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STRENGTH AND DEFORMATION PROPERTIES OF A GROUTED, DRY-STACKED,INTERLOCKING, CONCRETE BLOCK SYSTEM

R.G.Orysdale Professor Oept. ofCivil Engineering and Engineering Mechanics McMaster Uni versity Hamilton,Ontario Canada

ABSTRACT

E.A. Gazzola Senior Project Engineer Morrison Hershfield Limited Consulting Engineers Toronto, Ontario Canada

Results of tests on a new dry-stacked concrete block form of construction are reported. The concrete blocks are manufactured using a patented process to assure uniform block heights and smooth bed joint finishes to facilitate uniform bearing when placed in running bond. The masonry walls constructed with these units are stabilized and strengthened by filling the cells in the walls with grout.

Concentric compression tests were performed on four block high prisms for three different grout strengths as well as a sedes of ungrouted specimens. Eccentric compression tests and flexural tests were also performed on specimens constructed using the intermediate strength grout. In comparison with grouted concrete block masonry constructed with mortared joints, this form of construction had compressive strengths greater than those specified in North American codes. A reasonably linear stress-strain behaviour was found up to approximately half of the prism strength. The load carrying capacities under eccentric compression were found to be much larger than the theoretical predictions based on the concentric compression tests and indicated that beneficiaI strain gradient effects are much greater than are normally allowed for in building code provisions. Finally, the flexural tension strength of this grouted wall system was found to have safety facto r of over 6 compared to the allowable values listed in North American building codes.

INTROOUCTION

Structural tests on a new dry-stacked interlocking concrete block form of construction have been completed to investigate the applicability of existing masonry design codes1•2 . Also, beca use this construction provides a fully grouted wall system, it was anticipated that evidence of the previously identified benefits of grouting3,4 not currently recognized in these building codes would be found . Although surface bonding of the ungrouted dry-stacked units is also a viable type of construction, this is not dealt with in this paper.

Figure 1 is a drawing of a 20 cm standard stretcher unit which has nibs projecting above the two webs to provide interlocking and control of positioning for running bond. To assure uniform bearing between the dry-stacked units, a patented process is used to "miter" the face shell surfaces along the bed joints to provide uniform heights of blocks and smooth bearing surfaces.

PREPARATION ANO FABRICATION OF SPECIMENS

One block wide running bond prisms were constructed with every other course composed of half units cut from stretcher units with a diamond blade masonry saw. Thirty 4 block high prisms were dry-stacked in a row with polystyrene bead board separating the abutting open ends to provide a form for filling the open ends with grout. Five 2 block high prisms were dry-stacked in a similar manner.

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Groove in web for horizontal --~ reinforcement

1 I

'" ---L

'~

Figure 1 Dimensions of the Interlocking Dry-Stacking

Concrete Block

Figure 2 Set-up for Block Molded Prisms

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Prior to grouting the prisms, blocks were set up as shown in the photograph in Figure 2 to act as the forms for block molded grout prisms where paper towels taped to the blocks provided a bond break between the grout and the blocks. The blocks were arranged to provide a 207 mm X 137 mm cross-section area for the grout prisms which is equivalent to the average central core size of the block. Grout prisms for compressions tests were made 1 block high (203 mm) while those for flexural tests were made 2 blocks high (406 mm). These block molded prisms were fabricated in accordance with ASTM Standard CI0195 with the exception that the core area was used rather than the specified 76 mm square area this provided a better simulation ofthe effects ofwater absorption. For quality control comparisons, standard 152 mm diameter by 305 mm high grout cylinders were ma de in non-absorbent molds. Ten block molded prisms and five cylinders were prepared for each ofthe three grout strengths used. The grout specimens and twenty-five blocks set aside for testing were stored in the dry laboratory environment to be tested at the same time as the similarly cured block prisms.

Ali of the masonry prisms and block molded grout prisms were made on the same day using the grout proportions listed in Table 1. The grouts were batched by weight for better quality control and were adjusted to account for the difference between the natural moisture content and the saturated surface dry condition for the sand. The proportions and water to cement ratio (w/c) were selected to provide a 250 mm slump and to provide a large range of grout strengths.

Table 1. Grout Proportions and Strengths

Grout Proportions by Volume Compressi ve Strength (by weight) (MPa) Flexural

Grout Tensile Type Type 10 810ck Strength

Portland TypeN Concrete Molded (MPa)

Cement Lime Sand Water Cylinders Prisms

1 0.1 3.6 0.81 17.4 24.8 1.8

(1.0) (0.04) (4.02) (0.81)

2 1 0.1 3.3 0.69 23 .3 29.2 2.2

(1.0) (0.03) (3.20) (0 .69)

3 1 0.1 3.0 0.57 31.2 40.0 2.2

(1.0) (0.03) (2.71) (0.57)

TEST RESULTS AND DISCUSSION

Concrete 810ck Tests

Five concrete blocks were tested in compression in accordance with ASTM Standard C140.6 These units had the interlocking nibs on the top face knocked off to alIow the capping plate to be smoothly plastered to the block. AIso, because only the face shells ofthe top part ofthe block could be loaded due to the web configuration, the webs at the bottom of the block were covered with pape r towel to provide only face shell capping similar to the top ofthe block.

