ORIGINAL ARTICLE

Bond behavior between steel reinforcement and recycledconcrete

Sindy Seara-Paz • Belen Gonzalez-Fonteboa •

Javier Eiras-Lopez • Manuel F. Herrador

Received: 5 December 2012 / Accepted: 20 March 2013

� RILEM 2013

Abstract In this paper the bond behavior of recycled

aggregate concrete was characterized by replacing

different percentages of natural coarse aggregate with

recycled coarse aggregate (20, 50 and 100 %). The

results made it possible to establish the differences

between the conventional concrete bond strength and

the recycled concrete bond strength depending on the

replacement percentage. It was thus found that bond

stress decreases with the increase of the percentage of

recycled coarse aggregate used. In order to define the

influence of recycled aggregate content on bond

behavior, normalized bond strength was calculated

taking into account the reduced compressive strength

of the recycled concretes. Finally, using the experi-

mental results, a modified expression for maximum

bond stress (bond strength) prediction was developed,

taking into account replacement percentage and

compressive strength. The obtained results show that

the equation proposed provides an experimental value

to theoretical prediction ratio similar to that of

conventional concrete.

Keywords Recycled concrete � Bond strength � Pull

out test � Normalized bond strength � Time-dependent

compressive strength � Steel reinforcement

1 Introduction

In recent years, the use of recycled concrete has been

widely recognized as a means to solve, reduce or

minimize the environmental impact of construction

and demolition waste. Many countries promote sus-

tainable construction based on reuse of coarse aggre-

gates from crushed structural concrete, with the

objective of preserving natural resources and reducing

space for waste storage.

Aiming to popularize and encourage its use in

different application fields, many international studies

have been conducted about this topic [1–9]. Most of

them [1–5] state reductions in mechanical properties

for recycled concretes with a remarkable influence of

recycled aggregate content. This means that mechan-

ical strength decreases with the increase of replace-

ment ratio with recycled coarse aggregate. Some

findings [1, 3] note reductions up to 15–20 % in

S. Seara-Paz (&)

School of Building Engineering, Department of

Construction Technology, University of A Coruna,

E.U. Arquitectura Tecnica, Campus Zapateira s/n,

15071 La Coruna, Spain

e-mail: gumersinda.spaz@udc.es

B. Gonzalez-Fonteboa � J. Eiras-Lopez � M. F. Herrador

School of Civil Engineering, Department of Construction

Technology, University of A Coruna, E.T.S.I. Caminos,

Canales y Puertos, Campus Elvina s/n, 15071 La Coruna,

Spain

e-mail: bfonteboa@udc.es

J. Eiras-Lopez

e-mail: jeiras@udc.es

M. F. Herrador

e-mail: mherrador@udc.es

Materials and Structures

DOI 10.1617/s11527-013-0063-z

compressive strength or 40 % in modulus of elasticity

for 100 % replacement ratio, while tensile strength

shows random results, ranging from a slight increase

up to 10 % reduction. Nevertheless, recycled aggre-

gates from concrete demolition offer good enough

features for use in structural reinforced concrete;

indeed, with a recycled aggregate content up to

20–30 % the reduction in mechanical properties is

not conspicuous [6].

Presently, reinforced concrete is present in almost

all building and civil engineering works. This fact

makes concrete bond strength one of the most

important parameters in structural design. Thus, if

recycled aggregates are to be commonly used in

structural concrete, it is necessary to analyze the bond

behavior between reinforcing steel and recycled

coarse aggregate [7, 10–12].

2 Objectives

When designing reinforced concrete structures,

knowledge of bond strength is essential to determine

the necessary anchorage length. Both parameters are

closely related and depend on each other, so if bond

strength decreases, anchorage length must be

increased in the same extent and vice versa.

Numerous researches in the field of bond behavior

between steel bars and concrete have been carried out

in the last decades. As a result, parameters or variables

that have a noticeable influence on bond strength have

been identified. Most of these studies agree in pointing

out bar rib geometry [7, 10], concrete strength [12–

15], position and orientation of the bar during casting,

stress state, boundary conditions and concrete cover

[14, 16], as determining factors for bond design [17].

