CONTRIBUTION OF MORTAR COVERING ON COMPRESSIVE … · CONTRIBUTION OF MORTAR COVERING ON COMPRESSIVE BEHAVIOR OF NON-LOADBEARING CLAY BRICK SMALL WALLS Tavares, Samá de Andrade1;

15th International Brick and Block

Masonry Conference

Florianópolis – Brazil – 2012

CONTRIBUTION OF MORTAR COVERING ON COMPRESSIVE BEHAVIOR OF NON-LOADBEARING CLAY BRICK SMALL WALLS

Tavares, Samá de Andrade1; Sobrinho, Carlos Welligton de A. Pires2; Silva, Fernando Artur Nogueira3; Oliveira, Romilde Almeida4

1 MSc, Tecnological Institute of Pernambuco, [email protected] 2 MSc, Professor, State University of Pernambuco, Civil Engineering Department, [email protected]

3 DSc, Professor, Catholic University of Pernambuco, Civil Engineering Department, [email protected] 4 DSc, Professor, Catholic University of Pernambuco, Civil Engineering Department, [email protected]

Resistant masonry is a building technique which main feature is the use non-loadbearing clay or concrete bricks. More than six thousand multi-storey residential buildings were built in Recife Metropolitan Region using such technique and several collapsed spontaneously in last 18 years. Twelve others were demolished because it was not possible to perform retrofit works due to their fragility and a hundred of others buildings does not have conditions to occupation. The paper analyzes the hole played by mortar covering on the behavior of small clay walls subjected to compressive loading. Walls made with several types, mixes and thickness of mortar covering were studied in order one could formulate the understanding of the influence of such factors. Additionally, the effect of using welded meshes anchored with steel connectors embedded in the mortar coating was also studied. Tests were performed in a servo-controlled machine to make possible to capture the complete behavior of the specimen during all loading process, including post-peak behavior. Obtained results showed a trend in increasing loading capacity of tested walls with the increase of mortar quality and thickness. It was also observed changes on the overall behavior of the walls when tested with welded meshes embedded in the mortar coating.

Keywords: Non-loadbearing walls, retrofit in masonry, compressive behaviour of small walls

Theme: Restoration, repair, rehabilitation and refurbishment

INTRODUCTION

The five most populous cities of the Recife Metropolitan Region (RMR) have approximately three million inhabitants. About 10% of these inhabitants live in resistant masonry buildings up to four-stories high. These buildings, known locally as "caixão" type buildings, are structured with masonry elements using blocks that are characterized by standards as destined for use in non-loadbearing walls (Oliveira; Silva; Pires Sobrinho, 2008).

During the past two decades, more than ten buildings with these features presented a total or partial collapse and in two of these cases, the rupture resulted in human deaths (Oliveira and Pires Sobrinho, 2005). Today, a troubling scenario is present on the region that is characterized through the existence of several buildings that were demolished because they did not assured safety conditions for residents.

There is no technical code to deal with this type of building and besides its construction is usually made in an empiric way. Sometimes it is very often to identify the absence of important structural elements like concrete beams and columns. It is also common to find out an important variability in the building typology and foundations (Oliveira; Silva; Pires Sobrinho, 2008). These features, when combined with the poor quality of the construction materials used, can lead to critical situations that can result in suddenly rupture and progressive collapse of the whole building.

This paper analyzes the results of the influence of thickness and proportions of materials by volume of the mortar covering on the compressive behavior of clay masonry small walls with and without the incorporation of steel mesh.

The results obtained in the experimental program allowed knowing the real behavior of masonry wall subjected to compressive load, as well as understand the influence of plain or reinforced cement mortar coatings on the overall behavior of tested elements.

METHODOLOGY The methodology used to development the study is described in the following section

Design of Experiments An experimental program to investigate the compressive behavior of small clay masonry walls was performed. The small walls tested are elements that are used to represent a real wall because it approximates better the behavior of the wall than prisms since it contains all elements found in real walls, such bed and head joint.

