Recycled aggregate concrete (RAC) methodology for ...nopr.niscair.res.in/bitstream/123456789/10871/1/IJEMS 17(6) 449-462... · Recycled aggregate concrete (RAC) methodology for estimating

Indian Journal of Engineering & Materials Sciences

Vol. 17, December 2010, pp. 449-462

Recycled aggregate concrete (RAC) methodology for estimating its

long-term properties

Jorge de Brito & Ricardo Robles

Instituto Superior Técnico (IST), Technical University of Lisbon, Av. Rovisco Pais, 1049-001 Lisboa, Lisbon, Portugal

Received 12 January 2009; accepted 13 October 2010

In this paper, a methodology for prediction of long-term properties of recycled aggregate concrete is presented, based

on an extensive literature review of international experimental campaigns on this type of environment-friendly concrete. The

methodology presented is based on the previous determination of the main properties of the aggregates (density and water

absorption), primary and recycled, coarse and fine, and alternatively the 7-day compressive strength of concrete made with

those aggregates. The methodology is validated, based on a graphical analysis of the most important properties of hardened

concrete (compressive strength, modulus of elasticity, splitting and flexural tensile strength, shrinkage, creep, water

absorption, and carbonation and chloride penetration depth). It is concluded that the methodology can predict the long-term

performance of recycled aggregate concrete as compared with an equivalent conventional concrete and that this prediction

can be used to adapt structural design to this material.

Keywords: Recycled aggregates, Structural concrete, Performance early prediction, Sustainable materials

The construction industry is one of the economic

sectors that are more responsible for consuming

natural resources and generating waste. Within the

sector the activities related to the use of concrete,

from production to demolition, play a paramount role.

According to the North American organization, the

Strategic Development Council1, around 6000 million

tons of concrete is produced every year, which is

equivalent to 1 ton per inhabitant of this planet. To

guarantee these levels of concrete consumption

equivalent quantities of materials are needed that, in

“traditional” processes, are limited and non-renewable

natural resources, such as sand, gravel and other

aggregates, mostly from stone quarries. On the other

hand, the demolition of structures also causes a

considerable environmental impact. Masood et al.2

estimated that concrete demolition waste in the

European Union and the United States of America has

reached 100 million tons per year. The destination of

this waste is presently one of the greatest difficulties

and worries of the construction sectors, given the high

costs of dumping and transportation and the scarcity

of appropriate sites for receiving these materials.

The use of aggregates from construction and

demolition waste (CDW) in pavement beds is the

most usual way of reusing this material. Even though

considered as a valid reuse technique, it is not the best

economic valorization of this resource and it is

considered by many researchers to be a down-cycling

process that depreciates the capacities of the material.

But the production of structural concrete with

recycled aggregates (RA), however, offers great

potential and recycles the materials viably and

effectively.

This paper summarizes the knowledge acquired

through past experimental research with recycled

aggregate concrete (RAC) performed by researchers

from various countries and statistically and

graphically processing the published data3, aiming at

correlating the RAC properties with the properties of

the RA used to replace primary aggregates (PA). The

data collected is then systemized and some of the

results in the field of structural concrete are

interpreted. This paper describes the methodology

adopted in the data processing that leads to the

estimation of the long-term behavior of RAC. A

parallel study was performed using experimental

results from Portuguese researchers4.

The graphical analysis of the results of the

experimental campaigns followed a sequence of steps

in which the absolute values of the properties of the

recycled aggregate concrete (RCA) mixes were

converted into values relative to those of a reference

concrete (i.e. a concrete mix without any recycled

aggregates, designated RC) with the same size

distribution of aggregates, workability (slump value

INDIAN J. ENG. MATER. SCI., DECEMBER 2010

450

within a margin of ±10 mm) and effective

water/cement ratio. These relative values were used as

the ordinates of the graphs.

The abscissas of the graphs were the values (again

relative to the reference concrete mix) for three

different parameters: the density of the mixture of

aggregates used in the concrete mix, the water

absorption of the same mixture, and the 7-day

compressive strength of the concrete mix itself.

The advantages of this process are (i) it eliminates

the need to know the precise origin of the recycled

aggregates (e.g. ceramics, mortars or recycled

concrete), which is very often not known by the

recycling entity; this is not relevant, since what

matters is their density and water absorption, which

can be measured in a homogenized sample; (ii) the

replacement ratio of primary aggregates (PA) with

recycled aggregates (RA), as well as their density and

water absorption, are proportionally expressed in the

value of the weighed density and water absorption of

the mixture of aggregates used in the concrete mix

(for example, if only the coarse aggregates are

replaced, the fine portion has the same value in the

denominator and in the numerator).