The use of recycled concrete introduces new factors

into the bond behavior study due to the effect of the

adhered mortar present in recycled coarse aggregates,

which generates a new weak interface between

original mortar and new mortar. However, it has been

reported that the effect of recycled aggregates on bond

strength is lower than the effect of other factors such as

rebar geometry [10, 12] or boundary conditions [11,

18]. Different conclusions have been drawn from the

literature; while some authors noted reductions of

6–8 % [7, 10, 19], other authors obtained drops of 20

and 30 % [11, 18] for concretes with 100 % recycled

aggregate content compared to conventional ones.

Only Xiao et al. [12] observed similar bond strengths

(differences of 1 %) between the recycled and the

conventional concretes.

Regarding the amount and origin of recycled

aggregates, this research is focused on the use of

recycled coarse aggregate from demolition of concrete

structures to make concrete with different replacement

ratios: 20 % (highest percentage allowed by Spanish

codes [20]), 50 % (medium value) and 100 % (total

replacement of coarse aggregates).

On the other hand, the great number of researches

involving structural recycled concrete [21, 22] allows

to determine its behavior in structural members and to

establish trustworthy design guidelines. This is espe-

cially important for those countries whose codes and

standards do not yet include recommendations of use

for high replacement ratios. This paper attempts to

determine the recycled aggregate’s influence on bond

behavior, studying the effect of different replacement

percentages and its evolution over time.

3 Experimental program

For this study, two types of concrete have been

designed with different water to cement ratios, one

with w/c = 0.50, and the other with 0.65, named

respectively H50 and H65. The intention is to

encompass two types of recycled concretes, one in

the high strength range (w/c = 0.50) and another one

of lesser performance (w/c = 0.65).

Each type is constituted by four series of concretes

with different replacement ratios, 20, 50 and 100 %;

and a conventional or control concrete (replacement

ratio is considered as 0 %). Finally, eight different

concretes were obtained, which are referred to here-

after as H50-0, H50-20, H50-50, H50-100, H65-0,

H65-20, H65-50 and H65-100.

Firstly, consistency and density of fresh concrete

were identified. Then, density and water absorption

capacity were determined to define the basic proper-

ties of the hardened concretes. After that, compressive

strengths at 7, 28, 90 and 365 days were studied in

order to define a correct time-dependent curve of

compressive strength concretes with different replace-

ment ratios. Finally, pull-out tests at 7, 28, 90 and

365 days were performed to obtain the bond behavior

in each of the eight designed concretes at different

ages.

Materials and Structures

During the pull-out tests, the load and slip values

were measured and recorded for each specimen. These

measurements made it possible to determine the load–

slip curves and the bond strength of each designed

concrete. Finally, with these experimental results, the

bond behavior of recycled concrete with different

replacement ratios was identified.

4 Materials

The cement used was CEM I-42.5N/SR, which

according to Spanish standards has a minimum clinker

content of 95 % and 42.5 MPa compressive strength

(EN 197-1). A superplasticizer, SIKAMENT 500 HE,

was also used, as water reducing admixture in order to

obtain suitable workability.

Two types of coarse aggregates, natural and

recycled, were used. Two fractions of natural coarse

aggregates from crushed limestone were used; one

with aggregate size 4–12 mm (4–12N) and the other

with 8–20 mm (8–20N), whose fineness moduli were

6.20 and 7.37 respectively.

Recycled aggregates obtained from real demolition

debris of structural concrete were used. The fraction

used was 4–16 mm (4–16R) and its fineness modulus

was 7.15. These aggregates were made up mainly of

concrete.

Finally, for the fine fraction of aggregate, a natural

sand with a maximum aggregate size of 4 mm (0–4 N)

and a fineness modulus of 3.71 was used.

Table 1 summarizes the basic properties of the

aggregates used. Figures 1 and 2 respectively show

the grading curves of the aggregates (both natural and

recycled) and the composition of recycled coarse

aggregates. Since this experimental program consti-

tutes the third phase of a long research project, some of

these results have been more thoroughly presented in

previous papers [21, 23].

4.1 Test specimens

For this experimental program 35 cubic (10 9 10 9

10 cm) and 3 cylindrical (15 9 30 cm) specimens

were produced for each type of concrete. 15 of these

specimens were used to determine compressive

strength, in order to characterize the different con-

cretes designed. The remaining 20 cubic specimens

were embedded with a steel bar (diameter = 10 mm)

placed in the middle of the cross-section concrete and

tested according to the RILEM pull-out method [24].

