Estimation of in-situ Masonry Mortar Compressive Strength Using Hammer Pendulum Test Method

doi:10.21203/rs.3.rs-4862734/v1

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Estimation of in-situ Masonry Mortar Compressive Strength Using Hammer Pendulum Test Method

https://doi.org/10.21203/rs.3.rs-4862734/v1

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The compressive strength of in-situ masonry mortar was evaluated using the Pendulum Test Method (HPTM). A portable HPTM apparatus specifically designed for this purpose is utilized to indirectly evaluate the compressive strength of mortar by inserting steel nails into the horizontal mortar joints of the masonry and measuring the penetration depth. The discrete attributes of the test outcomes are analyzed to ascertain the repeatability and reliability of the HPTM results. A relationship between penetration depth and compressive strength is established. In comparison with other existing testing methods, it has been confirmed that HPTM is an alternative compression testing method for masonry mortar, with the advantages of high accuracy, minimal preparation work, and good repeatability. The mortar compressive strength estimated by HPTM is reasonably consistent with the mortar cubic compressive strength test results.

Hammer pendulum test

Regression analysis

mortar

Compressive strength

In-situ verification

The masonry structure comprises bricks bonded with mortar to toward regularising the transfer of local stresses of both vertical and horizontal loads. The compressive strength of masonry mortar is an important basis for evaluating the reliability of masonry structures, determining structural repairs, conducting accident analysis, and seismic reinforcement. The bending strength and shear strength of mortar are also key factors. The bonding strength between masonry components (bricks or stones) and mortar is also closely related to the compressive strength of mortar, which determines the mechanical properties of masonry. Consequently, compression testing is essential in evaluating these properties.

The compression test of prism samples is a standard technique commonly used in buildings and structures, and such samples are generally not suitable for evaluating the properties of mortar in existing structures. The testing of required samples collected from buildings based on representative statistical data is in contrast with the laboratory testing. Since the thickness of mortar joints is very small, prism-shaped samples required for standard testing can not be extracted and subsequently tested.

Based on this limitation, some scholars suggest using non-destructive testing (NDT). There are records in the literature on the estimation of the mortar compressive strength through in-situ NDT methods, such as rebound hammer [1], penetration testing method [2], pendulum rebound hardness [3], and torque penetration testing method [4], anti-drilling technology [5], indentation testing equipment [6], and scratch testing [7]. These methods are applicable to architectural heritage because of minimal damage to mortar and more testing data. The accuracy of the test results depends on the physical properties that are sensitive to changes in mortar strength. There is an absence of a reasonable and logical relation between the NDT physical quantity and the mortar strength. The empirical relationship established in the laboratory is not suitable for in-situ applications and is more sensitive to the mortar surface and testing conditions, which could lead to a decrease in estimated strength. Engineers typically use NDT as a preliminary testing method, which requires skilled operators for testing.

Given the above situation, there is a critical need for a fast and simple method to evaluate the strength of mortar. The method suggested in this article is a new one that could be easily operated without the need for skilled operators and ensure accurate results of the true strength in actual structures. Hammer pendulum test method (HPTM) specialized apparatus is used in this study to evaluate the mortar compressive strength. HPTM based on the penetration of steel pins through bed joints and measuring the penetration depth can be regarded as an NDT method for masonry structures. Through regression analysis of the correlation between unconfined compressive strength and mortar penetration depth, experimental data was analyzed. To prove the applicability, accuracy, and reliability of this method, the comparison of HPTM was made with other testing methods, and the effectiveness and reliability were verified through in-situ tests.

2.1 Test Apparatus

The specially developed HPTM apparatus is shown in Figure 1, with a total weight of 4.5 kilograms. A rigid vertical plate would not be deformed in testing. The yield strength of the frame steel exceeds 340MPa. Detailed of the HPTM apparatus is illustrated in Figure 2. The operating procedure of the HPTM apparatus is as follows:

1. Lift the hammer head to the top locking position, lock the swing rod at an angle of 175°, and insert the measuring pin into the nail holder on the vertical plate.

2. Attach the vertical plate of the pendulum tightly to the tested component. In case the component itself is not completely vertical, adjust the adjusting screw on the pendulum to center the liquid-level bubble and ensure that the plate is at a vertical position.

3. Press the handle of the pendulum tightly, press the activation rod with your thumb to make the hammer head swing freely, and drive the measuring nail in.

