A Comparative Study of Recron and Steel Fiber Reinforced Engineered Cementitious Composites

Dr. S. C. Patodi, Civil Engineering Department, Parul Institute of Engineering & Technology Limda, Vadodara; Dr. J. D. Rathod, Applied Mechanics Department, Faculty of Technology and Engineering, The M. S. University of Baroda, Vadodara

Fibers in the cementitious matrix tend to reinforce the composite under all modes of loading and the interaction between the fiber and matrix affects the performance of cement based fiber composite material. In the present experimental work, performance of a low modulus fiber (Recron 3S fiber) is compared with the performance of a high modulus fiber (Steel fiber) in producing the Engineered Cementitious Composite (ECC) with suitable mix design. Recron fibers are of polyester type which belongs to ester functional group. They exhibit strong interactions towards other polar substrates being polar in nature and because of this a very good bond is possible between fiber and cementitious matrix. Further, cross section of this fiber is substantial triangular due to which it is 2.2 times more effective than the fiber having circular cross section. On the other hand, corrugated steel fibers with flat geometry have optimized interface properties compared to other steel fibers and have modulus of elasticity more than ten times that of a Recron fiber. In the present paper, mechanical properties of Recron and steel fiber reinforced ECC under tension, compression, shear, impact and flexure are evaluated, with detailed parametric study, by testing different types of specimens. Recron fibers are found to give superb deformation performance under different types of loading with moderate strength enhancement. On the other hand, steel fibers are found to enhance strength of ECC under almost all types of loading but fail to demonstrate the required deformability.

Introduction

Strain hardening through multiple cracking in axial tension and tight crack width control are two unique properties that put Engineered Cementitious Composite (ECC) in a category of smart material [1]. Microstructure tailoring based on micromechanics [2] can lead to composite ductility of several percent in tension; a material property not seen before in discontinuous Fiber Reinforced Concrete (FRC). This makes ECC superior to that of ordinary concrete which is having brittleness in both tension and shear as inherent weakness. The lower fiber content combined with processing ease makes ECC economically and technically feasible to translate material performance to structural performance in the field.

The construction industry is highly cost-sensitive industry. The prime requirement of the society for any newly introduced material is that it must be beneficial, feasible and cost effective. Since, fibers increase cost of the composite and processibility is affected due to lack of workability, it is imperative to minimize the amount of fibers in the composite; yet the amount of the fibers must be good enough to just switch the material from a normal tension-softening FRC behaviour to a strain hardening ECC behaviour. Further, ECC mixture essentially consumes higher cement, typically two or three times than that of conventional concrete which is major factor that affects economy. The best possible way to address this issue is to substitute cement by industrial by-product well known as fly ash, provided the properties of resulting green ECC would still meet the performance requirement of ECC.

Details of a comprehensive study carried out for the performance evaluation of Recron and Steel fibers reinforced ECC (named as RECC and SECC specimens respectively) are given in this paper. Effect of size, C: S ratio, fly ash replacement and fiber volume fraction was studied under tension, compression, shear, flexure and impact type of loading. Briquette, coupon and dog bone type specimens were prepared to evaluate the effect of fiber orientation on tensile strength and strain performance. Indirect tension test was also carried out to establish relation with the direct tensile strength. Cubes were prepared for testing under compression. Inverted L-type shear specimens were prepared to facilitate testing under load and displacement control. Simply supported beams of various depths were tested under static four point loading arrangement to characterize size effect and fiber orientation effects on flexural performance. Also, cylindrical specimens were tested under drop weight test to evaluate impact strength.

For the preparation of specimens, Kamal brand 53 grade OPC, 300µ passing silica sand, w/c ratio of 0.35 and length of fiber as 12.5 mm were kept same in all ECC matrices. Cement: sand ratios of 1:0.5, 1:1, 1:1.5 and 1:2, different percentage volume fraction of Recron and steel fibers, processed siliceous pulverized fly ash confirming to IS 3812 (Part 1): 2003 up to 30% replacement of cement, variable dose of Glenium brand high performance concrete super plasticizer were included in the detailed parametric study performed to characterize ECC under different conditions.

Comparison of Performance Under Tension

Uniaxial tensile test though simple in concept is difficult to perform for concrete and cement matrix and requires attention to many test details. Amongst these is specimen alignment and post crack stability. Also, fiber orientation seems to play very crucial role along with the associated size effect. Testing with small cross section size specimens, in which fibers tend to be oriented favorable to the loading direction typically yields higher tensile strength and strain capacity in comparison to the true properties of the material when used in a structural member with larger dimensions. Therefore, in the present work three different types of specimens as shown in Fig. 1 were designed to give respectively 1D, 2D and 3D fiber orientation effect.


