Mechanical properties of self-compacting high-performance concrete with fly ash and silica fume
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This paper presents the results of experimental work on the mechanical properties of self-compacting high-performance concrete (SCHPC) containing fly ash (FA) and silica fume (SF). The paper focused on the feasibility of substituting the ordinary Portland cement with waste supplementary cementing materials to decrease the abundant disposal of FA likewise improving the mechanical properties of SCHPC. Six (6) different mixes were made with 0%, 25%, 40%, 50%, 60% and 75% cement replacement by FA and SF. Test on fresh concrete such as Slump flow, L-Box, and V-funnel was conducted to assess the fresh properties of SCHPC. The mechanical properties evaluated were development of compressive strength, splitting tensile strength and static modulus of elasticity. The results of slump flow, passing ability, and viscosity indicate that it satisfied the requirement of the Specification and Guidelines for Self-Compacting Concrete. Maximum compressive strength of 79.73 MPa was obtained by 25% PC; 65% FA; 10% SF at 28 days and the cement content of this mixture is only 146.88 kg/m3. The splitting tensile strength of SCHPC at the age of 28 days increases from 4.84 to 5.86 MPa, while the static modulus of elasticity values ranges between 64,685 MPa to 86,676 MPa. The results indicated that there is a synergy between FA and SF that can provide good alternative to produce SCHPC with smaller cement content and improved mechanical properties.
KeywordsSelf-compacting high-performance concrete Fly ash Silica fume Compressive strength Splitting tensile strength Static modulus of elasticity
In recent years the utilization of industrial by-products is gaining momentum with increasing environmental awareness and its likely hazardous effects . The addition of waste materials into construction product for roads, buildings and different infrastructure is amongst the commonly proposed beneficial applications. Besides decreasing the degree of waste disposed in landfills, this act reduces the utilization of naturally mined materials, thus, reducing the footprint and impact of the construction industry on the natural environment. Portland concrete (PC) is the most commonly used material for construction of built infrastructure worldwide. The production of PC consumes energy and leads to release of carbon dioxide; PC is liable for approximately 5% of global carbon dioxide (CO2) emissions, with a worldwide mean of 222 kg of carbon emitted per ton of PC manufactured [13, 33, 41, 45].
Self-compacting concrete (SCC) is a viscous mixture suitable for casting intricate structures and structures with congested reinforcement, with slight or without vibration, while maintaining a consistent flow free from segregation and bleeding [40, 42]. Self-compacting concrete was initiated by Okomora (1986), and the model of SCC was developed in 1988 at Tokyo University. The objective was to produce a durable concrete structure by improving quality in the construction industry . High-performance concrete (HPC) is a durable and abrasion-resistant concrete, which is made with low water-to-binder ratio (w/b) and cured adequately . The HPC is characterized by its fluidity, high strength, and durability. Both SCC and HPC are consist of similar component which when proportioned adequately can yield a desired class of concrete . Therefore, SCHPC is a new generation of concrete base on the concepts of SCC and HPC. It possessed adequate self-compact ability (i.e. filling ability, passing ability, and segregation resistance) of the SCC and the high strength and good durability of the HPC .
The SCHPC provided a beneficial working environment since it compacts without vibration, thus, leading to speedy construction and savings. However, due to its high cementitious content and chemical admixtures, the cost of production is high. Nevertheless, savings in labour cost and the use of mineral admixtures such as FA and SF might offset the increased cost. Moreover, the synergy of FA and SF in a ternary blend improved concrete properties and made SCHPC more economically and ecologically beneficial . The application of FA in concrete has gained momentum due to its high silica content which reacts with calcium hydroxide which is produced during hydration leading to pozzolonic action. This improves the concrete strength with time. FA application in high quantities might have an effect on the early strength of concrete but it can achieve similar strength at later age. Nevertheless the incorporation of SF enhances pozzolanic activity and early strength .
Fly ash is derived from burning pulverized coal in an electric generating power plant. Whiles SF is derived from smelting process in silicon and ferrosilicon industry [2, 20, 24]. The FA is grouped into two classes: (1) Class C; and (2) Class F. Class F fly ash is associated with higher pozzolanic properties with little or no cementitious properties. It is often used in the production of high-performance concrete. On the other hand, the Class C fly ash contained both cementitious and pozzolanic properties. However, the cementitious property of Class C FA is limited, unless the average particle size is equal to or less than that of cement . For example, Malaysia utilizes coal-fired power plants for generating electricity. One of the four coal power plants in Malaysia known as Tanjung Bin Power Plant produces 42,000 metric tons of FA per month from burning of coal ash . It is expected that the production of FA will increase as the coal-driven electricity generation is projected to increase by 2020.
