COMPRESSIVE STRENGTH OF MASONRY PRISMS MADE WITII AIR ENTRAINED MORTAR

1. ABSlRACf Philip 1. Evans 1

In Perth Western Australia, widespread use is made of air entraining adrnixtures, to improve mortar workability without regard to the influence on other mortar or masonry properties. This project examined the influence of air entrainment on the compressive strength of mortar and masonry prisms made using a vinsol resin adrnixture. The results indicated that whilst there was a trend of decreasing masonry compressive strength with increasing mortar air content, the maximum difference in masonry compressive strength resulting from changes in mortar type was less than 25%. Despite low mortar compressive strengths the characteristic masonry strength for each mortar type exceeded the rninimum value specified in the Australian Masonry Code.

2. INTRODUCTION

Normally, mortar rnixes are based on a cement, lime and sand composition, in the proportions by volume of 1:1:6 or 1:2:9. These rnixes have been found to provide the required performance with respect to strength, durability and workability without the necessity for the use of workability adrnixtures. The workability of these rnixes is assisted by both the lime component and the presence of a significant clay fraction in the mortar sand. When plotted on a mortar sand grading envelope (BSI, 1976), the grading curve for Perth mortar sands approaches the upper or fine grading curve. Consequent1y, mortars made with these local sands have comparatively poor workability. This lack of workability has resulted in widespread use of liquid workability admixtures which are described generally as air entraining agents. In Perth the most common type used is a neutralised vinsol resin (NVR) admixture.

3. SITE PRACTICE

Liquid air entraining adrnixtures are preferred to lime by local bricklayers since they are cheaper, easier to use and considered to give a more workable mortar when used with the local sands. Local mixes are usually specified as 1:5 or 1:6 by volume. The quality of mortars containing liquid admixtures is dependent upon accurate volume batching of the mix constituents on site and the addition of the maximum amount of admixture permitted.

Accurate volume batching is rarely employed on site and the accepted practice of shovel batching results in iocreased sand volumes and decreased cement volumes with consequent reductions in strength (McIntosh, 1970).

Additionally, to comply with the specified admixture dose rates, 25mls per 40kg of cement or 25mls per 20 litres of water, it is also necessary to accurately mix the liquid admixture

Keywords; Masonry; Mortar; Compressive Strength; Air.

1 Associate Professor and Head, Department of Construction Management, Curtin Universitv of Technolol!v. Perth. Western Australia .

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with the mixing water. The preferred local practice however is to add the admixture direct1y from the container to the mixer with no control on the amount added. The required amount being based on experience, or guess-work.

The influence of the absence of adequate site quality control procedures for mortar production is also suggested in the annual reports of the Westem Australian Builders Registration Board to the Minister for Consumer Affairs. (BRB 1985-1989). During the period 1983 to 1989, the majority of construction defects (15.5%), identified in Board orders, related to brickwork. Of these complaints it is estimated that 25% related to the quality and performance of mortar (BRB, 1990).

Whilst it is generally acknowledged that the use of air entraining adrnixtures will result in reduced strengths (McIntosh, 1970; Baker, 1984), there is liule in the literature quantifying the influence of ABA on important properties such as masonry compressive strength and flexural bond strength (SAA, 1988; Drysdale and Gazzola, 1985).

4. METIIOD AND MA TERIALS

The sand used in this investigation was a typical Perth mortar sand with a particle size distribution as shown in figure 1. It is essentially single sized with less than 2% passing the 150 micron sieve and with a frneness modulus of 1.84. An envelope of the particle size distributions of mortar sands sampled from 20 Perth construction sites is also shown on the figure. Type A ordinary portland cement and hydrated builders lime was used for comparison testing with lime mortars. The masonry units used were solid clay brick commons with a characteristic unconfined compressive strength of 33 MPa. The initial rate of absorption was 1.68 kg/m2/minute and the 24 hour cold water absorption value was 10%.

