Sustainable Construction—High Performance Concrete Containing Limestone Dust as Filler ()
1. Introduction
The quality of concrete has improved tremendously over the recent years due to ongoing research on newer materials and chemicals. High performance concretes with improved strengths and durability are readily available in the markets. Scarcity of natural aggregates and cement manufacturing raw materials has led to exploration of cheaper alternatives worldwide. Furthermore, sustainable and emerging green practices leading to environmentally friendly materials have led to use of many waste materials in manufacturing of cement and concrete. These have led to newer avenues especially in the developing/under developed countries where resource crunch is a major hurdle towards development. A number of studies have been carried out to find better/cheaper aggregates and cements along with better usage of waste materials in construction [4] - [11] .
Massive amounts of limestone and marble are quarried and used worldwide for various purposes. Marble is a refined form of limestone and dolomite formed by their metamorphism. During cutting and sawing process of limestone, huge amounts of fragments, fine powder and slurry are generated as waste product. Generally, it is believed that 60% to 70% of the stone is wasted in this process in the form of fragments, powder and slurry out of which around 30% is in the form of fine powder [1] . Large quantities of limestone processing waste worldwide poses disposal problems since it has no beneficial usage. Use of limestone dust waste in production of concretes for major construction projects will certainly reduce the costs and will hence result into cheaper construction and improved durability. Bulk use of this waste material can consume large quantities in construction industry, thereby reducing the burden on the environment in addition to producing environment friendly concrete. In the absence of any worthwhile research in this field, this study was undertaken to assess the properties of concretes produced by using limestone dust as partial fines replacement.
2. Research Significance
The significance of this research is to investigate the possible use of an abundantly available waste product in construction industry thereby solving its disposal problems along with possibility of obtaining a better product i.e. high performance concrete with improved characteristics. Environment is benefitted as a consequence.
3. Mix Design
Mix design for three high strength concrete mixes was carried out based on
Design of Normal Concrete Mixes method of Department of Environment― Transport and Road Research Laboratory, London, UK [12] . Linear projection of compressive strength versus w/c ratio from Design of Normal Concrete Mixes method was considered beyond the limiting w/c ratio of 0.3. Characteristic strengths selected for the trial concrete mixes were 60, 80 and 100 N/mm2. These mixes were designed using ordinary Portland cement, crushed limestone coarse aggregates (maximum 37.5 mm diameter) and medium grade sand partially replaced with 10% and 20% limestone dust. Control mix contained 100% medium sand. Limestone dust used was 100% passing #100 sieves. An initial estimate of density was made and later adjusted in the light of values actually obtained by the mix design method. The details of the quantities of materials used in the mixes are given in Table 1. Around 1% of super plasticizer with a maximum of 15.1 l/m3 was used to maintain workability and slump in the range of 90 to 120 mm.
4. Testing of Concrete
Three specimens each from three different batches were used in all tests. Tests and specimen used for different tests were as follows:
Compressive strength/density
Cubes-150 mm, Cylinders-150 mm diameter, 300 mm long.
Flexural strength
Beams―150 × 150 × 750 mm long.
Stress/strain behavior
Cylinders―150 mm diameter, 300 mm long.
Static modulus of elasticity
Cylinders―150 mm diameter, 300 mm long.
Dynamic modulus of elasticity
Beams―150 × 150 × 750 mm long.
Ultrasonic pulse velocity
Cubes―150 mm.
Initial surface absorption
Cubes―150 mm.
Sulphate and Chloride resistance
Cubes―150 mm. (Immersed in 5% H2SO4 and 5% HCl solutions for 90 days and measuring weight loss)
All specimens were cured in water at 20˚C for 28 days before testing.
5. Discussion of Test Results
The properties of the high-performance concretes produced are summarized in Table 2 and Table 3.
