Flexural bond strength of masonry using various blocks and mortars

Transcription

1 Matériaux et Constructions/Materials and Structures, Vol. 29, March 1996, pp Flexural bond strength of masonry using various blocks and mortars K. Venu Madhava Rao 1, B.V. Venkatarama Reddy 2, K.S. Jagadish 1 (1) Department of Civil Engineering, Indian Institute of Science, Bangalore , India. (2)Centre for Application of Science & Technology to Rural Areas, Indian Institute of Science, Bangalore , India. TECHNICAL NOTES A B S T R A C T This paper deals with an experimental study on flexural bond strength of masonry using various blocks in combination with different mortars. Flexural bond strength of masonry has been determined by testing stack-bonded prisms using a modified bond wrench test set-up. The effect of mortar composition and strength on the masonry s flexural bond strength using three types of masonry units (stabilized mud blocks, stabilized soil-sand blocks and burnt brick) has been examined. The effect of the masonry unit s moisture content on flexural bond strength has also been studied. Increases in mortar strength lead to increased flexural bond strength for cement mortar, irrespective of the type of masonry unit. It has been found that combination mortars, such as soil-cement mortar and cement-lime mortar, lead to better bond strength compared to cement mortars. The moisture content of the masonry unit at the time of casting has displayed significant influence on the flexural bond strength of the masonry. It has been found that for each type of masonry unit, an optimum moisture content exists, beyond which the flexural bond strength falls off quickly. R É S U M É On présente ici une étude expérimentale de l adhérence en flexion de maçonneries composées de blocs différents confectionnés à partir de divers mortiers. On a déterminé l adhérence sur des prismes essayés avec un dispositif en flexion modifié. On a examiné l effet de la composition du mortier et de la résistance sur l adhérence en flexion de la maçonnerie en utilisant trois types d unités (blocs de boue stabilisée, de terre/sable stabilisés et de brique). On a aussi étudié l influence de la teneur en humidité de la maçonnerie sur l adhérence. Pour le mortier de ciment, quel que soit le type d unité, l accroissement de la résistance du mortier entraîne une amélioration de l adhérence. Comparativement, on obtient une meilleure adhérence avec les mortiers combinés (terre/ciment et ciment/chaux). La teneur en humidité de l unité de maçonnerie au moment du coulage a une influence significative sur l adhérence en flexion. Pour chaque type de maçonnerie, il existe une teneur optimale au-delà de laquelle l adhérence en flexion diminue rapidement. 1. INTRODUCTION Construction with stabilized mud blocks is becoming popular in India and elsewhere. Some of the properties of stabilized mud block masonry under compression have been investigated in earlier studies [1]. However, the body of research on the behaviour of stabilized mud block masonry under various stress conditions is quite inadequate. Walls are frequently subjected to lateral loads as well as moments due to eccentric gravity loads. There is hence a need to consider the strength of masonry under flexure. Flexural bond strength (flexural tensile strength) may now be used as a measure of the bonding between two masonry materials (i.e. block/brick and mortar). The flexural bond strength is thus an important characteristic which warrants detailed study. The present investigation deals with the determination of the flexural bond strength of masonry using three types of masonry units. 2. EXPERIMENTAL PROGRAMME An experimental programme was planned to determine the flexural bond strength of masonry using different types of masonry units in combination with different mortars, taking account of the following effects : 1. Effect of mortar composition and strength on flexural bond strength of the masonry. 2. Effect of moisture content of the masonry unit on its flexural bond strength. 3. Effect of type of masonry unit on its flexural bond strength. The various masonry units used in this investigation are stabilized mud blocks (manual press), stabilized soil-sand blocks (motorized press) and burnt bricks ; their details are given below. The compressive strength and water absorption of these masonry units were determined as per the guidelines specified by IS: [2] /96 RILEM 119

