ASSESSMENT AND INTERPRETATION OF IN-SITU STRENGTH OF CONCRETE In-situ strength of concrete

Transcription

1 ASSESSMENT AND INTERPRETATION OF IN-SITU STRENGTH OF CONCRETE In-situ strength of concrete K.C.G. ONG and N. NANDAKUMAR Associate Professor and Postdoctoral Fellow, Centre for Construction Materials and Technology, Department of Civil Engineering, National University of Singapore, Singapore. Durability of Building Materials and Components 8. (1999) Edited by M.A. Lacasse and D.J. Vanier. Institute for Research in Construction, Ottawa ON, K1A 0R6, Canada, pp National Research Council Canada 1999 Abstract A series of penetration resistance tests (PRT) were conducted in the laboratory on concrete specimens to determine the in-situ characteristic strength of concrete. This was carried out in conjunction with core tests and cube compressive tests. It was found that the PRT and core tests tend to provide respectively an upper and lower bound estimate of the cube compressive strength. Major influencing factors including coarse aggregate content, proximity to reinforcement and carbonation that affect these two tests are reviewed and a method for the selection of an appropriate bias factor for each of the tests is proposed based on experimental studies carried out. Keywords: appraisal, concrete, core tests, in-situ strength, non-destructive testing, penetration, retrofit, ultrasonic pulse velocity. 1 Introduction Maintenance and upgrading of old RC structures are necessary for various reasons. Before any remedial action could be taken structural appraisal is essential. Such appraisal usually involve a process in which a structure is assessed by carrying out various tests to evaluate its existing condition to provide a basis for the recommendation of appropriate remedial measures to restore structural integrity, if necessary. NDT methods are usually fast and reasonably reliable. Hence, they are commonly used for structural assessment in conjunction with testing of cored concrete samples. One method currently used in Singapore is the penetration resistance tests (PRT). This test (Bungey 1982, 1989) involves driving a 75mm specially designed steel probe into the concrete with a standardised explosive cartridge, and the standards are set out in (ASTM C ). The principal physical limitations of this method are the need for adequate edge distance and probe spacing as well

2 as the requirement that a member thickness of at least twice the anticipated penetration is required. The effect caused by the presence of reinforcement in close proximity to a probe being driven is uncertain, and a minimum clearance of 50mm between probes and reinforcing bars has been recommended (Bungey 1982, 1989). The coarse aggregate content present in the mix will also contribute to a change in resistance against penetration of the probe. It is possible to correlate the PRT to the compressive strength of concrete provided that appropriate consideration is given to the influencing factors. (Lee et al. 1989) correlated the PRT with the compressive strength of concrete of a number of concrete mixes in an earlier study. A more direct method of obtaining an estimate of the in-situ compressive concrete strength of an existing structure is to drill cylindrical cores for testing in compression. Standards are available in most countries describing the procedures for trimming, testing and to a lesser extent interpretation of core strengths (BS 1881 Part ; ASTM C ; ACI ). When assessing the insitu strength of concrete by coring, the following factors, viz., reduction of strength due to coring process, effect of state of stress, shrinkage and sustained loading effects, and the difficulty in strength evaluation based usually on a very limited number of core samples, have to be considered. Although the need for in-situ measurements is clear, some of the features and especially the limitations of available test methods may result in unreliable values being used in structural appraisal. The objective of this investigation was to study some of the factors affecting the penetration resistance and core tests on concrete specimens. In particular, the following influencing factors affecting penetration resistance tests, namely, effect of age, effect of coarse aggregate content, effect of proximity of reinforcing bars and effect of carbonation are investigated. 2 Experimental programme 2.1 Materials used Ordinary Portland cement, river sand, crushed granite aggregates of sizes ranging from 0 to 20mm were used. Steel bars consisted of high yield steel (f y = 460 MPa) and links consisted of hot rolled mild steel (f y = 250 MPa) were used in the reinforced concrete specimens. Four mix proportions were chosen (Mix 1, Mix 2, Mix 3 and Mix 4), consisting of cement: fine aggregates: coarse aggregates: water, viz., 1.0:2.5:2.66:0.65, 1.0:2.50:3.16:0.65, 1.0:2.50:3.72:0.65 and 1.0:3.22:3.22: Test specimens The specimens were divided into six series (A to F). In Series A, three mix proportions were used to study the effect of coarse aggregate content on PRT measurements. For each mix, four 300mm cuboid blocks were cast, accompanied by twelve 150mm cubes. Two methods of discharging the probe in the low-power range, were also investigated. The two discharging methods I and II were conducted on the specimens at ages of 3 and 28 days. In Method I, a standard adaptor is cushioned by 50mm air-gap from the charge, while, Method II uses another adaptor in direct contact with the charge as described (Ngui 1992). Coring was not carried out in this series.

