Rev. IBRACON Estrut. Mater. vol.5 número5

© 2012 IBRACON

This paper presents the experimental results of a research program with ceramic block masonry under compression. Four different block geom-etries were investigated. Two of them had circular hollows with different net area. The third one had two rectangular hollow and the last block was with rectangular hollows and a double central webs. The prisms and walls were built with two mortar type 1:1:6 (I) and 1:0,5:4 (II) (proportions by volume of cement: lime: sand). One:three small scale blocks were used to test block, prisms and walls on compression. It was possible to conclude that the block with double central webs gave better results of compressive strength showing to be more eficient. The mortar didn´t inluenced the compressive strength of prisms and walls.

Keywords: ceramic blocks, structural masonry, geometry of the block.

Neste trabalho são apresentados os resultados experimentais sob compressão centrada de alvenaria estrutural de blocos cerâmicos. Foram es-tudadas quatro diferentes geometrias de blocos designados por A, B, C e D, sendo o primeiro e segundo (A e B) com septos arredondados com diferentes áreas líquidas, o terceiro bloco (C) com furos verticais no formato retangular e o último (D) com um duplo septo central. O programa experimental abrangeu os estudos de blocos, prismas e paredes. Os prismas e as paredes foram construídas com dois traços de argamassa 1:1:6 (tipo I) e 1:0,5:4 (tipo II) (proporções em volume de cimento:cal:areia). Para tanto, foram realizadas análises da resistência à compressão do bloco, prismas e paredes, construídas com essas geometrias em escala reduzida na proporção de 1:3 das dimensões reais. Com base na análise dos resultados experimentais, pode-se concluir que o bloco tipo D, que possui duplo septo central, é o bloco mais eiciente para o uso em alvenaria estrutural entre os blocos analisados. As paredes construídas com os blocos do tipo D apresentaram uma melhor capacidade de ab -sorver esforços verticais, devido à coincidência dos septos entre as iadas subseqüentes, provocadas pela existência de um duplo septo central. Além disso, não se veriicou inluência signiicativa da resistência da argamassa na resistência de prismas e paredes.

Palavras-chave: bloco cerâmico, alvenaria estrutural, geometria dos blocos.

Mechanical behavior analysis of small-scale modeling of

ceramic block masonry structures – Geometries effect

Análise do comportamento mecânico das alvenarias

estruturais de blocos cerâmicos utilizando modelos

físicos reduzidos – Efeito da geometria

E. RIZZATTI a

edu_rizzatti@yahoo.com.br

H. R. ROMAN b

humberto@ecv.ufsc

G. MOHAMAD c

gihad.civil@gmail.com

E.Y. NAKANISHI d

elizabete_nakanishi@hotmail.com

a _{Departamento de Estruturas e Construção Civil, Universidade Federal de Santa Maria, e-mail: edu_rizzatti@yahoo.com.br, Avenida Roraima,} Prédio 07, Centro de Tecnologia, Santa Maria, RS.

b _{Departamento de Engenharia Civil, Universidade Federal de Santa Catarina, e-mail: humberto@ecv.ufsc, Rua João Pio Duarte da Silva, 205,} Bairro Córrego Grande, CEP.: 88040-970, Florianópolis, SC.

c _{Curso Engenharia Civil, Universidade Federal do Pampa, UNIPAMPA, e-mail: gihad.civil@gmail.com, Av. Tiarajú, 810 – Bairro Ibirapuitã} CEP.: 97546-550, Alegrete, RS.

d _{Curso Engenharia Civil, Universidade Federal do Pampa, UNIPAMPA, e-mail: elizabete_nakanishi@hotmail.com, Av. Tiarajú, 810 – Bairro Ibirapuitã} CEP.: 97546-550, Alegrete, RS.

Received: 31 May 2011 • Accepted: 27 Jun 2012 • Available Online: 02 Oct 2012

Abstract

1. Introduction

The red ceramic industry is responsible for creating a number of jobs in Brazil. For instance, in the state of Santa Catarina, there are 742 pottery facilities with a production of approximately 100 million units per month that are responsible for 11,000 direct jobs and 30,000 indirect jobs. Therefore, this industry is socioeconomically very im-portant [1]. The pottery sector has approximately the same proile in almost all Brazilian states, showing both a high production potential but also the small-scale technological and investment capacity for creating new products. The red ceramic companies produce various products including tiles, bricks, and both structural and non-struc-tural blocks. The main masonry product used for partitioning space in construction are non-structural blocks containing horizontally ori-ented circular or rectangular hollows. A Brazilian standard deines the minimum compressive strength of non-structural blocks. There are different shapes and thicknesses of structural blocks that will be further discussed below with regard to their eficiency at stress distribution in masonry walls under compression.

