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Prediction on compressive strength concrete using modified pull-off testing method (MPTM)
Construction and Building Materials 250 (2020) 118834
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Prediction on compressive strength concrete using modified pull-off testing method (MPTM) Suhang Yang a, Zhifeng Xu b, Zhengning Bian a,⇑ a b
School of Civil Engineering and Architecture, Changzhou Institute of Technology, 666, Liaohe Road, Changzhou 213032, PR China College of Civil Engineering and Architecture, Shandong University of Science and Technology, Qingdao 266590, PR China
h i g h l i g h t s A modified pull-off test method is proposed with distinctive characteristics of high accuracy, fewer preparations, than the core-drilling method in-situ. Several influence factors upon concrete strength could be excluded, and a geometric correction factor is proposed. Good relationships between the MPTM strength and the concrete cube strength.
a r t i c l e
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Article history: Received 31 January 2019 Received in revised form 29 February 2020 Accepted 20 March 2020 Available online 28 March 2020 Keywords: Pull-off test In-situ test Concrete compressive strength Tensile strength Correlation
a b s t r a c t The classical pull-off test indicates that the compressive strength of concrete increases with its tensile strength. But it is difficult to obtain the effective data due to the loading eccentricity, which could produce combined bending plus axial stress instead of the uniaxial tension. The objective of this study is to introduce a modified pull-off testing method (MPTM) and improve the classical method by using a cup-shaped loading probe, so that the tensile strength of concrete can measured under pure axial-load. Experimental results indicate that several influential factors upon the MPTM strength could be excluded. There is a good relationship between the MPTM strength and the concrete cube strength (10–80 MPa) both at early age and later age. The repeatability of the MPTM is slightly better than the classical pulloff test method. The verification investigation shows that the MPTM provides a more accurate estimate of in-place cube strength and is easier to perform than the core-drilling method. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction The compressive strength is a principal property for assessment of concrete structures. Much research has been done on regarding the estimation of concrete strength by using nondestructive test methods (NDT), such as rebound hammer [1], nail penetration testing methods [2], ultrasonic pulse velocity method [3]. The NDT methods measure a test parameter (elastic modulus, penetration resistance, sound velocity and surface hardness) in a low-cost and nondestructive manner. Compressive strength is estimated by using an empirical correlation between the test physical quantity and concrete strength. However, the estimation of concrete strength from the measured physical quantity is sensitive to local changes in concrete, because the NDT test results are severely affected by the in-place environmental conditions, which may be different from that of the laboratory condition. The dispersion of compressive strength, obtained by NDT methods, using correlation ⇑ Corresponding author. E-mail address: [email protected] (Z. Bian). https://doi.org/10.1016/j.conbuildmat.2020.118834 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
curves, shows that increasing the number of data, the coefficient of variation does not necessary decrease [4].NDT is recommended for a preliminary structural inspection and checking the uniformity of structural concrete. The partial destructive test method (PDT) of drilling-cores and testing for concrete strength, and can provide more accurate results than using the NDT. But core-drilling may result in underestimation of the concrete strength because drilling invariably causes damage to concrete cores, and the inhomogeneous concrete may result in eccentric load under compression [5]. By comparison with available PDT testing methods, the widely accepted NDT methods should be able to provide reliable results with trained operators, and can estimate the in-place concrete strength using a portable apparatus. Many methods for this purpose have been reported in the literature, among them is the pull-off test method [6,7]. The pull-off test can be used to estimate the compressive strength by measuring the near-surface tensile strength of the concrete. It is already standardized in BS 1881-Part4 and ASTM C1583/ C1583M [8,9]. Large test loads are not needed for measuring the direct tensile strength of concrete, because the tensile strength of
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the concrete is a small fraction of the compressive strength. Compressive strength can be related to tensile strength using an empirical correlation. However, the inhomogeneity of concrete may cause eccentricity of the applied load, which can reduce pull-off strength by as much as 15% and produces a state of biaxial stress instead of the uniaxial tension [10]. The classical pull-off apparatus should be improved so as to apply uniform pure tensile stress within the specimen [11]. Some reviews indicate that factors affecting the tensile strength of concrete include the composition (e.g. type of aggregate and water/cement ratio), age, curing method, and size of the specimen, as well as the effects of boundary condition [12]. Some influential factors could be controlled to improve the accuracy of the test results [13,14]. The aim of this study is to use a modified pull–off method (MPTM) to estimate the compressive strength of concrete. The MPTM apparatus is intended to assure concentricity of loading, and thereby measure the true tensile strength. The failure mode of the specimen is analyzed to verify whether the MPTM is suitable for determining the uniaxial tensile strength of partial core specimens. Moreover, some influential factors of MPTM are studied and excluded to facilitate the method. Pull-off strengths and cube strengths are measured to determine whether a reliable correlation exists between them. This study investigates the adequacy of the correlation and the variation in the results achieved using the MPTM for a relatively large range of concrete cube strength (10–80 N/mm2), and promote the use of the MPTM in the assessment of concrete strength. 2. Modified pull-off testing method (MPTM) 2.1. Mechanism of the MPTM Fig. 1 compares the MPTM with the commonly used pull-off test procedure. The principle of the MPTM is fairly straightforward. A partial core specimen is erected in the concrete member to be tested. A self-aligned swivel joint is used to minimize the effect of misalignment, and to ensure the external force is parallel to the axis of the partial core. The top and side of the core are bonded to the inner-surface of a special cup. Then, the force gradually grows with distance from the surface, and bottom of the core specimen bears the maximum force. The thin-circular-tube with a cup shape feature is utilized to restrict the lateral deformation of the core, and to keep the axis of the core perpendicular to the concrete surface, so that a concentric of the axial load is applied uniformly
throughout the area. The core is pulled axially until the fracture occurs at the weakest base location. 2.2. Calculation and interpretation of results MPTM strength is calculated by dividing the maximum load by the area of the core as shown by Eq. (1), which assumes that the load is uniformly distributed over the area of the core.
r¼
4P 3:14D2
ð1Þ
where r is the tensile strength at failure (MPa) and identified as MPTM strength, P is the maximum tensile load applied to the test core (N), and D is the diameter (mm) of the core specimen. Visual examination of the failure mode is considered in evaluating the test results. If the core fails at its base, it is considered as the normal failure mode, and the tensile strength of concrete is effectively determined. If any unusual failure mode occurs anywhere other than at the core base, the test data must be discarded. 3. Specimen preparation 3.1. MPTM apparatus The MPTM testing apparatus used in this study comprises several components (Fig. 2). A reaction frame with sufficient stiffness is needed so that the load direction remains constant during testing. The applied force is transferred through the tripod reaction system. The dimensions of the cup-shaped loading probe are presented in Fig. 2b. The portable apparatus weights 3 kg. Loading is performed by using a hydraulic jack with a hand wheel. The tension load is applied gradually at the rates of 0.2–0.4 kN/s. A test should be completed within 20–50 s. The operating temperature range of the apparatus is from 10 to 50 °C. The peak load at failure is recorded with a digital gauge system, which is able to register loads with 1 N accuracy and a maximum load of about 30 kN. The inner diameter of the cup-shaped loading probe is 44 mm (Fig. 2b). The diameter of the probe is less than conventional stirrup spacing to avoid interference of reinforcing bars. According to BS Standard 1881, the thickness of the steel disc should be at least 40% of its diameter [9]. The probe can be reused after removing epoxy adhesive on the probe by heating to 100 °C in an oven. The 44 44 mm core size is equivalent to 1/12 of the volume of
Fig. 1. Schematic diagram of MPTM and Standard Pull-off test.
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Fig. 2. Schematic diagram of the portable apparatus for MPTM testing.
a standard 100 100 mm core, so the MPTM can be considered as a PDT method. 3.2. Coring equipment Before the cup-shaped loading probe is attached to the concrete surface with epoxy, a partial depth core has to be created by using a water-cooled coring machine (Fig. 4a). Concrete with lowstrength may be ruptured while drilling. The operation should proceed slowly to minimize disturbance to the core and the drilling depth should be measured and less than 3 mm beyond the end of the probe should be controlled. 3.3. Surface treatment and application of adhesive Sandpaper was used to prepare the concrete surface plate to improve the bonding of the epoxy. The concrete surface was abraded in a depth of about 3 mm until the coarse aggregate particles were exposed. Then, the cup-shaped loading probe was bonded to the concrete core using quick-setting epoxy adhesive. The entire contact surface between the probe and the concrete is bonded to ensure that the load is applied at the core base. The epoxy that was used attained its adhesive strength of 10 MPa after 2 h of curing. Table 1
aggregate, while crushed stone with particle sizes ranging from 10 to 25 mm was used as coarse aggregate. Concrete cube strengths ranging from 10 to 100 MPa were obtained. Two kinds of Portland cement (CEM I 42.5R and 52.5R), a crushed limestone, and a natural aggregate (river gravel) were used in concrete construction. Mixture proportions and properties of concrete mixtures are given in Table 2. Concrete cubes were cured outdoors and covered with a wet cloth, to protect the specimens from sunlight and rain. The curing temperature ranged from 10 °C to 40 °C. A core specimen with a height (h) of 44 mm and diameter (d) of 44 mm was selected as the standard specimen for comparison. All cube specimens were cured in the parallel environment to guarantee the comparability of the test results of the MPTM and cube compression tests. The compressive strengths of various concrete mixtures were obtained by testing three 150 mm cubes according to BS-EN 12390-3 [15].