After the selected experimental campaigns have

been briefly described, each of the individual

properties of hardened concrete is analyzed

graphically and the trends found are explained and

commented upon. The value of the correlation

indexes of the linear regression lines is classified in

categories that indirectly measure their degree of

confidence and the relative value of the slope of the

lines is also discussed since it measures the effect on

each property of the replacement of PA with RA.

Research Significance

This paper presents a methodology to estimate the

long-term properties of structural concrete made with

recycled aggregates (such as those resulting from

Construction and Demolition Waste) based on results

that can be obtained at a very early stage of the

construction process.

This innovative methodology provides the building

owner, the structural designer and the builder with

reliable information to render viable a process (the

reuse of inert waste in concrete production) that at the

moment faces practical technological limitations. It

shifts the reuse of these materials from the currently

dominant practices that in effect lead to their down-

cycling.

Objectives

Studies on the use of RA in concrete production

have generally demonstrated a decrease in the

mechanical and durability-related properties, when

compared with those of a concrete with only PA with

the same characteristics (composition, curing

conditions, workability, strength class, etc.). The

conclusion is also generally drawn that higher the

replacement rate of PA with RA greater differences in

the properties of RAC and the reference concrete

(RC), i.e., without recycled aggregates. To better

understand the behavior of RAC in the fresh and

hardened states, this study set out to collect

experimental data from various international

campaigns, and use it to correlate some of the

properties of the aggregates (density and water

absorption) and the 7-day compressive strength of

concrete with the most relevant properties of the

RAC. This approach is not an absolute novelty since

experimental studies on RAC usually embrace the

study of the RA’s properties. For example, in the

study by Limbachiya et al.5 on high performance

RAC, the authors start the conclusions by

characterizing the RA used in terms of density and

water absorption, relating these properties to those of

the PA. They pointed out that RA from different

origins should be analyzed independently.

As a matter of fact, the density and water

absorption allow the characterization of the

aggregates and their differentiation in terms of origin,

since RA usually show lower values of density and

much higher values of water absorption than those of

PA. The high water absorption of RA is due to the

greater porosity of the hardened mortar adhering to

the original particles and it also depends on the

original concrete and its water/cement ratio. The

aggregates’ water absorption increases for smaller

particles since the specific surface is greater. There is

also an increase in water absorption between concrete

RA and ceramic RA. The crushing process used also

has an influence on the absorption capacity of RA,

since it affects the quantity of adhered mortar that is

released during crushing.

Data research and selection

This study started with a bibliographic Internet-

based survey within the RAC topic, with emphasis on

the published results of international experimental

campaigns. This survey was guided by the criteria:

availability of the tests results performed on RA,

especially for water absorption and density;

BRITO & ROBLES: RECYCLED AGGREGATE CONCRETE

451

availability of the experimental values relative to the

greatest possible number of concrete properties in the

fresh and hardened states (mechanical and durability-

related), especially the 7-day compressive strength;

the greatest possible PA/RA replacement ratio

options; the greatest possible number of fixed

parameters (e.g. water/cement ratio, grading curve,

workability, curing method) during the experimental

production of RAC; existence of a reference concrete,

without which the corresponding campaign was

eliminated from the survey.

Of the numerous campaigns analyzed only a small

group was considered apt for data processing, since

most studies did not comply with some, and in the

majority of cases, most of the conditions stated above.

Methodology used

In order to establish correlations between the RAC

properties and the density and water absorption of the

aggregates used in the mix and the 7-day compressive

strength of the resulting concrete, a methodology of

graphical analysis was established, involving the

following steps: analysis and organization of the data

available from each experimental campaign, including

the properties of the PA, the RA, the RAC and the

RC; study of the composition of the concrete (RC and

RAC) and of the properties of the aggregates to

determine the values for density and water absorption

of the aggregates (RA and PA) used in the mixes;

graphical representation of the relationship between

the concrete properties and the PA/RA replacement

ratio; graphical representation of the variation in the

ratio of the properties between RAC and RC and the

replacement ratio of PA with RA; graphical

representation of the variation in the ratio of the

properties between RAC and RC and the ratio of the

weighed density of the various aggregates (PA and

RA) used in the concrete mixes (as shown in Eq. (1));

graphical representation of the variation in the ratio of

the properties between RAC and RC and the ratio of

the weighed water absorption of the various

aggregates (PA and RA) used in the concrete mixes

(as in Eq. (1)); graphical representation of the

variation in the ratio of the properties between RAC

and RC and the ratio of the respective 7-day

compressive strength values (similarly to Eq. (1));

superposition of the graphical results of each concrete

property for the various campaigns analyzed and

determination of the linear regression lines with the

respective correlation coefficient; correction of the

linear regression lines obtained, to make them

representative of the physical behavior under analysis,

forcing them to pass through the point that

corresponds to the RC, with the inconvenience of

lowering the respective correlation coefficient;

compilation of the data in a table, including the slope

of linear regression line and the respective correlation

coefficient. Table 1 gives the qualitative criteria

defined in terms of the correlation coefficient R2.

