Effect of polypropylene fibres on strength and durability performance of M-sand self compacting concrete

Abstract

The durability of the self compacting concrete is one of the prominent areas of concern in recent years since it has become prevalent due to its ease of handling in the construction industries. However, there is a lack of detailed durability study of M-sand self compacting concrete with polypropylene fibres in the literature. In this paper, self compacting concrete made by Portland pozzolana cement with natural sand and M-sand, different percentages of volume fraction (0%, 0.1%, 0.15% and 0.2%) polypropylene fibres with M-sand is thoroughly investigated for fresh properties, hardened properties, drying shrinkage, water absorption, permeability, acid resistance and corrosion crack initiation time. Further, SEM analysis was carried out to know the effect of polypropylene fibre on the concrete microstructure. The results confirmed that the use of M-sand has improved the fresh, hardened and durability properties of the self compacted concrete than natural sand. Also, it is observed that the addition of polypropylene fibres has reduced the fresh properties of self compacted concrete. However, M-sand with 0.15% polypropylene fibres has slightly improved the strength of self compacting concrete, compression strength by 6.97%, split tensile by 8.68% and flexure strength by 3%. Further, it has reduced the drying shrinkage by 40%, water absorption by 30%, water penetration by 33.33%, and increased the chemical resistance, surface crack initiation period than the natural sand self compacting concrete. Finally, this study concludes that M-sand with 0.15% polypropylene fibres improves the durability of self compacting concrete by reducing effect from aggressive environments and in turn contributes to increased corrosion resistance of reinforcements embedded in concrete. Thus, the study supports the use of self compacting concrete with M-sand and optimum amount of polypropylene fibres to achieve required sustainability in terms of strength and durability.

Keywords:

• compression strength
• durability
• flexural strength
• polypropylene fibre
• self compacting concrete

1. Introduction

Self compacting concrete (SCC) use in the construction industry is forefront these days due to its properties and ease of handling (Prakash, Raman, et al., 2021). The use of SCC can save the costs and manpower requirements for the work. It has gained popularity due to its advantages in recent days. SCC was first developed in Japan. Later, a detailed study on the SCC properties was conducted by Okamura and Ozawa (Okamura & Ozawa, 1996; Ouchi et al., 1996). SCC consists of a high amount of powder content compared to normal concrete, which helps the concrete to flow in its fresh state, increasing the cohesiveness and inter particle link. Also, it maintains uniformity while moulding the concrete and can pass through congested reinforcements. The key focuses of the new SCC standards are the capacity for filling, the capacity for passing, and the resistance to segregation. SCC ability is measured by a series of tests using distinguished apparatus in the fresh state while finalising the mix design by following the guidelines in EFNARC 2005 (EFNARC, 2005).

In fact, the concrete is weak in tension and its tensile strength is significantly lesser than the compression strength (almost 10% of the compression strength). Therefore, the crack propagation is rapid under tension. Also, the early age shrinkage of the concrete builds the tension in concrete and increases the possibility of cracks which affect the durability of concrete (Mac et al., 2021; Meyer & Combrinck, 2021). Hence, crack formation on the concrete depends on the loading and environmental effects like temperature changes and properties of raw materials (Zhou et al., 2022; Brue et al., 2017; Nasir et al., 2017). These cracks allow the ingression of external agents such as chemicals, CO₂, sulphates, etc., which affect the concrete durability (Prakash, Thenmozhi, et al., 2021). Sulphate attack damages the concrete by changing the pH value of the concrete. Sulphate contents may present in raw materials of concrete like water, aggregates admixtures or the surrounding soils with which concrete may come in contact. Sulphate attack reduces the cohesion and resisting capacity and increases the cracking percentage in concrete (Mehta, 1983).

Due to the more powder contents, SCC becomes more brittle than normal conventional concrete. This particular drawback of SCC could be overcome by the use of fibres in it. During the last decades, various types of fibres have been investigated in concrete to solve this problem (Das et al., 2020; Ahmadi et al., 2021; Monazami & Gupta, 2021; Blazy & Blazy, 2021; W. Wang et al., 2022). The polypropylene fibres(PPF) addition comparatively reduces the expansion in volume of the concrete, but when the concrete deterioration increases effectiveness of fibre also decreases (Behfarnia & Farshadfar, 2013; Lakshmi et al., 2022; Prakash et al., 2020). Fibre in SCC benefits in addition to the properties of SCC even though fibre addition affects the workability properties such as filling, passing and flowing ability of SCC (Y. Liu et al., 2021; Mastali & Dalvand, 2017). But the use of optimum fibre content in SCC reduces the cracks in the plain concrete with good workability. Cracking of the concrete reduces durability and serviceability. Optimum percentage usage of fibres in the SCC enhanced the ductility and flexural strength of concrete since they allow the effective transfer of stresses within the matrix. The use of fibres also increased stiffness of SCC (Rajesh Kumar et al., 2021). The minimum shear reinforcement can be substituted using PPF (Conforti et al., 2017), avoiding crack formation (Gokulnath et al., 2020). Many studies have been proved that the addition of PPF decreases the early age cracking due to shrinkage of concrete (Imperatore et al., 2022), adhesion of PPF with cement paste increases the capacity of stress transfer and able to bridge and reduce the growth of crack (Flores Medina et al., 2015). PPF concrete increases the toughness of concrete more than plain concrete (Guo et al., 2021). The use of PPF and fly ash reduced the unit weight of concrete, freeze and thaw resistance with the use of PPF (Karahan & Atiş, 2011).

