THE INFLUENCE OF MORTAR BEDDING ON THE COMPRESSIVE STRENGTH OF CONCRETE BLOCK MASONRY STRUCTURES

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1 THE INFLUENCE OF MORTAR BEDDING ON THE COMPRESSIVE STRENGTH OF CONCRETE BLOCK MASONRY STRUCTURES Izquierdo, Orieta Soto 1 ; Corrêa, Márcio Roberto Silva 2 ; Soto, Indara Izquierdo 3 1 PhD Student, EESC - University of São Paulo, Department of Structural Engineering, orieta@sc.usp.br 2 PhD, Associate Professor, EESC - University of São Paulo, Department of Structural Engineering, mcorrea@sc.usp.br 3 PhD Student, EESC - University of São Paulo, Department of Structural Engineering, indara@sc.usp.br Currently, structural masonry still has some advantages in the construction industry when compared with conventional systems. However, to use this system better, it should be further studied. This research studies the change in compressive strength of structural masonry of concrete blocks due to the type of mortar bedding, comparing full and face-shell bedding. To characterize the materials, experimental tests and compressive strength tests of the units, prisms, and wallettes were also conducted. All materials complied with the standard requirements for their correct use in the experimental tests. The results of the compressive tests of the elements showed that the bedding type significantly influenced the compressive strength of masonry, with full mortar bedding showing better performance. The specimens with full mortar bedding showed higher compressive strengths. Prisms and wallettes with face-shell bedding presented a more fragile failure, showing thick vertical cracks along the cross webs. Keywords: Mortar type, adjustment dimensional parts, contact area, structural masonry. INTRODUCTION Masonry is an assemblage of masonry units (bricks or blocks) bonded with mortar or mortar and grout. It is usually designed to resist compression or a combination of internal forces, and may contain reinforcement encased in concrete or mortar in the horizontal and / or vertical plane. There is currently a progressive search for streamlining processes in the construction market in order to increase productivity and reduce building costs. In this scenario, an increasing demand is seen for streamlined structural masonry buildings projects, enabling their widespread use today, a result of the characteristics of this process regarding execution speed, rigid quality control, modular coordination, reduction of waste and improvisations. The growing tendency in Brazil and in the rest of the world to use structural masonry for constructions requires implementing new technologies, therefore less use of manpower, less waste of materials and better working conditions. However, these aspects should be properly studied to make better use of all advantages, and the first step is promoting awareness of the technical aspects. In Brazil, scientific research is mostly directed at obtaining the best performance of walls, both structurally and functionally, hence improving the development of masonry in civil construction. 1

2 In the construction process, the type of mortar bedding can be decisive in the production and performance of the blockwork. The face shell bedding in the horizontal joints has been widely used as it reduces execution time. The effect of the mortar bedding type on stress distribution, cracking and failure of walls has been studied by some researchers and engineers. For example: Woldetinsae and Colville (1991) studied the influence of the mortar bedding type on the compressive strength of concrete masonry blocks. The authors concluded that the compressive strength of hollow prisms (mortar bedding area) is approximately 8% lower for prisms with face shell mortar bedding than for prisms with full mortar bedding. Ganesan and Ramamurthy (1992), using the finite element method to study the behavior of prisms with different mortar bedding types, concluded that only the placement of mortar on the side webs of the blocks may induce high stress concentration points in the transverse webs of the blocks. Mata (2006) also studied the influence of the mortar bedding type on concrete block masonry submitted to compression; the laboratory test results showed the loss of compressive strength capacity and fragile failure of the prisms and wallettes executed with face shell mortar bedding when compared against those with full mortar bedding. Pasquali (2007) investigated the influence of the mortar bedding type in the compressive strength of small structural ceramic masonry walls and concluded that the type of mortar bedding is a factor that can change the resistance of the masonry. Thus, the main objective of this paper is to investigate the behavior of the compressive strength of masonry walls with face shell and full mortar bedding, to confirm the previous findings and contribute with new results. EXPERIMENTAL PROGRAM The experimental program studied the behavior of masonry using two different specimens: prisms and wallettes made of concrete blocks, varying the mortar bedding type (face shell and full). The compressive tests were carried out applying distributed displacement-controlled monotonic loading. Two series were used for testing the walls: a) Series I full mortar bedding b) Series II face shell mortar bedding Complimentary tests were carried out to characterize the masonry: compressive strength testing of prisms, wallettes and cylindrical mortar specimens, all specimens were tested at the age of 28 days. 2

