Construction and Building Materials 25 (2011) 726–735

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Construction and Building Materials

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Concrete building blocks made with recycled demolition aggregate

Marios N. Soutsos a,*, Kangkang Tang b, Stephen G. Millard a

a Department of Engineering, University of Liverpool, Brownlow Street, Liverpool, L69 3GQ, UKb Arup, 12th floor the Plaza, 100 Old Hall Street, Liverpool, L3 9QJ, UK

a r t i c l e i n f o a b s t r a c t

Article history:Received 14 April 2010Received in revised form 14 July 2010Accepted 18 July 2010Available online 17 August 2010

Keywords:Recycling of materialsSustainabilityConstruction and demolition wasteConcrete blocksAggregatesEnvironmentLandfill

0950-0618/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2010.07.014

* Corresponding author. Tel.: +44 (0) 151 794 5217E-mail address: marios@liverpool.ac.uk (M.N. Sout

A study undertaken at the University of Liverpool has investigated the potential for using recycled demo-lition aggregate in the manufacture of precast concrete building blocks. Recycled aggregates derived fromconstruction and demolition waste (C&DW) can be used to replace quarried limestone aggregate, usuallyused in coarse (6 mm) and fine (4 mm-to-dust) gradings. The manufacturing process used in factories, forlarge-scale production, involves a ‘‘vibro-compaction” casting procedure, using a relatively dry concretemix with low cement content (�100 kg/m3). Trials in the laboratory successfully replicated the manufac-turing process using a specially modified electric hammer drill to compact the concrete mix into oversizesteel moulds to produce blocks of the same physical and mechanical properties as the commercial blocks.This enabled investigations of the effect of partially replacing newly quarried with recycled demolitionaggregate on the compressive strength of building blocks to be carried out in the laboratory. Levels ofreplacement of newly quarried with recycled demolition aggregate have been determined that will nothave significant detrimental effect on the mechanical properties. Factory trials showed that there wereno practical problems with the use of recycled demolition aggregate in the manufacture of buildingblocks. The factory strengths obtained confirmed that the replacement levels selected, based on the lab-oratory work, did not cause any significant strength reduction, i.e. there was no requirement to increasethe cement content to maintain the required strength, and therefore there would be no additional cost tothe manufacturers if they were to use recycled demolition aggregate for their routine concrete buildingblock production.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

About 275 million tonnes of new construction aggregates areextracted annually in the UK. By 2012, if UK demand for aggregatesincreases by an expected 1% per annum, an extra 20 million tonnesof aggregates will be needed each year. About 60% of extractedaggregate is crushed rock and 40% is sand and gravel [1]. Theseare essential materials for buildings and infrastructure but extrac-tion causes significant environmental damage. Government aimsare to reduce demand for primary aggregates by minimising thewaste of construction materials and maximising the use that ismade of alternatives [2]. An attempt to address the environmentalcosts associated with quarrying has been the introduction of theAggregates Levy in April 2002 [3].

The inert fraction or ‘‘core” construction and demolition waste(C&DW), which is essentially the mix of materials obtained whenan item of civil engineering infrastructure is demolished, i.e., thefraction derived from concrete, bricks and tiles, is well suited to

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being crushed and recycled as a substitute for newly quarried (pri-mary) aggregates. Although there are many potential uses for recy-cled demolition aggregate, most are currently used for low-valuepurposes such as road sub-base construction, engineering fill, orlandfill engineering. However, the costs for crushing the C&DW,which is estimated to be approximately £7 per tonne [4–7], isnot recovered when it is sold as road sub-base aggregate. The sell-ing price depends heavily on the demand and can vary between £2and £4 per tonne. Demolition contractors are still therefore re-quired to include for this difference and pay a recycling plant oper-ator for collecting C&DW from demolition sites. While recycleddemolition aggregate could be used for higher-value uses, poten-tial users are deterred by the perceived risks involved [8]. Needshave been identified to be:

� Increase research and development to improve the quality ofrecycled demolition aggregate.� Demonstrate where recycled demolition aggregates are com-

petitive with newly quarried ones. Confidence could be builtby identifying, undertaking and monitoring appropriate demon-stration projects and disseminating the results [5].

