AN INVESTIGATION INTO BLOCK SPECIMENS

752

THE CAPACITY AND BEHAVIOUR OF LOADED PARALLEL TO THE BED FACE

F.M. Khalaf* Department of Civil Engineering & Building Science

üniversity of Edinburgh, Scotland

ABSTRACT

CONCRETE

Some codes and standards give two methods to obtain f~ (ultimate compressive strength of blockwork masonry), (i) using a table of masonry strengths based on unit block strength and mortar types (ii) testing stack-bonded masonry prisms, with a height-to-thickness ratio (h/t) between 2 and 5, made of the materiais used in the actual construction. In both methods no consideration has been given to the way the units in method one should be loaded or the prisms in method two should be constructed and loaded to determine fffi for use in designing concrete blockwork masonry beams or walls in seismic zones where very high horizontal forces acting parallel to the units bed face are expected. This paper presents the results of an experimental and analytical investigation using two and three-dimensional finite element analysis for axially and eccentricaliy loaded single-block specimens and two block prisms loaded in a direction parallel to the unit bed face. Appropriate methods are recommended to determine fIm and the ultimate strain (Em ) values to be used in the strength design theory of masonry elements.

INTRODUCTION

Previous work on concrete block masonry [1,2,3 I indicated that in some instances ultimate strength design principies similar to those of reinforced concrete can be appiied to reinforced masonry. However, as this masonry is a four component element (concrete block, mortar, concrete infill and steei reinforcement) questions of continuity and homogeniety of materiais and the way the units are ioaded to determine f~ ,arise in concrete masonry beams and walls where standard hollow blocks are used in construction. Using this kind of biock also leads inevitably to incompletely filied voids between the individual biocks (Figure 1), because the voids are smali (approximately 51 x 102mm) and because the concrete infill poured after the blocklaying is completed. This inadequacy of construction leads to stress concentrations near the unfilled voids if the masonry element is loaded in its own plane, which is the case in masonry beams or walls in seismic zones.

*University of Technology, Baghdad. •

753

Some codes and standards [4,5,6,7], give two methods to obtain f~ , (i) using a table of masonry strengths based on unit block strength and mortar types; (ii) testing stack-bonded masonry prisms, with a height­to-thickness ratio (h/t) between 2 and 5, made of the same materiais as used in the actual construction. These references are somewhat incomplete in their recommendations for determining the values of fffi for use in the analysis and design of masonry beams or walls in seismic zones. This is because no consideration has been given to the way the units in method (i) should be loaded or the prisms in method (ii) should be constructed and loaded.

The British Standard, BS 5628: Part 2: 1985 [8], recognised that masonry is not a homogeneous material and suggests using either of the above methods, but states that if the compressive force in the element is parallel to the bed face, fffi, should be determined for the given mortar type from the block strength, tested in a direction parallel to the bed face of the unit. This standard recommends a prism test for this special case only for brick masonry. This paper presents the results of an experimental and analytical investigation using two and three-dimensional finite element analysis for axially and êccentrically loaded single-block specimens and two - block prisms loaded in a direction paralleJ to the unit bed face. Appropriate methods are recommended to determine f~ ,and the ultimate strain ( Em ) values to be used in the strength design theory of masonry elements.

MATERIAL PROPERTIES

Half block prisms, each three blocks high with a thin layer of between the half blocks were tested normal to the bed face to

plaster determine

the mechanical properties for the concrete blocks in this direction. Block moulded prisms of concrete infill measuring 76 x 76 x 152mm and type "s" mortar cubes measuring 51 x 51 x 51mm were prepared and tested in accordance with the requirements of ASTM standard C476 [9]. The mean results for each material used in the construction of the different types of single-block specimens and two-block prisms are given in Table 1. Table 2 gives the mechanical properties for the materiais used in the finite element analysis. Figure 2 shows the average stress-strain curves for the three materiais used.

EXPERIMENTAL PROGRAMME ANO TEST RESULTS

Single-Block Specimens: Five concrete filled single - block specimens were tested to failure under axial load applied in a direction parallel to the unit bed face. The observed mode of failure was, first, by local crushing at the inner face of the block legs, followed by shearing near the legs and lateral deflection of the block shells and finally, by complete disintegration of the specimen (Figure 3).