Notable efforts have been put forth to understand the properties of concrete with varying fibres concentrations. G.M. Sadiqul Islam and S. D. Gupta (Sadiqul Islam & Das Gupta, 2016) have studied the strength, permeability and plastic shrinkage of concrete with addition of PPF (0, 0.1%, 0.15%, 0.2%, 0.25% and 0.3% by volume). They found that after 0.1% fibre content concrete strength and permeability were reduced, the plastic shrinkage and concrete crack widths significantly reduced up to 0.3% PPF content. Arash Karimipour et al. (Karimipour et al., 2019) have studied the mechanical properties of SCC with PPF of length 12 mm using 0%, 0.1% and 0.3% by volume of concrete and found that 0.1% PPF addition increased the compression strength. Maciej Szeląg (Szeląg, 2018) studied the PPF of length 6 mm in cement paste by 0.5% of cement mass with three different w/c ratio (0.4, 0.5 and 0.6) and it is intuitive that, use of PPF did not affect the chemical composition of hardened cement matrix and PPF in cement matrix melt at temperature 150ºC. Jing jun Li et al. (Li et al., 2016) have investigated the high performance PPF (0%, 0.53%, 0.74%, 0.95%, 1.16% and 1.37% by volume of concrete) on mechanical properties of lightweight aggregate concrete, found that 0.95% is the optimum value of PPF which increases the mechanical properties of considered concrete and beyond this value strength properties decreased due to the poor dispersion of fibre with increased fibre volume. M Hatami Jorbat et al. (Hatami Jorbat et al., 2020) have done a comparison study of the concrete specimens with (0.2, 0.35 and 0.5 % volume) and without PPF for crack propagation and fracture toughness. They have concluded that fibre content of 0.35% volume fraction of concrete is the optimum value which improved the mechanical properties, more fracture toughness than the concrete specimens without fibres. M Usman Rashid (Usman Rashid, 2020) has done a durability study on steel and PPF fibre reinforced concrete under natural weathering action, found that PPF fibres negate the disadvantages of steel fibres and positively affect the durability of the concrete. Yuan and Y.Jia (Yuan & Jia, 2021) have experimentally studied the effect of glass fibre and PPF for 0.30 and 0.35 water binder ratio and 0.45%, 0.9% and 1.35% volume fraction of fibre content. They concluded that optimum fibre content was affected by water binder ratio, 0.45% addition of PPF reduced the water absorption by 10.6% and water absorption increased with fibre dosage. The higher percentage of PPF in concrete increases the water absorption percentage, porosity of concrete and correspondingly they affect the durability of concrete. Also, some studies showed that less percentage of micro synthetic fibres decreased the porosity in concrete. Among all fibres, the use of PPF improved the cracking resistance of concrete. Similarly, there is an increase in the flexural and tensile strengths of concrete when PPF is incorporated at a certain percentage (Ramezanianpour et al., 2013). As the PPF volume fraction increased, the chloride permeability and water penetration of concrete also improved.

Concrete durability deficiency increases the possibilities of steel corrosion in concrete due to the penetration of external agents such as chloride and sulphate (Rossi et al., 2020). The carbon dioxide, chloride and sulphate ions penetration alter the concrete’s pH value and increases the corrosion possibilities. The corrosion process develops expansive pressure, which leads to the microcracks formation on concrete around the steel bar (Šavija et al., 2015). This crack growth increases with time, reaches the surface and deteriorates by breaking the concrete and steel bond (L. Wang et al., 2017). Corrosion of the embedded steel has become the major cause of failures due to the deterioration of concrete in reinforced structures and prestressed structures exposed to the aggressive environment. Thus, failure due to corrosion depends mainly on the concrete quality. Reduction in the water absorption, permeability, shrinkage cracks in concrete and increase in density of the concrete can help to delay corrosion initiation and increase the durability and serviceability of concrete structure (Bhagwat et al., 2022). Water absorption of the concrete is the parameter considered to understand the durability of concrete; it is a measure of the resistance of concrete against the aggressive environment. When Portland cement was replaced with fly ash by 20%,water absorption of the concrete was reduced effectively, and reduction is increased when fly ash replacement is more than 20% (Leung et al., 2016).

