11th TNTERNATIONAL BRICKlBLOCK MASONRY CONFERENCE

TONGJI UNIVERSITY, SHANGHAI, CHINA, 14 - 16 OCTOBER 1997

FLEXURAL BOND STRENGTH OF PRESSED CEMENT EARTH BLOCK MASONRY

Peter Walker l

1. ABSTRACT

The paper outlines a study of flexural bond characteristics in cement stabilised pressed earth block masonry. Blocks were fabricated in the laboratory with a manual press using a blended building sand and clay composite soi!, stabilised with 10% cement. Bond test prisms were built using both soil based and cementlime:sand mortars. Flexural strengths were determined using the bond wrench test outlined in Australian Standard 3700 (1988). Experimental results outlining the effects of mortar type, moisture content, and age on flexural bond strengths are presented. In common with other masonry units bond strengths were dependent on unit moisture content at construction. Optimum unit moisture contents for maximum bond strength vary significantly depending on mortar type. Up to 28 days there was no increase in bond strength with age. The paper briefly outlines recommendations for mortar usage.

2. INTRODUCTION

In Australia stabilised pressed earth blocks are slowly gammg popularity as an altemative and environmentally sustainable building material. Although around one third of humanity lives in some form of unbaked earth building [I], current knowledge of many material characteristics, including the properties of pressed earth blocks, is limited. This paper outlines a study of flexural bond strength properties of pressed cement earth block masonry piers. Flexural strengths were determined using the bond wrench outlined in Australian Standard AS 3700 [2].

Keywords: Earth Blocks; Earth Mortars; Flexural Strength; Bond Wrench Testing.

I Lecturer, Department of Resource Engineering, University ofNew England, Armidale, NSW 2351, Australia

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Over the past 25 years much work has been undertaken to investigate the complexities and characteristics of the bond developed between cement mortar and various masonry units [3]. Bonding appears to be large1y mechanical in nature, relying on the formation of a layer of ettringite crystals at the unit/mortar interface. Formation of this layer and adequate bond strength is dependent on many factors, inc1uding: unit suction; unit moisture content; water retentivity of mortar; mortar consistency; quality of work; unit surface characteristics; sand grading; and, applied pre-compression [4]. Many of these factors are also likely to influence the bonding between cement earth blocks and their mortars.

Venu Madhava Rao et ai [5] recently reported on a limited series of flexural bond strength tests involving stabilised earth blocks and soil-cement mortars. The soil-cement mortars developed better bond strength than similar cementsand mixes. As expected, block moisture content at the time of construction proved very important in determining bond strength. Optimum moisture contents were around 85% of total water absorption values.

3. MATERIALS AND SAMPLE PREPARATION

Soils used for the project were formed by combining in two different mix proportions a building sand and a residual kaolinite c1ay soil (plasticity index = 38%). The grading and Atterberg limits ofthese two blended soils (A and B) are outlined in table I below. Use of blended soils, instead of a variety of natural deposits, allowed considerably greater control over experimental parameters. Stabilisation of c1ays with sand is a commonly employed technique in earth construction [1]. The blended soils were stabilised using a general purpose Portland cement. Cementlime:sand mortars were formed using the building sand, Portland cement and a hydrated building lime.

Table I. Characteristics ofblended soils

SoilA Soil B Characteristic (20% clay soil:80% (90% clay soil: 1 0%

building sand) building sand)

Grading (by mass) Fine Gravei fraction (2 - 6 mm) 8% 23% Sandfraction (0.06 - 2 mm) 77% 33% Siltfraction (0.002 - 0.06 mm) 4% 4% Clay fraction « 0.002 mm) 11% 40%

Liquid lirnit 21.9% 44.3% Plasticíty index Non-plastic 21.8% Linear shrinkage 0.4% 12.8%

Solid blocks for the i!westigation were fabricated using soil A combined with 10% Portland cement (by mass). Soil:cement mortars were formed by stabilising soils A and B with 5% and 10% cement (by mass) . In addition conventional 1:3:12 and 1:2:9 (cementlime:sand (by volume» mortars were prepared.

