STUDY OF THE INFLUENCE OF COMPRESSIVE ...2 MsC, Professor, Federal University of Alagoas, Sertão...

15th International Brick and Block Masonry Conference

Florianópolis Brazil 2012

ST UD Y O F T H E IN F L U E N C E O F C O M PR ESSI V E ST R E N G T H A ND T H I C K N ESS O F C APPIN G-M O R T A R O N C O M PR ESSI V E

ST R E N G T H O F PRISMS O F ST RU C T UR A L C L A Y B L O C KS

Lima , F lávio Barbosa1; L ima , A lexandre Nascimento2; Assis, Wayne Santos3 1 PhD, Professor, Federal University of Alagoas, Technology Center, [email protected] 2 MsC, Professor, Federal University of Alagoas, Sertão Campus, [email protected]

3 PhD, Professor, Federal University of Alagoas, Technology Center, [email protected]

Currently, structural masonry is one of the most commonly used structural systems in some countries, like Brazil. Among its components, the capping-mortar is seen as the weakest. This paper presents the results of a study that analyses the influence of the compressive strength and thickness of the bedding mortar on the mechanical performance of ungrouted prisms built with clay blocks, using a 3k factorial design and others statistical techniques, including the construction of response surface. Blocks of the same compressive strength were used. Tests were performed on prisms consisting of two blocks with three different thicknesses and three different compressive strengths of the bedding mortar. From the found results, the influence of these parameters was quantitatively verified and statistical models representing the individual and joint effects of variables were built.

Keywords: structural masonry, clay block, mortar

T heme: Research and testing IN T R O DU C T I O N The construction industry in Brazil has an urgent need to produce housing, since the housing deficit reaches 5.57 million households (Ministério das Cidades, 2010). Considering the serious shortage of housing, it is necessary to build faster and with lower cost, since the mostly of housing demand refers to less economically privileged classes. Obviously, adequate levels of quality, durability and safety must be preserved. Due to its characteristics, structural masonry construction system is a potential alternative to solve this problem. The masonry structure is the construction process that uses the walls as the main structural component of the building, being designed through rational calculation. The masonry structure is composed by the union of blocks (made of concrete, clay or calcium-silicate) with bedding mortar in horizontal and vertical joints. For its execution characteristics and suitability to the processes of rationalization and industrialization of construction, structural masonry presents a strong potential for growth in the Northeast of Brazil (especially in the state of Alagoas), where there is significant empiricism in the execution of structural masonry buildings.



L I T E R A T U E R E V I E W Studies analyzing the compressive strength of masonry (or prisms), relating it to their constituents, were found in the literature. Among the influencing factors the compressive strength, composition and geometry of the blocks influence the final strength of the set. Some studies highlight the great influence of the thickness of webs and outer shells (Santos et al., 2007) and the large influence of height (Carvalho, 2003). Studies have also concluded that the prisms built with more resistant to compression mortar showed higher strength than those with less resistant to compression mortar (Carvalho, 2003), and was evidenced the tendency of decrease in resistance with the increase in the thickness of the joint. (Leão & Perdigão, 2004). It has also demonstrated that the initial rate absorption of the block can interfere with the characteristics of prisms and walls (Beall, 2004). In the context of the construction of prisms and walls, was found great influence of the quality of the workmanship in the final results (Prudêncio Jr, 1986). Another influential factor is the bond between the mortar and the unit, which can be determined by the shear strength between them. This factor was found by Sarangapani et al (2005). E XPE R I M E N T A L D ESI G N Through the experimental design, it is possible to determine the variables that have greatest influence on process performance, reducing operational costs and improving the process time. The experimental design also affords an appropriate mathematical model to describe a phenomenon. Montgomery (2001) considers the factorial design is the most efficient experimental strategies. Was chosen to employ a factorial design 32, where the basis (3) represents the number of levels for each one of the factors in the exponent (2). The compressive mechanical behavior of structural masonry prisms (compressive strength and its mode of rupture) was adopted as the response variable (dependent). The factors selected were the compressive strength and thickness of the bedding mortar. Once in this plan each factor has 3 levels of variation, was chosen by the variation of the compressive strength of mortar as 50%, 100% and 150% of the strength of the block, considering the gross area. For the variation of the thickness of bedding mortar, it was decided by the thickness of 10 mm, 15 mm and 20 mm, small variations, to avoid significant changes in the slenderness ratio of the prisms. All other factors that could influence the mechanical behavior of the prisms were guaranteed uniform. After the evaluation of various proportions of mortars, were defined the three ones used in the prisms for the tests, depending on the compressive strength of the blocks. Five prisms were made for each combination strength/thickness of the mortar. This amount was enough to perform a statistical analysis. All tests were performed twice, providing a replica to allow the determination of experimental errors and obtain more accurate average effect.