The feasibility of methodology is explained by the

fact that, for conventional concrete as well, it is

possible to establish reliable correlations between the

various properties of hardened concrete and the

density (of which the water absorption is an indirect

measure) of the aggregates or the compressive

strength at early age.

Properties of the RA mixture

In order to obtain the weighed value of the density

of the aggregates present in a concrete mix, Eq. (1)

was used. Through this general equation the weighed

density value of the aggregates in the concrete mixes

with the various replacement ratios of PA with RA

(from 0% - reference concrete - to 100%) was

determined. Even though in most of the experimental

campaigns there was no replacement of the fine

fraction, i.e., its density is kept constant for all the

mixes, the weighed density of the mixture considers

them in its calculation.

100

FAD =

( )100

100

FRA FRA FRA FPAsubst d subst d × + − ××

( )100

100

FA−+

( )100

100

CRA CRA CRA CPAsubst d subst d × + − ××

… (1)

Where D is weighed density of the mixture of

aggregates in the concrete mix; FA is percentage of

fine aggregates used in the mixture; substFRA is

Table 1– Qualitative rating of the correlation coefficients

Rating Values range

Very good R2 ≥ 0.95

Good 0.80 ≤ R2 < 0.95

Acceptable 0.65 ≤ R2 < 0.80

Non-acceptable R2 < 0.65


452

replacement ratio (in percentage) of fine primary

aggregates with fine recycled aggregates; substCRA is

replacement ratio (in percentage) of coarse primary

aggregates with coarse recycled aggregates; dFRA is

density of the fine recycled aggregates; dFPA is density

of the fine primary aggregates; dCRA is density of the

coarse recycled aggregates; dCPA is density of the

coarse primary aggregates.

The transformation of the experimental absolute

results to relative values by comparison with the

RC allows the comparison between the different

campaigns performed. The analysis procedure

adopted for the density was also applied to the water

absorption of the mixture of aggregates by replacing

in Eq.(1) all the density values with the corresponding

water absorption values. In both equations it should

be noted that (i) both the fine and coarse fractions are

taken into account according to their relative

importance in the aggregates’ mix, (ii) both the

natural and recycled aggregates are taken into account

according to their relative importance in the same mix

and (iii) it is not necessary to classify or in any other

way thoroughly identify the nature of the recycled

aggregates, as long as it is possible to homogenize the

characteristics of each batch, which makes it much

easier in practice to use recycled aggregates in

concrete production.

Conclusions drawn from the bibliographic survey

The present study has revealed the high hetero-

geneity of the procedures adopted by international

researchers, which sometimes lead to difficulties

when performing such. It has been found that the

results of experimental campaigns, even in leading

international publications, are not always presented in

such a way as to allow their in-depth analysis due to

lack of specific data.

The survey of international experimental

campaigns has revealed great differences at the level

of procedures and of organization/presentation of the

results. In this process the analysis of several

campaigns had to be abandoned because of

unavailability of data. This is either due to existing

information not being included in the description, or

sometimes because relevant data were simply not

determined. Important examples of this type of

limitation are the absence of data on the properties of

aggregates, both RA and PA, or on the composition of

the concrete mixes tested. Another situation

concerned the water/cement ratio, since in some of the

campaigns it was not specified whether the ratio

concerned all the water in the mix or just the effective

water (the first one minus the water directly absorbed

by the recycled aggregates during mixing).

Another data collection problem was the variability

of the factors introduced in each campaign. In order to

allow a scientifically valid comparison, the campaigns

analyzed should be similar in the greatest numbers of

factors that affect the production of concrete. In order

to improve the quality of the comparison, the

following parameters are the ones that it is more

important to keep constant in each family of RC and

RAC: effective water/cement ratio (differentiating the

total amount of water introduced into the mix from

that which effectively contributes to the hydration of

the cement and the workability of fresh concrete);

workability (this property must be maintained by

using plasticizers or increasing the total quantity of

water without increasing the effective water/cement

ratio, for example by pre-saturating the recycled

aggregates); grading curve of the aggregates (when

replacing PA with RA this curve should be kept

exactly constant because any change leads to

uncontrolled shifts in almost every relevant property

of concrete).

This variability of the criteria from one campaign

to another is translated into a decrease in the linearity

of the trends detected (and of the respective

correlation coefficients) when the analysis of the

results progresses from the individual campaign to the

summation of results from various campaigns. The

process of comparison is thus rendered difficult,

leading to an artificial scattering of the results that

should not occur if the conditions stated above are

ensured in every campaign.

