Performance evaluation of low volume synthetic fibres in pozzolanic cement concrete

Abstract

Sustainable and resilient construction materials are key factors influencing the structural integrity and durability of concrete. Incorporation of polypropylene fibres has become a pivotal strategy to improve shrinkage reduction and crack resistance. The necessity of intensified exploration pertaining to synergistic use of polypropylene fibres with different cementitious materials led to the formulation of this study. 12 mm polypropylene fibres with low volume dosage 0.1%,0.2% and 0.3% with respect to concrete volume was optimized. The combination included commercially available binders like Ordinary Portland cement (OPC) and Portland pozzolana cement (PPC) for which fibrous concrete (M40 grade) behavior was investigated. Compressive strength had inverse relation with fibre dosage, 0.2% fibre dosage in OPC and PPC concrete showed marginal similarity with control non fibrous mixes with a strength of 52.24 MPa and 47.7MParespectively. Split tensile strength improved up to 3.7% and 6.4% for 0.3% fibre dosage in OPC and PPC concrete respectively. Overall improvement of flexural strength was marginal up to 2% and 8.2% for 0.3% fibre dosage in OPC and PPC concrete respectively. The results depicted the synergistic combination of pozzolans and fibres. Good quality concrete with pulse velocity range between 3.75 km/s–4.40 km/s was observed for all the set of variations reaffirming the suitability of concrete in structural applications. However, it is observed that permeability increase was within 20% for both the set of concrete with varying fibre dosages. Microstructure image depicted balling of fibres for 0.3% dosage of fibres.

Keywords:

• Polypropylene fiber
• concrete
• ordinary portland cement
• portland pozzolanic cement

Reviewing Editor:

• D. T. Pham
• Editor-in-Chief
• University of Birmingham
• UK

Subjects:

• Technology
• Engineering & Technology
• Civil, Environmental and Geotechnical Engineering
• Engineering Economics
• Engineering Education
• Materials Science

1. Introduction

One of the multifaceted problems, faced by world being climate change, is the raising issue faced globally. Being a significant contributor of climate change construction industry is responsible for carbon emissions, energy usage and environmental pollution. Cement production being an integral part of construction industry is responsible for carbon dioxide (CO₂) emissions. Energy consumption is high in ordinary Portland cement (OPC) production (12%–15%) as high temperature is required for raw material processing, the source of energy is combustion of fossil fuels and during the production decomposition of calcium carbonate liberates CO₂ ultimately contributing to anthropogenic emissions globally. Innovations are tried by researchers to implement materials that have less impact on nature. The selection of binder depends on the necessity of the structure. On the other hand utilization of pozzolanic materials in portland pozzolanic cement (PPC) production tends to have less environmental impact. When resource efficiency is considered utilization of industrial pozzolanic waste is a part of sustainable practice as this waste could have been an impact on environment posing problem pertaining to waste disposal. It is proven effective for use of industrial waste as concrete fabricated with it has an environmental friendly approach (Amin et al., 2020; Wang et al., 2023). Although PPC contributes towards sustainable development, there is reluctance by the construction industry towards usage due to delayed strength gain.

Concrete durability is a significant factor which determines the service life. Most of the times concrete fails to serve its predetermined service life. Durability is affected by exposure conditions, concrete grade, permeability, cement content, alternate wetting and drying cycles, corrosion, sulphate attack and alkali- aggregate reaction. Incorporation of natural pozzolans stands out as pivotal strategy in this era of growing environmental consciousness. The effect of pozzolan can vary based on size study by Al-swaidani (2021) observed that when nano natural pozzolans are used below 5% dosage and micro natural pozzolans when used upto 30% or more the concrete experiences higher resistance against chloride diffusion. Source based variation of pozzolans also exhibit varied features pertaining to durability study by Bediako et al. (Bediako et al., 2020) on study of clay- rice husk pozzolan observed sorptivity values in the range of 0.0055 mm/sec^1/2 and 0.0022 mm/sec^1/2 which was lower to that of Portland pozzolana cement revealing enhanced durability of concrete. Concrete is a quasi-brittle material whose brittleness and strength are directly proportional. The timeline of concrete strength development plays a vital role in structural soundness, Rahma & Jomaa (Rahma & Jomaa, 2018) on modelling of the cementitious effect of a locally available pozzolana ‘Tal Shihan’ revealed that in the range 28–56 days the pozzolana exhibited enhanaced effect on hydration. The study gave mathematical justification to the delayed strength gain in the pozzolanic concrete which can be used in further studies involving different types of pozzolans. This feature makes concrete less resistant to cracks formation and propagation. Fibres are extensively implemented in concrete to overcome cracking by improving ductility. The inherent limitation of traditional concrete is overcome by addition of these synthetic fibres, which are well known for high tensile strength and resistance to chemical degradation. Cracks constitute a source for water and corrosive material ingress, whose ingress deteriorates the concrete. From plain concrete as a basic concrete to composite concrete in their complex forms are being developed. Fibrous concrete, bacterial concrete, polymer concrete, alkali activated concrete, and so on. The investigations pertaining to the development of concrete technology are in progress.

