the effect of mortar bedding type and hollow concrete block ...

15th International Brick and Block Masonry Conference

Florianópolis – Brazil – 2012

THE EFFECT OF MORTAR BEDDING TYPE AND HOLLOW CONCRETE BLOCK GEOMETRY ON THE MECHANICAL

BEHAVIOR OF HIGH-STRENGTH STRUCTURAL MASONRY

Casali, Juliana Machado1; Oliveira, Alexandre Lima2; Sakamoto, Cintya3; Schankoski, Rudiele4; Prudêncio Jr., Luiz Roberto5

1 PhD, Professor, Federal Technological University of Paraná, Civil Construction Department, [email protected]

2 PhD, Professor, Federal Technological Institute of Santa Catarina, [email protected] 3 Student, Federal University of Santa Catarina, Civil Engineering Department, [email protected]

4 Master student, Federal University of Santa Catarina, Civil Engineering Department, [email protected] 5 PhD, Professor, Federal University of Santa Catarina, Civil Engineering Department,

[email protected]

In recent years the use of concrete block structural masonry for construction of tall buildings (above12 floors) has intensified in Brazil. In this type of masonry, the units used for the first floor are usually “high-strength” blocks (i.e., with compressive strength above 12MPa). To produce such blocks, some plants use a face-shell thickness of 32 mm cause the increase in net area makes it easier to obtain the desired compressive strength. Other plants produce blocks with a face-shell thickness of 25 mm that are lighter and easier to lay. However, in this case, a strong block machine is required. In addition, many construction firms use packaged-dry mortar to increase productivity and quality. This mortar contains cement, dry sand and admixtures. Only water and mixing are required at the jobsite. Another possibility is to use cement-lime mortar, commonly used for structural masonry. The present work aims to study the influence of both concrete block geometry and type of mortar on the prism/unit ratio, deformation and failure behaviour of structural masonry prisms. These were evaluated for two concrete block geometries (face-shell thickness of 25mm and 32mm) and four mortars (two packaged-dry mortars and two cement-lime mortars) with different compressive strengths. The results obtained show that concrete block geometry and type of mortar influenced mechanical behaviour. The highest prism/unit ratio was achieved in prisms with cement-lime mortar. The prisms made with packaged-dry mortar showed elevated deformation and crushing of the mortar joint (with 50% of rupture force). Concrete block geometry also influenced prism compressive strength. Concrete blocks with the smallest thickness resulted lower levels of compressive strength in prisms made with packaged-dry mortar. However, prisms with this same geometry but built with cement-lime mortar produced the best overall results.

Keywords: concrete blocks, bedding mortar, structural masonry



INTRODUCTION Structural masonry is a construction system whose purpose of increasing productivity, quality and efficiency. Due to these advantages and greater confidence in this system, builder currently has intensified the use of this building system for tall buildings (over 12 floors). In this type of structure, the concrete blocks used must presented compressive strength higher than those usually employed, especially in the first floor with compressive strength above 12MPa. To produce such concrete blocks with high-strength, some plants used different geometry of concrete blocks. If the plant has not a strong block machine for compressive the concrete, they choose to produce concrete blocks with face-shell thickness of 32mm cause the net area increase and it was easier to obtain the desired compressive strength. However, this option has some undesirable consequences: weight of concrete blocks increases (around 15%), overloading the foundations of buildings; productivity in laying the units decrease; more expensive transport costs. Another reason for using this type of concrete block is because there is less chance of involuntary changes of high-strength concrete block with others of lower strength once these blocks normally have thinner face-shell thickness. Others plants that have strong equipments, normally choose to produce concrete blocks with a face-shell thickness of 25mm. This preference is important because the plant does not need to stop the equipment for exchange the moulds when only the block compressive strength is changed. Usually the plants change the mixture proportion and the settings of the equipment. The required design strength of masonry is determined by the structural design requirements of the project. Masonry designer specifies this and define the compressive strength of prisms by each floor. The compressive strength of prism is expressed in net area according the ABNT NBR 15961-1/ 2011 Brazilian Standard. However, in the design, the tension generated on the walls is obtained by the ratio between the load on the walls and their gross area. Thus, for specifying the compressive strength of prism in net area, the masonry designer need to know the ratio of net area and gross area of concrete block that will be used. Another factor is that the use of concrete block with face-shell thickness larger (larger net area) can cause a compressive stress at the joint of lesser magnitude, which can totally alter the deformability of the structural masonry and even the failure mode. Currently there are many types of mortar with different properties and proportions that can be used in structural masonry. Generally, builders have opted for the use of packaged-dry mortar for increasing productivity and quality. This mortar contains cement, dry sand and admixtures. Only water and mixing are required at the jobsite. This mortar is used for both laying the unit and rendering. This option assumes that there is only one type of mortar that satisfies the minimum requirements for those different applications. Another possibility would be to use two different mortars: one for laying the units (higher compressive strength and modulus of elasticity) and other for rendering (lower compressive strength and modulus of elasticity).



