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SELECTION OF BLOCK SHAPES FOR DIFFERENT APPLlCATIONS IN MASONRY CONSTRUCTION

S. R. De S. Chandrakeerthy University 01 Moratuwa Moratuwa, Sri Lanka

ABSTRACT

Ahmad A. Hamid Drexel University Philadelphia, PA, USA

Although a vast array 01 block shapes is available, not ali blocks are suitable lor any given application. Hence, inlormation on selection 01 suitable blocks shapes is opportune and uselul. In order to select shapes suitable lor any particular application, a survey was conducted on various block shapes which are successlully used currently. These block shapes were then assessed against important considerations that aflect production 01 blocks, and construction and performance 01 block walls under likely prevalent conditions. Plain ended two-core block and flange ended two-core block are lound to be the best suited lor low strength and high strength applications , respectively . Multicellular blocks with an odd number 01 cavities are not suited lor reinlorced masonry or lor use with lul! mortar bedding. Open end units with minimum tapering 01 laceshells and web is more suitable lor reinlorced masonry construction.

INTRODUCTION

Concrete block is a widely used material lor building construction. It is made 01 a cement sand mix or a concrete mix with dense or lightweighl aggregate. Advances in manulacturing methods, and in the quality and unilormity 01 concrete block as wel! as easy availability 01 necessary raw materiais, suitable technology and appropriate equipment lor block production and the likely savings due to laster erection, have substantial!y changed the economics 01 its use as compared to other structural materiais. Thus, there is a demand worldwide lor lurther exploitation 01 this material.

Concrete masonry is hand-made with concrete blocks which are precas!. These blocks are proportioned so that their size enables delivery and laying, manually. Over lhe years many shapes of blocks were evolved to suit various specilic situations which resulted in a baflling array of block shapes to select lrom.

Many standards and building codes provided design inlormation on selection 01 block strength, mortar type and block dimensions. However, guidance on selection 01 block shape is not yet available , except lor indirect control 01 block shape to some extent by specifications such as minimum requirements for lace shell thickness and equivalent web thickness( 1,2) , or limits on percentage 01 solid material in the block(3.4), or limits on hollow portion and wall thickness(5l. Hence, inlormation and guidance to lacilitate the selection 01 block shape lor diflerent applications is opportune and uselul.

There are situations where the engineer has to select a block shape Irom many block shapes 01 same size available in the marke!. Decision becomes more demanding when cost and strength 01 each other are not very different, and inlormation on selection of block shapes will lacilitate proper selection .

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A new manulacturer commencing labrication 01 molds and installation 01 a block making, has to decide on block shapes he is going to produce. This is another instance where inlormation on selection 01 block shapes is indispensable.

Lack 01 guidance in building codes on selection 01 shapes and availability 01 wide variety 01 block shapes to choose Irom, create conlusion in the user's mind. Hence, consideration 01 more suitable block shapes lor use in dillerent applications will be opportune and benelicial.

The dimension herein is limited to loadbearing masonry units lor structural applications . Architectural units, special units and thermal and moisture considerations are not covered in this pape r.

OIFFERENT TYPES OF BLOCKS ANO THEIR USEFULNESS

General

Blocks are available in a variety 01 shapes which have been developed over the years, and each shape is the result 01 overcoming some shortcomings in either production or construction . More recently, many studies(6,7) were conducted to modily the shape 01 the block in order to improve the structural performance 01 masonry. There were also other studies(8,9) aimed to improve productivity 01 installation, out-ol-plan bending behavior, rain penetration resistance , esthetic appeal, thermal insulation, and sound absorption, and to reduce sound transmission . These studies have widened our understanding 01 the inlluence 01 block shape in masonry structures and also have produced some shapes with only a lew changes to the common block as well as other shapes which are radically differen!. Various block shapes and their lunctional advantages and disadvantages are presented below.

Conventlonal Blocks and Blocks Resembling Them

Solid block is probably the oldest type 01 concrete masonry uni!. Now, its use limited to special applications such as at the top or bearing course 01 loadbearing walls , lor increased lire protection, lor manhole construction, or lor boundary walls. It provides betler lire protection and sound insulation while it is easy to cast especially by hand casting . Its disadvantages are higher weight, difficulty in handling, extra material required to cast the block and extra mortar required lor block-Iaying.

Hollow block is one 01 the most important developments in concrete masonry and it is the most widely used block with many variations 01 shape. Its advantages are reduction in weight, betler thermal insulation, consumption 01 less material lor block as well as mortar, and easier handling . Disadvantages are: the need lor better quality control to prevent cracking during production and the need lor mixes with higher cement content lor casting blocks.