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In preparation the clay soil was thoroughly air dried, pulverised with a vibrating 'wacker' plate and passed through a 5 mm sieve. The building sand was also air dried and screened before use. The blended soils were mechanically dry mixed together for 2-to-3 minutes before addition of the cement. Once ali dry ingredients were mixed water was gradually added until the appropriate consistency was attained. For block production this was taken as the standard Proctor optimum moisture content [6] for the soil:cement mixo In mortar production sufficient water was added for a slump of 10-15 mm [7]. Blocks were compacted immediately after wet mixing for 2 minutes, however, in accordance with Australian Standard 2701 [7] wet mixing of ali mortars was stopped after only 60 seconds, left to stand for 10 minutes, and then recommenced for a further 60 seconds prior to use.

Blocks were produced using a manual constant volume press manufacturing units with nominal dimensions 295 mm (length) x 140 mm (breadth) x 120 mm (height). The compaction pressure was maintained at 2 MN/m2 throughout production. Once compacted the fresh blocks were extruded from the press and cured under polythene sheeting for 28 days. Approximately 500 blocks were produced for the experimental ' programo In accordance with appropriate standard methods, dry density [8], unconfined compressive strength [9], transverse strength (modulus of ruptun:) [10], drying shrinkage [10], initial rate of absorption [10], and wetting/drying durability [11] tests were undertaken on tive randomly selected samples for each parameter. Test results are summarised in table 2.

Table 2 Pressed earth block characteristics

Dry density (kgim3) 1775

Characteristic wet compressive strength (MPa) 4.26 Characteristic wet transverse strength (MPa) 1.02 A verage drying shrinkage 0.031%

Average initial rate ofabsorption (kg/m2/min.) 8.2 Wetting/drying durability (dry mass reduction 0.7% after 12 cycles)

4. FLEXURAL BOND TESTING

Piers for flexural bond strength testing were prepared in accordance with appendix A of Australian Standard 3700 [2], unless otherwise stated. Stack-bonded piers two courses high were prepared in the laboratory by an experienced mason. Ali joints were fully bedded, struck flush with the vertical faces, and maintained at a uniform 10 mm thick. Blocks were generally air dried in the laboratory before construction, although in the preparation of some tests blocks were oven-dried and/or soaked in water to pre­determined moisture contents for 48 hours before laying. After construction the piers were wrapped in polythene and left undisturbed for at least 5 days before further preparation for flexural bond wrench testing.

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A testing program was established to determine effects of mortar type, block moisture content at laying, and age at testing on flexural bond strengths. Piers were constructed using thr,ee different soil:cement mortars (soi! A with 5% and 10% cement, and soil B with 10% cement) and two cement:lime:sand mortar mixes (1:3 :12 and 1:2:9). Blocks were laid at four different moisture conditions, including oven-dry, saturated (after 48 hour immersion) and two intermediate values. Whilst in general bond wrench tests were undertaken at 7 days, one test series were completed at 1, 3 and 28 days. Piers were air­dried in the laboratory for 48 hours before testing.

Bond wrench tests were carried out in accordance with the procedure described in appendix A of AS 3700 [2]. Test equipment comprises a bond wrench and retaining frame to clamp the test specimen, figure 1. The lower block of the pier was initially clamped in the retaining frame, then the wrench attached to the upper course. To prevent damage surface contacts between the blocks and test equipment were packed with pieces of3 mm ply. Before loading the bond wrench arm was carefully set to horizontal. A container was then suspended from an appropriate notch of the wrench armo Using dry sand a vertical load was applied at a rate of 10 kg/min. until failure of the joint. Flexural bond strengths of individual specimens (f,p) are determined from:

fsp (1 )

Fig. 1 Bond wrench test

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,.

where: bending moment about centroid of joint total compressive force on testjoint section modulus of test joint cross-sectional area of test joint

Ten piers were tested as part of each test series. Characteristic flexural bond strengths.

(f' mt) are given by:

where:

f'mt

an_1

!sp(av.)

5. DISCUSSION OF RESUL TS

!sp(av.) -

unbiased standard deviation for test series average fll<xural bond strength for test series

(2)

Results of the flexural bond wrench tests. are summarised in table 3 below. In addition to mortar details and age at testing, the average block moisture contents at construction are provided .. For brevity only characteristic bond strengths are given. Co.efficients of variation for flexural bond strength ranged between 15.0% and 33.6%, which is fairly typical for this type of test.

