Engineered Cementitious Composite (ECC) is a class of fiber reinforced composites showing strain hardening behavior. The variation of cracking strength among various sections of a tensile specimen is ...a key factor governing the cracking behaviors of ECC. This study employs X-ray computed tomography method to investigate the correlation between cracking strength and flaw distribution in ECC, and identifies the dimensions of pre-existing flaws to be the main influencing parameters. Specifically, the largest cross-sectional area of a flaw perpendicular to stress can be well correlated with cracking strength. To validate the observation, the classic model on first crack strength is modified in this study by adopting a refined fiber bridging relation and employing iterated crack profile. The simulated cracking strength vs flaw size relation is in good agreement with test results. This study provides an improved understanding of the multiple cracking mechanism in ECC, which is useful for material design.
Engineered Cementitious Composites (ECC) are materials featuring strain-hardening behavior accompanied by formation of multiple cracks. The distribution of fiber orientation in ECC members is ...affected by member thickness due to the limited freedom of rotation for fibers near the surfaces. This paper first demonstrates how to acquire fiber orientation distributions for various member thicknesses from geometrical consideration. The distribution of fiber orientation is found to be between ideal 2D distribution and 3D distribution, so the tensile performance should be in between as well. Constitutive law for a single crack is computed based on obtained distributions. Stress-strain curves for tensile members are also simulated and compared with experimental results. This study reveals the theoretical effect of member thickness on ECC behavior. Compared to laboratory data obtained from small-size specimens, a thickness-dependent reduction factor for mechanical properties (mainly tensile strength and ductility) should be considered in the design of real structural members.
•SHCC reinforced by PET fibers showed poorer tensile performance than SHCC with PVA fibers.•In the modified Charpy impact test, PET-SHCC dissipated a similar amount of energy as PVA-SHCC.•Both ...modeling and tests show that a stronger interfacial bond doesn’t always lead to a higher impact energy absorption.
Strain Hardening Cementitious Composites (SHCC) are fiber reinforced composites exhibiting strain hardening and multiple cracking behaviors. The Polyvinyl Alcohol (PVA) fibers used in SHCC are expensive for normal civil engineering applications, so one kind of Polyethylene terephthalate (PET) fibers recycled from waste plastics are used in this study as a substitute of PVA fibers. Uniaxial tensile test was carried out on SHCC samples made with PVA or PET fibers where the PET-SHCC was found to behave poorer than the PVA-SHCC due to the weaker bond between PET fibers and matrix. Nevertheless, in the modified Charpy impact test which was designed to evaluate the impact energy absorption ability of SHCC, the PET-SHCC dissipated a great amount of energy comparable to that by PVA-SHCC, indicating that PET fibers can provide excellent impact resistance to cementitious composites. To reveal the mechanism of impact energy absorption, a physical model was developed to simulate the impact test, which can well support the experimental observations. Based on above findings, a hybrid mix of 1 vol% PVA fibers and 1 vol% PET fibers is recommended for practical applications to provide adequate tensile performance and excellent impact resistance with eco-friendly ingredients at low cost.
Strain-Hardening Cementitious Composite (SHCC) is a type of advanced construction material that can enhance the resiliency and durability of structures. However, the high cost of the constituents ...limits the wide application of SHCCs. To reduce the material cost and improve the sustainability, this study explores the potential of replacing commonly-used polyvinyl alcohol (PVA) fibers by recycled polyethylene terephthalate (PET) fibers. The potential of fiber hybridization was first evaluated using micromechanical modeling, and the ultimate tensile strain of hybrid-fiber SHCCs was estimated using a semi-empirical method. Then the tensile performance of SHCCs after standard curing and accelerated aging was experimentally evaluated, and the crack pattern development with increasing tensile strain was recorded. Satisfactory mechanical performance can be achieved even when 50% of PVA fibers are replaced by recycled PET fibers with surface treatment. In addition, using recycled PET fibers in SHCCs can significantly reduce the material cost and environmental impact.
•SHCCs with hybrid PVA/PET fibers are theoretically and experimentally studied.•A semi-empirical way to predict strain capacity of hybrid-fiber SHCCs is proposed.•The impacts of PET fiber surface treatment and curing time are explored.•SHCCs achieve satisfactory performance even 50% PVA fibers are replaced by PET fibers.•Using PET fibers in SHCCs effectively reduces material cost and environmental impact.