The constant effort of reducing the cement contents led to producing the blended cements using industrial by products such as fly ash and ground granulated blast furnace slag, and so on (Monteiro et al., 2017). Among other blended cements, fly ash blended cement, that is, Portland Pozzolana Cement is in use due to its cost effectiveness, significant contribution to targeted strength and durability (Golewski, 2022; Sangoju et al., 2011; Ginting, 2022; Kwon et al., 2017). The concrete’s ability to have chloride ion penetration is one of the main parameters for predicting the service life of concrete structures. Studies show that the use of fly ash reduced the chloride ion penetration and coulomb charge (Sangoju et al., 2015). This might be because of the spherical shape of fly ash particles, which improves the particle packing density of the concrete (Kumar Gangaram Singh & Kizhakkumodom Venkatanarayanan, 2020; Siddique, 2011).

The use of SCC is an effective solution to increase the concrete’s durability, since it has a dense microstructure than normal concrete (Ahmed et al., 2021). Use of mineral admixtures in SCC also further improves the quality of concrete microstructure and interfacial transition zones by reducing the pores formation (Afroughsabet et al., 2017; Leung et al., 2016; F. Liu et al., 2019), which in turn increases the durability of concrete. The natural sand deficiency inclined toward the use of M-sand as a substitute for fine aggregates. Also, from the literature studies (Mardani-Aghabaglou et al., 2018; Ramezanianpour et al., 2013; Yuan & Jia, 2021; Zhang & Li, 2013), it is evident that the use of a lower percentage volume of PPF positively contributes to the durability of concrete. In addition, the effect of PPF by considering 0.1%, 0.15% and 0.2% volume of concrete is not thoroughly investigated. Additionally, there are very few studies available on the durability of SCC using M-sand and PPF. To fill this gap, an experimental investigation is conducted to understand the mechanical and durability performance of SCC with natural sand and M-sand. Also, the optimum value of fibre content to improve the durability of SCC without negatively affecting the mechanical properties of SCC with M-sand and PPF is obtained in this study to improve the concrete performance. The main contributions of this study are mentioned as follows.

• Strength and durability properties of M-sand SCC show better performance over natural sand SCC.

• The 0.15% volume fraction of PPF has a positive effect on durability and strength performance of M-sand SCC.

• The linear regression based equation has been developed to estimate flow time in V-funnel using T₅₀₀ test results for PPF reinforced M-sand SCC.

The abbreviations used in the paper are listed in the Table 1.

Table 1. The abbreviations used in the paper

Abbreviation	Description
EFNARC	European Federation of National Associations Representing for Concrete
M-sand	Manufactured Sand
PPF	Polypropylene Fibre
PPC	Portland Pozzolana Cement
SCC	Self Compacting Concrete
SEM	Scanning Electron Microscopy
SCC_N	SCC mix with Natural sand
SCC_M	SCC mix with M-sand and No PPF
SCC_M_0.1F	SCC mix with M-sand and .1% PPF
SCC_M_0.15F	SCC mix with M-sand and .15% PPF
SCC_M_0.2F	SCC mix with M-sand and .2% PPF
UTM	Universal Testing Machine

2. Materials and methodology

The Portland Pozzolana Cement (PPC), two types of fine aggregates, coarse aggregate, polypropylene fibres (PPF), potable water and superplasticiser are used in the concrete mix. The PPC cement with 32% fly ash content confirming to IS 1489 (BIS:1489Part 1, 1991) with specific gravity of 2.92 and having a fineness of 4% (i.e. % weight retained on a 90 µm IS sieve) is used as a binder in SCC. The initial and final setting time of PPC after testing was found to be 41 minutes and 366 minutes, respectively.

The angular aggregate of crushed granite stone of size 4.75 to 12.5 mm is used as coarse aggregate. The size distribution of considered coarse aggregate from the sieve analysis in terms of percentage passing are 96.8, 67.09 and 5.45 for sieve sizes of 12.5 mm, 10 mm and 4.75 mm, respectively. River sand and crushed granite stone sand, that is, M-sand are used as fine aggregates in the study. The fine aggregate and coarse aggregates considered are free from all deleterious materials. The properties of the aggregate considered are tabulated in Table 2. The grain size distribution of the fine aggregates considered is shown in Figure 1. Aggregates are tested as per IS 383–1970 (IS:383, 1970).

Figure 1. Sieve analysis results of fine aggregates.

Table 2. The properties of aggregates considered in the study

Properties	Coarse Aggregate	Natural Sand	M-Sand
Specific Gravity	2.66	2.626	2.73
Fineness Modulus	6.3	2.56	2.6
Water Absorption	0.6%	0.8%	1.1%
Moisture Content	0.3%	2.35%	1%

To reduce the water content, polycarboxylate ether based superplasticiser is used as the high range water reducing agent. The admixture selected in this study acted as a water reducing agent and viscosity modifying agent, since it has both properties. As per the specification, the specific gravity of the considered admixture is 1.09. After many trials, the dosage selected for the present study is 0.7% of the binding material content. With this, the PPF used in this work has a diameter of 24 microns, length of 12 mm and a specific gravity of 0.9. Figure 2 shows the polypropylene fibre, and the properties of PPF are listed in Table 3. As mentioned in IS 456: 2000 (IS 456, 2000), potable water was used in this study.