One hundred and fifty small masonry walls measuring 0.09 m x 0.60 m x 1.20 m were built and tested. Eight holes clay bricks with dimensions of 9 cm x 19 cm x 19 cm laid with 1:1:6 cement, lime and sand mortar mix by volume. Table 1 below summarizes the data of the prototypes tested.

Table 1 – Tested small walls features

Type Specimen ID

Bed joint

mix

Base

lathing mix

Mortar

covering mix

Mortar covering on

each side (cm) Obs.

1 15 1 : 1 :6 - - - Apparent

2 15 1 : 1 :6 1: 3 - - Base lathing

3 15 1 : 1 :6 1: 3 1: 2: 9 1.5 w/o mesh

4 15 1 : 1 :6 1: 3 1: 2: 9 3.0 w/o mesh

5 15 1 : 1 :6 1: 3 1: 1: 6 3.0 w/o mesh

6 15 1 : 1 :6 1: 3 1: 0.5: 4.5 3.0 w/o mesh

7 15 1 : 1 :6 1: 3 1 : 1 : 6 3.0 w/o mesh

w/ admixture

8 15 1 : 1 :6 1: 3 1 : 2 : 9 3.0 w/ mesh

9 15 1 : 1 :6 1: 3 1 : 2 : 9 1.5 w/ mesh

10 15 1 : 1 :6 1: 3 1 : 1 : 6 3.0 w/ mesh

Figure 1 below shows the arrangement of the small walls groups for the situations in which they were tested.

Figure 1 – Scheme of different situations of tested small walls (Andrade, 2009)

Materials and Components Clay blocks used are characterized by standard NBR 15270-1:2005 as non-loadbearing blocks with eight prismatic holes measuring 9 cm x 19 cm x 19 cm. These blocks when tested according to NBR 15270-3:2005 showed a brittle behavior.

Figure 2 shows the test scheme and the typical rupture pattern for this type of block.

a)

b)

e)

c)

d)Figure 2 – Compressive test details: a) Beginning of the test; b) Pre-rupture; c) Rupture; d) Pos-

rupture; e) Load application scheme

Cement-lime-sand mortar mix with proportions of materials by volume of 1:1:6 was used to buttering. Three type of cement-lime-sand mortar mixes were used for mortar covering: 1:2:9; 1:1:6 e 1:0.5:4.5. Table 1 showed before summarizes this information.

Tested results from materials studied are showed in Table 2.

Table 2 – Test result for mortars and blocks (Andrade, 2009)

Material/Component Specimen features

Average compressive

strength

(MPa) Eight holes clay bricks 9cm x 19cm x 19cm 2.87

Mortar 1:2:9 Ф = 5cm, h=10 cm 4.00

Mortar 1:1:6 Ф = 5cm, h=10 cm 4.45

Mortar 1:0.5:4.5 Ф = 5cm, h=10 cm 6.23

Building and handling of the small walls The small walls were built is stages of three rows/day using bed joint with uniform thickness of 1.0 cm. They were built on 8” H steel sections in order to facilitate transportation and handling. The small walls reinforced with the steel mesh embedded in the mortar covering were laid over steel sections filled with concrete in order to allow mortar covering of the small wall could be made after the placement of the mesh. Figure 3 below shows the building stages of the small walls.

Figure 3 – Mortar covering, base lathing and floated coat stages (Andrade, 2009)

Figure 4 shows the building process used for the incorporation of the reinforcement of the small walls with steel mesh and recovering with mortar.

Figure 4 – Details of installation of the steel mesh (Andrade, 2009)

The preparation and treatment of the small walls for the tests demanded the development of special handling and transportation devices. Figure 5 shows a few examples of these devices.

Figure 5 – Handling and treatment details of small walls for testing (Andrade, 2009)

Compressive strength testing Compressive strength tests were performed in a reaction portal frame with load and displacement control using a servo machine. Structured as a self-reactive frame with three hydraulic jacks, each one with load capacity of 500 kN and a course 200 mm. Figure 6 shows the schematic layout of the reaction portal frame and details of the equipments and devices used in the tests.

