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\DOI{10.5802/crchim.330}
\datereceived{2024-02-24}
\daterevised{2024-05-10}
\datererevised{2024-07-03}
\dateaccepted{2024-08-26}
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%\makeatletter
%\def\TITREspecial{\relax}
%\def\cdr@specialtitle@english{Materials and Energy Valorization of Biomass and Waste : The Path for Sustainability and Circular Economy Promotion}
%\def\cdr@specialtitle@french{Valorisation \'energ\'etique et mati\`ere de la biomasse et des d\'echets : la voie de la durabilit\'e et de la promotion de l'\'economie circulaire}
%\makeatother

\CDRsetmeta{articletype}{research-article}

\title{Advancing energy efficiency: thermomechanical characterization
of plastic-infused concrete for sustainable building solutions}

\alttitle{Avancement de l'efficacit\'{e} \'{e}nerg\'{e}tique :
Caract\'{e}risation thermom\'{e}canique du b\'{e}ton infus\'{e} de
plastique pour des solutions de construction durables}

\author{\firstname{Mohammed Javeed} \lastname{Siddique}\CDRorcid{0000-0002-4465-7886}}
\address{School of Civil Engineering, Vellore Institute of Technology, Vellore, India}
\address{Civil and Environmental Engineering Department, A'Sharqiyah University, Ibra, Oman}
\email[M. J.Siddique]{mohammed.javeed@asu.edu.om}

\author{\firstname{Punitha Kumar} \lastname{Akhas}\CDRorcid{0000-0003-0248-7273}\IsCorresp}
\addressSameAs{1}{School of Civil Engineering, Vellore Institute of Technology, Vellore, India}
\email[A. Punitha Kumar]{punithakumar.a@vit.ac.in}

\begin{abstract}
Global energy demands within buildings are escalating, driven by
increased dependence on energy-intensive systems such as air
conditioning. This surge necessitates innovative solutions to reduce
energy consumption and promote sustainability in construction
practices. Simultaneously, the persistent problem of plastic waste
presents an environmental challenge, with recycling efforts lagging
behind the rapidly growing volumes of discarded materials. This study
addresses both issues by investigating the feasibility of using
recycled plastic materials---low-density polyethylene (LDPE), 
polypropylene (PP),
polyester, and high-density polyethylene (HDPE)---as partial
sand substitutes in concrete for roofing applications. The objectives
of this research include reducing air conditioning costs, decreasing
carbon emissions, and shortening the payback period for
energy-efficient building practices. Specifically, buildings situated
in the hot--dry climate of Muscat, Oman, and the composite climate of New
Delhi, India, were studied. The evaluation of the modified concrete's
mechanical and thermophysical properties indicates that the
incorporation of 30 wt\% LDPE yields the most
significant economic and environmental benefits. This modification
leads to the highest annual savings, calculated at
0.9946 \$/m\up{2} in Muscat and
0.9928 \$/m\up{2} in New Delhi, along with a marked
reduction in carbon emissions---19 kg/kWh in Oman and 18.2 kg/kWh in
New Delhi. Additionally, the use of 10 wt\% HDPE 
results in the fastest payback, recorded at 1.37 years.
\end{abstract}