3 Series B and C were conducted to study the effect of size and proximity of reinforcement on PRT. In addition to penetration resistance tests, core tests were carried out on the same specimens to establish a relationship between the tests. A mix containing a coarse aggregate content of 0.42% by volume, was used to cast both the series. Series B comprises 4 specimens, 600mm long and 300mm in cross-section. Series C consists of two specimens, 700mm long and 400mm in cross-section. The specimens were subjected to PRT (using Method I low-power technique) and cores drilled and tested at 3 days. UPV measurements and PRT were taken at the same location to check for consistency of pulse velocities. Series D and E using a coarse aggregate content of 0.42% by volume, were used to study the effect of proximity and neighbouring bars on PRT measurements. Tests carried out were similar to those on Series B and C, at 28 days using Method II in the low-power range of PRT. Tests were also carried out at 21 days using Method II on specimens in Series D. In Series F, the effect of carbonation on PRT was studied. Three cubes of size 200mm were used. The blocks were placed in an environmental chamber with the environment within maintained at a relative humidity and temperature of 70% and 20 C, respectively. The concentration of carbon dioxide gas present in the chamber was maintained at 7% by volume. Accompanying specimens were also left at ambient conditions in the laboratory (28 C and 82% R.H). All the specimens were tested at 63 days. The specimens were weighed and UPV measurements were taken across the PRT locations prior to testing to ensure consistency. 2.3 Specimen preparation Six cubes were cast for each mix to determine the compressive strength of concrete. Two 300mm size cuboid blocks were also cast for each mix to measure the penetration resistance of plain concrete. In each block, a 100mm diameter core was drilled vertically with respect to the casting surface right through the block. The core was then trimmed to obtain two cores, each measuring 100mm in length. The specimens were demoulded after one day and air cured in the laboratory till the age of testing. All specimens were tested dry at 3 and 28 days. In Series D, an additional block was cast to conduct PRT test at 21 days. Cubes, cylinders and core samples obtained from specimens and blocks, were weighed and subjected to UPV tests to check for consistency and for the presence of voids in the concrete before testing. UPV measurements were also taken across PRT locations on blocks and reinforced specimens. 3 Experimental results and discussion The UPV measurements made on reinforced specimens, blocks, standard cubes and cores did not show any sign of major voids or cracks. Typical average pulse velocity readings for Mix 1 and 3 at 3 and 28 days ranged from 3.37 to 3.69 km/s and 3.75 to 4.01 km/s respectively. The difference between the maximum and minimum depth of penetration of probes in each series of tests on blocks, cylinders and specimens averaged about 3mm. Thus, reflecting the consistency of the results obtained using the penetration resistance tests. Generally, tests carried out 3 days after casting indicated a greater variation in probe penetration than