Figure 6 – Schematic details of the reaction frame and test devices (Andrade, 2009)

RESULTS AND ANALYSIS Tables 3 to 5 show the obtained results from compressive tests of the small walls, listing the mean values, standard deviation, and coefficient of variation for each stage. Table 3 - Results – Load corresponding to the 1st crack on the blocks (Andrade,2009)

Prototype

Avg load for the 1st crack in the block

(kN)

Dispersion measures

Avg Deviation

(kN)

COV

(%)

Without mortar covering 4.14 1.16 27.9

With base lathing 6.79 1.96 28.8

Mortar covering mix: 1:2:9, thickness 1.5 cm 8.53 2.27 26.6



Mortar covering mix: 1:0,5:4,5, thickness 3.0 cm 17.03 5.42 31.8

Mortar covering mix: 1:2:9, thickness 1.5 cm + reinforced mortar 1:1:6 thickness 3.0cm & mesh

24.20 8.46 35.0

Mortar covering mix: 1:2:9 thickness 3.0 cm + reinforced mortar 1:1:6, thickness 3.0cm & mesh

25.48 5.75 22.6

Mortar covering mix: 1:1:6 thickness 3.0 cm + reinforced mortar 1:1:6 thickness 3.0cm & mesh

26.74 6.32 23.6

Table 4- Results – Load corresponding to the 1st crack of the mortar covering (Andrade, 2009)

Prototype

Avg load for the 1st crack in

the coating

(kN)

Dispersion measures

Avg Deviation

(kN)

COV

(%)

Without mortar covering --- ---- ----

With base lathing ---- ---- ----

Mortar covering mix: 1:2:9, thickness 1.5 cm Not observed ----- ----



Mortar covering mix: 1:0,5:4,5, thickness 3,0 cm 24.08 4.26 17.7

Mortar covering mix: 1:2:9, thickness 1.5 cm + reinforced mortar 1:1:6 thi 3.0cm & mesh

25.21 4.49 17.8

Mortar covering mix: 1:2:9 thickness 3.0 cm + reinforced mortar 1:1:6, thi 3.0cm & mesh

20.66 5.97 28.8

Mortar covering mix: 1:1:6 thickness 3.0 cm + reinforced mortar 1:1:6 thi 3.0cm & mesh

27.41 7.90 28.8

Table 5 - Results – Rupture load of the small walls (Andrade, 2009)

Prototype Avg rupture

load

(kN)

Dispersion measures

Avg Deviation

(kN)

COV

(%)

Without mortar covering 5.63 0.87 15.4

With base lathing 8.49 1.63 19.2




Mortar covering mix: 1:0.5:4.5, thickness 3.0 cm 26.22 4.27 16.3

Mortar covering mix: 1:2:9, thickness 1.5 cm + reinforced mortar 1:1:6 thickness 3.0 cm & mesh

32.10 4.77 14.9

Mortar covering mix: 1:2:9 thickness 3.0 cm + reinforced mortar 1:1:6, thickness 3.0 cm & mesh

36.70 4.93 13.4

Mortar covering mix: 1:1:6 thickness 3.0 cm + reinforced mortar 1:1:6 thickness 3.0 cm & mesh

41.71 6.30 15.1

The rupture in most of the small walls took place in the upper region of the specimens near the point where the load was applied and it was followed by cracks in the mortar covering and base lathing. Figure 5 shows details of the rupture feature usual profile.

Figure 5 – Rupture of the small walls with mortar covering (Andrade, 2009)

The observed behavior is due to the triaxial stress state on bed joints, which is a consequence of its confinement between the blocks. This stress state generates horizontal tensile stresses that

occur due to the bond mobilized between the mortar and the blocks. Therefore, the exact instant the stress value exceeds the limit tensile strength of the clay appears cracks (Silva, Oliveira and Pires Sobrinho, 2008). In the case of the small walls without the steel mesh reinforcement, the cracks in the septum of the blocks occurred before cracks in the mortar covering were observed which indicates an effective participation of the covering on the compressive behavior of the small wall. In the case of reinforced mortar with steel meshes, the initial crack was observed in the interface between the plain mortar covering and the reinforced mortar covering. This is possibly due to the increased deformability of the reinforced mortar layer and its position (without confinement) in relation to the confined and with covering masonry core and less bonding in the interface between the old and the new mortar covering. The following considerations can be made from the comparative analysis of the results.