Experimental campaigns selected for analysis

In the Carrijo6 campaign, at the University of São

Paulo, Brazil, river sand and basaltic gravel were used

as PA in the production of RC, and coarse ceramic

and recycled concrete as RA in the production of

RCA. Their water absorption was simplistically

considered nil (for comparative purposes, in the

present study the value was changed to 1%). Three

different values of the water/cement (w/c) ratio were

defined in the production of concrete: 0.4, 0.5 and

0.67). Only the coarse aggregates were always

replaced with a replacement ratio of 100%. This

strongly impairs a better understanding of the gradual

evolution of the properties as PA is replaced with RA.

Another parameter that was kept constant was the

quantity of water in the different concrete mixes. The


453

families of RAC were defined by the RA density, in

four categories. The hardened concrete was tested for

compressive strength, water absorption and modulus

of elasticity.

In the Kou et al.7 campaign, at the Hong Kong

Polytechnic University, where the RA used in the

production of RCA were coarse and undifferentiated,

three families of concrete were considered according

to the amount of fly ash added (0, 25% and 35% of

the initial amount of cement) to replace the cement,

leading to three different types of RC. The influence

of the method used to cure the concrete was also

analyzed. The traditional method was immersion in a

water tank at 27ºC (80.6ºF) after 24 h of cure in

natural conditions. In an alternative process the

specimens were exposed to water vapor at 65ºC

(149ºF) for 8 h and afterwards immersed in a water

tank until they were tested. The present study does not

include the comparison of results from the different

curing processes. The effective w/c ratio was kept

constant at 0.45. The hardened concrete was tested for

compressive strength, modulus of elasticity, chloride

penetration and shrinkage.

In the Leite8 campaign, at the Federal University of

Rio Grande do Sul, Brazil, where the RA used in the

production of RCA were coarse and fine ceramic and

recycled concrete, several replacement PA/RA ratios

were selected within each family of RCA, defined by

a predetermined w/c ratio (0.4, 0.45, 0.60, 0.75, and

0.80), leading to multiple linear regression equations

representative of each concrete property under

analysis. For comparative purposes with the other

campaigns analyzed within this study, only the

families corresponding to w/c of 0.40 and 0.45 were

selected, due to the contradictory nature of many of

the results of the other families where a high

percentage of fine RA was used. The hardened

concrete was tested for compressive strength, splitting

and flexural tensile strength and modulus of elasticity.

In the Soberón9 campaign, at the Technical

University of Catalonia, Spain, the RA were obtained

in laboratory by crushing a concrete produced for that

purpose. Two size distributions were obtained for

coarse RA, not strictly the same as for the PA. Each

family of RCA was defined in terms of age when the

tests were performed, since every other parameter

remained constant except for the replacement ratio (0,

15, 30, 60 and 100%) of coarse PA with equivalent

recycled concrete RA. The greater water absorption

capacity of RA compared with PA was taken into

account by pre-moistening the RA before mixing. The

effective w/c ratio was kept constant at 0.52. The

hardened concrete was tested for porosity, density,

permeability, compressive strength, splitting tensile

strength, modulus of elasticity, water absorption,

creep and shrinkage.

In the Cervantes et al.10

campaign, at the

University of Illinois, USA, the RCA families were

defined in terms of the addition of synthetic fibers in

the concrete production. Only the coarse fraction of

PA was replaced with different ratios of recycled

concrete RA (0, 50, and 100%). In order to maximize

the number of valid results, the family with 0.2% of

synthetic fibers was also considered in the present

study. The effective w/c ratio remained constant at

0.51. The hardened concrete was tested for

compressive strength, splitting tensile strength,

modulus of elasticity and shrinkage.

In the Katz11

campaign, at the Israel Institute of

Technology, the RA were produced in the laboratory

by crushing concrete specimens 1, 3 and 28 days old.

Three grades of aggregate groups were used. The PA

were replaced with RA for the following fractions:

2.36-9.5 mm (0.093- 0.374 in) and 9.5-25 mm (0.364-

0.984 in) (both coarse), and 0-2.36 mm (0-0.093 in)

(fines). Fine RCA was used in small amounts and

only to improve workability. Families were defined in

terms of the type of cement used (traditional and

white Portland) and the age of RA at crushing. The

traditional cement class was lower than the white

cement class, and that had some influence on the

results. The hardened concrete was tested for

compressive strength, splitting and flexural tensile

strength, modulus of elasticity, water absorption,

carbonation penetration and shrinkage.

Compressive strength

The compressive strength is the most common

tested property of hardened concrete, and for this

reason it was possible to obtain results from four

campaigns6-9

. The general trend identified for this

property indicates a reduction of strength with the

increase of the PA/RA replacement ratio.

