Water permeability of Eco-Friendly Ductile Cementitious Composites (EDCC) under an applied compressive stress

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Abstract

Permeability of concrete is one of its most important characteristics determining the durability. In this study, water permeability of a new class of fiber reinforced concrete materials called Eco-Friendly Ductile Cementitious Composites (EDCC) was studied, with and without an applied compressive stress. Hollow-core specimens were used and permeability tests were performed under full flow-equilibrium conditions. Four applied stress levels of 0.3fu, 0.4fu, 0.5fu and 0.6fu were investigated, where fu is the compressive strength of the EDCC in question. Permeability tests under identical conditions were carried out on plain control specimens without fiber reinforcement (termed Plain Cementitious Composites, PCC). The results indicated that the permeability of unstressed specimens declined over time due to the continuous hydration. A ‘critical’ compressive stress level (fcc) was identified and defined, which when exceeded, a dramatic increase in the coefficient of permeability occurred. The ‘critical’ compressive stress level (fcc) was noted to be between 0.5fu ~ 0.6fu for EDCC and 0.4fu ~ 0.5fu for PCC. In other words, EDCC was more damage tolerant than PCC, and even when fcc was exceeded, the impact of stress on the permeability of EDCC was far less pronounced compared to PCC.

Introduction

Concrete carries flaws and micro-cracks both in the material and at the interfaces even before an external load is applied. Under an applied load, distributed micro-cracks propagate, coalesce and align themselves to produce macro-cracks, resulting in greatly compromised durability of concrete structures [1]. Fortunately, the micro and macro-fracturing processes can be favorably modified by adding short, randomly distributed fibers of various suitable materials, resulting in fiber reinforced concrete. During the last two decades, significant efforts have been made towards developing high performance fiber reinforced cementitious composites (HPFRCC) that demonstrate significant increases in crack growth resistance as manifested by stress-strain curves that depict large ultimate strains. Engineered Cementitious Composites (ECC) is one promising type of HPFRCCs, depicting elasto-plastic and strain-hardening response with very tight crack widths [2]. These unique behaviors result from an elaborate design using a micro-mechanical model taking into account the interactions among fiber, matrix and the fiber-matrix interface [3].

The typical fiber used in ECC is polyvinyl alcohol (PVA) fiber with a diameter of 39 μm and a length of 6–12 mm [4]. The main problem of using PVA fiber in producing ECC is that PVA fiber tends to develop a very strong chemical bond with the matrix due to the presence of the hydroxyl group in its molecular chains. This high chemical bond leads to probable fiber ruptures which limits the tensile strain capacity [5]. In order to achieve strain-hardening behavior and high ductility, the strength of the chemical bond should be reduced. Li et al. [6] found that the fiber/matrix interfacial bond could be reduced by applying oil coating to the fiber surface. As a result, it is now generally accepted to prepare ECC with oil-coated PVA fibers. Moreover, matrix fracture toughness has to be limited to achieve strain-hardening. Therefore, the production of standard ECC mixtures has been restricted to the use of fine aggregate such as microsilica sand [7]. Plus, ECC materials contain considerably higher cement contents than conventional concrete. Such matrices and the use of surface-treated PVA fibers result in undesired high material costs and processing complications. They also increase the carbon footprint of ECC materials and make them unsustainable.

Preliminary work [8,9] was reported on a new class of ECC-like materials with ‘non-oiled’ PVA fibers and natural sand (1.19 mm maximum grain size) under the guidance of micromechanical principles. These materials also carried large amounts of supplementary cementitious materials (SCMs). With their high strain tolerance and minimal amounts of cement, we are calling these materials Eco-Friendly Ductile Cementitious Composites (EDCC). EDCCs are arguably an evolved variation on ECC and depict similar constitutive characteristics.

Due to its superior resistance to cracking, ECC is expected to demonstrate improved durability than normal concrete, especially under external loading. Permeability of concrete is regarded as a basic indicator of its durability [10]. Extensive work has been carried out to understand the permeability of normal concrete under an applied compressive stress [11,12]. It is shown that for normal concrete, the response is highly dependent on the level of applied compressive stress. At low levels of compressive stress, some researchers report a modest decrease in permeability [13,14], likely occurring due to pore-compression. This phenomenon however does not appear to be universal as some researchers observed no decrease in permeability under an applied compressive stress [15]. One universal agreement, however, is in the fact that when the compressive stress reaches a certain threshold value, often called the ‘critical’ stress level, permeability increases rapidly [13,16]. This is often explained by the creation of internal cracks that abruptly coalesce when the ‘critical’ stress level is attained causing a rapid increase in the permeability.

Lepech and Li [17] measured the permeability of a typical ECC mixture which was called ECC M45, and found its permeability coefficient at 28d was 8.18 × 10−12 m/s. This value is lower than the permeability coefficient of regular concrete, resulting from the relatively low water content of ECC. Several other studies have also studied the permeability of ECC under tension [[17], [18], [19], [20]]. It has been generally demonstrated that due to the narrow cracks, the permeability of ECC remained low even after the formation of numerous microcracks and tensile strain of up to 3%. No data exists on performance of ECC under compressive loading.

The present study aims to investigate the permeability of EDCC under an applied compressive stress. The specific objectives are: (1) to conduct permeability tests on EDCC specimens under full flow-equilibrium conditions; (2) to monitor the evolution of EDCC's permeability at very early ages (2d to 5d); and (3) to investigate the effects of compressive stress and fiber on permeability of EDCC.

Section snippets

Material composition

Based on preliminary tests [8], the mixture constituents and proportions of EDCC with “non-oiled” PVA fibers and natural sand were determined and are given in Table 1. The volume fraction of PVA fiber was 2% and its properties are shown in Table 2. Plain Cementitious Composite (PCC) without the fiber served as the control. Notice a slightly reduced amount of superplasticizer (1.7 l/m3) in PCC.

Compressive strength test

Cylindrical specimens (75 mm in diameter and 150 mm long) were cast to determine the compressive

Compressive strength

The compressive strengths of EDCC and PCC specimens as a function of age are shown in Fig. 3. The addition of PVA fibers did not alter the compressive strength much. EDCC marginally exceeded the strength of PCC at 28d. Overall, the compressive strengths of EDCC and PCC were very close.

Uniaxial tensile performance

The uniaxial tensile stress-strain curves of EDCC and PCC specimens at 28d are presented in Fig. 4. As expected, PCC specimens depicted a brittle response with a low tensile strength of about 1.83 MPa. On the

Conclusions

The study describes the development of low carbon footprint, elasto-plastic cementitious composites called Eco-Friendly Ductile Cementitious Composites (EDCC). Permeability of EDCC was measured under a compressive stress and compared with plain (unreinforced) cementitious composites (termed PCC). The following conclusions were drawn:

  • (1)

    With a high volume of fly ash and proper content of silica fume, EDCC mixture prepared with non-oiled PVA fibers and natural sand can attain strain-hardening

Declaration of competing interest

The authors declare that there are no known conflicts of interest associated with the submitted work and there has been no financial support for this work that could have influenced its outcome.

Acknowledgements

This work was supported by the Chinese Scholarship Council (CSC) and the Fundamental Research Funds for the Provincial Universities of Zhejiang (Grant no. 2019QN16), the experimental work was carried out in the materials lab, Department of Civil Engineering, University of British Columbia, Canada.

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