The increasing use of ceramic block masonry structures as con-struction systems in the Brazilian building market has been a factor in generating research projects focusing on development of mason-ry products that maintain a high eficiency capacity when subjected to external loads. The main goal of this work is experimentally ana-lysing and assessing the inluence of ceramic block geometry on the mechanical performance of structural walls under compression on a small-scale, allowing such blocks to potentially become an important component for the Brazilian ceramic industry.

2. Use of small physical models

in structural masonry

One of the greatest challenges in civil engineering is to develop reliable models for representing the behaviour of structures on a

full scale, thereby reducing the costs and the dificulties associated with “experimenting” with a full scale. The physical modelling of structures requires understanding of the similarity of the conditions of the models to truly to reproduce the behaviour of full-scale struc-tures with regarding to predicting the ultimate stress, failure modes and stiffness. Physical models therefore should reproduce the full-scale loading, geometry and material properties [2].

One of the irst authors to historically depict the use of physical models in structural masonry was ABBOUD et al. [2]. ABBOUD et al. reported that VOGT [3] carried out experimental studies on bricks masonry models at 1:4 and 1:10 scales, but failed to obtain consistent data regarding the behaviour of the material. ABBPID also cites that, in the 1960s, studies were performed at Melbourne University with limited success because of dificulties in manufac-turing bricks and constructing walls. ABBOUD also mentions that MOHR [4] achieved success in the execution of walls by using commercial units and prefabrication techniques at a 1:6 scale. The studies carried out by ABBOUD et al. [2] with concrete blocks units showed that there was reliability to be gained from using small models for predicting the complex behaviour of structural masonry. ABBOUD obtained excellent correlation between model results when compared to prototypes, but the standard deviation was smaller in prototypes. This result was obtained by reduction of the effect of stress volume.

In Brazil, CAMACHO [5] was the irst to perform studies on the compressive behaviour of block masonry. The author stat-ed that masonry is the oldest and most classical construction method used by man, while the implementation of the small model technique for studying structural behaviour is very recent. CAMACHO [5] afirms that studies were carried out at the Uni-versity of Bath and Karlsruhe UniUni-versity in Germany, regarding the behaviour of small masonry model walls made with ceramic bricks at scales of 1:2 and 1:4. CAMACHO states that those studies allowed the researchers to determine the strength cor-relations and deformations and therefore verify the parameters that would be affected by a scaling factor. Based on small-scale tests, it was concluded that masonry models can reproduce the failure mode and the ultimate strength when similar materials between the models and the prototypes are employed. How-ever, the value of the relation between the elasticity modulus based on compression strength was reduced with the decreas-ing of the scale, as shown in Figure 01.

CAMACHO [5] carried out experimental studies with hollow clay masonry blocks on full-scale model sand small scales of 1:3 and 1:5. The author performed compressive strength tests of prisms that were two, three and four blocks high as well as small walls. The compressive strength results for blocks, prisms (two, three and four blocks height) and small walls and the stress/strain results for blocks are shown in Figures 02 and 03, respectively. The author concluded in general that axial compression strengths between the small scale and the full-scale prototypes were similar but that for prisms and small walls, the small scale models presented compressive strength values 1.5 times higher for prisms and 1.3 times higher for small walls compared with the prototype. The strain at failure for the small scale and the prototypes were of the same order of magnitude, but the small-scale prisms had 2.4 times the prototype strain, while the small-scale small walls had strain values that were 4.5 times the prototype. In addition, the

rela-Figure � � �elationship �et�een modulus

of deformation/compression strength

704 IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 5

Figur���������u���������������ri�����������������������������r���������������������������

3. Factors that inluence

masonry strength

The main material responsible for the strength of masonry is the block. There are some Brazilian studies on the mechanical be-haviour of clay masonry [7 and 8]. In these studies, the blocks have different geometries, and in most cases, the results cannot be compared in terms of mechanical eficiency due to different as -pects such as the heterogeneous nature of the mixture, ineness of the clay, hole format and the iring temperature. Furthermore, in Brazil there have been few experiments using small-scale mod-els as a means of interpreting mechanical and physical phenom-enon. Thus, it is important to conduct technical studies on material behaviour to support the technical decisions for developing new structural products.

3.1 The block inluence on masonry strength

Clay blocks are structural masonry components with prismatic or circular hollows aligned in the direction of the load application, so that the block is set with the hollows oriented vertically. Bed -ding clay blocks are classiied as follows: either (a) structural clay tion between the prism and block strength was affected by the

scale factor with a value of 0.4, 0.5 and 0.6 for small scales of 1:1, 1:3 and 1:5, respectively. Although there were differences in the strength results, the failure mode presented by the pro-totype and the small-scale models were similar. CAMACHO concludes that it is possible to use small-scale models to de-termine the behaviour of clay block masonry.