Conventional concrete consists of location specific reinforcements, while fibre-reinforced concrete is a composite material consisting of cement matrix and randomly distributed fibres. Typically, fibres are available in following forms – glass, steel, synthetic and natural fibres. The versatility of fibrous concrete is due to availability of fibres in different material (Natural and synthetic), Mechanical properties and sizes (micro and macro). Fibrous concrete is extensively used by researchers to evaluate their positive and negative outputs. Addition of discrete fibres is proven to be a well-known strategy to improve mechanical properties, toughness, reduction of shrinkage (plastic or drying) and good impact resistance (Banthia & Gupta, 2006; Leong et al., 2020; Saje et al., 2011; Yousefieh et al., 2017). When the volume concentration of fibre is low, they are considered as secondary reinforcements as they only contribute in concrete crack control and not on load carrying capacity. Low-fibre volume concentrations are proved to be useful in crack control and are used in applications like slabs, tunnel linings and pavements. Deng et al. (2022) used three dosage of Polypropylene fibres (PPF)15 mm were used 0.05%, 0.10%, 0.15% to study the fibre pullout from concrete matrix. It was observed that fibre failure had two milestones, i.e., fibre rupture either full or partial and finally total rupture. On consideration of secondary reinforcements, fibres are specially implemented in concrete to improve energy absorption capacity and to reduce brittleness. Concrete toughness is improved by the addition of low modulus fibres (Guo et al., 2019). When synthetic reinforcement is considered, polypropylene fibres find its widespread application, as they are lightweight low-cost fibres with ease of handling, low thermal conduction and high alkali acid resistance. These fibres are hydrophobic in nature and the potential of chemical bonding is eliminated i.e., only mechanical bond exists between the fibres and matrix. Usually, the primary function of low modulus fibres is to control propagation and widening of cracks (Hannawi et al., 2016). The serviceability of fibrous concrete is judged based on its ability to absorb strain energy (elastic and plastic) by conducting the tensile stresses across the cracks. The threshold of initial cracking is raised by fibre addition as they restrain cracks by offering torturous path to propagation. The efficiency of the fibres with regard to post crack ductility depends on shape aspect ratio and orientation of fibres along with number of fibres per unit area and matrix strength. On consideration of combined working of pozzolans and polypropylene fibres within 400 °C, Qian et al. (2023) found the fibres did not attenuate the pulse velocity in UPVT instead Elevated temperature promoted secondary curing by continuing hydration and pozzolanic activity. This observation was contradictory to the observation made by Prasad and Singh (2023) who mentioned the capacity of fibres to scatter the pulses thereby reducing the pulse velocity. Bencardino et al. (2010) assessed the content and fiber type influence on fracture and mechanical properties. High modulus steel fibres and low modulus polypropylene fibres were used in consecutive proportions of 1% and 2% to study the fracture properties in comparison with plain concrete. The study was concluded by mentioning fibres contribute to stability, integrity and durability of concrete. Amin et al. (2020) investigated the effect of polypropylene fibres on mechanical properties of lightweight concrete. 21% and 41% increase in flexural strength was observed for 0.2% and 0.4% increase in fibre dosage, respectively. These fibres delayed unstable growth of cracks thus improving flexural strength. Ahmed et al. (2020) investigated the properties of high strength concrete with recycled aggregates and polypropylene fibres. 12 mm fibres with increments of 0.15% were used in varying dosages of 0.15%–0.9%. The bridging action of polypropylene fibres was observed due to their fineness whereas length variation prevented the microcrack formation. 0.6% was considered as optimum and increase was observed in compressive strength. Fibres are added to concrete in order to find the best fitting fibre that will satisfactorily serve the purpose without exploiting or affecting the homogeneity of the mix, strength and durability.

