PRELIMINARY ASSESSMENT OF CORRELATION BETWEEN MORTAR HARDNESS AND BRICK MASONRY STRENGTH

Transcription

1 PRELIMINARY ASSESSMENT OF CORRELATION BETWEEN MORTAR HARDNESS AND BRICK MASONRY STRENGTH Goran Simundic (1), Heber O. Sugo (2), Adrian W. Page (3) and Stephen J. Lawrence (4) (1) Professional Officer, School of Engineering, The University of Newcastle, Australia (2) Senior Research Associate, School of Engineering, The University of Newcastle, Australia (3) Emeritus Professor, School of Engineering, The University of Newcastle, Australia (4) Conjoint Professor, School of Engineering, The University of Newcastle, Australia Abstract The Masonry Research Group at the University of Newcastle has recently completed a major study of the use of the Australian Standard Scratch Test to measure mortar surface hardness and therefore the potential mortar durability for Australian conditions (where freeze-thaw issues are not critical). The variables studied included cement content, sand type, sand grading, joint surface finish, mortar type and environmental conditions. A total of 3024 individual scratch tests were performed on 432, four courses high brick masonry prisms located either in the laboratory or in an exposed marine environment. On the completion of this project, a supplementary study was carried out taking advantage of the large number of observations available, as well as the fact that destructive compression or bond wrench tests could now be performed on the masonry piers. A systematic study of the relationship between the site observed surface hardness and compressive/flexural strength of the masonry has been performed. The results of the supplementary study are presented and the reasons for the observed trends discussed. Keywords: mortar hardness, scratch test, durability 1 INTRODUCTION In Australia and similar temperate climates, the durability of masonry mortar is a question of resistance to abrasion from wind action, wetting/drying cycles and crypto-efflorescence from air-borne or water-borne salts. In a review paper, Bowler (1993) refers briefly to fretting of mortar by forces caused by salt crystallisation but does not give it any emphasis. To date, the focus in all European studies of mortar durability has been on freeze-thaw and chemical attack by sulphates. The key factors in resistance to these mechanisms include sand grading, cement content, air content, brick properties, pore size distribution and workmanship (Harrison & 673

2 Bowler, 1990). In temperate climates, on the other hand, freeze-thaw cycles do not occur and degradation does not involve a chemical reaction. The relevant mechanisms of attack are largely resisted by the surface hardness of the mortar and it is therefore logical to use surface hardness as a measure of mortar durability under these conditions. Test methods using rebound hammers (RILEM, 1997; Schmiedmayer 1997) have been directed at mortar compressive strength and pointing quality, and are not applicable to durability. The Australian Masonry Structures Standard AS 3700 (Standards Australia, 2001) includes a scratch test for assessing the surface hardness, and hence the durability potential, of masonry mortar, and includes deemed-to-satisfy criteria for various exposure conditions. Apart from its use in the standard as a control test, this test provides a means of assessing the relative effects of different factors on the mortar durability for a range of materials and construction techniques. The scratch test employs a mechanical tool to measure the penetration of a standard probe under a given force and rotation into the mortar surface. The probe has a diameter of 6 mm, the end ground flat with the specified cross-section and a range of movement of 10 mm into the mortar joint from the point of initial contact. The penetration depth is correlated with potential durability performance. A scratch index is derived as the average of five separate penetration measurements (in mm) distributed approximately uniformly throughout the mortar joint to be tested. The lower the index, the higher is the surface hardness and therefore durability. The technique determines the surface hardness of the mortar and can be used both on site to test insitu walls and in the laboratory to evaluate new mortars. The test method was specifically developed as a means of predicting the durability potential of the mortar when it is exposed to the physical degradation mechanisms found in the Australian environment. The development of the test method has been reported elsewhere (Lawrence & Samarasinghe, 1998 and 2000; Lawrence et al, 2004). This paper describes an investigation which was an extension of a durability project funded by Cement Concrete and Aggregates Australia, which was designed specifically to identify the factors affecting mortar durability in temperate climates and to measure their relative effects. The durability experiment was planned as a fully factorial design, so that the significance of the various factors could be examined by the analysis of variance (ANOVA). On the completion of the main project, a supplementary study was carried out taking advantage of the large number of observations available, as well as the fact that destructive compression or bond wrench tests could now be performed on the masonry piers. A systematic study of the relationship between the site observed surface hardness and compressive/flexural strength of the masonry has been performed. 2 INITIAL DURABILITY TESTING PROGRAM The original durability investigation involved a large number of variables likely to influence mortar durability. Variables included clay brick type, sand grading, cement type, mix proportions and joint finish. Scratch tests were performed on the joints of four high, stack bonded prisms which were exposed to two different environments laboratory and marine exposure and were tested over a period of three years. In total, 3024 measurements of scratch index were performed on 432 masonry prisms. As required by the test method, each measurement of scratch index was the average of five separate measurements with the test instrument. The variables and factors are summarised in Table 1. Two masonry prisms were constructed for each combination of masonry unit, sand, cement, mortar mix, and joint finish. One of each pair of prisms was exposed to each of the environments from 7 days after laying. Measurements were taken on each prism at seven ages from seven days to three years. Building 674