M A T E R I A LS The block chosen for the study was a clay structural block, hollow, usually sold in the city of Maceió, with nominal dimensions 14 cm wide, 19 cm high and 29 cm length (Figure 1).

F igure 1 C lay structural block used in this work .

For the characterization of the blocks were used the standard procedures of NBR 15270-3 (ABNT, 2005), allowing the determination of the dimensions, gross and net areas, water absorption (Table 1) and compressive strength of blocks (Table 2).

Table 1 - Average results of tests of clay structural blocks.

L ength (mm) Width (mm) H eight (mm)

G ross A rea (mm2)

Net Area (mm2)

Absorption (%)

293.0 140.5 190.0 41166 15418 9.4

Table 2 Results of compressive strength of clay structural blocks.

Feature G ross A rea Strength (MPa)

Net Area Strength

(MPa) Average Strength (MPa) 10.2 27.2

Standard Deviation (MPa) 1.52 4.07 Coefficient of var iation C V (%) 15.0 15.0

Outliers No In all tests, was attempted to identify anomalous values, which are individual values that cannot be considered as coming from the same probability distribution associated to the other values, being removed from the statistical analysis. To determine by Grubbs test (equation 1)

sxx

G a (1)

Where G is the value Grubbs test, ax

is the suspect value, x

is the average of the sample and

s is the standard deviation of sample.



The value is considered abnormal when it exceeds the tabulated value of G for a given confidence level (was adopted a confidence level of 95%). M O R T A R Taking into account the test results of compressive strength of the blocks, was obtained the desirable values (reference only) of the compressive strength of mortar: 5.1 MPa (50%) 10.2 MPa (100%) and 15.3 MPa (150%). In the experiment, was used mixed mortar (cement and lime). The type of cement used was the Portland with filler compound (CP-II-F-32, in Brazil) and hydrated lime type CH-I. According to the recommendations in the literature, were avoided sands with high percentage of large grains and very fine sands, which are unfavorable for adherence (Beall, 2004; Hendry et al. 2004). Sahlin (1971) recommends that the fineness modulus of sand should be between 1.6 and 2.5 and the maximum diameter should not exceed 1/3 or 1/2 the thickness of the joint. The chosen sand is commonly found in the city of Maceió. It was passed through the sieve 4.75 mm and had a maximum diameter of 2.36 mm and fineness modulus of 2.36. As highlighted by Steil (2003), the specification of structural masonry mortars should not be based solely on their compressive strength. Special attention should be given to the properties such as adhesion to the block and water retention. For each chosen mixing mortar proportion, was determined the amount of water to reach a consistency standard of 260 ± 5 mm, according to NBR 13276 (ABNT, 2005). The determination of the water retention of mortars was also executed, with the objective to use mortars with similar retention. The laboratorial procedures adopted are recommended by NBR 13277 (ABNT, 2005), being adopted the minimum value of 75%, according to NBR 8798 (ABNT, 1985). Finally, the compressive strength of the chosen mortars proportions was determined. The tests were performed following the procedures of NBR 13279 (ABNT, 2005), that determines the use of prismatic specimens with dimensions 4 cm x 4 cm x 16 cm. The three proportions chosen (by volume), taking into account the consistency, water retention and compressive strength, were: 1:0.5:4:1.1 (50%), 1:0.4:3.2:0.9 (100%) and 1:0.5:2:0.7 (150%). The test results of standard consistency, water retention and compressive strength are shown in Table 3.