Figure 2. Image of the polypropylene fibres.

Table 3. The properties of polypropylene fibres are considered in the study

Properties	Values
Shape	Triangular
Diameter	24 microns
Specific Gravity	0.9
Melting point	160–165ºC
Tensile Strength	320–490 MPa
Elongation	60–90%
Young’s Modulus	>4000 MPa
Alkaline Stability	Very good

The mix design for SCC is obtained by following EFNARC guidelines (EFNARC, 2005). Many trials were conducted to obtain the optimum mix design, checking for its fresh properties. Once it satisfied the requirements, the casting of the mix was done and tested for strength after curing. Once targeted strength was achieved, the mix was finalised and all the required specimens for the study was cast.

Proper mixing and curing are fundamental aspects of getting good quality SCC. Since SCC has more paste content, it requires intensive and thorough mixing. The mixes were prepared using pan mixer as studies suggest this gives a good mix of SCC (Hemalatha et al., 2015). Initially dry mix of cement, sand and coarse aggregate was done to get an adequate mix. The water was added in three steps to get good workability. With an optimum dosage of admixture, 50% of the water was mixed and kept. Initially 30% of water was added gradually to the dry mix and mixed thoroughly, later the water with admixture was added slowly and mixed for the next 2 minutes. The remaining 20% water is added gradually, looking at the consistency of the mix. After many trials, this particular method of adding water and admixture was found to give satisfying results for the used admixture in the study. Immediately fresh properties of concrete were checked with the part of the mix and the casting was done. For the fibre SCC, the PPF was made wet and added to the concrete in the dry mix phase to prevent lump formation and to uniformly distribute the fibres in the concrete. In this study, the SCC mix was made using natural sand for reference. Mix proportion for the reference mix is tabulated in Table 4. Other SCC mixes M-sand with PPF of 0%, 0.1%, 0.15% and 0.2% of volume fraction of concrete were adopted for the study. The samples are demolded after 24hrs of casting and kept for curing in the water pound at room temperature for 28 days. After the curing period of concrete, tested for its hardened properties and durability.

Table 4. The details of reference mix are considered in the study

Mix contents	Mix details
PPC content (kg/m^3)	650
Coarse Aggregate (kg/m^3)	738.09
Fine Aggregate (kg/m^3)	741
Water (kg/m^3)	201.5
Admixture dosage (%)	0.7
Polypropylene Fibres (%)	0

In this paper, mix with natural sand referred as SCC_N, mix with M-sand as SCC_M. The mixes with M-sand and polypropylene fibres with 0.1%, 0.15% and 0.2% of volume fraction of concrete are named as SCC_M_0.1F, SCC_M_0.15F and SCC_M_0.2F, respectively.

2.1. Fresh properties

Immediately after mixing, the fresh properties of the SCC were studied by conducting slump flow, T500, L box, V funnel tests to know the flowability, filling and passing ability, respectively. All the experiment to study fresh properties were conducted as per EFNARC (2005) guidelines (EFNARC, 2005), the test images are shown in Figure 3.

Figure 3. Tests conducted to determine the fresh properties of concrete.

2.2. Harden properties and durability tests

The hardened properties of SCC mixes were studied as per IS 516–1999 (IS 516, 1999), six specimens are considered for each test from every mix. The average values of the results are represented in terms of graph and tabulation. Compression strength was obtained at 7 and 28 days as per IS 516–1999 (IS 516, 1999) on cubes of size 100 mm. A load of 2.5 kN/s was applied gradually, and the ultimate load was noted. The modulus of elasticity of the concrete was obtained at the age of 28 days. The load was gradually applied to the cylindrical specimen using universal testing machine (UTM) of capacity 1200kN, and the corresponding change in length was noted using dial gauges fixed along the cylinder length. Following the procedure mentioned in IS 516 the modulus of elasticity was calculated. All specimens were tested in saturated surface dry conditions. The cylinder samples of size 150 mm diameter and 300 mm height were used for the split tensile test satisfying to IS:516–1999 (IS 516, 1999) and tested for 28 days. The load was applied gradually at the rate of 1.5 kN/s. The concrete prism of span 500 mm with cross section 100 × 100 mm was used for flexural strength test. The 4-point bending test was conducted on the 28th day after proper water curing. The load was applied at a distance of 1/3^rdof the effective span of the beam from the support till the failure of the beam. The effective span was considered as 400 mm. The center deflection corresponding to the load was measured by a reading dial gauge which was fixed at the center of the span.

To understand the PPF effect on drying shrinkage of SCC a drying shrinkage test was conducted as per IS 1199 (IS1199, 2012). The concrete specimen of size 75 × 75 × 300 mm was cast from all the considered mixes, after 24hrs specimens were demoulded and kept for curing. The drying shrinkage was measured using a length comparator after 56 days of curing. Initially, the reading of the specimen was noted on three days of curing for reference.

