16-08-2012, 10:02 AM
MECHANICAL PROPERTIES AND STRESS- STRAIN BEHAVIOUR OF SELF COMPACTING CONCRETE
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Abstract
The development of Self Compacting Concrete marks an important milestone in
improving the product quality and efficiency of the building industry. Self Compacting
Concrete is a recently developed concept in which the ingredients of the concrete mix are
proportioned in such a way that it can flow under its own weight to completely fill the
formwork and passes through the congested reinforcement without segregation and self
consolidate without any mechanical vibration. Several studies in the past have revealed
the usefulness of fibres to improve the structural properties of concrete like ductility,
post crack resistance, energy absorption capacity etc. Fiber reinforced self compacting
concreting combines the benefits of self compacting concrete in fresh state and shows an
improved performance in the hardened state due to the addition of fibers. In this
investigation Cem-FIL anti-crack high dispersion glass fibres were added to self
compacting concrete and Glass Fibre Reinforced Self Compacting Concrete was
developed. An attempt has been made to study mechanical properties and stress-strain
behaviour of self compacting concrete and glass fibre reinforced self compacting
concrete. A strength based mix proportion of self compacting concrete was arrived based
on Nan-Su method of mix design and the proportion was fine tuned by using Okamura’s
guidelines. Five self compacting concrete mixes with different mineral admixtures like fly
ash, ground granulated blast furnace slag and rice husk ash were taken for investigation
with and without incorporating glass fibres.
Introduction
Current scenario in the building industry shows increased construction of large and complex
structures, which often leads to difficult concreting conditions. When large quantity of
heavy reinforcement is to be placed in reinforced concrete members it is difficult to ensure
that the form work gets completely filled with concrete, that is fully compacted without
voids or honeycombs. Vibrating concrete in congested locations may cause some risk to
labour and there are always doubts about the strength and durability of concrete placed in
such locations. One solution for the achievement of durable concrete structures independent
of the quality of construction work is the employment of Self Compacting Concrete (SCC)
[1,2]. SCC is that concrete which is able to flow under its own weight and completely fill the
formwork without segregation, even in the presence of dense reinforcement, without the
need of any vibration whilst maintaining homogeneity.
Experimental Program
Five SCC Mixes with different types of admixtures were developed in the laboratory and
Cem-FIL anti-crack high dispersion glass fibres of 600 grams/m3 of concrete were added to
these SCC mixes and glass fibre reinforced self compacting concrete was developed. The
experimental programme consisted of casting and testing of SCC and GFRSCC elements(3
specimens each for each mix of SCC and GFRSCC) in compression , tension and in flexure.
Cubes of 100mm size and cylinders of 150 mm diameter and 300 mm long were cast for
testing in compression and in tension. Prisms of 100 × 100 × 500 mm were cast for testing
in flexure. 100 ×500 mm were cast for testing in flexure.
Specimen Preparation
After testing the SCC and GFRSCC in fresh state the concrete was poured in moulds of
cubes, cylinders and prisms. After the concrete has set in moulds the cylindrical specimens
which were to be tested in axial compression were capped with a thin layer of stiff neat
Portland cement paste. After 24 hours of casting the specimens were de-moulded and placed
in water for curing. After 28 days of curing the specimens were taken out from water and
allowed the surfaces for drying. For each SCC and GFRSCC mixes 3 cubes, 6 cylinders and
3 prisms were cast.
Analytical Stress-Strain Curves for SCC and GFRSCC
A number of empirical equations have been proposed to represent uniaxial stress-strain
behaviour of concrete [15], but most of them can be used for only ascending portion of the
curve. In 1985 Carriera and Chu [16] extended the empirical equation proposed by Popovics
[15] to include both ascending and descending portions of complete stress-strain curve.
Recently Hsu and Hsu [17] proposed an empirical equation which includes the effects of
steel fibres in concrete. NRD Murthy [18] also proposed an empirical equation which
includes the effects of steel fibres in concrete.
Conclusions
• All the SCC and GFRSCC mixes developed satisfied the requirements of self
compacting concrete specified by EFNARC
• Addition of 0.03% high dispersion glass fibres to SCC mixes did not have any
pronounced effect on the filling and passing ability when compared to SCC mixes.
• Incorporation of glass fibres by 0.03% ie 600 grams / m3 of concrete has increased the
strengths at 28 days by 2.0 to 5.5 % in compression,3.0 to 7.0 % in tension, and 11.0 to
20.0 % in flexure.
• Average value of Young’s modulus (EC) obtained for M30 grade SCC is 6.0% less than
ordinary concrete and follows the relationship EC = 4700√ fck,
• Addition of glass fibres have increased the EC value by 14.2% when compared to
ordinary M30 grade concrete and by 21.5% when compared with SCC and follows the
relationship EC = 5700√ fck