Table 3 - Water retention of mortars chosen. Proportion

(cem:lime :sand) in volume

Standard Consistency (mm)

Retention of water (%)

Compressive strength (MPa)

1:0.5:4:1.1 261 96.8 3.1 1:0.4:3.2:0.9 255 96.6 7.9 1:0.5:2:0.7 262 96.3 12.7

The proportions used were chosen with their strength a little lower than desired, since was expected that the changing of the mixing equipment used would produce an increased resistance.



PRISMS The prisms were made following the recommendations of NBR 15812-2 (ABNT, 2010), with two upright blocks with a single horizontal joint, see Figure 2.

F igure 2 Prism specimen built, the blocks were wetted before.

Following the recommendations of Beall (2004), Carvalho (2003) and Santos et al. (2007), the blocks had been wet before the building of the prisms, to prevent excessive absorption of water from the fresh mortar. To ensure the thickness of the bedding mortar, were used fixtures and a metal graduated scale. For all prisms, was allowed a variation of ± 3 mm in thickness of the bedding mortar, due to the imperfections of the blocks, ensuring the level of the prisms. The prisms received an identification number. For example, 100.15.1, means strength of mortar with 100%, thickness of 15 mm and first determination, 150.20.2, means strength mortar with 150%, thickness of 20 mm and the second determination (replica). The prisms remained in situation of dry curing until the day of the capping, being carefully transported to prevent vibrations or shocks. The capping of each face was done with cement paste. Once capped of the first face, the second face was capped after 24 hours. After hardening of the second face, at the age of 28 days of preparation, the prisms were measured and placed in the test machine to determine their strength. Table 4 shows the results of tests to determine the compressive strength of mortar and prisms, according to the combination strength/thickness of mortar. The results are the averages of six specimens of mortar and five prisms. The statistical parameters shown in the table already exclude outliers detected.



Table 4 Compression tests results.

Combination

1th determination Replica Mortars Pr isms

T M R*

Mortars Pr isms

T M R* Aver . (MPa)

Std. Dev.

(MPa)

Aver . (MPa)

Std. Dev.

(MPa)

Aver . (MPa)

Std. Dev.

(MPa)

Aver . (MPa)

Std. Dev.

(MPa) 50.10 7.5 0.60 6.9 0.67 MB* 7.4 1.01 7.0 0.32 MB* 50.15 7.2 1.13 6.4 0.53 MB* 7.5 1.12 6.9 0.53 MB* 50.20 7.5 1.33 4.8 0.99 M* 7.6 0.84 4.3 0.87 M*

100.10 10.4 1.30 7.6 0.76 B* 10.7 1.32 10.7 1.32 B* 100.15 10.4 1.30 6.8 0.84 MB 10.3 1.95 7.2 0.39 MB* 100.20 9.8 1.86 5.5 0.30 MB* 9.7 1.43 5.1 0.91 MB* 150.10 18.1 1.38 9.3 0.93 B* 19.1 2.42 9.5 1.02 B* 150.15 20.4 2.89 7.9 0.18 B* 19.1 2.41 7.4 1.10 B* 150.20 18.4 2.31 6.9 0.76 MB* 18.3 2.08 7.5 0.46 MB*

* TMR = typical mode of rupture M = mortar B = block MB = mortar/block

For comparison of the results, we performed an analysis of variance (ANOVA), using the Microsoft Excel 2007 spreadsheet software. For the results of mortar, and for the prisms, in each combination, The ANOVA results indicate there was no significative statistical difference between the means, for all combinations. A N A L YSIS O F R ESU L TS For the analysis of the results obtained in experiments with prisms, were used the mean strengths from results of compressive strength of mortars, and the mean of the compressive strength of prisms. The three typical modes of rupture are showed in Figures 3, 4 and 5. Table 5 summarizes the referred data.