Engineered Cementitious Composites (ECC) showing multiple-cracking and strain-hardening under tension are appealing for the construction industry, but the high cement content in conventional ECC ...leads to high environmental impacts. Recently, the limestone and calcined clay (LCC) blend has been proposed as a promising alternative of Supplementary Cementing Materials (SCM) to address the inadequacy of common SCM such as fly ash and blast furnace slag, which have been widely-used in ECC. This paper reports a feasibility study for the first time on using ultrahigh-volume LCC blend (HVLCC, 70% and 80% by weight of binder) to produce sustainable medium-strength ECC. Three water/binder ratios (0.30, 0.35 and 0.40) and two sand/binder ratios (0.2 and 0.4) were explored. HVLCC-ECC can achieve a tensile strain capacity of 0.57–1.58%, tensile strength of 3.24–5.19 MPa and compressive strength of 33–65 MPa at 28 days. HVLCC-ECC also have sufficient early strength at 3 days (22-38 MPa), and similar fresh properties and bulk densities under surface-dry condition (1900-2100 kg/m3), as compared to conventional ECC. Additionally, HVLCC-ECC show about 1/6 higher embodied energy and about 1/6 lower embodied carbon than a typical ECC mix with 55% fly ash in the binder, and ECC with fly ash perform much better than ECC with LCC blend in terms of embodied energy and embodied carbon per unit strength or strain capacity. Nevertheless, this new version of HVLCC-ECC with adequate mechanical performance are still attractive from the perspective of shortage of fly ash in the future due to the reduced consumption of coal.
•Medium-strength ECC with Limestone-Calcined Clay (LCC) blend are reported for the first time.•Sustainable PVA-ECC with ultrahigh-volume LCC blend (70–80 wt% of binder) were explored.•Sufficient workability, early strength and 28-day compressive strength of 33–65 MPa were obtained.•28-day tensile strength of 3.24–5.19 MPa and tensile strain capacity of 0.57–1.58% were achieved.•These ECC are attractive from the perspective of shortage of fly ash due to reduced consumption of coal.
Strain Hardening Cementitious Composites (SHCC) are materials exhibiting tensile hardening behavior up to several percent strain accompanied by the formation of fine multiple cracks. Their tensile ...ductility is governed by the spacing and opening of cracks, which depend on the stress transfer between the fibers and the matrix. In this article, a new analytic model which takes into consideration the effects of non-uniform matrix strength, post-cracking increase in fiber bridging stress and fiber rupture on stress transfer and multiple cracking behavior of SHCC is developed. Using material parameters within the range reported in the literature, simulation results can reach reasonable agreement with test data on SHCC for two different fiber contents. The effect of fiber length on tensile behavior of SHCC is then simulated to illustrate the applicability of the model to material design. The new model should be helpful to the micromechanics-based design of SHCC for various ductility requirements.
•UHVFA-cement mortar with 80% FA obtains a 28-day compressive strength over 65 MPa.•The total hydration heat of the UHVFA-cement mortar with 80% FA is reduced by 70%.•The C3S exothermic peak of ...UHVFA-cement mortar with 80% FA is delayed by 21 hours.•Aluminate gel content first increases and then decreases with increasing FA content.•Morphological & micro-aggregate effects of FA maintain adequate strength of UHVFA concrete.
Replacing Portland cement by fly ash in concrete has attracted extensive attention, as this approach is effective in controlling heat release rate, reducing material cost and enhancing greenness. However, only limited studies have been reported on the hydration and physical characteristics of ultrahigh-volume fly ash (UHVFA, fly ash/binder > 60 wt%) concrete. This study aims to explore these characteristics of fly ash–cement systems with low water/binder ratios and a wide range of fly ash replacement levels (from 20% to almost 100%). Even if 80% of the cement was replaced by fly ash, the 28-day compressive strength of the mortar reached over 65 MPa under normal curing conditions, and the total hydration heat was 70% less than that of conventional cement mortar. Moreover, the morphological and micro-aggregate effects of the fly ash were found to play important roles in maintaining adequate strength of the systems, especially for those cases with UHVFA. The findings of this study can support the future designs and applications of sustainable UHVFA concrete.
To investigate the impact of age-related change of macular and circumpapillary retinal nerve fiber layer (RNFL) measurements on evaluation of glaucoma progression.
Prospective, longitudinal study.
A ...total of 150 eyes of 90 patients with glaucoma and 72 eyes of 40 normal individuals.