4. Experimental programme and test procedure 4.1. Experimental programme The experimental programme comprises three series of tests as follows.
3.4. Test specimens (1) Series A The concrete was mixed in a drum-type mixer, placed into 150 mm steel cube moulds, and compacted on a vibrating table. Ordinary sand with a relative density of 2.5 was used as the fine
This tests is to eliminate the influence factors that do not need. Four grades of concrete strength, C20 ,C40, C60 and C80, were used,
Table 1 Proportions and some properties of concrete mixtures. Mixture
Mixture proportions, kg/m3
Some properties
Coarse aggregate
Fine aggregate
Cement
Water
Admixture
w/c
Aggregate type
Maximum Aggregate size, mm
28-day cube Compressive strength, MPa
MIX-A
1205
786
190
150
—
0.79
22
12.6
MIX-B
1190
745
286
180
—
0.63
22
21.2
MIX-C
1180
658
391
180
—
0.49
22
32.4
MIX-D
1195
649
367
180
3.6
0.46
22
41.2
MIX-E
1180
649
391
180
90
0.46
22
52.3
MIX-F
1120
574
450
150
140
0.33
22
61.8
MIX-G
1143
580
470
152
140
0.32
22
72.5
MIX-H
1051
643
450
150
160
0.33
Crushed Limestone Crushed Limestone Crushed Limestone Crushed Limestone Crushed Limestone Crushed Limestone Crushed Limestone Crushed Limestone
22
83.6
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(5) Series E
Table 2 Variability of standard pull-off test data and MPTM test data. Concrete mixture
C20 C30 C40 C50 C60 C70 C80
Test data coefficient of variation (CoV), % Test method MPTM test
Standard Pull-off test
7 8 8 9 11 10 13
9 9 12 11 13 14 18
This series test studied the comparison between standard Pulloff and MPTM methods. Pull-off and MPTM tests were performed on the same cube (Fig. 3(c)). The tests were done with MPTM in 28 day standard curing. 4.2. Test procedure The test procedure involves the following steps:
and all the specimens were tested at 28 days. When each influencing factor is tested, six MPTM tests of each concrete cube were performed as shown in Fig. 3 (a). (2) Series B This test is to check the data dispersion of MPTM. 2 groups of MPTM strength are obtained from the concrete cube with strength C20 and C60. 24 MPTM tests for each group were performed on four concrete cubes. The tests are done with MPTM in 28 day standard curing. (3) Series C MPTM and concrete cubes were tested at the early ages (12 h, 18 h, 24 h , 30 h, 36 h, 40 h, 48 h, 54 h, 66 h, 72 h, 84 h and 96 h). The relationship between MPTM test values and cube compressive strength was obtained at each ages. At each test age, Four cubes were selected from concrete cubes of each design strength. Six MPTM tests were performed on each concrete cube as shown in Fig. 3(a), and the effective data of each MPTM on the curve is the average of these six data. The remaining three of the two test cubes could be tested for compressive strength. (4) Series D MPTM, standard pull-off and concrete cubes were tested at the specified curing ages. After the cubes were made, the test shall be conducted every seven days until 360d. Cube compressive strength range from 10 to 80 MPa. The relationship between MPTM test values and cube compressive strength for core samples was obtained irrespective of the ages. At each test age, three cubes were selected from concrete cubes of each design strength. Six MPTM and standard pull-off tests were performed on each concrete cube respectively, as shown in Fig. 3(a) and Fig. 3(b). The effective data on the curve is the average of these six data. The remaining one of the three cubes shall be tested for compressive strength.
Step 1: Prepare concrete surface and carefully drill the partial depth core at the selected location (Fig. 4a). Six tests are performed on each cube; Step 2: Place freshly mixed epoxy resin on the inner-surface of the loading probe, (Fig. 4b) and allow epoxy to harden; Step 3: Apply load at a constant stress rate of about 0.05 N/ mm2/s following BS 1881-part207. After test is completed, the actual diameter of the core is measured at three positions to calculate the mean value. The diameter of the core is measured by a digital caliper, accurate to 0.01 mm (Fig. 4c). Step 4: Examine the failure surface and the failure mode visually. Report any unusual conditions, such as mis-alignment of testing surface and loading fixture, failure at the interface of concrete-steel, etc. Step 5: Obtain the maximum load values from the MPTM apparatus for the acceptance failure modes, accurate to 1 N. MPTM strength is calculated using Eq. (1). 5. Results and discussions 5.1. Failure mode A typical failure mode after the MPTM test is shown in Fig. 5a–c. It can be observed from Fig. 5a–c that the failure surface is at the core end, and is more planar than that of the standard pull-off test and almost parallel to the concrete surface. Fig. 5d shows that the typical failure mode occurs at the core end, this failure mode can be used to judge the validity of the test results. More than 95% of the failures occurred at the bottom of the probe and only a few failures were in the body of the core as shown in Fig. 6. No failure occurred in the adhesive-core interface for the MPTM tests. Detail failure modes of the core specimen are shown in Fig. 5(d). The concrete base is generally the weakest portion of the system, and tensile failure occurs in the concrete base. The above observations agree well with investigations of the standard pull-off test [16]. Non-typical failure modes in the MPTM test are shown in Fig. 6, which shows that the failure surface occurred deeper in the concrete. When the design strength of concrete is more than
Fig. 3. Testing position.
S. Yang et al. / Construction and Building Materials 250 (2020) 118834
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Fig. 4. Testing procedure.
Fig. 5. Typical failure mode of failure in the concrete interface.
ratio, core diameter and aggregate size. Four grades of concrete (20, 40, 60, 80 MPa) were used to investigate the effects of various factors. The compressive strength was measured at 28 days to derive appropriate correlations with MPTM strength. Statistical tests were used to determine if the effects of the various factors was statistically significant. (1) Effect of test direction
Fig. 6. Non-typical failure mode of failure in substrate.
70 MPa, the abnormal failure modes of specimens appear sporadically. This indicates that the weak position of high strength concrete is transferred from ITZ to aggregate itself. 5.2. Effects of influencing factors Series A test is performed to find out the influencing factors to be adopted for the MPTM. Based on previous studies, the factors that influence the MPTM strength include the height-diameter
Fig. 8 shows the MPTM strengths obtained from two testing directions (horizontal and vertical, Fig. 7) at different concrete strength. The test direction is always perpendicular to the cube face (Fig. 3), but only changes the orientation of the apparatus. The t-test was conducted on the average values for the two test directions. The significance level of the t-test values are exhibited in square brackets, and all group t-test values are larger than 0.05. Effect of testing directions is presented in the table attached to Fig. 8. The correction efficiency (Is) is defined as average vertical testing value divided by the average horizontal value of MPTM strengths. Each test result in the table is the mean value of six independent test results obtained from one concrete cube specimen. MPTM is independent of testing direction and without the effect of gravity. (2) Effect of specimen diameter (d) Three different specimen diameters (d = 38 mm, 44 mm, 49 mm) were investigated (Fig. 9). The one-way ANOVA of P-
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Fig. 7. Schematic diagram of the MPTM testing direction.
which indicates that there are no statistically significant differences in the mean value, due to aspect ratio is no significant. On average, MPTM test values for core specimens with h/d = 1.5 and h/d = 0.5 are 3.6% higher and 1.9% lower, respectively (see table in Fig. 11) than that of the standard core specimen with h/d = 1. These results are in accord with earlier findings, which demonstrate that the results are not so sensitive to the coring depth [13]. It could also be interpreted that the MPTM reduces the possibility of eccentric loading and the increase of h/d ratio did not affect the results. The internal tensile stress distribution in MPTM test is more uniform than the standard pull-off test. In addition, the increase of concrete strength results in higher dispersion, because of the increase in HSC brittleness, and it is common phenomenon for the dispersion of strength test results to increase with increasing average strength. (4) Effect of aggregate size
Fig. 8. Variation of MPTM load of two test directions for different concrete strengths.
