86_Use of Recycled Demolition Aggregate in Precast Products, Phase II,Concrete Paving Blocks

8/17/2019 86_Use of Recycled Demolition Aggregate in Precast Products, Phase II,Concrete Paving Blocks

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Use of recycled demolition aggregate in precast products, phase II: Concrete

paving blocks

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

a Department of Engineering, University of Liverpool, Brownlow Street, Liverpool L69 3GQ, UK b Arup, 12th Floor the Plaza, 100 Old Hall Street, Liverpool L3 9QJ, UK c Department of Civil Engineering, Xi’an Jiatong University, Suzhou, Jiangsu, China

a r t i c l e i n f o

Article history:

Received 7 August 2010

Received in revised form21 November 2010

Accepted 7 December 2010

Keywords:

Recycling of materials

Sustainability

Construction and demolition waste

Recycled demolition aggregate

Concrete paving blocks

Aggregates

Environment

a b s t r a c t

A study undertaken at the University of Liverpool has investigated the potential for using construction

and demolition waste (C&DW) as aggregate in the manufacture of a range of precast concrete products,

i.e. building and paving blocks and pavement flags. Phase II, which is reported here, investigated concrete

paving blocks. Recycled demolition aggregate can be used to replace newly quarried limestone aggregate,

usually used in coarse (6 mm) and fine (4 mm-to-dust) gradings. The first objective, as was the case with

concrete building blocks, was to replicate the process used by industry in fabricating concrete paving

blocks in the laboratory. The compaction technique used involved vibration and pressure at the same

time, i.e. a vibro-compaction technique. An electric hammer used previously for building blocks was

not sufficient for adequate compaction of paving blocks. Adequate compaction could only be achieved

by using the electric hammer while the specimens were on a vibrating table. The experimental work

involved two main series of tests, i.e. paving blocks made with concrete- and masonry-derived aggregate.

Variables that were investigated were level of replacement of (a) coarse aggregate only, (b) fineaggregate

only, and (c) both coarse and fine aggregate. Investigation of mechanical properties, i.e. compressive and

tensile splitting strength, of paving blocks made with recycled demolition aggregate determined levels of

replacement which produced similar mechanical properties to paving blocks made with newly quarriedaggregates. This had to be achieved without an increase in the cement content. The results from this

research programme indicate that recycled demolition aggregate can be used for this new higher value

market and therefore may encourage demolition contractors to develop crushing and screening facilities

for this.

2011 Published by Elsevier Ltd.

1. Introduction

Recycled demolition aggregate can be used to replace newly

quarried limestone aggregate, usually used in coarse (6 mm) and

fine (4 mm-to-dust) gradings in the production of paving blocks.

This study was done subsequently to phase I [1] which investi-

gated the use of recycled demolition aggregate in the manufacture

of concrete building blocks. Paving blocks were selected as a prom-

ising precast concrete product where large quantities of recycled

demolition aggregate could be used but also because:

Possible contamination from C&DW directly affecting reinforce-

ment is not an issue as paving blocks are unreinforced.

Unlike construction projects, paving block fabrication is essen-

tially a manufacturing process where supply of input materials

and 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 cost effec-

tive. Merseyside and more specifically Liverpool has been

selected as a realistic illustrative example of a major UK conur-

bation undergoing regeneration [2].

Resource supply or feed material can be guaranteed in an urban

area like Liverpool where replacement of infrastructure is

occurring, natural aggregate resources are limited, disposal

costs are high, and environmental regulations encourage

recycling.

Concrete block pavements (CBP) have been successfully used in

the UK for more than 30 years. Concrete paving blocks are com-

posed of 85–90% aggregates, mainly limestone and sand. Rectangu-

lar blocks, 200 100 60 mm in size appear to be the most

popular and they were therefore chosen to be investigated in this

project. Concrete paving blocks are precast products that are man-

ufactured in factories. This gives concrete paving blocks good con-

sistency and accuracy of dimensions. It even makes blocks from

0950-0618/$ - see front matter 2011 Published by Elsevier Ltd.doi:10.1016/j.conbuildmat.2010.12.024

⇑ Corresponding author. Tel.: +44 (0)151 794 5217; fax: +44 (0)151 794 5218.