3 Table 2 shows the total number of specimens for each series. Table 2 Number of specimens Series type Prisms Wallettes Cylindrical mortar specimens Series I Series II Total number of specimens The compressive strength of mortar The cylindrical mortar specimens (5cmx10cm) were molded and tested for compressive tests according to NBR (2005), after 28 days. To obtain regular and smooth surfaces they were rectified. This test was carried out in a hydraulic computerized ELE machine, model Autotest 2000, with 2000 kn capacity and 2.03 kn/s loading speed, shown in Figure 1. Figure 1 - Compressive strength test of the mortar The compressive strength of the concrete blocks Figure 2 shows the nominal dimensions of the units used in this study. Figure 2 Nominal dimensions of the blocks (in cm) Six blocks were tested for compressive strength, according to NBR (2007). A special soft material was used to cap the blocks, prisms, and wallettes. This is a typical ceiling panel material, manufactured by Eucatex Company (Figure 3). 3

4 Figure 3 Ceiling panel used for capping the specimens Figure 4 shows the compressive tests of the concrete blocks carried out in the hydraulic computerized ELE. Figure 4 Compressive strength test of the concrete blocks Compressive strength test of the prisms Figure 5 shows the nominal dimensions of the 2-block prisms. Figure 5 Nominal dimensions of the prisms (in cm) 4

12 prisms were built with full mortar bedding and 12 prisms with face shell mortar bedding. The compressive tests were performed in accordance with NBR 8215 (1983).

5 12 prisms were built with full mortar bedding and 12 prisms with face shell mortar bedding. The compressive tests were performed in accordance with NBR 8215 (1983). The prisms were tested in the servo-hydraulic universal INSTRON machine, model They were not instrumented, and the force was applied at a loading speed corresponding to a displacement increase of 0.01 mm/s. Figure 6 shows the test apparatus. Figure 6 Compressive strength test of the prisms Compressive strength test of the wallettes Figure 7 shows the dimensions of the 5-row wallettes. 6 wallettes were tested with full mortar bedding and 6 with face shell mortar bedding. Figure 7 Nominal dimensions of the wallettes (in cm) Similarly to the prisms, the compressive tests of the wallettes were carried out in the servohydraulic universal INSTRON machine and all the data results were read by the SYSTEM Four transducers were used in each wallette, two on each side. Figure 8 shows the test rig, indicating the dimensions and the instruments position (equal for both sides). 5

Figure 8 Compressive strength test of the wallettes (in cm) EXPERIMENTAL RESULTS AND ANALYSIS Compressive strength of the mortar Table 3 shows the compressive strength results of the mortar used to

6 Figure 8 Compressive strength test of the wallettes (in cm) EXPERIMENTAL RESULTS AND ANALYSIS Compressive strength of the mortar Table 3 shows the compressive strength results of the mortar used to build the prisms and wallettes of both series. Table 3 Compressive strength of mortar. Series Mean resistance Standard Deviation Coefficient of Variation (f am ) (MPa). (Sd) (MPa) (CV) (%) I II The results presented in Table 6 show that the compressive strength of mortar is consistent with mortar (ii) of BS 5628: Part 1 (2005), that predicts a mean resistance of 6.5 MPa in the laboratory with a volume ratio of 1:0.5:4.5. Compressive strength of the blocks Table 4 presents the results of the compressive strength of the blocks. Table 4 Results of the compressive strength of the blocks Mean strength Standard Coefficient of Characteristic strength Series (f bm ) (MPa). Deviation (MPa) Variation (%) (f bk ) (MPa) I e II Overall, the compression tests of the units showed a cone shaped failure, (Figure 9), typical of a compressed specimen confined by the platens at the top and bottom surfaces. 6

Figure 9 Typical compressive failure of the blocks Compressive strength of the prisms In general, the failure of the prisms was

The cracking width was larger for the face-shell bedding, as shown in Figure 10a.

in Figure 10b. (a) (b) Figure 10 Failure of prisms with face-shell mortar bedding (a); and full mortar bedding (b).

Table 5 Results of the compressive strength of prisms Series Mean resistance of CV Sd (MPa) prisms f pm (MPa) (%) Full mortar

7 Figure 9 Typical compressive failure of the blocks Compressive strength of the prisms In general, the failure of the prisms was characterized by a vertical crack along its thickness, leading to a separation of the units through the transverse web. The cracking width was larger for the face-shell bedding, as shown in Figure 10a. As for the prisms with full mortar bedding, the cracking process was not as severe and the block was usually crushed, as shown in Figure 10b. (a) (b) Figure 10 Failure of prisms with face-shell mortar bedding (a); and full mortar bedding (b). The compressive strength results of the prisms are shown in Table 5. Table 5 Results of the compressive strength of prisms Series Mean resistance of CV Sd (MPa) prisms f pm (MPa) (%) Full mortar bedding I Face-shell mortar bedding II Table 5 shows clear evidence that the prisms with face-shell mortar bedding presented lower resistance than the prisms with full mortar bedding, with an average difference of 36 %. 7

The statistical tests F and T were used to evaluate the means. The study concluded that, for a significance level of 5%, full mortar bedding prisms were stronger than those with face-shell bedding.

Table 6 Results of the compressive strength of wallettes Series Mean resistance of prisms f pm (MPa) Mean resistance of wallettes f m-p (MPa) Sd (MPa) CV (%) Full mortar bedding I 7.82 4.62 0.54 11.

