PENETRATION RESISTANCE (WINDSOR PROBE & PNR TEST)
his technique offers a means of determining relative strengths of concrete in the same structure or relative strength of different structures. Because of nature of equipments, it can not and should not be expected to yield absolute values of strength. ASTM C-803 gives this standard test method titled “Penetration Resistance of Hardened Concrete”.
Windsor Probe is penetration resistance measurement equipment, which consists of a gun powder actuated driver, hardened alloy of probe, loaded cartridges, a depth gauge and other accessories. In this technique a gunpowder actuated driver is used to fire a hardened alloy probe into the concrete. During testing, it is the exposed length of probe which is measured by a calibration depth gauge. But it is preferable to express the coefficient of variation in terms of depth of penetration as the fundamental relation is between concrete strength and penetration depth.
The probe shown in fig.1 has a diameter of 6.3mm, length of 73mm and conical point at the tip. The rear of the probe is threaded and screwed into a probe-driving head, which is 12.6mm in diameter and fits snugly along with a rubber washer into the bore of the driver. As the probe penetrates into the concrete, test results are actually not affected by local surface conditions such as texture and moisture content. However damage in the form of cracking may be cause to slender members. A minimum edge distance and member thickness of 150mm is required. It is important to leave 50mm distance from the reinforcement present in the member since the presence of reinforcing bars within the zone of influence of penetrating probe affects the penetration depth.
A pin penetration test device (PNR Tester) which requires less energy than the Windsor Probe system is given in fig.2.
Fig.1: Windsor Probe
Fig.2: Penetration Tester
Being a low energy device, sensitivity is reduced at higher strengths. Hence it is not recommended for testing concrete having strength above 28 N/sq.mm. in this a spring-loaded device, having energy of about 1.3% of that of Windsor probe, us used to drive 3.56mm diameter, a pointed hardened steel pin into the concrete. The penetration of pin creates a small indentation (or hole) on the surface of concrete. The pin is removed from the hole, the hole is cleaned with an air jet and the hole depth is measured with a suitable depth gauge. Each time a new pin is required as the pin gets blunted after use.
The strength properties of both mortar and stone aggregate influence the penetration depth of the probe in a concrete, which is contrastingly different than cube crushing strength, wherein the mortar strength predominantly governs the strength. Thus the type of stone aggregate has a strong effect on the relation of concrete strength versus depth of penetration as given in fig.3.
Fig.3: Effect of aggregate type on relationship between concrete strength and depth of probe penetration
For two samples of concrete with equal cube crushing strength, penetration depth would be more in the sample with softer aggregate than the one with harder aggregates. Correlation of the penetration resistance to compressive strength is based on calibration curves obtained from laboratory test on specific concrete with particular type of aggregates. Aggregate hardness is determined from standard samples provided along with the instrument. Aggregate size in the mix also influence the scatter of individual probe readings. This technique offers a means of determining relative strength of concrete in the same structure or relative strength of different structures. Because of the nature of equipment it can not and should not be expected to yield absolute values of strength. This test is not operator independent although verticality of bolt relative to the surface is obviously important and safety device in the driver prevents, if alignment is poor.
It is claimed an average coefficient of variation for a series of groups of three readings on similar concrete of the order of 4% may be expected. It has been observed that ±20% accuracy may be possible in strength determination of concrete. Fig.4 explains the approximate shape of failure during the test.
Fig.4: Approximate shape of failure zone in probe penetration test