Figure 1a shows the variation of the ratio between

the 28 and 90-day compressive strengths of concrete

(fc) and the ratio between the densities (D) of the

mixture of aggregates. The correlation coefficient is

considered good (according to the criteria defined in

Eq.(1)) and a linear relation between the parameters

can be identified. The reduction of the density and

mechanical strength of RA compared with PA, due to


454

the higher percentage of attached mortar in RA,

contributes to the reduction of the ratio between the

compressive strengths of concrete.

The same analysis was performed with the

variation of the ratio between the compressive

strengths of concrete and the ratio between the water

absorptions (wa) of the mixture of aggregates and is

presented in Fig. 1b. The correlation coefficient is not

acceptable, indicating however a tendency for a linear

behavior between the ratios.

Figure 1c shows the variation of the ratio between

the 28 and 90-day compressive strengths of concrete

and the ratio between the 7-day compressive strengths

of concrete (fc7). The lack of data about the 7-day

compressive strength of the concrete in the campaign

of Carrijo6 excluded the author from this particular

analysis. The negative values in the abscissa axis

mean that in the campaign of Soberón9, some of the

results for the 7-day compressive strength of the

concrete with RA were higher than the conventional

concrete. This particular behavior is not to be

expected and contradicts most of the research; the

values were nevertheless included in the analysis,

contributing to the reduction of the correlation

coefficient, which was, however, acceptable.

Modulus of elasticity

Modulus of elasticity results were obtained from

the campaigns of Carrijo6, Leite

8, Kou et al.

7 and

Soberón9. In every case, the modulus of elasticity

decreases when the PA/RA replacement ratio

increases. This behavior is mostly attributed to the

lower stiffness of RA compared with PA. The higher

porosity of RA is responsible for the higher

deformation of these aggregates when compared with

PA, and this effect is also reflected in the concrete

with RA when compared to conventional concrete.


the 28 and 90-day module of elasticity of concrete

(Ec) and the ratio between the densities of the mixture

of aggregates. The correlation coefficient is

acceptable. The same variation for the ratio between

the water absorptions of the mixture of aggregates is

presented in Fig. 2b. The correlation coefficient is

good. Figure 2c shows the variation of the ratio

between the 28 and 90-day module of elasticity of

concrete and the ratio between the 7-day compressive

strengths of concrete. The correlation coefficient is

not acceptable.

The comparison of the slopes of the correlation

lines between the module of elasticity and the

compressive strengths show similar values, despite

the fact that in the literature12-15

it is often stated that

the inclusion of recycled aggregates has a much

stronger influence on stiffness than on mechanical

strength.

Splitting tensile strength

The campaigns of Kou et al.7, Leite

8 and Soberón

9

tested concrete splitting tensile strength at different

ages (28 and 90 days). The results of Leite8 were only

for the age of 28 days and were very scattered.

Generally, the results of splitting tensile strength

graphs obtained confirm the scatter of test results for

this property in different campaigns. The general

Fig. 1 – Ratio between the 28 and 90-day compressive strengths

of concrete versus (a) the ratio between the densities, (b) the water

absorptions of the mixture of aggregates and (c) the 7-day

compressive strengths of concrete


455

trend identified for this property indicates a reduction

of strength with the increase of the PA/RA

replacement ratio. This is explained, as for

compressive strength, by the lower mechanical

characteristics of the mortar adhering to the primary

aggregates (in the case of recycled concrete RA) and

of the ceramics (when they were used as RA).


the 90-day splitting tensile strengths of concrete (fsp)

and the ratio between the densities of the mixture of

aggregates only for the campaign of Soberón. The

correlation coefficient is good. Figure 3b shows the

same correlation but for the water absorption of

mixture of aggregates. The correlation coefficient

obtained is also good. The variation of the ratio

between the 90-day tensile strengths of concrete and

the ratio between the 7-day compressive strengths is

shown in Fig. 3c. The correlation factor is good.

Judging by the slope of the correlation lines for all

three parameters, the splitting tensile strength is

slightly less affected by the PA/RA replacement than

the compressive strength, in accordance with existing

literature 12,16,17,18

.

Flexural tensile strength

For flexural tensile strength, the test results of

Leite8 at the age of 28 and 90 days are used. The

general trend is the same as for the splitting tensile

Fig. 2 – Ratio between the 28 and 90-day module of elasticity of

concrete versus (a) the ratio between the densitiesc, (b) the water



Fig. 3 – Ratio between the 90-day splitting tensile strengths of

concrete versus (a) the ratio between the densities, (b) the water




456

strength and for the same reasons. Figure 4,

representing the variation of the ratio between the 28

and 90-day flexural strengths of concrete (ft) and the

ratio between the three parameters, indicates the

existence of a linear relationship of the variation. In

the three graphs the correlation coefficients are all

good. However, the slope of the correlation lines is

much higher for all the reference parameters than for

the splitting tensile strength and even the compressive

strength, which contradicts the results of most

experimental campaigns reviewed in this study8,19,20

.

