Fly ash substitution to cement is a well-recognized approach to reduce CO2 emissions. Although fly ash concrete is prone to brittle behavior, researchers have shown that addition of fibers could reduce brittle behavior. Previous research efforts seem to have utlised a single type of fiber or two types of fibers. In this research, three types of fibers, steel, polypropylene, and basalt as 0%, 0.50%, 0.75%, and 1% by volume of concrete, were mixed in varying proportions with concrete specimens substituted with 50% fly ash (class F). All specimens were tested for compressive strength, indirect tensile strength, and flexural strength over a period of 3 to 56 days of curing. Test results showed that significant improvement in mechanical properties could be obtained by a particular hybrid fiber reinforcement combination (1% steel fiber, 0.75% polypropylene fiber, and 0.75% basalt fiber). The strength values were observed to exceed previous research results. Workability of concrete was affected when the fiber combination exceeded 3%. Thus a limiting value for adding fibers and the combination to achieve maximum strengths have been identified in this research.
Fly ash is generally an industrial waste obtained from burning coal. Fly ash substitution to cement is a well-recognized approach to reduce CO2 emissions. Malhotra [
In the literature, three types of fibers that were added to fly ash concrete are widely discussed: steel, polypropylene, and basalt. With the addition of steel fibers concrete toughness was observed to be proportional to fiber content in both static and dynamic loading conditions as shown by Zollo [
Addition of polypropylene fiber was found to improve the durability of concrete composites containing fly ash and silica fume but showed adverse effect on the workability of concrete. In addition, water permeability, dry shrinkage strain, and the depth of carbonation of concrete decreased gradually with the increase of fiber volume fraction [
The addition of basalt fiber was observed to improve the deformation and energy absorption properties without notable enhancement in dynamic compressive strength [
From the above review, it can be noted that steel fiber (high modulus fiber) is stronger and stiffer which improves the concrete strength, while polypropylene fiber (low modulus fiber) has the capacity to strengthen brittle cementitious materials and is more flexible and has the property to retain heat for a prolonged time which leads to improved toughness and strain capacity in the postcracking section and retard early cracks. Basalt fiber which is high in oxidation resistance and radiation resistance, fracture energy, and abrasion resistance leads to increase in the flexural strength.
In summary, various researchers have used steel, polypropylene, and basalt fibers mostly as individual additions and rarely as combinations to improve the properties of concrete. Therefore, it is logical to postulate that the combination of fibers may provide reasonable improvements overall and negate some of the disadvantages noted in the literature. This postulation requires an experimental investigation. Further sections of this paper present the experimental program and test results arising from the combination of fibers (steel, polypropylene, and basalt) in hybrid form added to high volume fly ash concrete.
Following materials were used in the experimental program: cement, Type I Portland Cement with specific gravity = 3.14; fine aggregate, river sand that passed through 4.75 mm sieve; and coarse aggregate, granite stone that passed through 12.5 mm IS sieve and retained on 10 mm sieve. Fly ash procured from the Tarong Power Plant was used and it was tested in the concrete laboratory at RMIT University and the results are presented in Appendix (Table
Control specimen is 50% cement and 50% class F fly ash with no fibers. Although the percentage of fly ash mix is dependent on a number of factors, there is sufficient agreement in the literature that brittle behavior is dominant once the fly ash percentage crosses 50%. The authors have used 50%-50% mix in our previous endeavors [
Specimen nomenclature.
Sl. number | Mix number | Fibers | Cement | Class F fly ash | ||
---|---|---|---|---|---|---|
Steel fiber | Polypropylene fiber | Basalt fiber | ||||
1 | C | 0% | 0% | 0% | 50% | 50% |
2 | S1 | 0.50% | 0% | 0% | 50% | 50% |
3 | S2 | 1% | 0% | 0% | 50% | 50% |
4 | P1 | 0% | 0.50% | 0% | 50% | 50% |
5 | P2 | 0% | 0.75% | 0% | 50% | 50% |
6 | B1 | 0% | 0% | 0.50% | 50% | 50% |
7 | B2 | 0% | 0% | 0.75% | 50% | 50% |
8 | B3 | 0% | 0% | 1% | 50% | 50% |
9 | X1 | 0.50% | 0.50% | 0.50% | 50% | 50% |
10 | X2 | 0.50% | 0.75% | 0% | 50% | 50% |
11 | X3 | 0.50% | 0.75% | 0.75% | 50% | 50% |
12 | X4 | 0.50% | 0% | 1% | 50% | 50% |
13 | Y1 | 1% | 0.50% | 0.50% | 50% | 50% |
14 | Y2 | 1% | 0.75% | 0% | 50% | 50% |
15 | Y3 | 1% | 0.75% | 0.75% | 50% | 50% |
16 | Y4 | 1% | 0% | 0.75% | 50% | 50% |
17 | Y5 | 1% | 0% | 1% | 50% | 50% |
18 | Y6 | 1% | 1% | 1% | 50% | 50% |
Chemical composition of fly ash.