Parande et al. (2011) performed corrosion study based on gravimetric weight loss and found Portland pozzolana cement (PPC) performed better compared to ordinary Portland cement. When matrix is considered type of cement plays a vital role, dense matrix implies durable and mechanically strong concrete. It is a well-known fact that density of Calcium silicate hydrate is responsible for strength development, in case of pozzolanic action secondary CSH is formed which densifies the matrix. Pozzolanic materials like flyash, silica fume, GGBS, rice husk ash are utilized in construction sector as mineral admixtures or as supplementary cementitious materials. Although significant drop in workability is observed with increase in dosage of fibres, in the presence of pozzolans the fibres are well dispersed (Lakshmi et al., 2022). The PP in the form of microfibres with smaller diameter and high surface area results in good mechanical bonding with surrounded matrix. As a matter of fact, these fibres with modulus of elasticity ranging from 3.5–4.9 GPa cannot be expected to increase strength of fully aged concrete with cementitious composite modulus of elasticity ranging from 15 to 30 GPa. The positive aspect of using fibres in concrete is it reduces crack formation due to shrinkage stresses thereby improving concrete quality which prevents premature deterioration of structure. On study of effect of PPC and OPC terms of flexure and fatigue Arora et al. (2019) observed the deflection for both the cement concrete was similar at first crack and varied in the range of 2–3mm for load_ultimate. Even after continuous loading and unloading the materials of concrete were intact, which implied similar behaviour of OPC and PPC concrete under fatigue which proved PPC is as promising as OPC. Pandit et al (Arunkumar et al., 2023; Poornachandra Pandit & Katta Venkataramana, 2014) on study of corrosion observed that the migration of chlorides in PPC concrete is lower than that OPC but the stiffness wise OPC concrete is superior to former at higher degree of corrosion. Saidani et al. (2016) studied the behavior of micro polypropylene fibres with varying dosages of 1%, 2% and 4% by the cement volume. There was drop down in workability and compressive strength. Tensile strength was 3.72 MPa and was closer to that of normal concrete with strength of 3.40 MPa with the failure mode being ductile. Guo et al. (2019) used 19 mm fibre with dosage of 0.12%, 0.17% and 0.22% in high strength concrete (M60). Splitting tensile strength decreased with increase in fibre content, i.e. compared to plain concrete 0.12% showed slight improvement in tensile strength and 0.17% showed minor drop. While 0.22% dosage resulted in major drop in split tensile strength. Babaie et al. (2019) investigated the influence of polymer fibres (40 mm) on mechanical properties of concrete. Polymer reinforcing fibres in spite of low elastic modulus (3.6 GPa) prevented crack widening and spreading. Usage of these fibres with high tensile strength (450 MPa) had great impact on betterment of performance of specimens in bending. Bencardino et al. (2010) assessed the content and fiber type influence on fracture and mechanical properties. Low modulus polypropylene fibres were used in consecutive proportions of 1% and 2% to study the fracture properties in comparison with plain concrete, and it was found that fibres contribute to stability, integrity and durability of concrete. Amin et al. (2020) investigated the effect of polypropylene fibres on mechanical and in lightweight concrete. 0.2% and 0.4% fibres with respect to concrete volume was utilized. These fibres delayed unstable growth of cracks thus improving flexural strength by 21% and 41%. Ahmed et al. (2020) investigated the properties of high strength concrete with recycled aggregates and polypropylene fibres. 12 mm fibres with increments of 0.15% were used in varying dosages of 0.15%–0.9%. 0.6% was considered as optimum with increase in compressive strength and tensile strength and beyond this dosage major reduction in workability was observed. Non-uniformity in fibre distribution led to accumulation of fibres resulting in voids that acted as weak points which affected density as well as reduced the compressive strength. Abu Sayed et al. (Akid et al., 2021) investigated the fresh, mechanical and durability of concrete containing 15% and 30% flyash (weight-based replacement for cement) and polypropylene fibres. 12 mm fibres were used in dosages of 0.06%, 0.12%, and 0.18%. Microstructural analysis indicated void packing feature of flyash concrete. 15% flyash with 0.12% PPF was considered optimum, as it represented lower sorptivity, chloride permeability and water penetration. 12%–22% compressive strength as improved for 90 days for this combination. Ninghui Liang et al. (Liang et al., 2022) investigated the corrosion resistance properties of polypropylene fibre (19 mm) reinforced concrete subjected to sulphate attack. On compressive loading fibrous specimen did not disintegrate due to the bridging action of PPF, multiple vertical cracks were observed on the surface. Compressive strength showed increment in fibrous concrete compared to plain concrete. Bridging action of fibres improved tensile strength. Deng et al. (2022) used three dosage of PP fibres 15 mm were used 0.05%, 0.10%, 0.15% to study the fibre pullout from concrete matrix. It was observed that fibre failure had two milestones, i.e., fibre rupture either full or partial and finally total rupture. Rangrazian et al. (2022) used 19 mm polypropylene fibres with 0.5% dosage and observed 6.8% increase in compressive strength compared to plain concrete made with OPC. These small fibres prevented small cracks and fibre length also effects flexural strength as this low modulus short length fibre did not contribute much to the flexural strength with 4.12 MPa for FRC and 3.22 MPa for plain concrete. Bhagwat et al. (2023) observed compression strength decreased beyond 0.15% fibre dosage while slight improvement was observed in split tensile strength, flexural strength and modulus of elasticity. Major improvement was observed in drying shrinkage. Portland pozzolanic cement and polypropylene fibres were proved to be effective combination against chlorides and acids. Based on the observations made by the previous studies, Table 1 pertaining to dosage of fibres three dosages of fibres – 0.1%, 0.2% and 0.3% with respect to concrete volume were utilized in the study.

Table 1. Observations made regarding properties of low volume dosage polypropylene fibre concrete.

Author	Polypropylene fibre	Major Observations
Drago Saje et al. (2011) (Saje et al., 2011)	0, 0.25, 0.5 and 0.75%	0.75% fibre specimen showed 25% less shrinkage than plain concrete
Vahid Afroughsabet (2015) (Afroughsabet & Ozbakkaloglu, 2015)	0.15%, 0.3%a nd 0.45%	Drop in Workability
(Mohseni et al., 2017)	0.30%	Lower water absorption
(Nosheen et al., 2018)	0.4% and 0.6%	Crack width increased for former and latter dosage was 7.5 mm and 9.5 mm.
(Sadrinejad et al., 2018)	0.10%	The maximum crack width with 0.1% PP was in the range between 4 mm-6mm.
(Deb et al., 2018)	2%	Flexural strength improvement
(Guo et al., 2019)	0.12%, 0.17% and 0.22% (19 mm)	Tensile strength - 0.12% slight improvement, 0.17% showed minor drop and 0.22% major drop
(Shen et al., 2020)	8kg/m^3 (60 mm, 54 mm and 42 mm)	Compressive and tensile strength increased with increase in fiber length
(Alwesabi et al., 2020)	1%	Drop in Workability
(Bencardino et al., 2010)	1% and 2%	Fibres contributed to stability, integrity and durability of concrete
(Leong et al., 2020)	0.15%, 0.3% and 0.50%	Higher dosage improved flexural strength up to 26.7% and drying shrinkage was also reduced
(Amin et al., 2020)	0.2% and 0.4%	21% and 41% increase in flexural strength respectively
(Rashid et al., 2019)	0.40%, 0.60%	lower absorption
(Ahmed et al., 2020)	0.15%–0.9%. (12 mm)	0.6% is considered as optimum with increase in compressive strength and tensile strength
(Akid et al., 2021)	0.06%, 0.12% and 0.18%.	0.12% optimum - lower sorptivity, chloride permeability and water penetration
(Liang et al., 2022)	0.9 and 6 kg/m^3 (19 mm)	Compressive strength showed increment
(Rangrazian et al., 2022)	0.5% (19 mm)	slight improvement in flexural strength with 4.12 MPa for FRC and 3.22 MPa for plain concrete