5.1. Compressive Strength
Compressive strength tests on cubes at 7 & 28 days showed that the rate of
Control―100% Medium Sand; A―10% Sand replaced with limestone dust; B―20% Sand replaced with limestone dust.
development of strength of concrete specimen containing 10% and 20% sand replaced with limestone dust was similar to normal concrete samples. The compressive strengths of high performance concrete with 10% and 20% sand replaced with limestone dust was higher than the control specimen varying between 4% to 12% with higher range for 20% sand replacement with limestone dust. Since low w/c ratios require lot of external water for hydration, it was observed that the compressive strengths of test samples kept increasing with time [13] . High performance concrete with 10% and 20% sand replaced with limestone dust both developed 80% to 85% of its 28 day characteristic strength in 7 days like normal concretes. During the testing, it was observed that the complete section of high performance concrete specimen tends to fail suddenly and explosively which is typical of high strength concretes [13] . Protective measures must be taken to prevent any damage due to such sudden failure of test specimen. To maintain safety from the explosive failure of high strength concrete specimen, a lower loading rate of 0.15 to 0.2 N/mm2/s was maintained during the testing as compared to 0.2 to 0.4 N/mm2 specified by BS1881:Part 116:1983. Table 2 gives the compressive strengths of the samples as well as the control specimen.
5.2. Flexural Strength
The flexural strength of high performance concrete with 10% sand replaced with limestone dust as well as specimen containing 20% sand replaced with limestone dust both were observed to be higher by 12% to 13% as compared to control specimen. Higher flexural strengths are certainly a consequence of higher compressive strength and increased density of concrete with 10% and 20% sand replaced with limestone dust as compared to the control mixes as shown in Table 2.
5.3. Stress/Strain Behavior
It was observed that the general slope of the stress/strain curve for high performance concrete with 10% sand replaced with limestone dust as well as specimen containing 20% sand replaced with limestone dust both were similar to the curve for control specimen. All the curves were observed to be virtually linear up to the point of failure which is typical of high strength concretes. Higher static and dynamic moduli of elasticity were observed for high strength concretes with 10% sand replaced with limestone dust as well as specimen containing 20% sand replaced with limestone dust both. Values for static and dynamic moduli are given in Table 3 while idealized stress/strain relationships for all samples are shown in Figure 1.
5.4. Static Modulus of Elasticity
Values for Static Modulus of Elasticity for high strength concrete with specimen containing 20% sand replaced with limestone dust was observed to be about 4 to
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Control―100% Medium Sand; A―10% Sand replaced with limestone dust; B―20% Sand replaced with limestone dust.
Figure 1. Idealized Stress―Strain Curves.
5% higher than the control specimen. For concrete with 10% sand replaced with limestone dust, the Static Modulus of Elasticity was observed to be 2% to 3% higher than the control specimen. Average values for Static Modulus of Elasticity was observed to be vary from 38,000 to 40,000 N/mm2 for high strength concrete with 20% sand replaced with limestone dust as compared to 36,800 to 38,000 N/mm2 for high strength concretes containing 100% medium sand. The Values for Static Modulus of Elasticity are given in Table 3.
5.5. Dynamic Modulus of Elasticity
Dynamic Modulus of Elasticity for concrete with 20% sand replaced with limestone dust was observed to be 4% higher than the control as compared to concrete with 10% sand replaced with limestone dust which was observed to be about 2% higher than the control specimen. Table 3 gives the values of Dynamic Moduli of Elasticity of various specimen.
5.6. Ultrasonic Pulse Velocity
Average pulse velocity across concrete with specimen containing 10% and 20% sand replaced with limestone dust was observed to be 5.2 and 5.4 km/s respectively as compared to an average velocity of around 4.8 km/s for control mixes. Hence ultrasonic pulse velocity in the case of concrete with 10% and 20% sand replaced with limestone dust was observed to be 10% to 12% higher than the control mixes. Higher pulse velocities are certainly due to better quality, higher density and reduced voids in the high strength concretes containing partial replacement of limestone dust with sand, as compared to the control mixes. The ultrasonic pulse velocities observed for different concretes are given in Table 3.
