Extended Abstract

STRUCTURAL CONCRETE WITH INCORPORATED

RECYCLED CONCRETE COARSE AGGREGATES

Influence of the curing conditions on the mechanica l behaviour

Nuno Miguel dos Santos Fonseca

ABSTRACT

This research aims at evaluating the effect of the incorporation of recycled concrete coarse

aggregates on the properties of the concrete, as well as analysing the influence of the curing

conditions on mechanical properties of recycled coarse aggregates concrete. In particular, the

relations between the compressive strength, the splitting tensile strength, the elasticity modulus

and the abrasion resistance are investigated and discussed in detail.

In order to accomplish these purposes, cylindrical and cubic specimens were cast, for testing

the aforementioned hardened properties of the concrete, with four different concrete mixes: a

conventional reference concrete and three concrete with substitution rates of 20, 50 and 100%

of natural coarse aggregates by recycled concrete coarse aggregates. All mixes were prepared

with a water / binder ratio of 0.43. Four curing methods were performed, namely, laboratory

curing, water curing, wet chamber curing and outer environment curing, to cure the cylindrical

and cubic specimens, until the day of testing.

KEYWORDS:

Construction and demolition waste; Concrete; Recycled concrete coarse aggregates;

Mechanical behaviour; Curing conditions.

January 2009

Structural concrete with incorporated recycled concrete coarse aggregates

2

1 INTRODUCTION

1.1 PRELIMINARY REMARKS

The use of recycled aggregates (RA) in concrete opens a whole new range of possibilities in

reusing materials in construction. Reuse of waste concrete as RA in new concretes is beneficial

from the view point of environmental protection and preservation of resources. This could be an

important breakthrough for society in its endeavour towards sustainable development.

Preceding studies were mainly engaged in the processing of demolished concrete, mix

proportion design, mechanical properties, durability aspects and improvements. Recently,

structural performances and economic aspects of using recycled aggregate concrete have also

been analysed. Some previous research results on the mechanical behaviour of recycled

aggregate concrete (RAC) have been reviewed by Hansen (1992) and Brito (2005). It was

shown that, in fact, none of those works indicated that RAC is unsuitable for structural

applications.

Recent investigation on the performance of concrete made with recycled concrete fine

aggregates (EVANGELISTA, 2007) and recycled concrete coarse aggregates (GOMES, 2007),

as well as on the influence of the pre-saturation of recycled concrete coarse aggregates

(FERREIRA, 2007), has given positive results, which further supports and encourages the

possibilities of applying RAC in civil engineering structures.

1.2 SCOPE AND METHODOLOGY OF THE INVESTIGATION

The mechanical behaviour of RAC depends on the characteristics of the RA, mix proportions

and curing conditions. Despite the fact that there are several studies concerning the first two,

there is a lack of information regarding the influence of the curing conditions on RAC properties.

As such, this dissertation aims at assessing the influence of the curing conditions on the

mechanical characteristics of recycled concrete coarse aggregate concrete (RCCAC), as well

as evaluating the effect of the incorporation of recycled concrete coarse aggregates (RCCA) on

the properties of the concrete. Compressive strength, splitting tensile strength, elasticity

modulus and abrasion resistance are investigated.

Proper curing maintains a suitably warm and moist environment for the development of

hydration products, thus reducing the porosity in hydrated cement paste and increasing the

density of the concrete’s microstructure. The hydration products extend from the surfaces of

cement grains, and the volume of pores decreases due to proper curing under appropriate

temperature and moisture conditions. If a concrete is not well cured, particularly at an early age,

it will not gain the desired properties and durability due to a lower degree of hydration, and will

undergo irreparable loss (RAMAN, 2007).

Researching international and national experimental campaigns was the primary stage of this

investigation. The collected information constituted a repository which refers the most important

properties of the aggregates, the experimental test results, and the conclusions of each

campaign. A common observation made in all the different works on this matter is that of a

generalized worsening of the mechanical properties of the RAC, with the increase of the

substitution rate of natural aggregates (NA) by RA, when compared with natural aggregate

Influence of curing conditions on mechanical behaviour

3

concrete (NAC) (concrete produced with NA only or regular concrete).