Class F FA usually offer good resistance to sulphate attack and alkali-silica reaction (ASR). Most FA, tend to lower the water demand of concrete and increase its resistance to fluid flow and the ionic diffusion. The beneficial effects of FA on permeability and diffusivity become apparent with time, particularly in the case of the more slowly reacting class F FA. For instance, Thomas and Shehata  observed that, at early ages of 5 years, concrete containing 8% SF provides increased resistance to chlorides compared to concrete with 25% FA due to its lower initial diffusion coefficient. Even so, at later ages variations between the chloride profiles for the two concretes become smaller because of the greater reductions in diffusivity with time for FA compared with SF. The concrete with the ternary mix has the advantage of a low diffusivity at early age and comparatibily large decay in diffusivity with time, which combine to improve the resistance to chloride penetration compared with concrete containing either FA or SF. However, drying-rewetting record of concrete before freezing and thawing has a significant effect on typical concrete, whereas silica fume concrete is comparatively unaffected. Khan and Siddique  in their study compared the frost resistance of Portland cement concrete to that of silica fume concrete. Results indicated that Portland cement concrete failed after 58 cycles, whereas silica fume concretes had durability factor in excess of 90% after 300 cycles. Concretes with 20%, 15% and 10% SF had durability factor of 92.8, 93.4, and 97.7, respectively. This performance of SF concrete was consequent of low degree of saturation because of self-desiccation during hydration. Likewise Cwirzen and Penttala  observed that Surface Scaling decreased with increasing SF amount and decreasing w/b. They concluded that moderate additions of SF seemed to densify the microstructure of the interfacial transition zone. Blending of plain concrete with 10–20% SF significantly improved the corrosion resistance. Moreover, Dotto et al.  also concludes that the addition of SF can be effectively used in protecting steel reinforcement against corrosion. In additition Lee et al.  investigates the effectiveness of SF in controlling the damage arising from sulfate attack. They observed that the incorporation of 10% SF in OPC concrete indicates no evidence of spalling and cracking up to about 1 year of exposure. Moreover, In concrete mixtures containing FA the depth of carbonation was slightly higher compared to control concrete. However the depth of carbonation was lower in concrete containg FA and SF, this is due to the fact that SF had little effect on carbonation .
Very few studies reported the static modulus of elasticity and splitting tensile strength of SCHPC with Class F fly ash and 10% silica fume. For instance, Safiuddin and Zain  investigated the individual and combine influence of SF and FA on fresh and hardened properties of high-performance concrete. Results indicated that SF improves the compressive strength, pulse velocity, dynamic modulus of elasticity. The result further revealed that it produced concrete with lowest levels of initial surface absorption and drying rate. While, on the contrary the properties of high-performance concrete do not improve significantly when FA alone was used, the combination of fly ash and silica fume improves the properties of high-performance concrete. Similarly, Askari et al.  studied mechanical properties of SCC containing FA and SF, fresh properties, compressive strength and splitting tensile strength of SCC. Results suggested that high volume FA content leads to compressive strength gain between 28 and 120 days of curing. This finding has confirmed continuity of pozzolanic activity of FA with time, however the tensile strength decreases with increase in FA. Nevertheless a 10% SF replacement of cement improves and maintained the tensile strength of SCC containing high volume FA. Furthermore, high-volume of FA content in SCC lowers the dosage of Superplastisizer (SP) required to achieved self-compact ability owing to the round shape of FA particles. Moreover, the unit weight of SCC mixtures decreases due to the low specific gravity of FA and SF compared to that of Portland cement.
In the same vein, Wongkeo et al.  explored the influence of adding 50, 60 and 70 wt% high calcium class C fly ash and SF as replacement of Portland cement on compressive strength and chloride resistance of SCC. It was drawn that FA and SF enhance the chloride resistance of SCC at high volume replacement of Portland cement. Likewise, Yazici  Studied the effect of replacing cement with class C FA at replacement levels of 30 to 60% on fresh properties, compressive strength, splitting tensile strength, modulus of elasticity and durability properties of SCC, with 10% addition of silica fume and a constant W/b ratio of 0.28. Results indicated that the addition of SF improves the tensile strength of SCC at all FA replacement levels. Moreover, an increase in FA up to 50% replacement level does not considerably influenced the modulus of elasticity of SCC. On the other hand, Samhitha et al.  developed high strength high volume FA concrete of grade M70 with the addition of 10% SF and lime, durability properties were explored. Results indicated that the water absorption and porosity of high strength high volume FA concrete reduced by 85% and 34% respectively.