E-mail address: [email protected] (M.N. Soutsos).

Construction and Building Materials 25 (2011) 3131–3143

Contents lists available at ScienceDirect

Construction and Building Materials

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o n b u i l d m a t

http://dx.doi.org/10.1016/j.conbuildmat.2010.12.024mailto:[email protected]://dx.doi.org/10.1016/j.conbuildmat.2010.12.024http://www.sciencedirect.com/science/journal/09500618http://www.elsevier.com/locate/conbuildmathttp://www.elsevier.com/locate/conbuildmathttp://www.sciencedirect.com/science/journal/09500618http://dx.doi.org/10.1016/j.conbuildmat.2010.12.024mailto:[email protected]://dx.doi.org/10.1016/j.conbuildmat.2010.12.024


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different manufacturers interchangeable. Pavements can be con-

structed with different colours, textures, shapes and paving pat-

terns [3].

Concrete paving blocks are manufactured from semi-dry mix-

tures with water–cement ratios less than 0.40. However, unlike

concrete building blocks, paving blocks must be fully compacted

to achieve a higher density. The manufacturing process involves

placing the fresh concrete into steel moulds which are then lev-

elled off before they are compacted by a combination of vibration

and compression force (>10 N/mm2). The concrete paving blocks

are demoulded immediately after casting and placed into a curing

chamber with humidity P80%. They are normally moist cured for

only 24-h and subsequently air cured up till 28 days.

Nowadays machines can be used for the laying of pavements

with paving blocks. Paving blocks have successfully been used

for areas where high loads are expected, e.g., airports and con-

tainer areas. BS 6717 [4] requires a minimum cementitious con-

tent of 380 kg/m3 in order to achieve the required compressive

strength of 49 N/mm2. Ghafoori and Mathis [5] suggested a min-

imum cement content of 395 kg/m3 to satisfy the ASTM specifica-

tions for the freezing and thawing resistance. BS EN 1338 [6].

which superseded BS 6717 [4] in 2003, requires a minimum char-acteristic tensile splitting strength of not less than 3.6 N/mm2. A

minimum cement content is not specified in BS EN 1338 [6]. Typ-

ical mix proportions used by industry to cast paving blocks are

shown in Table 1. The fine aggregate proportion, as a percentage

of the total, is 80%. This is much higher than used for concrete

building blocks but it is required to achieve a denser/better sur-

face finish.

Although concrete block pavements have been widely and, in

the majority of cases, successfully used since World War II, there

have also been instances where they have not performed satisfac-

torily. Such instances include: (a) surface damage – this is mainly a

durability problem which is caused by poor abrasion resistance,

arising from tyres, freeze–thaw damage or deterioration due to

an aggressive environment, etc., (b) spalling or cracking – thismay be caused by a heavy concentrated load from vehicles or

stacking loads, and, (c) excessive localized deformation – this ap-

pears to be a failure of part of the pavement.

Performance requirements for concrete paving blocks are given

in BS EN1338 [6] which classifies paving blocks under four classes,

see Table 2, according to the following mechanical properties: (a)

minimum tensile splitting strength or freezing/thawing resistance,

(b) minimum abrasion resistance, and, (c) minimum slip/skid resis-

tance. The effect of using recycled demolition aggregate as a partial

replacement of limestone aggregate on the compressive and ten-

sile strengths was investigated first. This was necessary in order

to identify acceptable mixes that could be used for full scale factory

trials. These are still being planned and it is anticipated that they

will provide specimens for testing abrasion and slip/skid resistancein the near future.

2. Aims and objectives

Precast concrete factories normally operate 24 h/day. Stoppage

in production is expensive and hence the investigation into the ef-fect of replacing quarried aggregate with recycled demolition

aggregate had to be done in the laboratory. The first objective

was to replicate the industrial casting procedures using laboratory

equipment. Once this was achieved, the effect of partially replacing

quarried with recycled demolition aggregates was investigated.

The industrial collaborators required that there should be no in-

crease in the cement content if recycled demolition aggregate

was to compete with quarried aggregate. The aim therefore was

to determine replacement levels that only caused small and insig-

nificant changes to the mechanical properties of the end products.