Three air entrained mixes were exarnined. They were a 1:5, 1:6 and 1:7 by volume. For comparison purposes 1:1:6 and 1:2:9 composition mortars were also batched. In the air entrained mixes the water cement ratio was kept constant and the workability was progressively increased by the addition of the liquid air entraining agent. For the composition rnixes the workability was increased by increasing the water cement ratio. A target flow of 110% plus or minus 5% was adopted however it was acknowledged that in accordance with local practice flows ranging from 90% to 130% were probable.

The admixture was initially added to the mixing water as specified by the rnanufacturer at a rate of 25m1s of adrnixture per 20 litres of mixing water. The admixture concentrations for each mix type were progressively increased by 2, 4,6,8 and 16 times the recommended amounts. At leveIs above 8 times the recommended amount the mortar tended to flow under the self weight of the masonry units and this concentration was adopted as the upper maximum practicallevel when making the prisms.

The mortar was prepared in 50 litre batches mixed with a 70 litre capacity rotating drum site mixer. The air content of the plastic mortar was deterrnined using both volumetric and gravirnetric methods.

The compressive strength of the mortar was deterrnined from prism specimens of nominal size 50mm x 25mm x 25mm tested in accordance with the provisions of part 4 of AS 2701 (SAA, 1984). Specimens were moist cured and tested in the saturated surface dry condition at 7, 28 and 90 days.

The compressive strength of the brickwork prisms was deterrnined in accordance with Appendix A of AS 3700 (SAA, 1988). The compression machine used was designed for the testing of standard concrete test cylinders and the specimen size had to be modified to accommodate the constraints of the platen sizes. The height between the pia tens was sufficient to allow 4 course stack bonded prisms to be constructed and tested.

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After manufacture the masonry prisms were wrapped in a vapor proof sheet and left undisturbed in the laboratory for testing at 7 and 28 days. Three specimens were tested for each mortar mix and the average unconfined compressive strength calculated. The actual compressive strengths have been reported with no correction applied for the height to thickness ratio of the specimen.

5. RESULTS AND DISCUSSION

5.1 Mortar Prism Compressive Strength Table 1 records the mean results of the compressive strengths for both the air entrained and composition mortars and the masonry prisms made with these mortars.

Figure 2 shows the compressive strength of the air entrained mortar prisms at 7 and 28 days plotted against the average air content of the mortar. Each point represents the mean of three results.

The graphs indicate a general trend of decreasing strength with increasing air contento The trend is not clearly discernable with the 1:7 mix, presurnably because of the limited range of air contents (20% to 22.7%) in the mixes tested despite a threefold increase in the admixture concentration. With the exception of the 1:7 mix, the strength loss with increasing air content was similar for both 7 and 28 day ages. There was a highly significant correlation between air content and 7 and 28 day mortar compressive strengths for the 1:5 and 1:6 mixes considered both individually and together. The relationships were best fitted with a negative linear function.

The equivalence of the compressive strengths of the mortar designations can be seen from figure 2 where the 7 and 28 day compressive strengths of the mortar test specimens have been plotted against mortar air contento The compressive strengths of the 1:5 and 1:6 mortars are similar to the 1:1:6 mortars whilst there is an equivalence also between the 1:7 and 1:2:9 mortars. It is considered that the low strengths of the project mortars have occurred as a result of the poorly graded single sized project sands. This effect has been reported by Currie and Sinha (1981) and de Vekey et al (1990). The low strengths appear to result from two factors . Firstly finer or poorly graded sands require increased water contents for a given workability resulting in lower strengths and secondly single sized sands do not "pack" as well as well graded sands resulting in void spaces which also in tum result in lower strengths. On the basis of the results of the compacted bulk density tests for the project sand (1636kg/m3) and assuming that the sand is composed of silica with a specific gravity of 2.635, the project sand would have a void content of (2635 -1636)/2635 or 38%.

5.2 Masonry Prism Compressive Strength The results of compression testing carried out on the four course stack bonded masonry prisms made with both the air entrained and composition mortars are shown in Table 1. The values represent the ultimate compressive strengths deterrnined from the mean of three tests. No aspect ratio reduction factor has been applied to the results. The range of adrnixture concentrations and water cement ratios were selected to represent the driest to the wettest practical mortar workabilities as assessed by the bricklayer during the trial mix evaluations.