The water absorption and Permeability tests are conducted to study the water penetration resistance and voids content in the cube. For the water absorption test, 100 mm cubes were used. The tests were conducted after 28 days of the curing period. The dry weight of the samples was noted after drying in the oven for 24hrs at a temperature of 70ºC, after keeping in water for 24 hrs wet weight was noted. Later percentage of water absorption was calculated. Permeability is the capacity of concrete to permit water to pass through it. Permeability test was conducted according to BS-EN12390-8, and 0.5N/mm² pressure was applied on the surface of a 150 mm cube for 72 hrs. After the test, the specimen was broken into halves along the water pressure applied direction, and the water penetration depth was measured. The water permeability test setup is shown in Figure 4.

Figure 4. Permeability test on concrete cube.

Chloride and Acid resistance was studied on a 100 mm cube. The cubes are kept in 5% Sodium Chloride solution and 2% concentration sulphuric acid solution for curing for the durability study. The initial weights of the cubes were noted before dipping into the solutions, and 56 days the samples were taken out, and final weight was noted. The durability study was done by considering the mass loss of the sample.

2.3. Corrosion monitoring by AC impressed current method

The accelerated corrosion method has been used since then to study the corrosion of rebar in concrete. To monitor the corrosion crack initiation time, 150 mm cubes were cast by keeping the steel bar at the center, leaving a 30 mm cover from the bottom of the cube. After 28 days of curing period, accelerated corrosion test was conducted adopting the impressed current technique. The specimen was kept in a 5% NaCl solution such that only 3/4th of the concrete should be dipped in the solution. Then connected to DC power supply keeping embedded steel bar of 12 mm diameter as anode and copper rod as cathode, 0.04 Amp current was passed constantly and maintained throughout the test. The current was applied till the appearance of a crack on the surface of the cube. The time taken for crack initiation was noted. The time taken for the appearance of the first crack was considered as a measure of the specimen’s relative resistance against corrosion of reinforcement and chloride permeability. Figure 5 represents the accelerated corrosion test setup.

Figure 5. Accelerated corrosion test.

SEM study was conducted to observe the microstructure and to understand the interface zone between the PPF, cement paste and aggregate. The concrete specimen was taken for SEM (scanning electron microscopy) test after 90 days of curing, the sample of size 6 × 6 × 3 mm was collected. The specimen with aggregates and fibres are used for study with proper care. Collected specimen is cleaned and dried at a temperature of 60–70ºC, golden coating was done by using a vacuum coating machine.

3. Results and discussion

SCC has a very dense microstructure since it has more powder content and less coarse aggregate. The compression strength of the SCC depends on the workability of the concrete. The low workability of the concrete increases the porosity in the concrete. Figure 6a shows the slump flow test results. The concrete mix SCC_M showed 2.9% more slump flow diameter than SCC_N. As the fibre content increases, SCC_M_0.1F showed 7.63%, SCC_M_0.15F showed 9.72% and SCC_M_0.2F showed an 11.25% reduction in the slump flow diameter. So, the result confirmed that concrete made of M-sand has a high flowing ability when compared with natural sand concrete for the same w/c ratio. And also, as the fibre content increased the slump flow was found to be reduced. L-box results, that is, H2/H1 values proved the inverse proportion with the fibres content. Very small variation between SCC_M and SCC_N was observed. And as the fibre content increased, H2/H1 values were found to be decreased. The L-box test results are shown in Figure 6(b).

Figure 6. Slump flow test and L-Box test results.

The T500 and V-Funnel test results are presented in Figure 7(a). It is observed that less time is taken by the concrete mix SCC_M compared to the SCC_N. The time taken in T-500 and V-Funnel tests is found to be inversely proportional to the percentage of polypropylene fibres. Less time represents greater flowability of the SCC. These results proved that M-sand improves the fresh properties of the SCC since it contains less silt content, proper gradation and also high specific gravity than natural sand. Also, the particle shape of the M-sand influences to get good flowability of SCC and to have improved fresh properties. But the use of fibres in SCC increases the surface area. The pastes in the concrete adhere to the fibres along with the aggregates, and internal friction increases between the aggregates and fibres as the paste is utilised to coat the fibre and aggregates. As the fibre content increased, the concrete becomes more viscous than the plain SCC and slump flow gets reduced. Similarly, with the flowing ability reduction in the other fresh properties of SCC.

Figure 7. Results of V-funnel, T500 tests and linear plot of the same.

Figure 7(b) represents the linear regression results of the V-funnel and T500 test of M-sand self compacted concrete. The coefficient of determination of 0.98 shows the best fit results. The linear regression is used to estimate the required coefficients of dependent variables of the predicted linear equation using one or more independent variables. It is observed from Figure 7(b) that the original V-funnel test results of 7.5, 10.31, 14, 20 are predicted as 7.2696, 10.111, 14.999, 19.35, respectively. V-funnel time can be estimated for the PPF M-sand SCC using the T500 test results using the linear regression results are also shown in Figure 7(b).