NETO [6] studied theoretical and experimental behaviour of ma-sonry walls with openings, using small-scale models of 1:3. The au-thor determined the mechanical behaviour of the components and elements. The studies used structural clay blocks with two rectan-gular hollows. The thickness of face shells and cross webs was the same, and the relation between the net area and gross area was 55%. The dimensions of the units were 9.82 cm x 6.59 cm x 4.65 cm (length x height x width). Table 01 presents the experimental test results and the corresponding strengths. The failure modes of prisms and small walls presented by NETO [6] showed crushing of the bending mortar joint combined with a splitting displacement at the contact point with mortar, with tensile cracks induced by the application of compression stress. Combined with the compressive strength results for prisms and blocks, it was possible to obtain the eficiency factors of experimental tests of NETO [6], whose values were f_p / f_b = 0.55 MPa and f_{small walls} / f_b = 0.38 MPa.

706 IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 5

Figur������Failur��mod��of�prisms�and�small�walls�of�NETO�[6]

Figur��

������r��i��������������������������

Figure � � �haracteristic com�ressive strength of block masonry with different relation

of h/t (height/width) relationshi� of ��� (a) and between ��� and ��� (b) � B������� ����

block with hollow walls; (b) structural clay block with solid walls; (c) structural clay block with solid walls but hollow internal walls; or (d) drilled structural clay blocks as in Figure 05 [9]. The compres-sion strength of blocks is the main factor that determines the com-pressive strength of masonry. The British Standard 5628-1 [10] can be used as reference because the existing Brazilian national rules do not present results that correlate the strength of the ma-sonry for different blocks and mortars, quoting only that strength should be determined on experimental tests of prisms with three blocks. BSI-5628-1 [10] presents graphs of the characteristic com -pressive strength of brick or block masonry for different classes of units and mortars, which is based on the design and the

propor-tions of cement, lime and sand by volume as follows: i (1:0.25:3), ii (1:0.5:4.5), iii (1:1:6) and iv (1:2:9). As shown in Figure 06, the ratio of the compressive strength of the walls in relation to the sive strength of blocks tends to decrease with increasing compres-sive strength of the block, and this ratio is higher for bricks than for blocks. The BSI 5628-1 [10] considers only the relation of the dimensions (height and width) of the block and does not taking into account the geometry and the arrangement of the hollows. For walls with relation between height (h) and width (w) of 0.6 to 2.0, the value of the compression strength of masonry should be obtained from Figure 06.

3.2 The inluence of block geometry on

the compressive strength of masonry

During the load application, the quantity and the arrangement of hollows and shapes may lead to a concentration of stress on the block that can decrease the potential strength of masonry, accord-ing to work performed by GANESAN and RAMAMURTHY [11]. The authors stated that it is necessary to understand the geometry effect of blocks to increase the eficiency of structural walls. The authors carried out some analytical studies using inite element methods to better understand the behaviour of concrete masonry blocks, taking into account the inluence of different geometries, arrangements and properties of mortars. GANESAN and RAMA -MURTHY [11] proposed the use of blocks with three types of ge-ometry, including one with a double central web, that is, where the thickness of the central web was twice the thickness of the face shell that provides the alignment of the hollows. The geometries were modelled with stack and running bond prisms with three courses, using three different geometries of concrete block: blocks with three hollows, blocks with two hollows and blocks with two hol-lows and a double central web. Four types of mortar were used in the masonry to compare the proportion between the elasticity mod-ulus of blocks (E_b) to that of the mortar (E_a), where the proportions were 1; 1.5; 2.0 and 2.8, while E_b was held constant. Establishing a stiffness ratio of E_b/E_a as a constant was found to affect the mortar and the failure mode of the masonry. It was used a heterogeneous elastic-linear behaviour in the model, by using a solid element of eight (8) nodes for determining stress on the face shells and cross

ypes of the blocks studied by

GANESAN and RAMAMURTHY [11],

with the dimension in mm tested

at Building Technology Laboratory

708 IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 5

webs of the blocks. The authors noted that there were no changes in the ratios between net and gross areas of the blocks.

The results showed that blocks with three vertical hollows pro-duced higher levels of stress than blocks with two vertical hollows. The stress level remained constant in the region close to the centre of the prism. Regarding the cross webs, the difference in behav-iour among the three types of blocks was more evident for running bond prisms. As a conclusion of the work of GANESAN and RA-MAMURTHY [11] on the mechanical behaviour of masonry, it can be veriied that the geometry of the block inluenced the distribution and the magnitude of the stress level. Moreover, mortar did not inluence the behaviour of masonry and stack bond prisms overes-timated the masonry strength. Another important conclusion was that the ratio of the compressive strength of walls to that of blocks depended on the block geometry and the type of laying mortar. The authors found that for some geometries and mortar, there are stress concentrations that reduce the compression strength of masonry. Figure 07 shows the geometries and the compressive strength of blocks and walls along with eficiency factors. For Type-A blocks the mortar was applied only on the faces shells while for others the mortar was applied on the entire surface of the block.

