01-12-2012, 02:36 PM
SUSTAINABLE DEVELOPMENT AND DURABILITY OF SELF-COMPACTING CONCRETES
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ABSTRACT
Self-compacting concretes (SCC) represent a move toward a sustainable material since they encourage the
use of waste and recycled materials. The high volume of very fine powder necessary to achieve deformability
and passing ability properties, in fact, permits SCC to consume large amount of fly-ash, very fine particles
generated by the recycling of demolished concrete structures, and huge amount of calcareous filler available
from the marble quarries. Moreover SCC turn out to be materials with an extended durability with respect to
conventional concretes. Since fresh properties of self-compacting concretes (SCC) are significantly different
from those of conventional concretes (CC) durability can be significantly improved when a SCC is used due
to a modification of the microstructure of the interfacial transition zone between aggregates and cement
matrix.
This paper presents results of an experimental study carried out to evaluate changes in microstructure of
interfacial transition zone (itz) and of bulk paste for both SCC and CC. Data on the influence of the
calcareous filler, a fundamental ingredients to achieve self-compactability, on the hydration process of
cement are also presented.
Data indicate that the decrease in internal bleeding, when self-compacting concrete is used, seems to favour
the formation of a stronger transition zone characterized by a less porous structure and with a limited amount
of microcracking responsible for higher compressive strength values for SCC with respect CC. No
differences were detected by EDAX analysis in the chemical nature of itz with respect the bulk matrix both
for SCC and CC.
INTRODUCTION
According to a study that has been carried out recently [1], in Europe innovation in construction is
concentrated in 4 main directions, which can be defined simplistically as follows:
• to build in a sustainable manner;
• to schedule appropriate building maintenance;
• to build safe and comfortable buildings;
• to build well and efficiently.
Of these directions, the first two ones are prevalent and are destined to represent the “new construction
ideology” of this millennium, at least for the next 30-50 years. In particular, to build in a sustainable manner
means to focus great attention on the following features:
• physical, environmental and technological resources;
• problems related to human health;
• energy conservation of new and existing buildings;
• control of construction technologies and methods.
Precast Concrete
In the near future the concrete industry is destined to serve the precast element plants, more than placement
on job-sites. In this way the realization of elements that can be assembled in the job-site will allow the
realization of components that can be mounted easily and, at the same time, can be replaced and recycled in a
very simple manner.
The production of these reinforced or prestressed concrete elements will be located close to urban areas, in
order to meet prevalent methodology of the construction works in the next 20-30 years (Table 1), which will
be made up of dispersed small projects, in form of buildings for commercial use and which will be in the
centres of the towns subjected to a strong urban renewal. This methodology of building works will encourage
the production of precast elements located close to urban areas requiring a small impact on the environment.
From this point of view, self-compacting concretes can satisfy sustainability since they can drastically
reduce acoustic environmental pollution, due to the possibility of casting precast elements without vibration.
Self-compacting concretes (SCC) also provide a significant opportunity for the use of waste and recycled
materials. The higher volume with respect to conventional concrete (CC) of very fine powder necessary to
achieve deformability and passing ability capacities permits them to incorporate high volumes, for instance,
of fly-ash [2]. This will represent a key-point since the amount of fly-ash available in the near future will
increase. In fact, some governments (i.e. Italy, USA, etc.) will build new carbon-based thermoelectric power
generation plants to resolve problems related to the increasing demand of energy. These demands partially
caused the black-outs in Italy and USA during the summer of 2003.
Improved Durability
Finally, self-compacting concretes turn out to be materials for the realization of building structures and
components, with a much durability than the one of the standard concretes. The greater durability of selfcompacting
concretes satisfies the request for sustainability, because it will be possible to delay the
maintenance and, therefore, limit the volumes of deteriorated concrete to be disposed of and lower use of
repair mortars based on special mixtures big consumers of non-renewable resources.
PARAMETERS AFFECTING DURABILITY
Many factors influence physical and mechanical properties of concrete, such as water/cement, type of
cement, use of materials having pozzolanic activity, type and particle size distribution of aggregates, type and
dosage of admixtures. But even other important parameters may have a strong influence on the
microstructure and, hence, on elastic and mechanical properties of concrete. These factors, less generally
focused on, are related to the fresh state of concrete and concern rheological properties (cohesion and plastic
viscosity), transportation, handling and placing of fresh concrete, segregation and bleeding, plastic settlement
and curing. Rheological properties, in fact, can significantly modify nature and structure of the concrete
interfacial transition zone (itz), between aggregate and concrete paste, which is responsible for the
mechanical properties, watertightness, fire resistance and durability of prestressed and reinforced concrete
structures [7].
Since fresh properties of self-compacting concrete (SCC) are significantly different from those of
conventional concrete (CC), formation, nature and microstructure of itz can change significantly when a SCC
is used instead of CC. For these reasons properties of hardened SCC can dramatically differ from those of CC
even for the same water/cement ratio, type of cement, type and maximum size of the aggregates.
This paper presents results of an experimental program carried out to evaluate the main changes of the itz in
SCC with respect that of CC.
INTERFACIAL TRANSITION ZONE
In freshly conventional compacted concrete, due to the presence of coarse aggregate, water films, initiated by
vibration, tend to accumulate close to the aggregate surface. These films determine an increase in the w/c
next to the aggregate compared to the bulk mortar [8]. As a consequence of the higher water/cement, the
crystalline hydration products ettringite and calcium hydroxide (CH) in the vicinity of the coarse aggregate
consist of larger crystals and therefore they form a more porous structure than in the bulk mortar matrix [9].
For these reasons, even for low w/cm, amount and size of voids in the transition zone will be larger than in
bulk mortar. Moreover, CH crystals tend to locate themselves with the c-axis perpendicular to the aggregate
surface. This results in a less adhesion capacity both for the lower surface area, and consequently weaker Van
der Walls forces, and also because of preferred failure sites for the oriented structure [10].