The compact ability of SCHPC is influenced by the properties of the constituent materials, therefore, when a high volume of cement is replaced, the strength development of the concrete must be investigated. Hence, this study aims to examine the influence of partial replacement of Portland cement by 25%, 40%, 50%, and 65% class F fly ash and 10% silica fume on the mechanical properties of SCHPC. It is against this background that the current study seeks to assess the mechanical properties of self-compacting high-performance concrete with FA and SF.
2 Experimental program
An ordinary Portland Cement of CEM 1 with 42.5 MPa strength and a specific gravity of 3.15 was used to produce the SCHPC mixtures. To achieve the desired workability and consistency of SCHPC and reduce the possibility of segregation and heat of hydration, low calcium FA equivalent to ASTM C618-12a  Class F with a specific gravity of 2.1, obtained from a power plant at Johor Bahru was added in different percentage. Densified SF with bulk density in the range of 550–650 kg/m3 and a specific gravity of 2.1–2.4 was added to compensate for the relatively low early strength of FA concrete. Ten percent (10%) SF was added as suggested by . Replacement of cement by 10% SF improveds and maintained the tensile strength of SCC containing high volume FA . Sadr Momtazi et al.  also observes that the paste containing 10% SF had uniform microstructure and compactness in cement matrix. In addition, moderate additions of SF seemed to densify the microstructure of the interfacial transition zone. Likewise, 10% SF addition improves frost resistance of concrete with durability factor of 97.7 . The incorporation of 10% SF in concrete indicates no evidence of spalling and cracking up to about 1 year of exposure to sulfate attack . Likewise, blending of plain concrete with 10% SF significantly improved the corrosion resistance .
Chemical composition of Portland cement and SCMs
2.2 Mix proportions and specimen preparation
Mixture proportion of SCHPC
W/B = 0.31
The recommended dosage of superplasticizer (SP) is 1-3% by weight of cement/powder. The unit of all constituent materials were in kg/m3.
Concrete was mixed using a rotary mixer of volume 0.03 m3. The mixing sequence is typical for powder type SCHPC and was based on the procedure established by Khayat et al. . After mixing, slump flow, L-box, and V-Funnel tests were conducted on the fresh concrete as recommended by EFNARC , to explore the features of the rheological properties of SCMs. Concrete cubes of size 100 × 100 mm were prepared for measuring compressive strength. The specimen was cured in water at a temperature of (20 ± 5) °C for 7, 28 and 56 days prior to testing.
3 Test methods
3.1 Test on fresh properties
The slump flow test was performed according to European Standard BS EN 12350-2 . The workability, consistency and filling ability of SCHPC was assessed through this test. The recommended spread diameter of the fresh concrete sample ranges from 550 to 850 mm EFNARC . The L-Box test was used to measure the passing ability of SCHPC to flow through tight openings including spaces between reinforcing bars and other obstructions without segregation or blocking . The viscosity and filling ability of SCHPC was assess using a V-funnel apparatus .
3.2 Compressive strength test
3.3 Splitting tensile test
3.4 Static modulus of elasticity test
4 Results and discussions
4.1 Fresh properties
Fresh properties of concrete mixes
Requirement limit by EFNARC 
Slump flow (mm)
L-box (H2/H1) ratio
The L-box passing ratio for the control mix is 0.76 which is approximately 0.8. The passing ratio is between 0.84 to 0.88 for all other mixes. The range of values for L-box passing ratio recommended by EPG-SCC  for passing ratio class PA2 is between 0.8 to 1.0. Therefore, all the SCHPC mixes satisfied the requirement for the passing ability class PA2. Moreover, the addition of FA into SCHPC improves the passing ability. This finding concurs with Nagaratnam et al. . The V-funnel flow time (Tv) of the fresh mixes varied between 10 to 12 s. Thus, the Tv for individual SCHPC lies within the range of 9 to 15 s as established by EPG-SCC  for VF2 viscosity class. The incorporation of SF increased the viscosity of the concrete. This finding also concurs with . Table 3 summarized the fresh properties of SCHPC.