M.N. Soutsos et al. / Construction and Building Materials 25 (2011) 726–735 727

� Expand specifications to accept more recycled demolitionaggregate where it has been shown to compete technically. Per-formance criteria for the finished product are preferable to ‘‘rec-ipe” based specifications if recycled demolition aggregate is tobe used more widely [2].� Facilitate the flow of market information.

Provisions for the use of recycled demolition aggregate inhigher grade, i.e., other than their current use for low-value pur-poses such as road sub-base construction, engineering fill, orlandfill engineering, and therefore high-value, i.e. applicationssuch as structural concrete have only recently been included innational standards. BS 8500-2:2006. ‘‘Specification for constituentmaterials and concrete” [8] only covers the use of recycled coarseconcrete aggregates (masonry content <5%), not recycled ma-sonry aggregate, and their use is restricted to the less severeenvironments. Detailed requirements, based on specified testmethods, for recycled masonry aggregates, such as acid solublesulphate, chloride content, alkali content and potential for alka-li-aggregate reaction, are restricting their use in concrete. Therestriction does not relate to mechanical properties; researchindicated that up to 30% coarse or 20% fine recycled concrete-de-rived aggregate had no effect on the strength of concrete [9].There is concern that gypsum plaster used on internal surfacescould accumulate in the fine recycled aggregates. It has beenshown that gypsum plaster, in amounts greater than the maxi-mum permissible sulphate content, can lead to delayed ettringiteformation. Concrete made with recycled aggregate (RA) or recy-cled concrete aggregate (RCA) should also be tested to confirmthat it has adequate freeze–thaw and sulphate resistance for itsintended use. The maximum strength class of concrete madewith RCA should not be more than C50 [8].

Blockwork appeared to be a promising product to begin investi-gations because:

� Possible chloride contamination from C&DW affecting rein-forcement is not an issue as common blocks are unreinforced.� Unlike construction projects, blockwork fabrication is essen-

tially a manufacturing process where supply of input materialsand storage of output are more easily managed.� There may be local circumstances that would make the use of

secondary and recycled materials for high-grade use costeffective.

The market for precast concrete blocks is very competitive. Thebasic concrete building block is a commodity product and the prof-it margin is low [10]. Large multi-national companies, that gener-ally own quarrying operations, dominate the sector. The rawmaterials used to manufacture blocks, whether they are virginaggregate, lightweight or man-made aggregate, are costly to trans-port and therefore most manufacturers have been faced with achoice between locating production close to the raw materials orclose to the market. In the majority of cases the decision has beentaken to locate the precast factory close to or even at the quarrysite. However, there is a significant number of factories that are lo-cated in urban regions. This is because the standard block is onlysold regionally; within a radius of 30 miles of the precast factory,as a result of the low profit margin [4]. There are approximately100 precast factories in the UK producing a wide variety of blocksfor the construction industry but the standard block weighingapproximately 20 kg continues to dominate sales, accounting forseven of every 10 blocks sold. Recent national construction statis-tics from the Department of Trade and Industry (DTI) indicate thatapproximately 360 million blocks are produced annually in the UK[11]. The estimated aggregate consumption can be based on theassumption that aggregate comprises 90% of each block, i.e. aggre-

gate consumption is 6.5 million tonnes per year. A single precastfactory can use up to 500 tonnes of aggregate per day.

Local circumstances favouring use of recycled demolition aggre-gate may include regeneration, which involves not only demolitionbut also major reconstruction, of major UK conurbations. An exam-ple of this is Merseyside, and more specifically Liverpool [12],where resource supply or feed material for a crushing plant canbe guaranteed due to ongoing infrastructure replacement. Naturalaggregate resources are limited in Liverpool, i.e. there are no aggre-gate quarries, and past surveys [13] have shown major movementsof quarry materials from West Midlands and North Wales to theNorth West of England. In considering future supply patterns tothe North West, assumptions will need to be made about suppliesfrom Wales, where planning policies for aggregates are now mat-ters for the devolved administration. It cannot therefore be as-sumed that past supply patterns will necessarily be maintainedin the future. It is not therefore surprising that the Regional WasteStrategy for the North West [14] aims to ‘‘promote the use of recy-cled construction and demolition waste in construction projectsand encourage developers and contractors to specify these materi-als wherever possible in the construction process”. Operators ofcrushing plants would welcome greater use of recycled demolitionaggregates especially for high-value applications, not only becauseof an increase in price per tonne but also because this may providea guaranteed constant/regular demand. Block making factories ap-pear to be interested in recycled demolition aggregate since theprice may be lower than that of quarried aggregate. This is in addi-tion to recycled demolition aggregate being supplied from localsources and thus reducing transport costs [10]. There was thereforescope for investigating a high-end value market, such as concretebuilding blocks, for recycled demolition aggregate.