All the specimens were stored in a climatic

chamber according to the EN 12390-2, at 20 �C of

temperature and 95 % of relative air moisture until the

age at testing.

4.2 Concrete mixtures

The concrete mixtures were designed according to the

Faury method [25] with a variable amount of water-

Table 1 Basic properties

of the aggregates used0–4N 8–20N 4–12N 4–16R

Density (EN 1097-6) g/cm3 2.67 2.66 2.61 2.57

Density in oven-dry conditions (EN 1097-6) g/cm3 2.52 2.56 2.47 2.25

Water Absorption (EN 1097-6) % 2.22 1.33 2.20 5.40

Los Angeles Abrasion (EN 1097-2) % – 23.10 – 34.28

Fineness module (EN 933-1) 3.71 7.37 6.20 7.15

Fines percentage (EN 933-1) % 11.54 0.42 1.49 0.30

Moisture content (EN 933-1) % 0.08 0.05 0.07 2.92

Fig. 1 Aggregate grading’s

Materials and Structures

reducing admixture, adjusted to achieve a slump

between 5 and 9 cm according to EN 12350-2, as

noted by other authors [26–28].

Recycled aggregates have a high absorption capac-

ity, due mainly to the adhered mortar [3, 4, 29–31].

This feature influences concrete properties, particu-

larly its consistency and workability, which are

reduced when recycled aggregates are introduced into

the mix. This decrease is due to the absorption of

mixing water by the recycled coarse aggregates.

In recent years, numerous attempts have been made

to define the best way to mitigate the effects of the

high absorption capacity of the recycled aggregates

and to prevent the decrease in the water to cement

ratio. As a result, several alternatives have been

proposed: working with dry aggregates while increas-

ing the amount of water incorporated in the mixer [2],

pre-soaking the recycled aggregates for 24 h, pre-

wetting them for 10 min [32] or sprinkling them [3].

The chosen option for this experimental program was

to pre-wet the recycled aggregates for 10 min before

mixing [32].

4.3 Test procedure

The test program included concrete characterization,

both in fresh and hardened state, and pull-out tests to

define the bond behavior of the recycled concretes.

Regarding fresh concrete, characterization included

the measurement of consistency and density according

to standards EN 12350-2 and EN 12350-6,

respectively.

For hardened concrete, compressive strength

(EN 12390-3) was determined at 7, 28, 90 and

365 days, using cubic specimens (fc,cub) and also

cylinder specimens (fc,cyl) at 28 days. Density

and water absorption tests (EN 12390-7) were

carried out at time of demolding, 1 day after

casting.

Pull-out tests were carried out using displacement

control, at a rate of 0.01 mm/s [33], following

RILEM TC9-RC 6 Bond Test guideline for rein-

forcement steel. The fixed rate was achieved using

feedback from a displacement transducer. Load–slip

measurements were taken with a load cell and

another displacement transducer with an accuracy

of ± 0.001 mm. All these instruments were con-

nected to a data acquisition system. Figure 3 shows

a specimen with the test equipment during a pull-out

test.

Thus, throughout these tests, load and slip were

continuously recorded, making it possible to define the

load–slip curve. Model Code-2010 [34] considers

1 mm of slip as the value where load reaches its

maximum and tends to stabilize on pull-out tests.

Therefore, in this experimental program, maximum

Fig. 2 Composition of

recycled coarse aggregates

(percentage by weight

according to EN-933-1)

Materials and Structures

slip value was established at 1.5 mm, in order to define

the constant branch of load–slip curve and the end of

pull-out tests.

5 Tests results

Compressive and bond behavior determined on the

recycled concretes can be compared with the conven-

tional one made with the same mix proportions

(Table 2).

5.1 Consistency and density

The basic properties of the different concretes are

collected in Table 3.

Fresh concrete consistency was obtained using the

slump-test (EN 12350-2) and the values were all

between 5 and 9 cm, as shown in Table 3.