The nail is 60 mm in length. The bottom diameter is 6 mm and a top diameter is 4 mm (Figure 3). The nail head is a hemispherical body with a radius of 4 mm. The nails are composed of high-strength steel with a Rockwell hardness of HRC45. The material, strength, and surface finish of the nails meet ASTM C803 [8].

2.2 Sample Preparation and Test Procedure

2.2.1 Materials

The grade of ordinary Portland cement is 32.5MPa in accordance with ASTM C150 [9]. Natural silica sand meets the requirements of ASTM C778 [10]. The nominal maximum particle size of sand is 2mm, and the average fineness modulus is 2.5. The size of ordinary clay bricks is 240×115×53mm, which complies with ASTM C62 [11]. The compressive strength of the bricks meets the requirements of ASTM C67-03a. Moisten the bricks 24 hours in advance before laying to prevent the bricks from absorbing moisture in the mortar and ensure that the cement in the mortar is fully hydrated. Mortar proportions were determined according to recommendations in ASTM C270-14a [12] (Table 1). Cement mortar consists of a mixture of portland cement and sand. Hydraulic lime material specified in ASTM C1506 [13] was adopted. The cement sand and lime are thoroughly mixed with water and then manually mixed with a shovel until the mortar reaches sufficient workability. The compressive strength of these mortar cubes in size of 50×50×50mm was tested by ASTM C109/C109M-08 (ASTM 2008b) [14]. The construction of specimens are shown in Fig. 4.

Table 1. Proportion of CM and CLM.

	Strength grade	Cement: kg/m³	Medium Sand: kg/m³	Water: kg/m³	Lime: kg/m³
Cement-mortar (CM)	M 0.4	222	1487	302	/
	M 2.5	239	1487	302	/
	M 5.0	256	1487	302	/
	M 7.5	264	1487	302	/
	M 10.0	298	1487	302	/
	M 15.0	330	1487	302	/
Cement-lime-mortar (CLM)	M 0.4	187	1480	250	122
	M 2.5	199	1480	250	111
	M 5.0	210	1480	250	99
	M 7.5	222	1480	250	88
	M 10.0	245	1480	250	65
	M 15.0	268	1480	250	42

2.2.2 Test Specimens

(1) Series A test

The dispersion of HPTM data is established tthrough A-series testing. The HPTM test was conducted in a 28 day curing environment to determine the coefficient of variation (CoV) caused by changes in mortar strength and number of tests. Six groups of mortar with M0.4, M2.5, M5.0, M7.5, M10.0 and M15.0 mortar strengths were selected for CM and CLM, respectively. A suitable number of mortar were selected and COV of HPTM test values for CM and CLM mixtures at 28, 45, 60, 90, 120, and 180 days were computed.

(2) Series B test

12 masonry specimens were prepared and two groups were divided : cement mortar (Group 1) and lime mortar (Group 2). All components were prepared by the same worker within 8 hours on the same day, placed outdoors, kept dry, and protected from direct sunlight and rainwater.

To test the compressive strength of mortar cubes, same samples as the corresponding member should be prepared for each member. All samples were cured in the same environment to ensure comparability of test results. Mortar boards are extracted from these members and tested with this HPTM when each masonry member is cured in the specific 28, 45, 60, 90, 120, and 180 days.

(3) Series C test

Series C is independently of A and B series. The mixing proportions of materials in Table 1 also applies to the materials tested in C-series. Masonry samples were tested simultaneously with different methods at the specified 28, 60, 90, and 180 days. The experimental results of HPTM and other methods were compared under the same conditions.

2.2.3 Test Procedure

Step 1: Clean and level the surface of the tested mortar. In case the surface is uneven, use a grinding wheel to smooth the mortar surface before dust removal, because the cleanliness and flatness of the horizontal joint surface in the masonry have a significant impact on the detection of penetration depth (Fig. 5a).

Step 2: Place the apparatus frame tightly against the masonry wall, keeping the frame straight. When the component itself is not completely straight, adjust the adjustable bolt on the pendulum hammer to center the leveling bubble and ensure that the frame is in a straight position. Lift the hammer head upwards and insert the measuring nail into its base. The fine end of the measuring nail should face the horizontal mortar joint of the masonry wall (Fig. 5b).

Step 3: Press the handle of the apparatus tightly, use your thumb to press the vibrator, so that the hammer can swing freely downwards, and knock the nail into the masonry mortar (Fig. 5c). The loading point should be at least 100mm away from each edge. After removing the nail, blow off the dust inside the hole and measure the penetration depth using a depth gauge (Fig. 5 d-e). The depth measured by HPTM is x, in millimeters, accurate to 0.01mm.

