Effect of steel fibres and low calcium fly ash on mechanical and elastic properties of geopolymer concrete composites (GPCC) has been presented. The study analyses the impact of steel fibres and low calcium fly ash on the compressive, flexural, split-tensile, and bond strengths of hardened GPCC. Geopolymer concrete mixes were prepared using low calcium fly ash and activated by alkaline solutions (NaOH and Na2SiO3) with solution to fly ash ratio of 0.35. Crimped steel fibres having aspect ratio of 50 with volume fraction of 0.0% to 0.5% at an interval of 0.1% by mass of normal geopolymer concrete are used. The entire tests were carried out according to test procedures given by the Indian standards wherever applicable. The inclusion of steel fibre showed the excellent improvement in the mechanical properties of fly ash based geopolymer concrete. Elastic properties of geopolymer concrete composites are also determined by various methods available in the literature and compared with each other.
Plain cement concrete suffers from numerous drawbacks such as low tensile strength, brittleness, unstable crack propagation, and low fracture resistance. Addition of steel fibres in plain cement concrete improves its mechanical and elastic properties. Hence, steel fibre reinforced concrete has been proved as a reliable and promising composite construction material having superior performance characteristics compared to conventional concrete.
The rate of production of carbon dioxide released to the atmosphere is increasing due to the increased use of Portland cement in the construction. Each ton of Portland cement releases a ton of carbon dioxide into the atmosphere. The greenhouse gas emission from the production of Portland cement is about 1.35 billion tons annually, which is about 7% of the total greenhouse gas emissions. On the other side, fly ash is the waste material of coal based thermal power plant available abundantly but this poses disposal problem. Several hectares of valuable land are acquired by thermal power plants for the disposal of fly ash. With silicon and aluminium as the main constituents, fly ash has great potential as a cement replacing material in concrete. The concrete made with such industrial wastes is eco-friendly. Although the use of Portland cement is still unavoidable, many efforts are being made in order to reduce the use of Portland cement in concrete. Davidovits [
Chindaprasirt et al. [
In the present study, an experimental investigation on mechanical and elastic properties of low calcium fly ash based GPCC has been carried out. Geopolymer concrete mixes were prepared with solution to fly ash ratio of 0.35. Crimped steel fibres having aspect ratio of 50 are used.
Experimental work is designed to study the effect of steel fibres on mechanical and elastic properties on geopolymer concrete. The materials used for making fly ash geopolymer concrete composite specimens are low-calcium fly ash, course and fine aggregates, steel fibres, alkaline solution, and water.
Fly ash is the residue from the combustion of pulverized coal collected by mechanical or electrostatic separators from the flue gases of thermal power plants. One of the important characteristics of fly ash is the spherical form of the particles. This shape of particle improves the flow ability and reduces the water demand. In this experimental work, the fly ash used is obtained from the silos of Eklahare Thermal Power Station, Nasik, Maharashtra, India, which is of low calcium, Class F (American Society for Testing and Materials). The fly ash which contains less than 10% calcium oxide is called Class F or low calcium fly ash. It is mainly pozzolanic and reacts with calcium hydroxide formed during the hydration process in moist condition to produce cementitious compounds when used with cement. Low calcium fly ash makes substantial contributions to the workability, chemical resistance, and reduction in thermal cracking. Table
Chemical composition of fly ash.
Chemical composition | SiO2 | Al2O3 | Fe2O3 | MgO | SO3 | Na2O | CaO | Total chlorides | Loss of ignition |
---|---|---|---|---|---|---|---|---|---|
Percentage (%) | 77.10 | 17.71 | 1.21 | 0.90 | 2.20 | 0.80 | 0.62 | 0.03 | 0.87 |
The laboratory grade sodium hydroxide (NaOH) in flake form and sodium silicate (Na2SiO3) solution were used as alkaline activators. The chemical compositions of both activators are given in Table
Chemical composition of sodium hydroxide and sodium silicate.