5.3 Air Entrained Masonry Figure 3 shows the 7 and 28 day prism compressive strengths plotted against the air content in the plastic mortar, determined from the average of the gravimetric and volumetric methods. The graphs generally show a decrease in prism compressive strength with increasing air content, with the rate of strength loss similar for each age at test for the three air entrained mixes.

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For each mix considered separately there was a significant correlation of prism compressive strength with mortar air content at each age of test with the exception of the 7 day 1:5 mix which was affected by an anomalous result at the 15.6% air content. In each case the relationship was best fitted by a negative linear function. The analysis of the combined air entrained mixes indicated a highly significant correlation of air content with masonry prism strength, the relationship again best fitted with a negative linear function.

Whilst there was a statistically significant relationship between mortar air content and masonry prism strength over the range of air contents considered, the results indicate that overall the compressive strength of the air entrained masonry is only marginally affected by the mortar type and air contento Regardless of air content and cement to sand ratio the masonry compressive strengths lie generally within the range 15 to 20MPa In figure 2 ali results have been plotted against mortar air content with the lower and upper values of air content representing the driest and and wettest workabilities respectively. Whilst there was a trend of decreasing mortar and compressive strength with increasing air content, apart from the anomalous results for the 1: 1:6 mixes which have occurred as a result of bleeding, both mortar and masonry strengths lie within a narrow band or range and there was no marked difference due to mix type. As expected there was an equivalence between the 1:5, 1:6 and 1:1:6 mix and the 1:7 and 1:2:9 mixes with respect to both mortar and masonry strengths. Over the range of practical site workabilities, the reduction in mortar and masonry compressive strengths were as shown in table 2.

The reductions in mortar strengths for both the 7 and 28 day results are similar and age does not appear to be a factor in strength loss. There is a similar trend for the reduction in masonry strengths. There also appears to be a proportional influence of mortar strengths on masonry strength, with the higher the mortar strength, the higher the prism strength. From table 2 it can be seen that in the case of the air entrained mixes the reduction in masonry prism compressive strength ranges from 0.44 to 0.67 the reduction in mortar strength.

The results generally confum the statement that mortar strength is weak1y related to masonry strength by a third or fourth root relationship as suggested by Sahlin (1971) and Hendry et al (1987).

For the stronger designations such as the 1 :5, 1:6 and 1: 1:6 mixes the relationship was approximated by a third or fourth root relationship but for the 1:7 and 1 :2:9 designations, the mortar strength was approximated by the fifth root of the masonry compressive strength. The unconfmed compressive strength of the unit also appears to have a strong influence on the masonry compressive strength regardless of mortar type. As shown in table 3, the ratio of characteristic masonry compressive strength to the root of the unit unconfined compressive strength indicated values slightly higher that those specified in Note 6 of Table 4.1 in the Australian Code.

As shown in figures 4 and 5 if the mortar strengths are plotted against the masonry strengths, for the range of mortar types considered, there is a significant linear correlation between the two properties. A comparison of the mean and characteristic masonry compressive strengths is shown in table 4 for each mix type. Coefficients of variations are shown in brackets. The design characteristic strength has been determined in accordance with clause 4.5.1 of the Masonry Code (SAA, 1988) based on a masonry unit unconfined compressive strength of 33.68 MPa. Figures 6 and 7 show the masonry prism strength plotted against mortar air content for the 7 and 28 day strengths. The characteristic strength based on the AS 3700 code provisions (AS 3700 CHAR) and characteristic strengths deterrnined from the actual coefficients of variation from the test results (ACV CHAR), together with a third characteristic value based on the strength exceeded by 95% of the test specirnens (SD CHAR), have been plotted.