5.7. Density of Hardened Concrete
The average saturated and oven-dried densities for high strength concrete with 10% and 20% sand replaced with limestone dust were 2580 and 2490 kg/m3 respectively, as compared to control mixes which were 2488 and 2461 kg/m3, respectively. Hence the saturated and dry densities of concrete with 10% and 20% sand replaced with limestone dust are about 4% & 2% higher respectively than the control mixes. Higher densities for concrete with 10% and 20% sand replaced with limestone dust are due to better packing of materials in concrete with sand replaced with fine limestone dust. In the presence of higher content of fines and cementitious material and low w/c ratios, most of the unhydrated cementitious material too acts as filler to densify the concrete, whilst the hydration process continues over longer duration.
5.8. Initial Surface Absorption (ISAT)
Initial surface absorption for concrete with 20% sand replaced with limestone dust was observed to be lowest with around 22% lower whilst for concrete with 10% sand replaced with limestone dust it was around 19% low as compared to the control. The values are compared with the guidelines given by the Concrete Society Technical Report # 31 [14] . Results of ISAT are given in Table 3.
5.9. Sulphate and Chloride Resistance
For chloride resistance, submersion of specimen in HCL solution resulted in average weight loss of 7% for control specimen as compared to 3.6% and 3.2% for concrete with 10% and 20% sand replaced with limestone dust respectively. For sulphate resistance, submersion of specimen in H2SO4 solution resulted in average weight loss for control specimen as 6% as compared to 2.7% and 2% for specimen containing 10% and 20% sand replaced with limestone dust respectively. Hence, the performance of concrete with partial replacement of sand with limestone dust was almost twice better in acidic environment and slightly over twice better in sulphate environment as compared to concrete with ordinary Portland cement control mixes. Better chloride and sulphate resistance for concrete with 10% and 20% sand replaced with limestone dust is mainly due to the reduced permeability and higher densities of concrete with partial replacement of sand with limestone dust.
5.10. Shrinkage
Shrinkage of all specimen was observed to be similar. No appreciable difference in shrinkage of specimen cast from concrete with partial replacement of sand with limestone dust and control mixes were observed for 90 days.
6. Conclusion
The performance of concrete with partial replacement of sand with limestone dust proved to be satisfactory with improved performance. Compressive strengths of 60/80/100 N/mm2 can be attained with up to 20% sand replaced with limestone dust in normal concrete mixes. Better packing, reduced voids, improved workability and better hydration due to fines dispersal in the mix resulted into better performance. 4% to 12% higher compressive strengths could be achieved with 10% and 20% sand replaced with limestone dust in normal concrete with higher range for 20% sand replacement with limestone dust. Flexural strength of high performance concrete with 10% and 20% sand replaced with limestone dust is observed to be higher by 12% to 13% as compared to control specimen. The average static modulus of elasticity for concrete specimen containing 20% sand replaced with limestone dust was observed to be about 4% to 5% higher than the control specimen while for concrete with 10% sand replaced with limestone dust, it was observed to be 2% to 3% higher than the control. The average dynamic modulus of elasticity for concrete with 20% sand replaced with limestone dust was observed to be 4% higher than the control as compared to concrete with 10% sand replaced with limestone dust which was observed to be about 2% higher. Ultrasonic pulse velocity in the case of concrete with 10% & 20% sand replaced with limestone dust was observed to be 10% to 12% higher than the control mixes. Saturated and dry densities of concrete with 10% & 20% sand replaced with limestone dust are about 4% & 2% higher respectively than the control mixes. The performance of concrete with partial replacement of sand with limestone dust was almost twice better in acidic environment and slightly over twice better in sulphate environment as compared to concrete with ordinary Portland cement control mixes. There is no appreciable difference in shrinkage of specimen cast from concrete with partial replacement of sand with limestone dust and control mixes were observed for 90 days.