2. Aims and objectives of project

Precast concrete factories normally operate 24 h per day. Stop-page in production is expensive and hence the investigation intothe effect of replacing quarried aggregate with recycled demolitionaggregate had to be done in the laboratory. The first objective wasto replicate the industrial casting procedures using laboratoryequipment. Once this was achieved, the effect of partially replacingquarried with recycled demolition aggregates was investigated.The Industrial Collaborators required that there should be no in-crease in the cement content if recycled demolition aggregatewas to compete with quarried aggregates. The aim therefore wasto determine replacement levels that only caused small and insig-nificant changes to the mechanical properties of the end products.

3. Materials and experimental methods

Specific gravity, absorption, fineness, and angularity are all important physicalproperties that need to be taken into consideration if recycled demolition aggregateis to be used in precast concrete products. The aggregate gradings for limestoneaggregate, supplied by a block making factory, as well as recycled concrete aggre-gate (RCA) and masonry derived aggregate (RMA) supplied by local demolitioncompanies are shown in Fig. 1. The concrete C&DW that was crushed to produceaggregates came from the foundations of a multi-storey reinforced concrete build-ing while the masonry C&DW came from the demolition of low-rise council houses(built mainly with concrete blocks for internal walls and clay bricks for externalwalls). It was expected that the use of RMA might have a greater detrimental effecton compressive strength than would RCA. It was therefore decided to investigatethe effects of using RCA and RMA separately, with the possibility of interpolatingto obtain the effects of a mixture of the two. The proportion of masonry in the mix-ture is likely to vary depending on what contract, whether multi-storey buildings ormasonry houses, the demolition contractor has secured.

4 mm-to-dust RMA, as delivered from the crushing plant, was found to be muchfiner than quarried limestone. The converse was found to be true for RCA. In orderto obtain a combined grading similar to that of natural limestone, the proportion ofmasonry fines needed to be reduced from 56% to 43% while that of concrete finesneeded to be increased from 56% to 61%. However, the initial trial mixes indicated

Fig. 1. Grading of quarried limestone aggregate and recycled demolition aggregate.

Fig. 3. Initial trials in the lab to cast blocks made use of an electric hammer drill anda cylindrical mould.

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that the concrete fines could be reduced from 61% to 45% and still get the same tex-ture on the blocks as those made with limestone aggregates. The overall gradings ofthe aggregates are shown in Fig. 2.

Both RCA and RMA had very high water absorption, see Table 1, which are sim-ilar to the behaviour of man-made lightweight aggregates. A mixing procedureadopted for making concrete using lightweight aggregates was trialled and foundto be successful when using recycled demolition aggregates, i.e. pre-mixing halfthe mix water with the aggregate and then adding the cement and the remainingwater.

Preliminary trials were carried out using standard 150 mm diameter cylindermoulds and a laboratory vibro-compaction hammer drill, see Fig. 3, to simulatethe industrial technique for making blocks. The texture of the cut surfaces of labo-ratory specimens, which was in addition to the mechanical properties, was ob-served to be similar to factory blocks, see Fig. 4, which indicated that theindustrial production technique could be replicated in the laboratory.

After having successfully replicated the industrial block-making procedure inthe laboratory, the replacement of quarried limestone with recycled demolitionaggregate could be investigated. The mix proportions of natural limestone aggre-gate used by a block making factory, Table 2, had to be converted to volume, re-

Fig. 2. Combined grading of quarried limestone and recycled demolition aggregate.

Table 1Water absorptions and densities of aggregates.

