 270 degree angles of circular face of each concrete cylinder. The data in this research were collected using a probe spacing of 50 mm (2 inches), instead of 38 mm (1.5 inches) as recommended by FDOT. The probe spacing could not be changed as it came from the manufacturer with 50 mm (2 inches) spacing. The whole experimental process took less than half hour to complete. Four cylinders were tested for each concrete mixture and altogether 32 data points (4 × 8 = 32 points) were collected for each mixture for surface electrical resistivity. The equipment measures the current flowing between the outer electrodes and the potential difference between the two inner electrodes. Assuming that the concrete cylinder has homogeneous semi-infinite geometry (the dimensions of the element are large in comparison of the probe spacing), and the probe depth is far less than the probe spacing, the concrete cylinder resistivity ρ can be computed as:
(1)
wherea is the probe spacing in mm;V is the applied voltage in volt;I is the current in ampere; andR1 is the surface resistance in KOhm.
2.3 Measurement of Bulk Electrical Resistivity
The Merlin conductivity tester was used to measure the bulk electrical conductivity, or its inverse, the bulk electrical resistivity, of water saturated concrete cylinders of 100 × 200 mm (4 × 8 in) in lime water tank. Bulk resistivity measurement was performed on the same cylinder sample as of surface electrical resistivity measurement. This test is also non-destructive and simple to perform. A test result was obtained within 2 s, and sample preparation and testing altogether takes less than 30 min. The conductivity of a saturated concrete specimen provides information on the resistance of the concrete to penetration of ionic species by the diffusion mechanism. The curing criteria and number of specimens were same as of surface electrical resistivity. Additionally, before testing, the tester verified with Merlin verification cylinder for rapid calibration purpose. A cylinder was placed on the support and two ends were wet with spraying bottle. This test method consists of applying a potential difference to the cylindrical specimen, thereby producing a current flow through the cylinder. The potential difference and resulting current can be utilized to obtain the electrical resistance. Two readings were obtained from data logger for each cylinder specimen by swapping the two end faces of a cylinder. From the measured currentI and voltageV, the bulk resistivity was calculated as follows:
(2)
where,A is the surface area,L is the length of the specimen andR2 is the bulk resistance. Figure 1a and b shows experimental set up for Wenner 4-probe meter and Merlin conductivity tester.
For 100 × 200 mm (4 × 8 inch) cylinder, surface areaA = ,d = 100 mm (4 inches),L = 200 mm (8 inches), and probe spacing ofa = 50 mm (2 inches). Finally, the ratio of theoretical surface and bulk resistance can be computed in Eq. (3).
(3)
Morris et al. (1996) developed the geometry correction factor for specific cylinder sizes and the ratio of two different types of resistivity is computed as 2.63. As a result, the ratio of theoretical surface and bulk resistance can be calculated as:
(4)