8 The statistical tests F and T were used to evaluate the means. The study concluded that, for a significance level of 5%, full mortar bedding prisms were stronger than those with face-shell bedding. Compressive strength of the wallettes Table 6 shows the results of the compressive strength of the wallettes. Table 6 Results of the compressive strength of wallettes Series Mean resistance of prisms f pm (MPa) Mean resistance of wallettes f m-p (MPa) Sd (MPa) CV (%) Full mortar bedding I Face-shell mortar bedding II The results in Table 6 show that the type of mortar bedding influenced the compressive strength of wallettes similar to what occurred in the prisms. The reduction of the compressive strength of wallettes with face-shell mortar bedding was 14% in average, compared to the specimens with full mortar bedding. Statistical analysis showed significant differences between these values for both series of wallettes. Failure type of the wallettes All wallettes with face-shell mortar bedding presented verticals cracks along their widths. These cracks appeared before the face cracks occurred and when the load was close to 85% of the failure load. With full mortar bedding the first cracks appeared at the front external faces with loads closer to the failure value (91%). The cracks on the transverse webs were less visible and, in some cases, did not appear. Figures 10 and 11 show examples of typical wallettes of each series, with their respective lateral and front faces. (a) (b) Figure 10 Side view (a) and front view (b) of the wallettes with full mortar bedding (Series I) 8

(a) (b) Figure 11 Side view (a) and front view (b) of the wallettes with face-shell mortar bedding (Series II) Stress-strain curves of the wallettes Figures 10 and 11 present the stress-strain curves

(a) Figure 10 -Stress-strain curves of the wallettes with face-shell mortar bedding (Series I) (b) Figure 11 -Stress-strain curves of the wallettes with full mortar bedding (Series II) Figures 10 and

9 (a) (b) Figure 11 Side view (a) and front view (b) of the wallettes with face-shell mortar bedding (Series II) Stress-strain curves of the wallettes Figures 10 and 11 present the stress-strain curves of the wallettes of both series. The strain records are the averages of the four LVDTs. (a) Figure 10 -Stress-strain curves of the wallettes with face-shell mortar bedding (Series I) (b) Figure 11 -Stress-strain curves of the wallettes with full mortar bedding (Series II) Figures 10 and 11 show that the wallettes with full mortar bedding presented higher compressive strength values and larger Young s modulus compared to the specimens with face-shell mortar bedding. CONCLUSIONS The mortar used in the experimental tests of this study was properly prepared, in accordance with the requirements of British Standards. The blocks showed compressive characteristic strength higher than 4.0 MPa, being classified as structural blocks according to Brazilian Standards. They generally showed a cone shaped failure, typical of specimens confined by the top and bottom platens. 9

10 Regarding the prisms, the type of mortar bedding significantly influenced the compressive strength; the specimens with full mortar bedding showed higher compressive strengths. Regarding the wallettes, the behavior was similar to the prisms. The specimens with full mortar bedding showed higher compressive strengths, with statistically significant differences. Both the prisms and the wallettes with face-shell mortar bedding showed a more fragile failure, characterized by the development of thick vertical cracks along their widths. ACKNOWLEDGEMENTS The Department of Structural Engineering School of Engineering of São Carlos - USP, for permission to realization of this work. REFERENCES ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. NBR 12118: Concrete hollow blocks for masonry - Test methods. Rio de Janeiro, (in Portuguese). NBR 8215: Prisms of concrete hollow blocks for simple structural masonry - Preparation and testing for compression - Test methods. Rio de Janeiro, (in Portuguese) BRITISH STANDARD INSTITUITION. BS 5628: Part 1: Code of practice for Structural use of masonry. Unreinforced masonry. London, Inglaterra, COLVILLE J. ; WOLDETINSAE, A.M. Compressive strength of grouted concrete masonry. Proceedings of the 9th International brick/block masonry conference. v. 1. Berlin, Germany, p , GANESAN, T.P.; RAMAMURTHY, K. Behavior of Concrete Hollow Block Masonry Prisms Under Axial Compression. Journal of Structural Engineering. v. 118, jul MATA, R.C. Influence of mortar bedding in the compressive strength of structural masonry prisms and mini-walls of concrete blocks p. Dissertation (Civil Engineering Master), Universidade Federal de Santa Catarina, Florianópolis, (in Portuguese) PASQUALI, I. Influence of the type of mortar bedding to the compretion resistence of small structure mansonry walls. Santa Maria, p. Dissertation (Civil Engineering Master), Universidade Federal de Santa Maria, Rio Grande do Sul, (in Portuguese) 10

Elements of Structural Masonry Reinforced with Sisal Fibers

Feb. 2013, Volume 7, No. 2 (Serial No. 63), pp. 141-146 Journal of Civil Engineering and Architecture, ISSN 1934-7359, USA D DAVID PUBLISHING Elements of Structural Masonry Reinforced with Sisal Fibers