3.1 Test numbers

Identify the dispersion of test data and the impact of test numbers on HPTM results using the coefficient of variation (CoV) through A-series testing. Figure 6 illustrates that CoV slightly decreases as the strength of the mortar increases. When the test number increases by six, the CoV of each mortar strength increases. Therefore, each value of HPTM obtained from the six test results was selected as the standard number of tests, less than 10% is acceptable.

The comparable CoVs for the mortar cube test, double impact test, spiral pull-out test, and pin penetration test are 11%, 25%, 20%, and 12%, respectively, and these tests were conducted at 28 days [15]. It is worth noting that a limitation of NDT technology is the high dispersion of experimental results, which is caused by the inherent discreteness of mortar and possible surface degradation. The dispersion of HPTM test results is close to that of PDT.

3.2 Stability of Test Results

The COVs of HPTM test values for CM and CLM mixtures at 28, 45, 60, 90, 120, and 180 days are given in Figure 7. Each COV at testing ages was derived from six samples. The CoV curve fluctuates as the age increases, and the corresponding CoV change range is between 5.5% and 10.5%, indicating that the age have less influence on the stability of the HPTM test results.

3.3 Relationship between HPTM strength and mortar cube strength

3.3.1 Format of equation

Series B test was performed to study the relation of cube strength with HPTM strength. The regression form y=ax^b is used as the best-fit line to represent the relationship. Symbol x represents the HPTM-tested depth of the mortar in mm, and y represents the compressive strength of the masonry cube, in MPa. The power function equation passes through the origin and is convenient for extrapolating the test results.

3.3.2 Regression of HPTM strength with mortar cube strength

Figure 8 exhibits the close correlation of CCTM with mortar cube strength, which is determined by a power function format and is independent of age or mortar strength. The correlation coefficient (R²) of the power function equations for CM and CLM are 0.96 and 0.81.

The comparison of curves between CM and CLM is shown in Figure 9. The ratio of HPTM strength to compressive strength decreases with the increase of strength. The HPTM strength increases with the decrease of mortar compressive strength. For any given cube strength, the HPTM test results in a lime mortar strength higher than CM. Due to the superior performance of CLM over CM and its better water retention than cement mortar, the density and HPTM value of CLM are higher. The hydration of lime in low strength mortar affects the hydration of cement, resulting in slightly lower mortar strength, while the hydration of lime in high-strength mortar leads to higher mortar strength.

3.3.3 Verification and Accuracy of HPTM

Series C test was carried out to compare with different methods, in-situ testing was conducted to verify the accuracy, applicability, and reliability of the proposed equations of HPTM testing. The rebound method, penetration method, LCTM, point-load test, crush test, and HPTM were used to estimate the compressive strength of the masonry mortar ( Fig. 10).

Table 2 summarizes the NDT and partial destructive test (PDT) results obtained at different ages. The maximum error of mortar compressive strength estimated by the rebound method is 35.51%, and the maximum error using the pin penetration method is 66.77%. After 60 days of curing, the rebound estimated value can reflect the trend of changes in the mortar compressive strength. The accuracy of the pin penetration test results is significantly higher than that of the rebound method. The limitation of NDT technology is that it can only evaluate the surface performance of mortar joints, which has a significant impact on the rebound method and penetration method.

The maximum errors of the crush test, LCTM, and PLT are 14.97%, 8.11%, and 8.11%, respectively. The mortar strengths evaluated by the point loading method, PLTM and crush test method are all significantly higher than the actual uniaxial compressive strength of mortar cube, especially when the mortar strength is higher. The compressive strength of mortars estimated by the crush method is the highest one.

For PDT, the limitation of NDT can be eliminated by the joints of masonry core samples extracted from deep joints. The mortar inside the masonry is denser than the mortar at the edges when squeezing. The excess water in the mortar inside the masonry joint is easily absorbed by the bricks, while the water in the edge of joint is not easily absorbed. The mortar at the edge of joint is more severely affected by the environment, so the curing quality of the mortar in joints is better compared to the surface mortar.

The C-series tests have verified that the accuracy of the PDT method is higher than that of the NDT method. The maximum error of HPTM is less than 10%, HPTM test can obtain the most accurate results compared with other methods. A strong correlation of HPTM strength with mortar cube strength exists. Surface irregularities for HPTM is not as sensitive as NDT methods. The accuracy of HPTM is close to, or higher than that of PDT, but its operational simplicity is similar to NDT.