Table 3. Flexural bond test results

Mortar Block Characteristic Test moisture Age at flexural bond

series Compro content testing strength Type strength when laid (f'mU

(MPa) (days) (MPa) I. 1:3:12 (cement:lime:sand) 1.6 1.7% 7 0.117 2. 1:3: 12 (cement:lime:sand) 1.6 6.2% 7 0.148 3. 1:3: 12 (cement:lime:sand) 1.6 10.4% 7 0.111 4. 1:3: 12 (cement:lime:sand) 1.6 15.2% 7 0.063 5. Soil A + 5% cement 0.82 0.3% 7 0.134 6. Soil A + 5% cement 0.82 6.3% 7 0.162 7. Soil A + 5% cement 0,82 9.2% 7 0.139 8. Soil A + 5% cement 0.82 14.9% 7 0.077 9. Soil B + 10% cement 1.9 0.6% 7 0.042 10. Soil B + 10% cement 1.9 2.8% 7 0.031 lI. Soil B -1; 10% cement 1.9 10.2% 7 0.121 12. Soil B + 10% cement 1.9 14.9% 7 0.087 13. Soil A + 5% cement 0.45 0.8% 1 0.089 14. Soil A + 5% cement 0.66 0.8% 3 0.116 15. Soil A + 5% cement 0.94 0.8% 7 0.091 16. Soil A + 5% cement 1.2 0.8% 28 0.069 17. Soil A + 10% cement 2.3 6.3% 7 0.192 18. 1:2:9 (cement:lime:sand) 3.4 6.3% 7 0.188

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As expected joint failure in flexure was sudden and brittle. Three distinctive modes of failure were observed. Firstly, failure of the joint interface, noticeable by a clean separation of the block and mortar along one of the interfaces, figure 2. Secondly and less commonly, the mortar joint itself split horizontally in two, with large amounts of mortar still adhered to both blocks. Thirdly, and usually in cases where stronger mortars were used, failure of the joint occurred due to failure of the block. Generally fracture was limited to a surface depth around 5 mm, although in extreme cases (but not observed with the blocks reported in this paper) significantly larger sections of block material can break-off. There seems little benefit in using higher strength mortars with comparatively weaker blocks.

Fig. 2 Failure ofblocklmortar interface

Relationship befween Inortar and bond strengths is variable, figure 3. For example, a 112.5% increase in strength of the cement:lime:sand mortars (mixes 1 :2:9 and 1:3: 12) resulted in a 27% increase in bond strength, table 3. For the soi! A based mortars (5% and 10% cement) there was a 18.5% increase in bond strength in response to a 180% increase in mortar strength. However, assessing the performance of piers using soil A mortar (+ 5% cement) and soil B mortar (+ 10% cement), relative bond strength decreased by over 80% compared to an increase in mortar strength of over 130%. Perhaps not surprisingly, high mOrtar c1ay contents would se em detrimental to bond strengths. Mortar strength alone is not indicative of likely bond strength.

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0.25

---ro p..

~ '-'

0.2 .s OD s::: <1) .... ti

"" 0.15 s::: o .o Cã .... ;:l >< 0.1 <1)

c;::;

.~ til

.i: E 0.05 u ro .... ro ..c:

U O

0.82 1.6 1.9 2.3 3.4

Mortar compressive strength (MPa)

Fig. 3 Distribution of flexural bond strength with mortilr strength

Comparing the performance of cement:lime:sand and soil based mortars, then low cJay content soil based mortars appear most suitable. Mortar using soil A with only 5% cement provided a 10% stronger bond compared to the 1:3: 12 mix, whilst using approximately 30% less cement. However, the differences in bond strength and cement usage between the soil A with 10% cement and 1:2:9 mortars are less marked, table 3.

Characteristic bond strengths are very cJear1y a function ofblock moisture content at the time of construction, figure 4. Behaviour is similar to that observed with conventional masonry units [4] and previous studies involving earth blocks [5]. Bond strengths are weakest when block moisture contents are very low (oven dry) and very high (saturated). Between these two extremes there is a unique optimum moisture content corresponding to maximum bond strength. When the block is very dry water is rapidly sucked out of the mortar preventing good adhesion and proper hydration of the cement. When the block is very wet mortar tends to float on the surface without proper adhesion.

Optimum moisture contents depend on mortar type. For both the 1:3 :12 and soil A mortars optimum moisture content is around 7%, ar approximately half the total water absorption value for the test blocks. However, soil B based mortar optimum moisture content is much higher, at around 11 .5%. This might be ascribed to the comparatively high cJay content of the soil B mortar. Observations of the failed joint after testing showed very little penetration ofthe sail B mortar into the block surfaces when laid very dry. High suction rates tended to rapidly dry mortar along the interface. Higher moisture contents ensured more efficient bonding, with greater penetration of the high cJay content mortar into the block surface.

