The principle of  windsor probe test is like the rebound hammer test.It is a hardness tester, and its inventors’ claim that the penetration of the probe reflects the precise compressive strength in a localized area is not strictly true. However, the probe penetration does relate to some property of the concrete below the surface, and, within limits, it has been possible to develop empirical correlations between strength properties and the penetration of the probe.

      The Windsor probe consists of a powder-actuated gun or driver, hardened alloy steel probes, loaded cartridges, a depth gauge for measuring the penetration of probes, and other related equipment. As the device looks like a firearm it may be necessary to obtain official approval for its use in some countries. The probes have a tip diameter of 6.3 mm, a length of 79.5 mm, and a conical point. Probes of 7.9 mm diameter are also available for the testing of concrete made with lightweight aggregates. The rear of the probe is threaded and screws into a probe driving head, which is 12.7 mm in diameter and fits snugly into the bore of the driver.
      The probe is driven into the concrete by the firing of a precision powder charge that develops energy of 79.5 m kg. For the testing of relatively low strength concrete, the power level can be reduced by pushing the driver head further into the barrel.

     The area to be tested must have a brush finish or a smooth surface. To test structures with coarse finishes, the surface first must be ground smooth in the area of the test. Briefly, the powder-actuated driver is used to drive a probe into the concrete. If flat surfaces are to be tested a suitable locating template to provide 178 mm equilateral triangular pattern is used, and three probes are driven into the concrete, one at each corner. A depth gauge measures the exposed lengths of the individual probes. The manufacturer also supplies a mechanical averaging device for measuring the average exposed length of the three probes fired in a triangular pattern. The mechanical averaging device consists of two triangular plates. The reference plate with three legs slips over the three probes and rests on the surface of the concrete. The other triangular plate rests against the tops of the three probes. The distance between the two plates, giving the mechanical average of exposed lengths of the three probes, is measured by a depth gauge inserted through a hole in the centre of the top plate. For testing  structures with curved surfaces, three probes are driven individually using the single probelocating  template. In either case, the measured average value of exposed probe length may  then be used to estimate the compressive strength of concrete by means of appropriate
correlation data.
       The manufacturer of the Windsor probe test system has published tables relating the  exposed length of the probe with the compressive strength of the concrete. For each exposed  length value, different values for compressive strength are given, depending on the hardness  of the aggregate as measured by the Mohs' scale of hardness. The tables provided by the  manufacturer are based on empirical relationships established in his laboratory. However, investigations carried out by Gaynor, Arni, Mallotra, and several others indicate that the  manufacturer's tables do not always give satisfactory results. Sometimes they considerably  overestimate the actual strength and in other instances they underestimate the strength.
     It is, therefore, imperative for each user of the probe to correlate probe test results with the type of concrete being used. Although the penetration resistance technique has been standardized the standard does not provide a procedure for developing a correlation. A practical procedure for developing such a relationship is outlined below.

  1. Prepare a number of 150 mm × 300 mm cylinders, or 150 mm3 cubes, and companion 600 mm × 600 mm × 200 mm concrete slabs covering a strength range that is to be encountered on a job site. Use the same cement and the same type and size of aggregates as those to be used on the job. Cure the specimens under standard moist curing conditions, keeping the curing period the same as the specified control age in the field.

  2. Test three specimens in compression at the age specified, using standard testing procedure. Then fire three probes into the top surface of the slab at least 150 mm apart and at least 150 mm in from the edges. If any of the three probes fails to properly penetrate the slab, remove it and fire another. Make sure that at least three valid probe results are available. Measure the exposed probe lengths and average the three results.

  3. Repeat the above procedure for all test specimens.

  4. Plot the exposed probe length against the compressive strength, and fit a curve or line by the method of least squares. The 95% confidence limits for individual results may also be drawn on the graph. These limits will describe the interval within which the probability of a test result falling is 95%.

   A typical correlation curve is shown in Fig. 7.1, together with the 95 confidence limits for individual values. The correlation published by several investigators for concrete made with limestone gravel, chert, and traprock aggregates are shown in Fig. 7.2. Note that different relationships have been obtained for concrete with aggregates having similar Mohs' hardness numbers.

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