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4 Elastic Modulus and Strength of Hollow Concrete Block Masonry With Refrence to the Effect of Lateral Ties

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Elastic Modulus and Strength of Hollow Concrete Block Masonry With Refrence to the Effect of Lateral Ties
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  Elastic modulus and strength of hollowconcrete block masonry with reference to the effect of lateral ties F. M. Khalaf,* BS~,   MSC PhD,   A. W. Hendry,@ ,   POD,   FICE , andD. R. Fairbairn,t   BS~,   PhD,   MICE NAPIER  U NIVERSITY : U NIVERSITY OF  E DINBURGH This paper presents the results ofan  experimental inves-tigation to derive  a formula  for the short-term static modulus of elasticity of unfilled  and filled  concreteblockwork masonry. The formula is  based on the mod-  us   of elasticity of each individual material used in theconstruction of blockwork masonry. The eflects   of using Iateral  ties as reinforcement on the behaviour, ductility and strength of concrete blockwork masonry are alsoinvestigated. Notation :   Eb 4 4   k 4 net area of concrete infill:   mm; gross area of prism: mm2 modulus of elasticity of brick unit: N/mm2 secant modulus of elasticity of hollow block,derived from unfilled three-course high half-  block prism with I-2mm dental plaster joints: N/mm2  (see Table 2)secant modulus of elasticity of concrete infill, derived from three-cube specimen: N/mm2  (seeTable 2) modulus of elasticity of aggregate:   N/mm2 modulus of elasticity of mortar joint: N/mm2 secant modulus of elasticity of 1Omm  mortar  joint, derived from unlYled  two-course high full-block prism: N/mm’  (see Table 2)modulus of elasticity of masonry: N/mm2 secant modulus of elasticity of mortar, derivedfrom a three-cube  specimen: N/mm2  (see Table 2 modulus of elasticity of cement paste: N/mm2 ‘Dqarbmt  of Civil and Transportation   Bngineerin&  Napier Uni-vmity,   hrchiston.   10   Colinton  Road, Bdinburgh,   EHlO   ZDT,  igLtmentofcivil~ngalldBuilding~univu‘m.~ ofEdiibur&.  The King’s Buildings.   Bdinburgh,   BH9   UL,  UK. g hb 2 fc x fm fm’ a f volume of aggregate per unit volume of mix brick/block height: mmmortar joint thickness: mmcube compressive strength of block material: N/mm2 cube compressive strength of concrete infIll: N/mm2 characteristic compressive strength of masonry: N/llUI12 cube compressive strength of mortar: N/mm2 ultimate compressive strength of blockwork  masonry: N/mm2   =  0.45  (for 1Omm  mortar joint) diameter of reinforcement: mm 0.25 lntro n tion Many previous attempts have been made to find aformula for the modulus of elasticity of brickwork and blockwork masonry. Sahlin’ related the modulus of elasticity of brickwork masonry to the moduli of boththe brick and the mortar using the following theoreti-cal equation ai 1  1   d)/Ej  + a/ ,’   G   Eb   (1) hba   h,+hj  2 In order to determine the modulus of elasticity Ej  of the mortar joint, Sahlin quoted the expression sug-gested by Hansen 2  for two-phase material (referring toconcrete) and given by the following equation i =  1   g ,   +   g/E (3)185  lvhalaf   et al. Table 1. Ehtic  modulus E;,  for concrete masonry  Nctamacomptwsivc 8trengtll  of mlits: N/mm’ 241.4   24  138 34.5 19310 22069 27.6 17931 20000 20.7 15 862 17241 17.2 15  17216552 13.8 12414 15 172 IO.3 10345 11035Modulus of eltMticity+ E,.:   N/mm’ TypcN mortar   Lii   intcqmlation  permitted. Most other researchers and standards relate the modulus of elasticity of masonry to the ultimate com-  pressive strength f,’   of masonry. Although relatingthe modulus of elasticity to the masonry strength isirrelevant theoretically, it does have some practical value. The British Code of Practice BS 5628 relates the short-term modulus of elasticity for clay, calcium sili-cate and concrete masonry, including reinforced mas-onry with infill  concrete, to fk   as follows’ Em  = fJW 4 The American Masonry Standard (AC1   530-88/ ASCE S-88) derives the modulw of elasticity of mas-onry from a table which relates the net area compres- sive strength of units and the type of mortar.4  This is shown as Table 1. E,,,   can also be determined from the secant modulusof elasticity taken between 0.OSf, and 33fd    by teston prisms in accordance with the prism test method of  AC1   530.1/ASCE   6-88  and ASTM E 447-84.The Canadian Standard CSA-CAN3-S304  recom-mends that the modulus of elasticity of unfilled mas- onry be expressed as follows’   l f ’   <  20685 N mm (5) Based on experimental data, Hatxinikolas  et al.”  recommended a conservative value for the modulus of  elasticity of unfilled masonry as follows   = 75Of, 0 Feeg et ~2 7  suggested that the modulus of elasticity of filled blockwork masonry be expressed as follows Em,   SOOf;  7) To study the effect of lateral ties, Feeg  et al tested reinforced blockwork masonry short columns under concentric load. All the columns were of 400mmnominal cross-section and 144m  high. Some of the col~nswereconstructedusing4OOmm  x 2OOmm  x200mm blocks. The column cross-section consistedof