A very common shape is a hollow block with two cores (Figure 1-a) . The cores 01 the hollow unit are tapered to permit ready stripping in the molding processo The taper also gives a broader base lor mortar bedding and lor better gripping by the mason. Sometimes the junctions 01 the central web and lace shell are thickened (Figure 1-a,d). This strengthens both the web and the lace shell and also minimizes cracking during stripping. However, the unit consumes more material and gripping the block with the center web becomes a little difficult.

Multicellular units contain more than two cores and the more common arrangement consists 01 three cores (Figure 1-b) . It is particularly use/ui when lhe length 01 the unit is more Ihan twice ils wiclth , easier to handle and more robust againsl breakage during handling. Its disadvantages are that it

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consumes more material, needs greater care in stripping and, olten, results in a heavier unit than a two core unit. Further, odd number 01 cores in a block leads to considerable misalignment 01 webs, reduces cross-section and, hence, not very suitable lar lull morta r bedding or reinlorced masonry.

Hollow unit with Ilanged ends (Figure 1) is tailor-made lor block laying with lace shell bedding. Its advantages are easy handling by masons and possibility 01 reinlorcing . Difficulty in stripping Irom molds, susceptibility to breakage, need lor special blocks (at corners, stopped ends, doar jambs , window jambs or piers) , and greater misalignment 01 webs (Figure 2) in a wall are some shortcomings(101.

Hollow block with plain ends (Figure 1) is more common(11) with smaller unit size such as the 1 OOmm width and the 150mm width sizes. Its advantages are: (i) it can be used lor stretcher, stopped end, corner or pier units, thus minimizing the need lor special blocks; (ii) greatly reduced misalignment 01 webs in a wall; (iH) easy to strip Irom the molds; (iv) lesse r likelihood 01 breakage during handling; and (v) provides greater area at head joint and hence it is better suited lor block laying with weak mortars. For larger units (200mm width or more) used with strong mortar where lace shell bedding is used, this unit may give rise to lower productivity and wastage 01 mortar.

Some core designs include a degree 01 Ilaring 01 the lace and web (Figure 1-c) to give a broader base lar mortar bedding, less mortar droppings into the cores 01 blocks, better gripping by the mason, and easy stripping Irom the mold. However, it consumes more material in lorming the block, and, when used lor reinlorced or grouted concrete masonry, causes difficulty in compaction and results in reduction in strength due to reduced grout area at the bed joint.

Compared to two-core hollow unit, the three-core hollow unit provides a weaker wall section at a head joint. To overcome this, lace shell are sometimes thickened at the center (Figure 5) to provide greater tensile strength at mid-section. This may lead to some wastage 01 mortar unless laid wilh greater care, and lorming hall units by breakage 01 lull units becomes difficult. Also, it is unsuilable lor lull mortar bedding due to considerable misalignment 01 webs.

Blocks Differing from Conventional Blocks

The core-aligned block (Figure 3-a) is an improvement 01 the conventional block(6,12) where block shaptl is altered to attain alignment 01 webs lor running bond construction . This is done by minimizing the taper 01 the webs and lace shells and increasing and reducing, respectively , the thickness 01 the middle and end webs. The wavy-core block (Figure 3-b) is a lurther improvemenl(6) 01 the above block where lurther thickening 01 the webs along the block's center line has the ellect 01 strengthening the block and postponing the onset 01 cracking along the head joints.

Fin block (Figure 3-c) is lighter and stronger, and it is laid in a lull bed 01 mortar unlike lace shell bedding(7) . It consumes less mortar lor block laying, produces vertically aligned cores and can be grouled wilh vertical reinlorcement. Its disadvantages are that horizontal reinlorcemenl is difficull lo be placed and, due to reduclion 01 compression zone, it has poor oul-ol-plane moment capacity.

Ivany block(12) and H block(11) (Figure 4) are developed in the Uniled Slales specilically lar reinlorced grouted masonry, and Ihey lacililale easy placement 01 both vertical and horizontal steel. The blocks act like lorms and rely more on lhe grouted area and reinlorcement lor structural action, and, hence, they are not suitable lor unreinlorced masonry construction or lightly reinlorced walls . These blocks are not as long and not as high as common blocks. Smaller size enables incorporation 01 only a lew short or notched webs in the block, which lacilitates placement in the wall and provides greater Ilexibility in reinlorcement positioning. However, smaller size leads to greater consumption 01 mortar and slower wall construction.