Recycled concretes have lower density than

conventional ones, mainly due to the adhered mortar

of the recycled coarse aggregates, as reported by

some authors in their researches [3, 23, 35]. As

shown in Table 3, the density values obtained

experimentally decrease as the replacement rate of

recycled aggregates increases, which agrees with the

conclusions obtained by the authors mentioned

above.

Fig. 3 Pull-out test

Table 2 Mix proportions 1 m3

0 % 20 % 50 % 100 %

Concrete H65

Cement kg 275.00 275.00 275.00 275.00

Water kg 178.75 178.75 178.75 178.75

0–4N kg 918.49 938.05 962.73 1005.18

8–20N kg 486.19 372.47 218.29 0.00

4–12N kg 457.65 350.60 205.48 0.00

4–16R kg 0.00 180.77 423.77 756.46

w/c 0.65 0.65 0.65 0.65

Admixture % 0.98 0.83 0.42 0.08

Concrete H50

Cement kg 380.00 380.00 380.00 380.00

Water kg 190.00 190.00 190.00 190.00

0–4N kg 781.43 794.31 811.37 838.29

8–20N kg 665.44 512.76 303.34 0.00

4–12N kg 307.93 237.28 140.37 0.00

4–16R kg 0.00 187.51 443.71 807.97

w/c 0.50 0.50 0.50 0.50

Admixture % 0.43 0.20 0.30 0.20

Materials and Structures

5.2 Compressive strength

Table 4 and Fig. 4 show the compressive strength of

the studied concretes for different ages at testing (7,

28, 90 and 365 days).

The compressive strength at 28 days of the H50

recycled concretes with a replacement ratio of 20, 50

and 100 % showed a decrease compared to conven-

tional concrete of 11, 18 and 31 %, respectively. If the

H65 concretes (at 28 days) are analyzed, drops of 2, 7

and 22 % in compressive strength for 20, 50 and

100 % replacement ratio are respectively observed.

Thus, with the increase of replacement ratio, the H65

recycled concretes underwent lower reductions than

the H50 ones, as already noted by other authors

[1, 36].

5.3 Time-dependent compressive strength

The time-dependent development of compressive

strength up to 365 days is shown in Fig. 4.

From 7 to 28 days, a different behavior pattern

between concretes was observed, dependent on the

recycled aggregate content, both for H50 and H65

series. On the one hand, the H50-0, H50-20, H65-0 and

H65-20 concretes underwent an increase of 11, 6, 13

and 29 %, respectively; while H50-50, H50-100, H65-

50 and H65-100 recycled concretes showed slightly

higher increases (16, 16, 21 and 25 %, respectively).

These increase percentages for recycled concretes

with 100 % replacement ratios are similar to those

reported by other authors [7, 10].

From 28 to 365 days age, there were hardly any

variations in compressive strengths and all the

concretes showed a similar trend of time-dependent

development.

Table 3 Basic properties of the concretes [17, 19]

0 % 20 % 50 % 100 %

Concretes H65

Slump values cm 6 7 6 5

Density of fresh

concrete

t/m3 2.38 2.36 2.33 2.29

Density of hardened

concrete

t/m3 2.38 2.36 2.32 2.28

Concretes H50

Slump values cm 5 6 8 9

Density of fresh

concrete

t/m3 2.39 2.37 2.32 2.29

Density of hardened

concrete

t/m3 2.39 2.36 2.31 2.28

Table 4 Mean values of compressive strength (MPa) and bond stress (MPa) at different ages and bond strength predictions at