4 those carried out at 28 days. However, the difference is not significant enough to affect the overall results. 3.1 Penetration resistance test results Effect of coarse aggregate content The influence of coarse aggregate (CA) content on penetration resistance test is more significant in young concrete. From Fig. 1, it is seen that by decreasing the coarse aggregate content, the depth of probe penetration is increased. For a given method, at 28 days, the different amounts of coarse aggregate in the three mixes did not cause any significant change in the estimated cube strength obtained by PRT. This could be due to an improved bond at the aggregate matrix interface. Fig. 1 also shows that by using PRT to estimate the strength of concrete at a younger age, corrections have to be made especially in Method II for lower CA content. The increase in depth of penetration of the probe is probably due to the less developed bond at the aggregate/matrix interface at a younger age. A decrease in the energy of the probe as it leaves the driver is evident when Method I is employed. The absence of an air gap in Method II allowed the probe to be driven further into the concrete Effect of proximity of reinforcement Fig. 2 shows the effect of proximity of reinforcement on PRT. A minimum spacing of 175mm has been suggested (ASTM C ) between probes and hence, the zone of influence of the probe being driven into a concrete mass is thus expected to radiate a distance 87.5mm from the centre of the probe. (Nasser and Al-Manaseer 1987) reported that as long as the nearest steel bars in concrete are located outside this zone of influence, the presence of steel bars in concrete did not affect the probe penetration. This is also reflected in Fig. 2 which indicates that probes driven at a distance of 75mm or more from the nearest reinforcement bar tend to provide a similar estimate of the cube strength. In general, overestimation of the concrete strength was evident by PRT when probes were driven 90mm or less and 50mm or less from the nearest bar respectively, when specimens tested by Method I and Method II Effect of carbonation (Nasser and Al-Manaseer 1987) reported that there is no effect of carbonation on PRT. However, the limited number of penetration resistance tests conducted on 200mm cubical blocks indicate a significant decrease in probe penetration as the depth of carbonation increases (Fig. 3). This could be due to the fact that carbonation affects the surface zones of the concrete only. If the depth of carbonation is significant compared to the penetration depth then the penetration depth of the probe decreases Velocity tests on probes Some results on the actual velocity of the probes as it leaves the driver is summarised briefly (Tan 1996). Two different Windsor probe driver units were used and the velocity of the probes measured were reported in Table 1. From Table 1, it is observed that there is no significant difference in the average velocities of probes discharged by the two driver units although they were purchased ten years apart. The first unit is of a model not in current production

5 and has been in use for about ten years. Method A used in measuring the discharge velocity of the probe, the two driver units have the same average discharge velocity. However, the new driver exhibited a slightly higher coefficient of variation when compared to the old driver. It is also observed that the average probe velocity measured ranged between 180 m/s and 187 m/s, which more or less confirms with the manufacturer s claim that this average is 183m/s. However, actual velocities measured can be as low as m/s or as high as m/s. Table 1: Velocities of the probe Type of experiments Method A - old driver (Tan 1996) Method B - new driver (Tan 1996) Method C - new driver (Tan 1994) Method D - new driver (Lee 1995) Average velocity (m/s) Coefficient of variation (%) Core test results The average compressive strength of cores drilled from corresponding specimens cast using the same mix were generally 10 to 40% lower than that obtained by crushing 150mm cubes. This could be caused by some of the factors discussed earlier namely the limited number of core samples and the damage caused during coring process Effect of coarse aggregate content Fig. 4 shows that the coarse aggregate (CA) content of concrete block specimens from which cores were extracted affects the ratio of the estimated cube strength to the actual cube strength at both 3 and 28 day strengths. The ratio is significantly low for all the mixes at 3 days. This may be due to weaker bond at the aggregate matrix interface at 3 days resulting in more damage caused by coring. 4 Analysis of test data 4.1 Interpretation of penetration resistance and core test results The data obtained from core tests and penetration tests can be used to estimate the characteristic strength, taken for the present study to be mean minus 1.64 times standard deviation. Since, the factors contributing to the statistical variation in the core tests and penetration resistance tests are different, their combined results are not cohesive, even, when testing specimens cast using the same batch of concrete. Hence, there is a need to calculate the characteristic strength obtained from cores and that from probes independently. The actual characteristic strength f k, may be derived from the individual characteristic strengths from cores and probes by multiplying the latter by bias factors for cores and PRT respectively. For the present study, these factors are assumed constant. v c f ck f k (1) v p f pk f k (2)

6 Estimated cube strength from PRT on blocks Actual cube strength METHOD I METHOD II M1 M2 M % Volume of coarse aggregate 30 DAYS 28 DAYS 28 DAYS 30 DAYS Fig.1 Effect of volume of coarse aggregate on PRT Fig. 1: Effect of volume of coarse aggregate on PRT Estimated cube strength from PRT on blocks Actual cube strength MIX 3 METHOD I 3 DAYS MIX 1 METHOD II 28 DAYS MIX 2 METHOD II 28 DAYS MIX 1 METHOD II 21 DAYS Distance from nearest reinforcement bar or link (mm) Fig. 2: Fig.2 Effect Effect of proximity of proximity of reinforcement of reinforcement on PRT on PRT