3. Materials and experimental methods

3.1. Materials

Specific gravity, absorption, fineness, and angularity are all important physical

properties that need to be taken into consideration if recycled demolition aggregate

is to be used in precast concrete products. The concrete C&DW that was crushed to

produce aggregates came from the foundations of a multi-storey reinforced con-

crete building while the masonry C&DW came from the demolition of low-rise

council houses. It was expected that the detrimental effect of masonry-derived

recycled demolition aggregate (RMA) on compressive strength would be higher

than that of concrete-derived recycled aggregate (RCA), due to the higher propor-

tion of fine, dusty content. It was therefore decided to investigate the effects of

RCA and RMA separately. The percentage of masonry in the mixture is likely to vary

depending on what contract, whether multi-storey buildings or masonry houses,

the demolition contractor has secured.

The aggregate gradings for quarried limestone aggregate, supplied by a block

making factory, as well as RCA and RMA, supplied by local demolition companies,

are shown in Fig. 1. The gradings of both 6 mmRCA and RMA were found to be very

similar to the quarried limestone. However, the4 mm-to-dust RMA wasfound to be

finer than natural medium grading sand. The opposite was found to be true for theRCA.

Both theRCA andRMA had very high percentages of water absorption, see Table

3, which are similar to the behaviour of man-made lightweight aggregate. Water

absorption values ranging from 4 to 15% have been reported in previous studies

[7–14]. A mixing procedure adopted for making concrete using lightweight aggre-

gates, i.e. pre-mixing of half the mix water with the aggregate first and then adding

the cement and the remaining water, had thus been trialled and found to be suc-

cessful when using recycled demolition aggregate. Two-stage mixing has also been

recommended by others [15,16].

3.2. Laboratory mixing and casting procedure for paving blocks

The technique used by industry for making paving blocks is similar to that for

building blocks [1], which was based on applying vibration and compaction at

the same time. A large bearing pad is brought down onto the top of the fresh mix

and is used to compress the concrete while it is vibrated. This procedure is similar

to that for building blocks which was replicated in the laboratory by the use of anelectric hammer, see Fig. 2a. It was believed that thesame technique could success-

Table 1

Mix proportions used by industry in casting concrete paving blocks (kg/m3).

Factory No. #01 Factory No. #02

Target density

(kg/m3)

2250– 2350 2350

Cement (kg/m3) CEM-I : 42.5, 380 CEM-I: 42.5 455

Fine aggregate

(kg/m3)

M grade sand 1520 Limestone dust & coarse sand

1290

Limestone (4 mm-to-dust) 600Coarse

aggregate

(kg/m3)

6 mm single sized

limestone 380

Admixture Superplasticiser 0.6% of

cement content

Concrete water reducer 0.25%

of cement content

Table 2

Performance requirements for concrete paving blocks [5].

Property Recommended

values

Dimension tolerance (when block thickness


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fully replicate the factory procedure for casting paving blocks as it did for building

blocks [1]. However, preliminary trials with varying water–cement ratios from 0.27

to 0.39 indicated that the required compressive and tensile splitting strengths of

49 N/mm2 and 3.9 N/mm2 respectively could not be achieved. This was despite

using similar proportions to those recommended by a precast concrete factory.

The tensile splitting strength vs. water–cement ratio also indicatedthat below a va-

lue of 0.30, full compaction was not being achieved. While the electric hammerwas

sufficient to compact the concrete building blocks it proved not to be sufficient for

paving blocks which require more compaction to achieve a denser block (about 8%

void content by volume). Several techniques were tried to improve the compaction.

This included increasing the compressive force and dispensing with vibration. High

pressure was applied to compact the specimens using the laboratory’s cube crush-

ing machine. A load of 15 N/mm2 was applied to the fresh concrete. The resulting

low compressive strength obtained from paving blocks cast in this way indicated

that compression alone wasnot sufficientand vibrationneeded to be applied simul-

taneously in order to get adequate compaction.

Efforts concentrated on modifying the previously used frame with the electric

hammer, so that the specimens could be vibrated froma source other than theelec-

tric hammer, while they were being compacted. A small metal table was modified

to a vibrating table by mounting a clamp-on-vibrator on it, see Fig. 2b. This was

used together with a plasticiser to improve the wet density of the paving blocks.

