bending capacity of dry stacked lightweight …

15th International Brick and Block

Masonry Conference

Florianópolis – Brazil – 2012

BENDING CAPACITY OF DRY STACKED LIGHTWEIGHT

CONCRETE BLOCK MASONRY

Molnár, Miklós1; Jönsson, Johan

2;

1 PhD, Assistant Professor, Lund University, Department of Building and Environmental Technology,

[email protected]; 2 PhD, Assistant Professor, Lund University, Department of Building and Environmental Technology,

[email protected];

Dry stacked light weight concrete block masonry is increasingly used due to its superior

productivity compared to standard block masonry. Low bending capacity parallel to the bed

joints is however a major drawback. This paper presents the results of an experimental study

which has been carried out to improve the bending capacity of dry stacked lightweight

concrete block masonry. The effects of following measures have been studied: 1) the blocks

have been provided with additional holes forming continuous vertical canals and grouted with

concrete; 2) both faces of the dry stacked block masonry have been reinforced with a steel

wire mesh embedded in render. The bending capacity has been determined in four-point

bending tests. Compared to standard dry stacked masonry the bending capacity increased by:

1) 20 % when the additional, continuous vertical canals were filled with concrete grout; 2)

more than five times when steel wire mesh embedded in render was used.

Keywords: bending capacity, dry stacked, lightweight concrete, grouting, steel wire mesh reinforcement

Theme: research and testing

INTRODUCTION

Labor costs constitute a substantial part of the total cost in masonry construction. Dry stacked

lightweight concrete masonry is increasingly used due to considerably higher productivity in

comparison with block masonry laid in mortar, Glitza (1990) and Anand & Ramamurthy

(2003). A major drawback with dry stacking is low bending capacity, especially in bending

parallel to the bed joints. Several existing dry stacked concrete block masonry systems aim at

diminishing this drawback by using a) hollow blocks with interlocking, b) vertical

perforations that are grouted, c) reinforcement placed in vertical holes d) surface bonding

mortars, Murray (2007).

In this paper results from an experimental study carried out to document and improve the

bending capacity of the Swedish NC-blocket, a dry stacked lightweight concrete block

masonry system, are presented.

The studied dry stacking system consists of solid light weight concrete blocks provided with a

central horizontal slot and vertical slot at the header face, see Figure 1. The horizontal slot can

be grouted and reinforced to improve the bending capacity perpendicular to the bed joints.

mailto:[email protected]

mailto:[email protected]


Masonry Conference


Figure 1: Lightweight concrete blocks used for dry stacked masonry – NC-blocket

system

The cavities that form at the header faces of the blocks are filled with concrete grout and

contribute to a locking of the blocks. This grouting also improves the bending capacity

parallel to the bed joints. The bending capacity of the present system is however low and

hinders its wider use.

In the present work two modifications expected to improve the bending capacity have been

studied. In the first case the blocks have been provided with additional holes forming

continuous vertical canals. These vertical canals were subsequently filled with a concrete

grout. In the second case, both faces of the dry stacked block masonry have been rendered and

reinforced with a steel wire mesh embedded in render.

EXPERIMENTAL DETAILS

Lightweight concrete blocks with compressive strength 2,2 N/mm2

have been used for all

specimens. The size and number of specimens in the testing scheme is shown in Table 1.

Table 1: Experimental plan masonry specimens

Series Execution type of masonry specimens Length*height*thickness

l*h*t (mm)

Number of

specimens

I Standard dry stacking (t=190 mm) 885*1000*190 3

II Standard dry stacking (t=400 mm) 900*1000*400 3

III Additional vertical canals grouted with concrete

(t=190 mm, canal diameter 60 mm, spacing 300 mm) 1475*1000*190 5

IV Reinforced render ( t=190 mm+25 mm) 885*1000*215 4

V Reinforced render (t=400 mm+25 mm) 900*1000*425 5

The blocks were 590*200*190 mm and 600*200*400 mm (l*h*t). The masonry specimens

were five courses high and 1,5 blocks long, excepting series 3 where they were 2,5 blocks

long. The specimens were manufactured in the Nyströms Cement factory and transported to

the Lund university laboratory for testing. Testing was carried out 28-30 days after

manufacturing of the specimens.