4.2 Compressive strength
In addition, results showed that the rapid and highest strength development occurred between the ages of 7 and 28 days as indicated by Figs. 5 and 6. Also, the compressive strength values at the age of 28 days varied between 51.11 and 79.73 MPa. Maximum compressive strength of 79.73 MPa was obtained by 25%PC; 65%FA; 10%SF at 28 days and the cement content of this mixture is only 146.88 kg/m3. Furthermore, the 7-days compressive strength of SCHPC with 50%FA;10%SF and 65%FA;10%SF is greater than the 7-days compressive strength of control specimen. Owing to the pozzolanic reaction and the finer particles of SF, it leads to the densification of the microstructure, the enhanced bond strength between the paste and aggregate and increase the compressive strength of SCHPC. Silica fume aids in dispersing flocculated cement grains in fresh concrete mixtures, thus, increasing the free water available for hydration .
The compressive strength of SCHPC at different curing ages of 7 days to 56 days is presented in Fig. 6. It is noticed that SF increases the early compressive strength of Portland–fly ash–silica fume concrete. However, the increase tends to decline in the long-term. The strength development within the cement composites with SF was principally due to the pore size refinement and matrix compaction, reduction of Ca(OH)2 content, and robust cement paste–aggregate surface bond. The formation of dense C–S–H gel and additional homogenised product at the surface zone results in the speedy strength development of concrete at the early ages. This has been primarily as result of the nucleation of CH crystals around SF particles in the mixture .
It has been noticed that the strength improvement of concrete with SF is fast during the early ages due to the pozzolanic action of SF. In concrete with lower w/b ratios, a large amount of unhydrated cementitious material was retained, even after a longer duration. At the early ages, with lower w/b ratios, the nonevaporable water content was higher with SF concrete than with plain concrete. However, the nonevaporable water content increased at higher w/b ratios and the development of strength continued even at the later ages. Thus, the pozzolanic reactions of the SF continued even in the later ages with the availability of adequate nonevaporable water content in the concrete. The nonavailability of nonevaporable water contents in concrete with lower w/b ratios led to the strength deterioration in concrete with SF .
4.3 Splitting tensile strength
4.4 Static modulus of elasticity
Static modulus of elasticity of SCHPC mixes
Static modulus of elasticity (MPa)
100%PC; 0%FA and 0%SF
50%PC; 40%FA and 10%SF
40%PC; 50%FA and 10%SF
25%PC; 65%FA and 10%SF
The stiffness of SCHPC increases as the compressive strength increases. This finding concur with . Moreover, the control specimen exhibits the highest modulus of elasticity of 86,676 MPa at 28 days. Likewise, the rate of increase of compressive strength is higher than that of elastic modulus which is related to high rate of hydration of concrete containing silica fume .
On the other hand, a certain level of cement replacement with pozzolans is not only advantageous in terms of cost, but also leads to energy efficiency, ecological and environmental benefits as well as durability properties. Nowadays, researchers have focused on the possibility of practical use of pozzolans as a partial replacement of cement in pursuit of improved long-term strength and durability properties .
The slump flow of the respective SCHPC lies within class SF1 of EFNARC which is indicative of good filling ability.
All the SCHPC mixes satisfy the requirement for the passing ability class PA2 and the V-funnel flow time for all respective SCHPC lies within viscosity class VF2;
The compressive strength of SCHPC containing 25% FA with 0% SF is 35% lower than that of the Portland cement control, this is due to the slow pozzolanic reaction and dilution effect of FA; Nevertheless, the addition of SF improved the compressive strength of Portland–fly ash–silica fume concrete;
The compressive strength values at the age of 28 days were in the range of 51.11 to 79.73 MPa. Maximum compressive strength of 79.73 MPa was obtained by 25% PC; 65% FA; 10% SF and the cement content of this mixture is only 146.88 kg/m3;
The splitting tensile strength of SCHPC at the age of 28 days increases from 4.84 to 5.86 MPa. The addition of FA and SF in the concrete mixes had a progressive response on the splitting tensile strength of concrete. The incorporation of 10% SF improves the splitting tensile strength.
The static modulus of elasticity values ranges between 64,685 MPa to 86,676 MPa and the control specimen exhibits the highest modulus of elasticity of 86,676 MPa.
Incorporating of class F FA and SF in concrete mixtures leads to economic and environmental benefits as well as reducing the heat of hydration of concrete.
The authors are grateful to the Ministry of Education (MOE), Universiti Teknologi Malaysia and Research Grant Reference No: PY/2017/01513.
Compliance with ethical standards
Conflict of interest
The authors pronounced that there is no conflict of interest associated with this article.
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