2390 kg/m3 was achieved compared to 2230 kg/m3 achieved by the electric ham-

mer alone. This was close to the compacted density achieved in factory production

and was considered to be acceptable. Compressive strengths greater than 49 N/

mm2 and tensile splitting strengths greater than 3.9 N/mm2 at the age of 28-days

were achieved using this method. The texture of concrete paving blocks cast in

the laboratory with the improved ‘vibro-compaction’ technique compared well

with that of paving blocks obtained from the factory. This was in addition to having

similar mechanical properties to those fromthe factory. It was concludedthat,since

the mix proportions used were the same as those used by the industrial collabora-tors, the factory casting procedure was successfully replicated in the laboratory.

Each series of mixes started with an initial cement content of 230 kg/m3. A

handful of the concrete mix was taken after mixing for 3 min. It had been found

from trials that if the concrete mix held together after it was squeezed tightly in

the hand then the mix would be of the required consistency which would enable

it to be compactedinto the moulds. If it did not hold together then additional water

was added. There is currently no widely accepted consistency test for such dry

mixes. Factories rely on their employees’ experience in judging consistency of con-

crete by squeezing it tightly in the hand.

Three paving blocks were cast and an increment of additional cement was then

added. The concrete was re-mixed for a further 2 min, and a visual inspection again

determined whether it was of the desired consistency to be compacted into the

moulds. Incremental increase of the cement content in this manner resulted in

blocks with various cement contents, water–cement ratios, and therefore compres-

sive strengths.

3.3. Compressive and tensile splitting tests for concrete paving blocks

BS 6717 [4] required concrete paving blocks to have a compressive strength of

not less than 49 N/mm2 at 28 days. The current BS EN 1338 [6] only requires the

characteristic tensile splitting strength to be more than 3.6 N/mm2. Concrete block

specimens were sawn into two equal pieces 100 100 60 mm. One of these was

used for tensile splitting test, see Fig. 3, and the other was used to determine the

durability characteristics using a water absorption test. Both tests were carried

out according to BS EN 1338:2003 [6]. Values shown on the figures are from three

replicate specimens.

It is now generally accepted that the variation in concrete strengths follows a

normal distribution [17]. This normal distribution curve is symmetrical about its

mean, has a precise mathematical equation and is completely specified by two

parameters, its mean m and its standard deviation s. The standard deviation is ameasure of the variability calculated from the equation:

Fig. 1. Grading of natural sand, quarried limestone, recycled concrete derived aggregate (RCA) and recycled masonry-derived aggregate (RMA).

Table 3

Water absorptions and densities of aggregates.

Fine aggregate (graded medium-fine sand) Coarse aggregate (5mm single size aggregate)

Sand Concrete Masonry Limestone Concrete Masonry

Particle density (SSDa) (kg/m3) 2440 2250 2420 2690 2380 2260

Particle density (oven-dry) (kg/m3

) 2410 1820 2010 2670 2270 2110Water absorption (% by mass) 1.5 13.56 13.42 0.65 7.24 8.83

a Saturated and surface dry.

M.N. Soutsos et al. / Construction and Building Materials 25 (2011) 3131–3143 3133


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s ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiPð xmÞ2

n 1

s

where x is the individual result, n the number of results and m is the mean of the nresults.

As a result of the variability of concrete in production, it is necessary to design

the mix to have a mean strength greater than the specified characteristic strength

by an amount termed the margin. Thus:

targetmean

strength

" #¼

specifiedcharacteristic

strength

" #þ margin

ðksÞ

The margin M can be derived from:

M ¼ k s

where k is the value appropriate to the ‘‘percentage defectives’’ permitted below the

characteristicstrength. It is derived from the mathematics or the normal distribution

and increases as the proportion of defectives is decreased. For 5% defectives k = 1.64

and s is the standard deviation.