values are shown in square brackets, and P-values of all groups are larger than 0.05. Thus there is no significant differences in the mean values for the different core diameters. Fig. 10 shows the MPTM strengths as a function of the diameter (d) of the core specimen. For a given compressive strength, MPTM strength decreases with the increase in core diameter. . .. The MPTM strengths for cores in 38 and 49 mm diameters are 22% higher and 19% lower than that of the standard core (d = 44 mm) respectively. This coincides with previous studies, the greater the core diameter, the smaller the strength is [17], because the greater the specimen diameter, the more flaws present. For HSC (C60, C80), the size effect decreases with the increase of concrete strength. Due to the strength of the interface transition zone (ITZ) between the aggregate and paste growing with the increase of concrete strength, the weak part of concrete has been transferred from ITZ to aggregate. (3) Effect of aspect ratio (h/d) Fig. 11 shows MPTM test results for different aspect ratios (h/d = 0.5, 1.0, 1.5) of the cores and different concrete cube strengths, h and d represent the height and diameter of the core, respectively. The MPTM test yields similar results for the core with the different h/d ratios. All group p-values are larger than 0.05,
The effect of maximum aggregate size (da) was evaluated using the reference core specimen (h/d = 1, d = 44 mm). Maximum aggregate sizes (da) of 25 mm and 15 mm were used. The t-test certified that there were no significant differences between each test group except for C20 group. Fig. 12 illustrates that the MPTM strength is greater for specimens with the 25 mm aggregate. MPTM strength is affected by enlarged da between 2% and 9%, with 5.2% on average. While the 25-mm aggregate resulted in higher strength, the increase is not statistically significant expectation for the tests on C20 concrete, which shows very low dispersion for the 15-mm aggregate. The factor can be disregarded because the results in Fig. 12 shows that maximum aggregate size does not have a statistically significant effect on the average MPTM strength. 5.3. Empirical relationship between MPTM and concrete cube strength The objective of this study is to find an empirical relationship between MPTM strength and cube strength ranging from 10 to 80 MPa. (1) Relationship between MPTM and concrete cube strengths at early age. Series D test was carried out to study the relationship between cube strength and MPTM strength at early age. Fig. 13 exhibits the relationship between concrete cube compressive strength and the MPTM strength at ages from 1 to 4 d. Several empirical functions
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Fig. 9. Specimens with different diameters.
Fig. 10. Variation of MPTM strength for different core diameters and cube concrete strengths.
Fig. 12. Variation of MPTM strength for different maximum aggregate sizes and concrete strengths.
where x = MPTM strength of concrete, in MPa; and y = cube compressive strength of concrete, in MPa. (2) Relationship between MPTM and concrete cube strengths Series D test was carried out to study the relationship between cube strength and MPTM strength. The regression form y = axb is adopted as the best-fit line to represent the relationship. Fig. 14 indicates a strong correlation between MPTM and cube strengths by using the power function equation, R2 is 0.953, and the residual standard error (RSE) is 8.0 MPa. The equation to estimate the cube compressive strength is as follows:
y ¼ 22:89x0:88
Fig. 11. Variation of MPTM strength for different aspect ratio(h/d) and concrete strengths.
were fitted to the data. The power function equation is slightly better and simpler than other formulas, the coefficient of determination (R2) is 0.98, and the RSE is 4.059 MPa. The compressive strength can be estimated by the following equation:
y ¼ 10:59x1:119
ð3Þ
ð4Þ
where x = MPTM strength of concrete, in MPa and y = cube compressive strength of concrete cube, in MPa. Fig. 14 also reveals that the increase of MPTM strength is associated with a decrease of the x/y ratio. This is consistent with the ordinary concrete tensile test rule. The concrete cube compressive strengths varied from 10.89 to 92.17 MPa and the MPTM strength varied from 0.34 to 6.22 MPa, whereas the ratio of MPTM/compressive strength varies from 3.1% to 6.7%. In previous research [18], the ratios of tensile/compressive strengths of the concrete in low, medium and high-strength classes were reported to be 10% to 11%, 8% to 9%, and 7%, respectively. Series B test is used to check the dispersion of the test data. The typical MPTM load histogram is close to a normal distribution (Fig. 15). Standard pull-off tests were also performed to compare
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Fig. 13. MPTM strength versus concrete cube strength at early age (1–4 days).
Comparison between MPTM and standard pull-off method are shown in Table 2, which is derived from series E test. Each coefficient of variation value of Table 2 is statistically analyzed with 24 test data from 6 concrete cubes with the same design strength. For each strength grade, the CoVs of MPTM is always lower than the pull-off test results. The coefficients variation for the MPTM testing results range from 7 to 13 percent. This compares well with other types of strength testing methods, with reported values for breakoff testing, pull-off testing, pull-out testing, and point-load testing of 10, 8, 10 and 9 percent, respectively [21–24]. In Fazlı et al.’s experimental study, the CoVs of pull-off tests range from 6.83% to 20.32%, which is higher than the maximum CoV obtained as 13% in this investigation [25]. Fig. 16 depicts curves compared with MPTM and pull-off test results. The curve of MPTM is derived from series C test. The curves indicates that significant reduction in the tensile strength results is observed by pull-off test. The scatter of standard pull-off tests results are less than that of MPTM test, the RSE of the two test methods are equal to 3 MPa and 8 MPa respectively. The pull-off method could be used to estimate the in-situ actual concrete strength depending on its correlation. Fig. 16 also demonstrates that the ratio of y/x for standard pulloff test is higher than MPTM tests, especially at HSC. MPTM failure load reduced in HSC is caused by higher elastic modulus of HSC, and it will result in higher stress-concentration than NSC.
Fig. 14. MPTM strength versus concrete cube strength.
with the MPTM tests, which were done using the probe as showed in Fig. 1a. The coefficients of variation of the standard pull-off test and the MPTM are summarized in Table 2. MPTM test data have good repeatability. (3) Comparison between MPTM and standard pull-off method
Fig. 16. Comparison of the Relationship between pull-off and MPTM versus concrete cube strength.
Fig. 15. Frequency distribution histogram (44 mm core-specimen).
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S. Yang et al. / Construction and Building Materials 250 (2020) 118834 Table 3 Comparison between the different empirical equations. No.
Formula format = = = =
8.65x1.29 [16] 28.89x0.938 22.89x0.88 5.68x2 [14]
1 2 3 4
y y y y
5 6
y = 11x [6] y = 11.31x + 8.221 [23]
jyexp yest j
Strength range (N/mm2)
Aggregate type
Correlation coefficient
RSE (MPa)
5–60 10–60 10–100 10–70
Crushed Crushed Crushed Crushed
0.911 0.901 0.953 0.819
4.647 3.0 8.0 10.128
9.7 5.5 13.0 8.9
10–45 30–45
Crushed basaltic Crushed limestone
0.991 0.964
2.945 /
8.7 4.4
gravel granite granite granitic
Table 3 compares the empirical formulas from the available literature [6,14,16,23] and from this study. Simple linear empirical formulas are also listed in Table 2 (No.5, No.6), and they are not practical due to the limited test data. 6. Verification and accuracy of the MPTM The applicability, accuracy and reliability of the MPTM are checked by the results obtained from 15-year and 24-year old existing structural concrete (Fig. 17). Concrete strength estimated by MPTM which calculated by Eq. (4) is compared with the compressive strength of 150 mm sawn-cubic specimens and the strength of 100 mm diameter cores with a height/diameter ratio equal to 1. Concrete members are manufactured into the 150 mm cubes were made by using the stone cutting machine, and the sawn-cube concrete strength is assumed to equal to that of a standard 150 mm cube. The height to diameter ratio is equal to 1 for the core sample in diameter of 100 mm. The effective data is taken as the average value of three core samples tested for compression at every test position. Core samples are ground after end cutting, and the compressive strength test shall be carried out after
yexp
100%
Test condition Pull-off test, (d = 50 mm,data num = 72); Pull-off test, (data num = 320); MPTM test, (data num = 980); Direct tensile test, Prism, (size = 100 100 500 mm,data num = 48); Pull-off test, (d = 50 mm,data num = 98) Pull-off test, (d = 50 mm,data num = 3)
the core sample is placed for 24 h. MPTM estimated concrete strength is derived from Eq. (4) in this paper. It is assumed that the compressive strength of a 100 100 mm core is equivalent to the compressive strength of a 150 mm cube of the same concrete strength in accordance with EN-13791 [26]. Column 5 in Table 4 illustrates that the ratio of MPTM estimated strength to cube strength varies from 0.85 to 1.18 and the average value is 1.05. MPTM seems effective in low-strength concrete (<30 MPa), whereas for increased concrete strength (>30 MPa), it tends to overestimate the cube strength. On average, the strength of the 100 mm drilled cores is equal to 0.82 of the 150 mm sawn cube strength (see Table 4). The strength obtained by core-drilling method is seriously underestimated. It is verified that MPTM test is more reliable and convenient than the drilling-core test because of its accurate results obtained. In the future, the accuracy of in-situ test could be partially enhanced by accumulating field test data to calibrate the empirical formula. Further research is recommended to evaluate the compressive strength of HSC by improving the MPTM apparatus to obtain the reasonable failure-mode. The effect of aggregate type should be investigated.