Mohamad, Lourenço and Roman (2007) verified that different types of mortars induce different failure modes in the masonry prisms and there is clear evidence that the failure of hollow concrete masonry starts after onset of mortar crushing. In addition, the masonry designer and builders should try to maximize the structural efficiency of the structural components by combinations of masonry units and mortar. Thus, it should be studied the best combination of mortar and concrete block that lead to the highest structural efficiency. This study’s objective is to examine the influence of both concrete geometry and type of mortar bedding on the prism/unit ratio, deformation and failure behavior of structural masonry prisms. EXPERIMENTAL PROGRAM To study the influence of mortar bedding and hollow concrete block geometry were employed four mortars (two packaged-dry mortars and two cement-lime mortars) and two types of concrete block (face-shell thickness of 25mm and 32mm). The hollow concrete block geometry and mortars are described below. MATERIALS BEDDING MORTAR Four mortars were used: two cement-lime mortars with mixture proportions of 1:0.61:5.13 (Mortar C1) and 1:0.27:3.62 (Mortar C2) (cement: lime: sand) in mass and two packaged-dry mortars. The definition of these mixture proportions aimed at obtaining two different compressive strength levels: approximately 7.0 MPa (Mortar C1) and 15.0 MPa (Mortar C2). Composed Portland Cement (CP II – Z – 32) (6-14% fly ash) and CH – III hydrated lime were used for the mortars. The characteristics of the cement and hydrated lime are detailed in Tables 1 and 2, respectively. Table 1: Characteristics of the cement

Physical Characteristics (%) Chemical Components (%) Age (day)

Compressive Strength (MPa) % water demand 28.30 MgO 5.51

Specific gravity (Kg/l) 3.00 SO3 2.38 1 12.79 Bulk density (Kg/l) 1.01 Loss on ignition 5.25 3 22.80

Initial setting time (minutes) Final setting time (minutes)

243 324

Free lime 0.80 7 27.56 Insoluble residue 13.13 28 37.21

Blaine specific surface (m2/kg) 344

Table 2: Characteristics of the hydrated lime Chemical Components (%) Chemical Components (%)

CaO 37.7 Insoluble residue 8.97 MgO 25.9 Bulk density (Kg/l) 0.67

Loss on ignition 26.55 Specific gravity (Kg/l) 2.40 The sand used for the mortars (Mortar C1 and Mortar C2) was natural. For the packaged-dry mortars, manufactured sand was used. Its basic characteristics are presented in Table 3.



Table 3: Characteristics of the sand used for the mortars (Mortar C1 and Mortar C2)

Sand Natural Manufactured Sieve / Size number ASTM/ABNT % Retained Cumulative % Retained Cumulative

4.75 mm / N° 4 0.0 0.0 2.36 mm / N° 8 1.0 0.0

1.18 mm / N° 16 11.2 1.4 0.60 mm / N° 30 32.1 17.3 0.30 mm / N° 50 62.2 36.3

0.15 mm / N° 100 85.1 57.2 Pan 100.0 100.0

Fineness modulus 1.92 1.12 Specific gravity (Kg/l) 2.632 2.795

One day before the production of Mortar C1 and Mortar C2, off-the-shelf hydrated lime was mixed with moist sand (10%) to make cement-lime based mortar. After the production of the mixture, the plastic mortar’s consistency was measured by means of the flow table consistency test (ABNT – NBR 7215/1996 Brazilian Standard). Water volume (presented in Table 4 for cement-lime based mortar – Mortar C1 and Mortar C2) was defined experimentally in order to obtain a satisfactory workability evaluated by GTec Test proposed by Casali and Prudêncio Jr. (2008). In this test, the amount of water in mortar has presented initial measure (Li) around 17.5 mm and number of jolts between 7 and 15 that are the limits recommended by the authors. The two packaged-dry mortars were made with the same materials and have similar compressive strength of cement-lime based mortars. The mortar P3 was commercially available and, according to the manufacturer, the compressive strength is around 7.0 MPa. Another mortar (Mortar P4) had higher amount of cement (more 5% in mass of cement in relation to Mortar P3) for obtaining a mortar with high compressive strength (around 15.0 MPa). The mortars were mixed according to the ABNT - NBR 13276/ 2005 Brazilian Standard. The amount of materials used for the mortars is described in Table 4. Table 4: Amount of materials used for the mortars