Base lathing influence It was observed that the simple application of the base lathing, with an average thickness of 5 mm, produced an average increase of 50.7% of the rupture load of the small walls, without changing, however, the way the walls collapse. Graph 1 below shows the average strength results of the small walls covering only with base lathing on each side of the wall.

Graph 1 – Rupture load of small walls without and with base lathing (Andrade, 2009)

Analyzing the compressive behavior of the tested small walls, see Graph 2, it is possible to observe that there is an increase in the slope of the load-displacement (stiffness) curve. The average stiffness of the small walls without base lathing is about 17 280 KNm/m, while in the small walls with base lathing it is about 21 600 KN.m/m, representing an increase of about 25%.

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a) Small walls without base lathing b) Small walls with base lathing Graph 2 – Compressive behavior of small walls, without and with base lathing (Andrade, 2009)

The results showed that the base lathing layer, even being very thin (5 mm), about 10% of the area of the blocks, caused an increase in the load capacity of the small wall at about 50% and the stiffness by 25% without changing, however, the way the walls collapse.

Mortar covering mix influence Performing a comparative analysis of the results in small walls with mortar covering having thickness of 3.0 cm with different mixes, 1:2:9 (weak), 1:1:6 (medium) and 1:0.5:4.5 (strong), it was possible to observe a slight increase in the strength capacity of the small walls, around 7.5 %, between weak and medium mortar, and a considerable increase of 55.8 %, between the strong and medium mortar. Graph 3 shows the behavior of the small walls with different mortar mixes.

Graph 3 – Rupture load of small walls with different mortar covering mixes (Andrade, 2009)

Analyzing the compressive behavior of the tested small walls, see Graph 4, it is possible to observe that there is an increase in stiffness of the small walls with mortar covering according to the higher cement content in the mortar. The small walls with mortar covering mix of 1:2:9 showed an average stiffness at about 48 000 KNm/m, those with mortar covering mix of 1:1:6 showed an average stiffness at about 58 000 KNm/m, an increase at about 21%. The small walls with mortar covering mix of 1:0.5:4.5 showed an average stiffness at about 64.000 KNm/m, representing a 10 % increase regarding to the previous one.

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a) Small wall with mortar covering with 3.0cm in thickness and mix of 1:2:9;

b) Small wall with mortar covering with 3.0cm in thickness and mix of 1:1:6;

c) Small wall with mortar covering with 3.0cm in thickness and mix of 5;

Graph 4 – Compressive strength behavior of small walls with different mortar covering mixes

(Andrade, 2009)

The increase in the cement content in the mortar covering mix produced an increase in the load capacity and stiffness of the masonry walls testes, without changing, however, the way the walls collapsed.

Influence of mortar covering thickness Performing a comparative analysis of average strength according to the thickness of the small walls with mortar covering mix of 1:2:9, with thicknesses of 1.5 cm and 3.0 cm, it can be observed that rupture load increased with the increase in thickness. Graph 4 shows that this increase was around 8.5 % between thicknesses of 1.5 cm and of 3.0 cm. The increases in rupture load regarding to the small walls with only base lathing layer were of 58 % and 72 % respectively.

Graph 5 – Rupture load of small walls with different thickness mortar covering (Andrade, 2009)

Influence of reinforcement with mortared steel mesh Small walls with mortar covering with 3.0 cm in thickness and mix of 1:2:9 and 1:1:6 were reinforced with 10 x 10 cm steel mesh with diameter of 4.2 mm, interconnected with steel bars of 6.0 mm at 20 cm space intervals and with mortar covering with mix of 1:1:6. Graph 6 shows a comparison of the load capacity of the small walls with mortar covering in the specified mixes, with and without reinforcement.

Graph 6 – Compressive strength of small walls with reinforced with steel mesh mortar covering

(Andrade, 2009)

The results showed that in addition to the increase of the load capacity, there were changes in the rupture behavior of the small walls. Graph 7 presents the typical rupture profile of the walls with mortar covering with and without reinforced mortar.