Fig. 17. Verification Test on In-Situ.
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Table 4 Comparison of the MPTM estimated strengths with cube and core strengths. No
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Coefficient of variation Mean Value
Sawn Cube Strength (MPa)
100 mm diameter core strength (MPa)
MPTM estimated Strength (MPa)
MPTM estimated Strength/ Sawn Cube Strength
Core Strength /Sawn Cube Strength
Relative Error
14.9 15.0 17.3 17.7 18.0 18.8 20.2 23.6 23.9 26.1 27.7 29.4 30.2 35.4 39.7 52.5 56.2 57.3
14.2 13.4 14.6 13.7 15.1 15.3 15.5 16.2 16.9 23.4 25.9 23.2 23.8 25.6 27.9 45.2 49.2 48.3
12.7 16.4 15.9 18.8 15.9 20.1 22.5 24.7 21.7 27.4 26.1 33.4 34.6 38.9 45.0 61.8 63.5 62.3
0.85 1.08 0.92 1.06 0.88 1.07 1.11 1.05 0.91 1.05 0.94 1.14 1.15 1.10 1.13 1.18 1.13 1.09 0.097
0.95 0.89 0.84 0.77 0.84 0.81 0.77 0.67 0.71 0.90 0.94 0.79 0.79 0.72 0.70 0.86 0.88 0.84 0.101
14.8 9.3 8.1 6.2 11.7 6.9 11.4 4.7 9.2 5.0 5.8 13.6 14.6 9.9 13.4 17.7 13.0 8.7
1.0467
0.815
10.0
jCubeMPTMj Cube
100%
7. Conclusions
Funding statement
The MPTM is recommended for the indirect estimation on the compressive strength of concrete. The MPTM value and cube strength are used to obtain a reliable and simple correlation between them. The conclusions drawn from the experimental study are as follows:
This work was supported by National Natural Science Foundation of China ( Grant No. 51908341).
(1) The MPTM produced more reliable and repeatable test results compared with the standard pull-off test method. Features of visible failure-mode can certify the correctness of the test result. The cup-shaped loading probe keeps the axis of the core parallel to the load. The tensile load produces true tensile stress in the specimen without the effect of an eccentric load. (2) MPTM test results are independent of the test direction, core diameter, core depth and aggregate size. The t-test was conducted to prove that there were no statistically significant differences in the mean value among the test groups. (3) Regression analyses shows that there is an acceptable relationship between the compressive strength and the MPTM strength regardless of concrete age and mixture proportions. The coefficients of determination R2 are found to be 0.8977 and 0.9874 for Eqs. (4) and (3), respectively. Eq. (3) is only used for the concrete in early-age of 4 days. MPTM can be used not only for monitoring the strength development of newly cast concrete, but also for assessing the strength of older concrete. (4) On average, the estimated in-situ cube compressive strength was approximately 5% greater than the strength measured from 150 mm cubes. This small fluctuation is believed to be of no practical significance for the purpose of strength evaluation of an existing structure. The mean error of core strength to cube strength ratio is greater than 15%, coredrilling may underestimate the concrete strength in-situ.
CRediT authorship contribution statement Suhang Yang: Data curation, Writing - original draft. Zhifeng Xu: Visualization, Investigation, Validation, Writing - original draft. Zhengning Bian: Conceptualization, Methodology.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] D. Breysse, Nondestructive evaluation of concrete strength: An historical review and a new perspective by combining NDT methods, Constr. Build. Mater. 33 (2012) 139–163. [2] H. Levent Selcuk, Suleyman Gökce, Kamil Kayabali, Osman Simsek, A nondestructive testing technique: nail penetration test, ACI Struct. J. 109 (2) (2012) 245–252. [3] V.M. Malhotra, N.J. Carino (Eds.), Handbook on Nondestructive Testing of Concrete, CRC Press Inc., Boca Raton, FL, 1991, p. 343. [4] Cristofaro, M.T., Pucinotti R., Tanganelli, M., De Stefano, M. The dispersion of concrete compressive strength of existing buildings. Computational Methods, Seismic Protection, Hybrid Testing and Resilience in Earthquake Engineering. G.P. Cimellaro et al. (eds), Geotechnical, Geological and Earthquake Engineering, Springer, 33 (2015): 275-285. [5] A. Neville, Core tests: easy to perform, not easy to interpret, Concr. Int. 23 (11) (2001) 59–68. [6] Long AE, Murray A. The pull-off partially destructive test for concrete. Proceedings of International Conference on In-situ/Non-Destructive Testing of Concrete, ACI SP-82; American Concrete Institute 1984: 327-350. [7] Y. Bai, P.A.M. Basheer, D.J. Cleland, State of the art applications of the pull-off test in çivil engineering, Int. J. Struct. Eng. 1 (1) (2009) 93–103. [8] ASTM C1583/C1583M-13. Standard test method for tensile strength of concrete surfaces and the bond strength or tensile strength of concrete repair and overlay materials by direct tension (Pull-off method). West Conshohocken, PA: American Society for Testing and Materials; 2013. [9] British Standard BS 1881: Part 207. Recommendations for the assessment of concrete strength by near-to-surface tests; 1992. [10] L. Courard, B. Bissonnette, A. Garbacz, A. Vaysburd, K. von Fay, G. Moczulski, M. Morency, Effect of misalignment on pulloff test results: numerical and experimental assessments, ACI Mater. J. 111 (2) (2014) 153–162. [11] Giancaspro J, Millman L, Goolabsingh R, MacDonald K, Yang QD.A Novel Bearing Swivel Joint and a Universal Joint for Concrete Pull-Off Testing Using a Material Testing Machine. Journal of Testing and Evaluation 2014; 42(1): 267275. [12] Weerheijm J. ‘‘Understanding the Tensile Properties of Concrete”, 2013, London: Woodhead Publishing Limited: 19-51. [13] J.H. Bungey, R. Mandandoust, Factors influencing pull-off tests in concrete, Mag. Concr. Res. 44 (158) (1992) 21–30.
S. Yang et al. / Construction and Building Materials 250 (2020) 118834 [14] W. Zheng, A.K.H. Kwan, P.K.K. Lee, Direct tension test of concrete, ACI Mater. J. 98 (1) (2001) 63–71. [15] BSI, BS EN 12390–3: Testing Hardened Concrete Compressive strength of test specimens, BSI, London, UK, 2009. [16] C. Chen, W. Jin, H. Ding, Y. Zhao, P.M. Basheer, Experimental research on concrete strength prediction by Limpet pull-off test in China, Int. J. Struct. Eng. 5 (1) (2014) 1–12. [17] M.R.A. van Vliet, J.G.M. van Mier, Effect of strain gradients on the size effect of concrete in uniaxial tension, Int. J. Fract. (1999) 195–219. [18] P.K. Mehta, P.J. Monteiro, Concrete Microstructure, Properties and Materials, McGraw-Hill, New York, NY, 2006. [21] A.E. Long, A.M.C. Murray, The pull-off partially destructive test for concrete, Special Publication 82 (1984) 327–350.