After this step, the experimental program was planned and executed. The RCCA and NA (fine

and coarse) were analysed, but their results were not listed in detail in this abstract. Four

different concrete mixes were produced, along with four different curing methods. In order to

establish a legitimate assessment between different mixes and/or different curing conditions,

fresh concrete tests analyses were carried out, so as to maintain the same slump and

workability. After the curing period, the hardened concrete tests were performed.

Subsequent to this stage, the experimental results were analysed, having been discussed in

detail. Correlations were established between the properties of the RCCAC and the density and

water absorption of the aggregates, the substitution rate of natural coarse aggregate (NCA) by

RCCA, and the curing conditions as well.

2 EXPERIMENTAL PROGRAM

2.1 MATERIALS

� recycled concrete coarse aggregates (RCCA): the RA were produced in a

laboratory, using a concrete jaw crusher; the primary concrete was industrially

manufactured and cast ‘in situ’ at the laboratory;

� natural aggregates (NA): the NA (limestone) were provided by the primary fresh

concrete supplier, in order to be identical to the ones used in the RA primary concrete;

� cement: ordinary CEM II A-L 42.5 R Portland cement was used; the cement required

was collected from the same batch of production of RA primary concrete to avoid

adding any further variables;

� water: tap water was used for mixing and curing.

2.2 MIX DESIGN

Four different concrete mixes were produced: a conventional reference concrete (NAC) and

three recycled concretes (RAC) with substitution rates of 20, 50 and 100% of NCA by RCCA. All

concrete mixes (NAC and RAC) were prepared based on an effective water / binder ratio of

0.43 and were balanced to have a slump of 80 ± 10 mm. The proportions of the materials were

determined on the basis of absolute volume of the constituents. The details of NAC’s mixture

proportions are given in Table 2.1. The characteristics of the reference concrete are:

� concrete class: C 30/37;

� slump class: S2;

� exposure class: XC3;

� binder: CEM II A-L 42.5 R Portland;

� aggregates’ maximum size: Dmax = 25.4 mm;

� chemical and mineral admixtures: none.

2.3 CURING CONDITIONS

The test specimens were subjected to four types of curing conditions, namely: laboratory

conditions curing (LCC); outer environment curing (OEC); wet chamber curing (WCC); water

immersion curing (WIC).


4

Table 2.1 – Mixture proportions of natural aggregat e concrete (NAC)

NAC 4 - 5.6 0.050 5.6 - 8 0.044

8 - 11.2 0.044 11.2 - 16 0.091

16 - 22.4 0.112

NC

A

22.4 - 25.4 0.043 Sand 1 0.062

Sand 2 0.202 Cement 0.144

Vwater 0.192 Vvoids 0.015

1.000

Regular tap water was used in WIC and the curing temperature was maintained at 16.3 ºC. The

WCC specimens were kept under a relative humidity of 100% and 20.0 ºC temperature. In the

case of OEC, the specimens were exposed to the weather, without any kind of protection, and

were continuously monitored with a thermo-hygrometer. Similarly to that, the LCC specimens

were preserved in laboratory, but protected from harsh weather changes.

2.4 TESTING OF AGGREGATES

The particle size distribution was determined in accordance with EN 933-1 (1997) and EN 933-2

(1995). The particle density and water absorption were measured following EN 1097-6 (2000).

The bulk density was determined in accordance with EN 1097-3 (1998). The aggregates’

resistance to abrasion was measured by the Los Angeles loss test following LNEC E-237

(1970). The water content was determined in accordance with EN 1097-5 (2008). The shape

index was measured following EN 933-4 (2008). Water absorption in time was determined

following the methodology established by Ferreira (2007).

