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FACTORS INFLUENCING GROUTEO MASONRY PRISM COMPRESSIVE STRENGTH

J C SCRIVENER Professor of Building

The University of Melbourne Parkville, Victoria, Australia 3052

L R BAKER Director, Masonry Research Centre

Deakin lIniversity Waurn Ponds, Victoria, Australia 3217

ABSTRACT

The paper reports results of a testing program on hollow grouted masonry prisms subjected to axial compressive loads to failure. Concrete and clay units, stack-honded and running-bonded, were used and the effect of changes in the compressive strength of the grout was investigated. The grout compressive strength was determined both by tests on grout poured into non-absorbent moulds and on grout removed from a hollow core after contact with the absorbent masonry. l\xi al stra i n measurements were taken and Young's Modulus of the prisms calculated.

The results and conclusions of the test program are comparerl with those of other researchers and the provisions of the draft masonry code in Austral i a.

INTROOUCTION

Some early work on the compress i ve strength of grouted hollow block concrete masonry was conducted in .l\ustralia by Isaacs [6J who made an empirical fit to experimental results in terms of block and grout strengths.

Later more extensive experimental programs were carried out by Drysdale et al [3,4,5,121. The f irst tests [31 showed that the strengths of ungrouted prisms ano grouterl portions could not be simply added to prerlict strengths of grnuted prisms. A failure criterion [5J tookinto account both the strength and strain di fferences of the units, mortar and grout. Unfortunately both the tests and the theory were on half blocks, fully bedded with al igning webs and the prisms had an aspect ratio of only three so that platen restraint could have harl a significant influence. Nevertheless the work clearly showed that·, tak ing the strength of the

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ungrouted prism as datum, the grout was only about 30% effective in the grouted prism. Subsequently Drysdale and Hamid [4J observed that the compressive strength of masonry assemblages will be less when the webs in successive courses do not align so work was carried out on running bond specimens [12J. Ungrouted prisms were face shell bedded while grouted prisms were fu11y bedded. Grouted prisms failed by the splitting of the face she11s leaving the grout undamagerl. Using the ungrouted prism strength as datum, the writers have interpreted the grout in the grouted prisms to be 46% effective in 2 high prisms ranging to 24% effective in 5 high prisms.

Priestley anrl Chai [81 argued that ohserved strain measurements indicate that grout reaches approximately 90% of its peak stress when the face shells reach their peak stress.

Using a finite element analysis Shrive [9J showed that capping and height/width affect the stress d-istribution within a prism. His recommendation that prisms of height/wirlth of five should be used to give the unconfined compressive strength has heen adopted in the draft Australian masonry code [lll.

Maurenbrecher [71 concluded that there is little rlifference in compressive strength between stack-honrled and running-bonded specimens.

The draft Australian masonry code [111 proposes that the strength of a grouted pri sm be expressedin terms of the strength of a face shell bedderl ungrouted pri sm and the strength of the grouted area. The foll owi ng program of testing was carried out hecause of the widely varying estimates af the effectiveness of the grout and the rlesirability of tests on Australian hollow units.

OESCRIPTION OF MATERIAlS ANO TESTS

The parameters investigaterl were:

(i) hollow concrete units and hollow clay units; (ii) stack-bonded prisms and running-bonded prisms; (iii) strengths of in-situ grout and of specimens constructed in non­

absorbent cylinder moulds ; (iv) vertical load deformation characteristics; (v) testing with full cross section plywoorl capping and with capping

covering the face shells only; (vi) the relationship between prism strength anrl unit strength.

In order to restrict the number of parameters to be investigated the following rlecisions were taken:

(i) One mortar mi x was used i n a 11 tests. flecause of the number of specimens and time factors it was necessary to mix mortars on severa 1 occas i ons and every attempt was made to achi eve consistency. The inevitable variation in mortar batches, as inrlicated ta some extent by the flow test results, was not considered to be a concern as prism strengths are not highly rlependent on mortar strength variations.

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(ii) 8ed thicknesses were kept constant at 10mm. (iii) Prisms of four units high were used throughout. While prism

height/least lateral dimension is a factor which influences the compressive failure load of piers, the splitting mode of failure always prevailed echoing the failure mode in masonry walls in compression.

The materials and test details follow.