Fine limestone Coarse limestone

Saturated surface dry relative density 2.30 2.69Oven dry relative density 2.24 2.67Moisture absorption (% of dry mass) 2.50 0.65

placed by an equal volume of recycled demolition aggregate and then convertedback into weight. This procedure ensured that the replacement was on a volumetricbasis, rather than weight, and was required in order to take into account the differ-ent densities of the recycled demolition aggregate compared to quarried limestoneaggregate. One of the critical parameters in achieving a target compressive strengthwas found to be the density of concrete blocks. A range of acceptable dry densities,i.e. 1850–2000 kg/m3, was provided by a precast concrete block manufacturer. Allthe factory-supplied blocks were found to be at the upper limit of this range andtherefore a target dry density of 2050 kg/m3 was set as the laboratory target. Waterloss during curing was considerable and therefore the relationship between dry andwet densities had to be determined in order to estimate the correct weight of wetmaterial that was to be placed in the mould before it was compacted, e.g., the wetdensity of limestone aggregate mix had to be 2125 kg/m3 for it to have a dry densityof 2050 kg/m3.

Each series of mixes started with an initial cement content of 100 kg/m3. Ahandful of the concrete mix was taken after mixing for three minutes, see Fig. 5.It had been found from trials that if the concrete mix held together after it wassqueezed tightly in the hand then the mix would be of the required consistencywhich would enable it to be compacted into the moulds. If it did not hold togetherthen additional water was added. In parallel with initial trial block-making proce-dures, purpose built moulds were designed and fabricated in the workshop to en-able full height but half-length block specimens to be made, see Fig. 6. Themoulds were over-sized in height to allow the uncompacted material to be placedin them. A ‘‘compaction rig” was designed and fabricated to produce specimens ofthe required density, size and dimensional tolerance. The rig allowed the vibration/compaction hammer drill to slide down guide rods to a pre-determined height. Thevibration/compaction hammer drill then compacted the material to the requiredheight of 215 mm, the height of factory produced blocks, see Fig. 7. Two to threeblocks were cast and an increment of additional cement was then added. The con-crete was re-mixed for a further two minutes, and a visual inspection again deter-mined whether it had sufficient workability to be compacted into the moulds.Incremental increase of the cement content in this manner resulted in blocks withvarious cement contents, water–cement ratios, and therefore compressivestrengths.

The factory has a curing chamber that is heated to around 43 �C and evaporatingwater from the blocks provides a naturally humid environment. Thus a warm moistcuring environment is provided for the first 24 h of production. However, a compar-ison of laboratory made blocks that were (a) air cured at ambient laboratory condi-tions, see Fig. 8, and (b) cured in an environmental chamber with 90% humidity, at atemperature of 43 �C for 24 h and then air-cured showed very little difference instrength. The manufacturers use a pre-determined ratio between the strength of

Fine masonry Coarse masonry Fine concrete Coarse concrete

2.24 2.30 2.36 2.411.90 2.11 2.01 2.22

18.00 9.15 17.50 8.50

Fig. 4. Initial trials in the lab aimed at producing blocks with the same texture as those obtained from the factory.

Table 2Mix proportions used for blocks.

Mix type Cement (kg/m3) Coarse aggregate(6 mm)

Fine aggregate(4 mm-to-dust)

Total water (kg/m3) Free W/C Density (wet) (kg/m3) Compressive strength

Limestone (kg/m3) Limestone (kg/m3) 7-day 28-day(N/mm2) (N/mm2)

Limestone (control) A.1 100 1000 1250 100 0.62 2125 7.9 8.4A.2 120 1000 1250 100 0.50 2125 9.1 9.6A.3 139 1000 1250 106 0.44 2125 9.6 10.2A.4 158 1000 1250 106 0.39 2125 11.2 11.9A.5 177 1000 1250 112 0.35 2125 11.9 12.6A.6 215 1000 1250 119 0.33 2125 12.2 12.9

Fig. 5. Laboratory mixing, casting and testing – the concrete is being squeezed inthe hand to determine its consistency.

Fig. 6. Laboratory mixing, casting and testing – the pre-determined weight ofmaterial is placed in the over-sized half block mould.

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28-day mortar-capped blocks and that of 7-day blocks tested with fibreboard pack-ing on the ends to simplify and speed up the testing procedure, see Fig. 9. This ratio,which was quoted as 1.06, was also confirmed in the laboratory. All the blocks weretested at 7-days using fibreboard end packing and the conversion factor of 1.06 wasused to convert this strength to the ‘‘equivalent” 28-day strength. All the values onthe figures are the ‘‘equivalent” 28-day strengths.