Shrinkage

Shrinkage results were obtained from the

campaigns of Cervantes et al.10

and Soberón 9 at the

age of 28 and 90 days, respectively. The general trend

is that of a progressive increase of shrinkage with the

inclusion of recycled aggregates in the concrete

mixes, which is explained by the higher absorption

and lower stiffness of the RA compared with the PA.


the 28 and 90-day shrinkages of concrete and the ratio

between the densities of the mixture of aggregates.

The correlation coefficient obtained is not acceptable.

Figure 5b shows the same correlation but for the

water absorption of the mixture of aggregates. The

correlation coefficient is not acceptable. Since the 7-

day compressive strength results for the campaign of

Cervantes et al.10

are considered inconsistent, the

analysis of the correlation with this parameter is not

presented. Therefore, Fig. 5c shows the variation of

Fig. 4 – Ratio between the 28 and 90-day flexural tensile strengths

of concrete versus (a) the ratio between the densities, (b) the water



Fig. 5 – Ratio between the 28 and 90-day shrinkages of concrete

versus (a) the ratio between the densities, (b) the water




457

the ratio between the 90-day shrinkages of concrete

and the ratio between the compressive strengths of

concrete for the campaign of Soberón. The correlation

coefficient obtained is good.

In the literature12,14,21,22

, shrinkage is often

considered the single property of concrete most

affected by the replacement of PA with RA. However,

in this study the slopes of the correlation lines for the

90-day shrinkage are similar to those of the modulus

of elasticity and of the compressive strength, even

though they significantly increase for the 28-day

shrinkage (see Table 2). The results are not conclusive

because there are not enough to make them

representative.

Creep

Soberón9 tested the 90-day creep of concrete. The

reduction of the stiffness of RA compared with PA

contributes to a higher creep with the increase of the

PA/RA replacement ratio. Furthermore, a hypothetical

increment of the w/c ratio, to balance the higher water

absorption of RA compared with PA, can contribute

to higher values of creep in the concrete with RA.

Figure 6 shows the ratio between the 90-day creep

values of concrete and the ratio of the densities and

Table 2 – Summary of the correlation trend lines between the different concrete properties and the densities and water absorptions of

the mixture of aggregates and the 7-day compressive strengths of concrete (Robles 2007)

Aggregates

density

Aggregates

water absorption

Concrete 7-day

compressive strength

Property Campaigns R2 slope R2 slope R2 slope

fc28 Carrijo / Kou / Leite / Soberón 0.8697 -1.8354 0.6692 -0.0369 0.6339 -1.3551

Kou / Leite / Soberón - - - - 0.7616 -1.0539 fc90

Leite / Soberón 0.8152 -1.4169 0.7716 -0.0190 - -

Carrijo / Kou / Leite / Soberón 0.837 -1.7693 - - 0.6664 -1.2045 fc

Carrijo / Leite / Soberón - - 0.6250 -0.0308 - -

Ec28 Carrijo / Kou / Soberón 0.7591 -1.9224 0.7300 -0.0506 0.6356 -1.3738

Ec90 Kou / Soberón 0.7565 -4.9946 0.6593 -0.0934 0.4215 -0.9771

Carrijo / Leite / Soberón 0.7791 -2.1506 0.8250 -0.0457 - - Ec

Carrijo / Kou / Soberón - - - - 0.5295 -1.1755

Leite / Soberón 0.4906 -1.3441 0.4693 -0.0180 - - fsp28

Soberón / Kou - - - - 0.6356 -0.6321

Soberón / Kou - - - - 0.8969 -0.7122 fsp90

Soberón 0.8669 -1.5773 0.8660 -0.0294 - -

Leite/ Soberón 0.5358 -1.3530 0.5094 -0.0183 - - fsp

Soberón / Kou - - - - 0.7858 -0.6721

ff Leite 0.8248 -2.9245 0.8240 -0.0392 0.8143 -2.8241

Shrinkage28 Cervantes 0.6280 3.7310 0.7402 0.2117 - -

Shrinkage 90 Soberón 0.7397 2.1448 0.7402 0.0400 0.8910 1.0630

Shrinkage Soberón / Cervantes 0.5890 3.1945 0.3626 0.0525 - -

Creep90 Soberón 0.9943 3.6548 0.9943 0.0682 0.7454 1.0672

Absorption28 Soberón 0.9606 2.8785 0.9601 0.0537 0.6363 0.7900

Carbonation7 Katz 0.7616 6.506 0.7616 0.1780 0.8291 1.5452

Chloride28 Kou 0.8101 14.0860 0.8888 0.4108 0.8903 1.8266

Chloride 90 Kou 0.8821 16.8710 0.8455 0.2744 0.8460 1.2199

Chloride Kou 0.8498 11.3010 0.8109 0.3426 0.8117 1.5233

correlation coefficient acceptable (0.65 ≤ R2 < 0.80)

correlation coefficient good (0.80 ≤ R2 < 0.95)

correlation coefficient very good (R2 ≥ 0.95)