The focus of the present study is to compare the behaviour of low volume polypropylene microfibres in the matrix of OPC and PPC. The objective of the research is to highlight the variation in contribution of polypropylene fibres at low volume dosage to the PPC concrete. Fiber-reinforced concrete (low volume fibre dosage) is a composite material with the capacity to vary micro structural property. Micro cracks formed in concrete results in crack propagation under stresses and loads leading to form crack path resulting in brittle fracture. According to research works this can be aided by fibrous inclusions. In this aspect, the fibre and the matrix interaction play vital role in modifying the activities of FRC. Short fibres tend to limit the spread of cracks. There exists a conflict in results while mechanical properties are considered based on dosage of fibres with regard to improvement of tensile and flexural strength and effect on compressive strength. When the volume concentration of fibre is low, they are considered as secondary reinforcements and they only contribute in concrete crack control and not on load carrying capacity. The hypothesis driving this investigation is that polypropylene fibres when combined with pozzolanic properties inherent in PPC will result in concrete formulations of moderate mechanical and durability features. The polypropylene fibres being synthetic in nature are highly resistant to alkalis and acids. They only involve in mechanical bonding in the matrix due to which based on the cohesive nature of matrix variation in behaviour of composite concrete is observed. Further hypothesis suggests a dosage dependent relation which promoted a systematic exploration of fiber content 0% to 0.3%. Based on the results, it is observed that increase in fibre content correlates with improved properties up to the threshold in low volume dosage of fibres beyond which diminish in properties are observed.

2. Materials and methodology

The mechanical properties of fibre were provided by the manufacturers. 12 mm micro fibres with tensile strength of 300 MPa–400 MPa and elastic modulus of 3.5–4.8 GPa were used in the present study. The density of fibre was 0.91 g/cc with aspect ratio of 545. Two types of commercially available cements OPC and PPC confirming to IS8112:1989 (Bureau of Indian Standard (BIS), 2013) and Bureau of Indian standards IS 1489 - 1991 (Bureau of Indian Standards, 1489, p. 1991), respectively. M40 grade concrete was designed as per IS 10262:2009 (Bureau of Indian Standards 10262:2009), and trial mixes were casted separately for two cement types with constant water to cement ratio of 0.36. Table 2 represents the mix designations for the three dosages of fibres in respective concrete sample. Figure 1 represents the experimental program Standardized tests have been carried out to determine fresh and hardened concrete properties

Figure 1. Experimental program.

Table 2. Concrete mix designations.

Cement type	Fibre dosage (%)	Designation	Cement (kg/m^3)	Fine Aggregate (kg/m^3)	Coarse aggregate (kg/m^3)	Water (kg/m^3)	Admixture (%)
Ordinary Portland cement (0)	0	0 (O)	420	840	840	151.2	0.55
	0.1	0.1 (O)	420	840	840	151.2	0.55
	0.2	0.2 (O)	420	840	840	151.2	0.55
	0.3	0.3 (O)	420	840	840	151.2	0.55
Portland pozzolana cement (P)	0	0 (P)	450	720	1327.5	162	0.5
	0.1	0.1 (P)	450	720	1327.5	162	0.5
	0.2	0.2 (P)	450	720	1327.5	162	0.5
	0.3	0.3 (P)	450	720	1327.5	162	0.5

1. Fresh property – Desired slump was 100 mm and slump test as per Bureau of Indian standards IS 1199-1959 (Bureau of Indian Standards, 1199-1959) was carried out on freshly mixed concrete

2. Mechanical properties – Average of the compressive strength values have been reported after 7 and 28 days of curing. 150 mm × 150 mm × 150 mm cubes were tested for compressive strength as per Bureau of Indian standards IS 516:1959 (Bureau of Indian Standards, 516:1959). 150 mm × 300 mm cylinders for split tensile strength and 500 mm × 100 mm × 100 mm beams for flexural strength were tested on 28th day as per Bureau of Indian standards IS 5816:1999 (Bureau of Indian Standards, 5816:1999).

3. Ultrasonic pulse velocity Test (UPVT) BS EN 12504 -4 2004 (BS EN12504-4 2004.) - Non-destructive tests for assessing the quality of the concrete is ultrasonic pulse velocity test. 150 mm × 150 mm × 150 mm cube samples were used. Basic principle involved in the technique is interpretation of results based on wave velocity. Good quality specimens with minimum discontinuities within like pores or void spaces exhibit higher wave velocity.

4. Permeability test - Principle involved application of water pressure of 5kg/cm² for 3 days. In the end, specimens were split into two halves, and water penetration was measured. On standard specimen of 150 mm × 150 mm × 150 mm cube samples as per DIN: 1048 (part 5) (Gebhard et al., 1991).

5. Microstructure - The effect polypropylene fibres on ITZ (Interfacial Transition Zone) of concrete was observed. 28th day of concrete maturation samples were extracted for SEM analysis. Sample size used for the study was 10 × 10 × 3 mm with unpolished surface.