4. Results and discussion

The experimental work involved two main series of tests, i.e.blocks made with RCA and RMA aggregate. In addition, (a) com-bined coarse and fine fraction replacement, and (b) contaminationby masonry of RCA with RMA have also been investigated. Mix pro-portions selected from the laboratory work were used for full scalefactory trials. These trials are reported at the end of this section.

4.1. Series I – RCA

Blocks made with RCA had marginally lower wet densities thanquarried limestone blocks as a result of the volumetric rather thanweight based replacement procedure. For example, a block with100% replacement of both coarse and fine fractions of limestoneaggregate with RCA, shown as 100C + F in Fig. 10a, had a wet den-sity of 1890 kg/m3 which was lower than the 2125 kg/m3 for ablock with limestone aggregate only. Fig. 10a shows that 100%replacement of both coarse 6 mm and fine 4 mm-to-dust quarriedlimestone aggregate with RCA has a considerable detrimental ef-fect on the compressive strength. The cement content would needto be increased from 100 kg/m3 to approximately 130 kg/m3 in or-der for the strength to be at least 7 N/mm2. This was not acceptableto precast concrete factories as the cost of the additional cement

Fig. 7. Laboratory mixing, casting and testing – the alignment/compaction rig.

Fig. 8. Laboratory mixing, casting and testing – demoulding of the concrete blocksimmediately after casting.

Fig. 9. Laboratory mixing, casting and testing – fibreboard used for testing ofconcrete building blocks for compressive strength at 7-days.

Fig. 10. Compressive strength versus cement content for blocks made with fine andcoarse RCA.

730 M.N. Soutsos et al. / Construction and Building Materials 25 (2011) 726–735

would negate any advantages arising from the use of recycleddemolition aggregate. A ‘‘green” block made with one hundred per-cent recycled demolition aggregate was also impractical becausethere was simply not the required supply; six crushing plantsavailable in Merseyside would not be able to supply one precastconcrete factory and that is assuming that they had a continuoussupply of C&DW. The aim of the research therefore had to be toidentify suitable levels of replacement that had very small detri-

mental effect on the mechanical properties so as not to requirean increase in the cement content. Subsequent studies thereforeaimed to replace either the coarse fraction or the fines fractiononly, but not both, in order to quantify the relative effects of eachfraction. It was believed that the fines fraction, i.e. 4 mm-to-dust, isthe one that has the biggest detrimental effect on the compressivestrength. Promising results were obtained for a 60% replacement ofthe coarse fraction with RCA, i.e. there was no detrimental effect onthe compressive strength. However, increasing the coarse fractionreplacement to 100% appeared to have the same detrimental effectas replacing both the coarse and the fine aggregate fractions withRCA, shown as 60C in Fig. 10a. Fig. 10b shows that replacing the

M.N. Soutsos et al. / Construction and Building Materials 25 (2011) 726–735 731

fine aggregate fraction only with RCA has more of a detrimental ef-fect on strength than the coarse aggregate replacement. The rec-ommendation is therefore to limit the fine aggregatereplacement to less than 30%.

The results from the above mixes have also been plotted ascompressive strength versus water–cement ratio and are shownin Fig. 11. Lower water–cement ratios are needed if RCA is to havethe same strength as quarried limestone blocks. Associated withthe lower water–cement ratios is an increase in cement contentas it appears that the consistency of the mix depends to a large ex-tent on its ‘‘free” water content, i.e. the free water content needs tobe the same for high and low water–cement ratios. This trend re-lates very well with the design of normal concrete mixes [15].Fig. 12 shows the compressive strength versus the percentage of

Fig. 11. Compressive strength versus water–cement ratio for blocks made with fineand coarse RCA.

Fig. 12. Strength versus % replacement level of limestone aggregate with coarse andfine RCA – (all mixes had 100 kg/m3 of cement).

replacement of limestone aggregate with coarse and fine con-crete-derived aggregates for all the mixes with 100 kg/m3 of ce-ment. It was concluded that reasonable replacement levels wouldbe 60% for the coarse fraction and 20% for the fine fraction.