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SELECTION OF BLOCK SHAPES FOR DIFFERENT APPLlCATIONS

General

In the search for suitable block shapes it was necessary to classify applications into dilferent categories. The most profound variation in block shape occurred between low slrenglh and high strength applicalions and, hence, it was considered lo be lhe best basis for lhe ma in classification. Concrele masonry with greater esthetic appeal, reinforced concrete masonry, concrele masonry with improved insulation, and concrele masonry for fast construction were also found to be significant categories worthy of separate treatment. However, il was found more apt to consider these categories as subdivisions within the main classification based on slrength.

Applications which utilize blocks of compressive strenglh not less than 6.0 N/mm2 are considered as high strength applications in this study. These usually include ali applications, excepl Ihose using lightweight blocks, in many developed countries. Low slrength applications are considered as Ihose utilizing blocks weaker than above. Many applications in some developing countries fali into the latter category, where weaker blocks are use to benefit from less harsh exposure conditions such as absence of freezing and thawing, absence of earthquakes, smaller temperature variations, low wind speeds , as well as lower loads from low rise buildings.

Low Strength Applications

The face shell bedding (Figure 5 ) consumes less mortar and accelerales lhe speed of construction, but, does not utilize ali the available structural strength in a wall as the elfective wall cross-section is reduced, except in fully grouted walls. Hence, face shell bedding is ideally suited for strong blocks and mortar used in high strength applications.

In face shell bedding, mortar is placed in the bed joint as two bands above lhe face shells which are also continued into the head joint( Figure 5-a). As head joint mortar is applied on the block beforehand and then shoved into posilion to form the wall, block shape Ihal provides for easy handling during shoving and placement and high strenglh mortar which readily attains compaction under moderale pressure are preferred. This favors a flange ended block rather than a plain ended block (Figure 1) .

Adequacy of a usual slrelcher block lo meet a variety of situalions in concrele masonry conslruclion such as jambs, stop ends, corners elc. is a useful consideration thal can minimize wasle or loss, and can save the burden of ordering and holding slocks of many types of accessory blocks. Here a plain ended block is superior to a flange ended block.

Units with less misalignment of webs in a wall provide improved structural performance(7,12), Hence, blocks with plain ends are more desirable than blocks with flanged ends as regards reduction of web misalignment.

Drying shrinkage is an important consideration in high strength applications due to the higher cement content in both the block and the morta r. A block subjected to differential drying shrinkage may cause cracking or distortion, and it can be minimized by using it in construction after adequate curing . It is influence by block shape and grealer care is needed in a symmetric blocks. Masonry elements can show drying shrinkage, even if well cured blocks are used, due to that of mortar resulting in wall cracking. Blocks for easy splitting and blocks with odd numbered cavities where

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middle lace shells are not thickened ,are more prone to vertical tensile cracking across a line 01 head joints . Wetting expansion is 01 lesse r signilicance as it is generally lower than the drying shrinkage (unless clinker aggregate is used) and causes compression in masonry, when restrained. Hence its main adverse effect is conlined to minor distortion in a symmetric blocks.

Cracking 01 block lace shells can be minimized( 10) by concentrating the main area 01 the block in the lace shells, or by use 01 multicellular blocks, or by strong blocks which induce stepped-wise cracking around the blocks. Cracking at head joints can be minimized by provision 01 grooves, or tongues and grooves on end laces or end Ilanges 01 the block. Cracking 01 mortar bed joints by tension or shear lorces gives rise to tensile debonding or shear debonding along the mortar-block interfaces, and greater effective cross-section at mortar bed joints, provided by lull mortar bedding or lace shell bedding with blocks 01 thicker lace shells, minimizes such cracking . Thus lor high strength applications, where block is strong, two core block with thicker lace shells containing grooves, or tongues and grooves at end flanges or end laces 01 the block, is prelerred as regards minimizing the incidence 01 cracking .

Concrete masonry is increasingly used with cavities lilled, or reinlorced and grouted. For these applications, plain ended blocks are better than Ilange ended blocks as misalignment 01 cores is smaller giving continuous grout cores 01 larger sectional area. In addition, compaction 01 grout is easier with the absence 01 smaller cavities at the head joints. When a closer spacing 01 reinlorcement is anticipated, multicellular block with an even number 01 cavities is more advantageous than a two core block provided cavities satisly minimum dimensional requirements. However, when masonry is heavily reinlorced, Ivany or H block (Figure 4) is prelerred with the latter suited lor reinlorcing at very elose spacings. Blocks with an odd number 01 cavities are unsuitable lor reinforced masonry due to considerable misalignment 01 webs leading to obstruction 01 cavities.

