11th INTERNATIONAL BRICKlBLOCK MASONRY CONFERENCE

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

CHARACTERISTICS OF PRESSED EARTH BLOCKS IN COMPRESSION

Peter WalkerJ

1. ABSTRACT

The paper reviews current compressive strength test procedures and performance criteria requirements for cement stabilised pressed earth blocks. An experimental program undertaken to assess the influence of constituent materiaIs, specimen moisture content at testing, and unit geometry on compressive strength is outlined. Cement stabilised pressed earth blocks have been fabricated using a range of blended soil mixes and compacted using a constant volume manual press. The compressive strength of cement stabilised blocks is shown to improve with binder content and be impaired by increasing clay content. Soaking in water typically reduces compressive strength by 50%. Nominal block compressive strengths are closely rclated to unit aspect ratios (heightJthickness). Correction factors, to provide unconfined strength values, originally developed for fired clay and concrete masonry seem applicable to cement stabilised pr::ssed earth blocks.

2. INTRODUCTION AND REVIEW

As an environrnentally sustainable building material pressed earth blocks offer many advantages, including reduced energy consumption and a decreasing reliance on non­renewable resources. Pressed earth blocks maximise use of locally available materials, require relatively simple technology for their manufacture, and offer favourable insulating characteristics. Although around one-third of the world' s population live in some form ofunbaked earth structure [1], to date there have been very few enginecring studies of earthen wall characteristics. Like many other low cost low energy building materiaIs much research and development work is required. This paper discusses some

Keywords: Earth Blocks; Compression; Testing; Aspect Ratio; Unconfined Strength.

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

ofthe issues relating to compressive strength testing and properties of cement stabilised pressed earth blocks.

Compressive strength test procedures for earth blocks generally follow those used for concrete and c1ay masonry units [2]. Confined uniaxial compression tests are typically undertaken on samples sandwiched between two thin plywood sheets, although some specifications prefer that blocks are capped with mortar or plaster. Specified loading rates, similar to those used for other masonry units, vary between 1.0 and 4.0 MPalmin.

Minimum compressive strength requirements are often specified using wet ('saturated') values, varying between 1.0 and 2.8 MPa [1,3-9], although some prefer testing under air dry conditions [10]. Testing when dry is more indicative of in service moisture conditions, and for unstabilised earth materiais is the only suitable moisture condition. However, as in-service stresses are typically only a fraction of strength values, compressive strength requirements are more an indication of general suitability for construction (e.g. handling, robustness, durability) than load carrying capacity.

Many of the test procedures reviewed use a minimum average batch value for strength specification, with little consideration given to statistical variation. However, in common with the standard for other masonry units [2] Australian document Bulletin 5 [10] uses a 95% characteristic strength for each test batch. Other publications have applied a similar approach [3,5,7,11]. Recommended rates for testing typically require 5 samples from every consignrnent ofbetween 5000 and 20 000 blocks manufactured.

Remarkably only one of the documents reviewed takes into consideration the influence ofblock aspect ratio on compressive strength [10]. However, unless a standard test unit geometry is adopted specifications for compressjve strength performance have limited general application. In order to relate specimen strengths irrespective of their geometry correction factors for aspect ratio are applied to confined test results to yield a standard 'unconfined' value. Correction factors used in Bulletin 5 have been taken directly from predecessors of the Australian masonry unit standard AS 4456.4 [2], and represent results of tests undertaken on ca1cium silicate bricks and masonry [12] and earlier wall tests [13]. These correction factors are outlined below during discussion of the experimental results. However, other work has suggested that these correction factors may not be generally suitable for cement stabilised pressed earth blocks [14].

Results of an experimental program undertaken to assess the influence of constitUent materiais (c1ay content, cement content), specimen moisture content at testing (wet or dry), and unit geometry (aspect ratio) on unit compressive strength are outlined be1ow. Experimental results are discussed in light of the current situation and proposals to standardise testing procedures outlined.