4.2. Series II – RMA

The replacement of quarried limestone aggregate with RMA hasbeen investigated independently from RCA. The lower density ofRMA was expected to be problematic. Replacement of limestonewith RMA on an equal weight basis was not possible. The increasedvolume of material, resulting from the different densities, couldnot be compacted into a block of the required dimensions. Replace-ment again had to be on a volumetric basis, rather than weight, inorder to take into account the different densities of the materials.

Fig. 13 shows that 100% replacement of either the coarse 6 mmor/and the fine 4 mm-to-dust quarried limestone aggregate withRMA had a considerable detrimental effect on the compressivestrength. However, a lower percentage replacement of either thecoarse or fine fraction showed only a small detrimental effect. Thisis also apparent in Fig. 14, which shows the compressive strengthversus water–cement ratio. It appears from this graph that there isa greater detrimental effect at the higher water–cement ratios, i.e.,with low cement contents. This detrimental effect decreases withdecreasing water–cement ratios, i.e. increasing cement contents.

The high percentage of water absorption of recycled aggregateand the difficulty in measuring this accurately may have contributedto the curves of strength versus water–cement ratio being so closetogether for replacement levels up to 60%. The ‘‘true” effect of replac-ing quarried limestone with RMA can be seen in Fig. 15. It appearsthat the detrimental effect varies almost linearly with the percent-age replacement level. However, up to a 20% replacement level bycoarse and fine RMA aggregate can be recommended as it can stillproduce blocks with compressive strengths above 7 N/mm2.

4.3. Series III – combined coarse and fine fraction replacement

The decision was taken on the basis of the results of Series I andII mixes to focus on a mix where, in the case of RCA, the coarse

Fig. 13. Compressive strength versus cement content for coarse and fine fractionreplacement (%) of limestone with RMA.

Fig. 14. Compressive strength versus water–cement ratio for coarse and finefraction replacement (%) of limestone aggregate with RMA.

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fraction of quarried limestone be replaced by 60% but the fine frac-tion be replaced by 30% instead of 20%. The corresponding replace-ment levels for RMA were 20% and 20% for fine and coarsefractions. These percentage replacement levels were based onmixes that had either the coarse or the fine fraction replaced butnot both. It was therefore necessary to investigate the effect ofvarying the percentage of fine fraction replacement for a mix thatalready had either 60% or 20% of the coarse fraction replaced withRCA and RMA aggregate, respectively.

Fig. 16 shows that the effect of increasing replacement level offine fraction has a similar detrimental effect on the 60% coarse RCAmix as it did if all the coarse aggregate was limestone. The similar-

ity of Fig. 16 with Fig. 12 confirms that 60% replacement of thecoarse fraction with RCA has little effect on the strength and thatthe detrimental effect is mainly due to the fine fraction. It appearsthat the increasing the replacement of the fine fraction with 30%RCA may have been on the high side and that a 20% replacementlevel is more appropriate as indicated in Series II mixes.

Similarly for RMA, Fig. 16 shows that the detrimental effect ofincreasing fine fraction replacement on the 20% coarse RMA mixis similar to the effect it had on the coarse limestone aggregatemix. The similarity of Fig. 16 with Fig. 15 confirms that 20%replacement of the coarse fraction with RMA has negligible effect

Fig. 15. Strength versus % replacement level of limestone aggregate with coarse andfine RMA – (all mixes had 100 kg/m3 of cement).

Fig. 16. Strength versus % of fine fraction replacement in 60% coarse RCA and 20%coarse RMA mixes.

Fig. 17. Strength versus % of RMA contamination in RCA.

M.N. Soutsos et al. / Construction and Building Materials 25 (2011) 726–735 733

on the strength, and that the detrimental effect is mainly due to thefine fraction.