\begin{altabstract}
{Les demandes \'{e}nerg\'{e}tiques mondiales dans les b\^{a}timents
sont en augmentation, stimul\'{e}es par la d\'{e}pendance accrue aux
syst\`{e}mes \'{e}nergivores tels que la climatisation. Cette
mont\'{e}e n\'{e}cessite des solutions innovantes pour r\'{e}duire la
consommation d'\'{e}nergie et promouvoir la durabilit\'{e} dans les
pratiques de construction. Parall\`{e}lement, le probl\`{e}me
persistant des d\'{e}chets plastiques pr\'{e}sente un d\'{e}fi
environnemental, les efforts de recyclage ne suivant pas le volume
croissant de mat\'{e}riaux jet\'{e}s. Cette \'{e}tude aborde ces deux
probl\`{e}mes en explorant la faisabilit\'{e} d'utiliser des
mat\'{e}riaux plastiques recycl\'{e}s---poly\'{e}thyl\`{e}ne
basse densit\'{e}, polypropyl\`{e}ne (LDPE), polyester et
poly\'{e}thyl\`{e}ne haute densit\'{e} (HDPE)---comme
substituts partiels du sable dans le b\'{e}ton pour des applications de
toiture. Les objectifs de cette recherche incluent la r\'{e}duction des
co\^{u}ts de climatisation, la diminution des \'{e}missions de carbone
et la r\'{e}duction de la p\'{e}riode de retour sur investissement pour
les pratiques de construction \'{e}co\'{e}nerg\'{e}tiques. Plus
pr\'{e}cis\'{e}ment, des b\^{a}timents situ\'{e}s dans le climat chaud
et sec de Mascate, Oman et le climat composite de New Delhi, Inde ont
\'{e}t\'{e} \'{e}tudi\'{e}s. L'\'{e}valuation des propri\'{e}t\'{e}s
m\'{e}caniques et thermophysiques du b\'{e}ton modifi\'{e} indique que
l'incorporation de LDPE \`{a} une proportion de poids de 30\% offre les
avantages \'{e}conomiques et environnementaux les plus significatifs.
Cette modification conduit aux \'{e}conomies annuelles les plus
\'{e}lev\'{e}es, calcul\'{e}es \`{a} 0,9946 \$/m}\up{2}
{\`{a} Mascate et 0,9928 \$/m}\up{2} {\`{a} New Delhi,
ainsi qu'\`{a} une r\'{e}duction notable des \'{e}missions de
carbone---19 kg/kWh \`{a} Oman et 18,2 kg/kWh \`{a} New
Delhi. De plus, l'utilisation de HDPE \`{a} une proportion de poids de
10\% r\'{e}sulte dans la p\'{e}riode de retour sur investissement la
plus rapide, enregistr\'{e}e \`{a} 1,37 ans.}
\end{altabstract}

\keywords{\kwd{Sustainable building materials}
\kwd{HVAC energy load}
\kwd{Carbon mitigation}
\kwd{Waste plastics in construction}
\kwd{Hot--dry and composite climates}}

\altkeywords{\kwd{Mat\'{e}riaux de construction durables}
\kwd{Charge \'{e}nerg\'{e}tique des syst\`{e}mes HVAC}
\kwd{Att\'{e}nuation du carbone}
\kwd{Utilisation des plastiques usag\'{e}s dans la construction}
\kwd{Climats chauds et secs et composites}}
\begin{DefTralics}
\newcommand{\up}[1]{\textsuperscript{#1}}
\end{DefTralics}