3.3 The mortar inluence on masonry strength

Development of units (blocks) with greater compression capacities requires a proportional strength increase in the mortar joint, due to a failure mechanism of masonry that is closely related to interaction among these components, as it is shown in Figure 08. Several stud-ies were carried out in Brazil to determine the inluence of mortar, in which the studies carried out by GOMES [7] stand out. GOMES concludes that mortar strength should be between 0.7 to 1.0 times the block strength measured over the gross area. GOMES state that when mortars with a compression strength close to that of the block

are used, the masonry will display an excessively fragile failure with subsequent instability of the structure. MENDES [8] also conducted studies on hollow clay block prisms that were 140 mm wide x 290 mm long x 190 mm high (shape of Figure 05-b), where the relation-ship between the net and gross areas was 0.52. Experiments were conducted on grouted and non-grouted prisms with two compres-sion strengths of mortar. Based on the studies of MENDES [8], it can be observed that failure of non-grouted prisms is due to the crushing of mortar joints generating tensile concentrations in the blocks and splitting the contact surface between the block and mor-tar. The failure types of non-grouted prisms were fragile for prisms with mortar A1 and the crushing of the block lateral walls for mortar A3. For grouted prisms, all walls of the block (face shell and cross webs) were separated. The separation was caused by the lateral ex-pansion of the grout creating tensile concentrations that separated the face shell and the cross webs. Figure 09 presents the individual results for the block (B1), mortars (A1 and A3), grouts (G1, G2 and G3) and the different strength combinations between non-grouted and grouted prisms. The failure modes of grouted and non-grouted prisms are presented in Figure 10, as well as block geometry and the failure process for grouted prisms. Regarding the recommenda -tions of BSI-5628-1 [10] in Figure 06 and the experimental results of GOMES [7] and MENDES [8], it can be concluded that mortar strength did not signiicantly inluence the compressive strength of masonry for block strengths from 2.5 to 10 MPa. However, for blocks with compressive strength greater than 10 MPa it was veriied that the mortar inluenced the compressive strength of masonry.

4. Experimental Program

An experimental program was carried out with prepared clay unit blocks and masonry components using small-scale models with proportions of 1:3.

Figure �� � Fai�ure mode of grouted and ungrouted prims (MENDES [8])

4.1 Clay for laboratory production of ceramic units

One of the irst challenges of this work was to study the ideal clay composition for block fabrication. The clay mixture should have plasticity when mixed in water, so it can be shaped, contain suf-icient strength for keeping that shape and be able to fuse particles at high temperature. The plasticity of the clay and the inluence of the drying and burning protocols depended on the particle size and

the minerals present in the clay. To produce units on a small scale, clays were composed of colloidal particles with diameter smaller than 0.005 mm. The inal product, (i.e., the clay blocks) should have physical properties such as aspect, dimension, squareness and latness that meet the according to the standardized recom -mendations presented in Table 02 of NBR 15270-2 [9].

LINDNER [12] helped to develop a clay mixture for these stud -ies. Two types of clay were used for fabrication of the units. The clays were subjected to blending, grinding and homogenization.

Ta�ela � � Dimensional tolerances related to the average of effective dimensions

DIMENSION

Dimensional tolerances

related to the individual

measurements (mm)

Dimensional tolerances

related to the average

(mm)

Thickness (T)

± 5

± 3

Heigth (H)

± 5

± 3

Length (L)

± 5

± 3

Deviation from square (D)

3

710 IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 5 luorescence carried out at “Centro de Tecnologia em Cerâmica” (Masonry Technology Centre) in Criciúma, Santa Catarina state. In this study, blocks were fabricated with different geometries on a scale of 1:3. The ceramic mass was conformed through an extrud-er, where the mass is pushed through an opening called a mouth-piece in the geometry of the desired block shape. The extruder was equipped with a vacuum chamber to facilitate removing air from the block.

4.2 Mechanical characterisation of blocks

and mortars

In the evaluation of the inluence of block geometry on the mechan -ical behaviour of masonry, experimental studies on the compres-sion strength of units (blocks), prisms and walls at a small scale of 1:3 were carried out using two types of mortar. The small-scale blocks were 4.67 x 6.33 x 9.67 cm. Figure 11 shows the different geometries of the blocks as well as the dimension and an image of the small-scale prisms. The main goal of the experimental program was to use the small-scale models to investigate the inluence of the block geometry on structural masonry when submitted to com-pressive stress, and to determine the potential use of the small scale to represent masonry behaviour. Four different types of block geometry designated type A, B, C, and D were used.