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[22] W.C. Stone, N.J. Carino, C.P. Reeve, Statistical methods for in-place strength predictions by the pullout test, ACI Mater. J. 83 (5) (1986) 745–756. [23] D.N. Richardson, Point load test for estimation of concrete compressive strength, Mater. J. 86 (4) (1989) 409–416. [24] E. Pereira, M.H.F. de Medeiros, Pull Off test to evaluate the compressive strength of concrete: an alternative to Brazilian standard techniques, Rev. Estrut. Mater. 5 (6) (2012) 757–780. [25] H. Fazli, A.M. Yassin, N. Shafiq, W. Teo, Pull-off testing as an interfacial bond strength assessment of CFRP-concrete interface exposed to a marine environment, Int. J. Adhes. Adhes. 84 (2018) 335–342. [26] BS EN 13791-2007 Assessment of in-situ compressive strength in structures and pre-cast concrete components.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Prediction on compressive strength concrete using modified pull-off testing method (MPTM) Suhang Yang a, Zhifeng Xu b, Zhengning Bian a,⇑ a b
School of Civil Engineering and Architecture, Changzhou Institute of Technology, 666, Liaohe Road, Changzhou 213032, PR China College of Civil Engineering and Architecture, Shandong University of Science and Technology, Qingdao 266590, PR China
h i g h l i g h t s A modified pull-off test method is proposed with distinctive characteristics of high accuracy, fewer preparations, than the core-drilling method in-situ. Several influence factors upon concrete strength could be excluded, and a geometric correction factor is proposed. Good relationships between the MPTM strength and the concrete cube strength.
a r t i c l e
i n f o
Article history: Received 31 January 2019 Received in revised form 29 February 2020 Accepted 20 March 2020 Available online 28 March 2020 Keywords: Pull-off test In-situ test Concrete compressive strength Tensile strength Correlation
a b s t r a c t The classical pull-off test indicates that the compressive strength of concrete increases with its tensile strength. But it is difficult to obtain the effective data due to the loading eccentricity, which could produce combined bending plus axial stress instead of the uniaxial tension. The objective of this study is to introduce a modified pull-off testing method (MPTM) and improve the classical method by using a cup-shaped loading probe, so that the tensile strength of concrete can measured under pure axial-load. Experimental results indicate that several influential factors upon the MPTM strength could be excluded. There is a good relationship between the MPTM strength and the concrete cube strength (10–80 MPa) both at early age and later age. The repeatability of the MPTM is slightly better than the classical pulloff test method. The verification investigation shows that the MPTM provides a more accurate estimate of in-place cube strength and is easier to perform than the core-drilling method. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction The compressive strength is a principal property for assessment of concrete structures. Much research has been done on regarding the estimation of concrete strength by using nondestructive test methods (NDT), such as rebound hammer [1], nail penetration testing methods [2], ultrasonic pulse velocity method [3]. The NDT methods measure a test parameter (elastic modulus, penetration resistance, sound velocity and surface hardness) in a low-cost and nondestructive manner. Compressive strength is estimated by using an empirical correlation between the test physical quantity and concrete strength. However, the estimation of concrete strength from the measured physical quantity is sensitive to local changes in concrete, because the NDT test results are severely affected by the in-place environmental conditions, which may be different from that of the laboratory condition. The dispersion of compressive strength, obtained by NDT methods, using correlation ⇑ Corresponding author. E-mail address: [email protected] (Z. Bian). https://doi.org/10.1016/j.conbuildmat.2020.118834 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
curves, shows that increasing the number of data, the coefficient of variation does not necessary decrease [4].NDT is recommended for a preliminary structural inspection and checking the uniformity of structural concrete. The partial destructive test method (PDT) of drilling-cores and testing for concrete strength, and can provide more accurate results than using the NDT. But core-drilling may result in underestimation of the concrete strength because drilling invariably causes damage to concrete cores, and the inhomogeneous concrete may result in eccentric load under compression [5]. By comparison with available PDT testing methods, the widely accepted NDT methods should be able to provide reliable results with trained operators, and can estimate the in-place concrete strength using a portable apparatus. Many methods for this purpose have been reported in the literature, among them is the pull-off test method [6,7]. The pull-off test can be used to estimate the compressive strength by measuring the near-surface tensile strength of the concrete. It is already standardized in BS 1881-Part4 and ASTM C1583/ C1583M [8,9]. Large test loads are not needed for measuring the direct tensile strength of concrete, because the tensile strength of
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the concrete is a small fraction of the compressive strength. Compressive strength can be related to tensile strength using an empirical correlation. However, the inhomogeneity of concrete may cause eccentricity of the applied load, which can reduce pull-off strength by as much as 15% and produces a state of biaxial stress instead of the uniaxial tension [10]. The classical pull-off apparatus should be improved so as to apply uniform pure tensile stress within the specimen [11]. Some reviews indicate that factors affecting the tensile strength of concrete include the composition (e.g. type of aggregate and water/cement ratio), age, curing method, and size of the specimen, as well as the effects of boundary condition [12]. Some influential factors could be controlled to improve the accuracy of the test results [13,14]. The aim of this study is to use a modified pull–off method (MPTM) to estimate the compressive strength of concrete. The MPTM apparatus is intended to assure concentricity of loading, and thereby measure the true tensile strength. The failure mode of the specimen is analyzed to verify whether the MPTM is suitable for determining the uniaxial tensile strength of partial core specimens. Moreover, some influential factors of MPTM are studied and excluded to facilitate the method. Pull-off strengths and cube strengths are measured to determine whether a reliable correlation exists between them. This study investigates the adequacy of the correlation and the variation in the results achieved using the MPTM for a relatively large range of concrete cube strength (10–80 N/mm2), and promote the use of the MPTM in the assessment of concrete strength. 2. Modified pull-off testing method (MPTM) 2.1. Mechanism of the MPTM Fig. 1 compares the MPTM with the commonly used pull-off test procedure. The principle of the MPTM is fairly straightforward. A partial core specimen is erected in the concrete member to be tested. A self-aligned swivel joint is used to minimize the effect of misalignment, and to ensure the external force is parallel to the axis of the partial core. The top and side of the core are bonded to the inner-surface of a special cup. Then, the force gradually grows with distance from the surface, and bottom of the core specimen bears the maximum force. The thin-circular-tube with a cup shape feature is utilized to restrict the lateral deformation of the core, and to keep the axis of the core perpendicular to the concrete surface, so that a concentric of the axial load is applied uniformly
throughout the area. The core is pulled axially until the fracture occurs at the weakest base location. 2.2. Calculation and interpretation of results MPTM strength is calculated by dividing the maximum load by the area of the core as shown by Eq. (1), which assumes that the load is uniformly distributed over the area of the core.
r¼
4P 3:14D2
ð1Þ
where r is the tensile strength at failure (MPa) and identified as MPTM strength, P is the maximum tensile load applied to the test core (N), and D is the diameter (mm) of the core specimen. Visual examination of the failure mode is considered in evaluating the test results. If the core fails at its base, it is considered as the normal failure mode, and the tensile strength of concrete is effectively determined. If any unusual failure mode occurs anywhere other than at the core base, the test data must be discarded. 3. Specimen preparation 3.1. MPTM apparatus The MPTM testing apparatus used in this study comprises several components (Fig. 2). A reaction frame with sufficient stiffness is needed so that the load direction remains constant during testing. The applied force is transferred through the tripod reaction system. The dimensions of the cup-shaped loading probe are presented in Fig. 2b. The portable apparatus weights 3 kg. Loading is performed by using a hydraulic jack with a hand wheel. The tension load is applied gradually at the rates of 0.2–0.4 kN/s. A test should be completed within 20–50 s. The operating temperature range of the apparatus is from 10 to 50 °C. The peak load at failure is recorded with a digital gauge system, which is able to register loads with 1 N accuracy and a maximum load of about 30 kN. The inner diameter of the cup-shaped loading probe is 44 mm (Fig. 2b). The diameter of the probe is less than conventional stirrup spacing to avoid interference of reinforcing bars. According to BS Standard 1881, the thickness of the steel disc should be at least 40% of its diameter [9]. The probe can be reused after removing epoxy adhesive on the probe by heating to 100 °C in an oven. The 44 44 mm core size is equivalent to 1/12 of the volume of
Fig. 1. Schematic diagram of MPTM and Standard Pull-off test.
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Fig. 2. Schematic diagram of the portable apparatus for MPTM testing.
a standard 100 100 mm core, so the MPTM can be considered as a PDT method. 3.2. Coring equipment Before the cup-shaped loading probe is attached to the concrete surface with epoxy, a partial depth core has to be created by using a water-cooled coring machine (Fig. 4a). Concrete with lowstrength may be ruptured while drilling. The operation should proceed slowly to minimize disturbance to the core and the drilling depth should be measured and less than 3 mm beyond the end of the probe should be controlled. 3.3. Surface treatment and application of adhesive Sandpaper was used to prepare the concrete surface plate to improve the bonding of the epoxy. The concrete surface was abraded in a depth of about 3 mm until the coarse aggregate particles were exposed. Then, the cup-shaped loading probe was bonded to the concrete core using quick-setting epoxy adhesive. The entire contact surface between the probe and the concrete is bonded to ensure that the load is applied at the core base. The epoxy that was used attained its adhesive strength of 10 MPa after 2 h of curing. Table 1
aggregate, while crushed stone with particle sizes ranging from 10 to 25 mm was used as coarse aggregate. Concrete cube strengths ranging from 10 to 100 MPa were obtained. Two kinds of Portland cement (CEM I 42.5R and 52.5R), a crushed limestone, and a natural aggregate (river gravel) were used in concrete construction. Mixture proportions and properties of concrete mixtures are given in Table 2. Concrete cubes were cured outdoors and covered with a wet cloth, to protect the specimens from sunlight and rain. The curing temperature ranged from 10 °C to 40 °C. A core specimen with a height (h) of 44 mm and diameter (d) of 44 mm was selected as the standard specimen for comparison. All cube specimens were cured in the parallel environment to guarantee the comparability of the test results of the MPTM and cube compression tests. The compressive strengths of various concrete mixtures were obtained by testing three 150 mm cubes according to BS-EN 12390-3 [15].