Table 2. Comparison of compressive strength of mortar using four test methods.

		Rebound test [16]		Penetration test [17]		HPTM		Crush test [18]		Point-load test [19]		LCTM [20]		Compressive strength of mortar (MPa)
		Estimated strength (MPa)	RD (%)	Estimated strength (MPa)	RD (%)	Tested strength (MPa)	RD (%)	Estimated Strength (MPa)	RD (%)	Tested strength (MPa)	RD (%)	Estimated strength (MPa)	RD (%)	Compressive strength of mortar (MPa)
28d	M10 CM masonry	10.10	-15.11	11.21	-5.81	11.58	-2.66	10.76	-9.58	10.73	-9.87	10.89	-8.45	11.9
	M7.5 CLM masonry	6.70	-19.25	5.59	-32.55	7.23	-12.89	7.78	-6.24	7.45	-10.25	7.051	-15.04	8.3
	M5 CM masonry	4.47	-31.24	5.17	-20.39	5.87	-9.65	6.05	-6.87	5.66	-12.89	7.1	10.24	6.5
60d	M2.5 CLM masonry	1.98	-38.22	2.33	-27.06	2.95	-7.71	2.94	-8.26	2.73	-14.65	3.03	-5.29	3.2
	M5 CM masonry	4.82	-32.07	5.79	-18.41	6.64	-6.41	8.47	19.24	6.76	-4.73	6.5	-7.62	7.1
	M10 CM masonry	11.11	-17.12	13.91	3.80	12.68	-5.36	12.11	-9.65	14.43	7.66	11.33	-15.43	13.4
90d	M5.0 CM masonry	5.81	-15.84	6.04	-12.48	6.16	-10.75	7.84	13.56	7.44	7.84	5.69	-17.55	6.9
	M5.0 CLM masonry	7.89	-10.25	7.28	-17.32	8.25	-6.25	9.69	10.21	9.47	7.61	7.41	-15.83	8.8
	M2.5 CM masonry	2.92	-21.16	3.62	-2.032	3.41	-7.87	3.53	-4.72	3.38	-8.52	3.37	-8.98	3.7
180d	M15 CM masonry	20.54	9.23	20.03	6.53	17.74	-5.62	20.72	10.21	20.05	6.63	20.65	9.83	18.8
	M7.5 CM	9.57	-8.81	11.49	9.48	9.64	-8.21	11.84	12.76	10.75	2.35	11.10	5.76	10.5
	M10 CLM masonry	10.10	9.05	11.21	4.32	11.58	-6.25	10.76	13.83	10.73	6.38	10.89	8.56	13.8
	Mean of absolute value of RD		18.95		13.35		7.47		10.43		8.28		10.72

Note: Relative deviation RD=100×(Estimated compressive strength of mortar cube--actual compressive strength of mortar cube) / actual compressive strength of mortar cube

The in-situ testing verifies the applicability, reliability, and accuracy of PLTM testing in practical projects. HPTM was implemented in a brick building in the town of Lidian, in Jiangsu Province, China ( Figure 11a). The building was built with clay-fired bricks of 240×115×53 mm and cement mortar.

HPTM was utilized to quickly evaluate the strength of mortar in horizontal joints in-situ. HPTM testing at four distinct walls of each story of the building was conducted to estimate the spatial variability of the properties of the used materials ( Fig. 11b). Mortar cubes are comprised of the same material as the masonry building during construction and placed under the same environment as the masonry structure for curing. Three cube samples at each selected location were tested in the laboratory, and the strength of the tested cubes was compared with the evaluated HPTM values. The test values obtained using PLTM were converted into the cube strength by the curve mentioned in Section 3.2.

The results obtained at different positions of the masonry building are given in table 3. The in-situ verification indicates that the HPTM-tested cube compressive strength was approximately 5.08-19.49 %, lower than the measured strength of the mortar cubes. This suggests that the quality of mortar tested in-situ is always worse than that tested in the laboratory. This may lead to a significant reduction in estimated strength, as empirical relationships established in the laboratory are more susceptible to mortar surface and testing conditions.