Reduction of the geometric scale was applied for the bedding mor -tar joint and for the vertical joints of prisms and walls. To keep the properties of the joint equal to the real scale, a reduction in the particle size distribution of the mortar sand was performed. A sand was selected that best it the particle size limits in the British The two clays were dosed and blended in the feeder, which breaks

up the mixture prior to the horizontal mixer. In the inal phase of the production process, water was added to adjust the moisture content for optimum extrusion. Table 03 presents the clay chemi-cal composition determined by chemichemi-cal analyses by using X-ray

Table � � Chemical composition

of the clay

Chemical compounds

Percent by weight

SiO

2

61,46%

Al O

2 3

19,73%

Fe O

2 3

7,00%

CaO

0,05%

Na O

2

0,18%

K O

2

2,13%

MnO

0,08%

TiO

2

0,91%

MgO

0,97%

P O

2 5

0,22%

Loss on ignition

7,27%

standards as shown in Figure 12. Bedding mortar used in the ex -perimental tests followed the recommendation of BSI-5628-1 [10], where the proportions of cement:lime:sand by volume were 1:1:6 (Mortar – I) and 1:0.5:4 (Mortar - II). Mortars recommended in the British standards were used because they presented minimum

Figur��������r��i�g��i�i��������r��r�����

mechanical characteristics for each type of proportioning. The wa-ter/cement relation was adjusted to achieve a ixed consistency of 270 mm ± 10 mm when measured on a low table. The bed-ding mortar was prepared using a mixer with a vertical axis. For each mortar, six cylindrical specimen 5 cm in diameter and 10 cm in height were moulded for 28-day compression testing following procedures in NBR 13279 [13]. The specimens were cured in a laboratory environment for 28 days to reproduce the conditions of prisms and walls.

Ta�le � � Ma�� unit� Portland Cement� Hydrated lime and �and u�ed in t�e mortar�

Material

Portland cement CP II F-32

Hydrated lime CH III

Natural Sand

3

Mass unit (kg/dm )

1,12

0,64

1,33

Figure �� � �elations�i� bet�een net

and gross area for different blocks

712 IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 5 The granulometric distribution of sand used in the experimental

tests followed the recommendations of BS 1200 [14]. Portland ce-ments CP II- F-32 and CH-III-type hydrated lime were used. Deter -mination of the unitary mass of the cement and the lime followed the procedures described by NBR 7251 [15]. Table 04 shows the values of the unit masses of cement, lime and sand.

The geometries of the blocks had the following characteristics: 1 – Block type A –a model with two rectangular hollows similar to

the format of concrete block;

2 – Block types B and C – both types with two circular hollows. The block type B maintains the same thickness for the face shell and cross webs, resulting in a higher net area. For block type C the net area was maintained equal to the block type A; 3 – Block type D –a model with two rectangular hollows. The

in-ternal cross webs thickness is double the thickness of the face shell plus that of the mortar joint. This causes a meeting on the vertical joints of the mortar.

The relationships between the different net areas of the blocks are presented in Figure 13, where it can be observed that the relation-ship between the net and gross area of blocks A and C (BA/BC)

of the influence of geometry on masonry

(dimensions in cm)

are 1.0, that is, both blocks have the same relationship between the net and gross area.

The walls were built with an apparatus that ensured that the blocks were level, aligned and vertical in each row, following the recom-mendation of NBR 8949 [16]. Table 14 presents the irst and sec -ond rows with the apparatus for execution of the wall. For the dif-ferent types of blocks, ive prisms with and without vertical joint and three walls were built for each type of mortar, as shown in Figure 15. Table 05 presents the descriptions of the different tests of blocks, prisms and walls. The designation PA1 indicate the prisms with block type A and mortar I while the designation PPB2 indicates a masonry wall built with block B and mortar type II. The designation code is as follows: A, B, C, D = block, P = prism; PP = masonry wall; 1 = mortar type 1:1:6; and 2 = mortar type 1:0.5:4. Due to the dificulties of implementing tensile tests on the blocks, it was decided to obtain the tensile strengths of the blocks indirectly by dia-metric compression as shown in American Standard ASTM C1006-84 [17]. The cylindrical steel bars required for the tests were between 1/8 and 1/12 of the height of the sample and had lengths greater than their widths. The bars were aligned with the central crossing web in each block. The load applied at a rate of 0.33 MPa/min. The tensile strength was then determined by using Equation 01.

(1)

f

t

=

_

_.

2

_L

.

P

_.

_H

Where: f_t= tensile strength by diametric compression (MPa); P = applied load (kN); L = length (mm); and H = height of the sample. Values of the tensile strength determined by diametric compres-sion are presented in Table 06, together with a depiction of the test device.

Sixteen blocks of each geometry were randomly selected for the compression tests. Blocks were prepared for testing by the follow -ing procedure:

– the top and bottom of the blocks were covered with a mixture of 70% cement paste plus 30% sand retained in the 0.15 mm sieve to avoid cracking caused by shrinkage;

– after the capping of the top and bottom of the blocks the speci-mens were immersed in water for 24 hours;

– excess water was removed with a dry rag before the tests Compression tests were performed by incrementally applying the load at a rate of 0.5 MPa/second. The compressive strength of the gross area gives a standard strength for a constant area that is independent of geometry effects.