\dateposted{2024-11-22}
\begin{document}
\maketitle

\twocolumngrid

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{
\section{Introduction}

The phenomena of urbanization and increasing populations are two
pivotal factors shaping the contemporary world. Earth's clay and sand
serve as essential raw materials in the construction industry; however,
their extensive use may result in the depletion of vital resources.
Consequently, researchers are increasingly motivated to explore both
innovative materials and recycling strategies for waste generated
across various industries~\cite{1}. Plastics are ubiquitously present
in the modern world, and their degradation, particularly in materials
like polypropylene (PP), polycarbonate, and PVC, can take up to
500 years~\cite{2}. The Environmental Protection Agency estimates that
only approximately 7\% of plastic waste is currently recycled
annually~\cite{3}. Utilizing plastics in brick production not only
mitigates plastic pollution but also yields both environmental and
economic benefits~\cite{4}. India and China are dominant forces in the
brick manufacturing industry, with their combined annual production
surpassing 240 billion bricks~\cite{5}. The traditional approach to
brick manufacturing, reliant predominantly on sand and cement,
significantly contributes to carbon emissions and depletes crucial
natural resources~\cite{6}. In contrast, the incorporation of plastic
materials into brick production is gaining recognition as a sustainable
and eco-friendly alternative in civil engineering~\cite{7}. A study was
conducted that employed glass powder and PP particles in the
manufacture of fired clay bricks. The results indicated that bricks
incorporating waste glass demonstrated superior mechanical properties,
as evidenced by compression tests when compared to bricks utilizing
plastic waste~\cite{8}. Plastic, a material with both detrimental and
beneficial attributes, poses challenges to long-term sustainability
efforts due to its resistance to biodegradation. In light of this,
studies have investigated the use of thermoplastic materials such as
high-density polyethylene (HDPE) and PP for fabricating
bricks through physical recycling techniques. One such study showed
that bricks made from HDPE displayed a compressive strength of 11.19
N/mm$^{2}$, exceeding the 10.5 N/mm$^{2}$ strength found in
high-quality traditional bricks~\cite{9}. Further research has
broadened the scope to include the utilization of plastic waste and
other thermoplastic materials in construction applications. For
example, Balasubramanian et~al.\ examined the effects of\break replacing
electronic waste in concrete at various proportions (10\%, 20\%, and
30\%), concluding that a 20\% replacement yielded optimal
results~\cite{10}. Similarly, Muthu et~al.\ found that the maximum
compressive strength achieved for blocks made from e-waste plastic was
20.48 N/mm$^{2}$, outperforming both sand-based and fly-ash
blocks~\cite{11}. Few researchers have concentrated on evaluating the
impact of both polymer and nonpolymer admixtures on concrete roofing.
These studies primarily examine the prospective advantages of
minimizing air conditioning costs and mitigating carbon
emissions~\cite{12}. Ibrahim et~al.\ employed polyethylene terephthalate
(PET) as a \mbox{partial} replacement for sand in a series of six concrete
mixtures, with substitution ratios ranging from 0\% to 50\%. The
experimental results showed that the inclusion of a sand substitute
influenced the mechanical properties of the building materials at
varying levels~\cite{13}. The research by Pratap et~al.\ demonstrated
that substituting 3\% of sand with waste PET led to an optimal
compressive strength of 36.66 N/mm$^{2}$, surpassing that of
conventional mixtures~\cite{14}. Similarly, Prashanth et~al.\ 
incorporated burned solid waste and recycled plastic waste 
as alternative materials in concrete, proving its practicality in
creating more cost-effective construction materials and significantly
reducing building costs~\cite{15}. Vivek et~al.\ conducted an extensive
study to explore the properties of concrete blocks made with a partial
replacement of fine aggregate using low-density polyethylene (LDPE).
The outcomes of this study suggest that the integration of plastic
elements not only aids in conserving invaluable natural resources but
also acts as an effective strategy in mitigating the proliferation of
plastic waste in the environment~\cite{16}. In a similar vein, Ali 
et~al.\ undertook research on the recycling of polyethylene and
PP waste, targeting the production of plastic blocks. Their
findings indicate that the use of recycled plastic waste as a cement
substitute in the production of plastic-reinforced blocks provides a
viable and green, eco-friendly alternative~\cite{17}. This approach not
only mitigates the accumulation of plastic waste but also yields
sustainable and economically viable building materials. Idrees et~al.\ 
focused on the integration of eco-friendly methodologies in the
fabrication of low shrinkage fired clay blocks, achieved through the
incorporation of recycled plastic dust as a primary component. The
studies indicated that the compressive strength decreased as the
proportion of\break recycled plastic dust increased~\cite{18}. Such an
approach confers advantages like reducing the consumption of natural
soil resources and enabling the production of bricks with improved
weight and volumetric stability. Arya et~al.\ investigated the
replacement of conventional fine aggregates with a blend of e-waste,
specifically keyboards, and other discarded materials such as
eggshells, fly ash, and silica fumes. Preliminary results indicate that
these alternative materials show promise in both mitigating
environmental impact and enhancing \mbox{sustainable} \mbox{construction}
practices~\cite{19}. Modifications in the \mbox{mechanical} and functional
attributes of concrete pavement blocks were observed when sand was
substituted with plastic; a 4\% replacement of sand by plastic led to
an improvement in compressive strength. However, as the proportion of
plastic increased, a corresponding decline in structural integrity was
noted~\cite{20}. Haque et~al.\ conducted a study focusing on the use of
recycled PET plastic bottles in the fabrication of bricks~\cite{21}.
The prevailing literature largely emphasizes the critical examination
of compressive strength while also considering factors such as reducing
cooling expenses, lowering carbon emissions, and evaluating the
significance of the payback period (PBP)~\cite{22}. A few studies have delved
into the thermal transmission characteristics of insulation materials
in construction, finding that smaller surface factors in these
materials result in a slower response to short-wave radiation, while
larger surface factors lead to time delays~\cite{23}. Saboor et~al.\ 
primarily examined the conversion of waste materials such as rice husk,
sawdust, coir pith, and fly ash into viable construction materials.
Their findings revealed that walls constructed from rice husk mud
bricks offer the highest reduction in air conditioning costs, amounting
to 1.74~\dollar/m$^{2}$, along with an acceptable PBP of 1.17
years and higher annual carbon emission mitigation values of 
33.35~kg/kWh~\cite{24}. Few studies have focused on challenges associated
with moisture-induced damage such as rutting resistance
absorption~\cite{25} and wetness sensitivity~\cite{26}. A study on
obsidian mortar, using waste brick powder and reinforced with
PP fibers, demonstrated that criteria for rutting resistance
and wetness sensitivity were satisfied at a temperature of\break
25~\textdegree C~\cite{27}.\looseness=1