4. Experimental programme and test procedure 4.1. Experimental programme The experimental programme comprises three series of tests as follows.
3.4. Test specimens (1) Series A The concrete was mixed in a drum-type mixer, placed into 150 mm steel cube moulds, and compacted on a vibrating table. Ordinary sand with a relative density of 2.5 was used as the fine
This tests is to eliminate the influence factors that do not need. Four grades of concrete strength, C20 ,C40, C60 and C80, were used,
Table 1 Proportions and some properties of concrete mixtures. Mixture
Mixture proportions, kg/m3
Some properties
Coarse aggregate
Fine aggregate
Cement
Water
Admixture
w/c
Aggregate type
Maximum Aggregate size, mm
28-day cube Compressive strength, MPa
MIX-A
1205
786
190
150
—
0.79
22
12.6
MIX-B
1190
745
286
180
—
0.63
22
21.2
MIX-C
1180
658
391
180
—
0.49
22
32.4
MIX-D
1195
649
367
180
3.6
0.46
22
41.2
MIX-E
1180
649
391
180
90
0.46
22
52.3
MIX-F
1120
574
450
150
140
0.33
22
61.8
MIX-G
1143
580
470
152
140
0.32
22
72.5
MIX-H
1051
643
450
150
160
0.33
Crushed Limestone Crushed Limestone Crushed Limestone Crushed Limestone Crushed Limestone Crushed Limestone Crushed Limestone Crushed Limestone
22
83.6
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(5) Series E
Table 2 Variability of standard pull-off test data and MPTM test data. Concrete mixture
C20 C30 C40 C50 C60 C70 C80
Test data coefficient of variation (CoV), % Test method MPTM test
Standard Pull-off test
7 8 8 9 11 10 13
9 9 12 11 13 14 18
This series test studied the comparison between standard Pulloff and MPTM methods. Pull-off and MPTM tests were performed on the same cube (Fig. 3(c)). The tests were done with MPTM in 28 day standard curing. 4.2. Test procedure The test procedure involves the following steps:
and all the specimens were tested at 28 days. When each influencing factor is tested, six MPTM tests of each concrete cube were performed as shown in Fig. 3 (a). (2) Series B This test is to check the data dispersion of MPTM. 2 groups of MPTM strength are obtained from the concrete cube with strength C20 and C60. 24 MPTM tests for each group were performed on four concrete cubes. The tests are done with MPTM in 28 day standard curing. (3) Series C MPTM and concrete cubes were tested at the early ages (12 h, 18 h, 24 h , 30 h, 36 h, 40 h, 48 h, 54 h, 66 h, 72 h, 84 h and 96 h). The relationship between MPTM test values and cube compressive strength was obtained at each ages. At each test age, Four cubes were selected from concrete cubes of each design strength. Six MPTM tests were performed on each concrete cube as shown in Fig. 3(a), and the effective data of each MPTM on the curve is the average of these six data. The remaining three of the two test cubes could be tested for compressive strength. (4) Series D MPTM, standard pull-off and concrete cubes were tested at the specified curing ages. After the cubes were made, the test shall be conducted every seven days until 360d. Cube compressive strength range from 10 to 80 MPa. The relationship between MPTM test values and cube compressive strength for core samples was obtained irrespective of the ages. At each test age, three cubes were selected from concrete cubes of each design strength. Six MPTM and standard pull-off tests were performed on each concrete cube respectively, as shown in Fig. 3(a) and Fig. 3(b). The effective data on the curve is the average of these six data. The remaining one of the three cubes shall be tested for compressive strength.
Step 1: Prepare concrete surface and carefully drill the partial depth core at the selected location (Fig. 4a). Six tests are performed on each cube; Step 2: Place freshly mixed epoxy resin on the inner-surface of the loading probe, (Fig. 4b) and allow epoxy to harden; Step 3: Apply load at a constant stress rate of about 0.05 N/ mm2/s following BS 1881-part207. After test is completed, the actual diameter of the core is measured at three positions to calculate the mean value. The diameter of the core is measured by a digital caliper, accurate to 0.01 mm (Fig. 4c). Step 4: Examine the failure surface and the failure mode visually. Report any unusual conditions, such as mis-alignment of testing surface and loading fixture, failure at the interface of concrete-steel, etc. Step 5: Obtain the maximum load values from the MPTM apparatus for the acceptance failure modes, accurate to 1 N. MPTM strength is calculated using Eq. (1). 5. Results and discussions 5.1. Failure mode A typical failure mode after the MPTM test is shown in Fig. 5a–c. It can be observed from Fig. 5a–c that the failure surface is at the core end, and is more planar than that of the standard pull-off test and almost parallel to the concrete surface. Fig. 5d shows that the typical failure mode occurs at the core end, this failure mode can be used to judge the validity of the test results. More than 95% of the failures occurred at the bottom of the probe and only a few failures were in the body of the core as shown in Fig. 6. No failure occurred in the adhesive-core interface for the MPTM tests. Detail failure modes of the core specimen are shown in Fig. 5(d). The concrete base is generally the weakest portion of the system, and tensile failure occurs in the concrete base. The above observations agree well with investigations of the standard pull-off test [16]. Non-typical failure modes in the MPTM test are shown in Fig. 6, which shows that the failure surface occurred deeper in the concrete. When the design strength of concrete is more than
Fig. 3. Testing position.
S. Yang et al. / Construction and Building Materials 250 (2020) 118834
5
Fig. 4. Testing procedure.
Fig. 5. Typical failure mode of failure in the concrete interface.
ratio, core diameter and aggregate size. Four grades of concrete (20, 40, 60, 80 MPa) were used to investigate the effects of various factors. The compressive strength was measured at 28 days to derive appropriate correlations with MPTM strength. Statistical tests were used to determine if the effects of the various factors was statistically significant. (1) Effect of test direction
Fig. 6. Non-typical failure mode of failure in substrate.
70 MPa, the abnormal failure modes of specimens appear sporadically. This indicates that the weak position of high strength concrete is transferred from ITZ to aggregate itself. 5.2. Effects of influencing factors Series A test is performed to find out the influencing factors to be adopted for the MPTM. Based on previous studies, the factors that influence the MPTM strength include the height-diameter
Fig. 8 shows the MPTM strengths obtained from two testing directions (horizontal and vertical, Fig. 7) at different concrete strength. The test direction is always perpendicular to the cube face (Fig. 3), but only changes the orientation of the apparatus. The t-test was conducted on the average values for the two test directions. The significance level of the t-test values are exhibited in square brackets, and all group t-test values are larger than 0.05. Effect of testing directions is presented in the table attached to Fig. 8. The correction efficiency (Is) is defined as average vertical testing value divided by the average horizontal value of MPTM strengths. Each test result in the table is the mean value of six independent test results obtained from one concrete cube specimen. MPTM is independent of testing direction and without the effect of gravity. (2) Effect of specimen diameter (d) Three different specimen diameters (d = 38 mm, 44 mm, 49 mm) were investigated (Fig. 9). The one-way ANOVA of P-
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Fig. 7. Schematic diagram of the MPTM testing direction.
which indicates that there are no statistically significant differences in the mean value, due to aspect ratio is no significant. On average, MPTM test values for core specimens with h/d = 1.5 and h/d = 0.5 are 3.6% higher and 1.9% lower, respectively (see table in Fig. 11) than that of the standard core specimen with h/d = 1. These results are in accord with earlier findings, which demonstrate that the results are not so sensitive to the coring depth [13]. It could also be interpreted that the MPTM reduces the possibility of eccentric loading and the increase of h/d ratio did not affect the results. The internal tensile stress distribution in MPTM test is more uniform than the standard pull-off test. In addition, the increase of concrete strength results in higher dispersion, because of the increase in HSC brittleness, and it is common phenomenon for the dispersion of strength test results to increase with increasing average strength. (4) Effect of aggregate size
Fig. 8. Variation of MPTM load of two test directions for different concrete strengths.
