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 Council 1 , 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 data 3 , 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 researchers 4 . 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
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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;