4.4. Series IV – contamination by RMA in RCA

Structures are commonly constructed using concrete structuralelements but with masonry cladding. Complete separation of theconcrete and masonry prior to demolition may not be possible ifcontract deadlines are to be adhered to. The Industrial Collabora-tors expressed concern about the homogeneity of the recycleddemolition aggregate. Contamination of RCA with RMA is a concernsince RMA has been shown to have a greater detrimental effect on

strength. Consistency of mechanical properties of concrete build-ing blocks may as a consequence of the variability of supplied recy-cled aggregate be also affected. The mix using 60% and 30% RCAreplacement of the coarse and fine fractions was selected as thecontrol mix. It was then assumed that the coarse and fine RCA frac-tions were contaminated with RMA. Fig. 17 shows that increasingpercentage of RMA in the RCA does have a detrimental effect. De-spite the scatter of results it can be concluded that the percentageof masonry should be limited to 10%. This is also the amount of ma-sonry permitted in recycled aggregate if it is to be classified as‘‘concrete-derived aggregates – RCA” in BS 8500 Guidance for EN206-1 [8].

4.5. Series V factory trials

Sufficient concrete and masonry C&DW was initially crushed tobe used for both laboratory work and factory trials. However, thequantity that remained after the laboratory work was sufficientfor only one factory trial mix. The precast concrete manufacturerrequested that there should be enough aggregate crushed for sev-eral trials rather than just one. It was therefore necessary to re-quest from the crushing plant operator to source and crushadditional material. In total, 10 tonnes of recycled demolitionaggregate was delivered to the Forticrete factory at Buxton. Thiscomprised RMA (2 tonnes of 6 mm and 2 tonnes of 5 mm-to-dust)and RCA (4 tonnes of 6 mm and 2 tonnes of 5 mm-to-dust). Sam-ples obtained from these batches of aggregates indicated that thegradings were all comparable to those used in the laboratory withthe exception of the coarse, i.e. 6 mm, RCA. This was not ‘‘single-sized” but an ‘‘all-in” aggregate. It was therefore decided that boththe 60% coarse fraction and the 30% fine fraction of RCA should bereplaced by the ‘‘all-in” aggregate. The remaining fine RCA wasthen used for an additional, i.e. replacing 30% of only the fine frac-tion of a quarried limestone mix. A further factory trial mix was toinvestigate replacement with RMA and this went ahead as origi-nally planned, i.e. 20% of the coarse fraction and 20% of the finefraction were replaced with coarse and fine RMA. The mix propor-tions used for the factory trials are shown in Table 3.

The factory trials had to be carried out between shifts and whenthere were sufficient storage bins empty to hold the recycled

Table 3Mix proportions and compressive strengths of blocks cast during factory trials.

Mix type Cement (kg/m3) Coarse aggregate(kg/m3)

Fine aggregate(kg/m3)

Free water(kg/m3)

Free W/C Density(wet) (kg/m3)

Compressivestrength (N/mm2)

LimestoneRCAe RMA

LimestoneRCA RMA

7-Day 28-Day 7-Day 28-Day

60% Coarse & 30% fine aggregatereplaced by RCA

L1 93.59 471.30 669.62 211.42 2.26 1967 1917 7.9 10.1

808.66 00 0

L2 151.03 444.45 631.47 179.76 1.19 1888 1849 9.6 11.5762.59 0

0 0L3 209.56 435.64 618.96 181.97 0.87 1880 1852 11.2 14.5

747.48 00 0

30% Fine aggregate replaced by RCA L4 92.76 1129.88 651.98 149.47 1.61 1971 1935 7.7 10.00 254.810 0

L5 158.51 1103.30 636.65 143.89 0.91 1955 1906 12.4 13.40 248.820 0

L6 215.98 1061.99 612.81 136.52 0.63 1961 1927 13.4 16.60 239.500 0

20% Coarse and 20% fine aggregateeplaced by RMA

L7 92.93 791.44 852.21 135.10 1.45 1894 1874 7.0 9.3

0 0157.36 201.24

L8 158.43 778.46 838.24 138.98 0.88 1932 1892 12.2 13.90 0

154.78 197.94L9 211.80 731.29 787.45 143.91 0.68 1911 1886 15.7 16.7

0 0145.40 185.95

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material. The recycled demolition aggregate were delivered in onetonne builders’ bags and were placed in the hoppers. The mix pro-portions/weights were then input into the computer of the batch-ing plant. The first trial required three additions of water beforeapproval was given for the blocks to be cast. This resulted in theblocks from this first batch to be slightly wetter than the norm.Nonetheless, the same amount of water was maintained for thehigher cement contents. The cement contents investigated wereapproximately 100, 175 and 250 kg/m3. The blocks cast were la-belled and one of the blocks from each batch was weighed. This en-abled an accurate estimate of the cement content, see Table 3.There were concerns that the red colour of RMA would be apparentin the blocks. However, it was only after careful inspection of thebuilding blocks that one might find the odd masonry aggregateparticle appearing on the surface. The cement paste covered theRMA effectively and the colour of the blocks was the normal darkgrey.