In series I and II (standard specimens) the cavities formed at the end of the blocks were filled

with a concrete grout with compressive strength 30 N/mm2. This grout with low consistency

fills also approximately 100 mm of the horizontal slot, forming a T-shaped body, see Figure 2

and 3. In each course, the spacing of the grouted cavities is 600 mm. The grouting of these

cavities assures that the dry stacked masonry obtains a bending capacity higher than zero.


Masonry Conference


Figure 2: Specimen with standard dry stacked blocks

Figure 3: Grouting of the cavities formed at the header face of the blocks

In series III the blocks were provided with additional circular holes with diameter 50 mm

forming vertical canals, see Figure 4. The vertical canals were filled with a concrete grout

with compressive strength of 40 N/mm2. The spacing of the continuous vertical canals was

300 mm, see Figure 5.


Masonry Conference


Figure 4: Blocks with additional circular holes

Figure 5: Series III specimen with grouted vertical canals

Series IV and V consisted of standard specimens provided with a mesh-reinforced render on

both faces. The render consisted of cement lime mortar with compressive strength 5,5 N/mm2

and tensile strength 1,0 N/mm2, Jönsson (2011). The thickness of the render was 15 mm

(exterior face) and 10 mm (interior face).

The reinforcement which was applied centrically in the render consisted of a zinc coated steel

mesh with thread diameter 1,1 mm and mesh spacing of 19 mm. The reinforcement area per

meter of wire-mesh is 50 mm2/m. The tensile strength of the steel threads is 400 N/mm

2.

The bending capacity was determined in four-point bending test in accordance with EN 1052-

2, see Figure 6.


Masonry Conference


Figure 6: Test set-up

The load was applied by a hydraulic jack. The force and displacement at mid height of the

specimens were registered.

RESULTS

The specimens failed by formation of a large horizontal crack in either of the two central bed

joints. The failure was quasi-brittle in Series I - II and brittle in series III – V. Typical failure

modes and load-mid-span deformation curves for standard dry stacked and rendered mesh-

reinforced specimens are shown in Figures 7-8.

Figure 7: Typical failure mode for dry stacked masonry in bending – series I


Masonry Conference


Figure 8: Load-mid-span deformation curves for rendered reinforced specimens – Series

IV

The maximum bending moments Mu obtained in the tests are shown in Table 2.

Table 2: Results of the flexural tests

Specimens Maximum bending

moment Mu (Nm/m)

Mean

(Nm/m)

C.V.

(%)

Formal flexural

strength fx1 (N/mm2)

Series I - Standard dry stacking,

t=190 mm

651

682 4 0,11 690

704

Series II - Standard dry stacking,

t=400 mm

2 598

1 989 29 0,07 1 436

1 932

Series III -Additional vertical

canals grouted with concrete

(t=190 mm, canal diameter 60

mm, spacing 300 mm)

831

769 9 0,13

821

753

656

782

Series IV - Reinforced render

( t=190 mm+25 mm)

5 138

5 251 3 0,68 5 138

5 337

5 390

Series V - Reinforced render

(t=400 mm+25 mm)

10 164

10 367 3 0,34

10 291

10 767

10 544

10 070

The formal flexural strength was calculated dividing the maximum bending moment with the

elastic section modulus.


Masonry Conference


ANALYSIS AND DISCUSSION

The bending capacity parallel to bed joints of dry stacked masonry without any grouting is

close to zero, Uzoegbo and Ngowi (2004). Grouting of the vertical slots at the header face of

the blocks gives rise to a certain bonding between the dry stacked block courses. This bonding

resulted in formal flexural strength values amounting to 0,07 – 0,11 N/mm2.

From a phenomenological point of view it is however questionable to use flexural strength to

characterize transversal load resistance capacity of dry stacked masonry. During bending a

force couple is formed between the edge (compression) and the grout in the cavities formed

by the slots (tension). The bending capacity depends on the tensile force that can be resisted

by the grouted slots and the lever arm.

