of using aggregate from demolished concrete was reported
by many researchers and they concluded that the compressive
strength of concrete decreased when recycled aggregate
was used [5,6]. Chen et al. [7] reviewed the published works
on the effect of recycled aggregate on the compressive
strength of concrete. They found that the compressive
strength of concrete containing 100% of recycled coarse
aggregate could be as low as 60% of concrete made from
normal aggregate. Poon et al. [8] suggested that concrete
containing not more than 50% of recycled coarse aggregate
in the air-dried state should be optimum for normal
strength recycled aggregate concrete production.
It is noted that most of the study on recycled aggregate
in concrete has been focused on replacing either the coarse
or fine aggregate, and it is limited to a low level of replacement
of the recycled aggregates. This is due to the low compressive
strength of concrete containing recycled
aggregates.
In this study, all recycled aggregate concretes were made
from 100% of recycled coarse aggregate (RCA), then recycled
fine aggregate (RFA) was used to replace river sand at
0%, 50%, and 100% by weight of fine aggregate. In addition,
ground RHBA with high fineness was used as a pozzolanic
material to replace Portland cement type I in
recycled aggregate concretes. The purpose of using RHBA
as a cement replacement in recycled aggregate concretes is
not only to increase the ultimate strength of recycled aggregate
concrete, but also to reduce the amount of RHBA at
landfills. The mechanical properties of recycled aggregate
concretes such as slump loss, compressive strength, splitting
tensile strength, and modulus of elasticity were investigated
and compared to those of the conventional
concrete (concrete produced from normal aggregates). Utilization
of the two waste materials, RHBA and recycled
aggregates, will reduce the waste disposal volume and
make a better environment. Moreover, the use of recycled
aggregate also helps the conservation of natural aggregates.
2. Experimental program
2.1. Materials
2.1.1. Cement
Portland cement type I was used in the conventional
concrete and recycled aggregate concretes. Its physical
and chemical properties are reported in Tables 1 and 2,
respectively.
2.1.2. Rice husk–bark ash (RHBA)
Rice husk–bark ash was collected from a biomass power
plant, at which 65% of rice husk and 35% of eucalyptus
bark by weight were burnt together as a fuel at controlled
temperature about 800–900 C. Since the particle size of
RHBA from the biomass power plant is much larger than
that of the cement particle, thus it is improved the reactivity
by grinding until the particles retained on a sieve No.
325 are less than 5% by weight. Then, ground RHBA