458

water absorptions of the mixture of aggregates and the

7-day compressive strengths of concrete. The

correlation coefficients obtained are very good for the

variation with the ratio between the properties of the

aggregates in the mixture and acceptable for the

variation with the ratio between the 7-day


There are very few studies5,22,23,24

on creep of RCA,

and these point to a slightly better behavior than for

shrinkage. However, the slopes of the correlation lines

obtained from the Soberón study are higher than those

in Fig. 5a and 5b (the exception is Fig. 5c concerning

the 7-day compressive strength of concrete). The

small number of valid results for both shrinkage and

creep does not allow definitive conclusions.

Water absorption by immersion

The water absorption by immersion of concrete

was tested by Soberón9 according to the UNE 83-310-

90 norm. It is expected that increasing the PA/RA

replacement ratio will increase the water absorption of

concrete, mostly because of the much higher water

absorption of RA compared with PA (due to the

porosity of the mortar attached to the first, in the case

of recycled aggregates RA, or the intrinsic porosity of

the ceramic RA).

Figure 7 shows the variation of the ratio between

the 28-day water absorptions by immersion of

concrete and the ratio of the densities and water

absorptions of the mixture of aggregates and the

Fig. 6 – Ratio between the 90-day creep values of concrete versus

(a) the ratio between the densities, (b) the water absorptions of the

mixture of aggregates and (c) the 7-day compressive strengths of

concrete

Fig. 7 – Ratio between the 28-day water absorptions by

immersion of concrete versus (a) the ratio between the densities,

(b) the water absorptions of the mixture of aggregates and (c) the

7-day compressive strengths of concrete


459

7-day compressive strengths of concrete. The

correlation coefficients are very good for the variation

with the ratio between the properties of the mixture of

aggregates and not acceptable for the variation with

the ratio between the 7-day compressive strengths of

concrete.

As stated above, concrete water absorption by

immersion is very strongly linked with the porosity of

the aggregates and therefore is expected to be one of

the characteristics of RCA most affected, in relation

to RC12-14,16,25

. The slopes of the correlation lines in

Fig. 8 are similar to those obtained for the corres-

ponding parameters for creep (Fig. 6) thus corro-

borating the point.

Carbonation penetration depth

The experimental campaign of Katz11

analyzed the

carbonation effect on concrete for a conventional

concrete and a concrete with recycled aggregates

only. For this study, and in order to collect the

maximum amount of results, values obtained in the

three areas of the concrete specimen tested (top,

bottom and sides) were used. Like the resistance to

chloride penetration, the carbonation penetration

resistance is reduced with an increase of the

replacement ratio of RA for PA. This behavior is

mostly justified by the higher porosity of RA

compared with PA.


the 7-day carbonation depths of concrete and the ratio

of the densities and water absorptions of the mixture

of aggregates and the 7-day compressive strengths of

concrete. The correlation coefficients are acceptable

for the variation with the ratio between the properties

of the mixture of aggregates and good for the

variation with the ratio between the 7-day


Even though this conclusion is based solely on the

results of a single campaign, the fact that the slopes of

the correlation lines in Fig. 8 are the highest for all the

properties analyzed so far gives a good indication that

RCA are especially susceptible to this mechanism of

degradation, which initiates the steel reinforcement

corrosion process, as reported in existing

literature12,21,26,27

.

Chloride penetration depth

The chloride penetration results were obtained by

Kou et al.7 through the test defined in the ASTM

C1202-94. This norm establishes the relationship

between the electric charge across concrete over a

certain period of time and the chloride penetration

resistance of concrete. The higher values of the

electric charge correspond to a lower resistance to

chloride penetration. The chloride penetration

resistance is expected to decrease with the increase of

the PA/RA replacement ratio, for the same reason as

for carbonation penetration, i.e., the higher porosity of

RA compared with PA.


the 28 and 90-day electric charges measured in

concrete (Elect) and the ratio between the densities

and water absorptions of the mixture of aggregates

and the 7-day compressive strengths of concrete. The

correlation coefficients obtained are all good,

expressing a linear trend.