Specimens in series I and III have the same cross section but different amount of grouted

cavities. In series I the number of grouted cavities per meter length is 3, while in series III

their number is 4. According to the mentioned force transfer mechanism, the bending capacity

of series III should be approximately 4/3 = 1,33 times larger than in series I. The observed

difference in bending capacity from the tests is 769/682 = 1,13, see Table 2. The lower

bending capacity observed in the tests might be caused by high water absorption of the blocks

and not entirely filled slots.

The effect of grouted slots and holes is dependent on the tensile strength of the concrete used

as grout. The theoretical cross section area of the grouted slots and holes is approximately

2000 mm2 per slot/hole. The tensile stress σt at the contact surface at failure can be

approximated from:

(1)

where Mu is the bending moment, Agrout is the cross section of the grouted area and z is the

cantilever arm. Using the results from series I the mean value of the bending moment is

Mu=682 Nm/m, the lever arm z=0,190/2=0,095 m and the number of grouted slots 3. For this

case the tensile stress at failure at the contact surface is calculated to σt = 1,3 N/mm2. This

value is lower than the expected mean tensile strength of the concrete grout (for concrete C30

fctm = 2,9 N/mm2). As mentioned above, the difference might depend on difficulties to fill the

cavities entirely with grout, which results in lower cross section area than assumed.

Considering these factors, the correspondence between experimentally deduced and

theoretical bending capacity seems reasonable.

In absolute terms the increase of the bending capacity due to grouting of additional vertical

holes is limited. More substantial increase of the bending capacity should be possible by

increasing the bonding area by providing the blocks with larger slots and holes. Another

possibility is to place vertical reinforcement in the continuous vertical canals.

When dry stacked block masonry is provided with mesh-reinforcement embedded in the

render at both faces of the blocks, the main contribution to the flexural strength is obtained

from a force couple formed between the compressed and tensioned faces of the masonry. The

flexural strength will in this case depend on the tensile strength of the render and

reinforcement. A failure mode implying crushing of the compressed render and parts of the


Masonry Conference


block should also be possible if the amount of the reinforcement on the tension side is in

excess. Throughout the analysis it is assumed that de-bonding of the render from the block

substrate cannot occur.

In the present study all the rendered reinforced specimens failed in tension in a brittle manner.

Given the presence of steel reinforcement this was somewhat unexpected. In order to find an

explanation to the observed brittle failure, the maximum tensile forces that can be carried by

the render and the mesh reinforcement respectively are compared, see Table 3. Corresponding

bending moment for blocks with thickness 190 mm and render 10+25 mm on either side is

also shown.

Table 3: Maximum tensile forces in the render and the steel reinforcement

Tensile

strength

(N/mm2)

Area (mm2)

Maximum force

(N)

Bending moment

capacity (Nm)

render 1,0 15*900 13 500 2 902

steel 400 50*0,9 18 000 3 870

Series IV 5 251

From Table 3 it can be seen that the ultimate load that can be carried by the mesh

reinforcement is only 33 % larger than the maximum load in the render. As the tests were

carried out under load control, cracking of the render might have caused dynamic effects that

were large enough to cause brittle failure in the mesh reinforcement. In the tests the mean

bending moment was 5251 Nm. The fact that the bending moment in the test was 35 % larger

than the bending moment based on the steel tensile strength can be explained by the dynamic

nature of the failure. According to Ansell (2006) the ultimate tensile strength of steel can

increase by 30-40 % due to dynamic effects.

In order to avoid brittle failure of the tensioned reinforcement, use of renders with lower

tensile strength might be advisable. If cracking of the render occurs at lower load levels,

dynamic affects will be more limited. This would make possible the steel mesh to fail in a

ductile manner. Other possible measures to increase the bending capacity parallel to the bed

joints could be gluing of the blocks and application of reinforcement in vertical slots sawn in

the dry stacked lightweight concrete block masonry.

Reinforcing the render can be used as an efficient measure to increase the bending capacity of

dry stacked lightweight concrete block masonry. The bending capacity could, for the studied

product configurations, be increased by a factor 5 to 7. This result can be compared with an

increase by factor 3 to 4 of the bending capacity due to rendering of traditional block masonry

observed by Ramamurthy et al (1996).