3. EXPERIMENTAL DETAILS

Blocks were fabricated using a range of blended soil mixes, formed by combining in varying proportions a residual c1ay soil and building sand. The c1ay soil had liquid limit = 69% and plasticity index = 38%. Partic1e size distributions ofboth soils determined in

2

accordance with AS 1289.3.6.2 [15] are outlined in Table 1 below. The clay soil and building sand were combined in varying proportions by mass between 90% clay soil:l0% building sand and 20% clay soil:80% building sand. Each blended soil mix was stabilised using a general purpose Portland cement in soil:cement proportions of 20:1 (5%) and 10:1 (10%) by mass.

Table 1. Soil Particle Size Distributions

Grading Clay soil Building Sand Fine graveI fraction (2 - 6 mm) 25% 4% Coarse sand fraction (0.6 - 2 mm) 20% 36% Medium sand fraction (0.2 - 0.6 mm) 3% 31% Fine sand fraction (0.06 - 0.2 mm) 4% 22% Silt fraction (0.002 - 0.06 mm) 4% 4% Clay fraction « 0.002 mm) 44% 3%

'.,'

Fig. 1 Block press

3

A . ~anually operated constant volume press · was used for production of the blocks, figure 1: Because of their comparativeiy low cost, simple construction, ease of maintenance and high mobility, such pressesare ideally suited to developing countries. A comparison of costs and production rates has indicated that low compaction presses often prove the most economical solution [16]. Under normal operation the single acting ram presses are capable of developing compaction pressures up to 4 MN/m2

• For research. described in this paper the compaction pressures were maintained throughout at 2 MN/m2

• The press used for this study produces blocks with nominal dimensions 295 mm (length) x 140 mm (width) x 120 mm (height). For smaller scale blocks appropriate spacers and dividers are placed inside the main mould.

Ali block production was undertaken in the Dept. of Resource Engineering laboratories at the University of New England. Prior to mixing the clay soil was broken down after air drying and passed through a 5 mm screen sieve. Macro-organic matter (roots, leaves) was removed at this stage. After air-drying the sand was also passed through the 5 mm screen sieve. The soils were initially mixed together in a 70 litre capacity drum concrete mixer prior to adding the cement. Once ali dry materiaIs had been thoroughly mixed the water was gradually added until the standard Proctor optimum moisture content for the mix was attained

Once ali mixing was completed block pressing commenced immediately. On extraction from the press each fresh block was stored for curing under polythene sheeting. During their first seven days the blocks were moistened daily, and thereafter they were stacked under the plastic until 28 days old, when they were prepared for testing. Both full-scale (295 x 140 x 120 mm) and small-scale blocks were tested in compression. Small-scale blocks were obtained indirectly from cutting up full-scale blocks, and fabricated directly with the press using spacers and dividers in the mould.

Five blocks from each mix consignrnent were randomly selected for wet uniaxial compressive strength testing and five for dry testing. In preparation for wet testing blocks were immersed in a curing tank for 24 hours. Prior to dry testing blocks were oven dried to constant mass. Ali blocks were loaded in their as pressed aspect in uniaxial compression uncapped between two 4 mm thick sheets of plywood. Stress was applied continuously without shock up to failure at a rate of 3.5 MPa/min. Individual unit compressive strengths were obtained from maximum load divided by average bed face area. Characteristic compressive strengths for the batches were determined from [10]:

Characteristic strength = A verage strength - 1.65 x Unbiased standard deviation.

4. DISCUSSION OF RESUL TS

A program of full-scale block tests were undertaken to assess the influence of constituent materiaIs and moisture content at testing on compressive strength. A later program of small-scale tests were conducted to assess the influence of unit aspect ratio on compressive strength values. Results from these two programs are discussed separately below.