Target mean compressive and tensile splitting strengths were 49 N/mm2 and

3.9 N/mm2, respectively, at 28-days. These were set for this project after consulta-tion with industrial collaborators. A margin of 0.3 N/mm2 was allowed for so that

the characteristic tensile splitting strength (>3.6 N/mm2) reported in BS EN

1338:2003 [6] could be converted to mean tensile strength (3.9 N/mm2). Blocks

were tested at 7- and 28-days with fibreboard packing on the ends to simplify

andspeedup thetesting procedure. The28-day/7-day ratio of 1.1,which wasdeter-

mined from all strength results cast in the laboratory, was used to determine 7-day

target mean compressive and tensile splitting strengths. These were 45 N/mm2 and

3.5 N/mm2, respectively. All the results shown on figures are mean compressive/

tensile strengths and not characteristic strengths.

3.4. Durability characteristics of concrete paving blocks

In addition to compressive and tensile splitting strengths, with which paving

blocks need to comply, there are some other requirements: (a) slip resistance and

(b) water absorption.

Concrete paving blocks need to show a satisfactory slip/skid resistance during

the design life of a pavement. The measure of unpolished slip resistance value

(USRV) is required by BS EN 1338 [6]. The pendulum friction test rig incorporates

a spring loaded slider made of a standard rubber attached to the end of the pendu-

lum. On swinging the pendulum the frictional force between thesliderand test sur-

face is measured by the reduction in length of the swing using a calibrated scale. A

USRV value of at least 63 is considered to be ‘acceptable to good skid resistance’

[18]. This test was not conducted in the laboratory but is going to be used for spec-

imens cast during factory trials in the near future.

The weathering resistance of concrete paving blocks is believed to be related to

the water absorption. BS EN 1338 [6] requires paving blocks to have less than 6%water absorption. The weathering resistance can also be determined by the

freeze–thaw resistance test according to BS EN 1338 [6]. A maximum mass loss

of 1.0 kg/m2 is required for the class three concrete paving blocks, after soaked in

a 3% NaCl solution for 28 freeze/thaw cycles from 20 to 20 C. This test was not

conducted in this project because such a low temperature cyclic freezer was not

readily available in the laboratory. It is anticipated that this as well as abrasion

and slip/skid resistance tests will be carried out on factory cast specimens by the

factory’s quality control laboratory.

4. Results and discussion

The experimental work involved two main series of tests, i.e.

blocks made with RCA and RMA. In addition to the mechanical

properties, i.e. compressive and tensile splitting strength, recycled

demolition aggregate also affects the water absorption of pavingblocks and this is reported at the end of this section.

Fig. 2. Electric hammer used to make paving blocks with clamped-on vibrator used together with an electric hammer.

Fig. 3. Tensile splitting strength test setup. 1 = Packaging pieces, 2 = rigid bars, and

3 = concrete paving bolcks.

3134 M.N. Soutsos et al. / Construction and Building Materials 25 (2011) 3131–3143


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4.1. The effect of recycled demolition aggregate on mechanical

properties

4.1.1. Series I: The effect of RCA on mechanical properties

After successfully having replicated the industrial block-making

procedure in the laboratory, the replacement of quarried limestone

with concrete-derived aggregates was investigated. The mix pro-

portions of natural limestone aggregate used by several block mak-

ing factories are shown in the Table 1. The mix proportions used at

Factory No. #01 were selected for this study as it is anticipated that

this is where full scale factory trials will take place. Each series of

mixes started with an initial cement content of 230 kg/m3 and this

was raised incrementally up to 380 kg/m3, the content identified as

being used at the Factory No. #01. Paving blocks are required to

Fig. 4. 7-day strength vs. cement content for paving blocks with coarse fraction replaced with recycled concrete derived aggregate (RCA).

Fig. 5. 7-day strength vs. free W /C ratio for paving blocks with coarse fraction replaced with recycled concrete derived aggregate (RCA).



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have 28-days compressive and tensile strengths of 49 N/mm2 and

3.9 N/mm2 respectively. It was believed that the fines fraction

(4 mm-to-dust) would have the biggest detrimental effect on

strength. Studies therefore aimed to replace either the coarse or

the fine fraction only but not both in order to quantify the relative

effects of each.

The results from mixes with the coarse aggregate fraction re-

placed with RCA have been plotted as compressive and tensile

splitting strength vs. cement content, see Fig. 4, and vs. water–ce-

ment ratio, see Fig. 5. It is seen that lower water–cement ratios are

needed if blocks using RCA are to have the same strength as blocks

using quarried limestone aggregate. Associated with the low

Fig. 6. 7-day strength vs. cement content for paving blocks with fine fraction replaced with recycled concrete derived aggregate (RCA).