values are shown in square brackets, and P-values of all groups are larger than 0.05. Thus there is no significant differences in the mean values for the different core diameters. Fig. 10 shows the MPTM strengths as a function of the diameter (d) of the core specimen. For a given compressive strength, MPTM strength decreases with the increase in core diameter. . .. The MPTM strengths for cores in 38 and 49 mm diameters are 22% higher and 19% lower than that of the standard core (d = 44 mm) respectively. This coincides with previous studies, the greater the core diameter, the smaller the strength is [17], because the greater the specimen diameter, the more flaws present. For HSC (C60, C80), the size effect decreases with the increase of concrete strength. Due to the strength of the interface transition zone (ITZ) between the aggregate and paste growing with the increase of concrete strength, the weak part of concrete has been transferred from ITZ to aggregate. (3) Effect of aspect ratio (h/d) Fig. 11 shows MPTM test results for different aspect ratios (h/d = 0.5, 1.0, 1.5) of the cores and different concrete cube strengths, h and d represent the height and diameter of the core, respectively. The MPTM test yields similar results for the core with the different h/d ratios. All group p-values are larger than 0.05,
The effect of maximum aggregate size (da) was evaluated using the reference core specimen (h/d = 1, d = 44 mm). Maximum aggregate sizes (da) of 25 mm and 15 mm were used. The t-test certified that there were no significant differences between each test group except for C20 group. Fig. 12 illustrates that the MPTM strength is greater for specimens with the 25 mm aggregate. MPTM strength is affected by enlarged da between 2% and 9%, with 5.2% on average. While the 25-mm aggregate resulted in higher strength, the increase is not statistically significant expectation for the tests on C20 concrete, which shows very low dispersion for the 15-mm aggregate. The factor can be disregarded because the results in Fig. 12 shows that maximum aggregate size does not have a statistically significant effect on the average MPTM strength. 5.3. Empirical relationship between MPTM and concrete cube strength The objective of this study is to find an empirical relationship between MPTM strength and cube strength ranging from 10 to 80 MPa. (1) Relationship between MPTM and concrete cube strengths at early age. Series D test was carried out to study the relationship between cube strength and MPTM strength at early age. Fig. 13 exhibits the relationship between concrete cube compressive strength and the MPTM strength at ages from 1 to 4 d. Several empirical functions
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S. Yang et al. / Construction and Building Materials 250 (2020) 118834
Fig. 9. Specimens with different diameters.
Fig. 10. Variation of MPTM strength for different core diameters and cube concrete strengths.
Fig. 12. Variation of MPTM strength for different maximum aggregate sizes and concrete strengths.
where x = MPTM strength of concrete, in MPa; and y = cube compressive strength of concrete, in MPa. (2) Relationship between MPTM and concrete cube strengths Series D test was carried out to study the relationship between cube strength and MPTM strength. The regression form y = axb is adopted as the best-fit line to represent the relationship. Fig. 14 indicates a strong correlation between MPTM and cube strengths by using the power function equation, R2 is 0.953, and the residual standard error (RSE) is 8.0 MPa. The equation to estimate the cube compressive strength is as follows:
y ¼ 22:89x0:88
Fig. 11. Variation of MPTM strength for different aspect ratio(h/d) and concrete strengths.
were fitted to the data. The power function equation is slightly better and simpler than other formulas, the coefficient of determination (R2) is 0.98, and the RSE is 4.059 MPa. The compressive strength can be estimated by the following equation:
y ¼ 10:59x1:119
ð3Þ
ð4Þ
where x = MPTM strength of concrete, in MPa and y = cube compressive strength of concrete cube, in MPa. Fig. 14 also reveals that the increase of MPTM strength is associated with a decrease of the x/y ratio. This is consistent with the ordinary concrete tensile test rule. The concrete cube compressive strengths varied from 10.89 to 92.17 MPa and the MPTM strength varied from 0.34 to 6.22 MPa, whereas the ratio of MPTM/compressive strength varies from 3.1% to 6.7%. In previous research [18], the ratios of tensile/compressive strengths of the concrete in low, medium and high-strength classes were reported to be 10% to 11%, 8% to 9%, and 7%, respectively. Series B test is used to check the dispersion of the test data. The typical MPTM load histogram is close to a normal distribution (Fig. 15). Standard pull-off tests were also performed to compare
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S. Yang et al. / Construction and Building Materials 250 (2020) 118834
Fig. 13. MPTM strength versus concrete cube strength at early age (1–4 days).
Comparison between MPTM and standard pull-off method are shown in Table 2, which is derived from series E test. Each coefficient of variation value of Table 2 is statistically analyzed with 24 test data from 6 concrete cubes with the same design strength. For each strength grade, the CoVs of MPTM is always lower than the pull-off test results. The coefficients variation for the MPTM testing results range from 7 to 13 percent. This compares well with other types of strength testing methods, with reported values for breakoff testing, pull-off testing, pull-out testing, and point-load testing of 10, 8, 10 and 9 percent, respectively [21–24]. In Fazlı et al.’s experimental study, the CoVs of pull-off tests range from 6.83% to 20.32%, which is higher than the maximum CoV obtained as 13% in this investigation [25]. Fig. 16 depicts curves compared with MPTM and pull-off test results. The curve of MPTM is derived from series C test. The curves indicates that significant reduction in the tensile strength results is observed by pull-off test. The scatter of standard pull-off tests results are less than that of MPTM test, the RSE of the two test methods are equal to 3 MPa and 8 MPa respectively. The pull-off method could be used to estimate the in-situ actual concrete strength depending on its correlation. Fig. 16 also demonstrates that the ratio of y/x for standard pulloff test is higher than MPTM tests, especially at HSC. MPTM failure load reduced in HSC is caused by higher elastic modulus of HSC, and it will result in higher stress-concentration than NSC.
Fig. 14. MPTM strength versus concrete cube strength.
with the MPTM tests, which were done using the probe as showed in Fig. 1a. The coefficients of variation of the standard pull-off test and the MPTM are summarized in Table 2. MPTM test data have good repeatability. (3) Comparison between MPTM and standard pull-off method
Fig. 16. Comparison of the Relationship between pull-off and MPTM versus concrete cube strength.
Fig. 15. Frequency distribution histogram (44 mm core-specimen).
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S. Yang et al. / Construction and Building Materials 250 (2020) 118834 Table 3 Comparison between the different empirical equations. No.
Formula format = = = =
8.65x1.29 [16] 28.89x0.938 22.89x0.88 5.68x2 [14]
1 2 3 4
y y y y
5 6
y = 11x [6] y = 11.31x + 8.221 [23]
jyexp yest j
Strength range (N/mm2)
Aggregate type
Correlation coefficient
RSE (MPa)
5–60 10–60 10–100 10–70
Crushed Crushed Crushed Crushed
0.911 0.901 0.953 0.819
4.647 3.0 8.0 10.128
9.7 5.5 13.0 8.9
10–45 30–45
Crushed basaltic Crushed limestone
0.991 0.964
2.945 /
8.7 4.4
gravel granite granite granitic
Table 3 compares the empirical formulas from the available literature [6,14,16,23] and from this study. Simple linear empirical formulas are also listed in Table 2 (No.5, No.6), and they are not practical due to the limited test data. 6. Verification and accuracy of the MPTM The applicability, accuracy and reliability of the MPTM are checked by the results obtained from 15-year and 24-year old existing structural concrete (Fig. 17). Concrete strength estimated by MPTM which calculated by Eq. (4) is compared with the compressive strength of 150 mm sawn-cubic specimens and the strength of 100 mm diameter cores with a height/diameter ratio equal to 1. Concrete members are manufactured into the 150 mm cubes were made by using the stone cutting machine, and the sawn-cube concrete strength is assumed to equal to that of a standard 150 mm cube. The height to diameter ratio is equal to 1 for the core sample in diameter of 100 mm. The effective data is taken as the average value of three core samples tested for compression at every test position. Core samples are ground after end cutting, and the compressive strength test shall be carried out after
yexp
100%
Test condition Pull-off test, (d = 50 mm,data num = 72); Pull-off test, (data num = 320); MPTM test, (data num = 980); Direct tensile test, Prism, (size = 100 100 500 mm,data num = 48); Pull-off test, (d = 50 mm,data num = 98) Pull-off test, (d = 50 mm,data num = 3)
the core sample is placed for 24 h. MPTM estimated concrete strength is derived from Eq. (4) in this paper. It is assumed that the compressive strength of a 100 100 mm core is equivalent to the compressive strength of a 150 mm cube of the same concrete strength in accordance with EN-13791 [26]. Column 5 in Table 4 illustrates that the ratio of MPTM estimated strength to cube strength varies from 0.85 to 1.18 and the average value is 1.05. MPTM seems effective in low-strength concrete (<30 MPa), whereas for increased concrete strength (>30 MPa), it tends to overestimate the cube strength. On average, the strength of the 100 mm drilled cores is equal to 0.82 of the 150 mm sawn cube strength (see Table 4). The strength obtained by core-drilling method is seriously underestimated. It is verified that MPTM test is more reliable and convenient than the drilling-core test because of its accurate results obtained. In the future, the accuracy of in-situ test could be partially enhanced by accumulating field test data to calibrate the empirical formula. Further research is recommended to evaluate the compressive strength of HSC by improving the MPTM apparatus to obtain the reasonable failure-mode. The effect of aggregate type should be investigated.