An extensive review of the existing literature highlights a significant gap
in research on the use of plastic waste to improve the structural and
thermal\break properties of concrete roofing. This gap extends to the
potential of these materials to reduce air conditioning costs and
shorten the PBP for buildings. This study employs an
innovative approach by leveraging the dynamic thermal properties of
building materials to decrease energy consumption associated with air
conditioning. It investigates the cost-saving benefits of concrete
samples incorporating various types of plastics such as LDPE, PP,
HDPE, and polyester. The research also assesses the reduction in carbon
emissions and determines the PBPs for the modified concrete
samples. This investigation contributes to the development of
environmentally sustainable building materials by integrating recycled
plastics into concrete.

\section{Materials and methods} 

\subsection{Materials}

The production of concrete samples involves mixing fine aggregate,
coarse aggregate, cement, and water. The inclusion of recycled plastic
scraps is anticipated to enhance the thermal properties of the
resultant material, based on observations made during this study.
Utilizing concrete that incorporates waste plastics is economically
viable as it reduces the demand for virgin resources, which can be
costly to produce and transport. In this study, four types of plastic
materials---LDPE, HDPE, PP, and polyester---are considered as
potential substitutes for sand in the mix. Figure~\ref{fig1} illustrates the
plastic materials used. Ordinary Portland cement of grade 53 was
utilized in accordance with IS 12269: 2013. The mix design for
M30-grade concrete was formulated following the guidelines set forth in
IS 10262. Subsequently, three distinct concrete specimens for each
type of plastic were prepared (Figure~\ref{fig2}); the specimens had dimensions of
100 ${\times}$ 100 ${\times}$ 100~mm. After a 24~h setting period,
the samples were carefully demolded to maintain their structural
integrity. These specimens then underwent a rigorous 28 day
water-curing phase. The roofing concrete generally consists of a
0.15~m thick reinforced cement concrete covered with 
0.015~m of cement plaster on both its top and bottom surfaces.

\begin{figure*}
\includegraphics{fig01}
\caption{\label{fig1}(a) Low-density polyethylene, (b) high-density
polyethylene, (c) polypropylene, and (d)~polyester.}
\vspace*{1em}
\end{figure*}

\begin{figure*}
\includegraphics{fig02}
\caption{\label{fig2}Preparation of concrete samples.}
\end{figure*}

