Handbook of nondestructive testing of concrete：Penetration Resistance Methods.pdf

2-1 2 Penetration Resistance Methods* 2.1Introduction 2-1 2.2Probe Penetration Test System. 2-2 Principle · Description · Method of Testing · Correlation Pocedure 2.3Evaluation of the Probe Penetration Test 2-4 Mechanism of Concrete Failure · Correlations between Probe Test Results and Compressive Strength · Variability of the Probe Penetration Test · Variations in the Estimated Strength Values · Nondestructive Nature of the Probe Penetration Test · Use of the Probe Penetration Test for Early Form Removal · Probe Penetration Test vs. Core Testing · Probe Penetration Test vs. Rebound Hammer Test · North American Survey on the Use of the Probe Penetration Test · Advantages and Disadvantages of the Probe Penetration Test 2.4Pin Penetration Test 2-13 2.5Standardization of Penetration Resistance Techniques 2-15 2.6Limitations and Usefulness of Penetration Resistance Methods. 2-15 This chapter reviews the development of penetration resistance methods for testing concrete nondestruc- tively. These are being increasingly used for quality control and strength estimation of in situ concrete. Among the penetration techniques presently available, the most well known and widely used is the Windsor probe test. The principle of this method, the test equipment and procedures, and the preparation of calibration charts are described in detail. Factors affecting the variability of the test are discussed. Correla- tions that have been developed between the Windsor probe test results and the compressive strength of concrete are presented. A pin penetration test was developed in Canada for the purpose of determining safe form removal times. The advantages, limitations, and applications of the penetration methods are outlined. The chapter concludes with a list of pertinent references. 2.1Introduction Penetration resistance methods are based on the determination of the depth of penetration of probes (steel rods or pins) into concrete. This provides a measure of the hardness or penetration resistance of the material that can be related to its strength. *©Minister of Supply and Services Canada, 1989. *Deceased. V. Mohan Malhotra Department of Natural Resources Canada, Ottawa Georges G. Carette* Department of Natural Resources Canada, Ottawa 2-2Handbook on Nondestructive Testing of Concrete: Second Edition The measurement of concrete hardness by probing techniques was reported by Voellmy1 in 1954. Two techniques were used. In one case, a hammer known as Simbi was used to perforate concrete, and the depth of the borehole was correlated to the compressive strength of concrete cubes. In the other technique, the probing of concrete was achieved by Spit pins, and the depth of penetration of the pins was correlated with the compressive strength of concrete Apart from the data reported by Voellmy, there is little other published work available on these tests, and they appear to have received little acceptance in Europe or elsewhere. Perhaps the introduction of the rebound method around 1950 was one of the reasons for the failure of the above tests to achieve general acceptance. In the 1960s, the Windsor probe test system was introduced in the United States and this was followed by the pin penetration test in Canada in the 1980s. 2.2Probe Penetration Test System Between 1964 and 1966. a device known as the Windsor probe was advanced for penetration testing of concrete in the laboratory as well as in situ. The device was meant to estimate the quality and compressive strength of in situ concrete by measuring the depth of penetration of probes driven into the concrete by means of a powder-actuated driver. The development of this technique was the joint undertaking of the Port of New York Authority, New York, and the Windsor Machinery Co., Connecticut. This development was closely related to studies reported by Kopf.2 Results of the investigations carried out by the Port of New York Authority were presented by Cantor3 in 1970. Meanwhile, a number of other organizations had initiated exploratory studies of this technique,48 and a few years later, Arni9,10 reported the results of a detailed investigation on the evaluation of the Windsor probe, while Malhotra1113 reported the results of his investigations on both 150 × 300-mm cylinders and 610 × 610 × 200-mm concrete slabs. In 1972, Klotz14 stated that extensive application of the Windsor probe test system had been made in investigations of in-place compressive strength of concrete and in determinations of concrete quality. The Windsor probe had been used to test reinforced concrete pipes, highway bridge piers, abutments, pavements, and concrete damaged by fi re. In the 1970s, several U.S. federal agencies and state highway departments reported investigations on the assessment of the Windsor probe for in situ testing of hardened concrete.1519 In 1984 Swamy and Al-Hamed20 in the U.K. published results of a study on the use of the Windsor probe system to estimate the in situ strength of both lightweight and normal weight concretes. 2.2.1Principle The Windsor probe, like the rebound hammer, is a hardness tester, and its inventors claim that the penetration of the probe refl ects the precise compressive strength in a localized area is not strictly true.21 However, the probe penetration relates 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 pene- tration of the probe. 2.2.2Description The Windsor probe consists of a powder-actuated gun or driver (Figure 2.1), hardened alloy-steel probes, loaded cartridges, a depth gauge for measuring the penetration of probes, and other related equipment. The probes have a tip diameter of 6.3 mm, a length of 79.5 mm, and a conical point (Figure 2.2). Probes of 7.9 mm diameter are also available for the testing of concretes 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 fi ts snugly into the bore of the driver. The probe is driven into the concrete by the fi ring of a precision powder charge that develops an energy of 79.5 m-kg. For the testing of relatively low-strength concrete, the powder level can be reduced by pushing the driver head further into the barrel. Penetration Resistance Methods2-3 2.2.3Method of Testing The method of testing is relatively simple and is given in the manual supplied by the manufacturer. The area to be tested must have a brush fi nish or a smooth surface. To test structures with coarse fi nishes, the surface fi rst must be ground smooth in the area of the test. Briefl y, the powder-actuated driver is used to drive a probe into concrete. If fl at 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. The exposed lengths of the individual probes are measured by a depth gauge. The manufacturer also supplies a mechanical averaging device for measuring the average exposed length of the three probes fi red 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 center of the top plate. For testing structures with curved surfaces, three FIGURE 2.1 A view of the Windsor probe equipment. (A) Driver unit. (B) Probe for normal-weight concrete. (C) Single probe template. (D) Calibrated depth gauge. (Adapted from Reference 11.) FIGURE 2.2 A view of probe for normal-weight concrete before and after assembly. (Adapted from Reference 11.) 