3 the stack-bonded prisms four courses high provided enough joint length in each prism to perform the tests for all ages. The three sand types were chosen to represent the range commonly used for clay brickwork in Australia. Table 1. Variables Included in the Investigation Variable Levels Details Clay Brick 2 Low and high rates of absorption. Sand 3 Two with low clay content (Dune and Coarse) and one with high clay content (Fatty). Cement 3 General Purpose Portland (GP), Fly Ash Blend, and Slag Blend. Mortar Composition 4 M2 (1:2:9), two M3 (1:0:5 and 1:1:6) and M4 (1:¼:3). Joint Tooling 3 Struck flush, ironed and raked. Exposure Environment 2 Laboratory (internal) and marine (exposed). Age 7 Tests carried out at 7, 28, 90, 180, 365, 720 and 1095 days. The test specimens stored in the laboratory environment are shown in Figure 1. Those in the exposed marine environment are shown in Figure 2. Figure 3 shows the scratch test instrument in use on a masonry wall. Detailed analysis of the results of this study have been reported elsewhere (Lawrence et al 2008). Figure 1. Test Specimens in the Laboratory Environment 675

4 Figure 2. Test Specimens in the Exposed Environment Figure 3. A Scratch Test Performed on a Wall 676

5 3. EXAMINATION OF MASONRY STRENGTH PROPERTIES On the completion of the durability project, a supplementary study was carried out taking advantage of the large number of observations available, as well as the fact that destructive compression or bond wrench tests could now be performed on the masonry piers. A systematic study of the relationship between the site observed surface hardness and compressive/flexural strength of the masonry has been performed. If reasonable correlations were obtained, there would be potential to establish a non-destructive site test to estimate the in-situ masonry compressive and bond strengths. As indicated in Table 1, there were a large number of variables considered in the original investigation. For the supplementary study, some of these results were pooled to obtain samples of a reasonable size (see Table 2). Consistent with the provisions of the Australian Masonry Code, no discrimination was made between sand types, cement types and joint finish, with the variables now being the brick type (pressed or extruded) and mortar composition (M1 to M4 with progressively increasing proportions of cement). To test the general applicability of this approach, the results for internal and external exposure were also pooled. Since the original testing program had been completed, the site specimens were transported to the laboratory and all tested at an age in excess of three years. Compression tests on prisms and bond wrench tests on individual joints were performed in accordance with the Australian standard test procedures. Table 2. Summary of Variables Variable Levels Details Clay brick 2 Low and high rates of absorption (extruded and pressed bricks respectively) Mortar composition 4 M2(1:2:9), two M3(1:0:5 and 1:1:6) and M4 (1:0.25:3) In comparing the scratch index results with the masonry compressive and flexural strengths it is useful in those comparisons to consider the proportion of cement in each mortar mix. The nominal proportions of cement by volume for each mortar are given in Table 3. Table 3. Proportions of Cement in Each Mortar Mix. Mortar Type Cement Content (%) M2 (1 : 2 : 9) 8.3 M3a (1 : 1 : 6) 12.5 M3b (1 : 0 : 5) 23.5 M4 (1 : 0.25 : 3) RESULTS AND DISCUSSION The results pooled as described above for the prism and bond wrench tests are presented in Tables 4 and 5 for the pressed and extruded bricks, respectively. 677

6 Table 4. Compression Prism and Bond Wrench Results Pressed Bricks Mortar Compressive Strength Flexural Strength Type Number of Mean Number of Mean Specimens Specimens (MPa) (MPa) M2 (1:2:9) M3a (1:1:6) M3b (1:0:5) M4 (1:0.25:3) Table 5. Compression Prism and Bond Wrench Results Extruded Bricks Mortar Compressive Strength Flexural Strength Type Number of Mean Number of Mean Specimens Specimens (MPa) (MPa) M2 (1:2:9) M3a (1:1:6) M3b (1:0:5) M4 (1:0.25:3) The scratch index (and the corresponding surface hardness) is expected to decrease as the mortar cement content increased as established by Lawrence et al, It is also likely that the flexural strength and compressive strength of the masonry would be influenced by the type of mortar. The scratch index values obtained from tests on the joints of the compression and bond wrench prisms are presented separately for the pressed and extruded bricks in Tables 6 and 7, respectively. By assuming the proportions of cement by volume for each mortar type given in Table 3, the relationship between masonry compressive and flexural strengths and cement content can be obtained for the pressed and extruded brick specimens (see Figures 4 and 5). Some increase in strength with cement content is evident in both cases. Note that the pressed bricks had higher suctions than the extruded bricks and that the M3b results in each case are for the 1:0:5 mortars containing no lime. Table 6. Scratch Index Compression and Bond Wrench Prisms (Pressed Bricks) Mortar Compressive Strength Prisms Flexural Strength Prisms Type Number of Scratch Number of Scratch Specimens Index Specimens Index M2 (1:2:9) M3a (1:1:6) M3b (1:0:5) M4 (1:0.25:3)