F igure 3 Mortar mode of rupture.



F igure 4 Mortar/block mode of rupture.

F igure 5 Block mode of rupture.

Table 5: Average strength and modes of rupture of the prisms tested.

Combination Average

strength of mortar (MPa)

Thickness of mortar (mm)

Average strength of

pr isms (MPa)

Typical mode of rupture

50.10 7.5 10 6.9 Mortar/block 50.15 7.5 15 6.6 Mortar/block 50.20 7.5 20 4.5 Mortar 100.10 10.2 10 7.5 Block 100.15 10.2 15 7.0 Mortar/block 100.20 10.2 20 5.3 Mortar/block 150.10 18.9 10 9.4 Block 150.15 18.9 15 7.7 Block 150.20 18.9 20 7.2 Mortar/block

Analyzing the results presented in the table, is possible to identify the growing strength of the prisms with increasing the strength of the bedding mortar and decreases with increasing thickness of the bedding mortar. Regression analysis is one of the most used techniques for data analysis. Once the appropriate model has been made, it can be used to make predictions, calculate probabilities, etc. The



regression models are used in all areas of knowledge. In the area of statistics, most methods of analysis use the theory of regression. To determine the function, was used the software DataF it version 9059. With this software, is possible to determine the significance of effects, the standard error and correlation coefficients. Was obtained as the best model, i.e., the model with lowest standard error and best correlation coefficient, the curve whose function is shown in equation 2. The proposed model has an estimated standard error of 0.42 MPa, and correlation coefficient of 91.1%.

( ) ( ) ( )42.0±23.0ln3.2+7.4=, aaaap efeff (2) Where pf is the strength of prism, in MPa, af is the strength of bedding mortar, in MPa and

ae is the thickness of bedding mortar, in mm.

It was decided to also use the response surface methodology, which is a collection of mathematical and statistical techniques for modeling and analyzing problems in which a response of interest is influenced by several variables (Montgomery, 2001). For the generation of response surface and data analysis was also used software DataF it. The response surface for the model suggested in equation 2 can be seen in Figure 6. Each color in the response surface indicates a range of strength values of the prism.

F igure 6: Surface response for the model adopted.

It can be seen, from the response surface, the trend of the direct influence of increased compressive strength of mortar and the negative influence of the increased thickness of the mortar in the compressive strength of the prism.



C O N C L USI O NS As a general result of the analysis, it was observed that the increase of the compressive strength of the bedding mortar, remaining constant the thickness of the bedding mortar, produces an increase in the compressive strength of the prism, for all thicknesses of bedding mortar. Analysis of the effect of the variation of the compressive strength of mortar was considered statistically significant. On the other hand, increasing the thickness of the bedding mortar caused a decrease in the compressive strength of the prisms, for all strength tested. The analysis of the effect of thickness variation in the bedding mortar was also considered statistically significant. The test results showed that the combined effect of variations of the compressive strength and thickness of the bedding mortar is negligible (there is no interaction between these variables). The response surface is restricted to the parameters of Brazilian standards, as the ultimate strength of the mortar (restricted to a maximum of 70% of the resistance of the block, calculated on net area). At the other extreme, the restriction to the thickness of the bedding mortar is at least 10 mm, as specified in Brazilian standards. The interpretation of the response surface shows the importance of worry about the excessive variability of the joint, which requires a strict control on workmanship. A factor analysis with replication was efficient in achieving the objectives of this work, able to determine the influence of the factors studied on the mechanical performance of the not grouting prisms of structural clay blocks and the construction of the calculation model. The model obtained is in accordance with the literature, demonstrating a natural logarithm relationship of the strength of the prism with the compressive strength of mortar and the linear relationship of the strength of the prism with the thickness of the bedding mortar (Aryana, 2006). It was shown also that the increase of the compressive strength of mortar is not as crucial in the compressive strength of the prism. The increased strength of the mortar of 7.5 MPa to 18.9 MPa (152%) caused a maximum increase of only 60% in the strength of the prism. However, this study noted that the strength of the mortar affects the failure mode of the prism. Mortars with compressive strength lesser than the tensile strength of the blocks, in confined condition, lead to a fragile rupture of the prism (crushing of the mortar). Stronger mortars lead to a more ductile rupture of the prism (splitting tensile). R E F E R E N C ES Aryana, S. A. Statistical analysis of compressive strength of clay brick masonry prisms. Dissertação de Mestrado. Universidade do Texas, Arlington, 2006. Associação Brasileira de Normas Técnicas. NBR 8798: Execução e controle de obras em alvenaria estrutural de blocos vazados de concreto. Rio de Janeiro, 1985.