Axially Loaded Two-Block Prisms: Eleven concrete filled two-block prisms with different mortar and concrete infill compressive strengths were tested under axial load parallel to the unit bed face. Two modes of failure ~lere observed. The first mode was by local crushing at the inner face of the mortar joints followed by shearing and lateral deflection of the block shells at the mortar joints (Figure 4(a)). The second mode was similar to the first mode, but there were

serious between

754

tensile cracks at the centre of the webs near the unfilled void the blocks (Figure 4(b)). The particular mode of failure depends

on the mortar compressive strength.

Eccentrically Loaded Two-Block Prisms: Four concrete filled prisms were tested under eccentric load with eccentricity of t/6 using PIN/PIN end conditions. The modes of failure were essentially by crushing and lateral deflection of the block sheel at the mortar joint. There was little evidence of tensile cracks at the centre of the webs near the mortar joints. However, vertical cracks were observed between the masonry and the concrete infill (Figure 5).

A summary of ali the tested single-block specimen and two-block prism proerties, failure loads, and stresses are given in Table 1. Figure 6 shows the location of "Demec" points used to monitor the strain.

FINITE ELEMENT ANALYSIS MODELS

Planar and three - dimensional stress analysis [10,11], were performed for the single - block specimens and two -block prisms using the finite element analysis technique. The analyses were used to find and study the deflections and stresses distribution and also used to predict cracks initiation and modes of failure. The computer two and three-dimensional finite element models (Figure 7) built up by taking into consideration the following factors:

1. The advantages of symmetry.

2. The nodal points which fali on the axis of symmetry were fixed against horizontal movement and were free to move vertically.

3. The interfaces between the blocks and mortar were assumed to be rigid because frictional forces created by compression prevent slipping [12] and perfect bond was assumed between the blocks and concrete infill (no concrete infill shrinkage).

4. The tapering of face shells and webs was ignored, instead the average shell thickness was used.

5. The elastic material properties were assumed to be the same for ali models (Table 2 and Figure 2).

DISCUSSION OF TEST AND ANALYSIS RESULTS

Single - block specimens failed with a percentage reduction of 34% to the block material. This reduction was attributed to the differences in the deformation characteristics of the concrete infill and block material, where the concrete infill undergores a larger lateral expansion due to an increased Poisson's ratio near ultimate strength [13]. Concrete infill is also responsible for the beam action, near the unfilled voids, where by a high localized vertical stresses is created at the inner faces of the block legs.

•

755

AXially loaded two-block prisms with lower mortar (fM = 17.8 Mpa) failed with a percentage reduction of 17% compared to the average strength of 21.7 Mpa for the prisms with higher mortar (on average f M = 23.6 Mpa). The reason for the reduction was the lower mortar strength. Figure 8 shows typical load-strain curves for prisms with high mortar strength (prism number 8). The curves tend to show, for the same level of load, a higher strain reading at the inner face of the mortar joints (curves 2 and 3,sides A and C) than at the outer face (curves 1 and 4, sides A and C,and all the curves on sides B and D). These differences are the result of the high localized vertical stresses at the inner faces, caused by the beam action near the unfilled void. Also evident are high tensile strains (curves 5 and 6, sides A and C) at the centre of the webs near the unfilled void between the two blocks.

Figures 9 and 10 relate the compressive strength results of the axially loaded prisms to the mortar and concrete infill strengths respectively. It can be seen in Figure 9 that the mortar strength has an important effect in determining the strength of the prisms loaded in a direction parallel to the bed face. To a certain extent, however, because after a specific mortar strength the splitting of the block at the centre of the webs, near the unfilled void, dominates the strength and type of failure (figures 4(b), 8, 14(b», and the prisms compressive strength, stabilized on a constant plateau. On the other hand, Figure 10 shows that there is almost no effect on the prism strength caused by using two concrete infills with 45% difference between their strangths [13,14].

The maximum extreme fibre compressive strength for the eccentrically loaded prisms were calculated using an idealized compressive stress distribution for the 10mm mortar joint obtained from the average stress­strain curves for the axially loaded prisms. Zero stress on the tension side were assumed and also that plane sections remain plane (Figure 11). These prisms on average showed a 4% increase in strength compared to the axially loaded prisms with high mortar. The increase was attributed to the strain gradient effect [15). The stress-strain curve for the 10mm mortar joint (Figure 11) tends to indicate increasing stiffness in the elastic region, caused by the high horizontal confinement stresses resulting from the differences in the deformational characteristics of the mortar and the block material [16]. Figure 12 shows the load-strain curves for prism number 13. The curves on the compression side show higher strain readings at the inner face of the mortar joints than at the outer faces. This was due to the same reason as for the axially loaded prisms. The curves also show a high tensile strain at the centre of the webs near the unfilled void.