Fig. 17. Verification Test on In-Situ.
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S. Yang et al. / Construction and Building Materials 250 (2020) 118834
Table 4 Comparison of the MPTM estimated strengths with cube and core strengths. No
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Coefficient of variation Mean Value
Sawn Cube Strength (MPa)
100 mm diameter core strength (MPa)
MPTM estimated Strength (MPa)
MPTM estimated Strength/ Sawn Cube Strength
Core Strength /Sawn Cube Strength
Relative Error
14.9 15.0 17.3 17.7 18.0 18.8 20.2 23.6 23.9 26.1 27.7 29.4 30.2 35.4 39.7 52.5 56.2 57.3
14.2 13.4 14.6 13.7 15.1 15.3 15.5 16.2 16.9 23.4 25.9 23.2 23.8 25.6 27.9 45.2 49.2 48.3
12.7 16.4 15.9 18.8 15.9 20.1 22.5 24.7 21.7 27.4 26.1 33.4 34.6 38.9 45.0 61.8 63.5 62.3
0.85 1.08 0.92 1.06 0.88 1.07 1.11 1.05 0.91 1.05 0.94 1.14 1.15 1.10 1.13 1.18 1.13 1.09 0.097
0.95 0.89 0.84 0.77 0.84 0.81 0.77 0.67 0.71 0.90 0.94 0.79 0.79 0.72 0.70 0.86 0.88 0.84 0.101
14.8 9.3 8.1 6.2 11.7 6.9 11.4 4.7 9.2 5.0 5.8 13.6 14.6 9.9 13.4 17.7 13.0 8.7
1.0467
0.815
10.0
jCubeMPTMj Cube
100%
7. Conclusions
Funding statement
The MPTM is recommended for the indirect estimation on the compressive strength of concrete. The MPTM value and cube strength are used to obtain a reliable and simple correlation between them. The conclusions drawn from the experimental study are as follows:
This work was supported by National Natural Science Foundation of China ( Grant No. 51908341).
(1) The MPTM produced more reliable and repeatable test results compared with the standard pull-off test method. Features of visible failure-mode can certify the correctness of the test result. The cup-shaped loading probe keeps the axis of the core parallel to the load. The tensile load produces true tensile stress in the specimen without the effect of an eccentric load. (2) MPTM test results are independent of the test direction, core diameter, core depth and aggregate size. The t-test was conducted to prove that there were no statistically significant differences in the mean value among the test groups. (3) Regression analyses shows that there is an acceptable relationship between the compressive strength and the MPTM strength regardless of concrete age and mixture proportions. The coefficients of determination R2 are found to be 0.8977 and 0.9874 for Eqs. (4) and (3), respectively. Eq. (3) is only used for the concrete in early-age of 4 days. MPTM can be used not only for monitoring the strength development of newly cast concrete, but also for assessing the strength of older concrete. (4) On average, the estimated in-situ cube compressive strength was approximately 5% greater than the strength measured from 150 mm cubes. This small fluctuation is believed to be of no practical significance for the purpose of strength evaluation of an existing structure. The mean error of core strength to cube strength ratio is greater than 15%, coredrilling may underestimate the concrete strength in-situ.
CRediT authorship contribution statement Suhang Yang: Data curation, Writing - original draft. Zhifeng Xu: Visualization, Investigation, Validation, Writing - original draft. Zhengning Bian: Conceptualization, Methodology.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] D. Breysse, Nondestructive evaluation of concrete strength: An historical review and a new perspective by combining NDT methods, Constr. Build. Mater. 33 (2012) 139–163. [2] H. Levent Selcuk, Suleyman Gökce, Kamil Kayabali, Osman Simsek, A nondestructive testing technique: nail penetration test, ACI Struct. J. 109 (2) (2012) 245–252. [3] V.M. Malhotra, N.J. Carino (Eds.), Handbook on Nondestructive Testing of Concrete, CRC Press Inc., Boca Raton, FL, 1991, p. 343. [4] Cristofaro, M.T., Pucinotti R., Tanganelli, M., De Stefano, M. The dispersion of concrete compressive strength of existing buildings. Computational Methods, Seismic Protection, Hybrid Testing and Resilience in Earthquake Engineering. G.P. Cimellaro et al. (eds), Geotechnical, Geological and Earthquake Engineering, Springer, 33 (2015): 275-285. [5] A. Neville, Core tests: easy to perform, not easy to interpret, Concr. Int. 23 (11) (2001) 59–68. [6] Long AE, Murray A. The pull-off partially destructive test for concrete. Proceedings of International Conference on In-situ/Non-Destructive Testing of Concrete, ACI SP-82; American Concrete Institute 1984: 327-350. [7] Y. Bai, P.A.M. Basheer, D.J. Cleland, State of the art applications of the pull-off test in çivil engineering, Int. J. Struct. Eng. 1 (1) (2009) 93–103. [8] ASTM C1583/C1583M-13. Standard test method for tensile strength of concrete surfaces and the bond strength or tensile strength of concrete repair and overlay materials by direct tension (Pull-off method). West Conshohocken, PA: American Society for Testing and Materials; 2013. [9] British Standard BS 1881: Part 207. Recommendations for the assessment of concrete strength by near-to-surface tests; 1992. [10] L. Courard, B. Bissonnette, A. Garbacz, A. Vaysburd, K. von Fay, G. Moczulski, M. Morency, Effect of misalignment on pulloff test results: numerical and experimental assessments, ACI Mater. J. 111 (2) (2014) 153–162. [11] Giancaspro J, Millman L, Goolabsingh R, MacDonald K, Yang QD.A Novel Bearing Swivel Joint and a Universal Joint for Concrete Pull-Off Testing Using a Material Testing Machine. Journal of Testing and Evaluation 2014; 42(1): 267275. [12] Weerheijm J. ‘‘Understanding the Tensile Properties of Concrete”, 2013, London: Woodhead Publishing Limited: 19-51. [13] J.H. Bungey, R. Mandandoust, Factors influencing pull-off tests in concrete, Mag. Concr. Res. 44 (158) (1992) 21–30.
S. Yang et al. / Construction and Building Materials 250 (2020) 118834 [14] W. Zheng, A.K.H. Kwan, P.K.K. Lee, Direct tension test of concrete, ACI Mater. J. 98 (1) (2001) 63–71. [15] BSI, BS EN 12390–3: Testing Hardened Concrete Compressive strength of test specimens, BSI, London, UK, 2009. [16] C. Chen, W. Jin, H. Ding, Y. Zhao, P.M. Basheer, Experimental research on concrete strength prediction by Limpet pull-off test in China, Int. J. Struct. Eng. 5 (1) (2014) 1–12. [17] M.R.A. van Vliet, J.G.M. van Mier, Effect of strain gradients on the size effect of concrete in uniaxial tension, Int. J. Fract. (1999) 195–219. [18] P.K. Mehta, P.J. Monteiro, Concrete Microstructure, Properties and Materials, McGraw-Hill, New York, NY, 2006. [21] A.E. Long, A.M.C. Murray, The pull-off partially destructive test for concrete, Special Publication 82 (1984) 327–350.
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[22] W.C. Stone, N.J. Carino, C.P. Reeve, Statistical methods for in-place strength predictions by the pullout test, ACI Mater. J. 83 (5) (1986) 745–756. [23] D.N. Richardson, Point load test for estimation of concrete compressive strength, Mater. J. 86 (4) (1989) 409–416. [24] E. Pereira, M.H.F. de Medeiros, Pull Off test to evaluate the compressive strength of concrete: an alternative to Brazilian standard techniques, Rev. Estrut. Mater. 5 (6) (2012) 757–780. [25] H. Fazli, A.M. Yassin, N. Shafiq, W. Teo, Pull-off testing as an interfacial bond strength assessment of CFRP-concrete interface exposed to a marine environment, Int. J. Adhes. Adhes. 84 (2018) 335–342. [26] BS EN 13791-2007 Assessment of in-situ compressive strength in structures and pre-cast concrete components.