Mona Qouqa Supervised by Dr. Andrew J. Boyd

Department of Civil Engineering and Applied Mechanics

Electrical surface resistivity of saturated concrete is a novel approach

to estimating the susceptibility of concrete to deterioration. Surface

resistivity, or the resistance to electrical current flow, correlates well

with the rate of penetration of ions into the concrete, which is directly

proportional to the rate of deterioration. The lower the resistivity, the

faster that ions can penetrate into and through the concrete, resulting

in faster damage accumulation.

HOW DOES IT WORK?

The apparatus used for measuring surface resistivity of concrete

was a Wenner Probe resistivity meter.

The meter has four equally spaced pins ending in water saturated

wooden tips.

The two outer pins apply an AC current through the saturated

concrete. This results in a potential difference between the two

inner pins which is then measured.

The resistivity of the concrete is then calculated using the applied

current, the generated potential difference between the two inner

pins and the sample’s surface area and length.

INTRODUCTION

ACKNOWLEDGEMENTS

I would like to thank my supervisor Dr. Andrew J. Boyd for his invaluable guidance throughout the project. I would also

like to thank the Civil Engineering Department and the lab technicians for their support.

In this research, four different curing/exposure solutions were studied:

o Tap water

o Lime water

o 5% sodium sulphate solution

o 5% sodium chloride solution

Four sets of samples were prepared and cured continuously in each of the curing solutions for about 8 months. Each

set consisted of three 100 mm diameter x 200 mm long cylinders, three 150 mm diameter x 100 mm long pucks, and

three 75 x 75 x 275 mm prisms.

RESULTS AND DISCUSSION

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Re

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Time (Days)

Sulphate-cast surface

Sulphate-finishedsurface

Chloride-cast surface

Chloride-finshed surface

Lime water-cast surface

Lime water-finshedsurface

Tap water-cast surface

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Resis

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Time (Days)

Sulphate

Chloride

Lime water

Tap water

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Time (Days)

Sulphate

Chloride

Lime water

Tap water

Figure 3: Resistivity development over time for: (a) puck-shaped, (b) prismatic, and (c) cylindrical samples.

(a) (b) (c)

As shown in Figure 3,the strength of the samples increased for all curing solutions. Subsequent to the concrete reaching

its ultimate strength as a result of curing over long periods of time, stresses start to form as a result of the formation of

chemical reaction by-products, inducing deterioration levels assessed through surface resistivity for samples cured in

sulphate and chloride solutions. Samples cured in tap and lime water solutions, continued to show an increase in

resistivity over the entire period of curing.

CONCLUSION

Amongst the samples that were continuously cured it was shown that the puck-shaped samples were the most resistant,

followed by the cylinders and, finally, the prisms. The rate of increase in their resistivity was higher than both the

cylindrical and prismatic samples

The best combination of sample geometry and curing solution was puck-shaped samples cured in tap or lime water.

Since chloride exposure does not actually damage concrete, the reduction in resistivity due to chloride exposure

represents a change in the basic resistivity of the concrete.

Figure 1:Wenner Probe

Figure 2: Completed circuit as probe comes into

contact with the water saturated concrete.