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In accordance with the code, the design characteristic strength of the masonry constructed f TOm units with an unconfmed compressive strength of 33.68 MPa is 8.12 MPa for the 1:5, 1:6, and 1:1:6 mortars and 6.38 MPa for the 1:7 and 1:2:9 mortars. On the basis of the test results each masonry type exceeds the minimum specified compressive strength. In addition characteristic strengths based on the actual average coefficients of variation from the test results (6% compared to the assumed 15% code value) have also been plotted. A third characteristic value based on the strength exceeded by 95% of the test specimens has also been plotted. In this expression the characteristic strength factor from the code has been taken as unity and the lower five percentile value for the set of test results measured f TOm a relative cumulative frequency distribution of the test data has been calculated assuming the frequency curve is normally distributed (Mean - 1.65 SD). Again each of these characteristic values exceed the maximum specified values.

The relationship between the mean strength and the code characteristic strength (fm) generally confinn the target strength expression in section 8.5.4 of the code (Compressive Strength = 1.4 fm).

Figures 8 and 9 show the characteristic masonry compressive strength plotted against the mean masonry strength for the 7 and 28 day ages. The correlation is highly significant for each age of test Neglecting the intercept on the vertical axis and considering the inverse of the slope of the curve, gave a target average strength factor of 1.57 for the 7 day test and 1.85 for the 28 day tests. The 1.57 value compares favorably with the code value of 1.4.

6.0 CONCLUSIONS

The compressive strength of the masonry prisms decreased with increasing mortar air content but the maximum difference between prism compressive strength for any mortar designation was 5 MPa. The reduction in prism compressive strength ranged from 44% to 67% of the reduction in mortar strengths which occurred as a result of increased air contento

For the range of mortar types and masonry unit selected there was a strong correlation between mortar strength and masonry prism strength. The unconfined compressive strength of the masonry unit had a strong influence on masonry strength with the ratio of masonry strength to the root of unit strength indicating factors higher than those specified in AS 3700.

Despite low values of mortar compressive strength, the characteristic strength of the masonry prisms exceeded the minimum values specified in AS 3700 and there was a strong correlation between masonry prism strength and characteristic strength.

7. ACKNOWLEDGEMENTS

The author wishes to acknowledge the support of the Midland Brick Company, Cockburn Cement Ltd., and Cement Aids in providing the materiaIs used in this project, and the support of the School of Architecture and Planning of the Curtin University of Technology.

8.0 REFERENCES

British Standards Institution., "Specification for Building Sands from Natural Sources," BS 1200, 1976. McIntosh, I .D., "Specifying the Quality of Bedding Mortars," Proceedings, British Cerarnic Society, No. 17. February,1970, pp 65-81. Builders Registration Board. , "Annual RepoTt," Perth, W.A., 1985-1989. Builders Registration Board., "Private Communication," Perth, W.A.,1990.

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Baker, L.R. (Ed) "Masonry Code of Practice," Association of Consulting Engineers, N.S.W., Sydney, 1984. Standards Association of Australia, "S.AA. Masonry Code," AS. 3700, S.A.A. North Sydney, 1988. Drysdale, R.G. and Gazzola, A.E., "Influence of Mortar Properties on the Tensile Strength of Brick Masonry," Proceedings 7th International Brick Masonry Conference, Melbourne, Vic. 1985, pp 191-203. Standards Association of Australia, "Methods of Sampling and Testing Mortars for Masonry Construction," AS. 2701, S.A.A. North Sydney, 1984 Currie, D. and Sinha, B., "Survey of Scottish Sands and Their Characteristics which Affect Mortar Strength," Magazine of Chemistry and Industry September,1981, pp 639-645. De Vekey, R.C, Edgell, G.J and Dukes, R., "The Effect of Sand Grading on the Performance and Properties of Masonry," Proceedings, British Masonry Socíety. No 4 1990, pp 152-159. Sahlin, S.,"Structural Masonry," Prentice-Hall, New Jersey. 1971. Hendry, A.W., Sinha, B.P. and Davies, S.R., "Load Bearing Brickwork Design," Ellis Horwood, Chichester, U.K. 1987.