2 Matériaux et Constructions/Materials and Structures, Vol. 29, March Stabilized mud blocks These are pressed, stabilized mud blocks prepared using a manually-operated machine called ITGE VOTH. Earlier studies [1] indicate that sandy soils are best suited for stabilized mud blocks. Hence, a local soil was reconstituted by adding sand in the ratio of 1 part soil to 1.75 parts sand by weight. The soil composition of the reconstituted soil is 15% clay, 10% silt and 75% sand. Masonry prisms for flexural bond strength were cast using 28-day cured stabilized mud blocks prepared using reconstituted soil and cement. Stabilized mud blocks of dimensions 305 mm x 143 mm x 100 mm were prepared using 6% cement by weight. The wet compressive strength and water absorption (by weight) of the stabilized mud blocks are 4.6 MPa (standard deviation = 0.8 MPa) and 9.0% (standard deviation = 1.1%) respectively. These values represent the average of 10 specimens. 2.2 Stabilized soil-sand blocks Stabilized mud blocks prepared using a motorized press in a factory are also available locally. The block size is 229 mm x 109 mm x 70 mm. These blocks were used in the investigation. The average wet compressive strength and water absorption (by weight) of these blocks are 9.1 MPa (standard deviation = 1.5 MPa), and 12.0% (standard deviation = 1.4%) respectively. These values represent the average of 10 specimens. 2.3 Burnt bricks Locally available burnt bricks, called table-moulded bricks, of dimensions : 224 mm x 105 mm x 75 mm were used. The average compressive strength of 10 specimens was found to be 5.6 MPa with a standard deviation of 1.5 MPa. The water absorption (by weight) was 9.8% (standard deviation = 1.2%). Details of the frog present on the surfaces of the three types of masonry units are shown in Table 1. Stabilized mud blocks have frog on both faces, whereas burnt bricks and stabilized soil-sand blocks have frog on only one face. Stabilized soil-sand blocks have bigger and deeper frog, compared to that on stabilized mud blocks and burnt bricks. 2.4 Masonry mortars Cement mortar, soil-cement mortar and cementlime mortar were used, in combination with various masonry units. Details of the proportions and properties of these mortars are shown in Table 2. The compressive strength of the mortar was determined through cubical specimens of size 70 x 70 x 70 mm. Rather high water cement ratios were chosen for better workability. Rich cement mortars such as 1:4 were used for purposes of comparison. Soil-cement mortar has better workability and plasticity, hence it was also used to examine flexural bond strength of masonry. Table 1 - Details of frogs on different masonry units Area of frog Area of frog Sl. Type of Type of frog (mm 2 ) Face area No. masonry unit Face 1 Face 2 Face 1 Face 2 Face 1 Face 2 In the form 1 Brick of company No name frog RRB 2 Stabilized mud Rectangular Rectangular block shape shape 1 No. 2 No. 3 Stabilized Rectangular No soil-sand shape frog block 1 No. Table 2 - Properties of masonry mortars Mortar Water Compressive Standard Designation Proportion cement Strength (MPa) deviation (by weight) ratio (N = 9) MPa A 1:4 (cement:sand) B 1:6 (cement:sand) C 1:10 (cement:sand) D 1:1:6 (cement: soil:sand) E 1:1:10 (cement:lime:sand)

3 Venu Madhava Rao, Venkatarama Reddy, Jagadish Fig. 1 - Modified bond wrench test set-up. 2.5 Test procedure Various test procedures have been recommended [3, 4, 5, 6] for the determination of flexural bond strength of unreinforced masonry. ASTM C1072 specifies a bond wrench test for the flexural bond strength of masonry. This method has been adopted with minor modifications [7]. The modified test set-up is shown in Fig. 1. ASTM C1072 guidelines are considered while preparing the test specimens Prism casting and testing Stack bonded prisms of 4-block height were used for the determination of flexural bond strength. The masonry units were dried in air and then soaked in water for one minute prior to casting the prism. Hence, it was ensured that the moisture content of the brick or block be kept constant for all cases, except when the effect of moisture content of the masonry unit on flexural bond strength was examined. Prisms for studying the effect of moisture content of the masonry unit on flexural bond strength were prepared separately. The moisture content of the masonry units was varied modifying soaking time in water. The masonry units removed directly from the oven and used for prism casting were treated as units with zero moisture content. In the case of saturated moisture content, the masonry units soaked in water for 48 hours were used for prism casting. A mortar joint thickness of 10 mm was maintained for all cases. The prisms were cured for 28 days by remaining moist thanks to a wet burlap at a room temperature of C. Three prisms were prepared in each combination. The prisms were tested in a wet state by soaking in water for 48 hours prior to testing. The procedure, outlined in ASTM C1072, has been followed for testing the prisms. 3. RESULTS AND DISCUSSION The effects of the mortars composition and compressive strength, and the moisture content of the masonry unit at the time of casting on the flexural bond strength of masonry have been studied. 3.1 Effect of mortar composition and strength on flexural bond strength Three types of masonry units, with five mortar combinations, have been investigated. Table 3 gives details of flexural bond strength results of masonry using different masonry units and mortar combinations. The table also provides details of the type of masonry unit, mortar type, flexural bond strength, standard deviation and coefficient of variation Flexural bond strength of stabilized mud block masonry All five mortars discussed earlier were used for stabilized mud block masonry. Table 3 displays detailed results of flexural bond strength of masonry using stabilized mud blocks for different mortar combinations. It is clear from the table that flexural bond strength increases with increases in mortar strength for cement mortars. The bond strength varies between 0.02 MPa and 0.23 MPa for cement mortars. There is a 119% increase in 121