Inlluence 01 block shape on masonry elements subjected to temperature effects , is similar to that 01 drying shrinkage and wetting expansion. 11 suffices to state that, in a restrained wall , the effect of a drop in temperature resembles that subjected to drying shrinkage while the effect 01 a rise in temperature resembles that subjected to wetting expansion, which were treated earlier.

Alteration 01 existing structures or their demolition olten involves dismantling, initially, at least one block Irom the masonry element. In reinlorced or grouted walls, this task is difficult irrespective 01 block shape. In unreinlorced concrete masonry, lace shell bedded blocks are easier to dismantle, by chiseling away the mortar joint, than lull mortar bedded blocks. Also , a flange ended block is easier to remove by breaking a Ilanged lace shell portion or by prying the block using a Ilanged lace shell portion. Hence, unreinlorced concrete masonry made 01 face shell bedded, Ilange ended blocks are easier to aijer or demolish.

Productivity 01 conventional block-Iaying depends mainly on efficiency in spreading of mortar, laying blocks in position, and adjustment 01 blocks to line, levei and plumb. In high strength applications, flange ended block has a decisive advantage over pia in ended block as it is easier to position and also, to apply the mortar. Still laster construction is possible with interlocking blocks.

In high strength applications which enable the use 01 rich mortars, head joint mortar is applied on the block prior to pressing it in position. The Ilange ended block is easier to grip for pressing and positioning than other types . Block shape is immaterial , as regards gripping, in interlocking blocks as they use dry stacking at least initially.

For blocks 01 similar compressive strength, plain ended block is robust , hence, superior to flange ended block as regards to the likely damage during transportation and handling.

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Low Strength Applications

Full mortar bedding (Figure 5) is lavored lor low strength blocks and lean mortars, as change in cement content 01 lean mixes does not signilicantly allect wall strength, while the increase in effective wall cross-section by using this type 01 mortar bedding signilicantly increases wall strength.

Use of low consistency lean mortar and lull mortar bedding requires a differenl block-Iaying lechnique consisting 01 applying a lull bed 01 mortar, placing the block in position, and placing and compacling mortar at the head joint by slicing movement 01 the trowel. Rain penetration resistance is improved by this technique as it involves minimum movement 01 lhe block afler it has been placed in contact with Iresh mortar(13). This block-Iaying technique also lavors pia in ended blocks as mortar wastage is considerable wilh Ilange ended blocks. The "buttering" method is ineffective as il produces poor bond at head joinls due lo insufficient and variable compaclion, and entrapped air ai blocklmortar interface caused by higher waler-cement ratios and inadequate compactive eflort.

As regards ordering and holding slocks 01 many !ypes 01 accessory blocks, plain ended block is superior to Ilange ended block as mentioned earlier lor high strength applications.

For low strenglh applications, plain ended blocks are desirable Ihan Ilange ended blocks lor the same reasons attributed to high strength applications earlier, as regards misalignment 01 webs.

As amounl 01 cemenl used in blocks or mortar 01 low strength applications is low, drying shrinkage and wetting expansion are only of minor importance in selection 01 the block shape.

In unreinlorced low strenglh block walls, tensile slrength parallel lo bed joinls helps the wall lo span horizontally between cross walls or pilasters or in unexpecled situations 01 differenlial settlement. Fully mortared head joints 01 plain ended blocks will be prelerred as they provide greater tensile strenglh.

Cracking 01 block shell can be minimized by concentrating the main area 01 the block in the lace shells(1 O). Strong blocks induce stepwise cracking around lhe blocks and multicellular blocks resist diagonal cracking 01 the block beller(10). Cracking of mortar bed joints by tension or shear lorces gives rise lo tensile debonding or shear debonding along the mortar block interfaces, and lull mortar bedding and grealer area 01 blocks minimize such cracking. Thus, mullicellular blocks with an even number 01 cavities will perform beller in such a situation.

Cracking at head joints can be minimized by provision 01 grooves or longues and grooves on end faces or end Ilanges 01 lhe block. As a lull head joinl is more advantageous against head joint cracking, plain ended blocks even withoul grooves will perform beller.

Allhough the most common applicalion lor low strength blockwork is with cavities unlilled, Ihere is some scope lor blockwork with cavities Iilled and lor reinlorced blockwork. For lhe laller Iwo applications, plain ended blocks are beller Ihan Ilange ended blocks as misalignment 01 cores is smaller giving continuous graut cores 01 larger sectional area. In addition, compaction 01 graul is easier wilh the absence 01 smaller cavities at the head joints .