Figure 8(a) represents the compression strength of all mixes for 7 days and for 28 days. Mix SCC_M showed a good result when compared to the mix SCC_N. Since manufactured sand was well graded and free from the very fine powder content, it influenced the compression strength positively. Results show that very small increase in compression strength for the addition of 0.1% and 0.15% fibre content, but it was decreased for 0.2% fibre content. The addition of PPF does not much affect the compression strength of SCC, but a very small increase up to the optimum value of fibres addition was observed. As the fibre contents increases, it forms the cluster and leads to voids formation. The weak interfacial bonding between fibre and cement paste also leads to a decrease in the compression strength of concrete.

Figure 8. Compression strength and split tensile test results.

The modulus of elasticity of concrete is found to be more for mix SCC_M compared to SCC_N. And with PPF content, the modulus of elasticity value was found to be a little higher for the PPF content 0.1% and 0.15%, for 0.2% it is found to be decreased. So, the addition of PPF causes very slight variations to the modulus of elasticity of SCC up to 0.15% content, later modulus of elasticity inversely varied with the percentage increase of PPF.

Figure 8(b) shows the split tensile strength of the considered concrete mixes. It is seen that SCC_M shows 25.5% less strength than SCC_N. M-sand makes the concrete comparatively brittle when compared with the concrete mix made by natural sand. But the addition of fibres with m-sand concrete increased the split tensile strength considerably. The mix SCC_M_0.1F showed 28.35%, and SCC_M_0.15F showed a 36.45% improvement in split tensile strength compared with SCC_M. Later, a sudden decrease in the strength was noticed for SCC_M_0.2F, so from this variation it is clear that the split tensile strength of concrete increased with the addition of PPF up to an optimum percentage of addition and later it decreased. As the fibre content increases difficult to reduce the balling effect and uniform distribution of PPF becomes a problem, so the strength reduces.

The flexural behaviour of the SCC specimens for all the mixes considered under four-point loading are shown in Figure 9. From the graph, it is noticed that the flexural strength of specimens of SCC_M is 4.65% more than SCC_N. Also, the flexural strength was found to be increased for SCC_M_ 0.15F by 7.84% than SCC_N and 3.04% more than SCC_M. With the increase in the fibre content, flexural strength also improved slightly. But for SCC_M_ 0.2F, flexural strength was found to be reduced. It is also observed that mid span deflection is increased with the increase in the fibres percentage, so PPF addition improved the tensile capacity of concrete. In comparison, a very small increase in the flexural strength for using M-sand and PPF is observed in the present study. Flexural strength improvement in the concrete due to fibre also depends on the length of the fibre. The fibres used in this study are macro fibres of 12 mm in length, so there is a very small increase in the strength.

Figure 9. Results of flexural test on plain concrete with fibre and without fibre.

The evaporation of the moisture from the concrete leads to the drying shrinkage of concrete. The drying shrinkage is measured as the change in length of the specimen when measured in the length comparator. Figure 10 shows the change in the length of the specimen at 56 days after curing. The mix SCC_M had 17% less shrinkage than SCC_N. The use of M-sand in concrete reduces the voids content in the concrete because of its particle shape, so entrapped water is less so, correspondingly reduction in the drying shrinkage of the SCC. Also, SCC_M_0.1F showed a 20.5% reduction, SCC_M_0.15F had 27.7% and SCC_M_0.2F had a 51.8% reduction when compared with the SCC_M. Thus, it is clear that the use of PPF in SCC significantly reduces the drying shrinkage with the increase in the percentage of volume fraction of concrete. This shows that the PPC cement with M-sand and PPF can be used to reduce the drying shrinkage.

Figure 10. Comparison of Drying Shrinkage at 56^th day of all the concrete mixes considered for study.

Water absorption is one of the tests considered to understand the durability of concrete. The concrete comes in contact with the water directly or indirectly. If it is not potable water, then there is a chance of chemical attack through water absorption, and they may change the properties of the concrete. The water present in the pore undergoes a freeze and thaw effect due to the influence of temperature changes, which leads to the cracking of the concrete. These crack developments allow the external agents to penetrate inside the concrete and affect, decrease the durability. So, water absorption test was conducted on a 100 mm cube after 28 days of curing time.

Figure 11(a) shows the water absorption test results after 28 days of curing. Water absorption percentage was found to be less in SCC_M when compared with the SCC_N, but the reduction percentage is considerably less. For SCC_M_0.1F, small reduction in the water absorption percentage compared with SCC_M. But when fibre content increased to 0.15%, that is, for SCC_M_0.15F, the water absorption was found to be decreased by 1.15% than SCC_M. After 0.15% of fibre content, the water absorption percentage increased with the increase in the fibre content. Uniform spreading and thorough mixing of the fibres are required to reduce the porosity of the concrete. But with an increase in fibre content, keeping uniformity in mixing fibres becomes a difficult task and clustering of fibres may cause and make the concrete porous. No proper interaction between the fibre and cement paste makes the concrete get crack and correspondingly increases the water absorption of the concrete. Water absorption is directly proportional to the voids present. The optimum percentage of fibre content optimises the pore structure of the concrete and reduces water absorption (Blazy & Blazy, 2021). So, results show that the fibre geometrical properties finely bind the concrete mix with improved properties of SCC.