Both eyes were imaged by the Cirrus HD-OCT (Carl Zeiss Meditec, Dublin, CA; optic nerve head and macular scans were taken every 4 months for a mean of 45.8 months (range, 35.4-60.6 months). The mean age-related rates of change of macular (including the ganglion cell and inner plexiform layer GCIPL, inner retina IR, outer retina OR, and total macular thicknesses) and circumpapillary RNFL measurements were estimated with linear mixed models in the normal group. Macular and RNFL progression were then evaluated in individual eyes in the glaucoma group, with trend analysis before and after accounting for age-related change using the lower 95% confidence intervals (CIs) of the mean age-related rates of change as cutoffs. The survival probability was evaluated with the Kaplan-Meier estimator, and the agreement of progression detection among the structural parameters was calculated with Kappa statistics.
Detection of glaucoma progression and survival probability of macular and RNFL parameters.
Before accounting for age-related change, 50.0% (75 eyes) showed progression by the GCIPL thickness, 50.0% (75 eyes) showed progression by the IR thickness, 30.0% (45 eyes) showed progression by the total macular thickness, 27.3% (41 eyes) showed progression by the circumpapillary RNFL thickness, and 10.0% (15 eyes) showed progression by the OR thickness. The survival probability of GCIPL and IR thicknesses were significantly worse compared with circumpapillary RNFL thickness (P ≤ 0.001). After accounting for age-related change, the proportions decreased to 14.7%, 20.0%, 16.0%, 26.7%, and 1.3%, respectively, with the circumpapillary RNFL thickness demonstrating the worst survival probability. The agreement of progression detection between RNFL and macular measurements was poor with (kappa range, -0.055 to 0.185) or without (kappa range, -0.046 to 0.173) taking age-related change into consideration.
Age-related change of macular and circumpapillary RNFL measurements can be detected in normal eyes and can affect the analysis of glaucoma progression. The impact is more substantial in analyzing macular progression than circumpapillary RNFL progression.
•Hybrid-fiber SHCCs with PVA/steel fibers at a fixed total fiber fraction are studied.•Superposition method is used to model crack-bridging relations of hybrid-fiber SHCCs.•Interaction between ...different types of fibers is considered by the matrix micro-spalling.•Positive synergetic effect is observed between PVA and steel fibers in SHCCs.
As tensile crack-bridging constitutive relations play an important role in the multiple cracking behaviors of Strain-Hardening Cementitious Composites (SHCCs), careful control of the crack-bridging relations is the key to a successful design of the materials. This study theoretically explores the crack-bridging relations of SHCCs with fixed total volume fraction (2.5%) of hybrid polyvinyl alcohol (PVA) and steel fibers. Since a large number of experiments at the single-fiber level are needed to determine the parameters for the micromechanical model, the snubbing coefficient, fiber strength reduction factor and Cook-Gordon parameter for mono-fiber composites were theoretically calibrated rather than experimentally obtained in this study. With these calibrated parameters, the crack-bridging relations of hybrid-fiber SHCCs were then modeled and compared to the test results. The superposition principle was used to address the contributions of different types of fibers, and the interaction between PVA and steel fibers was considered through the matrix micro-spalling in the modeling. The theoretically modeled crack-bridging relations of hybrid-fiber SHCCs were in good agreement with the test curves in terms of the tensile strength and the corresponding crack opening. The findings in this study provide a better understanding of fiber hybridizations in SHCCs.
•This study explores the feasibility of recycling PET solid wastes as fibers in SHCC.•Recycled PET fiber is designed and treated following a micromechanical model of SHCC.•An effective treatment ...method is developed to improve the PET/matrix frictional bond.•The alkali resistance of PET fibers is improved by the same treatment method.•It offers a promising way to dispose hazardous PET wastes in construction industry.
As an important portion of the total plastic waste bulk but lack of reuse and recycling, the enormous amounts of polyethylene terephthalate (PET) solid wastes have led to serious environmental issues. This study explores the feasibility of recycling PET solid wastes as short fibers in Strain-Hardening Cementitious Composites (SHCCs), which exhibit strain-hardening and multiple cracking under tension, and therefore have clear advantages over conventional concrete for many construction applications. Based on micromechanical modeling, fiber dispersion and alkali resistance, the size of recycled PET fibers was first determined. Then the hydrophobic PET surface was treated with NaOH solution followed by a silane coupling agent to achieve the dual purpose of improving the fiber/matrix interfacial frictional bond (from 0.64 MPa to 0.80 MPa) and enhancing the alkali resistance for applications in alkaline cementitious environment. With surface treatment, recycling PET wastes as fibers in SHCCs is a promising approach to significantly reduce the material cost of SHCCs while disposing hazardous PET wastes in construction industry.