Masonry Uni ts Mediumweight and heavyweight hollow concrete units 200mm high x 190mm wide x 390mm 10n9 with two rectangular cores 122mm x 145mm and hollow clay units 150mm, high x 127mm wide x 263mm long with two rectangular cores 68mm x 83mm were Ilsed.

Table 1 gives the unconfined compressive strengths of units averaged from six specimens. No adjustment was necessary from the failure loads of the units to account for height/thickness effects according to Table 2, which gives the aspect ratio factors of the draft Austral ian code [l1l. However adjustments were made for the grouted prisms.

TARLE 1 Unconfined compressive strengths of units

Materi al Full Capping Strip Capping

Co-eff • Co-eff. Load Stress of Load St ress of (kN) (MPa) variation (kN) (MPa) variation

Medium weight concrete 490 o.1i 0.10 371 15.8 0.09

Heavy weight concrete 1120 15.1 0.19 738 31.5 0.08

f:lay 512 15.3 0.08 387 23.3 0.09

"

Height/thickness ratio Aspect ratio factor

o

o

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TABLE 2 Aspect Ratio Factor

0.4 1.0

0.50 0.70

5.0 or more

1.00

Notes: The thickness used for evaluating the height/thickness ratio is: (a) for solid or cored masonry units, or for pr i sms made from such units -the overall width of the unit. (h) for hollow masonry units or for prisms made from such units - the mi nimum thickness of the face shell of the unit. This table is taken directly from the draft Australian masonry code [111.

Grouts Four different strength Portland cement grouts of high fluidity were supplied by a ready mix concrete planto The prism cores and 100mm di ameter x 200mm high cylinders were filled one day after the prisms were laid. The grout strengths are given i n Table 3.

Desi gnation

A S C O

TABLE 3 Unconfined compressive strengths of grouts

Cylinder Strength

UWa)

0.3 3.0

lli.4 22.0

Core Strength (MPa) Mediumwe i ght Heavyweight

Concrete Concrete Units Units

0.4 3.4

13.3 20.8

0.4 3.0

12.3 18.8

Clay Units

0.3 3.1

17.7 20.3

Note: Cylinder strengths are the average from six tests on 100mm diameter x 200mm high specimens formed in non-absorhent moulds. Six specimens were obta i ned by knock i ng out or cutt i ng from grouted cores of the unit S. The strengths recorded are the averages adjusted for equi val ence with specimens of aspect ratio two using ASTM correction factors [IJ. Specimens were cured as the prisms and released from the 'moulds' just prior to testing at 21 days age.

Mortar A mix of 1 part Portland cement to 1 part lime to 6 parts mortar sand was used throughout. The six separate mortar batches gave flows (see [10J) of 198,200,201,201,202 and 202% with average compressive strengths from three cylinders at 21 days of 6.3,6.8,7.6,6.8,7.3 and 7.6 MPa respectively.

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Unit Reddi ng Un it 5 we re face she 11 bedded except that the end webs were s 1 i ght 1 Y buttererl to contain the grout. The bottom unit of each prism was lairl in mortar over the full cross sectional area.

Pri sm Bondi ng The runni ng-bonderl pri srns had a full unit on the base and thi rd courses with two half units on the second and top courses. The stack-bonrlerl pr-isrns were constructed norrnally.

Pri sms Five prisms of each unit and of each grout were const r ucted and cured in t he arnhient conditions of the laboratory.

Prism Testing At 21 rlays age the prisms were testedin a Universal Test machine with hall seats at top anrl bottom. The grouterl pri Srns were pl aster capped and testerl with 3mm plywood over the full cross section while strips over the face shells wer,e used for the ungrou t ed prisms (thisis the stanrlarrl test procedure of the draft Australian corle [111). The compressive load, measured with a load cell, was applierl axially at the unifonn rate of lS0kN per minute.

Strain Measurement On some pr-isms longitudinal strains were recorded using linear rlisplacernent transdrlcers over the height of a unit inclurling a central mortar joint (see Figure 1). Four transducers were placed symmetrically on the faces of a pr i sm anrl the deformations averaged to eliminate any eccentricity effects.

PRISM TEST BEHAVIOUR

Types of fail ure The pri sm fai 1 ures were predomi nant 1 y i nducerl by ve rt i ca 1 sp 1 it ti ng in the central two courses with occasional spalling in a unit adjacent to a platen. While most fa i lures originated from a crack on a unit end or ends (see Figures 2 and 3) there were some originating from cracks on a unit face particularly with prisms of higher grout strength. Of the 45 runn i ng-bond prisms tested, the four face shell failures originated in the perpend between the two half units.