3. Results and discussion

3.1 Effect of fibre dosage on fresh concrete

Initial challenge pertaining to usage of polypropylene fibres was non-uniform distribution of fibres due low density (0.91 g/cc), these fibres rose up in fresh concrete mix. To overcome this issue, fibres were initially thoroughly mixed with aggregates. This criterion improved the final mix but there was gradual decrease in slump value with increase in fibres due to thixotropic effect of fibres and is in agreement with the study by Akid et al. (2021; Leung & Balendran, 2003). The workability of fibrous concrete depends on aspect ratio fibre dosage and spreading of fresh concrete was limited due to sticking action of hydrophobic fibres to the surrounding cement paste thus workability deterioration was observed in fresh concrete. Alwesabi et al. (2020) mentioned that for 1% fibre dosage the workability decreased and greater the number of fibers in the mix, more is the contact surface which increases the viscosity thereby lowering the workability. Unlike natural fibres the synthetic fibres do not require special treatments with matrix. Devadiga et al. (2020) used baggase fibre, where the fibres were subjected to various treatments like alkaline and silane treatment to improve the adhesion with the matrix. In order to modify the slump characteristics, polycarboxylate-based superplasticizer was used. Conventional observations were made pertaining to workability, for the present study graphical representation of slump loss with OPC and PPC is presented in Figure 2. The results obtained are in agreement with the previous studies by Afroughsabet and Ozbakkaloglu (2015) and Saidani et al., (2016). Control mixes exhibited 110 mm slump and consistent drop in workability were observed for fibrous concrete irrespective of cement type, only variation of result was observed at 0.2% dosage of fibres where the workability of PPC concrete was 10 mm greater than that of OPC concrete this can be attributed to the high amount of cement content adopted in PPC concrete. Karahan & Atiş (2011) too observed slump loss and harsh mix for 0.3% dosage of polypropylene fibres. This is basically due to the hydrophobic nature of fibres.

Figure 2. Workability characteristics of PPC and OPC.

Figure 3. The variation of Concrete strength, quality and with respect to fibre dosage.

3.2. Evaluation of effect of low volume synthetic fibre dosage on hardened concrete - strength, quality and durability

As these fibres are hydrophobic in nature the potential of chemical bonding is eliminated i.e only mechanical bond exists between the fibres and matrix. Usually, the primary function of low modulus fibres is to control propagation and widening of cracks (Hannawi et al., 2016). The serviceability of fibrous concrete is judged based on its ability to absorb strain energy (elastic and plastic) by conducting the tensile stresses across the cracks. The threshold of initial cracking was raised by fibre addition as they restrain cracks by offering torturous path to propagation. The efficiency of the fibres with regard to post crack ductility depends on shape aspect ratio and orientation of fibres along with number of fibres per unit area and matrix strength. In Fibrous concrete the term, “Critical fibre volume” (Hannant, 2000) played a vital role in deciding the mechanical and durability properties of concrete. Being low modulus fibres along with playing the role of secondary reinforcement, these fibres effectively controlled generation and expansion of cracks by partially offsetting the stresses generated which is explained in the following sections.

Figure 3 represents the variation of Concrete strength, quality and permeability with respect to fibre dosage as independent variable influencing the above-mentioned dependent variables. Origin pro was used to perform regression analysis regarding strength, quality and durability on the basis of varying fibre dosage. P- values and Co- relation co-efficients are presented in Tables 3 and 4 for PPC and OPC fibrous concrete specimens, respectively. In the subsequent sections, linear trend of relations of compressive strength, tensile strength, flexural strength, ultrasonic pulse velocity and water penetration, respectively, with fibre dosage can be observed. The fitting functions calculated through regression are used to correlate the above-mentioned factors with our independent variables i.e. fibre dosage in two types of cement.

Table 3. Correlation coefficients of PPC concrete.

Portland pozzolana cement		Fibre dosage(%)	Compressive strength (N/mm2)	Split Tensile strength (N/mm2)	Flexural strength (N/mm2)	Ultrasonic Pulse velocity (km/s)	Permeability (mm)
Fibre dosage(%)	Pearson Corr.	1	−0.85693	0.97206	0.92582	−0.96471	0.95618
	p-value	–	0.14307	0.02794	0.07418	0.03529	0.04382

2-tailed test of significance is used.

PPC tends to exhibit higher standard deviation values compared to OPC in most of the parameters as mentioned in Tables 5 and 6 demonstrates former cement exhibits greater variation in mechanical properties compared to latter. Although this minor variation indicates potential challenge in achieving consistent performance while using PPC particularly regarding mechanical strengths and permeability. The observations made tend to guide future researches and practical applications by highlighting the performance variability between OPC and PPC.

Table 5. Standard deviation for PPC concrete.

	Mean	SD	Min	Max
Fibre dosage (%)	0.15	0.1291	0	0.3
Compressive strength (N/mm2)	47.4525	0.97473	46.3	48.63
Split tensile strength (N/mm2)	3.2225	0.0943	3.11	3.31
Flexural strength (N/mm2)	4.84	0.16733	4.6	4.98
Ultrasonic pulse velocity (km/s)	4.035	0.26764	3.79	4.4
Permeability (mm)	13	1.08012	12	14.5

Table 6. Standard deviation for OPC concrete.

	Mean	SD	Min	Max
Fibre dosage (%)	0.15	0.1291	0	0.3
Compressive strength (N/mm^2)	52.105	0.8985	50.8	52.78
Split tensile strength (N/mm^2)	3.3425	0.06131	3.28	3.4
Flexural strength (N/mm^2)	5.8625	0.05315	5.8	5.92
Ultrasonic pulse velocity (km/s)	4.0575	0.24254	3.76	4.27
Permeability (mm)	13.55	0.8226	12.5	14.5