AIl the prisms tested show very high strain readings. The highest strain recorded was 0.01464, which is very high compared to the ultimate strain for concrete ( Em= 0.003) given by the ACI-318M83 standard [17].

The finite element analyses on the other hand showed very good agreement with alI the tested single and two-block specimens. The two dimensional analytical results for the PIN/FIX single and two-block models show an approximate similarity in the elastic vertical and shear stresses, with the higher vertical stresses tend to occur at the inner face of the block legs and the mortar joints respectively, (Figures 13(a), 14(a», while the shear stresses, tending to concentrate near the block legs (Figure 14(c». These high vertical and shear stresses are mainly due to the beam action.

756

Appreciable differences were evident between the single and two-block models elastic tension stresses, with the two block model showing a very sharp increase in the tensile (splitting) stresses at the centre of the webs, near the unfilled void, compared to the single block model (Figures 13(b), 14(b)). These high tensile stresses are clearly explained by the magnified deflected shape of the PIN!FIX two-block model (Figure 15). The differences between the single and two-block models tensile (splitting) stresses, leads to the conclusion that the only way to determine f~ is by testing a two-block prism, where these stresses become more critical than the compressive stresses.

The three-dimensional stress analysis tends to confirm the test result for the eccentrically loaded prism,showing higher elastic vertical stresses at the inner face of the mortar joints rather than at the outer face

The analytical results show tensile stresses at the centre of the webs near the unfilled void However, these tensile stresses do not have such serious implications for the eccentrically loaded prisms. The intensity of these stresses depends on the position of the neutral axes in the compression zone of a beam, the lower the neutral axes, the higher the possibility of failure by splitting which is the case with axially loaded prisms. This phenomenon has been demonstrated by a comparison of the heavily and lightly reinforced masonry beams [2]. In the first case, the compression zone acts as it would in an axially loaded prism, causing dangerous solitting in the beam. In the second case, the compression zone acts as it wou1d in an eccentrica11y loaded prism, which also causes splitting in the centre of the beam; but in this case, it is not serious.

Both the two and three - dimensional analytical results show a confinement stresses at the mortar joints (Figures 14(b)) These stresses are the results of the differences between the deformational characteristic of the mortar and the block material [16].

CONCLUSIONS AND REMARKS

1. The codes and standards discussed in this study, other than the BS 5628: Part 2 : 1985, are somewhat incomplete in their recommendations for the values of f~ used for the analysis and design of blockwork masonry beams or walls in seismic zones, where high horizontal forces are expected. Because, when ffi is determined from the unit strength and mortar type, the recommended values are based on tests of units under axial load normal to the bed face. Whereas when determined by prism testing, these are constructed and loaded as wall elements.

2. For the analysis and design of concrete filled blockwork masonry beams or walls in seismic zones, using ultimate strength design theory, and the same type of block units used in this study. The ultimate blockwork compressive strength fffi must be determined as follows:

(a) lf the mortar used has a lower compressive strength than the filled single - block units tested in a direction parallel to the bed face, then f~ is determined from a rational method relating the filled single-block unit strength to the mortar type.

(b) lf the mortar used is of higher strength, then f~ is determined by testing filled two-block prisms laid in running bond, and loaded in a direction parallel to the unit bed face .

757

3 . The mortar compressive strength has a major effect on determining the compressive strength of the filled prisms. On the other hand the concrete infill strength has no significant effect.

4. A stress-strain curve for the 10mm confined mortar joint are suggested, where the curve tends to indicate increasing stiffness in the elastic region, caused by the high horizontal confinement stresses resulted from the differences between the deformational characteristics of the mortar and the block material.

5. Strain gradient does fibre ultimate strength, parallel to the bed face.

not have much effect on increasing the for the eccentrically loaded prism,

extreme tested

6. The maximum usable strain ofEm = 0.0048 must be used, that is when the mortar yields and the redistrubition of stresses occurs.

ACKNOWLEDGEMENTS

The author wishes to express his appreciation and thanks to Professor J.I. Glanville for his guidance and advice during the experimental and analytical stages of this investigation, where this study was conducted under his supervision at Manitoba University, Canada.