Ta�le � � Denomina�ions o� �locks, mor�ar, prisms and walls

Block

Type A (BA)

Type B (BB)

Type C (BC)

Type D (BD)

Prism

Mortar I

PA1

PB1

PC1

PD1

Mortar II

PA2

PB2

PC2

PD2

Table � � Tensile strength by diametral compression

Block Type

Tensile strength by diametral

_{compression (MPa)}

Mean

1,81

A

s.d

0,23

c.v (%)

12,89

Mean

1,57

B

s.d

0,16

c.v (%)

10,05

Mean

1,67

C

s.d

0,11

c.v (%)

6,41

Mean

1,80

D

s.d

0,17

c.v (%)

9,30

- s.d is the standard deviation and c.v (%) is the coefficient of variation in percentage.

4.3 Mechanical characterization of prisms and walls

Five stack bond prisms that were three blocks high were built with each of the two types of mortar (I and II) and ive running bond prisms with an intermediate row composed of two half-blocks and a vertical joint were built only with mortar type I. The three-block height was selected because of the effect of coninement stress produced by the plate so that the intermediate block did not de-velop shear stress. The prisms have a full mortar bedding (face shell and cross webs) and were built on a levelled granite table and covered with a plastic with oil. The thickness of the mortar joint remained constant on the order of 3 ± 0.1 mm. Levelling was maintained during prism construction while plummeting was maintained during construction of masonry walls. The prisms were tested 28 days after construction. Before the compression tests the prisms were capped with a mixture of 70% cement paste and 30% sand retained on 0.15 mm sieves. Prisms and walls were tested using a servo-controlled machine (SHIMADZU, se -ries UH) with a 200 ton capacity, at a loading rate of 0.05 ± 0.01 MPa/s. Table 07 shows the compression strength results for six mortar type I and II specimens.

Ta��e � � �es��ts of mortar compression strength

BLOCK

The average compressive strength

_{of mortar I (MPa)}

The average compressive strength

_{of mortar II (MPa)}

A

3,08

5,21

B

3,17

5,53

C

3,43

5,46

D

2,56

5,15

fmortar

fmortar

714 IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 5

T�ble � � Res�lts of compression strength of prisms

Block

Prism

Type

PRISM

PRISM

PRISM

MORTAR I

MORTAR II

MORTAR I

f A

net.

f A

gross

f /f

p b

f A

net.

f A

gross

f /f

p b

f A

net.

f A

gross

f /f

p b

f

p

20,48

f

p

f

p

10,56

0,36

24,50

12,64

0,43

24,27

12,52

0,43

A

s.d

2,10

s.d

4,30

s.d

1,29

c.v

10,30

c.v

17,57

c.v

5,32

f

p

23,64

f

p

f

p

13,61

0,42

26,59

15,31

0,47

25,98

14,96

0,46

B

s.d

2,05

s.d

2,83

s.d

1,03

c.v

8,67

c.v

10,65

c.v

3,96

f

p

23,04

f

p

f

p

11,18

0,37

26,03

12,64

0,41

28,59

13,88

0,45

C

s.d

3,98

s.d

0,96

s.d

4,30

c.v

17,30

c.v

7,61

c.v

15,00

f

p

20,30

f

p

f

p

11,67

0,35

22,99

13,22

0,39

26,25

14,88

0,45

D

s.d

1,42

s.d

1,74

s.d

3,34

c.v

7,00

c.v

7,56

c.v

12,70

Where: f is the prism strength (MPa); f /f is efficiency factor between prism strength in relation to the block; s.d. is the standard p p b deviation (MPa); c.v is the coeficiente of variation (%); f A is the strength in the net area; f Anet gross is the strength in the gross area.

The experimental results of compression tests of stack and run-ning bond prisms with mortar types I and II are presented in Table 08 along with the standard deviations and coeficients of variation. The compressive strength results for prisms and blocks were ob-tained for both the net and gross areas (ES 772-1 [18]). There

were no signiicant differences in the compressive strength results for prisms using the different block types A, B, C and D. The small differences in the strength values are likely due to the superposi-tion of the face-shell and the cross webs of the blocks for the two types of prisms. Table 08 presents the eficiency factor between the compressive strength of prisms and blocks (f_p/f_b). It was ob -served that there was a reduction in the compressive strength be-tween prisms and blocks of approximately 55% to 65%. The failure mode of prisms were similar to those obtained by MENDES [8] who observed a failure caused by crushing of the bedding mortar joint and the splitting of the surfaces between blocks and mortar. The circles in Figure 17 depict the failure modes of prisms. The tests showed that scale-reduced prisms had failure modes similar to those obtained for full-scale prisms.