2.5 TESTING OF FRESH CONCRETE

The fresh concrete was produced using a revolving drum concrete mixer. Immediately after the

mixing, it was tested for slump and density. The slump was determined according to Abrams’

slump test following EN 12350-2 (1999). The concrete’s fresh density was measured according

to EN 12350-6 (1999).

2.6 TESTING OF HARDENED CONCRETE

The 7, 28 and 56-day compressive strength of the concrete was determined in accordance with

EN 12390-3 (2001). The 28-day tensile splitting strength was measured following EN 12390-6

(2000). Young’s modulus / elasticity modulus in compression was measured following LNEC E-

397 (1993). The abrasion resistance was determined by Böhme’s grinding wheel wear test, in

accordance with DIN 52108 (2002).


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3 RESULTS AND DISCUSSIONS

3.1 COMPRESSIVE STRENGTH

The development of compressive strength with age is illustrated in Figure 3.1. The test results

at 7, 28 and 56-day are presented in Figure 3.2. Detailed results are accessible in Table 6.1, in

the Appendix.

Figure 3.1 – Compressive strength evolution with ag e

OEC LCC WCC WIC

Concrete age = 7 days

39

40

41

42

43

44

45

46

0 20 40 60 80 100

Ratio o f NCA substitution by RCCA (%)

f cm (

MP

a)


47

48

49

50

51

52

53

54

55

0 20 40 60 80 100

Ratio of NCA substitution by RCCA (%)


47

48

49

50

51

52

53

54

55

0 20 40 60 80 100

Ratio o f NCA substitution by RCCA (%) Figure 3.2 – Variation of the compressive strength with the ratio of NCA substitution by RCCA


6

With all curing methods, concrete’s compressive strength increased with age. The averages of

the compressive strength (fcm) at 7, 28 and 56-day are, respectively, 42.8, 49.8 and 51.6 MPa.

Generally speaking, after 7 days of curing the specimens revealed 80% of their 56-day

compressive strength and 95% after 28 days.

It was expected that compressive strength would decrease linearly with the substitution of NCA

by RCCA. This, however, was not the case. In fact, the compressive strength of all the different

concretes typologies differs by no more that 7.5%, in relation to NAC. Therefore, no distinct

relation can be established between the compressive strength and the proportion of RA in the

concrete mix. For the same reason, RCCAC do not seem to be more, or less, affected by curing

conditions than conventional concrete.

It is considered that the properties of the RCCA are similar to the concrete’s binder matrix, thus

not constituting a weak spot. Therefore, the particle size distribution, shape and surface texture,

have a huge effect on the concrete’s compressive strength. The upper values of compressive

strength for RAC can be explained by the higher porosity and roughness of the RCCA, which

balance their lesser strength.

3.2 SPLITTING TENSILE STRENGTH

The results for the 28-day splitting tensile strength of concrete are presented in Figure 3.3 and,

in detail, in Table 6.2, in the Appendix.

2.0

2.5

3.0

3.5

4.0

0 10 20 30 40 50 60 70 80 90 100


Spl

ittin

g te

nsile

str

engt

h (M

Pa)

OEC LCC WCC WIC

Figure 3.3 – Variation of the splitting tensile str ength with the ratio of NAC substitution by RCCA

Generally, the 28-day splitting tensile strength decreased with the improvement of the

incorporation of RCCA. It varied from 2.37 to 3.88 MPa for different RCCA incorporation

percentages and curing methods. All RAC100 typologies exhibit the lower values of splitting

tensile strength, with the exception of WCC specimens that reveal a slightly higher value than

NAC-WCC.

The tensile strength results are very inconstant and, as a result, the correlation coefficients are

not acceptable. Nevertheless, the OEC specimens’ results reveal a very good determination

coefficient (R2 = 0.808).