12.7 mm7.9 mm Plastic Fletching 79.4 mm 38.1 mm25.4 mm12.7 mm 25.4 mm PROBE BEFORE ASSEMBLY 95.2 mm 6.3 mm Driving Head ASSEMBLED DRIVING HEAD AND PROBE 2-4Handbook on Nondestructive Testing of Concrete: Second Edition probes are driven individually using the single probe locating 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. 2.2.4Correlation Procedure The manufacturer of the Windsor probe test system has published tables relating exposed length of the probe with compressive strength of concrete. For each exposed length value, different values for com- pressive 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,7 Arni,9 Malhotra,1113 and several others8,16,2224 indicate that the manufacturers tables do not always give satisfactory results. Sometimes they considerably overestimate the actual strength11,2022 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 × 300-mm cylinders, or 150-mm cubes, and companion 600 × 600 × 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 specifi ed control age in the fi eld. 2. Test three specimens in compression at the age specifi ed, using standard testing procedure. Then fi re three probes into the top surface of the slab at least 150 mm apart and at least 150 mm from the edges (Figure 2.3). If any of the three probes fails to properly penetrate the slab, remove it and fi re 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 fi t a curve or line by the method of least squares. The 95% confi dence 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 Figure 2.4, together with the 95% confi dence limits for individual values. The correlation published by several investigators for concretes made with limestone gravel, chert, and traprock aggregates are shown in Figure 2.5. Note that different relationships have been obtained for concretes with aggregates having similar Mohs hardness numbers. 2.3Evaluation of the Probe Penetration Test 2.3.1Mechanism of Concrete Failure There is no rigorous theoretical analysis of the probe penetration test available. Such analysis may, in fact, not be easy to achieve in view of the complex combinations of dynamic stresses developed during penetration of the probe, and the heterogeneous nature of concrete. The test involves a given initial amount of kinetic energy of the probe, which is absorbed during penetration, in large part through crushing and fracturing of the concrete, and in lesser part through friction between the probe and the concrete. Penetration of the probe causes the concrete to fracture within a cone-shaped zone below the surface with cracks propagating up to the surface (Figure 2.6). Further penetration below this zone is, in large part, resisted by the compression of the adjacent material, and it has been claimed25 that the Windsor Penetration Resistance Methods2-5 FIGURE 2.3 A view of the Windsor probe in operation: a 600 × 600 × 200-mm slab under test for correlation purposes. (Adapted from Reference 21.) FIGURE 2.4 Relationship between exposed probe length and 28-day compressive strength of concrete. (Adapted from Reference 12.) 28-DAY COMPRESSIVE STRENGTH, psi - Y MPa mm 30405060 6,000 5,000 4,000 3,000 2,000 1,000 0 AGGREGATE TYPE = LIMESTONE SIZE OF CYLINDERS = 150 × 300 mm SIZE OF SLABS PROBED = 610 × 610 × 200 mm 1.01.21.41.61.82.02.22.42.6 EXPOSED PROBE LENGTH, in - X Y = 5333 + 5385 X, psi S.E. = 327 psi CORRELATION COEFFICIENT = 0.963 41 35 28 21 14 7 0 95% CONFIDENCE LIMITS 2-6Handbook on Nondestructive Testing of Concrete: Second Edition probe test measures the compressibility of a localized area of concrete by creating a subsurface compaction bulb. Further, it has been claimed that the energy required to break pieces of aggregate is a low percentage of the total energy of the driven probe, and the depth of penetration is not signifi cantly affected. However, these claims have never been proven. Notwithstanding the extent to which the above claims may be true, it nevertheless appears clear that the probe penetrations do relate to some strength parameter of the concrete below the surface, which makes it possible to establish useful empirical relationships between the depth of penetration and compressive strength. 2.3.2Correlations between Probe Test Results and Compressive Strength The usefulness of the probe penetration test lies primarily upon its being able to establish suffi ciently accurate relationships between probe penetration and compressive strength. A factor, long recognized, that affects FIGURE 2.5 Relationship between exposed probe length and 28-day compressive strength of concrete as obtained by different investigators. (Adapted from References 8, 9, and 12.) FIGURE 2.6 Typical failure of mature concrete during probe penetration. 28-DAY COMPRESSIVE STRENGTH, psi MPa 6,000 5,000 4,000 3,000 2,000 1,000 41 35 26 21 14 7 0 0 30405060 mm 1.01.21.41.61.82.02.22.4 EXPOSED PROBE LENGTH, in. TRAPROCK, MOHS HARDNESS 7.0 LAW AND BURT ARNI MALHOTRA CHERT, MOHS HARDNESS 7.0 GRAVEL, MOHS HARDNESS 6.5 LIMESTONE, MOHS HARDNESS 5.5 SIZE OF CYLINDERS = 150 × 300 mm Fracture ZoneFracture Zone Compression ZoneCompression Zone Probe Penetration Resistance Methods2-7 this relationship is the hardness of the coarse aggregate, and this is taken into account in the correlation tables provided by the equipment manufacturer. However, as previously mentioned, the use of the manu- facturers tables has been found by several investigators not to be satisfactory. This is probably because factors other than aggregate hardness, which also affect probe penetration, have not been considered. There appears to have been no systematic attempts to determine the relative infl uences of these factors that could affect the probe penetration test results. Ho