28 days

H50-0 H50-20 H50-50 H50-100 H65-0 H65-20 H65-50 H65-100

fcm,cub, 7 days 58.03 53.72 45.54 38.05 47.68 41.04 41.23 33.63

fcm,cub, 28 days 64.13 57.19 52.74 44.20 53.85 52.83 49.85 42.04

fcm,cub, 90 days 64.58 60.73 55.02 47.73 54.97 54.87 48.50 42.89

fcm,cub, 365 days 69.87 61.51 48.98 47.99 57.02 53.21 49.20 41.54

fcm,cyl, 28 days 53.47 48.56 43.11 35.21 44.45 44.28 40.52 35.58

sb,max, 7 days 24.16 20.44 15.13 15.52 20.33 17.25 16.58 15.07

sb,max, 28 days 25.32 22.97 21.22 18.39 21.62 20.67 19.11 16.90

sb,max, 90 days 24.94 22.12 18.65 18.28 22.33 20.45 18.73 17.32

sb,max, 365 days 24.32 23.54 18.55 17.40 21.83 20.62 18.99 16.96

sb,m, 7 days 15.64 13.55 9.06 9.56 14.68 10.75 11.25 9.29

sb,m, 28 days 17.23 14.61 13.63 11.93 14.92 13.38 12.44 11.08

sb,m, 90 days 15.22 13.73 11.25 11.45 15.56 13.41 12.01 10.58

sb,m, 365 days 16.17 14.85 11.59 10.48 14.34 13.58 12.68 10.47

sb,max, Kim et al. 17.31 16.34 15.15 13.27 15.75 15.58 14.67 13.35

sb,max, MC-2010 18.28 17.42 16.41 14.83 16.67 16.64 15.91 14.91

sb,max, suggested 18.28 16.99 15.40 13.00 16.67 16.22 14.93 13.07

Materials and Structures

5.4 Bond stresses

The main goal of this paper is, as noted in the

introduction, to define the bond behavior of recycled

concrete and its time-dependent development. With

this aim, pull-out tests were carried out at different

ages (7, 28, 90 and 365 days). During pull-out tests,

load and slip were continuously recorded for each

specimen. The mean values of these measurements

were obtained, for each of the eight concretes, as the

average of the experimental results obtained from five

specimens tested at 28 days (Fig. 5).The constant

bond stresses were calculated using experimentally

recorded load (Q) and contact surface, according to the

following direct expression:

sb ¼ Q=ðp � / � lbÞ

In this equation, sb stands for constant bond stress

(MPa), Q is load (kN), / is steel bar diameter (10 mm)

and lb is bond length (50 mm).

Due to the high amount of recorded data for each

concrete type and age, some reference values for bond

stress have been selected for this research. Besides the

maximum bond stress (sb,max) included in almost

every bond study [10, 14, 16], stresses which produce

slips of 0.01, 0.1 and 1 mm on the free end of the

specimen, and whose average allows to obtain the

mean bond stress (sb,m) [13], have also been consid-

ered. These results are collected in Table 4 for the

different ages.

At 28 days, bond stress (mean and maximum)

showed behavior similar to compressive strength. These

values decrease as the recycled aggregates content

increases, in almost the same extent as compressive

strength. Bond strength (maximum bond stress) was

reduced by 9, 16, 27 %, for the recycled concretes H50-

20, H50-50, H50-100, and 4, 12 and 22 % for H65-20,

H65-50 and H65-100, respectively. Again, H65 recy-

cled concretes underwent smaller declines than H50

with the recycled aggregate replacement.

Fig. 4 Evolution of compressive strength versus time on cubic specimens

Fig. 5 Load–slip curves at 28 days

Materials and Structures

5.5 Time-dependent bond strength

Focusing on time-dependent development from 7 to

28 days, bond strength increases of 5 % for the

conventional concrete H50-0 and of 12, 40 and

19 %, for the recycled concretes H50-20, H50-50,

H50-100, were respectively observed. Meanwhile,

H65 concretes underwent increases of 6 % for the

H65-0 and of 20, 15 and 12 % for the H65-20, H65-50

and H65-100, respectively. From 28 to 365 days, bond

strength variation was barely noticeable for both

concrete series, between 1 and 2 % for H65 concretes

and slightly higher values for H50 (2–6 %). It was thus

noted a similar trend to time-dependent development

of compressive strengths at these ages (from 7 to

28 days and from 28 and 365 days).

In addition, through the analysis of load-slip curves,

failure mode can be detected. As explained on the test

procedure section, if slip reaches 1.0 mm value on the

curve traced, pull-out failure is produced; otherwise,

splitting is considered [37]. In this experimental

program, all the specimens failed in pull-out mode,

as shown in Fig. 6.

5.6 Normalized bond strength

It is widely accepted that a low mechanical strength

will provide a worse bond performance of concretes;

so, the decrease of compressive strength of recycled

concretes has to be taken into account if it attempts to

define the influence of recycled aggregate on bond

strength.