Mortar Proportion (in mass)

Cement (g)

Lime (g) Sand (g) Water

(ml) Dry materials

(g) H (%)**

C1 1: 0.61: 5.13 2600 1586 13340 3510 - 20.0 C2 1: 0.27: 3.92 3600 972 13032 2400 - 13.4 P3 - - - - 2326 16000 14.5 P4 - 750 - - 2481 15000 15.8

** - water/dry materials ratio After the production of the mixtures, the properties of fresh mortar were evaluated: the specific gravity and entrained-air content (measured through the density method as per ABNT – NBR 13278/2005); workability, measured with the GTec Test, consistency with the flow table consistency test (as per ABNT – NBR 7215/1996) and water retentivity (as per ABNT – NBR 13277/ 2005).



Compressive strength and modulus of elasticity was measured in (5x10) cm specimens (as per ABNT – NBR 7215/1996) and flexural and compressive strength with (4x4x16) cm specimens (as per ABNT – NBR 13279/2005) after 28 days. HOLLOW CONCRETE BLOCKS The hollow concrete blocks studied were (140 x 190 x 390) mm (width x height x length) and their nominal characteristic compressive strength was 15.0 MPa. These hollow concrete blocks were made in two different plants. The geometry of hollow concrete blocks are detailed in Figure 1.

Figure 1: Geometry of the hollow concrete blocks used in the experimental program

PRISM CONSTRUCTION For each type of mortar, four 3-block high prisms were built for different combinations of bedding mortar (Figure 2a). All prisms were capped with a cement/fine-sand paste at least 24 hours before being assembled. The prisms were constructed according to the Brazilian Standard, with a joint thickness of (10±1) mm. After construction, the prisms were stored inside the LMCC (Laboratory of Building Materials of Federal University of Santa Catarina - UFSC). Prism compressive strength and modulus of elasticity were measured after 28 days demonstrated in Figure 2b. The secant elastic modulus of the mortars was determined by drawing a straight line connecting the points corresponding respectively to 0.5 N/mm2 and 33% of the maximum compressive stress on the stress–strain experimental curves.



(a) (b)

Figure 2: (a) Details of the prisms made with 3-block high and (b) details of the apparatus to measure the deformation

RESULTS AND DISCUSSION Table 5 presents the characteristics of the plastic mortar: consistency, workability (GTec Test) and specific gravity. The result of GTec Test are presented in Initial Measure (Li) (indicate the consistency) and number of jolts (indicate the plasticity). Table 5: Consistency, workability and specific gravity

Results

Mortar

Flow Table (mm)

Theory Specific gravity (g/cm3)

Real Specific gravity (g/cm3)

Entrained-air content (%)

Workability (GTec Test)

Initial Measure (Li) (cm)

Number of jolts (n)

C1 225 2.08 2.04 1.96 18.0 7 C2 215 2.23 2.08 6.83 18.0 6 P3 200 2.28 1.88 17.52 17.5 9 P4 220 2.25 1.87 17.04 18.5 12

As expected, the flow table values in Table 5 are different for each mortar since the workability varied from 200 to 225 mm. The ABNT - NBR 8798/1995 Brazilian Standard recommends a flow table limit of (230±10) mm for concrete block masonry bedding mortars. The mortar P3 and mortar C2 did not satisfy this requirement. However all mortar had adequate workability for laying concrete blocks in the subjected evaluation of the masons, confirming the suitability of the GTec Test (the workability parameters measured by GTec Test showed in Table 5 are almost the same and within the limits recommended by Casali and Prudêncio Jr. (2008)). This means that there is no direct relationship between the flow table results and mortar workability. The flow table can be used as an alternative for measuring consistency, but only for mortars of the same type. Bowler, Jackson and Monk (1996) and Casali and Prudêncio Jr. (2008) reached with this same conclusion. The values of entrained- air content for mortars P3 and P4 are higher than those for mortar C1 and C2 as can be observed in Table 5. This behavior can be explained by the composition of the mortars. The mortars P3 and P4 are packaged-dry mortar and contain cement, dry sand and admixtures.