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The average capacity of 851 kN corresponds to a compressi ve strength of 30.4 MPa based on the average face shell thickness of 34.5 mm. The average splitting tensile strength of the face shells was 2.6 MPa based on the same average face shell thickness. The respective coefficients of variation were 5.8% and 9.5%.

GroutTests

The results ofthe three series oftests performed on the three strengths of grout are listed in Table 1. The first series were compression tests of grout placed into standard 152 mm X 305 mm non-absorbent cylinder molds. The series of compression tests on block molded grout prisms with the same cross-sectional dimensions as the central core of the block and 'a 203 mm height produced higher strengths. These strengths best represent the actual in-place strength of the grout as afTected by absorption of water by the blocks. The final series of grout tests were flexural tensile tests ofblock molded grout prisms which had the same cross-sectional dimensions as the central core of the block and a 406 mm length. A two-point loading scheme with 127 mm shear spans was used to provide the constant moment region for these tests.

Prism Compressive Strengths

Five series of compressive tests were performed on the concrete block prisms. The average strengths and coefficients ofvariation are listed in Table 2.

Series No.

2

3

4

5

Table 2. Summary ofthe Concentric Tests of the 8lock Prisms

Prism Height (Courses)

4

4

4

4

2

Grout Type

N/A

2

3

2

Average Strength*

(M Pa)

11.5

15.9

16.5

19.5

20.9

Coefficient of Modulus of Variation Elasticityt

(%) (MPa)

15.8 N/A

2.7 17700

11.2 18900

11.7 20200

6.4 14100

* The compressive strength was based on the gross cross-sectional area of 82418 mm2 except that the average face shell thickness of 34.5 mm was used for the ungrouted prisms in Series 1.

t Average secant modulus of elasticity determined at half ofthe prism strength.

The first series of prism compression tests were on ungrouted four-block high, one block wide, running bond prisms. Ali five prisms failed by splitting in the webs of the concrete blocks (as seen in Figure 3) due to the lateral tensile stresses developed . There was also some splitting of the face shells in a few of the tests as also seen in Figure 3. The average failure load for the five tests was 322 kN which corresponds to 3.9 MPa based on gross area. 8ased on an average net area for a face shell thickness of 34.5 mm, as was used for the concrete unit tests, this load corresponds to a strength of 11.5 MPa. Obviously, due to end platen restraints' in the block tests,

Figure 3 Typical Compression Failure Figure 4 Typical Compression Failure

for an Ungrouted Prism for a Grouted Prism

cn co

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the prism strength would be expected to be much lower than the compressive strength of the block.

The second, third and fourth series of prism compression tests were on fully grouted, four-block-high, one block wide, running bond prisms with the three different grout strengths. Five repetitions for each test were performed. There was relatively little difference in the prism strengths for the lower strength grouts, but for the higher strength Type 3 grout, the average prism strength was 23% higher compared to the prisms with Type 1 grout. However, this increase was far from being directly proportional to the comparable 79% increase in grout compressi ve strength.

Ali the grouted prisms exhibited a similar type of failure mode, irrespective of the grout strength. An hourglass type of shearing failure, as is typical for concrete prisms, was observed with some shearing off of face shells, as is common for grouted masonry. Figure 4 is a photograph ofthis typical combination failure.

The fifth series of compression tests on two-block high, one block wide, running bond prisms constructed with the Type 2 grout were performed to determine whether there was a signi­ficant difference between strengths using two-high and four-high block prisms. A compressive strength of 20.9 MPa obtained for the two-block high prisms represents a 27% increase over the four block high strength. Although this increase represents the significant effect of end platen restraint, the failure ofthe prisms was very much the same as for the four block high prisms.

Modulus of Elasticity

The stress-strain behaviour for each grouted compression test was recorded by means of strain transducers fastened to the sides of the prism. There were four transducers, two on each face, placed on each prism in the direction of the applied load. A minimum of ten load increments were taken for each specimen. For each load increment, the load and strain were recorded on a computer via the data acquisition system. The load was converted to stress and the strain from the four transducers was averaged to give the final stress-strain behaviour.

The average stress-strain curve and modulus of elasticity for each series of five com­pression tests were calculated by performing a least squares analysis on the normalized stress versus strain data for ali five tests in a series. Each stress was normalized by dividing the stress levei by the compressive strength of the prism. Then the data from the five tests were combined and a least squares fit analysis provided the average stress-strain curve. The secant moduli of elasticity listed in Table 2 were determined at stresses of half ofthe average capacities.