Figure 11. Water absorption and water penetration depth results.

The depth of water penetration depends on the porosity of the concrete. Figure 11(b) shows the permeability test results. They present that as the fibres content increased, water penetration depth was found to be decreased and penetration depth for SCC_M was 8.3% less than the SCC_N. Also, for the SCC_M_0.1F by 9.09%, SCC_M_0.15F by 27.27% and SCC_M_0.2F by 36.36% than SCC_M. The addition of fibres reduces the cracking percentage of concrete, it bridges the microcracks and slows down crack propagation. The more powder content in SCC and presence of fly ash makes the dense concrete and reduces the voids.

3.1. Chloride and acid resistance

The deteriorated weight due to chloride and acid attack are shown in Figure 12. The chloride test results show that the mass loss of the cubes after curing in 5% NaCl solution after 56 days is very less. The pozzolanic action of the fly ash present in PPC cement and the PPF action together reduce the porosity of SCC (Afroughsabet et al., 2017; Leung et al., 2016; F. Liu et al., 2019; Siddique, 2011; Usman Rashid, 2020). Also, more powder content in SCC gives a denser structure (Siddique, 2011), so the effect of chloride penetration was found to be less when tested on 56 days of curing.

Figure 12. Change in weight of concrete cube in Chloride and Acid resistance study.

Curing in a 2% solution of sulphuric acid showed comparatively more deterioration at 56 days of the curing period. The percentage change of weight for SCC_N is 1.72% and for SCC_M is 1.59%, so the percentage of deterioration of concrete with M-sand is found to be comparatively less. The percentage change in weight of SCC_M_0.1F is 1.48%, SCC_M_0.15F is 1.4% and for SCC_M_0.2F is 1.53%. With the increase in the PPF content, deterioration due to acid attack was found to be reduced, but for 0.15% PPF deterioration percentage was found to be increased when compared with others. So, from the above study, it is observed that the use of an optimum percentage of PPF can increase the durability of SCC by reducing the deterioration from chloride and acid attacks.

3.2. Corrosion monitoring by AC impressed current method

Accelerated corrosion test is considered the indirect method of studying rebar corrosion in concrete, helpful in understanding concrete resistivity for the rust pressure that develops inside the concrete. Here the comparison is made for different specimens of concrete mixes considered in this study, the average time taken for crack initiation is shown in Figure 13 for a constant current of 0.04Amp to the rebar embedded in the concrete cube. Many studies on corrosion show that the greater the time taken for corrosion initiation greater is the resistance of concrete for corrosion pressure. The results proved that the small variation in the crack initiation time of all the mixes. Since SCC_M has brittle nature, it took less time for crack initiation when compared to SCC_N. But it is also seen that the addition of PPF to the concrete increased the crack initiation. Bridging effect of fibre delay the crack initiation time in concrete. Mix SCC_M_0.15F have taken a maximum time (522 hr) to initiate crack so this particular mix can be considered as more resistant to corrosion. SCC_M_0.2F comparatively less time, that is, 492 hr. because of the decrease in the concrete strength due to uneven distribution of fibre in concrete. Other studies showed that concrete strength directly proportional to the crack initiation period. Chloride penetration initiates the steel corrosion in concrete when the specimen is dipped in NaCl solution with the current supply. The ionic moment inside the concrete and reaching to steel leads to the corrosion, so the less crack initiation time shows more penetration of chloride inside the concrete in a similar environment (constant current and concrete strength environment). The initiation and growth of corrosion product on steel embedded in concrete depends on the concrete permeability, cover thickness provided, concentration of the solution used, applied current in the study and the temperature.

Figure 13. Average time taken for surface crack initiation in concrete cube considered for accelerated corrosion test.

3.2.1. Microstructure study

The SEM images of the samples considered for the study are shown in Figure 14. Not many voids were observed in the SEM images of the samples from concrete mix SCC_N and SCC_M which are without fibres in Figures 14(a,b). The proper gradation of aggregates reduces the voids in the concrete. Since PPC is used, the optimum blending of fly ash with cement causes dense microstructure and good bonding between the aggregates and cement paste. The properties of the fibre reinforced concrete depend on the quality of interface between the fibre and cement matrix. The keen observation and analysis showed the protuberance on the fibre surface due to the adhesion of hydration products on the surface of the fibre (Figure 14(d)) (Huang et al., 2020). Formation of this uneven surface on fibre forms the interlocking between the fibre and cement matrix and improves the strength at interface of the concrete and correspondingly contributes to enhancing the strength of the concrete (Yuan & Jia, 2021). The comparison of similar magnified SEM images showed that the addition of fibres decreased the crystallisation in concrete. The presence of fly ash and PPF reduced the amount of gypsum and ettringite formation. Also, micro voids present in the SCC_M are found to be reduced with the increase in the fibre content. Since the PPF is chemically not active, there was no chemical action of fibres observed. Fibres were bound with the paste. There were no voids around the fibres and interfacial transition zone cracks were seen. Also, it is evident that the water binder ratio can affect the optimum fibre content (Yuan & Jia, 2021). In the present study, a water binder ratio of 0.31 and the fibre content of 0.15% showed a perfect bonding with less voids formation in the concrete. Use of optimum fibre content reduces the microcracks and voids in the concrete. After that with an increase in fibre content non uniform spreading enhances the weakest bond of the internal structures and increases the voids content.