The first cracking usually occurred between 0.9 and 1.0 of the failure 1 oarl. However with one cl ay pri sm the fi rst crack was at O.R1 of the failure load.

Stress-Strain 8ehaviour Strain was reasonably l i near with stress up to an average of 77 't. and 68% failure load for the concrete masonry and clay prisms respect i ve l y. The clay prisms showed greater non-linearity than the concrete prisms.

The Young's Modulus figures quoted in Table 4 are for secant modulus from zero loarl to the load where the strain departed significantly from 1 i nearity.

Figure 1. Prism under test shawing transducers in pasitian

Fi gure 2. Vertical splitting failure af prism

Figure 3. Vertical crass-sectian af prism at splitting crack

00 --.I \O

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Unconfined Compressive Strengths of Prisms Tahle 4 gives the uncanfinerl campressive strengths of prisms averaged from six tests.

TARLE 4 Unconfinerl Compressive Strengths of Prisms

Cylinder Grout Prism Co-eff Young's E/Prism

Strength Loarl af Modulus E St re3s Material Ronding (MPa) (kN) Variation (MPaxI0 3) (xl0 )

Medium- Stack 389 0.09 weight 0.3 337 0.14 9 2.0 concrete 3.0 541 0.02 9 1.2

16.4 593 0.12 9 1.1 ?2. O 582 0.10 6 0.8

Running 347 0.11 16.4 512 0.08 8 1.1 22 .0 577 0.08 11 1.4

Heavy- Stack 614 0.16 weight 0.3 435 0.08 15 2.5 concrete 3.0 602 0.04 14 1.7

lfi .4 1040 0.12 ;10 1.5 2;1.0 846 0.20 19 1.6

Running 481 0.09 16.4 779 0.13 15 1.5 22 .0 799 0.11 19 1. 8

Clay Stack 350 0.12 0.3 286 0.10 I) 0.7 3.0 393 0.06 7 0.1)

16.4 360 0.20 7 0.6 22 .0 464 0.12 7 0.5

Running 360 0.07 11;.4 392 0.03 8 0.7 22.0 402 0.07 8 0.7

Note: The pri sm l oads have been adjusted for aspect ratio facto r according to Table 2.

881

DISCUSSION OF RESULTS

Pri sm St rengt h In Figure 4 the experimental results for unconfined compressive strengths of mediumweight concrete prisms are plotted against grout strengths. The results show that only a small proportion of the grout strength is effective in the determination of the prism strength and generally support the tests of Drysdale et al [3,4,5,121. The results compare favourably with the draft Australian masonry code [111 proposal that

Grouted prism strength Fg ungrouted prism strength Fug +

grout area Ag x square root of grout stress fg

which recogn ises that the higher the grout strength the less effective as a percentage of its own strength the grout is on the prism strength. Figures 5, 6 and 7 for prisms of mediumweight concrete, heavyweight concrete and cl ay units respecti vely compare the Austral i an code proposal and the experimental results for both stack-bonded and running-bonded prisms.

While running-bonded prisms in concrete masonry have 75 to 99% of the strength of comparable stack-bonded prisms (see Figures 5 and 6), with c 1 ay pri sms there are on 1 y mi nor st rength diffe rences due to a bond i ng change (see Figure 7). The clay results confirm those of Maurenbrecher [71. With concrete rnasonry it appears that the perpend joint weakens the prisrn. On the basis of so few tests and for the sake of such small di fferences a change from the simpl e stack-bonderl pri sm test to the more comp l icated running-bonded prisrn test i s not warranterl.

Table 4 and Figures 5, 6 and 7 show that with very weak grout (0.3 MPa) the grouted pri sm strength i s reduced below the ungrouted pri sm strength which confirms the findings of Brown and Whitlock [2J.

The strength of ungrouted prisrns is more closely indicated by tests on units with face shell capping than units with full capping .

Prism Behaviour While the Young's Modulus results show considerable scatter it can be seen that, apart from the concrete prisms with very low grout strength, concrete masonry prisms exhibit an E slightly higher than 1000 x prism strength, the generally recognised val ue. On the other hand, for brick prisrns E/prism strength tends to be much less than 1000.