Evaluation of strength in axial tension requires complicated test setup, time consuming testing procedure and expensive experimentation. Split tensile test seems to be the best option to get tensile strength indirectly for the cementitious composite. It is accepted since long for concrete as it is not only easy to cast the cylindrical specimen but also it is simple to perform avoiding all the precautions as discussed above. Therefore, cylindrical specimens were also tested as shown in Fig. 1(d) to know the indirect tensile strength of ECC.


Failure pattern of various RECC tensile test specimens is depicted in Fig. 2 and that of SECC specimens is depicted in Fig. 3. Direct tensile test specimens were tested on MTS machine under displacement control using Multi-purpose Testware whereas indirect tensile test specimens were tested on compression testing machine under load control.

Load-displacement curves for briquette specimens for different volume fraction of Recron fiber are shown in Fig. 4 whereas Fig. 5 depicts the load-displacement graphs for briquette specimens having different percentage of steel fibers and fly ash.


Tensile strength of Recron fiber reinforced ECC is found less than the steel fiber reinforced ECC for the same fiber volume fraction irrespective of fiber orientation or fly ash replacement. Increase in tensile strength of SECC over RECC ranges between 40 to 60% in different types of tension specimens. Twisting mechanism of Recron fibers dominated over well known size effect of the specimen whereas size effect is more predominant in case of steel fiber reinforced ECC as performance is governed by contribution of cementitious material.

Performance of RECC is focussed on 4% fiber volume fraction being optimized [3] whereas performance of SECC is limited to 3% fiber volume fraction [4]. Maximum strain exhibited by 4% Recron fiber reinforced ECC with c: s ratio of 1:0.5 is found to be 1.53% in briquette specimen in contrast to 0.92 in the same cementitious matrix with 3% steel fiber volume fraction. Approximately 40% strain is reduced in this case. It is important to note here that strain performance of SECC is not at all consistant. It is highly unpredictale and insignificant in most cases.

The key issue to be addressed over here is the excellent strain hardening performance exhibited by 4% Recron fiber reinforced ECC with c: s ratio of 1:0.5 comparable to that with metal. Fly ash replacements in ECC could also maintain the true strain hardening performance with some reduction. Thus, it can be said that the main aim of this investigation to develop truly strain hardening material is satisfied by the use of 4% Recron fibers in ECC. None of the specimens could exhibit strain hardening performance by the use of steel fibers even upto 3% fiber content. Therefore, it can be concluded that the steel fiber reinforced cementitious composite should not be designated as ECC but it can be considered as regular FRC. Fiber volume fraction of more than 6% may convert FRC performance into true ECC performance in case of steel fibers.

Another criterion of ECC performance is also satisfied fully in tensile specimens of RECC i.e. multiple cracking. Density of multiple cracking reduced with increase in fly ash replacement. Combined action of pull out and rupture of the fibers governed the failure mechanism. In contrast, multiple cracking is not seen in any of the specimens in case of steel fiber reinforced ECC. All the specimens failed by formation of single crack in which fracture failure and matrix splitting governed the failure mechanism.

Fiber orientation related with size effect of the specimen plays critical role in triggering strain hardening performance at minimum fiber volume fraction. Strain hardening performance in RECC is geared up at minimum fiber volume fraction of 2% for 1D, 3% for 2D and 4% for 3D specimens. However, only 4% Recron fiber reinforced ECC is selected for further study as most of the specimens tested in the subsequent sections have 3D fiber orientation.

Comparison of Response Under Compression

ECC has shown its compatibility with the concrete and reinforcement. This property of ECC leads to use it in many structural applications where it has to interact with the concrete which is designed for compression. Therefore, one can not neglect this fundamental property of the cementitious material as the end users in the construction industry recognize the construction material by its compressive strength.

For finding the compressive strength of Recron and steel fiber reinforced ECCs, cubes of 150 mm size were tested using a compression testing machine; the failure patterns of the same are shown in Fig. 6.


Maximum compressive strength in Recron fiber reinforced ECC without fly ash is found as 36.77 N/mm² at the c: s ratio of 1:0.5 with 4% fiber content. This strength could be elevated to 39.84 N/mm² by addition of 30% fly ash leading to 8.34% increase in strength. Cement: sand ratio of 1:0.5 proved to be better in Recron and steel fiber reinforced ECC. Average density of 2250 Kg/cm² and 2350 Kg/m³ respectively is obtained in RECC and SECC. The ratio of compressive strength of cube to the cylinder specimen is about 1.35 in RECC whereas it is found to be 2.5 for SECC.

Maximum compressive strength in SECC without fly ash is found as 49.90 N/mm² at the c: s ratio of 1:0.5 with 3% fiber content. This strength could not be elevated by addition of fly ash; the strength remains almost the same. Use of fly ash renders only economy in case of SECC while it renders economy and strength in RECC. Maximum enhancement of compressive strength in steel fiber reinforced ECC is 25% over Recron fiber reinforced ECC.