4

4.1 Full-scale tests

The mode of failure in compression was similar to that commonly observed in standard cube tests. All blocks exhibited the c1assic hour-glass shape afier spalling of the four vertical sides with onset of failure. Compressive strength results for the full-scale tests are presented in table 2 below. For constant compactive effort block compressive strength is a function of cement content and c1ay content. There is a steady, near-linear, decrease in strength with increasing soil plasticity index, figure 2. Although a small quantity is necessary for production c1ay content is c1early, and as expected, detrimental to the wet compressive strength of cement stabilised pressed earth blocks.

Table 2. Results of full-scale tests

Cement Soil Average Characteristic Characteristic content plasticity dry density Wet Dry

index compressive strength compressive (By Mass) (kg/m3

) (MPa) strength (MPa) 21.8 1452 1.84 3.80 19.8 1520 2.50 4.65

10% 15.7 1620 2.99 6.63 7.2 1671 4.45 7.97 3.1 1757 5.22 9.60

N.P. 1775 6.35 12.54 21.8 1408 0.08 1.72 19.8 1503 0.30 1.79

5% 15.7 1584 1.23 3.48 7.2 1659 2.24 5.51 3.1 1697 · 2.77 6.31

N.P. 1733 3.36 6.88

Reduction in strength with increasing plasticity index may be attributed to weakening of the bond between the cement paste and sand/gravel structure by c1ay minerais. Although cement has a stabilising effect on c1ay minerais, the colloidal/cement structure will not be as strong as that of the sand/gravel and cement paste. Stabilisation is also impaired by the presence of uncemented cohesive aggregations which may forIIÍ during wet mixing. As expected wet compressive strength is improved by increasing cement content, table 2 and figure 2. Past studies support a linear relationship between cement content and wet compressive resistance [16].

For ali blended soil mixes dry compressive strengths exceed the corresponding wet values. The reduction in strength with soaking was typically between 40% and 60%, but for some high c1ay and 5% cement mixes the drop in strength increased to as much as 95%, table 2. This decrease in strength can be attributed to pore water pressures and liquefaction of unstabilised c1ay minerais. Despite the drop in strength with wetting, the effects of cement and plasticity on dry strength are similar to those outlined for wet performance. In this particular study the Q.j fferences in wet and dry strengths corresponded to a 50% reduction in cement content, figure 2.

5

14r-----------------------------------____ ~

l 12

6 t 10

~ ~ 8

' ;n CIl

t .~ ti

j U

4

2

-' . .......

. -x - _____ . __ )(.

". " .

" .-.

- - + - -10% Cement (Wet)

- . ~ .. -5% Cement (Wet)

---+-10% Cement (Dry)

-x- 5% Cement (Dry)

o+-------~------~------_+------~~~--~ o 5 10 15 20 25

Plasticity index

Fig. 2 Variation of compressive strength with plasticity index: fvll-scale blocks

Even from these limited data it is c1ear that no consistent relationship exists between dry and wet strengths. Dry testing cannot therefore provi de a re1iable indication of wet performance. Saturation by immersion for 24 hours is readily controlled and an easily attained moisture condition for laboratory testing. However, testing samples when wet may run the risk of rejecting blocks which are perfectly adequate under drier service moisture conditions.

Due to effects such as platen restraint and composite material interaction the standard compressive strength test for masonry units is not representative of in-service stress conditions. Wetting of a sample is therefore no less representative of in-service conditions than application of a uniaxial compressive stress at a constant rate through plywood sheets or plaster/cement caps. Therefore, uniaxial compressive strength testing of cement stabilised pressed earth blocks when saturated by immersion is preferred, as the method yields a minimum lower bound performance at a readily attained and controlled moisture condition. The test may also prove indicative of durability. However, wet testing is c1early not suitable for unstabilised earthen materiais such as adobe.