Fig. 7. 7-day strength vs. free W /C ratio for paving blocks with fine fraction replaced with recycled concrete derived aggregate (RCA).



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water–cement ratios is an increase in cement content as it appears

that the consistency of the mix depends to a large extent on its free

water content, i.e. the free water content needs to be the same for

high and low water–cement ratios. This trend compares very well

with the design of normal concrete mixes.

The effect of fine RCA on mechanical properties is shown in Figs.

6 and 7. Concrete paving blocks, unlike concrete building blocks,

use a much higher fine/coarse aggregate ratio. A typical value of

4:1, compared to about 1:1 for building blocks, is used to get a bet-

ter surface finish. This caused concern since the fine fraction was

Fig. 8. Effect of replacing coarse fraction with recycled concrete derived aggregate (RCA) on the strength of concrete paving blocks (all mixes had 380 kg/m3 of cement).

Fig. 9. Effect of replacing fine fraction with recycled concrete derived aggregate (RCA) on the strength of concrete paving blocks (all mixes had 380 kg/m3 of cement).



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shown to have a bigger detrimental effect on the compressive

strength of concrete building blocks than the coarse aggregate

[1]. Figs. 6 and 7 however show that although there is some detri-

mental effect this is similar to the coarse fraction.

Figs. 8 and 9 show the compressive strength vs. the percentage

of replacement of limestone aggregate with RCA (all mixes had

380 kg/m3 of cement). It can be concluded that reasonable replace-

ment levels would be up to 60% for the coarse fraction. It is not sur-

prising, since the coarse aggregate proportion is only 20% of the

total aggregate, that a high percentage replacement only causes a

small detrimental effect on strength. Dhir et al. [19] reported a

19% concrete strength reduction when 100% coarse RCA was used.

Fig. 10. 7-day strength vs. cement content for paving blocks with coarse fraction replaced with recycled masonry-derived aggregate (RMA).

Fig. 11. 7-day strength vs. free W /C ratio for paving blocks with coarse fraction replaced with recycled masonry-derived aggregate (RMA).



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The reduction was attributed to the lower strength of the recycled

aggregates which was determined from the Los Angeles test. Fig. 9

shows that the detrimental effect due to a fine aggregate replace-

ment is surprisingly small and similar to that of coarse aggregate

replacement. This is the case for even high replacement levels.

The tensile strength obtained showed considerable variability.

The results show that the tensile strength does not only show con-

siderable variability but that it is also affected more than the com-

pressive strength. Therefore a conservative recommendation of

60% maximum replacement was made.

It was regrettable that subsequent to the above experiments the

fine RCA ran out. A new delivery was prohibited because a small

amount of asbestos was detected in the samples. This problem is

expected to diminish in the future because asbestos, as a thermal

Fig. 12. 7-day strength vs. cement content for paving blocks with fine fraction replaced with recycled masonry-derived aggregate (RMA).

Fig. 13. 7-day strength vs. free W/C ratio for paving blocks with fine fraction replaced with recycled masonry-derived aggregate (RMA).



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insulation material, was banned in new construction after 1985

[20].

4.1.2. Series II: The effect of RMA on mechanical properties

The replacement of newly quarried limestone aggregate with

RMA has been investigated separately from RCA. The lower density

of fine RMA (4 mm-to-dust) was expected to be problematic. Poon

and Chan [21] reported a 10% reduction in density of paving blocks

when 75% crushed clay was used. The test results have been plot-

ted as compressive strength and tensile splitting strength vs.

water–cement ratio. It is seen that a lower water–cement ratio is

needed if blocks using RMA are to have the same strength as blocks

using quarried limestone. Associated with the lower water–cement

ratios is an increase in cement content as it appears that the con-

Fig. 14. Effect of replacing coarse fraction with recycled masonry-derived aggregate (RMA) on the strength of concrete paving blocks (all mixes had 380 kg/m3 of cement).