7 Table 7. Scratch Index Compression and Bond Wrench Prisms (Extruded Bricks) Mortar Compressive Strength Prisms Flexural Strength Prisms Type Number of Scratch Number of Scratch Specimens Index Specimens Index M2 (1:2:9) M3a (1:1:6) M3b (1:0:5) M4 (1:0.25:3) Mean Compressive Strength (MPa) M3b Pressed Extruded Cement Content (%) Figure 4 Masonry Compressive Strength versus Cement Content 679

8 Mean Flexural Strength (MPa) 0.4 M3b Pressed Extruded Cement Content (%) Figure 5. Masonry Flexural Strength versus Cement Content Mean Compressive Strength (MPa) M3b Scratch Index for Compressive Strength Specimens Figure 6 Scratch Index versus Masonry Compressive Strength 680

9 0.8 Mean Flexural Strength (MPa) M3b Scratch Index for Flexural Strength Specimens Figure 7 Scratch Index versus Masonry Flexural Strength From a site assessment and design perspective, it would be extremely useful if an estimate of the masonry compressive strength and/or masonry flexural bond strength could be obtained using the non-destructive scratch index test. The respective relationships between compressive strength and flexural strength with scratch index for both brick types considered together are shown in Figures 6 and 7. It can be seen from Figure 6 that there does appear to be a trend for the compressive strength values with a consistent increase in compressive strength with decreasing scratch index. This would be expected, given the well-known correlation of masonry compressive strength with cement content of the mortar, and the demonstrated correlation between the scratch index and cement content (Lawrence & Samarasinghe, 1998; Lawrence et al, 2008). With the exception of the highest scratch index result, a similar trend can also be seen from Figure 7 for flexural strength. Further investigation of a potential relationship for flexural strength appears warranted. 5. SUMMARY AND CONCLUSIONS The results of a previously reported mortar durability study were used to investigate a possible relationship between scratch index and masonry compressive and flexural strengths with a view to providing a non destructive site test for assessing these strength parameters. Consistent with the provisions of the Australian Masonry Code, the results for different cement types, sand types and joint finish as well as exposure condition were pooled for the supplementary study. Although the original experiment was not designed with the supplementary study in mind, the study did reveal a potential relationship between scratch index and masonry compressive 681

10 and flexural strengths. Given the potentially useful nature of a non destructive site test such as the scratch index in predicting the in-situ masonry strength parameters, a further more systematic study is certainly warranted. ACKNOWLEDGEMENTS The original durability (scratch index) study was funded by Cement Concrete and Aggregates Australia. The subsequent analysis also had the support of Think Brick Australia. The support of both organisations is gratefully acknowledged. REFERENCES [1] Bowler, G.K., 1993, Deterioration of Mortar by Chemical and Physical Action, Masonry International, Vol.6, No.3, [2] Harrison, W.H. & Bowler, G.K., 1990, Aspects of Mortar Durability, British Ceramic Society Transactions and Journal, Vol.89, No.13, [3] RILEM, 1997, MS-D.7: Determination of Pointing Hardness by Pendulum Hammer, Materials and Structures, Vol.30, [4] Schmiedmayer R., 1997, Non-destructive In Situ Determination of Mortar Load Capacity Using a Modified Schmidt Rebound Hammer. Proceedings of the 11th International Brick/Block Masonry Conference, Shanghai, China, [5] Standards Australia, 2001, AS3700 Masonry Structures, Sydney. [6] Lawrence, S.J. & Samarasinghe, W., 1998, A New Method for Assessing the Service Life of Masonry Mortars, Rehabilitation of Structures: Proceedings of the 2nd International RILEM/CSIRO/ACRA Conference, Melbourne, [7] Lawrence, S. & Samarasinghe, W., 2000, Assessing the Durability of Masonry Mortars, Proceedings of the 12th International Brick/Block Masonry Conference, Madrid, [8] Lawrence, S.J.; Samarasinghe, W. & Guirguis S., 2004, Mortar Durability Development and Standardization of Test Methods, Proceedings of the 13th International Brick/Block Masonry Conference, Eindhoven University of Technology. [9] Lawrence S.J., Testone T., Sugo H.O. & Page A.W., An Investigation of Factors Affecting the Durability of Masonry Mortar, Proc. 14IB2MAC, Sydney, Australia, Feb