All blocks were cured for one day in the factory’s humiditychamber. Five or six blocks were tested for compressive strengthat 7- and 28-days. The mixes with RCA and approx. 100 kg/m3 ofcement achieved strengths higher than the 28-day target meanstrength of 7 N/mm2 even at 7-days. The masonry mix withapproximately 100 kg/m3 of cement achieved exactly 7 N/mm2 at7-days. However, the 28-day/7-day strength ratios varied from1.06 to 1.30. The factory is using a revised ratio of 1.08 whichseems to be on the low end of the range obtained. On the otherhand, it may be that because of the higher water absorptions ofthe recycled aggregate, there is sufficient water for hydration avail-able beyond 7-days. This effect will however need to be consistentfor all mixes and production batches if the manufacturer is to takeadvantage of the later age strength development.

The concrete strengths obtained are also shown graphically inFig. 18. It is seen that the industrial vibro-compaction techniquewas more efficient than the laboratory technique and producedhigher compressive strengths throughout. As a result the relation-ships between strength and cement contents were shifted up-wards. The strengths obtained confirmed that the replacementlevels recommended from the laboratory work did not cause sig-nificant strength reduction, i.e. there was no requirement to in-crease the cement content to maintain the required strength, andhence there would be no additional cost to the manufacturers ifthey were to use recycled aggregates. Overall, it was a very satis-factory factory trial.

5. Conclusions

The laboratory part of this work has shown that the industrial‘‘vibro-compaction” technique for casting concrete blocks can bereplicated using an electric hammer drill. This enabled investiga-tions, i.e. the effect of recycled demolition aggregate on the com-pressive strength, to be carried out in a laboratory. Conclusionsfrom the laboratory studies were:

s The physical characteristics of recycled demolition aggregatesmay adversely affect the mechanical properties of the blocks.However, levels of replacement of quarried limestone aggre-gates with recycled demolition aggregates have been deter-mined that will not have significant detrimental effect on thecompressive strength.

s The maximum replacement levels for RCA were determined tobe 60% for the coarse fraction, i.e. 6 mm, and 20% for the finefraction, i.e. 5 mm-to-dust.

Fig. 18. Twenty-eight day strengths of factory blocks made with recycled demo-lition aggregate.

M.N. Soutsos et al. / Construction and Building Materials 25 (2011) 726–735 735

s The maximum replacement levels for RMA were determined tobe 20% for the coarse fraction, i.e. 6 mm, and 20% for the finefraction, i.e. 5 mm-to-dust.

s Even if partial replacement takes place for both the coarse andfine fractions, there is still no significant effect on the compres-sive strength of blocks.

s Contamination of RCA with RMA is a concern since RMA hasbeen shown to have a greater detrimental effect on strength.For this reason, it is recommended that the level of masonrypermitted in RCA should be limited to 10%.

Factory trials showed that there were no practical problemswith the use of recycled demolition aggregate. The strengths ob-tained confirmed that the replacement levels selected, based onthe laboratory work, did not cause any significant strength reduc-tion, i.e. there was no requirement to increase the cement contentto maintain the required strength. Therefore there would be noadditional cost to the manufacturers if they were to use recycledaggregates for their routine concrete building block production.

Acknowledgements

The authors are grateful to the Veolia Environmental Trust andthe Flintshire Community Trust Ltd. (AD Waste Ltd.) for funding

this project. The authors would also like to thank the followingindustrial collaborators for their assistance with the project: CleanMerseyside Centre, Marshalls Ltd, Forticrete Ltd, Liverpool CityCouncil, Liverpool Housing Action Trust (LHAT), Cemex Ltd, WFDoyle & Co. Ltd. and DSM Demolition Ltd. However, the views gi-ven in this discussion are those of the authors and do not necessar-ily represent those of the funders, regulatory bodies or commercialinterests.

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