Structural Composites

Soft computing approaches to homogenized properties of inclusion-modified concrete mixtures: Application to XLPE-modified concrete

Gérard-Philippe Zéhil, Chady Ghnatios and Rawad Himo

Journal of Building Engineering

Journal article, 2020

Abstract

This work focuses on determining the homogenized elastic properties of concrete mixtures containing randomly oriented chopped inclusions using soft computing techniques, to identify optimal inclusion ratios. It also addresses the efficient resolution of the inverse problem, to identify the elastic properties of the constituents from those of the mix. A solution manifold is constructed using computationally efficient 2D axially-symmetrical finite element models of a typical concrete cylinder, at the mesoscale, with randomly generated structures of round aggregates and flat inclusions. To overcome the prohibitive cost of solving the reverse component property identification problem, more efficient analytical and machine learning approaches are proposed and tested for their ability to learn the numerical manifold. The best model is used to determine the elastic properties of the constituents of a set of real cross-linked polyethylene (XLPE) modified concrete mixtures, from their observed homogenized behavior. A maximum XLPE inclusion ratio of about 230 kg/m3 is also determined to maintain an apparent stiffness consistent with structural applications.

Feasibility of concrete mixtures containing cross-linked polyethylene waste materials

Gérard-Philippe Zéhil and Joseph Assaad

Construction and Building Materials

Journal article, 2019

Abstract

Post-consumer plastics made of thermosetting polymers such as crosslinked polyethylene (XLPE) cannot be easily fused or dissolved during recycling. As a result, such wastes are often incinerated or landfilled with adverse consequences on the environment. This work addresses the feasibility of incorporating XLPE waste materials in concrete by testing the fresh and hardened properties of several sets of mixtures prepared with various cement and free water contents. The XLPE was shredded to different sizes and incorporated up to 8% of cement mass. Test results showed that concrete workability and air content are marginally affected by XLPE inclusions, while the unit weight decreases due to the difference in density between aggregates and XLPE shreds. XLPE inclusions also result in superior concrete performance in terms of water permeability. Concrete strengths and shrinkage suffer moderately from increasing XLPE content and particle size. However, these shortcomings can be mitigated by decreasing the water-to-cement ratio. The thermal degradation of XLPE following exposure to heat reduces the residual compressive strengths of XLPE modified concrete mixtures.

Effects of polypropylene fibers on the physical and mechanical properties of recycled aggregate concrete

Pierre Matar and Gérard-Philippe Zéhil

Journal of Wuhan University of Technology - Mater. Sci. Ed.

Journal article, 2019

Abstract

The viability of using polypropylene fibers (PPF) in concrete was largely studied. Yet, few of the existing research studies investigated the effects of PPF on the properties of concrete containing recycled concrete aggregate (RCA). Mixes with different RCA replacement ratios and different PPF content were designed and tested in this study. Test results showed that the addition of PPF did not change significantly the compressive strength and the density of the concrete, but slightly decreased its modulus of elasticity and Poisson’s ratio. The drop in the splitting tensile strength and the flexural strength due to RCA inclusions was to a large extent compensated by the PPF addition. The water absorption decreased and the percent voids increased with increased PPF addition. Correlations between the RCA content, the PPF content and the properties of concrete were studied. Useful regression models were proposed to predict the properties of concrete in relevant ranges of RCA and PPF content.

2D modeling of the thermal conductivity of XLPE-modified concrete

Rawad Himo, Gérard-Philippe Zéhil and Chady Ghnatios

Société Française de Thermique

SFT 2019 Conference Proceedings

Abstract

Cross-linked polyethylene (XLPE) is a thermoset polymeric material extensively used in the industry, including in firefighting systems, hot water and sanitary piping, and for the insulation of electric cables. The cross-linking of polyethylene increases its performance while reducing its recyclability. A recent approach considers processing XLPE waste materials in the form of small flakes to partially replace natural aggregates in concrete mixtures. Ongoing experimental and numerical studies addressing the homogenized mechanical properties of such composite mixtures are showing their feasibility. Despite a decreased stiffness and strength due to the presence of XLPE inclusions (which are typically weaker than traditional aggregates) the modified concrete mixtures can still maintain working structural properties. In this vein, it is also interesting to characterize the role of XLPE inclusions in improving the thermal insulation of XLPE-modified concretes. A virtual concrete wall is thus modeled numerically, in 2D at the mesoscale, to study the statistical effect of varying the volume fractions of XLPE inclusions, and aggregates, on the homogenized thermal conductivity of the modified concrete. The equivalent thermal conductivity of the wall keq typically drops as more aggregates are replaced by XLPE inclusions, thus achieving better thermal insulation. The dependence of keq on the area fraction of XLPE is roughly linear, while the statistical variability is very low.