two blocks laid in running bond. Other columnswere built using 400 mm x 400 mm x 200mmsingle-core pilaster concrete block units. Face-shell bedding was used and a mortar joint thickness of  1Omm  was maintained throughout. The block unitthickness restricted the placement of the horizontalreinforcement to a spacing of 200mm; on the other hand, the mortar joint thickness restricted the size  of  the tie reinforcement to be placed in the mortar joint. Tie diameters used in this investigation were 3.77, 4.76  and 6.35  mm. The variables investigated were tic. diameter and tie location within the mortar joint, either in contact with the vertical reinforcement or in the mortar joint between the block outer shells. The test results showed that increasing the tie dia-meter increased the strength of the column, compared with columns with no ties. This increase in strength was also accompanied by a decrease in the amount of vertical cracking at failure. No significant difference instrength was observed between columns having ties in contact with the vertical reinforcement and those which did not, although tie strains were larger for tiet located in the mortar joint. Rust was also noted on ungalvanixed ties after failures of columns where they had been placed in the mortar joint. Sahm”  tested under axial load blockwork masonry  prisms with helical confinement reinforcement at thecore. The test specimens were constructed from con- crete block units, 200 mm square and 200mm high,The units were horizontally laid, and the masonry  prisms were built to have flush mortar joints of nomi-nal thickness 9.5  mm. After construction, helical rein- forcement, consisting of mild steel wire of diameter  52mm  with a core diameter of 108 mm centre-tc- centre was placed inside the prism and the core wss grouted. The observed mode of failure for the ungrouted  prisms was tensile splitting, which was initiated in the central concrete block. The mode of failure for the grouted prisms (splitting of the units and compressivefailure of the core) was similar to that of the ungrout- ed prisms, but not as explosive. Cracking was initiated at 70-75 of the ultimate capacity. Cracking of  prisms, reinforced with confinement wire, started atthe top or bottom block at about 45-55 of ultimateload. The sudden failure was replaced by a more ductile failure. An increase in the compressive strengthof the prism of between 30 and 38 ,  compared with unreinforced prisms, was achieved by introducinghelical confinement reinforcement. Sturgeon et aL9  carried out tests on concentric load-ed nine-course high blockwork masonry columns. The short columns were constructed using 400mm x400mm x 200mm singlecore  pilaster units, andwere tested to failure. Full mortar bedding was usedand a joint thickness of 1Omm  was maintained. Thehorizontal reinforcement was placed within themortar joint of the cross-section. All lateral ties were  fabricated from 6mm diameter plain steel. Theauthors concluded that the introduction of lateral reinforcement resulted in an increase in the ultimatestrength of the column of 8-28 . This Paper presents the results of an experimentalinvestigation carried out to derive a formula for theshort-term static modulus of elasticity E,,,   of concrete blockwork masonry. The results of tests on eighteenaxially loaded unfilled, filled and laterally reinforced blockwork masonry columns are reported. Experimental programme The short stack-bonded blockwork masonry col-umns were constructed and tested under axial load to study the mechanism of failure, to determine the strength, to derive a formula for the modulus of elas-ticity, and to study the effect of lateral ties. The col-umns were divided into two main series: full-block (390mm x 190mm) cross-sections; and half-block  (190 mm x 190 mm) cross-sections. The oltins were all six-course high with a slenderness ratio of  6.26. One type of mortar (1: O-25  : 3),  and one type of  concrete infill  (1: 3 : 2) were used in the construction of the columns. The columns were constructed by anexperienced mason, thus ensuring the complete filling of the 1Omm  horizontal mortar joints between the concrete blocks. The block at the base of each columnwas a bond beam type (Fig. l(a)) in which the end shells and mid-web had been removed to make it  possible to remove the column of mortar remaining after  construction.’ The type of block was provided bythe supplier; the rest of the column blocks were either full- or half-blocks. The full-blocks for the laterallyreinforced columns were provided with 20mm wide and  20mm deep grooves, cut with a diamond saw atthe sides of the block mid-webs (Fig. l(b)), to accom-modate the lateral ties. It was not possible to obtain blocks with such grooves from the supplier, but blockswith a 20 mm or larger dip in the mid-web (Fig. l(c)), can  easily be produced by using a steel mould withsuch dips, as is the case with various different shapes and  types of concrete block on the market. The placement of lateral ties at the mortar joints was avoided because previous studies6 had shown that placing lateral ties at the mortar joints produces a highconcentration of tensile splitting stresses around the tics, resulting in a reduction in the compressive strength of the masonry assemblage. Also, placing thelateral ties at the mortar joints means that concrete blocks with thicker shells are required to comply withthe required concrete cover to the reinforcement.’The lateral ties were placed in every course duringthe construction of the columns, including the top and bottom sides, to prevent any local failure. This gives anominal spacing of 189 mm. The lateral ties, for thefull-block columns, were placed in the 20mm wide Fig. 1. T&es  of concrete block wed  in columnconstruction: (a) bond beam; (6)   stan&.vd;   (c)   standkrd with mid-web dip and 20mm deep grooves during construction. In thecase of half-block columns, taking advantage of block tapering, the dimensions of the lateral ties were made slightly smaller than the half-block wide core. This enabled the ties to be held in the hollow cores, fhst  byfriction and then by mortar after the construction.  Two steel brackets, 25 mm wide and 6 mm thick,were placed in prepared positions at the first and fifthmortar joints. These brackets were used later to mounttwo electrical displacement transducers (LVDTs)  on both sides of the column to measure changes in lengthwith the load increments.After construction the six-course high columns wereleft for four days under polythene sheeting to allow the mortar joints to gain in strength. The columns were then filled with concrete which was batched byvolume, mixed to a high slump of 150 mm, then cast in two layers. Each layer was compacted using a25mm poker vibrator until full compaction was attained. The surface of the concrete inftll  was thentrowelled level. After casting, the specimens were leftto cure under polythene sheeting for fourteen days.The polythene was then removed, and the specimensleft for a further fourteen days to cure under ambientconditions in the laboratory before testing.Twelve of the eighteen full- and half-block columns built were unreinforced columns, tested either unfilledor filled, under axial load to determine the short-termstatic modulus of elasticity E of the blockwork mas-onry. The rest were filled reinforced columns testedunder axial load to study the effect of the lateral ties.The mechanical properties for the materials used inthe construction of the blockwork masonry columnswere determined as follows (Table 2). Concrete block The mechanical properties for hollow blocks were determined by testing three blockwork masonry Table 2. Material me mica1   propertiesMatelial Block SolidtPrism$ Half-block  24.3   21.9   25.7   cbncrete 1:5:21:3:21:1:2 8.8 8.3 225 17.2 42.1 37.8 Mortar 1:1:6 1:@5:4~5 1:025:3 8.18.016.5 14.6 26.6  25-0   2113   2127   21277.2 1978 165 2034 32.9 2286 69 1798 12.5 1875 21.7 1948 1omm   nmarjoiar1:1:6 1:0.5:4*5 l:Q25:3   + kant  modulus of ehaticity  at 3   maximum eomprrasive   &engtb  of spaSma. tCt~bc   comprcssivc   stmgtb  of block materials   =   24.3N/mm2. UniYlaI   thrceeoursc  high half-block prism with 1-2mm dental plaster jolntn.spcciInens  or singlc-cubcatrcngtb: N/l2ldllmecllbes specimenstrength: N/Uld  prisms. Each consisted of three-course high half-block   prisms separated and capped with a I-2mm dental  plaster joint inserted between the half-blocks and bet-ween the prisms and the machine platens. This thick-ness was achieved by mixing the dental plaster withwater in a plastic bag to the desired workability. Aspirit level was used to adjust the specimen. The soft dental plaster was then compressed by the machine toaccomplish the desired thickness.” Double-axis (vertical and horizontal) elect&l strain gauges were mounted at mid-prism height ontwo opposite sides of the prism. A computer strain logger was used to record the strain continuouslythroughout the test until prism failure. Unit half-blocks were tested in compression to determine their mechanical properties. Solid full-blocks,cast at the same time as the hollow blocks, were also compressed parallel to the bed face to compare their  mechanical properties with the hollow units. Solid blocks, sawn to the dimensions of a 190mm x 190 mm x190 mm cube, were tested in compression.The average compressive strength was then adjustedfor specimen sire” to determine the block materialcube strength fb. Concrete n ll To determine the contribution of concrete infill  tothe modulus of elasticity II,,,  of blockwork masonry,eighteen three-course high prisms, nine full-block andnine half-block, were constructed using a 10 mm thick  polystyrene sheet inserted between the blocks to sim- ulate a zero strength mortar joint. The prisms were Density: kg/m’ Tangentsecant* modulus  of  modulus of  elasticity: elasticity: N/mm* N/lid Poisson ratio Initial stlws Maximum strws 1911813014  O 2 25 33 100 28977  0.15  2 38 054 38 054 0.13 0.186032 4674  044 0.3316033 11444  0.18  4 28 320 17 180 0.14  0.225603 1025014119 2m 41005500 3696  0.26  040 5ooo O-22 0.358140 0.18  0.251232 2652 4037  
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