ASTM C1202-RCPT Test
ASTM C1202 “Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration” is actually a test of electrical conductance, rather chloride permeability as is often stated. As discussed on page 88, electrical conductivity is related to the diffusion coefficient. In this test, a water- saturated concrete specimen, nominally 100 mm diameter and 50 mm thick, is positioned in a test cell (right and pg. 121) containing fluid reservoirs on both ends of the specimen. One reservoir is filled with a 3 % NaCl solution and the other with a 0.3N NaOH solution. An electrical potential of 60 VDC is applied across the cell. The negative terminal of the potential source is connected to the electrode in the the NaCl solution and the positive terminal is connected to the electrode in the NaOH solution. The negatively charged ions will migrate towards the positive terminal resulting in current through the specimen. The current is measured.
The more permeable is the concrete, the more negative ions will migrate through the specimen, and a higher current will be measured. The current is measured for 6 hours. The area under the curve of current versus time is determined, which represents the total charge or Coulombs passed across the specimen. Test results are corrected for a standard specimen diameter of 95 mm. The Coulomb values are used for classifying the concrete as follows (ASTM C1202):
|w/c* > 0.5
|w/c = 0.4 to 0.5
|w/c < 0.4
*w/c = water-cement ratio
NT Build 492-Chloride Migration Test
To use PROOVE´it for the chloride migration test described in NT Build 492 “Chloride Migration Coefficient from Non-Steady State Migration Experiments”, the reservoir surrounding the negative terminal is filled with a 10 % NaCl solution and the reservoir surrounding the positive terminal is filled with a 0.3N NaOH solution. A 30 VDC potential is applied across the specimen, and the initial current is measured. Based on the measured initial current, the test voltage and test duration are selected accordingly. For example, if the initial current is between 120 and 180 mA, the test voltage is 15 VDC and the test duration is 24 h, but if the initial current is less than 5 mA, the test voltage is 60 VDC and test duration is 96 h. After the test is completed, the specimen is split, and the chloride ion penetration is measured by spraying the split surface with a 0.1 M silver nitrate solution, which precipitates as white AgCl where chlorides are present. From the penetration depth and test conditions, the chloride ion migration coefficient is calculated. It is important to maintain a constant temperature in the solutions in the reservoirs. Therefore, cells with cooling fins (Part No. PR-1100) are recommended.
There is also an AASHTO test method TP-64, “Standard Method of Test for Predicting Chloride Penetration of Hydraulic Cement Concrete by the Rapid Migration Procedure,” that uses the same procedure as NT Build 492. The test result, however, is reported as a rate of penetration, by dividing the depth of penetration, in mm, by the product of applied voltage (V) and the test duration (h).
ASTM C1760-Bulk Electrical Conductivity
ASTM C1760, “Standard Test Method for Bulk Electrical Conductivity of Hardened Concrete,” involves the same basic testing procedure as ASTM C1202, with the following exceptions: 1) both reservoirs contain the 3 % NaCl solution; 2) the specimen length can be up to 200 mm; and 3) the current is measured at 1 minute after turning on the power supply. The bulk electrical conductivity is calculated using Eq. (2) on page 89. As was discussed on page 90, different ranges of bulk electrical conductivity correspond to different ranges of charge passed in accordance with ASTM C1202.
Accuracy and Variability
At 60 VDC, the accuracy of the PROOVE´it microprocessor power supply is within ± 0.1 mA for a current between 30 mA and 300 mA. The repeatability of the RCPT or Coulomb Test is reported to be about 12 % (ASTM C1202), and the repeatability of the migration test is reported to be about 9 % (NT Build 492).
PROOVE’it System Features
The computer-controlled microprocessor power supply and the Windows®-based software for testing and report preparation offers the following key features:
- Testing up to 8 cells simultaneously
- Voltage settings of 5 to 60 VDC in 5 V increments
- Programmable testing time as required
- Temperature measurement and recording
- Cyclic testing option for effect of curing duration
- Measure concrete conductivity at 60 VDC in 1 min
- Predicted 6-h Coulomb value every 5 min
- Documentation of each test result
A complete system composed of coring and slicing equipment, vacuum desiccator, vacuum pump, watertight test cells, microprocessor power supply, and software are presented below. The cells are easy to assemble, simple to maintain, and watertight.