9.0 TABLES AND FIGURES

Table I; Compressive Strength Results

MIX AIR COlVIPRESSIVE STRENGTH (:liIPa) MORTAR MAS01'."RY

(%) 7DAY 28 DAY 7DAY 28 DAY 1:5+2A 15.8 1.99 2.85 19.69 20.94 1:5+4A 15.6 2.07 2.82 16.80 .- 21.20 1:5+6A 22.0 1.49 1.94 15.90 18.14 1:5+8A 20.6 1.61 2.27 16.99 18.52

1:6+2A 14.2 2.06 2.73 17 .80 19.80 1:6+4A 22.2 1.04 1.58 14.24 16.15 1:6+6A 19.3 1 1.25 2.05 16.96 19.49 1:6+8A 21.1 1.24 2.08 1370 15.50

1:7+2A 20.5 1.06 2.16 13.90 18.32 1:7+4A 22.7 0.71 1.45 14.65 16.75 1:7+6A 20.0 1.02 1.82 1 16.26 21.17 1:7+8A 21.7 1.15 1.50 16.00 19.05

I 1: 1:6 7.4 I l.721 2.64 1 19.22 23.03 1: 1:6 ij 8.5 1 1.63 2.86 1 22.06 23.11 1: 1:6 , 4.2 . 2.09 309 1 17.58 20.33 1:1:6 i 2.6 l.87 2.961 16.26 21.04

~ 1:2:9 ! 10.1 1.12 1.75 12.96 17.90 1:2:9 ~ 11.2 1.17 1.68 12.641 18.68 1:2:9 I 8.4 0.96 1.41 14.90 18.30 1:2:9 I 6.5 0.85 1.31 13.15 19.81

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Table 2; S trength Red uction

Mix Type Age Air Range Mortar Masonry Strength Strength Reduction Reduction

Days % % %

1:5 7 6.2 28.0 I 19.2 28 6.2 25.0 13.4

1:6 7 8.0 49 .5 23.0 28 8.0 42.0 I 21.7

1:7 7 2.2 33.0 14.5 28 2.2 33.0 20.9

Table 3; Ratio Prism / Unit Strength

lVlix Project Results Code Provision

Masoruy Masonrv

-JfJrut -JfJrut 1:5 1.64 IA

1:6 1.43 IA

1:7 1.40 1.1 1:1:6 1.79 1.4 1:2:9 1.24 1.1

Table 4; Characteristic Compressive Strengths

ylix AS 3700 l 7 Day Comp. ()'fPa) 28 Day Comp. C\IPa) Value Character Mean Character 1 :'.lean

1:5+2A 8.12 1 11.68 19.69(6.2) 12.28 1 20 . 9~ (8.7)

1:5+4A 8.121 10.191 16.80(31) 12.08 1 21.20(9.3) 1:5+6A 8.121 9.~9 1590(3.2) 10.98 18.H(2.2)

1 1:5+8A 8.12 1 9.78 16.9900.1 ) 11.48 18.52(0 1) I I

1:6+2A 8.12 I 10.17 17.80(7.3) llA8 19.8016.5) 1:6+4A 8.12 i 8.48 1~.24(56) 9.88 1 16.15( 1.5) 1:6+6A 8.12 1 8.881 16.9603.5) 12.18 19.49\2.2 ) 1:6+8A 812 1 8.38 1 13.70(1.8) 9.49 15.50(1.6)

1 1:7+2A 6.38 1 7.29 13.9005.6) 11.98 18.32(8.2) 1:7+4A 6.381 7.79 14.65(14.3) 9.59 16.75(11.2) 1:7+6A 6.381 9.68 16.26(3.4) 12.68 21.17(4.8 ) 1:7+8A 6.38 1 10.39 16.00(3.9) 11.68 19.05(1.0)

I 1 1: 1:6 8.12 1 11.28 19.22(9.2) 13.97 23.03(2.1 ) 1: 1:6 8.12 13.38 22.06(3.1) 13.68 1 23.1 H6..1 ) 1: 1:6 8.12 10.69 1 17.58(2.7) 11.48 1 20.33( 12 .6) 1: 1:6 8.12 9.49 16.26(8.4) 11.68 1 21.ü4( 10.0)