In Figure 3, it can be observed the value of water retentivity with the time for the four mortars studied.

Figure 3: Water retentivity by the time for mortars

The water retentivity of all mortar was higher than 75% (Figure 3). As expect, the water retentivity for mortar C1 was higher than that of mortar C2 because the relation between lime and sand is higher in mortar C1. Table 6 presents flexural and compressive strength values obtained for the mortars, as well as their modulus of elasticity. Table 6: Flexural and compressive strength and modulus of elasticity of mortars

Mortars

Properties Strength Modulus of

elasticity Flexural Compressive (4x4x8) cm

Compressive (5x10) cm

Average (MPa)

Sd (MPa)

Average (MPa)

Sd (MPa)

Average (MPa) Sd (MPa) Average (MPa)

C1 3.07 0.34 10.09 2.02 8.52 0.52 6.23 C2 4.91 0.47 15.44 1.78 16.14 0.75 13.01 P3 3.46 0.35 6.85 1.83 7.01 1.40 7.51 P4 4.03 0.34 9.38 0.68 8.72 1.91 8.06

Regarding the results of compressive strength, mortar C1, C2 and P3 obtained results within the expected range (mortar C1 – 8.52 MPa; mortar C2 – 16.14 MPa and mortar P3 – 7.01 MPa in specimens 5 x 10 cm). In the mortar P4, increasing the amount of cement (10%) did not increase the level of compressive strength as expected. In previous studies, mortar presented compressive strength around 15.0 MPa with this cement content. Thus, in this studied the amount of cement did not significantly affect the compressive strength.



In the modulus of elasticity, mortar C2 showed higher values compared to other mortars because of its greater compressive strength. The other mortars had values of modulus of elasticity between 6.0 to 8.0 GPa. The behavior of mortar on the flexural strength was similar to the compressive strength. As expected, mortar C2 presented value of flexural strength significantly higher (4.91 MPa), while the other mortars, this result was between 3.0 to 4.0 MPa. It should be noted that the relationship between the flexural strength and compressive strength for packaged-dry mortars was higher (around 46%) compared to cement-lime mortar (about 30%). This different behavior is due to the presence of admixtures in packaged-dry mortars. Table 7 shows the average results of compressive strength in gross area of concrete block 1 and concrete block 2. Both concrete blocks had levels of high compressive strength (up to 20.0 MPa in gross area) and compressive strength characteristics exceeding 15.0 MPa, based on the definitions of Brazilian standard - NBR 6136 (2007). Table 7: Compressive strength of hollow concrete blocks (gross area)

Compressive Strength Concrete block 1* Concrete block 2** Average (MPa) 20.9 22.2

Sd (MPa) 3.53 0.53 C. O. V (%) 16.9 2.39 Fbk

*** (MPa) 17.3 21.3 * - Hollow concrete block with face-shell thickness of 25.0 mm ** - Hollow concrete block with face-shell thickness of 32.0 mm *** – Compressive strength characteristics of the blocks, determined using Brazilian standard NBR 6136 (2007). Table 8 presented the average results of compressive strength, prism/unit ratio and modulus of elasticity of the prisms with all mortars studied. Table 8: Average of compressive strength, modulus of elasticity and prism/unit ratio of prisms

Mortars

Hollow concrete blocks Concrete block 1 Concrete block 2

Compressive strength (MPa)

Modulus of elasticity

(GPa) F.E.* Compressive

strength (MPa)

Modulus of elasticity

(GPa) F.E.*

C1 15.15 10.40 0.72 15.87 10.60 0.71 C2 15.74 12.56 0.75 14.08 15.35 0.63 P3 8.53 13.83 0.41 11.74 11.15 0.53 P4 10.07 12.06 0.48 13.09 16.94 0.59