The RAC specimens kept in OEC conditions appear to be more harmed by this curing method

than regular concrete. The LCC and WIC curing methods exhibit a similar development with the


7

increase of the incorporation of RCCA; therefore, they do not seem to be more, or less, affected

by the curing conditions than conventional concrete. On the other hand, WCC specimens reveal

an unusual variation, with splitting tensile strength increasing with the NCA substitution by

RCCA. It must be noted that the correlation in this curing condition is very low (R2 = 0.106), for

which reason no clear conclusion can be reached.

3.3 ELASTICITY MODULUS

The results for the elasticity modulus in compression of concrete are presented in Figure 3.4

and, in detail, in Table 6.3, in the Appendix.

The modulus of elasticity decreased, with the increase in the incorporation of RCCA. It varied

from 30.6 to 43.4 GPa for different RCCA incorporation ratios and curing methods.

28

30

32

34

36

38

40

42

44

46

0 10 20 30 40 50 60 70 80 90 100


Ela

stic

ity m

odul

us (

GP

a)

OEC LCC WCC WIC

Figure 3.4 – Variation of the elasticity modulus wi th the ratio of NAC substitution by RCCA

The LCC specimens display the lowest elasticity modulus values. In view of the fact that this

curing method involved only minor relative humidity, this reduction is related to moisture

movement from the specimens. Since moisture moved out with the increase in age and the

concrete specimens were dried with increasing exposure lengths, the microstructure of concrete

remained porous and resulted in a lower modulus of elasticity.

The variation of the LCC specimens’ elasticity modulus (Figure 3.4) suggests that RCCAC is

less affected by this curing condition than regular concrete (decrease with RCCA incorporation

still exists, but with a slighter rate). However, the divergence is minimal and not clear. The

remaining curing conditions (OEC, WCC and WIC) had all included elevated relative humidity

and all exhibit extremely similar correlations with RCCA incorporation. Therefore, they do not

seem to be more, or less, affected by the curing conditions than conventional concrete.

3.4 ABRASION RESISTANCE

The results for the abrasion resistance of concrete are presented in Figure 3.5 and, in detail, in

Table 6.4, in the Appendix.

Since curing conditions strongly affect concrete’s surface layer, it is noted that the test

specimens (71x71x50 mm3) were obtained by sawing larger concrete cubes (100 mm edge)


8

after curing. This action was done in order to avoid the existence of the concrete’s surface

finishing as a variable in the test. Thus, the test surface is the cutting surface itself, i.e., an

internal plane of the concrete element, composed by aggregates and binder mix, and not an

outer surface.

0.8

0.9

1.0

1.1

1.2

0 10 20 30 40 50 60 70 80 90 100


∆l B

AR / ∆

l BR

OEC LCC WCC WIC

Figure 3.5 – Variation of the abrasion resistance w ith the ratio of NAC substitution by RCCA

The irregular variation of abrasion resistance values, in all curing conditions, does not allow the

establishment of a clear relation between this property and the incorporation of RCCA.

Excluding the RAC50-WCC, which reveals a 10% higher wear than NAC, the abrasion

resistance of all others concretes’ typologies differs by no more than 5.4%, in relation to NAC,

which is not statistically significant from an experimental point of view.

On the other hand, it must be noted that all RAC100 specimens reveal the lowest loss of

thickness and subsequently higher abrasion resistance. For that reason, it can be concluded

that the incorporation of RCCA leads to a better performance, in what concerns abrasion

resistance. This can be explained by the better connections established between the binder and

the RCCA, in view of their higher porosity.

In what concerns the curing conditions’ influence, no clear conclusion can be reached.

Nevertheless, the lower values of variations suggest that RCCAC do not appear to be affected

any differently than conventional concrete. These results indicated that the performance of

mixes incorporating RA is comparable to the concrete mix in which 100% NCA was used.

4 CONCLUSIONS

The use of RAC should always take into consideration that they have, in most cases, a lower

performance when compared to conventional concrete. Still, RCCAC can acquire adequate

quality as structural concrete. The following conclusions can be drawn based on the

experimental results and the respective discussion of the study:

(1) Compressive strength does not seem to be affected by RCCA incorporation or by different

curing conditions, when compared with conventional concrete.