As mentioned earlier, productivity 01 block-Iaying depends mainly on efliciency in spreading 01 mortar, laying blocks in position, and adjuslment 01 blocks to line, levei and plumb. For low strenglh applications, bolh the main types 01 blocks are similar with none having an edge over lhe other. Flange ended block is easier to posilion but, dillicult to lorm the head joinl (as buttering prior lo

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placement is ineftective with lean mortars) and the opposite is true for plain ended block. Interlocking blocks are not very viable for low strength applications as tongues and grooves in them are liable to easy breakage during handling and transporto

Full rnortar bedding and the method of forming head joints , used in low strength applications , make block shape unimportant as regards ease of gripping the block for placement in the wall during construction.

As regards to the likely damage during transportation and handling of low strength blocks , plain ended block is far more robust than the flange ended block.

CONCLUSIONS

The following conclusions can be made from this study:

(a) A vast array of blocks shapes can be used, but the most effective block shape depends on whether the application is of high strength or low strength and other special conditions desired such as high quality of insulation, need for fast construction , or use of reinforced masonry;

(b) Plain ended blocks are better suited for low strength applications, while flange ended blocks are better suited for high strength applications;

(c) Two-core arrangement is preferable to the multi-core arrangement ; (d) Structural and construction considerations are not always decisive in selection of a

suitable block shape; and (e) Multicellular blocks with an odd number of cavities should be avoided for reinforced

masonry or for use with full mortar bedding.

REFERENCES

(1) ASTM C90-85 , "Standard Specification for Hollow Load-bearing Concrete Masonry Units", Annual Book of ASTM standards, Vol. 04, 05., American Society for Testing and Materiais, Philadelphia,1985.

(2) AS 2733 :1984, "Concrete Masonry Units", Standards Association of Australia, Sydney, NSW, 1984.

(3) IS 2185:1967, "Specification for Hollow Cement Concrete Blocks", Indian Standards Institution, New Delhi, 1967.

(4) SLS 855:1989, "Specification for Cement Blocks", Sri Lanka Standards Institution, Colombo,1989.

(5) JIS A 5406-1972, "Hollow Concrete Blocks", Japanese Standards Associatio, Tokyo,1972.

(6) Shrive, N. G., and Jessop, E. L., "Hollow Concrete Blocks with Enhanced Structural Efticiency and A Compatiable Grout", Magazine of Concrete Research , Vol. 39 , No. 140, September 1987.

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(7) Shrive, N. G., Lorrendo, T., and Jessop, E. L., "Design 01 Efficient Loadbearing Concrete Blocks Leading to the Development 01 A New Concrete Block Masonry Building System", Proceedings 01 the 41h Canadian Masonry Symposium, Fredericlon, Canada, June 1986.

(8) Whelan, M. L., "Hollow Concrete Masonry Unit Shape Modilication to Improve Productivity 01 Placement: Results 01 the Preliminary Research Effort", Proceedings 01 the Third North American Masonry Conlerence, Arlington, Texas, June 1985.

(9) Gage, M., and Kirkbride, T. , "Design in Blockwork", The Architectural Press Limited, London, 1976.

(10) Drysdale, R. and Hamid, A.," Inlluence 01 the Characleristics 01 lhe Unils on the Strength 01 Block Masonry", Proceedings 01 the Second North American Masonry Conlerence, University 01 MAryland, College PArk, Augusl 1982

(11) Ganesan, T. P., Kalyanasunderam , P., Ambalavanan, R., and Ramamurthy , K., "Structurally Efficient Hollow Concrete Blocks in Loadbearing Masonry", Proceedings 01 lhe 8th International BricklBlock Masonry Conlerence, Dublin, Ireland, 1988.

(12) George R. Ivany and Associales , Inc., "Ivany Block-An Innovalion in Concrete Masonry", Technical Brochure, Cleveland, Ohio.

(13) Eppell,F.J.," State 01 the Art Report : Rain Penetration 01 Masonry", Proceedings 01 the Second Canadian Masonry Symposium, Ottawa, June 1980.

ReQulor stretcher

(ol Two · core 8 x8xl6·in units

Both ends ploln

(double Corner ar pIe, 1

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Requlor stretcher

(b) Three-core 8x8x I6·in. unils

Thlckened foce shell

Both ends plom (double Corne, ()( Pler)

Iroor . (e) eross sectians

Figure 1· Typlcal Hollow Unlts

Figure 2· MIsallgnment of Webs

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~ ~ I I I I I o o

(4) lhe core-allgned block.. (b) lhe wavy-core blod:. (c) Th. ftn block syst.m.

Figure 3 - Core-aligned Block, Wavy-core Block and Fin Block

Ivany block for reta l n1nç walls

Figure 4 - Ivany Block and H Block

Face- shell mor1()1" beddinq

Figure 5 - Types oI Mortar Beddlng