3.2.1. Compressive strength

On evaluation of results based on effect of fibres are considered for both the cement types based on p-value in Tables 4 and 5 polypropylene fibres have no influence on compressive strength. But increase in fibre dosage decreased compressive strength in both the set of specimens. Figure 4 represents concrete mixing, testing and failed specimen. It is vital to note that control samples of OPC concrete represented 52.78 MPa whose strength dropped 0.34%, 1% and 3.75% for consecutive three fibre dosages respectively. Whereas PPC concrete with initial strength of 48.60 MPa experienced a drop in the range of 2.98%,1.91% and 4.79% for consecutive three fibre dosages respectively. Therefore, it can be clearly stated that 0.3% fibre dosage despite being a low volume caused fibre balling which affected the compressive strength. Unlike steel fibres whose intrinsic rigidity is responsible for strength enhancement, increase in fibre dosage did not contribute to compressive strength development which is in according to represented in Figure 5. Compared to control specimen all the fibrous sample showed a drop in compressive strength unlike study by Bhagwat et al. (2023) where the strength increased at 0.15% fibre dosage so in present study 0.2% was concluded as optimum dosage which can be explained on the basis of marginal increment (less than 10%) of compressive strength observed owing to the bridging action of fibres thereby narrowing down the transverse deformations. However, multiple vertical cracks formed did not allow the fibres to potentially improve compressive strength. Observations made are in agreement with the previous studies made by Afroughsabet and Ozbakkaloglu (2015) and Blazy and Blazy (2021). It confirms the previous observation made by Ahmed et al. (2020) that the bridging action of polypropylene fibres due to its fineness and length variation prevented the micro crack formation. Strong correlation represents inverse relationship between fibre dosage and both the cements. It is observed from Figure 4(d) that fibres are responsible for connecting the matrix through fastening action due to well dispersed 3-D network of fibres as discussed by Aishwarya (Lakshmi et al., 2022) that fibres are well dispersed in the presence of pozzolans.

Figure 4. Concrete compression test. a. Mixing, b. Concrete collection, c. Casted cubes, e. Compression test, d. Post failure sample.

Figure 5. Compressive strength variation of fibrous concrete.

Table 4. Correlation coefficients of OPC concrete.

Ordinary Portland cement		Fibre dosage (%)	Compressive strength (N/mm2)	Split Tensile strength (N/mm2)	Flexural strength (N/mm2)	Ultrasonic Pulse velocity (km/s)	Permeability (mm)
Fibre dosage (%)	Pearson Corr.	1	−0.90521	0.94763	0.99586	−0.96343	0.97304
	p-value	–	0.09479	0.05237	0.00414	0.03657	0.02696

2-tailed test of significance is used.

1. Mixing.

2. Concrete collection.

3. Casted cubes.

4. Compression test

5. Post failure sample.

There was no disintegration of specimen after failure as brittle nature of concrete was optimized by fibres to minor extent. Fibres in combination with PPC showed good matrix bonding attributed to pozzolanic action. The observed result can be explained the phenomena of secondary hydration triggered by supplementary cementitious material decreasing the brittleness of concrete in blended cement composition as observed in the study by Golewski (Golewski, 2023c). As a matter of fact, shear modulus of fibre is lower than that of concrete and there was no substantial contribution of fibres to shearing action. Figure 6 represents the compressive strength data with increase in fibre content beyond optimum led to trivial decrease in strength, thus 0.2% fibre implementation was considered optimum. However, the influence of fibre dosage cannot be neglected as the trend of dependence is negative.

Figure 6. Evolution of compressive strength of OPC concrete (left) and PPC concrete (right).

3.2.2. Split tensile strength

As a matter of fact, presence of fibres improved the tensile nature of both the cement concrete which can be observed in the graphical analysis presented in Figure 7. Strong positive correlation was observed between independent and dependent variables. Plain concrete being brittle in nature, after the occurrence of first matrix crack the crack path propagates interconnecting the existing cracks.

Figure 7. Evolution of Split tensile strength of OPC concrete (left) and PPC concrete (right).

Relative changes of splitting strength were observed between fibrous and non- fibrous concrete. This enhancement of strength in is attributed to the fibres which are present in the crack path and act as a barrier in matrix thereby obstructing the crack propagation. Control OPC concrete represented 3.28 MPa whose strength increased in the range 0.6%, 3.60% and 3.65% for consecutive three fibre dosages respectively. Whereas control PPC concrete with initial strength of 3.11 MPa experienced an increase in the range of 2.25%,5.78% and 6.4% for consecutive three fibre dosages, respectively. On comparison of p-values in Tables 3 and 4 to understand the influence of fibre dosage on OPC and PPC concrete, split tensile strength of PPC concrete has got slightly superior influence of fibre dosage, though splitting strength improvement was marginal (within 10%), but the mode of failure was ductile for fibrous concrete compared to fibrous OPC, i.e., there was no separation of half cylinder in former while latter exhibited splitting of halves in brittle manner. Therefore, the results of split tensile strength of fibrous sample confirms the earlier results given by Mohammad amin 2020 (Amin et al., 2020) that Non fibrous samples exhibited brittle failure of concrete cylinders during split tensile testing whereas fibrous samples exhibited no separation after reaching peak tensile strength In 0.2(P) bridging effect of fibres aided the stress dispersion by transferring the loads uncracked region. These hydrophobic fibres had mechanical interaction with matrix which resulted in fibre – matrix adhesion, this feature prevented pullout of fibres. This criterion is true when the number of fibres present in concrete matrix is optimum. 0.3(P) was observed with flocking of fibres at random locations. This can be explained based design mix with lower water cement ratio and a greater number of fibres, despite of low volume dosage for the given mix the number of fibre can be considered too much. Beyond optimum levels the effect of excess fibres included harsh concrete mix in fresh state and lowered mechanical properties in hardened state Figure 8.

Figure 8. Cylinders post failure –0.3% fibre dosage.