I 1:2:9 6.38 1 7.64 12.96(6.1) 10.68 1790(5.9) 1:2:9 6.38 l 7.69 12.64( 1.7) 11.23 1868(3.0) 1:2:9 6.38 1 898 14.90(2.5) 10.88 18.30(3.7) 1:2:9 6.38 1 6.79 13.15(14.6) 12.08 19 8H2.3)

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1:5 25

20

15

COMPRESSIVE

10

O

15 li 1~ li 206 22

PERCENT PASSING

0.05 OC8 0 .1 5 0_3 0<6.3 0 .6 lol ! 2 _38 ".15

PARTICLE SIZE (mm)

I O- FINE LlMIT -fr COARSE LlMIT - PROJECT SANO I

Figure l ' Sand Grading

1:6 1:7 1:1:6

AIR CONTENT ("lo)

<2> 7 Day Mor '" 28 Day Mor O 7 Day Mas {'; 28 Day Mas

Figure 2; Mortar a~d Prism Strength Comparison

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1 :2'9

" ~

20 o

Q.

~ 19 1: ... Cl 16 z W cr: 17 .... U> a. :;; 16 O U > '5 cr: z O ,. U> <t :;; '3

Figure

70AY COMPAESSIVE

STRENGTH (MP.)

4·

25

15

10

COMPAESSlVE STRENGTH

(MPa)

1;5 1:6 1:7

. l~~. ~I ! <f'~ ~ ~.

J . 01 L.....--...-~ . .. _ . .. .. . . . -----....JH!

AIR CONTENT (~.)

07 DAY 628 DAY

Figure 3; Strength v Air Contem

AIR óNTRAJNED MIXES AIR ~NTRA:NEO MIX::S

y = 12.0659.2.8802:1 R = 0.7.

;/ c o

c

~ 7DAY C

, MORTAR COMP STRENGTH (MPa)

7 Day Mortar v Masonry Strength

t :5

156 158 206 22 142 19 J 21 I 22 2

22 o a. ~ 21 :I: .... Cl Z

20 w a: 19 ... U> o. :;; 18 O u

17 > cr: z O , 6 U> <t :;; '5

Figure

20 205 21.7 227

AIA CONTENT .,~

y = i4 .1239 + 2.2393x R = 0.58 c

t:J o c

c c

c 28 DAY

MORTAR COMP STRENGTH (MPa)

5; 28 Day Mortar v Masonry

1:1 :6

26 42 7 J 85 65 84 10 I 112

Figure 6; 7 Day. Air Contem v Prism Strength

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o a

Strength

28 DAY COMPRESSIVE

STRENGTH (MP.)

20

15

10

15.6 158 20.6 22 14 2 193 21.1 222 20 20521.7 227

AJR CONTENT (-J.)

1:1:6

26 <42 7.4 6 .5

OMEAN {':, A53700 CHAA 050 CHAA ~ ACV CHAA

Figure 7· 28 Day, Air Content v Prism Strength

~ o-~ J: >-" z w a: >-

'" o-::; o u a: « J: u

Figure

~ 0-

~ J: >-

" z w ~ '" o-::; o u a: « ::t: u

AIA :NTRAINEO & CCMPosmON MIXES 14~--~------------------------,-~

12

10

12

8;

15

" 13

12

11

10

14

Y ~ ·0.8186 + 0.6358x R = 0.95

~ ~

~

(J . (J

c ~ 7DAY

14 16 t8 20 22 MASONRY COMP STRENGTH (MP.)

24

7 Day, Mean v Characteristic Strength

AIR ENTRAINED a. COMPOSI71CN MIXES

y • 1.0392. 0.5427x R. 0.95

28 DAY ~

16 18 20 22 24 MASONRY COM? STRENGTH (MP.)

Figure 9; 28 Day, Mean v Characteristic Strength

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1·2:9

6.5 8.4 10.1 11 .2