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a) Small walls with mortar covering mix of 1:2:9, thickness=3.0cm, without reinforcement

b) Small walls with mortar covering mix of 1:2:9, thi=3.0cm, with reinforcement Mesh 4.2mm#10cm + 1:1:6 thickness=3.0cm

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c)Small walls with mortar covering mix of 1:1:6, thickness=3.0cm, without reinforcement

d)Small walls with mortar covering mix of 1:1:6 thi=3.0cm,with reinforcement Mesh 4.2mm, #10cm + 1:1:6, thickness=3.0cm

Graph 7 – Compressive behavior of the small walls, with and without reinforcement (Andrade, 2009)

Observing the post-peak behavior of the small walls reinforced with steel mesh, the importance of the connectors between steel meshes was noted. As long as the hooks of the connectors did not open, the small walls maintained a strength capacity of the order of the walls without reinforcement. CONCLUSIONS Mortar covering on masonry walls produced an increase in load capacity and in stiffness of the walls tested. This increase was due to the thickness of and the higher cement content of the mortar mixes. The incorporation of mortar covering did not change the rupture way of masonry small walls tested, which happen always in suddenly manner, but it was possible to note that there was an effective participation of the mortar covering on the compressive behavior of walls. Reinforcement with steel mesh, with effective locking and mortar covering, in addition to increasing load capacity of the walls, also produces changes in their rupture ways the walls collapsed. Connectors, which are the bars that interconnect the reinforcement steel meshes, play an important role in the post-peak behavior of the masonry walls tested. It must be taken into account that the increases in load capacity of the walls expressed in percentages depend on the strength of the block used. For lower strength blocks, the increases were high, and the opposite will be true for blocks of high strength.

ACKNOWLEDGMENTS The authors would like to thank FINEP – Financiadora de Estudos e Projetos (Studies and Projects Funding) for the financial support granted through Notice MCT/FINEP/FVA-HABITARE - 02/2004, Agreement nº. 01.04.1050.00, which made possible the research this study is part of. REFERENCES

Andrade, S.T. Influence of coating on the resistance of resistant ceramic blocks masonry small walls (in portuguese). Master Thesis, Federal University of Pernambuco, (2009): pgs. 80

Brazilian Technical Standards Association. NBR 8949 - Structural Masonry Walls: Single Compression Test (in portuguese): (1985). Rio de Janeiro.

Brazilian Technical Standards Association. NBR 15270-1 Ceramic blocks for sealing and structural masonry – Terminology and Requirements (in portuguese) (2005). Rio de Janeiro

Brazilian Technical Standards Association. NBR 15270-3 Ceramic blocks for sealing and structural masonry – Testing Methods (in portuguese) (2005). Rio de Janeiro

Oliveira, R. A.; Silva, F. A. N.; Pires Sobrinho C. W. Buildings constructed with resistant masonry in Pernambuco – Current situation and future outlook (in portuguese). In: Bernardo Silva Monteiro and José Afonso P. Vitório. (Org.). The SINAENCO-PE and the Production of Knowledge – Selection of Technical Articles. 1 ed. Recife: , 2008, v. 1, p. 233-263

Oliveira, R. A.; Sobrinho, C. W. A. Accidents with resistant masonry buildings in the Recife metropolitam region (in portuguese). In: DAMSTRUC, (2005): pp 285-292, João Pessoa: JP (BR).

Silva, F. A. N.; Oliveira, R. A. ; Pires Sobrinho, C.W.A. Influence of Coating on the Resistance of Resistant Ceramic Blocks Masonry Walls (in portuguese) in: JORNADAS SUDAMERICANAS DE INGENIERIA ESTRUCTURAL (2008): Santiago, Chile.

Documents

CONTRIBUTION OF MORTAR COVERING ON COMPRESSIVE … · CONTRIBUTION OF MORTAR COVERING ON COMPRESSIVE BEHAVIOR OF NON-LOADBEARING CLAY BRICK SMALL WALLS Tavares, Samá de Andrade1;