__________. NBR 13276: Argamassa de assentamento e revestimento de paredes e tetos Preparo da mistura e determinação do índice de consistência. Rio de Janeiro, 2005. __________. NBR 13277: Argamassa para assentamento de paredes e revestimento de paredes tetos Determinação da retenção de água Método de ensaio. Rio d Janeiro, 1995. __________. NBR 13279: Argamassa para assentamento e revestimento de paredes e tetos Determinação da resistência à compressão à tração e à compressão. Rio de Janeiro, 2005. __________. NBR 15270-3: Componentes cerâmicos Parte 3: Blocos cerâmicos para alvenaria estrutural e de vedação Métodos e ensaio. Rio de Janeiro, 2005. __________. NBR 15812-2: Alvenaria estrutural Blocos cerâmicos. Parte 2: Execução e controle de obras. Rio de Janeiro, 2010. Beall, C. Masonry and Concrete for Residential Constrution. McGraw-Hill, 2004. Carvalho, J. N.; Desempenho estrutural de prismas de blocos cerâmicos com diferentes formas e dimensões. Dissertação de Mestrado. Universidade Federal de Santa Catarina, 2003 Hendry, A. W.; Sinha, B.P.; Davies, S. R. Design of Masonry Structures. E & FN Spon, Londres, 2004. Leão, G. F. B.; Perdigão, R. S. Alvenaria estrutural: Influência da espessura da camada de assentamento no sistema bloco-prisma. Trabalho de conclusão de curso. Universidade Federal de Alagoas, 2004. Ministério das Cidades. Déficit Habitacional 2008. Nota. Brasília, 23 de julho de 2010. Disponível em: www.cidades.gov.br. Acessado em outubro de 2010. Montgomery, D. C. Design and Analysis of Experiments. 5th edition. John Wiley & Sons, New York, 2001. Prudêncio Jr, L. R. Resistência à compressão da alvenaria e correlação entre a resistência de unidades, prismas e paredes. Dissertação de Mestrado. Escola de Engenharia da Universidade Federal do Rio Grande do Sul, 1986. Sahlin, S. Structural masonry. Pretence-Hall, New Jersey, 1971. Santos, M. D. F.; Carvalho, M; Bremm, L. C.; Silva, G. M. Desempenho de prismas e paredes construídas com diferentes geometrias de blocos cerâmicos. In: 51º Congresso Brasileiro de Cerâmica (Anais). Salvador, 2007. Sarangapani, G; Venkatarana Reddy, B. V.; Jagadish, K. S. Brick-mortar Bond and masonry compressive strength. In: Journal of Materials in Civil Engineering. ASCE, 2005.



Steil, R. O. Efeito da geometria do bloco de concreto e do tipo de argamassa no desempenho à compressão de prismas de alvenaria não grauteados. Dissertação de Mestrado. Universidade Federal de Santa Catarina, 2003.

STUDY OF THE INFLUENCE OF COMPRESSIVE ...2 MsC, Professor, Federal University of Alagoas, Sertão...

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