Chemical composition | |||
---|---|---|---|
Sodium hydroxide | Percentage (%) | Sodium silicate | Percentage (%) |
Sodium hydroxide (min. assay) | 97 | Na2O | 16.37 |
Carbonate | 2 | SiO2 | 34.31 |
Chloride | 0.01 | Total solid | 50.68 |
Sulphate | 0.05 | Water content | 49.32 |
Potassium | 0.1 | — | |
Silicate | 0.05 | — | |
Zinc | 0.02 | — |
Locally available river sand is used as a fine aggregate (FA) and crushed basalt stones of nominal maximum size of 20 mm and 12.5 mm are used as coarse aggregates (CA). The physical properties of CA and FA are shown in Table
Properties of aggregates.
Physical properties | Coarse aggregate (CA) | Fine aggregate (FA) | |
---|---|---|---|
CA-I | CA-II | ||
Type | Crushed | Crushed | River sand |
Maximum size | 20 mm | 12.5 mm | 4.75 mm |
Specific gravity | 2.641 | 2.639 | 2.563 |
Water absorption | 0.59% | 0.82% | 1.56% |
Moisture content | Nil | Nil | Nil |
Grading of course aggregates.
Serial number | IS Sieve size (mm) | CA-I |
CA-II |
CA-I : CA-II |
Required grading as per |
---|---|---|---|---|---|
1 | 40 | 100 | 100 | 100 | 100 |
2 | 25 | 100 | 100 | 100 | — |
3 | 20 | 90.60 | 100 | 93.89 | 90–100 |
4 | 16 | 6.80 | 100 | 39.42 | — |
5 | 12.5 | 0.40 | 96.5 | 34.04 | — |
6 | 10 | 0.00 | 76.4 | 26.74 | 25–35 |
7 | 4.75 | 0.00 | 0.90 | 0.32 | 0–10 |
8 | 2.36 | 0.00 | 0.00 | 0.00 | — |
Crimped steel fibres with aspect ratio (
Fly ash, coarse aggregates, fine aggregate, and steel fibres are initially mixed together in dry state and then the liquid component of the mixture is added to prepare wet mix until it gives homogeneous mix. Mix proportion and fibre quantity of geopolymer concrete per cubic meter are given in Tables
Mix proportion of geopolymer concrete per cubic meter.
Fly ash | NaOH | Na2SiO3 | FA | CA | Extra water |
---|---|---|---|---|---|
300 | 52.5 | 52.5 | 722.24 | 1341.30 | 61.46 |
1 | 0.175 | 0.175 | 2.40 | 4.4771 | 0.20 |
Quantity of fibres for various mixes.
Steel fibre (%) | 0.0 | 0.1 | 0.2 | 0.3 | 0.4 | 0.5 |
---|---|---|---|---|---|---|
Fibre quantity (kg/m3) | 0.0 | 2.53 | 5.06 | 7.59 | 10.12 | 12.65 |
The workability of fresh geopolymer concrete is measured by flow table apparatus as per IS: 1199–1959. After making the homogeneous mix, concrete was placed in two layers; each layer should be compacted 20 times with the tamping rod and then level the top surface. Lift the mould away from the concrete one minute after completing the mixing operation. Then apply fifteen jolting and measure the average diameter of subside concrete at minimum six equal intervals expressed as a percentage of the original base diameter. Examination of Table
Workability, wet density, and dry density of GPCC Mixes.