6

4.2 Small-scale tests

Small-scale blocks were obtained indirectly from cutting up full-scale samples of the 6.35 MPa (wet strength) blocks outlined in table 2. The cut blocks had a standard nominal cross-section 40 x 20 mm and six different heights varying between 12 and 107 mm, table 3. The different sized blocks were cut randomly from the full-scale units but orientated as pressed. Small-scale blocks were also directly fabricated by pressing samples using the manual press. A blended soil mix comprised of 40% clay soil:60% building sand (PI = 7.2%) was chosen and stabilised with 5% cement. The pressed blocks had standard nominal cross-section 140 x 70 mm with four different heights varying between 23 and 113 mm, table 3. Ali blocks were compression tested in their as pressed aspect using the procedure outlined above for full-scale units. At each aspect ratio five blocks from both the cut and pressed samples were randomly chosen for both wet and dry testing.

Table 3. Results of small-scale tests

Cement Soil Average Aspect Av. dry Charac. Charac. Comment content PI height ratio density wet dry

strength strength (mm) (a,) (kg/m3

) (MPa) (MPa) 23 0.33 1644 3.40 7.47

5% 7.2 47 0.67 1620 l.l8 2.96 Pressed 77 l.l0 1616 0.95 2.24 blocks

113 1.61 1567 0.54 0.96 12 0.62 5.68 7.77 22 l.l0 4.70 6.43

10% N.P. 41 2.04 1775 3.99 2.92 Cut 64 3.20 3.06 5.00 blocks 86 4.30 2.14 3.41

107 5.35 2.62 2.29

As platen restraint effects reduced with increasing aspect ratio then apparent compressive strength values decreased, table 3. Figure 3 shows experimental characteristic values attained at each aspect ratio together with best-fit curves. As expected strength reduction was most rapid when specimen aspect ratios were less than 2. There was little change in compressive strength once aspect ratio values had exceeded 3-to-5. This behaviour was similar irrespective of moisture condition at testing. For aspect ratios less than 2 the observed mo de of failure was in the manner outlined above for the full-scale samples. However, at higher aspect ratios there was often a noticeable diagonal shearing of the specimen as well as the material spalling associated with crushing.

Compressive strength "'behaviour for the fabricated blocks was also influenced by reduction in dry density. 'R"eduction in compressive strength with change in geometry may thus be ascribed to both aspect ratio and decreasing material density effects, This might explain the more rapid strength decreases observed compared with the cut blocks, figure 3, Although ali fabricated blocks were subject to equal compactive effort, density

7

reduction might be attributed to greater side friction effects during compaction as the specimen height increasedo

~

~

1 '" ~ otn

'" ~ o o o o;::

'" OI::

J U

10r---~------------------------------~ __ --, I

8

6

4

2

I I I

" I \: xl t! I

I " \: \'

I x \

" + \ \

"\ \: j: " \:

.,\ \I " .

.... --:,

x

x

---

- +-. Pressed (Wet)

..•. o Pressed (Dry)

-+- Cut (Wet)

- ·x - Cut (Dry)

---;-----~---------~----~----------------o+o--~

I·· ..• '\ . ......... . . . .. ....... .. ........ .. ........ .. .... .. .......... .... ........ .. .................... .. ....

X +

'~'-,. -.~.-.-.-.-.-.-.-.-.- .-.-.-.- . -.-.-.-•

O+-------~----_+------~------~------~----~ O 2 3 4 5 6

Aspect Ratio (a,.)

Figo 3 Variation of compressive strength with aspect ratio : small-scale blocks

Figure 4 is a plot of correction factor (K. = unconfined strength/confined strength) o against aspect ratio, where unconfined ~ r:ngth is the experimental best-fit compressive

resistance when a,. = 50 Theoretically K. ~ I , however with experimental variation values exceeding this limit were recordedo The correction factors currently in use [10] provide generally conservative indications of the experimental behaviouro Despite significant experimental variation there seems little justification for general adoption of the altemative correction values proposed specifically for soil blocks [14]0 However, further work is required to assess the influences of specimen size, further variations soil type and the fabrication processo

50 SUMMARY AND CONCLUSIONS

From this work the following conclusions may be drawno

• Pressed earth block compressive strengths are impaired by increasing clay content, and reducing cement content. A near-linear relationship between decreasing strength and increasing soil plasticity index wasrecordedo

8

• In general saturation by immersion in water reduces unit compressive strength by 50% in comparison with corresponding oven dry values. However, at higher plasticity indices reduction in strength with wetting can be as high as 95%. Strength reduction may be attributed to pore water pressures and liquefaction of unstabilised soil.