For each type of block and mortar, three walls were built and test-ed. The blocks were wetted before bedding so that water would not be removed from the mortar, making it available to hydrate the cement. The blocks at the top and bottom of the walls were capped with the cement paste and sand mixture described above before testing. Walls were tested 28 days after construction, and remained in the laboratory environment between construction and

testing. Table 09 presents the compressive strength results for walls with different block geometries and two types of mortar along with the standard deviations and coeficients of variation measured on both the net and gross areas. Figure 19 presents the individual compressive strength results for blocks, mortars, prisms and walls.

Table � � Results of co��ression strength of structural walls

Where: f is the wall strength (MPa); f /f is efficiency factor between wall strength in relation to the block; s.d. is the standard wall wall b

deviation (MPa); c.v is the coeficiente of variation (%); f A is the strength in the net area; f A is the strength in the gross area.net gross

BLOCK

COMPRESSIVE STRENGTH OF

MASONRY WALL WITH MORTAR I

COMPRESSIVE STRENGTH OF

MASONRY WALL WITH MORTAR II

f A

net.

f A

gross

f /f

wall b

f A

net.

f A

gross

f /f

wall b

f

wall

9,62

f

wall

4,76

0,17

9,72

4,80

0,17

A

s.d

0,69

s.d

1,36

c.v

7,2

c.v

13,94

f

wall

10,27

f

wall

6,03

0,18

10,32

6,06

0,18

B

s.d

0,65

s.d

0,60

c.v

6,3

c.v

5,85

f

wall

8,72

f

wall

4,41

0,14

9,99

5,05

0,16

C

s.d

1,70

s.d

2,67

c.v

19,50

c.v

26,34

f

wall

14,29

f

wall

7,99

0,25

15,48

8,59

0,27

D

s.d

1,37

s.d

1,15

c.v

9,5

c.v

7,4

716 IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 5 Figure 19 shows the values of eficiency factors between the prisms

and walls relative to the blocks, where: fPA1/fB = compressive strength of prism with mortar I divided by the compressive strength of block; fPA2/fB = compressive strength of prism with mortar II di-vided by the compressive strength of block; fPPA1/fB = compressive strength of wall with mortar I divided by the compressive strength of block; fPPA2/fB = compressive strength of wall with mortar II divided by the compressive strength of block. Figure 19 also presents the ef-iciency factor of masonry walls, which are 1.00 when the compres -sive strength of the wall is equal to that for clay blocks. The experi-mental results showed that there was a signiicant reduction in the

Figure �� � �fficiency factor of prisms

and walls for the two mortar traces (I and II)

eficiency factor of prisms and walls with different clay blocks. For the walls using the blocks of types A, B, C and D, the eficiency fac-tors did not depend on the type of mortar (I and II). According to the experimental results, the geometry of block D presented the best ef-iciency, close to 0.25. The improvement in the vertical distribution of stress over the face shell and the cross web due to the geometry of block D, where the longitudinal wall was twice as thick as the block’s wall thickness plus the thickness of the mortar joint, increased the compression eficiency of the masonry. No differences were found for the compressive strength of prisms for different block geometries either with or without the presence of half blocks at an intermediate course. That is, the prisms failed to show inluence of block geom-etry. Thus, it is possible to conclude that the geometry of block D presents a better compression performance compared to the others. Figure 20 shows the failure mode under compression of walls built with different types of blocks. No differences were observed in the failure mode of walls with the block type. The cracks were basically vertical with failure caused by the crushing of the bedding mortar joint and splitting the surface of the block. The axial strain was mea-sured with a mechanical extensometer, namely a “demec-gauge” following the procedures of NBR 8522 [19], as shown in Figure 21. The experimental results are the averages for three samples for each type of block. The results led to the relationship between the elasticity modulus and the compressive strength of masonry, the so-called “Ritter constant” (k) for different block geometries as shown in Equation (02).

(2)

E

wall

= k. F

wall

Table 10 presents the average results of the elasticity modulus of walls built with mortar type I. The elasticity was obtained at a

Figure �� � �etermination of elasticit�

modulus of the wall under compression

stress level of 30% of the compression strength. For measure-ments of the initial stress and strain, different characteristics of elasticity modulus of the walls depending of the block geometry were observed, especially for block types B and C. This differ -ence between blocks B and C was a factor of 1.8. This differ-ence is not thought to be due to the geometric shape of the block but is rather due to the level of conining stress in the mortar caused by friction between the block surface and the bedding mortar joint. For this level of stress, the mortar joint is the main cause for the deformation of walls. The value of “Ritter constant” was lower than those speciied in Brazilian and international standards. The Brazilian Standard NBR 15812-1 [20], shows a Ritter constant of 600 while ES 6 [21] recommends a value of 1000. This fact may be related to the degree of compaction of the mortar joint dur-ing the bedddur-ing of the units, as suggested by ABBOUD [2] and CAMACHO [5].