Tables~\ref{tab1} and~\ref{tab2} furnish detailed information on the physical
and chemical composition of the cement. Crushed aggregate with a
particle size of 20~mm was used for the concrete casting, meeting the
specifications set forth in IS 383. The aggregate has a density
 of 2.78 {g/cm$^{3}$,} a water absorption rate of 0.83\%, and a
fineness modulus of 7.22. Density was ascertained by measuring the mass
and volume of the material~\cite{28}. The fine aggregate used comprises
natural river sand with a maximum particle size of 2.36~mm. The
specified plastic materials---LDPE, PP, polyester, and HDPE---are
integrated into the concrete samples at varying percentages of 10,
20, and 30, substituting sand to investigate their thermophysical
properties. These plastic materials are reduced to a finely sized,
rounded powder form with an average particle size ranging from 0.7 to 
1~mm, using a pulverizer. Two distinct climatic zones, Muscat and New
Delhi, were selected based on the American Society of Heating,
Refrigerating and Air-Conditioning Engineers (ASHRAE) meteorological
data~\cite{29}. Thermoeconomic analysis was conducted, focusing on key
parameters such as annual savings in air conditioning and electricity
expenses, cost benefits over time, PBPs, and carbon
emission reduction~\cite{30}. Figure~\ref{fig3} displays the monthly
degree-hours for cooling and heating in both Muscat and~New~Delhi.
\mbox{Thermophysical} properties of the modified concrete samples were then
measured at room\break temperature.}

\begin{table}
\caption{\label{tab1}Percentage oxide composition of\newline cement}
\begin{tabular}{cc}
\thead
Composition &amp; \% \\ 
\endthead
SiO$_{2}$ &amp; 20.72 \\ 
Al$_{2}$O$_{3}$ &amp; \04.88 \\ 
Fe$_{2}$O$_{3}$ &amp; \02.95 \\ 
CaO &amp; 61.83 \\ 
MgO &amp; \01.39 \\ 
Na$_{2}$O &amp; \00.19 \\ 
K$_{2}$O &amp; \00.67 \\ 
SO$_{3}$ &amp; \02.33 \\ 
LOI &amp; \03.17
\botline
\end{tabular}
\end{table}

\begin{table}
\caption{\label{tab2}Properties of cement}
\tabcolsep=3pt
\begin{tabular}{cccc}
\thead
Material  &amp; Color &amp; Blaine fineness &amp; Density \\ 
\endthead
Cement &amp; Gray &amp; 351~m$^{2}$/kg &amp; 3.15 {g/cm$^{3}$}
\botline
\end{tabular}
\end{table}

\begin{figure*}
{\vspace*{-4pt}}
\includegraphics{fig03}
{\vspace*{-6pt}}
\caption{\label{fig3}Monthly degree-hours for cooling and heating of
Muscat and New Delhi.}
{\vspace*{-6pt}}
\end{figure*}

\subsection{Analytical method}

The admittance method is utilized to compute transient thermal
transmittance using matrix algebra, as described in the Chartered
Institution of Building Services Engineers guide (CIBSE, 2006). Mathematical
calculations for concrete roofing materials were performed, ensuring
that the program variance remains below 1\%~\cite{31,32}. The
heat transfer properties were analyzed using the one-dimensional heat
diffusion equation to determine their effectiveness in \mbox{various} concrete
compositions:
{\begin{equation}\label{eq1}
\frac{\partial ^{2}T}{\partial {V}^{2}}=\frac{1}{\alpha }
\frac{\partial {T}}{\partial \tau}
\end{equation}}\unskip
The boundary condition at the roof's inner surface: 
{\begin{equation}\label{eq2}
\left(\frac{\partial {T}}{\partial V}\right)_{{V}=0}=
h_{\mathrm{i}}[{T}_{V=0}(\tau )-{T}_{\mathrm{i}}]
\end{equation}}\unskip
The boundary condition at the roof's outer surface: 
{\begin{equation}\label{eq3}
\left(\frac{\partial {T}}{\partial V}\right)_{V=L}=
h_{\mathrm{o}}[{T}_{\mathrm{s}}(\tau )-{T}_{V=L}(\tau )]
\end{equation}}\unskip
Here, 
{{${T}_{V = 0} $}}
represents the inner surface temperature of the
roof/wall,  
{{${T}_{V = L}$}}
represents the outer surface temperature of
the roof/wall, ${T}_{\mathrm{i}}$ is the indoor temperature, and  $T_{\mathrm{s}}$ is the
sol-air temperature. According to CIBSE guidelines, the internal and
external heat transfer coefficients for roofs are 10 and 25~W${\cdot}$m$^{-2}{\cdot}$K$^{-1}$,
respectively.
{\begin{eqnarray}\label{eq4}
{T} &amp;=&amp; [M\sinh (Vl+\mathrm{j}Vl)+N\cosh (Vl+\mathrm{j}Vl)]\nonumber\\
&amp;&amp;\qquad \times\, \exp(\mathrm{j}2\rmpi \tau /P)
\end{eqnarray}}\unskip
where $V =\sqrt{\rmpi \rho c_{p}/ks}$ 

