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Penetration resistance test is conducted on concrete structures using Windsor Probe test machine. In this test method, a steel probe is fired on the concrete surface by a sudden explosion. The penetration is inversely proportional to the strength of concrete. The result of the test is influenced by aggregate strength and nature of formed surfaces of concrete.
The purpose of the penetration resistance test is used to determine the uniformity of concrete, specify the poor quality or deteriorated concrete zones, and evaluate the in-place strength of concrete. It is sometimes necessary to estimate the strength of concrete on-site for early form removal or to investigate the strength of concrete in place because of low cylinder test results.
Due to the nature of the equipment, it cannot and should not be expected to yield absolute values of strength. The penetration resistance test on hardened concrete can be carried out based on the procedures and specifications of the Standard Test Method for Penetration Resistance of Hardened Concrete (ASTM C 803/ 803M- 97) or British Standard (BS 1881 Part 207).
Purpose of Penetration Resistance Test
- Determine the uniformity of concrete
- Specify exact locations of poor quality or deteriorated concrete zones
- Assess in-place strength of concrete
Application of Penetration Resistance Test
Penetration resistance test is conducted to estimate the strength of concrete on-site for early form removal or to investigate the strength of concrete in place because of low cylinder test results.
Probe consists of driver unit used to drive the probe into the concrete and probe manufactured from alloy-steel rod plated for corrosion protection, with a blunt conical end that can be inserted into the driver unit and driven into the concrete surface.
Probes of 79.4-mm overall length and 7.9-mm diameter, with the penetrating end diameter reduced to 6.4 mm for approximately 14.3 mm in length, is suitable for testing concrete with a unit weight of 2000 kg/m3 or greater. Pins can be used instead of probe when the penetration resistance test is carried out using this tool.
2. Measurement Equipment
Measurement equipment such as a Vernier caliper or depth gauge to measure the exposed length of a probe to the nearest 0.5 mm. The measuring equipment shall include a reference base plate which is supported on the concrete surface at three equally spaced points at least 50 mm from the probe to be measured.
3. Positioning Device
A single device or a triangular device with holes at the three corners can be used for positioning and guiding the probe and driver unit.
- Concrete to be tested should have gain enough strength so that the probe would not penetrate more than one half the thickness of the concrete member
- Maximum spacing between probes is 175 mm
- Minimum spacing between probes is 100 mm
- Minimum spacing between concrete and the edge of a concrete surface is 100mm.
- A minimum of three firmly embedded test probes in a given test area constitute one test.
- Place the positioning device on the surface of the concrete at the location to be tested.
- Mount a probe in the driver unit
- Position the driver in the positioning device
- Fire the probe into the concrete.
- Remove the positioning device and tap the probe on the exposed end with a small hammer to ensure that it has not rebounded and to confirm that it is firmly embedded.
- Place the measuring base plate over the probe and position it so that it bears firmly on the surface of the concrete without rocking or other movement.
- If the probe is sloped with respect to the surface of the concrete, take four measurements equally spaced around and parallel to the probe and average them to get the measurement.
- If the probe is not firmly embedded then the test is not valid and hence it should be repeated.
- Similarly, the test should be repeated if the range of depth of penetration for three tests is more than 8.4mm in concrete made with 25mm maximum aggregate size, and 11.7 in concrete made with 50mm maximum aggregate size.
- When tests are to be made on concrete having a density of approximately 2000 kg/m3 or less, and on all concrete with strengths less than 17 MPa, decrease the amount of energy delivered to the probe by the driver or use a larger-diameter probe, or both.
The penetration resistance of concrete is computed by measuring the exposed length of probes driven into concrete. In order to estimate concrete strength, it is necessary to establish a relationship between penetration resistance and concrete strength.
Such a relationship must be established for a given test apparatus, using similar concrete materials and mixture proportions as in the structure. Procedures and statistical methods provided in ACI 228.1R can be used for developing and using the strength relationship.
Factors Influence the Penetration Resistance Test Result
Nature of the formed surfaces for instance wooden forms versus steel forms. That is why correlation testing should be performed on specimens with formed surfaces similar to those to be used during construction. Probe penetration resistance is affected by concrete strength as well as the nature of the coarse aggregate.