4 Matériaux et Constructions/Materials and Structures, Vol. 29, March 1996 Table 3 - Flexural bond strength of various types of masonry Type of Mortar Flexural Bond Standard Coeff. of Masonry type Strength Deviation variation Unit (MPa) (N=9) (MPa) A Stabilized B mud C block D E A Stabilized B soil-sand C block D E A B Burnt brick C D E bond strength when the mortar proportion was changed from 1:6 to 1:4. Combination mortars D & E showed better bond strengths. It is to be noted that in case of soilcement mortar (type D), the bond strength is relatively high. Even though the compressive strength of 1:6 cement mortar (type B) and 1:1:6 soil-cement mortar (type D) is in the same range, there is a 68% increase in bond strength for type D mortar. This result may be attributed to the better grading of mortar due to the presence of the soil thereby leading to more uniform and less porous mortar, hence more contact area. Addition of lime to 1:10 cement-sand mortar leads to an increase in mortar strength as well as bond strength. Lean mortar C showed poor bond strength. The failure mode of the joint under flexure depends mainly on the strength of the block/brick and the mortar properties. Two types of failure were observed. Failure takes place at the block-mortar interface. It could be either complete bond failure or a combination of bond failure and block failure due to bending tension. There was a complete bond failure in the case of lean mortar, such as type C. Whereas in the case of mortars A and B and combination mortars D and E, the failure is a combination of bond failure and block failure. At the block-mortar interface, a portion of the block fails in flexure and gets attached to the mortar joint. The percentage of the block material sticking to the failure joint surface varies depending upon the type and strength of the mortar. This percentage varies between %. Here, 100% means complete block failure in flexure. It was high for mortar types A and D, where the bond strength is high, and low for types B and E. Failure took place on the one frog surface of the block for all the cases Flexural bond strength of stabilized soil-sand block masonry Table 3 gives detailed results of the flexural bond strength of stabilized soil-sand block masonry for different mortar combinations. It is clear from the table that flexural bond strength increases with increases in mortar strength for cement mortars. Bond strength varies between 0.05 MPa and 0.29 MPa for cement mortars. There is a 148% increase in bond strength when the mortar proportion changes from 1:6 to 1:4. Low bond strength values have been observed for leaner mortars (type C). Combination mortars D and E showed better bond strengths. Even though the compressive strength of mortar D is less than half that of mortar A, the bond strengths using these mortars are equal. Similarly, addition of lime to cement mortar (type E) leads to increases in mortar strength as well as bond strength. Bond failure has occurred for all types of cement mortars. This may be attributed to the higher strength of the masonry unit. Whereas in case of combination mortars (types D and E), complete bond failure as well as the combination of bond failure and block failure were observed. The combination failure is again due to better bond because of more contact area at the block-mortar interface. The percentage of material sticking to failure joints varies between 10-75%. All incidences of failure took place on the non-frog surface of the block Flexural bond strength of burnt brick masonry Details of flexural bond strength test results for burnt brick masonry are also shown in Table 3. It is once again clear that flexural bond strength increases with increases in mortar strength for cement mortars. Bond strength varies between 0.05 MPa and 0.10 MPa for cement mortars. There is a 35% increase in the bond strength when mortar proportion is changed from 1:6 to 1:4. Leaner mortar such as 1:10 lead to low flexural bond strength. Combination mortars D and E showed better bond strengths. Bond strengths were found to be equal for mortars A and D. Lime mortar and soil-cement mortars also yielded better bond strengths for brick masonry. Failure modes were similar to those observed in block masonry. Failure takes place at the brick-mortar interface. In the case of lean mortars B and C, there was a complete bond failure. In case of rich cement mortar (type A) and combination mortars D and E, however, failure results a combination of the both bond and brick. The percentage of brick material sticking to the failed joint varies between %. All incidences of failure took place on the nonfrog surface of the brick. Table 3 also compares the flexural bond strength of masonry using different types of masonry units with various mortar combinations. It is apparent from the Table that the stabilized soil-sand block masonry shows higher bond strength compared to stabilized mud block and brick masonry for all types of mortars considered, except for 1:10 122