Comparison of Performance Under Shear

Shear failure is generally brittle in concrete structures. The beam-column connection and the base of shear wall are likely to be subjected to intensive shear during earthquake loading. Typical diagonal crack patterns observed in the catastrophically failed structures due to shear in Bhuj earthquake suggest that the structural shear load induces local tensile failure of material. Thus, tensile strength plays a significant role in shear mechanism for brittle materials whereas compressive and tensile strength together governs shear mechanism for ductile material. The present study is primarily concerned with translation of pseudo strain hardening property of ECC in axial tension from material level to the structural level under intensive shear load.

Most of the researchers have carried out the testing of conventional reinforced beams to study the effect of shear span to effective depth ratio, fiber type, fiber volume fraction and aspect ratio and the longitudinal steel content on shear capacity. But the experience shows that the two planes failing simultaneously in double shear for beam specimens is rarely observed in reality and hence shear strength calculated in this manner could be erroneous. Also, an attempt to get one failure plane in shear in beam and column specimen directly under compression testing machine invites undue eccentricity since one portion of the specimen needs to be held fixed with respect to the other part. Hence, there exists no standard, reliable and simple method to get direct shear strength. An attempt is, therefore, made in the present study to get direct shear strength of ECC material by preparing inverted L-type specimens and testing under single shear arrangement as shown in Fig. 7. Recron fibers being polymeric tend to pull out with some resistance or rupture in the matrix. Pull out mechanism of these fibers in the shear zone is very complex in nature whereas it has definite mechanism in tension. Therefore, Recron fibers do not enhance the strength performance up to the extent it is observed in case of steel fibers. However, three dimensional effect because of more uniform distribution of fibers makes it very beneficial to undergo large shear displacement representing pure shear failure. The sliding shear mechanism observed in case of Recron fiber reinforced ECC (Fig. 7(a) is unique in nature and very difficult to get in any other cementituious composites.


Advantage of Recron fibers could be quantified to a large extent in shear behavior in comparison to the axial tension. Once again, ECC with c: s ratio of 1:0.5 without fly ash and 4% fiber volume fraction proved to be superior in performance as it could exhibit enhanced strength and strain performance.

Steel fibers being very strong in shear make it possible to resist large amount of the load if fibers are available in the resisting plane. It leads to very uncertain behavior in shear specimens due to this reason only. In some of the specimens none of the fibers could be seen in the failure plane. The specimen looked like plain cementitious matrix. Therefore, it is very difficult to get proper failure pattern or predict ultimate load. Some of the specimens could exhibit shear strength 58% more than the maximum strength obtained in RECC. However, this enhancement is not consistent and can not be guaranteed.

Comparison of Performance Under Flexure

The flexural strength of unreinforced cement-based materials and their fiber composites is important for many applications such as buildings, road pavement slabs, airfield runways, roofing tiles, and architectural wall panels. In addition, using a simple flexure test, an important property of cement-based materials, namely their tensile strength, can be deduced from their flexural strength.

Flexural behavior of plain cementitious matrix is affected by well-known size effect. Fiber orientation and distribution is also affected by size effect. Thus, size effect plays dual role in fiber reinforced composite behavior and therefore it should be taken into account seriously. Also, ECC material composition, which satisfies performance criteria of strain hardening through multiple cracking in axial tension, will definitely satisfy in flexure but not vice versa. Therefore, material optimization strategy in flexure is different than that of axial tension. Lower fiber volume fraction in ECC may satisfy performance criteria under flexure.


In the present work, 500 mm long beam samples having 100 mm x 20 mm and 100 mm x 45 mm cross sections were tested under four point loading arrangement as shown in Fig. 8. Load-displacement response of RECC and SECC beams is shown in Fig. 9 (a) and 9 (b) respectively whereas failure pattern of Recron fiber and steel fiber reinforced beam samples is shown in Figs. 10(a) and 10(b) respectively. Comparison between performance of 4% Recron and 3% steel fiber reinforced ECC beams is made with 20 mm and 45 mm thick specimens in Table 1.


One of the significant differences in RECC and SECC beams is that none of the SECC beams could exhibit strain hardening (Fig. 9(b)) and multiple cracking (Fig. 10 (b)). All the specimens behaved like plain cementitious matrix. Reserved strength and deflection hardening are not obtained, therefore, they are not considered for the comparison. Ultimate strength for all cases is found to be lower in SECC beams in comparison to RECC beams.