Figure 14. SEM images of the samples from the concrete mixes considered for the study.

From all the results it is seen that, M-sand with 0.15% of PPF reduced the drying shrinkage by 40%, water absorption by 30%, water penetration by 33.33%, deterioration due to acid attacks by 17.5% and increased the surface crack initiation period due to corrosion by 3.57% than the SCC with natural sand. So, the study confirms that M-sand with 0.15% of PPF improve the durability of the concrete practically by reducing the ingression of external agents like chemical, sea water etc. in SCC correspondingly safeguard the embedded reinforcements in concrete from corrosion. Enhancing the durability of reinforced concrete is connected with the protection against corrosion of steel bars and sulphate attack, water and ions penetration through cracks and pores (El et al., 2019). It is well known that plastic shrinkage at young ages is one of the main causes of concrete cracks (Sun et al., 2022). As a result, use of polypropylene fibres and M-sand in place of river sand by reducing cracks and increasing the durability, appear to be particularly advantageous from the perspective of sustainable development.

4. Conclusion

The following conclusions are drawn from the study done on SCC with natural sand, M-sand and M-sand with PPF on the fresh properties, hardened properties and durability.

• The SCC with M-sand improved the fresh, hardened properties and durability of the concrete.

• Addition of PPF lowers the workability of the SCC considerably. But the optimum percentage of PPF content improved the mechanical properties and durability with good workability.

• PPF percentage not much affected the compression strength of SCC, but after 0.15% of PPF content compression strength was found to be decreased. Split tensile strength, flexural strength and modulus of elasticity also showed slight improvement with the increase in the percentage of PPF.

• The use of M-sand in concrete reduced the drying shrinkage. And with the increase in the PPF percentage in SCC, the reduction in the drying shrinkage also increased.

• Water absorption and water penetration depth also decreased with an increase in fibre content, but water absorption value increased after 0.15% PPF.

• Chloride and acid resistance improved by adding PPF with PPC cement.

• Study concluded that PPF of 0.15% by volume fraction of concrete improved the mechanical properties and durability of the SCC with better fresh properties. So, this percentage is considered as the optimum value, which increases the resistance against deterioration and corrosion of steel in SCC.

• Optimum PPF addition improved the crack initiation time of concrete due to corrosion pressure.

Despite the obtained results, a more detailed investigation and analysis need to be carried out to understand the durability under thermal effect. Also, the variations in microstructure of concrete should be carefully analysed to understand the change in composition with time due to the environmental effect. Therefore, this study can be extended further to understand strength and durability properties of SCC with different PPFs to enhance serviceability of structures for engineering applications. With this, study recommends to use M-sand in SCC instead of natural sand to achieve sustainable development in terms of strength and durability.

Acknowledgements

Authors really thankful to Er. Anil Baliga, Proprietor, Concrete Solutions, Mangalore, for providing the chemical admixture for this study.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Notes on contributors

Yamuna Bhagwat

Yamuna Bhagwat received her Master of Technology in Structural Engineering from KLE DR. M. S. Sheshgiri College of Engineering and Technology, Belagavi, India. She is currently pursuing her Ph.D in Department of Civil Engineering, Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal, India. Her research interests include concrete technology, durability of concrete structures, structural analysis, artificial intelligence and optimization algorithms.

Gopinatha Nayak

Gopinatha Nayak received his Master of Technology in Structural Engineering and Ph.D from National Institute of Technology, Surathkal, Karnataka, India. Presently he is working as a Professor in the department of Civil Engineering, Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal, India. He has published several papers in reputed journals and conferences. Currently he has been guiding 9 Ph.D students. His research interests include concrete technology, recycling plastic wastes, Durability of concrete structure, Structural engineering.

Poornachandra Pandit

Poornachandra Pandit received his Master of Technology in Structural Engineering and Ph.D from National Institute of Technology, Surathkal, Karnataka, India. Presently he is working as an Assistant Professor in the department of Civil Engineering, Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal, India. His research interests include concrete technology, Reinforcement corrosion, Durability of concrete structure, Structural engineering.

Aishwarya Lakshmi

Aishwarya Lakshmi received her Master of Technology in Construction Technology from NMAM Institute of Technology, Nitte, India. She is currently pursuing her Ph.D in Department of Civil Engineering, Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal, India. Her research interests include concrete technology, durability of concrete structures.