The Modulus of Elasticity values increase as the grout strengths in the compression prisms increased. The value of the Modulus of Elasticity is in the area of 1100 times the compressive strength for 4 block high prisms.

Eccentric Compression Tests on Grouted Prisms

Two series of eccentric compression tests were performed on grouted four-block high, one block wide, running bond concrete block prisms constructed with Type 2 grout. The applied eccentricity, average failure loads and mean strengths, and the coefficients ofvariation are listed in Table 3.

The first series of compression tests for eccentricity of ti6 (34 mm) had typical com­pression failures of face shell crushing and spalling as was observed for the concentric tests. The calculated strength represents a 66.5% increase over the value for the concentric tests ofthe Type 2 grouted prisms. The second series of compression tests with an eccentricity of ti3 (68 mm) would theoretically have flexural cracking halfway through the grouted prisms, assuming that the

*

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Table 3. Eccentric Compression Tests ofGrouted 810ck Prisms

Grout Eccentricity Type

ti6 2

ti3 2

Average Failure Load (kN)

1130

574

Average Strength* (MPa)

27 .4

27 .9

Coefficient of Variation (%)

7.7

18.3

Strengths are based on linear elastic analysis of the gross cross-section assuming zero tensile strength.

prisms had no tensile strength. There were flexural cracks on the tension face of ali the prisms at one or ali three of the joints in the prism. Although these cracks did propagate halfway through the prism in some cases, the eventual failure, often explosive, was by compression. The calculated compression strength was 69 .0% higher than the comparable result from the concentric tests .

Flexural Tensile Strengths ofGrouted Prisms

Flexural tensile tests on blockwork prisms grouted with the Type 2 grout comprised the final test series. Fifteen tests were performed using the bond wrench test method. The a verage flexural tensile strength, based on the gross cross-sectional dimensions of 203 mm X 406 mm, was 1.7 MPa with a coefficient variation of 10.4%. This va lue is less than the flexural tensile strength ofthe grout which is 2.2 MPa. However it should be recognized that this calculation was based on the gross area whereas it is obvious that the tension face shell will not contribute to the flexural capacity. Neglecting the tensile face shell results in a calculated flexural tensile strength of 2.55 MPa.

CONCLUSIONS

In comparison with standard grouted concrete block masonry, for a block compression strength of 30.4 MPa, the block prisms constructed with the three grout strengths have f' m values greater than those specified in CSA S304-M842, with the Type 3 grout prism having a compressive strength higher than specified for the hol\ow concrete block prism strength.

The stress-strain curves are reasonably linear up to a stress near half of the prism strength. Therefore, the Secant Modulus of Elasticity values listed provide a reasonable estimate ofresponse under service loads. The values are slightly higher than predicted by the code value of 1000f'm·

For the eccentric load tests, the capacities indicate that the practice of al\owing higher compressive stresses for flexure to al\ow for the strain gradient effect is applicable and even exceeded by these tests. This extra al\owance can be expressed both in terms of increases in the eccentricity coefficient andJor in higher al\owable compressive stress for bending.

The flexural tensile strength of 1. 7 MPa provides a factor of safety of 6.8 compared to the allowable flexural tensile stresses listed in CSA S304-M84 for tension normal to the bed joint for grouted concrete blockwork.

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ACKNOWLEDGEMENT

The research repor ted in this paper was supported by TCG Mate riaIs Limited , Burlington, Ontario, Canada.

REFERENCES

1. ACI-ASCE Committee 530, "Building Code Requirements for Masonry Structures (ACI 530-88/ASCE 5-88)", ACI, Michigan, V .S.A., 1988, 18 pages.

2. Canadian Standards Association, "Masonry Design for Buildings (CAN3-S304-M84)", CSA, Ontario, Canada, 1984, 69 pages .

3. Hamid, A.A. and Drysdale , R.G. , "Flexural Tensile Strength ofConcrete Block Masonry", Journal ofthe Structural Division, ASCE, Vol. 114, No. 1,Jan. 1988, pp. 50-66.

4. Drysdale, R.G . and Hamid, A.A., "Effect of Grouting on the Flexural Tensile Strength of Concrete Block Masonry", The Masonry SocietyJournal, Vol. 3, No . 2, July-Dec. 1984, pp. 710-719.

5. ASTM Standard C1019-89acl , "Standard Method of Sampling and T es ting Grout" , Annual Book of ASTM Sta nda rds , Vol. 04.05, 1990, pp. 628-630.

6. ASMT Standard C140-75 (Reapproved 1988), "Standard Method ofSampling and Testing Concrete Masonry Vnits", Annual Book of ASTM Standards, Vol. 04.05 , 1990, pp. 87-89 .