(2) Splitting tensile strength decreases with the increase in RCCA incorporation. Recycled

concrete specimens maintained in OEC conditions seem to be more harmed than

specimens of conventional concrete.


9

(3) Elasticity modulus decreases with the increase of RCCA percentage. Recycled concrete’s

specimens maintained in LCC seem to be slightly affected by RCCA incorporation. The

elasticity modulus of RCCAC kept in the other curing conditions (OEC, WCC and WIC)

does not seem to be more, or less, affected than conventional concrete.

(4) Abrasion resistance test values reveal an erratic variation; therefore, no correlation can be

established. Nevertheless, all RAC100 typologies present the lowest wear. The lower

values for variation suggest that the performance of mixes incorporating RA is comparable

to conventional concrete, independently of curing conditions.

While this field presents many possibilities (and necessities) of investigation if the behaviour of

recycled aggregates concretes is to be fully understood, it can be concluded from the results of

this experimental study that these aggregates do indeed reveal a potential for being used in the

production of structural concrete.

5 REFERENCES

BRITO, J. (2005) – Recycled aggregates and their influence on concrete’s properties (in

Portuguese). Public lecture within the full professorship in Civil Engineering pre-admission

examination, Lisbon.

DIN 52108 (2002) – Testing of inorganic non-metallic materials: Wear test with the grinding

wheel according to Böhme.

EN 933-1 (1997) – Tests for geometrical properties of aggregates. Part 1: Determination of

particle size distribution. Sieving method.


particle size distribution. Test sieves, nominal size of apertures.


particle shape. Shape index.

EN 1097-3 (1998) – Tests for mechanical and physical properties of aggregates. Part 3:

Determination of loose bulk density and voids.


Determination of the water content by drying in a ventilated oven.


Determination of particle density and water absorption.

EN 12350-2 (1999) – Testing fresh concrete. Part 2: Slump test.

EN 12350-6 (1999) – Testing fresh concrete. Part 6: Density.

EN 12390-3 (2001) – Testing hardened concrete. Part 3: Compressive strength of test

specimens.

EN 12390-6 (2000) – Testing hardened concrete. Part 6: Tensile splitting strength of test

specimens.

EVANGELISTA, L. (2007) – Performance of concrete made with fine recycled concrete

aggregates (in Portuguese). MSc Dissertation in Civil Engineering, Instituto Superior Técnico,

Lisbon.


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FERREIRA, L. (2007) – Structural concrete with incorporation of coarse recycled concrete

aggregates: Influence of the pre-saturation (in Portuguese). MSc Dissertation in Civil

Engineering, Instituto Superior Técnico, Lisbon.

GOMES, M. (2007) – Structural concrete with incorporation of concrete, ceramic and mortar

recycled aggregates (in Portuguese). MSc Dissertation in Civil Engineering, Instituto Superior

Técnico, Lisbon.

HANSEN, T. (1992) – Recycling of demolished concrete and masonry. Report of technical

committee 37-DRC, Demolition and Reuse of Concrete, Taylor & Francis, London.

LNEC E-237 (1970) – Aggregates: Los Angeles abrasion test.

LNEC E-397 (1993) – Concrete: Determination of elastic modulus in compression.

RAMAN, S.; SAFIUDDIN, MD.; ZAIN, M. (2007) – Effect of different curing methods on the

properties of microsilica concrete. Australian journal of basic and applied sciences, 1(2), pp. 87-

95, INSInet Publication.