Many researchers have revealed the trend of improved tensile strength of concrete (Guo et al., 2019; Bhagwat et al., 2023; Deng et al., 2022) is ultimately due to the ‘crack arresting’ effect of fibres resulted in gradual failure of fibrous concrete despite like non fibrous concrete that failed in brittle manner. Concrete with lower dosage of fibres failed in brittle manner with sudden splitting of concrete into two halves. On cylinder testing failure pattern included micro crack formation near the center along the line of loading which on load increase extended to form a continuous straight crack path as shown in Figure 9.

Figure 9. Cylinder testing (left) and Cylinders post failure –0, 0.1% and 0.2% fibre dosage (right).

3.2.3. Flexural strength

As an obvious case OPC concrete performed better compared to PPC. Figure 10 represents the beam specimen and test. Unlike plain concrete fibre reinforced concrete did not fail suddenly after the first crack appeared instead failed in ductile manner with multiple crack appearance. The results obtained is in confirmation with study by Rangrazian et al. (2022) where these low modulus fibres did not contribute much to the flexural strength. Control PPC concrete with initial strength of 4.60 MPa experienced an increase in the range of 5.65%, 6.9% and 8.2% for consecutive three fibre dosages, respectively. It should be noted that encompassing of fibres by surrounding matrix worked really well for pozzolanic cement concrete. The delay of strength gain in PPC should be considered as factor of influence on p-value in Table 3, the result reveals statistically significant influence of fibres. It is not just the virtue of fibres, which has to be demarked on observation of regression coefficient in Figure 11, and the strength development was limited below 10%, results observed in Table 4 and Figure 11 represents statistically significant relation between fibre dosage and flexural strength. Thereby making PPC concrete sensitive and receptive to the properties exhibited by fibres. This improvement can be attributed to the high tensile strength of these secondary reinforcing fibres, which in spite of their shear modulus being low likely contributed to crack prevention. As the load increases fibres tend to transfer additional stresses to the matrix in the form of bond stress when this stress surpasses the bond strength multiple crack pattern appears.

Figure 10. Flexure test on beam specimen.

Figure 11. Evolution of Flexural strength of OPC concrete (left) and PPC concrete (right).

It is important to note that along with following factors of fibre like – type of fibre, aspect ratio, distribution, orientation, length and dosage even type of cement has got marginal influence on behaviour of concrete and the results obtained are in line with (Bhagwat et al., 2023; Deb et al., 2018; Joshi et al., 2018; Mohseni et al., 2017). Fibre slenderness played a vital role with improving bond quality in fibrous concrete along with contribution of pozzolanic action. Although the presence of pozzolans made the matrix heterogeneous they successively modified the pores (Golewski, 2023c). Thus, employment of fibres despite improving flexure properties in PPC concrete also delayed crack formation in extreme concrete fiber.

3.2.4. Ultrasonic pulse velocity (UPV)

UPV being a function of material density and elastic property represented strong correlation between fibre dosage with inverse relationship. Increase in porosity tends to decrease the pulse velocity as the pulse strength is attenuated on encountering of discontinuities. Figure 12 shows the specimen testing. P-value observed in Tables 3 and 4 can be justified based on the fact that fibres made concrete partially porous as matrix – fibre were bonded mechanically with both the cement types due to hydrophobic nature of fibres and synthetic fibres. The results were in accordance with study by Benaicha et al. (2015). Mohseni et al. (2017) mentioned the inability of matrix densification by low volume synthetic fibres. The minor drop in pulse velocity was observed due to void spaces around the fibres. On the other hand, these factors did not severely hinder the quality of concrete which is a good sign. Optimum dosage of fibres improved other characteristics of concrete with satisfying the criteria of good quality concrete. In both cement concretes and fibres affected the quality of concrete to certain extent due to which pulse velocity decreased on encounter of void spaces in both the cases of OPC and PPC. The effect of fibre dosage on concrete quality was similar with both the types of cement the drop in pulse velocity range was between 3.75 km/s–4.40 km/s and is considered as good quality and the values are shown in Figure 13.

Figure 12. Ultrasonic pulse velocity test.

Figure 13. Evolution of ultrasonic pulse velocity for OPC concrete (left) and PPC concrete (right).

3.2.5. Permeability

Both the cements have got equal contribution towards permeability from the fibres. Figure 14 is the test on concrete cubes. The higher fibre dosage demonstrated increase in permeability In PPC, concrete pozzolanic action delayed strength gain but formation of secondary CSH gel results in compact matrix formation along with strength gain. Permeability being a durability parameter is found to have strong positive correlation presented in Figure 15 and that there does not exist a remarkable difference in permeability values for two cement types, though PPC concrete exhibited lower permeability homogenization of matrix takes place due to secondary hydration Golewski (Golewski, 2018) which counteracted porosity caused by fibres. This is the reason for increase in the water penetration with increase in fibre dosage irrespective of cement type with fibre dosage which is proven to be detrimental to the concrete durability. It can be concluded that irrespective of cement type, increase in dosage of polypropylene fibres increased penetration as the fibres change the distribution and structure of pores as the fibres are hydrophobic in nature their bonding with the matrix is extremely affected, p-value represents Portland pozzolanic concrete with fibres performed better as increase in water absorption was comparatively less as presence of pozzolanic material acts as reactive powder resulting in controlled absorption. The reason for this is modification of microstructure in the presence of fly ash particles (Golewski, 2023b), and the fibres which were uniformly distributed in well composed matrix due to pozzolanic action and minimized cracks due to the presence of fibres gave less channel to the entry of water compared to OPC combinations Meanwhile on consideration of cement type presence of pozzolanic material acted like reactive powder with well-defined matrix fibre bonding resulting in lowered absorption compared to ordinary Portland cement concrete. Overall performance of PPC with and without fibres was better compared to OPC with and without fibres. Fibres were uniformly distributed in well composed matrix due to pozzolanic action and minimized cracks due to the presence of fibres gave less channel to the entry of water compared to OPC combinations. Polypropylene fibres being hydrophobic in nature tend to bond poorly with the matrix, this drawback was aided by pozzolanic action of PPC.