Most authors have defined some kind of bonding

ratio in their respective researches, which allows to

obtain relative bond strength and to establish a

comparative analysis between concretes, regardless

of its mechanical strength.

Most authors consider the square root of compressive

strength as the most suitable parameter to normalize the

bond strength [12, 15, 16, 19, 38–41]. Sometimes, when

specimens are made with a variable concrete cover [11,

14] or are designed to splitting failure [13], the linear

relationship with fc was used to normalize the bond

strength. Regarding codes and standards, the MC-2010,

ACI-08 and EHE-08 [20, 34, 42] predict the bond

strength with the square root of compressive strength;

the MC-2010 also uses the fc0.25 if splitting failure mode

occurs, and only the EC-2 [43] appeals to the fct.

In order to further develop this issue, and taking

into account that when pull out failure occurs most

authors and codes consider that the square root of the

compressive strength is the best parameter to normal-

ize the bond strength, this has been the parameter used

to normalize the experimental results of bond strength

at different ages. Figure 7 shows the time develop-

ment for maximum and mean values of normalized

bond strength.

Despite taking into account the lower compressive

strength of the recycled concretes, normalized bond

strength generally decreases as the replacement ratio

of recycled aggregates increases. Only the H50-50

underwent fluctuations at certain ages, showing

slightly lower normalized bond strength than H50-

100 at 7 and 90 days.

At 28 days, the normalized bond strength of the

recycled concretes underwent a similar behavior in

both concretes series. It was obtained a decrease,

compared to conventional concrete, of 4, 8 and 13 %

for H50-20, H50-50 and H50-100, and of 3, 8 and

12 % for H65-20, H65-50 and H65-100, respectively.

It is thus observed that recycled aggregates content

influences bond strength.

6 Bond strength prediction

Numerous studies have been developed about bond

strength prediction between concrete and steel. As a

Fig. 6 Pull-out failure mode

Materials and Structures

result, most of them have contributed to define

standardized expressions, which are currently included

in international codes, such as Model Code-2010 [34],

whose theoretical prediction is calculated from the

square root of the compressive strength.

The Model Code (MC-2010) expression for bond

strength prediction has been applied to the experi-

mental data obtained in this research (Table 4; Fig. 8).

Thus, it is attempted to define the suitability of this

code estimation for the different replacement percent-

ages of the recycled concretes. In addition, the

equation suggested by Kim et al. [19] has been

included in this analysis, mainly due to the use of the

replacement ratio of recycled fine and coarse aggre-

gates as a parameter of calculation. This expression

predicts bond strength taking into account the square

root of the compressive strength, the steel bar diam-

eter, the concrete cover and replacement ratio of

recycled aggregates.

sb;max ¼ 2:5ffiffiffiffiffiffi

fcm

p

Model Code expressionð Þ ðRef:34Þsb;max ¼ 0:614

ffiffiffiffiffiffi

fckr

p

ðc=d� 0:55Þ� ð0:4203e0:0172S

þ 0:007GÞ ½Kim et al: expression ðRef: 19Þ�

In these expressions, sb,max is the maximum predicted

bond strength in MPa, fckr/fcm is the mean value of

cylinder compressive strength experimentally obtained

for each concrete (in MPa), c is the distance from the

steel bar core to the concrete surface, d the diameter of

steel bar and S and G the replacement ratios of recycled

fine and coarse aggregates, respectively.

Within each theoretical prediction, the experimen-

tal compressive strength obtained for the different

concretes was included. Figure 8 shows the bond

strengths experimentally obtained and their corre-

sponding theoretical predictions.

It can be seen that predicted bond strength for

recycled aggregate concretes (RAC) is lower than for

Fig. 7 Development of normalized bond stress versus time

Fig. 8 Experimental bond strength versus theoretical predictions

Materials and Structures

conventional ones, mainly due to their lower com-

pressive strength. Even so, once the relationship

between experimental and estimated values is calcu-

lated for each type of concrete, a deterioration of the

ratio ‘‘experimental value to theoretical prediction’’ is

observed as the recycled aggregate content increases

(Fig. 9) It is thus evident that recycled aggregate

content has influence on bond behavior and it should

be taken into account for its theoretical prediction.