Fig. 15. Effect of replacing fine fraction with recycled masonry-derived aggregate (RMA) on the strength of concrete paving blocks (all mixes had 380 kg/m3 of cement).



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sistency of the mix depends to a large extent on its free water–ce-

ment ratio.

The effect of coarse RMA on mechanical properties is shown in

Figs. 10 and 11. Higher cement contents would be needed if blocks

using RMA are to have the same strength as natural aggregate

blocks. Dhir et al. [19] reported a 35% compressive strength reduc-

tion in concrete mixes with 100% coarse RMA. The increase in ce-

ment content required to counteract this was considerable, i.e.

from 214 to 310 kg/m3. A similar increase, i.e. 230–380 kg/m3,

would have been needed for paving blocks with 75% coarse RMA.

Counteracting the strength reduction of higher percentages may

not be economically achievable withan increase in cement content.

The effect of fine RMA on mechanical properties is shown in

Figs. 12 and 13. The fine fraction, i.e. 4 mm-to-dust, was expected

to have an even bigger detrimental effect than RCA fines. Again

Figs. 12 and 13 show that higher cement contents would be needed

if blocks using RMA are to have the same strength as natural aggre-

gate blocks. The correlation between compressive strength and ce-

ment content is also affected. The reduction in the slope indicates

that the detrimental effect of RCA fines will be higher for paving

blocks of higher compressive and tensile splitting strengths. Indus-

trial collaborators have also commented that the scatter in the val-

ues for tensile splitting strength was a lot higher in the laboratory

than they achieve in their factories.

Fig. 14 shows that although there is some detrimental effect, a

reasonable replacement level would be up to 60% for the coarse

fraction with RMA. Similarly to concrete derived aggregate,

Fig. 15 shows that although there is some detrimental effect from

the replacement of the fine fraction with RMA this is not very dis-

similar to the coarse fraction. Tensile splitting strength is again af-

fected more than compressive strength. There is also an indication

that there may be a ceiling value to the compressive strength that

can be obtained with 100% RCA fines. It can be recommended that

a reasonable replacement level would be up to 40% for the fine

fraction with RMA. This recommendation is based on the compres-

sive strength results. It is expected that the industrial casting pro-

cedure will be more efficient for compacting specimens as it was

shown in phase I for concrete building blocks [1].

In order to maximize the recycling, an investigation of the com-

bined effect, i.e. replacement of both coarse (set at 60%) and vary-

ing fine fraction with RMA, was conducted in the laboratory. With

up to 60% of the coarse and 20% of the fine fractions replaced with

RMA, the target compressive strength was still achieved at the age

of 28 days, see Fig. 16.

4.2. Water absorption of paving blocks

The weathering resistance of concrete paving blocks is believed

to be related to the water absorption. 7-day old specimens were

cured in a water tank until they reached constant mass. They were

then oven dried to constant mass. The loss in mass is expressed as

a percentage of the mass of the dry specimen.

The high water absorption of recycled demolition aggregate ap-

pears to influence adversely the concrete water absorption, see

Fig. 17. Similar and even higher values of water absorption have

been reported by Poon and Chan [21]. BS EN 1338 [6] requires pav-

ing blocks to have less than 6% water absorption and this can only

be achieved with the replacement levels indicated in Table 4.

Industry cast limestone aggregate specimens had on average a

water absorption of 4%. The critical or deciding factor for the level

of replacement of newly quarried limestone aggregates with recy-

cled demolition aggregate may have to be the water absorption

rather than the strength. It is fortunate that the ‘‘allowable’’ per-

centage replacement level of coarse RCA is only slightly less than

that determined based on compressive strength of paving blocks.

The recommended 60%, based on strength, may have to be conser-

vatively reduced to 55% for blocks to comply with the requirement

for less than 6% water absorption. Replacement with fine RCA is

however more problematic. The recommended 60% replacement

level based on strength may have to be conservatively reduced to

Fig. 16. Effect of replacing fine fraction (Inc. 60% coarse aggregate replacement) with recycled masonry-derived aggregate (RMA) on strength (all mixes had 380 kg/m3 of cement).