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6 APPENDIX Table 6.1 – Compressive strength at 7, 28 and 56-da y

3rd FASE 4th FASE OEC

fcm 28 (MPa) ∆ (%) fcm 7 (MPa) ∆ (%) fcm 28 (MPa) ∆ (%) fcm 56 (MPa) ∆ (%) NAC 49.7 - 42.8 - 51.6 - 51.6 -

RAC20 48.3 -2.8 39.6 -7.5 51.3 -0.5 51.8 0.4 RAC50 47.6 -4.2 42.0 -2.0 50.4 -2.2 51.7 0.2

RAC100 47.9 -3.6 41.8 -2.3 49.3 -4.3 49.1 -4.7

3rd FASE 4th FASE LCC fcm 28 (MPa) ∆ (%) fcm 7 (MPa) ∆ (%) fcm 28 (MPa) ∆ (%) fcm 56 (MPa) ∆ (%)

NAC 48.7 - 42.7 - 50.3 - 53.1 - RAC20 45.0 -7.5 43.0 0.6 49.1 -2.3 53.3 0.5 RAC50 48.0 -1.4 42.1 -1.5 49.8 -0.9 52.0 -2.0

RAC100 45.1 -7.2 45.2 5.8 51.3 2.1 53.7 1.2

3rd FASE 4th FASE WCC fcm 28 (MPa) ∆ (%) fcm 7 (MPa) ∆ (%) fcm 28 (MPa) ∆ (%) fcm 56 (MPa) ∆ (%)

NAC 43.6 - 43.1 - 49.3 - 49.0 - RAC20 44.8 2.8 43.0 -0.2 48.7 -1.2 48.6 -0.8 RAC50 46.8 7.4 42.0 -2.6 48.4 -1.8 51.1 4.3

RAC100 44.7 2.6 42.8 -0.8 47.7 -3.3 50.6 3.2

3rd FASE 4th FASE WIC fcm 28 (MPa) ∆ (%) fcm 7 (MPa) ∆ (%) fcm 28 (MPa) ∆ (%) fcm 56 (MPa) ∆ (%)

NAC 47.9 - 44.0 - 51.0 - 53.3 - RAC20 45.3 -5.3 43.9 -0.2 49.4 -3.2 52.5 -1.5 RAC50 48.5 1.3 43.7 -0.7 49.5 -3.0 52.0 -2.4

RAC100 45.7 -4.4 43.0 -2.4 49.4 -3.2 52.7 -1.0

Table 6.2 – Splitting tensile strength

OEC LCC WCC WIC

fctm 28 (MPa) ∆ (%) fctm 28 (MPa) ∆ (%) fctm 28 (MPa) ∆ (%) fctm 28 (MPa) ∆ (%) NAC 3.88 - 2.89 - 2.85 - 3.17 -

RAC20 3.05 -21.4 2.62 -9.1 2.37 -16.9 3.45 8.6 RAC50 3.22 -17.0 2.90 0.6 2.60 -8.5 3.33 4.9

RAC100 2.40 -38.0 2.44 -15.5 2.86 0.6 2.77 -12.7

Table 6.3 – Elasticity modulus

OEC LCC WCC WIC

ECm 28 (GPa) ∆ (%) ECm 28 (GPa) ∆ (%) ECm 28 (GPa) ∆ (%) ECm 28 (GPa) ∆ (%)

NAC 40.3 - 36.3 - 41.5 - 43.4 -

RAC20 39.6 -1.7 35.2 -3.1 41.1 -0.8 37.1 -14.6 RAC50 39.0 -3.3 35.6 -1.8 37.9 -8.5 38.7 -11.0

RAC100 31.9 -20.9 30.6 -15.7 32.4 -22.0 33.5 -22.9

Table 6.4 – Abrasion resistance

OEC LCC WCC WIC

∆lm (mm) ∆ (%) ∆lm (mm) ∆ (%) ∆lm (mm) ∆ (%) ∆lm (mm) ∆ (%) NAC 1.7 - 2.1 - 1.9 - 1.8 -

RAC20 1.5 -13.9 1.9 -9.6 1.9 2.3 1.8 -2.6 RAC50 1.8 5.4 1.8 -11.4 2.1 10.2 1.9 3.9

RAC100 1.4 -15.5 1.8 -11.4 1.8 -4.1 1.6 -9.8

Documents

Extended Abstract