\noindent $p$~$=$~period

\noindent $l$~$=$~finite thickness

\noindent $M$~$=$~heat flux

\noindent $N$~$=$~thermal resistance.
{\begin{equation}\label{eq5}
\left[\begin{array}{c}
T_{\mathrm{i}}\\
q_{\mathrm{i}}
\end{array}\right]
=\left[\begin{array}{cc}
\cosh ({c}+\mathrm{j}c) &amp; (\sinh ({c}+\mathrm{j}c))/b\\
b\sinh ({c}+\mathrm{j}c) &amp; \cosh ({c}+\mathrm{j}c)
\end{array}\right]\left[\begin{array}{c}
{T}_{\mathrm{o}}\\
q_{\mathrm{o}}
\end{array}\right]
\end{equation}}\unskip
where cyclic thickness $(c) = Vl$

\noindent \textit{T} $=$ periodic temperature 

\noindent \textit{q} $=$ periodic heat flux

\noindent slab admittance $(b) = \sqrt{\mathrm{j}} 2\rmpi k\rho c_{p}/s$.
{\begin{equation}\label{eq6}
\left[\begin{array}{cc}
r_{1} &amp; r_{2}\\
r_{3} &amp; r_{1}
\end{array}\right]=\left[\begin{array}{cc}
{B}+\mathrm{j}C &amp; ({D}+\mathrm{j}E)/b\\
b(-{E}+\mathrm{j}D) &amp; {B}+\mathrm{j}C
\end{array}\right] 
\end{equation}}\unskip
\mbox{}
{$$
\begin{array}{ll}
B=\cosh c\cos c  \\
C = \sinh c\sin c \\ 
D = (\cosh c\sin c+\sinh c\cos c)(1/\sqrt{2}) \\
E = (\cosh c\sin c-\sinh c\cos c)(1/\sqrt{2}) 
\end{array}
$$}\unskip
The surface resistance is given by
{\begin{eqnarray}
&amp;\hspace*{-1em}R_{\mathrm{si}}=\left[\begin{array}{cc}
1 &amp; -1/h_{\mathrm{i}}\\
0 &amp; 1
\end{array}\right]\quad  R_{\mathrm{so}}=\left[\begin{array}{cc}
1 &amp; -1/h_{\mathrm{o}}\\
0 &amp; 1
\end{array}\right]\label{eq7}\Seqnsplit
&amp;\hspace*{-1em}\left[\begin{array}{c}
T_{\mathrm{i}}\\
q_{\mathrm{i}}
\end{array}\right]
=\left[\begin{array}{cc}
1 &amp; -(1/h_{\mathrm{i}})\\
0 &amp; 1
\end{array}\right]
\left[\begin{array}{cc}
r_{1} &amp; r_{2}\\
r_{3} &amp; r_{1}
\end{array}\right]
\left[\begin{array}{cc}
1 &amp; -(1/h_{\mathrm{o}})\\
0 &amp; 1
\end{array}\right]
\left[\begin{array}{c}
T_{\mathrm{o}}\\
q_{\mathrm{o}}
\end{array}\right]\label{eq8}\Seqnsplit
&amp;\hspace*{-1em}\left[\begin{array}{c}
T_{\mathrm{i}}\\
q_{\mathrm{i}}
\end{array}\right]
%\end{array}
=\left[\begin{array}{cc}
A_{1} &amp; A_{2}\\
A_{3} &amp; A_{1}
\end{array}\right]
\left[\begin{array}{c}
T_{\mathrm{o}}\\
q_{\mathrm{o}}
\end{array}\right]\label{eq9}
\end{eqnarray}}\unskip
Unsteady thermal transmittance (U) considers both thermal mass and
thermal insulation components. It can be mathematically calculated
using Equation~(\ref{eq10}): 
{\begin{equation}\label{eq10}
U = \left| -\frac{1}{A_{2}}\right|
\end{equation}}\unskip