*Average prism compressive strength (using gross area) / average block compressive strength It can be seen in Table 8 that the cement-lime mortar (mortar C1 and C2) showed a superior performance (higher prism/unit ratio) compared packaged-dry mortar (mortar P3 and P4) for both concrete block 1 and concrete block 2. However, the increase in the level of compressive strength of cement-lime mortar (47%) did not result in a significant increase in performance of the prisms. In the case of prisms made of concrete blocks with face-shell thickness of 32 mm (concrete block 2), this increased compressive strength in the mortar resulted in reduced prism/unit ratio of the prisms. It is worth emphasizing that the mortar C1, P3 and P4,



having similar levels of compressive strength when tested in specimens of (5 x 10) cm presented different performances in prism. It can be observed that the values of prism/ unit ratio obtained for prisms with mortar C1 were significantly higher than the prism with packaged-dry mortar (P3 and P4). Besides the difference in the composition of the mortars, the decrease in prism/ unit ratio have been caused by increased levels of entrained air which reduced the performance together with the reduction of the extent of bond. Another important fact is the high deformation rate exhibited by all prisms made with packaged-dry mortars when the load reaches around 50% of the rupture load indicating the begging of the process of joint crushing (Figure 4).

Figure 4: Stress-strain experimental curves for the prisms with concrete block 1

This behavior can be explained to the fact that packaged-dry mortars present a higher entrained air content according to their composition (presence of admixtures). In addition, immediately after the crushing of joints, the other parts of the concrete block that were in contact with the mortar began spalling, initiating the breakup of the concrete blocks for this region. This fact may be responsible for the performance difference of the prisms made with the different concrete blocks (face-shell thickness of 25mm and 32mm), since the difference in face-shell thickness of the transverse and longitudinal walls was approximately 6.5 mm. The concrete blocks with thicker face-shell thickness resisted longer the effect of spalling without compromising the performance of the prisms. It is noteworthy that the mortar C2 has presented a value of modulus of elasticity significantly higher than the other mortars tested (see Table 6). This effect was not reflected in the behavior of prisms, indicating again that the properties of mortar determined in sealed molds not represent the behavior of the mortar joint, when the material is applied to an absorbent substrate.



CONCLUSION Based on the results presented in this work, we can conclude that: - The properties of mortars obtained by sealed molds do not represent the behavior of the mortar joint, when the material is applied to an absorbent substrate in the prisms. - The increase of the compressive strength of mortar does not result in a significant increase in performance as the compressive strength of prisms. - The mechanism of rupture of packaged-dry mortars when applied together with concrete blocks of high compressive strength was found to be different from that observed in the literature as the mortar joints have suffered crushing 50% of breaking load. - Prisms made with concrete block with face-shell thickness of 25 mm were superior performance compared to prisms with concrete block face-shell thickness of 32 mm, when the mortar joints have suffered crushing. This is due to increase the face-shell thickness, and for this reason the effect of greater compressive strength to spalling of the concrete blocks caused by the joint. REFERENCES Association Brazilian of Standards (NBR). Hollow concrete clocks for concrete masonry - Requirements. NBR 6136. Rio de Janeiro, Brazil. 2007. Association Brazilian of Standards (NBR). Portland cement – Determination of compressive strength. NBR 7215. Rio de Janeiro, Brazil. 1996. Association Brazilian of Standards (NBR). Mortars applied on walls and ceilings - Determination of the water retentivity. NBR 13277. Rio de Janeiro, Brazil. 2005. Association Brazilian of Standards (NBR). Mortars applied on walls and ceilings - Determination of the specific gravity and the air entrained content in the fresh stage. NBR 13278. Rio de Janeiro, Brazil. 2005. Association Brazilian of Standards (NBR). Mortars applied on walls and ceilings - Determination of the flexural and the compressive strength in the hardened stage. NBR 13279. Rio de Janeiro, Brazil. 2005. Association Brazilian of Standards (NBR). Structural masonry – Concrete blocks Parte 1: Design. NBR 15961. Rio de Janeiro, Brazil. 2011. Bowler, G. K., Jackson, P. J. and Monk, M. G., “The measurement of mortar workability”. Masonry International, Vol 10, N°1, 1996. pp 17-23. Casali, J.M., Prudêncio Jr., L.R. A new test method for the evaluation of the workability of concrete block masonry bedding mortars. In: 14th International brick and block masonry conference - 2008, Sydney. Australia: 14IBMaC, 2008. Mohamad, G.; Lourenço, P.B.; Roman, H. R. Mechanics of hollow concrete block masonry prisms under compression: Review and prospects. Cement & Concrete Composites 29, 2007b.

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