Figures 2 and 3 give the vertical splitting failure pattern most common 1 y obse rved in these test s. It i si nte rest i ng to observe that the vertical split continues through the grouted cores, which is in contrast with failures of Wong and Drysda le [121 where though the fa i lures originated "in the masonry shells the grout remained undamagerl.

Grout St rengt hs Overall, the strength of grout in the cores of the units, Table 3, tended to be slightly lower than the strength of grout in non-absorbent cylinders, though this did not apply in all cases. This is a surpnslng result as it has been presurned by many that the suction of the masonry

1000

500

z :::. ..c: ... : 1J' C Q) ... +' (fJ

E Ul . .., ... p., ,

O

Fg=Fug + f'g Ag

-- F g=Fug + 0.3 f' g Ag

Fg=Fug + ~ Ag

Stack bonded prisms

10 20 Grout Strength (MPa )

Figure 4. Effectiveness of grout in prisms of mediumweight

concrete units 700

)C

./

Fg=Fug + ~ Ag z ~

..c: +' 1J' C Q) ... +' Experimental points (fJ

E X Stack bonded Ul . .., ... p.,

o Running bo nded L

o 10 20

Grout Strength (MP a )

Figure 5. Prism strengths in med i umwe i ght concrete

units

882

1000

500

z :::. ..c: +' 1J' C Q) ... +' [J)

E Ul . .., ... p.,

I

O

)(

G ~

, - é\ .... po~é\e

.... '- ~!\).!\q )'l\J.'>

Fg=Fug + 1.2 [f7; Ag

Experimental points

X Stack bonded

o Running b onded

10 20 Grout Strength (MPa)

Figure 6. Prism strengths in heavy­weight concrete units

700

~

..c: +' 1J' C (l) ... +' (fJ

E Ul . .., ...

p.,

O

- -

10

)(

o 0 )c

~Ag

20

Grout Strength (MPa)

Figure 7. Pri sm strengths in clay units

1

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unit had the effect of reducing the water content of grout in -situ with the consequent rising of the grout strength. It is a convenient result as the strengths obtained from simple cylinder tests would appear to reflect in-situ grout strength most adequately.

CONCLUSIONS

From the results of the prism tests reported, the fo11owing conclusions are reached.

1. The experi menta 1 resu lts con fi rm that on 1 y asma 11 proport i on of grout strength is effective in the determination of prism strength.

2. The ratio of Young's Modulus to prism strength appears to be slightly greater than 1000 for concrete unit prisms and somewhat less than 1000 for clay prisms.

3. The strength of grout within the cores of ho11ow units is slightly lower than the strength of grout poured in non-absorbent moulds.

4. The strength of running-bonded prisms of grouted concrete masonry is slightly below equivalent stack-bonded prisms. With clay prisms the difference is minoro

REFERE"CES

1. American Society of Testing Materials. C42-84a, ASTM 1984. 2. Rrown, R.H. and Whitlock, A.R. Compressive strength of grouted hollow

brick prisms. ~~asonry: Materials, Properties and Performance, ASTM STP 778, 1982.

3. Orysdale, R.G. and Hamid, A.A. Rehaviour of concrete block masonry under axial compression. ACI ,Journal, Proc. V.76, June 1979, 707-21.

4. Drysdale, R.G. and Hamid, A.A. Influence of the characteristics of the units on the strength of block masonry. Proc. Second North American Masonry Conf., Maryland, Aug. 1982, 13p.

5. Hamid, A.A. and Drysdale, R.G. Suggested failure criteria for grouted masonry under axial compression. ACI Journal, Proc. V.76, Oct. 1979, 1047-6l.

6. Isaacs, H. The ultimate strength of grouted ho11ow concrete block masonry. Constructional Review, May 1975, 36-47.

7. Maurenbrecher, A.H.P. Effect of test procedures on compressive strength of ITlasonry prisms. Proc. Second Canadian Masonry Symp., Ottawa, June 1980, 119-32.

8. Priestley, M.J.N. and Chai, Y.H. Prediction of masonry compression strength from constituent properties. New Zealand Concrete Construction, V.28, March and April 1984.

9. Shrive, N.G. The prism test as a measure of masonry strength. Proc. Rritish Ceramic Society, A3A-12.

Ir). Standards Association of Austral ia. Masonry cement. AS 1316-1972. 11. Standards Association of Australia. Draft SAA Masonry Code, 1987. 12. Wong, H.E. and Orysdale, R.G. Stress-strain characteristics of

concrete hlock masonry. Source unknown.