Testing and modeling the behavior of sandwich lightweight panels against wind and seismic loads

Joseph Assaad, Elie Chakar and Gérard-Philippe Zéhil

Engineering Structures

Journal article, 2018

Abstract

The behavior of non-structural sandwich lightweight panels against wind and seismic loadings is not well understood. Such panels are typically composed of two calcium-silicate board (CSB) wythes separated by a core infill made of expanded polystyrene lightweight concrete (LWC). This paper provides understanding regarding the mechanisms of panel connectivity by tongue-and-groove, including transfer of forces through steel dowels to the existing structure. Series of LWC mixtures having 470 to 975 kg/m3 density were tested using suitably established testing procedures to determine the flexural strength, core shear strength, interfacial bond between LWC and CSB, and pullout forces transmitted through the steel dowels. Special emphasis is placed on modeling the effect of wind and seismic loads on a 3.6 × 3 m2 partition wall constructed using LWC sandwich panels. The highest stresses and deformations generated from the model are compared to those determined experimentally, which allowed establishing different charts that can predict the safety factors as a function of LWC density and type/magnitude of the lateral loading applied.

Exploring XLPE-Concrete as a novel sustainable construction material

Gérard-Philippe Zéhil and Daisy Saba

AIP Conference Proceedings

Submitted manuscript, 2018

Abstract

Cross-linked polyethylene (XLPE) is extensively used but complex to recycle. Alternative methods are thus needed to better recycle XLPE waste. It is also well known that the extraction of natural aggregates for the production of concrete has negative impacts on the environment. It is therefore here proposed to replace 5%, 10%, 15% and 20% of the medium sized aggregates in structural concrete by XLPE waste shreds. The mechanical and physical properties of XLPE-Concrete are investigated and compared to those of a control mix without XLPE inclusions. As the XLPE replacement ratio increases, the unit weight of concrete decreases, the amount of superplasticizer needed to maintain the same level of concrete workability increases and the modulus of elasticity tends to decrease. The tensile strength increases with the XLPE content, up to a replacement ratio of 15 %, and then starts to drop. The compressive strength also increases initially, for a replacement ratio of 5%, and then decreases gradually as the XLPE content is further augmented. Nevertheless, all XLPE-Concrete mixes maintain a level of strength that is compatible with structural applications.

A combined analytical and computational approach to the structural behavior of composite tubes

Gérard-Philippe Zéhil

IEEE Xplore Digital Library, 2016

Submitted manuscript, 2016

Abstract

In this paper, we touch on the mechanical behavior of casing-infill composite tubes, as potential new lightweight structural elements. The axial and the torsional behavior of composite tubes of circular cross-section, comprising a casing and an infill, is addressed analytically, to develop physical insight into the mechanical interactions between these two components, in the linear range. The influence of the material parameters of the infill, relative to those of the casing, and of geometry – such as the ratio of wall-thickness to diameter of the casing – on the structural stiffness and capacity of the composite are revealed and presented in a novel graphical form. It is shown that significantly improved overall stiffness and capacity at yield can be obtained by bonding a moderately softer, but highly auxetic, infill to the casing, which highlights the need to develop new lightweight auxetic materials, without compromising their stiffness. These predictions are verified in torsion and confirmed in bending by means of suitable computational models based on the finite element method.

Computer-aided measurements of the electrical resistivity fields in concrete mixtures with and without polyethylene terephthalate

Naji Khoury, Yara Maalouf, Sophia Ghanimeh and Gérard-Philippe Zéhil

IEEE Xplore Digital Library, 2016

Submitted manuscript, 2016

Abstract

This study aims at determining the electrical resistivity and mechanical properties of concrete mixes with and without polyethylene terephthalate (PET). 2-D Electrical Resistivity Tomographies (ERTs) were obtained using 14 electrodes spread on the surface of the concrete cylinder. AGI Supersting R1/IP was used for data acquisition and data were processed by an iterative finite element based inversion algorithm with EarthImager 2D. 1D testing was also performed on concrete cylinders with two stainless steel plates located at the ends acting as a source and a receiver. ERTs and 1D results showed that the electrical resistivity of concrete was lower in specimens with PET than in those without PET. After ERT testing, compressive strength tests were conducted on specimens. Findings revealed an increase in the compressive strength up to a PET content of 5% then a decrease up to a PET content of 20%. Findings also showed that the inclusion of PET increased the probability of corrosive action in PET concrete.