When the Kim equation is used the ratio obtained in

this research goes on decreasing as the percentage of

recycled aggregate is increased, although in a lower

extent than when the MC-2010 equation is used.

Therefore, a modified expression for theoretical

estimation of the bond strength of recycled concretes

may be proposed.

The expression proposed in this research is based

on the MC-2010 equation. In order to achieve an

‘‘experimental value to theoretical prediction’’ ratio

for recycled concretes similar to that obtained for

conventional concretes, the following expression,

adjusted by regression (R2 = 0.903), is proposed:

sb;max ¼ 2:5ffiffiffiffiffiffi

fcm

p

ð1� 0:124G=100Þ

In this expression, sb,max stands for bond strength in

MPa, fcm is the mean value of cylinder compressive

strength experimentally obtained for each concrete (in

MPa), and G is the replacement ratio of recycled

coarse aggregates (as a percentage of total coarse

aggregate).

Theoretical bond strength for recycled concrete

computed with the equation proposed in this research

provides a similar ‘‘experimental value to theoretical

prediction’’ ratio as the one calculated with the Model

Code 2010 [34] for conventional concrete (Fig. 9).

7 Conclusions

The mechanical and bond behavior of recycled

concretes with different replacement ratios of recycled

coarse aggregate at different ages is determined in this

research. Based on these experimental results the

following conclusions can be drawn:

1. As already noted by different authors, the density

and the compressive strength of RAC decreases as

the recycled aggregates content increases. When

the compressive strength of 100 % replacement

RACs is analyzed, these reductions reach 31 and

22 % for H50 and H65 concretes, respectively.

Recycled concretes whose control concrete shows

lower compressive strength (named H65 in this

research) undergo lower strength drops than those

concretes with higher compressive strengths

(H50).

2. From 7 to 28 days, the conventional concretes and

the recycled concretes with a replacement per-

centage of 20 % showed lower increase of their

compressive strength than the recycled concretes

with high replacement ratios (50 and 100 %).

From 28 to 365 days, compressive strength

showed almost no increase, with a similar trend

for conventional and recycled concretes.

3. The bond strength at 28 days declined with the

increase of recycled aggregate content showing a

behavior similar to compressive strength. For

Fig. 9 Ratio ‘‘experimental value to theoretical prediction’’ of bond strength

Materials and Structures

recycled concretes with 100 % replacement, bond

strength was respectively reduced 27 and 22 %

for H50 and H65 concretes. Again, recycled

concrete with lower compressive strength, H65,

underwent smaller bond strength drop compared

to conventional concrete.

4. The time-dependent development of bond

strength followed the same trend as compressive

strength. From 7 to 28 days, recycled concretes

underwent a slightly higher increase than con-

ventional ones. From 28 to 365 days, bond

strength variation was barely noticeable for both

concrete series: around 1–2 % for H65 concretes

and slightly higher values for H50 (2-6 %).

5. Normalized bond strength was calculated taking

into account the square root of the experimental

compressive strength obtained with each concrete

at each age. It showed a decrease with the increase

of recycled aggregate content. It was thus stated

that the amount of recycled aggregate does

influence bond strength.

6. Using these experimental results, a modified

expression was proposed to predict the maximum

bond stress of the recycled concretes. This equation,

based on MC-2010 expression, includes a new term

to take into account the replacement percentage of

recycled aggregates and to guarantee an ‘‘experi-

mental value to theoretical prediction’’ ratio similar

to that of conventional concretes.

Acknowledgments The study is part of two projects

entitled:.‘‘Clean, efficient and nice construction along its life

cycle (CLEAM)’’ funded by the Centre for the Technology and

Industrial Development (CDTI) and led by the Group of

Economical Interest CLEAM-CENIT, AIE comprising by the

country’s largest construction companies (Acciona, Dragados,

Ferrovial, FCC, Isolux Corsan, OHL and Sacyr) and some

PYME (Informatica 68, Quilosa and Martınez Segovia y

asociados).‘‘Bond and anchorage of passive reinforcement

steel in concrete (ADHAN)’’ funded by the Ministry of

Science and Innovation.The experimental program was carried

out at the Construction laboratories of Technological Innovation

Centre of Building and Civil Engineering (CITEEC) and Civil

Engineering School, of A Coruna University.

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