REFERENCES

1. Hatzinikolas, M., Longworth, J. and Warwaruk, J., Concrete masonry walls, Struct. Eng. Report No. 70, Department of Civil Engineering, University of Alberta, Edmonton, 1978.

2. Khalaf, F.M., An investigation of flexure in reinforced masonry beams, Master's thesis, University of Manitoba, Winnipeg, 1981.

3. Suter, G.T. and Fenton, G.A., Flexural capacity of reinforced masonry members, ACI J., 1986,83,1,127-136.

4. American Concrete Institute, Building code requirements for concrete masonry structures, ACI-531R-79, 1981.

5. Canadian Standards Association, Masonry design and construction for buildings, CSA-CAN3-S304, 1979.

6. Uniform Building Code, International Conference of Building Officials, Whittier, 1979.

7. American Society for Testing and MateriaIs, Standard test methods for compressive strength of masonry prisms, ASTM-E447-80, 1981.

8. British Standards Institution, Structural use of reinforced and pre­stressed masonry, BS5628: Part 2: 1985.

9. American Society for Testing and MateriaIs, Spec. for mortar and grout for reinforced masonry, ASTM-C476-71, 1980.

10. Doherity, W.P., Wilson, E.L. and Taylor, R.L., Stress analysis of axisymmetric solids utilizing higher-order quadrilateral finite ele­ments, Struct. Eng. Lab., Report No. SESM 69.3, University of California, Berkeley, 1969.

11. Bathe, K.J., Wilson, E.L. and Peterson, F.E., SAP IV - A structural analysis program for static and dynamic response of linear systems, Earthquake Eng. Research Center, Report No. EERC 73-11, College of Engineering, University of California, Berkeley, 1973.

12. Hamid, A.A., Behaviour characteristics of concrete masonry, Ph.D. thesis, University of McMaster, Hamilton, 1978.

758

13. Drysdale, R.G. and Hamid, A.A., Behaviour of concrete block masonry under axial compression, ACI J. 1979, 76,6,707-721.

14. Cheema, T.S. and Klingner, R.E., Compressive Strength of Concrete Masonry prisms, ACI J., 1986,83, 1,88-97.

15 Drysdale, R.G. and Hamid, A.A., Capacity of concrete block masonry prisms under eccentric compressive loading, ACI J., 1983,80,2,102-108.

16. Hilsdorf, H.K., Investigation in to the failure mechanisms of brick masonry loaded in axial compression, Designing Eng. and Consto with Masonry Products, Ed., Gulf Publishing, Texas, 1969.

17. American Concrete Institute, Building code requirements for reinforced concrete, ACI-318M-1983.

Table 1 Summary of the material properties and test data for the specimens tested

Specimen come. Strenath_N/mm~ l'1aterial Comp . Strenqth _ N/m m2

Specirnen 5pecimen P (U1 t. ) Based on Average 5.D . f • f' f' G M B

descriotion No. (kN) net area (Grout) (Mortar) !B1ockl

391 .4 19.9 Standard 458. I 23.3

filled block 373 .6 19.0 20.0 2.6 27.7 30.4

4 320.3 16.3

5 422 . 6 21. 5

A xially ~oaded 1 334.9 18.8 Filled Prisms, 2 340.3 19.1 18.1 1.5 28.4 17.8 30.4 Failure 292.5 16.4 Node I

4 373 :"6 20 . 9 5 409.2 22.9 22.0 1.0 18.2 24.3 30.4

A xially loaded 6 398.1 22.3 F i11 ed Pr i sms. '-í-- --- - '39 S ~õ ---2 2 ~l---- -- - - - - - - - - - -- - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - ---Failure 8 387.9 21.8 Node 11 9 358.5 20.1 21.4 1.2 32.8 22.9 30.4

lO 409.2 22.9 11 361.2 20.3

Eccentricallly 12 305.5 22.8 Loadeà filled 13 289.2 21.6

22 . 5 Prisms 14 299.8 22 .4 0 . 6 32.8 22.6 30 . 4

15 307.9 23.0

Table 2 Material properties used in the F.E.M. analysis

Compresllv8 Secant modulul Sh88r modulul Palltllon 'I Itrength 01 elalUclty of al.sUelty ratlo

Mal.rla' (N/mm 2) ( N/mm2 ) ( N/mm 2 )

Morta, 18 14469 6132 0.175

Coner.t. Inflll 31 21359 8957 0.166

Coneret. malo"ry unlta 30 30316 13091 0.170

Figure 1 Unfilled voids between filled standard blocks.

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