Figure 14. Permeability test.

Figure 15. Evolution of water penetration for OPC concrete (left) and PPC concrete (right).

3.2.6. Interfacial transition zone

Aspect ratio and dosage of fibres play a major role in slump characteristic of concrete as both the parameters increase leads to balling of fibres. (Fang et al., 2017). As a matter of fact, when durability is concerned PPC concrete is corrosion resistant compared to that of OPC concrete (Pandit, 2019; Poornachandra Pandit & Katta Venkataramana, 2014).

It has been observed that Figure 19 Polypropylene fibres appear in the form of round filaments which act as secondary reinforcement with poor bonding with matrix due to its hydrophobic nature and has the tendency to draw air bubbles into ITZ. Thus, synthetic fibres tend to introduce porosity in ITZ Randomly oriented yet uniformly distributed fibres improve crack path arresting phenomena. Meanwhile overdosage of fibres beyond optimum leads to balling effect Figure 19. Being low modulus fibres (3.4 GPa–4.5 GPa) while playing the role of secondary reinforcement these fibres effectively control generation and expansion of cracks by partially offsetting the stresses generated.

The effect of polypropylene fibres on concrete was observed. 28th day of concrete maturation samples were extracted for SEM analysis. Specimens (OPC concrete and PPC concrete) with optimum results (0.2% dosage of fibres) pertaining to mechanical and physical properties were chosen for examination. Bridging effect of fibres was observed in SEM images which depicted the improvement in tensile and flexural behaviour. On initiation of hydration as result of interaction between calcium, sulphate, aluminate and hydroxyl ions within few minutes calcium tri sulphoaluminate hydrate – ‘Ettringites’ formed which appear as needle shaped crystals in Figure 19. As a matter of fact, binding material calcium silicate hydrate (CSH) gel and aggregate appear in brighter and darker hue respectively, fine aggregate appears in semi dark hue. Meanwhile in PPCP2 matrix seems inhomogeneous and voids are observed but there exists accountability between fibres and CSH gel which surrounded the fibre making the matrix look compact with lower amount calcium hydroxide as its reaction with pozzolans enhanced CSH formation (Golewski, 2023a), secondary CSH formation is responsible for this. It is noted that the results obtained are in confirmation with the studies made on ITZ by other experiment works by Golewski (2023d), where compactness is observed due to addition coal fly ash 10% greater than that of binder of Consumed Calcium Hydroxide led to secondary CSH formation. Meanwhile bridging effect of fibre is resulted due to encompassing of gel around fibres i.e void packing behaviour of flysh present in binder aided the compactness of the matrix which was also an observation made by Extreme heterogeneity of concrete made it difficult to identify the properties of fibrous concrete. The use of fibres increased the initial cracking threshold but in Figures 16 and 17 porous ITZ was observed in fiber-matrix interface which affected compressive strength. Plain concrete microstructure was compact and uniform nevertheless, on observation of fibrous PPC concrete its observed that CSH gel has encompassed the fibres and aggregates to certain extent, therefore the binder role was well played by PPC cement despite of delayed strength gain.

Figure 16. Pores observed in concrete matrix.

Figure 17. Matrix of semi-porous and non-fibrous PPC concrete.

It is observed from the images CSH gel in the form of small fibrous crystals begin to fill the space which was formerly occupied by water and dissolving cement paste thereby encompassing the fibres but hydrophobic nature of fibres led to the formation of a void zone around it when sufficient CSH gel was not available and this phenomenon was observed in OPCP2 which justifies its increased permeability and absorption properties. Although fibres are encompassed by CSH gel Figure 18 shows minor incompatibility as water pocket is formed between cement paste and fibres which establishes the relation between increased permeability due to increase in the fiber dosage. The water pocket formation around these hydrophobic fibres negatively affect the durability as it acts like voids which on loading. Mechanical bridging of fibres in matrix is the reason for improved mechanical properties (Figure 19). This resulted in pore blocking effect and decreased permeability compared OPC fibrous concrete. Overall, fibrous concrete had more air voids compared to plain concrete.

Figure 18. Water pocket formation around fibres.

Figure 19. Balling of fibres at 0.3% dosage of PPF.

Conclusion

• The integration of PPC with low volume polypropylene fibres emerged as the suitable choice in uniting strength, quality and durability in moderate strength concrete. Making them a synergistic combination.

• The combination failed to enhance compressive strength of concrete while marginal improvement (less than 10%) of split tensile strength up to 6.4% and flexural strength up to 8.2% was observed compared to control specimens.

• There is no variation in concrete quality with implementation of synthetic fibres up to optimum 0.2(P) beyond this, balling effect attracted air bubbles around them and porous network was observed in matrix.

• For the 0.2% fibre dosage, the failure mode shifted from brittle to quasi- brittle in the presence of in Portland pozzolanic cement concrete hence it was considered as optimum dosage.

• The porous microstructure between fibre and matrix was justified by the hydrophobic nature of synthetic fibres which increased the permeability irrespective of cement type.

Acknowledgement

Authors convey their heartfelt thanks to Er. Anil Baliga, Proprietor, Concrete Solutions, Mangalore, for providing the chemical admixture for this study.

Disclosure statement

The authors declare that they do not have any conflict of interests.