Comparison of Response Under Impact

Industrial flooring subjected to heavy machine vibration, cavitations and erosion damage in dams and other hydraulic facilities, ability of shotcrete to absorb energy, specialized cementitious coating for repair, fiber reinforced castable refractory, military and blast resistent applications etc. are some of the practical examples where impact test results help to access the performance of the material qualitatively.


Generally, one cylinder of 150 mm diameter and 300 mm height as shown in Fig. 11(a) is casted and three specimens of 64 mm height are cut from it. This method of sampling was adopted for Recron fiber reinforced ECC which could be cut easily. Cylinders prepared from flat steel fibers, however, could not be cut properly by the available concrete cutting machine. Therefore, directly the samples of 64 mm height were prepared by using the mould as shown in Fig. 11 (b) and were tested using the test set up for the impact test as per the ASTM standard as shown in the Fig. 11 (c). The hammer of weight 4.54 kg was dropped through a height of 457 mm on a steel ball held firmly in the center of the specimen and number of blows required to cause the first visible crack on the top surface were recorded.


Performance of 4% RECC is compared here with 3% SECC. Results of 30% fly ash are also included for the comparison. With 4% RECC, 1550 percentage enhancement of impact strength over M40 concrete is found as shown in Table 2. 30% fly ash replacement reduced the enhancement to 1016% which indicates negative effect of fly ash in case where impact energy is to be absorbed. In case of SECC, addition of 30% fly ash resulted in number of blows less than the concrete which is a debatable issue.


Failure patterns of impact specimens prepared from Recron and steel fiber reinforced ECC are shown in Fig. 12. Failure pattern of RECC shown in Fig. 12 (a) clearly reveals ultra high ductility by the extensive deformation (penetration) of the hard steel ball at the centre of the specimen. There is a perfect splitting crack and large deformability without disruption, disintegration or spalling of cementitious material. On the other hand, the failure pattern of SECC as shown in Fig. 12 (b) is very similar to that of concrete for being brittle in nature.

Energy dissipation in the composite is mainly attributed to the compatible and connected fiber action. Fractured surface of impact specimens is shown in Fig. 13 which indicates that more number of fibers are evenly distributed in RECC compared to SECC. Good strength performance could be obtained under static load in SECC due to fiber geometry, surface deformation and stiffness. However, impact performance of the SECC is very poor. It is almost similar to the M40 concrete; 5.5 percent enhancement in the impact strength can be considered as negligible.

Conclusions

• Workability aspect of steel fiber reinforced ECC is an appreciable issue as satisfactory workability is observed without use of any chemical admixture. Whereas, 2% dose of high performance concrete super plastisizer is a must to have good workability in case of 4% Recron fiber reinforced ECC.
• Although, tensile strength of SECC is found more than RECC for the same fiber volume fraction irrespective of fiber orientation or fly ash replacement, none of the specimens could exhibit strain hardening performance and multiple cracking pattern by the use of steel fibers even upto 3% fiber content. Fiber volume fraction more than 6% may lead to its characterization as ECC but such a high volume of steel fibers will make it highly uneconomical.
• Compressive strength of 4% Recron fiber reinforced ECC is comparable to medium strength concrete whereas compressive strength of 3% steel fiber reinforced ECC is 25% more than RECC. However, material strength can not be directly correlated to the structural strength which seems to increase with the higher ductility. Hence good structural performance requires a balanced material strength and ductility which RECC is able to confirm through ductile failure patterns.
• Maximum shear strength obtained in SECC is approximately 5 times the strength obtained in ordinary concrete. Also, its shear strength is more than the maximum shear strength of RECC. However, results of SECC are not consistent and most of the specimens failed by fracture control after formation of the first crack. Steel fibers being very strong and stiff in shear makes it possible to resist large amount of the load only if fibers are available in the resisting plane.
• Ductile nature of failure pattern is observed in RECC while fracture controlled and brittle nature of failure similar to the concrete is exhibited by the SECC under flexural loading which is the major difference between the two.
• Large difference in the impact strength value between the RECC and SECC may be correlated to the performance of both the composites in axial tension. RECC undergoes extensive deformation with significant stain hardening performance indicating super ductile behavior in axial tension whereas, the SECC fails suddenly after the formation of the first crack similar to the concrete indicating brittle nature. Therefore, there is not much difference in the impact strength of the steel fiber reinforced ECC and M40 concrete.
• Steel fibers enhance strength of ECC under almost all types of loading but fail to demonstrate deformability. On the other hand, Recron fibers demonstrate superb deformation performance under different types of loading with moderate strength enhancement. This situation gives rise to the hybrid concept of combining steel and Recron fibers to obtain better performance in both strength and strain in the composite.

Acknowledgement

The first author would like to acknowledge with thanks the support provided by the Gujarat Council on Science and Technology (GUJCOST), Gandhinagar to carry out a part of the research work reported here.

References

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