The CoVs of PLTM at each test position range from 6.48 to 20.90, with a maximum of 20.90 at wall 2 of the first story, it is related to locally superficial anomalies caused by environmental degradation. HPTM can accurately detect the variability of mechanical performance at different positions of buildings, and in-situ test results of HPTM are safe and acceptable. The predicted results are consistent with the actual consequences of cube strength testing, proving the effectiveness of the HPTM proposed in this paper for in-situ mortar strength testing.

Table 3. In-situ verification for existing masonry buildings with HPTM

Story	Parameter	Wall 1 (MPa)	Wall 2 (MPa)	Wall 3 (MPa)	Wall 4 (MPa)	Cube strength
First-story	HPTM estimated strength	10.2	10.0	8.2	9.6	11.2
		9.6	8.9	9.7	8.8
		9.2	8.3	8.5	8.5
		11.3	7.3	8.8	10.8
		10.6	12.6	9.2	10.3
		9.8	11.4	10.4	11.2
	Average	10.1	9.7	9.1	9.9
	Relative deviation^d (%)	9.82	13.39	18.75	11.61
	Standard deviation	0.83	2.03	0.59	0.97
	CoV (%)	8.22	20.9	6.48	9.79
Second-story	HPTM estimated strength	8.2	9.6	8.7	9.9	11.8
		9.7	11.3	10.5	11.2
		9.1	10.8	8.3	11.6
		10.2	11.6	8.9	11.7
		12.5	11.9	9.8	11.6
		8.9	10.7	10.6	10.9
	Average	9.8	11.0	9.5	11.2
	Relative deviation^d (%)	16.95	6.78	19.49	5.08
	Standard deviation	1.61	0.90	0.89	0.75
	CoV (%)	16.43	8.18	9.37	6.69

This article aims to investigate the applicability of HPTM for evaluating the strength of mortar. The HPTM apparatus can directly test samples by penetrating steel nail into bed joints. HPTM can be proficiently operated by trained technicians at a low cost, and is simple and easy to implement, making it very suitable for structures that require extensive testing. The conclusions are as follows:

(1) HPTM is an innovative method which by freely swinging the hammer head, the test nail is inserted into the material to be inspected, and the compressive strength of the material is estimated in basis of the penetration depth of the test nail. Insert the steel rebar into the joint of the base with a hammer, measure the HPTM strength and cube strength, and explore the reliable and simple correlation between the two. It is recommended to use HPTM to indirectly evaluate the mortar compressive strength.

(2) The HPTM testing apparatus is portable and convenient for in-situ use. Within the strength range from M0.4 to M15.0, the compressive strength of mortar cubes is closely related to the compressive strength tested using HPTM. The regression equation in power form is used, and the correlation coefficients (R²) of CM is 0.925, and that of CLM is 0.901. Compared with other testing methods, the accuracy of HPTM test results can be verified with an error of less than 10%.

(3) The compressive strength test curve is suitable for obtaining the compressive strength of mortar with nail penetration depths ranging from 3.04 to 13.0 mm. When the nail penetration depth exceeds this range, the strength test curve is no longer applicable. Further research is needed to verify its use for other mortar at different curing ages.

(4) The mortar strength tested with HPTM is correlated with the compressive strength of mortar cubes cured under the same conditions, and their correlation has been verified. HPTM can accurately estimate the compressive strength of masonry mortar of masonry structures. Its operation can be programmed and standardized to avoid human errors, making it more accurate and convenient than other testing methods.

DPT testing is considered more reliable for that purpose.

Author Contribution

Yang and Hu. wrote the main manuscript text and Xu prepared figures. All authors reviewed the manuscript.

ACKNOWLEDGMENTS

This study is supported by the Opening Foundation of Shandong Key Laboratory of Civil Engineering Disaster Prevention and Mitigation (Grant No. CDPM2019KF10), the National Natural Science Foundation of China (Grant No. 51908341) and the National Key R & D Program of China (Grant No. 2018YFD1100402-05), Opening Foundation of Key Laboratory of concrete and prestressed concrete structure, Ministry of education, Southeast University (Grant No. CPCSME2019-06).

DATA AVAILABILITY STATEMENT

Some or all data, models, or codes that support the findings of this study are available from the corresponding author upon reasonable request.

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No competing interests reported.

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Estimation of in-situ Masonry Mortar Compressive Strength Using Hammer Pendulum Test Method

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Abstract

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1. Introduction

2. Test Programme

3. Test Results and Discussion

4. In situ verification of HPTM

5. Conclusions

Declarations

Author Contribution

ACKNOWLEDGMENTS

DATA AVAILABILITY STATEMENT

References

Additional Declarations

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