• As compressive strength testing of earth blocks is largely an indication of overall quality and suitability, testing of cement stabilised blocks in a wet state is proposed as the preferred method of testing.

• Unit compressive strength reduces with increasing aspect ratio as the effects of platen restraint are reduced. Platen restraint has minimal effects once unit height exceeds between 3 and 5 times its thickness. At constant compactive effort the effects of increasing aspect ratio are further enhanced by the fabrication process due to a reduction in unit density.

• Aspect.ratio correction factors developed for fired cJay and concrete masonry units and prisms would seem equally suitable for pressed earth blocks.

1.2

• •

• ~

0.8 ~ • '-' .... "" B ;'

o ;' • .s ..- • 0.6 / d ~ o . /. "B . / o ,/ t:::: 0.4 o u /

/ ---+- Bulletin 5 ./

0.2 • - - - Heathcote et ai /

/ 'I

O+-----~~----_+------~------~----~------~

o 2 3 4 5 6

Aspect Ratio (a.-)

Fig. 4 ReJationship between correction factor (KJ and aspect ratio

6. ACKNOWLEDGEMENTS

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

9

7. REFERENCES

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

2. Standards Australia, "Masonry units and segmenta I pavers - Test methods", AS 4456.4, Sydney, 1997.

3. Fitzmaurice, R., "Manual on stabilised soil construction for housing", United Nations, New York, USA, 1958.

4. International Labour Office, "Small-scale manufacture of stabilised soil blocks", Technical Meniorandum No. 12, Geneva, SwitzerIand, 1983 .

5. Indian Institute of Science, "The Stabilised Mud Block", Indian Institute of Science, Bangalore, India, 1989.

6. Jagadish, K.S., Ventakarama Reddy, B.V. and Yogananda, M.R. , "Specifications for the use of mud blocks for building construction", Proceedings National Seminar on Application of Stabilised Mud Blocks in Housing and Building, Bangalore, India, 1988, pp. 19-22.

7. Indian Standards Institution, "Specification for soil-cement blocks used in general building construction", IS 1725, New Delhi, India, 1960.

8. UNCHS (Habitat), "Kenya Specification for stabilised soil blocks", Nairobi, Kenya, 1989.

9. Govemment of the Republic of Zimbabwe, "Mo dei Building By-Laws", Harare, Zimbabwe, 1977.

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

II.Hubbell, E., "Earth brick construction", US Department of the Interior, Office of Indian Affairs, Washington, USA, 1943 .

12.Page, A.W. and Marshall, R., "The influence of brick and brickwork prism aspect ratio on the evaluation of compressive strength", Proceedings ofthe 7th International Brick Masonry Conference, Melbourne, Australia, 1985, pp. 653-664.

13.Krefeld, W.J. " Effect of shape of specimen on the apparent compressive strength of brick masonry", Proceedings of the American Society of Materiais, Philadelphia, USA, 1938, pp. 363-369.

14.Heathcote, K.A. and Jankulovski, E., "Aspect ratio correction factors ' for soilcrete blocks", Australian Civil Engineering Transactions, Institution of Engineers Australia, Canberra, Australia, Vol. CE34, No. 4, 1992, pp. 309-312.

15.Standards Australia, "Methods of testing soils for engineering purposes", AS 1289.3.6.2, Sydney, 1995.

16.Gooding, D. "Quasi-Static Compression Forming of Stabilised Soil-Cement Building Blocks", Development Technology Unit Working Paper No. 40, Departrnent of Engineering, University ofWarwick, UK, 1993.

10