Fig. 8 – Ratio between the 7-day carbonation depths of concrete

versus (a) the ratio between the densities, (b) the water




460

The slopes of the correlation lines in Fig. 9 are not

directly comparable with the ones in the equivalent

graphs for the other properties (because the property

analyzed is not the one directly measured). However,

it is very clear that, as for carbonation penetration,

RCA are very susceptible to degradation phenomena

related to the presence of chlorides in the environment

surrounding concrete elements, as reported in the

literature5,12,22,26,28

. Table 2 shows all the correlation

trend lines presented in this paper. The correlation

coefficient classification is identified by different

colors.

Conclusions The search for international experimental results

for this study revealed great differences in the test

procedures and organization of the published

information. Most of the campaigns accepted the

variation of more than one parameter, including the

w/c ratio, making the analysis of the effect of the

replacement ratio impracticable. These obstacles

meant that a great number of campaigns could not be

used in the graphical analysis developed.

This study managed to analyze some of the most

important properties of concrete and to develop the

concept initially devised of finding correlations

between the relative values of these concrete

properties and the weighed density and water

absorption of the mixture of aggregates and the

relative 7-day compressive strength of concrete. The

fundamental objective was thus fulfilled, even though

there is room for further development through the

analysis of other experimental campaigns. This would

serve to consolidate this innovative procedure of

anticipating the properties of RAC relative to those of

a corresponding RC based on the properties of the RA

and early strength of the resulting RAC. The

innovative character of this methodology, which is the

subject of a Portuguese patent (No. PT103756 -

“Methodology to estimate the properties of recycled

aggregates concrete”), lies in the expedient and

economic way by which data is collected (density

and water absorption of the RA and 7-day

compressive strength of the RAC) by the construction

sector agents, enabling an early prediction of the long-

term properties of RAC, given the replacement ratio

of PA with RA and the characteristics of the

aggregates.

Based on a selection of six campaigns, it was

possible to analyze nine different properties of

hardened concrete. The relationship between these

properties and the ratio between densities and water

absorptions of the mixture of aggregates and the

7-day compressive strengths of concrete allowed the

following conclusions to be drawn:

(i) With very few exceptions, it is possible to

establish a linear relationship for the variation of

the ratio between the concrete properties and the

ratio concerning the three parameters mentioned.

(ii) Generally, the density of the mixture of

aggregates showed higher correlation coefficients

in the graphical analysis for the hardened concrete

properties.

Fig. 9 – Ratio between 28 and 90-day electric charges measured in

concrete versus (a) the ratio between the densities, (b) the water




461

(iii) The 7-day compressive strength of concrete

seems to be the least suitable parameter to

estimate the long-term concrete properties, since,

in general, the lowest correlation coefficients

were obtained with this property; this can be

explained by the influence of the variation of

mixture procedures from one campaign to the

other and by the higher scatter of results for

young concrete.

(iv) The lowest results were obtained for the splitting

tensile strength and can be justified by the greater

variability of this property compared with the

compressive strength, for example (a trend

common to conventional concrete).

(v) For every property of hardened concrete analyzed

the performance of RCA is worse than that of the

RC and the difference increases with the value of

the replacement ratio.

(vi) Based on the slope of the correlation lines for

either of three reference parameters, density and

water absorption of the mixture of aggregates and

7-day compressive strength of concrete, the effect

on RCA of the inclusion of RA instead of PA

grows from the mechanical properties

(compressive strength, splitting and flexural

tensile strength and modulus of elasticity) to those

related to its rheologic behavior (shrinkage and

creep), and from these to the durability-related

properties (water absorption, carbonation and

chloride penetration).

Notwithstanding the variability of factors

introduced by each researcher in the experimental

procedures, it was still possible to validate a

methodology to estimate the properties of the concrete

with recycled aggregates. The major advantage of this

procedure is related to the low cost and short time

needed to obtain the results to estimate the long-term

properties of hardened concrete. The generalization of

this methodology could, in the future, allow

construction developers to decide cheaply and quickly

on the use of RA in the construction of new concrete

structures.

A practical example of this application, once it has

been fully developed, would be the chance for a

building owner, who is going to rebuild after

demolishing an existing structure, to decide how to

reuse the waste resulting from the demolition in the

new structure, and provide the structural designer

with the necessary data on the expected evolution of

the properties of the RAC to be used in the

calculations. With this data, the designer can adjust

the reinforced concrete project, allowing a

considerable saving in the whole process and a

contribution to the effective sustainability of the

sector.

Acknowledgements The authors would like to acknowledge the support

of the ICIST Research Institute from IST, the

Technical University of Lisbon and the FCT

(Foundation for Science and Technology).

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