Characteristics | Tarong fly ash | ASTM class F fly ash |
---|---|---|
SiO2 | 65.9 | The sum of SiO2 + Al2O3 + Fe2O3 (min 70%) |
Al2O3 | 28.89 | |
Fe2O3 | 0.38 | |
| ||
TiO2 | 1.97 | |
MnO | 0 | |
MgO | 0.15 | |
CaO | 0.06 | |
Na2O | 0.05 | |
K2O | 0.26 | |
P2O5 | 0.08 | |
SO3 | 0.03 | Max, 5% |
LOI | 1.24 | Max, 6% |
Cylindrical specimens were cast with dimensions of 100 mm diameter × 200 mm length for compressive strength test, cylindrical specimens with dimensions of 150 mm diameter × 300 mm length for indirect tensile strength test, and beam specimens with dimensions of 350 mm × 100 mm × 100 mm for flexural strength test. The numbers of specimens used for this research are compressive strength tests, 270 cylinders; indirect tensile strength tests, 54 cylinders; flexure strength tests, 54 beams.
Methodology for concrete mixing involved the following: Aggregates and sand were washed with water and completely dried. Then both were placed in the concrete mixer and dry mixed for 2 minutes. Cement, fly ash, and lime powder (5%) were added in the mixer with aggregates and sand and dry mixed for 2 minutes. Then fibers (steel fiber, polypropylene fiber, and basalt fiber) were added one after another and dry mixed for 1 minute. Then normal water (85%) is added and mixed for approximately 2 minutes. Remaining mixing water (15%) and plasticizer were added to the mixer and mixed for 3 minutes. Then the mixed concrete was cast into the specimen moulds and vibrated simultaneously in the vibrator for 1 minute to remove any air remaining entrapped mainly to avoid voids. Each specimen was allowed to stand for 24 hours in concrete laboratory before demolding.
The compressive strength test was carried out in accordance with Australian Standard [
The Indirect tensile strength test was carried out in accordance with Australian Standard [
The indirect tensile strength (
The modulus of rupture development of concrete test was carried out in accordance with Australian Standard [
The modulus of rupture (
The modulus of rupture (
As can be seen from Figures
Comparison of compressive strength between control concrete (50% cement, 50% fly ash, and no fibers) and hybrid fiber mixes at 7 days.
Comparison of compressive strength between control concrete (50% cement, 50% fly ash, and no fibers) and hybrid fiber mixes at 28 days.
As Y3 mix is observed to have the greatest strength from Figure
Comparison of compressive strength for hybrid fiber mix Y3 and control mix with compressive strength of OPC from Çolak [
Figure
Comparison of indirect tensile strength for hybrid fibers at 28 days.
Comparing indirect tensile strength test results, Y1 (1% steel fiber, 0.5% polypropylene fiber, and 0.5% basalt fiber) and Y3 (1% steel fiber, 0.75% polypropylene fiber, and 0.75% basalt fiber) in hybrid form give higher tensile strength when compared to control concrete and individual fibers.
Figure
Comparison of flexural strength for hybrid fiber mixes at 28 days.
Clearly the above figures have shown that the mix Y3 has significantly higher values across the testing regime. Brief investigations of the contribution of different mix proportions to the three strengths were analysed using strength values obtained at 28 days. First, the contribution of individual fibers is presented in Figure
Comparison of compressive strengths in individual fiber mix.
A comparison of compressive strengths with hybrid combination is presented in Figure
Comparison of compressive strengths with fiber mix at different proportions.
This paper has shown that three fibers (steel, polypropylene, and basalt) can be added in a hybrid combination to achieve compressive strengths higher than the normal OPC. In particular, a hybrid combination of 1% steel, 0.75% polypropylene, and 0.75% basalt provided the highest strength results in terms of common mechanical properties. The paper has also shown that significant increase in flexural strength and tensile strengths can be observed simultaneously leading to the suggestion that some of the negative effects of individual fiber additions can be avoided using the hybrid fiber combination. A limiting combination of 3% overall has also been determined based on the workability of concrete. Further work may include the analysis of specimens in terms of matrix formulations, cracking behavior, and concrete toughness.
See Table
Specimens cast and ready for testing.
Crack patterns at ultimate failure load: control specimen (a) and specimen (Y3) (b). Reduced cracks in Y3 specimen.
The authors declare that there is no conflict of interests regarding the publication of the paper.