5. Conclusions

Based on the results, the following can be concluded:

n Blocks with a double central web presented the best

perfor-mance in wall compression compared to the others. This con-clusion is conirmed by the eficiency factor of the set (block and mortar);

n Compression tests on stack-bonded prisms with a half-block

in the intermediate course were not adequate for checking the inluence of the geometry. For the cases studied, the eficiency factor ranged randomly from 0.35 to 0.47 without

demonstra-tion of similar behaviours for the same shapes of hollows;

n There were no signiicant differences in the compressive

strength of walls caused by increasing mortar strength, with it likely the case that the scale reduction decreased the inlu-ence on the strength most likely the reduction on the scale de-creased this inluence;

n The strength and the corresponding eficiency factors showed

that strength potential among prisms and walls decreased;

n Three-block prisms with an intermediate layer consisting of two half-blocks did not produce lower eficiency factors compared to three-whole-block prisms;

n D-shaped blocks provided a more uniform distribution of

trac-tion tension than the other geometry shapes due to the coinci-dence of the webs in transversal walls of the block;

n It was found by the results between the deformation modulus

by compression strength (“constant of ritter”) that the values were signiicantly lower on the small scale. This inding was demonstrated by the mortar bedding degree at the moment of unit bedding;

n The failure mode of prisms and walls on a small scale were similar to the ones found in the literature cited in this work. It can therefore be stated that the study of small models is able to reproduce trials at a scale that is an eficient, practical alter -native to full-size trials. The failure mode of prisms and walls was the crushing of bedding mortar joints followed by contact cracking of blocks and bending mortar.

6. References

[01] BRDE. Banco regional de desenvolvimento do extremo sul. Cerâmica vermelha: informe setorial. Florianópolis, SC, Dezembro, 1994, 14 P.

[02] ABBOUD, B. E.; HAMID, A.A.; HARRIS, H.G. Small-Scale modeling of concrete block masonry structures. ACI Structural Journal, Detroit, v.87, n.2, p.145-155, mar/apr. 1990.

[03] VOGT, H. Consideration and investigation on the basic principle of model tests in brickwork and masonry structures. Garston: Building Research Station, 1956. 30p.

[04] MOHR, G. A. Slender load bearing brickwork walls with returns. Parkville: University of Melbourne/Civil Engineering Department, 1970. Thesis (MESc)-University of Melbourne, 1970.

[05] CAMACHO, J.S. Contribuição ao estudo de modelos físicos reduzidos de alvenaria estrutural cerâmica. Tese de doutorado, Universidade de São Paulo, 1995. [06] NETO, J. A. do N. Estudo de painéis com abertura

constituídos por alvenaria estrutural de blocos. Tese de doutorado, Escola de Engenharia de São Carlos, Universidade de São Paulo, 2003.

[07] GOMES, N. S. A resistência das paredes de alvenaria. Dissertação de mestrado, Escola politécnica da universidade de São Paulo, 1974.

718 IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 5

[09] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Blocos cerâmicos para alvenaria estrutural – terminologia e requisitos. NBR 15270-2, Rio de Janeiro, 2005.

[10] BRITISH STANDARD INSTITUTE. Structural use of unreinforced masonry. BSI 5628-1, London, 1992. [11] GANESAN, T. P., RAMAMURTHY, K. Behavior

of concrete hollow-block masonry prisms under axial compression. Journal of Structural Engineering, vol. 118, July, 1992.

[12] LINDNER, G. Uso de modelo reduzido para pesquisa e desenvolvimento de blocos cerâmicos estruturais. Dissertação de mestrado. Programa de pós-graduação em engenharia civil da Universidade Federal de Santa Catarina. Florianópolis, SC, 95 P, 2001. [13] ASSOCIAÇÃO BRASILEIRA DE NORMAS

TÉCNICAS. Argamassa - Determinação da resistência à compressão – Método de ensaio. NBR 13279, Rio de Janeiro, 1995.

[14] BRITISH STANDARD INSTITUTE. Speciication for buildings sand from natural sources. BSI 1200, 1976.

[15] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Agregado em estado solto - Determinação da massa unitária. NBR 7251, Rio de Janeiro, 1982.

[16] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Paredes de alvenaria estrutural. Método de ensaio. NBR 8949, Rio de Janeiro, 1985.

[17] AMERICAN SOCIETY FOR TESTING AND MATERIALS. Standard test methods of splitting tensile strength of masonry units. ASTM C 1006-84, Philadelphia, 1984. [18] EUROPEAN STANDARD. Methods of test for masonry

units – Part.1: Determination of compressive strength. ES 772-1, 2000.

[19] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Concreto - determinação dos módulos estáticos de elasticidade e de deformação e da curva tensão e deformação. NBR 8522, Rio Janeiro, 2003.

[20] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Alvenaria estrutural – Blocos cerâmicos – Parte 1: Projetos. NBR 15812-1, Rio Janeiro, 2010. [21] EUROPEAN STANDARD. Design of masonry