12/13

as low as 25% for blocks to comply with the requirement for less

than 6% water absorption. The same trend has also been shown

with fine RMA derived aggregate with the recommended level of

replacement having to be reduced from 55%, based on strength

down to 20%, based on water absorption. Tests are planned for fac-

tory cast specimens to confirm these findings. It is also believed

that, because of the high water absorption of the recycled demoli-tion aggregate, the water absorption by the blocks may not be

indicative of their durability. This will be confirmed with freeze–

thaw tests to be carried out on factory cast specimens.

5. Conclusions

The electric hammer which was sufficient to compact the con-

crete building blocks proved not to be sufficient for paving blocks,

which required more compaction to achieve a denser block (about

8% void content by volume). The previously used frame with the

electric hammer was placed on a vibrating table so that the speci-

mens could be vibrated from a source other than the electric ham-

mer, while they were being compacted. The texture of concrete

paving blocks cast in the laboratory with the improved ‘vibro-com-paction’ technique compared well with that of paving blocks ob-

tained from the factory. The factory technique was successfully

replicated in the laboratory and therefore the effect of replacing

newly quarried aggregate with recycled demolition aggregate

could be investigated.

Concrete paving blocks use a much higher fine/coarse aggregate

ratio than building blocks (4:1 vs. 1:1), in order to get a better sur-

face finish. The detrimental effect from using high replacement

levels for both coarse and fine fraction replacement of newly quar-

ried aggregate with RCA was small. However, the tensile strength

obtained showed considerable variability, and a conservative rec-

ommendation of up to 60% replacement maximum for both coarse

and fine fractions was made.

The use of RMA leads to some detrimental effect at very high

replacement levels. However, because of the coarse content being

low, a replacement level of 60% for the coarse fraction with ma-

sonry-derived aggregates can still have the same strength as natu-

ral aggregate blocks. Reasonable replacement level with masonry-

derived aggregates as the fine fraction could be up to 40% if newly

quarried aggregate is used as the coarse aggregate. However, this

replacement level needs to be reduced down to 20% when 60% of

the coarse fraction is also replaced with RMA if the target 28-day

tensile splitting strength is to be achieved without increasing the

cement content.

The high water absorption of paving blocks made with recycled

demolition aggregate is however a concern. This may be attributed

to the higher water absorption of the recycled demolition aggre-

gate and may therefore not be a good indicator for durability;

the freeze–thaw resistance of paving blocks needs therefore to be

investigated in order to determine whether this, in addition to

slip/skid resistance, will be a determining factor on the replace-

ment level with recycled demolition aggregate.

The research showed that selection of appropriate replacement

levels of newly quarried with recycled demolition aggregate can

lead to paving blocks with similar mechanical properties without

the need to increase the cement content. It is expected therefore

that there will be significant cost savings where recycled demoli-

tion aggregate can be supplied to the block manufacturer at a pricebelow that of newly quarried aggregates. This price may still be set

higher than that currently obtained for road sub-base aggregate.

This may encourage demolition contractors to develop crushing/

screening facilities for this new added/higher value market.

Acknowledgements

The authors are grateful to the Veolia Environmental Trust and

the Flintshire Community Trust Ltd. (AD Waste Ltd.) for funding

this Project. The authors would also like to thank the following

industrial collaborators for their assistance with the Project: Clean

Merseyside Centre, Marshalls Ltd., Forticrete Ltd., Liverpool City

Council, Liverpool Housing Action Trust (LHAT), Cemex Ltd., WF

Doyle & Co. Ltd., DSM Demolition Ltd. However, the views given

in this discussion are those of the authors and do not necessarily

represent those of the funders, regulatory bodies or commercial

interests.

References

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[3] Tang K. Precast concrete paving products made with recycled demolition

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[4] BSI. BS 6717-1:1993 Precast, unreinforced concrete paving blocks –

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Fig. 17. Effect of recycled demolition aggregateon the water absorption of concrete

paving blocks.

Table 4

Standard requirements vs. replacement level (%).

Aggregate type Compressive strength >49 MPa &

tensile splitting strength >3.6 MPa

(%)

Water

absorption

>6% (%)

Concrete

derived

aggregate

Coarse

aggregate

60 55

Fine

aggregate

60 25

Masonry-

derived

aggregate

Coarse

aggregate

60 55

Fine

aggregate

40 20



13/13

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