|
Flow (mm) | Decrease in flow (%) | Degree of workability | Density (kg/m3) | Increase in density (%) | ||
---|---|---|---|---|---|---|---|
Wet | Dry | Wet | Dry | ||||
0.0 | 62.93 | 0.00 | High | 2583 | 2453 | 0.0 | 0.0 |
0.1 | 60.40 | 4.02 | High | 2592 | 2460 | 0.34 | 0.28 |
0.2 | 57.33 | 8.89 | High | 2616 | 2479 | 1.27 | 1.05 |
0.3 | 54.13 | 13.98 | High | 2634 | 2505 | 1.97 | 2.11 |
0.4 | 51.73 | 17.79 | High | 2641 | 2515 | 2.24 | 2.52 |
0.5 | 48.40 | 22.72 | Medium | 2660 | 2530 | 2.98 | 3.13 |
The tests on hardened geopolymer concrete composites are carried out according to the relevant standards wherever applicable. New expressions for mechanical properties of GPCC are proposed in this investigation. Comparison of various strengths obtained using experimental work is presented in Table
Comparison of compressive strength (
|
Strengths (MPa) | Percentage increase in strength | ||||||
---|---|---|---|---|---|---|---|---|
|
|
|
|
|
|
|
| |
0.0 | 28.89 | 3.20 | 3.36 | 12.10 | 0.00 | 0.00 | 0.00 | 0.00 |
0.1 | 33.55 | 3.68 | 3.87 | 12.82 | 16.13 | 15.00 | 15.17 | 5.95 |
0.2 | 37.55 | 4.16 | 4.37 | 14.05 | 29.98 | 30.00 | 30.05 | 16.11 |
0.3 | 33.11 | 3.87 | 4.06 | 13.59 | 14.61 | 20.90 | 20.83 | 12.31 |
0.4 | 32.22 | 3.52 | 3.70 | 13.33 | 11.53 | 10.00 | 10.11 | 10.16 |
0.5 | 31.5 | 3.31 | 3.48 | 12.94 | 9.03 | 3.43 | 3.57 | 6.94 |
The compressive strength of concrete increases with respect to fibre content up to 0.2% and then it decreases because higher percentage of fibre content reduces the workability of GPCC. The maximum increase in compressive strength of GPCC is 29.98% over that of normal geopolymer concrete. Expression for compressive strength in the third degree polynomial in terms of
Flexural test is carried out on beams of size
From Table
Cylinders of size 100 mm diameter and 300 mm length are used to obtain split-tensile strength. The split-tensile strength is obtained by using
The maximum increase in split-tensile strength is 30.05% with 0.2% of fibre content over that of normal geopolymer concrete. Expression for split-tensile strength in the third degree polynomial in terms of
The bond strength test was carried out according to IS 2770 [
Expression for bond strength in the third degree polynomial in terms of
Optimum fibre content for various strengths of geopolymer concrete is presented in Table
Optimum fibre content and maximum percentage increase in various strengths.
Strength |
|
Maximum strength (MPa) | Percentage increase in strength (%) |
---|---|---|---|
|
0.2 | 37.55 | 29.98 |
|
0.2 | 4.16 | 30.00 |
|
0.2 | 4.37 | 30.35 |
|
0.2 | 14.05 | 16.11 |
Compressive strength (
|
Strength (MPa) | Percentage increase | ||||||
---|---|---|---|---|---|---|---|---|
|
|
|
|
|
|
|
| |
0.0 | 28.59 | 3.15 | 3.32 | 12.00 | 0.00 | 0.00 | 0.00 | 0.00 |
0.1 | 34.71 | 3.82 | 4.01 | 13.14 | 20.35 | 21.26 | 20.78 | 9.50 |
0.2 | 35.83 | 4.01 | 4.21 | 13.66 | 25.32 | 27.30 | 26.80 | 13.83 |
0.3 | 34.18 | 3.87 | 4.07 | 13.70 | 19.55 | 22.85 | 22.59 | 14.16 |
0.4 | 31.98 | 3.58 | 3.76 | 13.40 | 11.85 | 13.65 | 13.25 | 11.66 |
0.5 | 31.47 | 3.32 | 3.45 | 12.98 | 10.07 | 5.39 | 3.91 | 8.16 |
Variation of compressive strength (
Variation of flexural strength (
Variation of split-tensile strength (
Variation of bond strength (
Elastic properties such as modulus of elasticity, modulus of rigidity, and Poisson’s ratio are the important parameters in the analysis of structures. These properties are obtained by various methods available in the literature.