5 Venu Madhava Rao, Venkatarama Reddy, Jagadish Fig. 2 - Effect of moisture content of masonry unit on flexural bond strength. cement mortar. The higher bond strength of the stabilized soil-sand block masonry could be attributed to the presence of a deeper and wider frog. In the case of 1:10 cement mortar, burnt brick masonry and stabilized soilsand block masonry have similar bond strengths, whereas stabilized mud block masonry has comparatively lower bond strength. Combination mortars such as soil-cement and cement-lime mortars (types D and E) lead to better bond strengths compared to 1:6 cement mortar, for all types of masonry considered herein. Stabilized mud block masonry has better bond strength than the burnt brick masonry except for 1:10 cement mortar. It may also be observed that in all cases, failure of masonry under flexure was initiated on the non-frog surface or the surface with less frog area of the block/brick. Hence, the presence of frog and its magnitude significantly influence the masonry s flexural bond strength. Examining the effect of Table 4 - Properties of masonry units Type of Wet compressive Water absorption Masonry unit strength (MPa) by weight (%) (N = 10) (N = 10) Burnt brick Stabilized mud block frog size and shape on the flexural bond strength of masonry would be a necessary subsequent step. 3.2 Effect of moisture content of the masonry unit on bond strength Earlier studies [8] have indicated that the moisture content of the masonry unit at the time of casting has significant influence on the bond between masonry unit and mortar. The effect of the masonry unit s moisture content on flexural bond strength has been studied for two types of masonry units, i.e. burnt brick and stabilized mud block with 2 mortar proportions (types A and D). The properties of the masonry units are given in Table 4. The bricks and stabilized mud blocks used for this study were selected from a different lot, hence their properties are different from those used in studying the effect of mortar strength on bond strength. The mortar properties have been already discussed and displayed in Table 2. In all, 5 moisture percentages varying between zero and saturation were examined. The results of the effect of moisture content on flexural bond strength are shown in Fig. 2 for all cases. The following observations can be drawn from these results. 123

6 Matériaux et Constructions/Materials and Structures, Vol. 29, March Flexural bond strength increases initially with increases in moisture content (at the time of casting) up to a certain point and then drops suddenly for all cases considered. 2. There is an optimum moisture content that leads to a maximum bond strength for all types of masonry. The optimum moisture content varies with the type of masonry unit and mortar. The optimum moisture content values are the same for a given type of masonry unit with different mortars. The optimum values for burnt brick masonry and stabilized mud block masonry are 13% and 11% respectively, for both 1:4 cement mortar and 1:1:6 soil-cement mortar. 3. When the masonry unit is saturated, the flexural bond strength values are equal, irrespective of the type of masonry unit and mortar. 4. The flexural bond strength of stabilized mud block masonry is greater than that of burnt brick masonry, except at saturation. 4. CONCLUSIONS The following conclusions can be made from the investigations carried out on flexural bond strength of masonry using different masonry units in combination with various mortars. (1) In general, the flexural bond strength increases with increases in mortar strength for the cement mortar, irrespective of the type of masonry unit. (2) Flexural bond strength is very low for lean mortars, such as 1:10 cement mortar, compared to 1:4 and 1:6 cement mortars. (3) Combination mortars such as 1:1:6 soil-cement mortar and 1:1:10 cement-lime mortar show better bond strength than 1:10 and 1:6 cement mortars. Bond strengths using 1:1:6 soil-cement mortar and 1:4 cement mortar are approximately the same for all specific types of masonry considered herein. (4) Higher bond strengths were obtained for masonry units with deeper and wider frog. (5) The moisture content of the masonry unit (at the time of casting) has a significant influence on the flexural bond strength. There is an optimum moisture content that leads to a maximum bond strength. Optimum moisture content values for burnt brick masonry and stabilized mud block masonry are 13% and 11% respectively, for both 1:4 cement mortar and 1:1:6 soil-cement mortar. However, bond strength falls very rapidly as moisture content increases beyond the optimum level. REFERENCES [1] Venkatarama Reddy, B.V., Studies on static soil compaction and compacted soil-cement blocks for walls Ph.D thesis (Dept. of Civil Engineering, Indian Institute of Science, Bangalore, India, April 1991). [2] IS: 3495, Indian standard code of practice for methods of tests for burnt clay building bricks (Bureau of Indian Standards, 1976). [3] AS CA47, Australian code of masonry, Australia, [4] BS 5628, Code of practice for structural use of masonry, Part 1. Unreinforced masonry (British Standards Institution, 1978). [5] ASTM E518, Standard test methods for flexural bond strength of masonry (American Society of Testing and Materials, 1980). [6] ASTM C1072, Standard method for measurement of masonry flexural bond strength (American Society of Testing and Materials, 1986). [7] Venu Madhava Rao, K., Some studies on flexural and compressive strength of masonry, M. Sc (Engg.) thesis (Dept. of Civil Engineering, Indian Institute of Science, Bangalore, India, 1993). [8] Sinha, B.P., Model studies related to load bearing brick work, PhD Thesis (University of Edinburgh, 1967). 124