Behaviour of Fibre Reinforced Geopolymer Concrete
B. Prabu, Assistant Professor, Civil Engineering, V S A Group of Institutions, and A. Shalini, Assistant Professor, Civil Engineering, K.S.R College of Engineering, Salem, Tamil Nadu
Concrete is the world’s most versatile, durable and
reliable construction material next to water. Geopolymer
Concrete is a growing field in the construction industry
for the effective use of byproducts from various industries and
solid wastes. This study involves investigation on the mechanical
properties of fly ash based geopolymer concrete partially
replaced with Ground Granulated Blast furnace Slag (GGBS) by
10% in the mass of total binder.
For the geopolymerization, sodium hydroxide (NaOH) and
sodium silicate(Na2Sio3) were used as activators, mass ratio of
Na2Sio3/NaOH is 2.5, NaOH 10M, mass ratio of alkaline liquid to
binder is kept as 0.4. The increase in GGBS content decreases the
workability and enhances the setting property of geopolymer
concrete. In addition, steel fibres were added with the controlled
specimens with different volume fractions as 0.25% and 0.50%
with aspect ratios as 45. The specimens were prepared and cured
for 28 days at ambient conditions and it results in increased
compressive strength. Totally 3 beams specimens were cast to find
the flexural behaviour of GPC beams. The FRGPC beam behaviour
is compared with controlled beam specimens.
Concrete is the construction material that requires large quantities of Portland cement. The production of cement is highly intensive. It also emits a large amount of CO2 into the air, which causes global warming and also OPC durability is limited due to inherit brittleness. Hence, it is necessary to find an alternative material for cement. Davidovits, a chemistry professor invented Geopolymer in 1978; further researches were carried out by B V Rangan. Geopolymer concretes (GPCs) are a new class of building materials that have emerged as an alternative to Ordinary Portland Cement Concrete (OPCC) and possess the potential to revolutionize the building construction industry. So far, many researches have been carried out on geopolymer concrete, which enhances the strength and durability as compared to OPC concrete and proves its environmental benefits. Geopolymer concrete is a combination of alkaline liquids and source materials, which may be rich in silicon and aluminium. The reaction that takes place between silicon and aluminium in the source materials and alkaline liquids is referred to as polymerization process, similar to the reaction that takes place between OPC and water is known as hydration process . By the polymerization process, the three dimensional network of aluminate and silicates were created showed the enhanced and excellent properties of concrete. The source materials may be the solid waste or byproduct from various industries. Fly ash, Rice Husk Ash, Metakaoline, Silica fume and GGBS can be used as source materials. Sodium silicate and sodium hydroxide or potassium hydroxide and potassium silicate can be used as alkaline liquids as per the requirements, availability, usage and cost. While comparing the different source materials, fly ash is abundantly dumped as a waste material in thermal power stations and it is produced during the combustion of coal. Partha Sarathi Deb et al reported the addition of GGBS enhances the setting of concrete at ambient temperature and workability of geopolymer concrete decreased with the increase of GGBS content together with fly ash in the binder when the other mixture binder variables remained the same. This is mainly because of the accelerated reaction of the calcium and the angular shape of the slag as compared to the spherical shape of the fly ash particles .
Steel Fibre Reinforced Geopolymer Concrete
Fibre reinforced concrete is a composite material made of cements, water, fine and coarse aggregate, and a dispersion of discontinuous, small fibres. These short discrete fibres are uniformly distributed and randomly oriented. It is well known that plain concrete is brittle and weak under flexural loads. To eliminate the disadvantages, plain concrete is added fibres into concrete mix. Commonly basalt, polypropylene, steel fibres are used. In this paper hook end steel fibres were used. Steel fibres are generally Hook ended, Flat, Crimpled type in shape. N. Ganesan et.al used steel fibres in the percentages of 0.25%, 0.5%, 0.75% and 1%. And they declared that the addition of fibres improved the mechanical properties of Steel Fibre Reinforced Geopolymer Concrete (SFRGPC). The increase was found to be nominal in the case of compressive strength (8.51%), significant in the case of splitting tensile strength (61.63%), modulus of rupture (24%), modulus of elasticity (64.92%) Poisson’s ratio (50%) at 1% volume fraction of fibres . The shape of Hook End steel fibre is shown in fig 1.