The modulus of elasticity is obtained by various methods available in the literature as given below.
As per IS: 456 [
Modulus of elasticity of fibre reinforced composites can be calculated using law of mixtures as suggested by Hannant [
The equation of modulus of elasticity in terms of compressive strength (
For average value of specific gravity, (
The modulus of elasticity obtained by using (
Elastic properties of geopolymer concrete composites (GPCC).
|
Modulus of elasticity ( |
Poisson’s ratio ( |
|||
---|---|---|---|---|---|
Equation ( |
Equation ( |
Equation ( |
Equation ( |
Equation ( | |
0.0 | 26.874 | 36.931 | 26.516 | 26.046 | 0.1107 |
0.1 | 28.96 | 27.881 | 28.455 | 28.069 | 0.1096 |
0.2 | 30.63 | 32.603 | 29.300 | 29.690 | 0.1107 |
0.3 | 28.77 | 31.76 | 30.46 | 27.884 | 0.1168 |
0.4 | 28.38 | 32.37 | 27.986 | 30.050 | 0.1092 |
0.5 | 28.06 | 31.90 | 29.713 | 27.198 | 0.1050 |
Modulus of elasticity and shear modulus using Halpin Tsai equations [
|
|
|
|
|
|
|
|
---|---|---|---|---|---|---|---|
0.0 | 0.063 | 0.694 | 26.87 | 26.87 | 26.87 | 10.07 | 26.874 |
0.1 | 0.058 | 0.675 | 46.02 | 35.24 | 93.28 | 14.56 | 35.353 |
0.2 | 0.054 | 0.661 | 64.40 | 44.63 | 52.04 | 19.21 | 41.632 |
0.3 | 0.058 | 0.678 | 80.22 | 50.81 | 61.84 | 22.73 | 43.286 |
0.4 | 0.059 | 0.681 | 97.66 | 60.25 | 74.28 | 27.27 | 44.568 |
0.5 | 0.060 | 0.683 | 115.71 | 71.70 | 88.21 | 32.39 | 44.106 |
The behavior of geopolymer concrete is very complex. So, the modulus of elasticity calculated by various methods does not match with each other. But it is observed that the modulus of elasticity increases with incorporation of steel fibres up to 0.2% by volume and then decreases with increase in fibre content but higher than plain geopolymer concrete. This means that fibre improves the elastic behavior of geopolymer concrete which is more brittle than cement concrete.
The Poisson’s ratio (
Table
Geopolymer concrete is a new invention in the world of concrete in which cement is totally replaced by industrial waste which contributes towards the global worming by reducing use of cement and utilisation of byproducts like fly ash. Since geopolymer concrete is more brittle than conventional concrete, steel fibres are used to make it an elastic one.
This paper presents the effect of steel fibres on mechanical and elastic properties of geopolymer concrete. From the experimental results it is concluded that the wet and dry densities of geopolymer concrete composites increased continuously with increase in fibre content, whereas the workability of geopolymer concrete composites reduced with increase in fibre content. Optimum fibre content for the maximum value of various strengths of geopolymer concrete composites is 0.2%. The maximum percentage increase in compressive strength, flexural strength, split tensile strength, and bond strength is 29.98%, 30%, 30.05%, and 16.11%, respectively. The proposed equation for modulus of elasticity yield excellent result and Poisson’s ratio varies between specified limit.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The support from Saurabh Shah, Sanket Sakalkar, Amol Gite, Ritesh Gajare and Chetan Shahare, Graduate students, Department of Civil Engineering, SRES’s College of Engineering, Kopargaon, Maharashtra, India, for this study is gratefully acknowledged.