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0.2 ,....., o:l o..

::E '-" 0.16 t c Q) -" ~ ... til

. -o 0.12 c o .D

~ ~ x 0.08 Q)

t;:: <.)

'';:: til

'C E 0.04 - x ~ .. ~ ... x ~

.J: U

O

O

-- ..... - ..... ,..,.-- "

4

..... .,$ ..... , .. .... .....

.x::-", " " o

_____ 1:3:12

..... "o ..... 00

..... 00

' ..... 000

..... , 0"")( ..... ..

- .. - Soil A + 5% cement

" " ·x· " . Soil B + 10% cement

8 12

Block moisture content (%)

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Fig. 4 Relationship between block moisture content and flexural bond strength

A limited series of bond tests were also undertaken at 1, 3, and 28 days age. Whilst mortar strength increased with age as expected, figure 5, there was little significant change in bond strength after day 1. Indeed after 3 days strengths decreased, although with the typical variation in performance this decrease is not considered significant. Similar behaviour has been reported with both concrete and clay unit masonry [12]. The behaviour here may be due to block strength goveming joint failure, although surface fracture was not widely observed. Perhaps growth and strength development of the mechanical bond ceases after the initial set. Further work on this aspect of behaviour is warranted.

6. CONCLUSIONS

Following this work the following conclusions may be drawn.

• There are three distinctive modes of failure in flexure depending on whether joint interface strength, block strength, or mortar strength govems. In cases where higher strength mortars are matched with weaker blocks, joint capacity is likely to be govemed by the out-of-plane flexural strength of the unit. There is little or no strength gain in using leaner mortars with weak earth blocks.

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• Relationship between mortar strength and flexural bond strength is variable. Bond capacity is highly dependant on mortar type. High mortar clay contents appear to be especially detrimental to bond strength.

• To optimise bond strength low c1ay content soi! mortars may be preferred to similar cement:lime:sand mortars.

• B10ck moisture content has a significant influence on final bond strength. Optimum moisture content are mortar type dependent. A higher c1ay content in the mortar has raised the optimum block moisture content.

• After the first 24 hours from construction, no improvement in bond strength was recorded up to 28 days age.

1.5 -r-------------------------,

---- Bond strength i 0.5 -o- Mortar stren~

OG--------+---~---+_------~

o 5 10 15 20 25 30

Age (days)

Fig. 5 Development of bond strength with age

7. ACKNOWLEDGEMENTS

The author wishes to acknowledge the financiaI support of the Australian Research Council (SmaU Grant), and thank staff in the Department of Resource Engineering, University ofNew England for their assistance with this project.

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8. REFERENCES

1. Houben, H. and Guillaud, H., "Earth Construction: A Comprehensive Guide", Intermediate Technology Publications, London, UK, 1994.

2. Standards Australia, "Masonry code", AS 3700, Sydney, 1988. 3. Hendry, A., "Structural Masonry", Macmillan, London, UK, 1990. 4. Sinha, B.P., "Factors affecting the brickJmortar interface bond strength",

International Journal ofMasonry Construction, 3, No. 1, 1983, pp. 14-18. 5. Venu Madhava Rao, K., Ventakarama Reddy, B.V., and Jagadish, K.S ., "Flexural

bond strength ofmasonry using various blocksand mortars", MateriaIs & Structures, 29, March 1996, pp. 119-124.

6. Standards Australia, "Methods of testing soils for engineering purposes", AS 1289.5.1.1 , Sydney, 1993.

7. Standards Australia, "Methods of sampling and testing mortar for masonry construction", AS 2701 , Sydney, 1984.

8. Standards Australia, "Masonry units and segmental pavers - Methods of test", AS 4456.8, Sydney, 1997.

9. Middleton, G.F. (revised by Schneider, L.M.), "Earth-Wall Construction", Bulletin 5, CSIRO Division ofBuilding, Construction and Engineering, Fourth Edition, Sydney, Australia, 1992.

1 O.Standards Australia, "Concrete masonry units", AS 2733 , Sydney, 1984. 11 .American Society for Testing and MateriaIs, "Wetting and drying compacted soil­

cement mixtures", D559, Philadelphia, USA, 1989. 12.Sise, A., Shrive, N.G., and Jessop, E.L., "Flexural bond strength of masonry stack

prisms", Proceedings ofthe British Ceramic Society, 2,1988, pp. 103-107.

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