The significance of the present investigation was to study the performance of GPC with and without steel fibres. Totally three beam specimens were cast to find the flexural behaviour. The steel fibres were added in 0.25% and 0.50% volume fractions in beam specimen. The specimens were cured in ambient condition. The structural parameters such as load deflection behaviour, first crack load, ultimate load were investigated.
The following materials were used for the preparation of test specimens:
– Low calcium fly ash obtained from Mettur thermal power plant and ground granulated blast furnace slag was used as a source material to prepare Geopolymer Concrete.
– Locally available river sand conforming to grading zone III of IS 383-1970 with specific gravity 2.5 as fine aggregate.
– Crushed blue granite stone aggregates of maximum size 12mm and graded as per IS 383-1970 with specific gravity 2.68 as coarse aggregate.
– Sodium hydroxide and sodium silicate solutions are used as activators.
– Distilled water is used to dissolve sodium hydroxide pellets in 10M.
– Hook end steel fibres are used in concrete. The properties of steel fibres are shown in Table 1
B. Mix proportions
The design mix can be arrived at by assuming the density of geopolymer concrete as 2400 kg/m3. The total volume occupied by fine and coarse aggregate is around 77-80% and the various materials used in this project are:
– Density of aggregate is 77%.
– The alkaline liquid to fly ash ratio is kept as 0.4.
– Sodium hydroxide is taken as 10 Molarity.
– The ratio of sodium silicate to sodium hydroxide is kept as 2.5.
– Extra water is added as 15% of cementitious material.
– Super Plasticizer is added as 3% of cementitious material.
– The materials required for 1m3 of geopolymer concrete are given in Table 2 .
C. Casting of Specimens
Cubes of size 100mm x 100mm x 100mm, cylinders of size 300mm height and 150mm dia., and prisms of size 500mm x 100mm x 100mm were cast to find out the compressive strength, split tensile strength, and flexural strength respectively. The fresh concrete mix was filled in the steel moulds in three equal layers and each layer was well compacted using table vibrator. After de-moulding they were kept at room temperature for 28 days. Fig 2 shows casting of controlled cube, cylinder and prism specimens for test. Beams of size 1600mm x 150mm x 100mm were cast to find out the first crack load, ultimate load and load deflection behaviour. Beams were kept at room temperature for curing up to test period of 28 days.
The compressive strength, split tensile and flexural strength of the controlled and fibre reinforced geopolymer concrete specimens were tested at the age of 28 days. Three identical specimens were tested in all the mixtures.
E. Experimental setup for testing of beams
The experimental setup for testing of beams are shown in fig 4. The beam was supported on the two simply supported edges and tested in 250KN of the loading frame in the laboratory. The load was gradually applied to the specimen by using hand operated hydraulic jack. The intensity of load was measured by the load cell having capacity of 50KN. The load is applied at an increment of 5 KN up to the failure of the specimen. A Linear Variable Differential Transformer (LVDT) was used to measure the deflection at mid span and two dial gauges were used at l/3 distance from supports under the load acting point. At each increment of load, deflection and crack pattern were recorded. The failure mode of the specimen was observed.
Results and Discussion
Fibre reinforced Geopolymer concrete possesses enhanced mechanical properties than conventional geopolymer concrete. The experimental results show that the compressive strength increases with the addition of steel fibres at various volume fractions. At 7 days the compressive strength of geopolymer concrete with 0.25% and 0.5% of steel fibres was 1.16 and 1.07 times that of the conventional geopolymer concrete cubes. At 28 days the compressive strength of geopolymer concrete with 0.25% and 0.5% of steel fibres was 1.04 and 1.1 times that of the conventional geopolymer concrete cubes. The split tensile strength of geopolymer concrete with 0.25% and 0.5% of steel fibres is 1.24 and 1.59 times that of the conventional geopolymer concrete. The flexural strength of geopolymer concrete with 0.25% and 0.5% of steel fibres is 3.34 and 3.54 times that of the conventional geopolymer concrete.
The compressive, split tensile, flexural strength for different mix proportions were shown in fig 5, fig 6 and fig 7.
Based on the investigation, the flexural behaviour of the GPC beams were compared with FRGPC beams and the following conclusions were arrived at. When compared to the flexural behaviour of FRGPC with GPC beams, the FRGPC beams are more ductile in nature. The first crack, ultimate load and maximum deflection of Fibre Reinforced Geopolymer Beams (FRGPC) beams are better than Geopolymer Concrete (GPC) beams.
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