MAIN ROADS Western Australia Document 71/04/622.1 Issue Date: 13 January 2012 D10#280791

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RESISTIVITY OF CONCRETE 1 SCOPE This method describes the procedure for determining the electrical resistance (resistivity) of in situ concrete. 2 SAFETY This method does not attempt to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this method to establish appropriate occupational health and safety practices that meet statutory regulations. 3 THEORY The rate of corrosion of steel reinforcement embedded in concrete is governed by the magnitude of the ionic corrosion current which flows in the concrete between the anodic and cathodic areas on the reinforcement. The magnitude of this current is dependent on the potential difference between the anode and the cathode and the electrical resistance of the concrete. The electrical resistance of the concrete is determined using a four-probe resistivity meter (Wenner probe) whereby an AC current is applied between the two outer probes and the potential difference measured across the two inner probes is determined. 4 APPARATUS (a) A suitable soil resistance meter NOTE: A modified 4 pin soil resistance meter, such as the Nilsson Model 400 has been shown to be suitable. See Figure 1. (b) Four steel probe pins NOTE: It is desirable for the probe pins to have a maximum diameter of no more than 1/10th of the probe spacing (a). Probe pins made of 6mm mild steel have been found to give reasonable results. Pins should be cylindrical in shape to ensure good electrical contact with a slightly tapered end for the ease of insertion. (c) 300 mm steel rule (1 mm graduations) (d) Masonry drill and bit (e) Hammer (f) Marking pen (g) Covermeter

5 PROCEDURE (a) Locate a representative area of concrete, free of major defects, such as cracking or spalling, within the designated area for resistivity measurement. (b) Measure and mark in an approximately straight line, using a ruler and marking pen, the position of four points of designated separation (a) (Fig 1). The line joining these points shall be located centrally between and parallel to the adjacent reinforcing bars. The probe spacing (a) must be larger than the maximum aggregate size but small enough to prevent the steel reinforcing bars from influencing the result. A 50 mm spacing has been found appropriate in most situations NOTES:

i. The designated separation (a) is normally specified

by the client.

ii. The line joining these points may cross a single steel reinforcing bar at right angles without significant effect. Measurements should not be taken within 2a of the edge of the concrete element. (c) Drill a hole using a suitable masonry drill and bit at each marked point, of sufficient diameter to allow a tight fit of each probe pin, to a depth of approximately 20 mm. (d) Wet each hole and probe with water and insert a probe into each hole. Secure the probes firmly by tapping with a hammer to a depth of approximately 20 mm. NOTE: An effective electrical contact between the probe and the concrete is essential to prevent high contact resistances from affecting the measurement. The depth of hole may need to be increased in cases of significant carbonation to beyond the carbonation depth (e) Check the calibration of the resistance meter using a set of standard 10 , 100 and 1000 resistors at the start and finish of each days readings or where results are in doubt (Fig 2). (f) Connect the resistivity meter to the installed probe pins by means of lead wires (Fig 1). (g) Set the sensitivity of the meter to low and adjust the meter to null.

TEST METHOD WA 622.1 – 2012

MAIN ROADS Western Australia Document 71/04/622.1 Issue Date: 13 January 2012 D10#280791

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TEST METHOD WA 622.1 – 2012

(h) Set the sensitivity to high and refine the null adjustment. (i) Record the resistance in ohms necessary to achieve the null reading. 6 CALCULATONS Calculate the Resistivity using the following formula: = 2.a. Where: = Resistivity in ohm – cm a = Spacing between probes in cm = Meter reading ohms

7 REPORTING (a) Report the Resistivity in ohm – cm to the nearest

100 cm (b). The following information shall be recorded:

Structure identification Position of structure Date of Test Surface condition of concrete Location of steel reinforcements in the vicinity of the probe pins The probe separation (a)

8 FIGURES AND DRAWINGS

FIGURE 2: Resistance Meter Test Plate

FIGURE 1: Resistance Meter

MAIN ROADS Western Australia Document 71/04/622.1 Issue Date: 13 January 2012 D10#280791

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TEST METHOD WA 622.1 – 2012

9 ISSUING AUTHORITY

10 REVISION STATUS RECORD

Page No. Section Revision

Description / Reference

All All Complete Revision and Re-issue of Test Method

Document Owner:

Manager Materials Engineering

Delegated Custodian:

Pavements Manager