7 Depth of carbonation (mm) Depth of probe penetration (mm) NON CARBONATED BLOCK AFTER 1 W EEK EXPOSURE MIX 4 METHOD I 63 DAYS AFTER 2 W EEKS EXPOSURE 20 Fig. Fig.3 3: Effect Effect of carbonation of carbonation probe penetration probe penetration Estimated cube strength from PRT on blocks Actual cube strength DAYS DAYS 0.6 M1 M2 M % Volume of coarse aggregate Fig.4 4: Effect of volume of volume of coarse of aggregate coarse on cores aggregate on cores

8 Where, v c and v p are constant bias factors. If f k is underestimated by the core results and overestimated by the probe results, then, thus f ck < f k < f pk (3) 1.0 < v c < f pk /f ck (4) f ck /f pk < v p < 1.0 (5) Bayesian statistical approach was employed to provide a range of bias factors for strength estimation, obtained from PRT and core test. The steps involved are as follows: 1. The characteristic strength from corresponding cubes was obtained and compared with the uncorrected characteristic strength from cores and PRT. 2. The maximum and minimum ranges of bias factors were also obtained. By predicting a range of bias factors limited by factors affecting both PRT and cores, an estimated range of characteristic cube strength is obtained. Since the characteristic strength obtained from cubes is known then the actual bias factors (assumed constant) for PRT and cores can be determined. Bias factors and characteristic strength of specimens tested is reported in Table 2. All these values obtained using low-power method I are within the expected range of bias factors reported in an earlier study (Lee et. al. 1989). Observing the bias factors obtained using low-power method II, it is assumed that the bias factor for PRT should be less than 1.0. Table 2: Bias factors and characteristic strength for specimens tested. Test specimen Mix 1 Mix 2 Mix 3 Mix 3 Mix 1 Mix 2 Method of driving Low-power I Low-power I Low-power I Low-power I Low-power II Low-power II Age v c v p f k (MPa) 28 days days days days days days Conclusions The following conclusions can be drawn from this investigation: 1. An increase in coarse aggregate content increases the ratio of estimated cube compressive strength from PRT to actual cube strength. 2. The presence of reinforcement may affect the depth of probe penetration depending on the proximity of probe location to the nearest reinforcing bar. It was observed that the presence of reinforcing bars within the proximity of 50mm generally cause an overestimation of the in-situ cube compressive strength using PRT. 3. An increase in the depth of carbonation in concrete decreases the depth of probe penetration.

9 4. An increase in coarse aggregate content increases the ratio of estimated cube compressive strength from cores to actual cube strength. 5. The estimation of in-situ characteristic cube strength of concrete can be correlated with PRT and core test results using the Bayesian statistical approach. However, the nature of influencing factors need to be identified and the selection of the appropriate overall bias factor for each situation need to be systematically explored so that specific guidelines may be proposed to aid engineers engaged in this field of work. 6 References ACI 318. (1995) Building Code requirements for Reinforced Concrete. American Concrete Institute, Detroit. ASTM C803. (1996) Test for penetration resistance of hardened concrete. American Society for Testing and Materials, Philadelphia. ASTM C42. (1994) Standard test method for obtaining and testing drilled cores and sawed beams of concrete. American Society for Testing and Materials, Philadelphia. BS 1881 Part 120. (1983) Method for determination of the compressive strength of concrete cores. British Standards Institution, London. Bungey, J.H. (1982, 1989) The testing of concrete in structures. Surrey University Press. Lee, S.L., Tam, C.T., Ong, K.C.G., Swaddiwudhipong, S. and Quek, S.T. (1989) Appraisal and repair of concrete structures. Journal of the Institution of Engineers, Singapore, Vol. 29, No. 2, pp Lee, T.K. (1995) Improved windsor probe test for in-situ concrete strength assessment. BEngg Thesis, National University of Singapore. Nasser, K.W. and Al-Manaseer, A.A. (1987) Comparison of non-destructive testers of hardened concrete. ACI Materials Journal, Vol. 84, No. 5. pp Ngui, A. (1992) Assessment and interpretation of in-situ strength of concrete using penetration resistance, ultrasonic pulse velocity and core tests. BEngg. Thesis, National University of Singapore, 106 pp. Tan, H.S. (1996) Improvement of a velocity sensor for an impact concrete tester. BEngg. Thesis, National University of Singapore, 52pp. Tan, K.L. (1994) Evaluation and modification of impact testing of concrete. BEngg. Thesis, National University of Singapore.

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