In a realistic design situation the brittle nature of failure in bending has to be considered. If

the failure of the dry stacked masonry is expected to occur by a brittle tensile failure of the

reinforcement, design values of the steel strength shall be calculated with partial factors for

masonry material. These factors are usually twice as large as those for steel, which means that

the design will be on the safe side.


Masonry Conference


To facilitate transportation between the factory and the laboratory, the height of the specimens

was limited to five courses, which is the lowest number of courses allowed by the test method

EN-1052-2. Accordingly, the slenderness ratio of the specimens was 5,3 respectively 2,5

which is not optimal for bending tests. Arching effects due to the short span might have

resulted in an overestimation of the bending capacity. In the optimal case the slenderness ratio

should have been chosen larger than 6. This circumstance might be taken into consideration

when test results are used in design. More stringent requirements concerning the lowest

admissible slenderness ratios should be introduced in EN-1052-2.

CONCLUSIONS

In this work, dry stacked lightweight concrete block masonry has been tested in bending

parallel to the bed joints. The objective of the work was to study and quantify the effects of

additional grouting and of surface reinforcement embedded in render on the bending capacity

parallel to bed joints. Main findings and conclusions:

- Providing light weight concrete blocks with additional smaller holes that can be filled

with concrete grout has a positive, though limited effect. Larger increase in bending

capacity can be expected if larger continuous cavities are provided and filled with

concrete grout. This measure may however result in increased costs and lower thermal

insulation.

- Surface reinforcement of steel wire mesh embedded in render is an efficient measure

to increase the bending capacity parallel to the bed joints.

- When renders with high tensile strength are used, dynamic effects will result in a

brittle failure of the steel mesh. In practical design partial factors accounting for brittle

failure should be used.

- Using renders with lower tensile strength can limit dynamic effects allowing the steel

mesh reinforcement to fail in a ductile manner. Smaller crack width would be an

additional positive effect.

- The bending capacity of rendered reinforced dry stack masonry parallel to bed joints

can be anticipated by using a force couple analogy and knowledge about wall

geometry and tensile strength of the render and mesh reinforcement.

ACKNOWLEDGEMENTS

The research was carried out with financial support from Nyströms Cement AB. Arne Cajdert

from AC Byggkonsult contributed with technical advice during planning of the tests. All

contributions are kindly acknowledged.

REFERENCES

Anand, K. B. and Ramamurthy. K., "Laboratory-Based Productivity Study on Alternative

Masonry Systems", Journal of Construction Engineering and Management, Vol. 129, No 3

(2003), pp 237-242.


Masonry Conference


Ansell, A. ”Dynamic testing of steel for new type of energy absorbing rock bolt”, the Journal

of Constructional Steel Research, 62, 2006, pp 501-512.

EN 1052-2 “Methods for test of masonry – Part 2: Determination of flexural strength”,

Swedish Institute of Standards, 2000.

Jönsson, J. “Analysis of parameters influencing cracking of renders”, Research report (draft),

Lund University, Lund, Sweden, 2011.

Glitza, H. ”State of the art and tendency of development of masonry without mortar” (in

German), Proceedings of the 9th International Brick/Block Masonry Conference, Berlin,

Germany, 13-16 October 1991, Vol. 2, pp 1028-1033.

Murray, E.B. ”Dry stacked surface bonded masonry – structural testing and evaluation”, MSc

thesis, Brigham Young University, USA, 2007.

http://contentdm.lib.byu.edu/ETD/image/etd2188.pdf, accessed 2011-08-04.

Ramamurthy, K., Radhakrishnan, R. and Rajasekhar, G. “Flexural strength behavior normal

to bed joint of plastered concrete block masonry”, Masonry International, Vol. 9, No. 3, 1996,

pp 91-94.

Uzoegbo, H.C. and Ngowi, J.V. ”Lateral strength of a dry-stack wall system”, Masonry

International, Vol. 17, No. 3, 2004, pp 122-128.

http://contentdm.lib.byu.edu/ETD/image/etd2188.pdf

bending capacity of dry stacked lightweight …

Documents