A MATLAB program has been formulated and subsequently verified by
conducting a thorough assessment using the information provided in the
CIBSE guidelines~\cite{35}. The convective heat transfer coefficient
for the external surface ($h_{\mathrm{o}}$) is determined to be
25 W${\cdot}$m$^{-2}{\cdot}$K$^{-1}$, whereas the convective heat transfer coefficients for the
internal surfaces ($h_{\mathrm{i}}$) are established as 7.7 and 10
W${\cdot}$m$^{-2}{\cdot}$K$^{-1}$ for walls and roofs, respectively, in accordance with the
guidelines set forth by the CIBSE (2006).

\subsection{Energy-economic analysis}

Heating degree-hours (HDH) and cooling degree-hours (CDH) serve as
essential metrics for quantifying the annual energy requirements for
heating and cooling operations, respectively. In the context of 
thermal engineering,
the performance of mechanical systems is fundamentally tied to a base
temperature that maintains thermal comfort within the boundaries of a
structure. The sol-air temperature is the temperature that gives the
combined effect of outdoor temperature distribution and incident solar
radiation. Energy-economic analyses are utilized to obtain key metrics
such as payback times, carbon emission reductions, and savings in air
conditioning costs~\cite{36,37}. The monthly degree-hours for
Muscat (23.6086~\textdegree  N, 58.9754~\textdegree  E) and New Delhi
(28.57~\textdegree  N, 77.12~\textdegree  E) are also documented in this
study. 

The HDH and CDH can be calculated using {Equations}
(\ref{eq11}) and (\ref{eq12}), respectively:
{\begin{eqnarray}
\mathrm{HDH} &amp;=&amp; \sum _{1}^{\mathrm{HD}}(T_{\mathrm{b}}-T_{\mathrm{s}})=N_{\mathrm{HD}}\rmDelta T\label{eq11}\Seqnsplit
\mathrm{CDH} &amp;=&amp; \sum _{1}^{\mathrm{CD}}(T_{\mathrm{s}}-T_{\mathrm{b}})=N_{\mathrm{{CD}}}\rmDelta T\label{eq12}
\end{eqnarray}}\unskip
Here, $N_{\mathrm{HD}}$ and $N_{{\mathrm{CD}}}$ represent the number of heating and
cooling hours, respectively. According to ASHRAE standards, the minimum
base temperatures ($T_{\mathrm{b}}$) of 18~{\textdegree}C and
23.3~{\textdegree}C for the winter (HDH) and summer (CDH) months have
been considered,\break respectively.

Heating cost savings (HCS) and cooling cost savings (CCS) collectively
constitute the overall A/C cost reduction ($C_{\mathrm{s}}$). 
The cost of electricity ($C_{\mathrm{el}}$) and the cost of natural gas
($C_{\mathrm{ng}}$) have been considered as per the local rates in New
Delhi and Muscat.

{For New Delhi}, $C_{\mathrm{el}} = 0.082$ \dollar/kWh and
$C_{\mathrm{ng}} = 0.014$~\dollar/kWh.

{For Muscat}, $C_{\mathrm{el}} = 0.026$ \dollar/kWh and
$C_{\mathrm{ng}} = 0.01$~\dollar/kWh.
{\begin{eqnarray}
&amp; \displaystyle \mathrm{HCS}=\frac{0.001\cdot\rmDelta U\cdot\mathrm{HDH}\cdot C_{\mathrm{ng}}}{\eta}\label{eq13}\Seqnsplit
&amp; \displaystyle \mathrm{CCS}=\frac{0.001\cdot\rmDelta U\cdot\mathrm{CDH}\cdot C_{\mathrm{el}}}{\mathrm{COP}} \label{eq14}
\end{eqnarray}}\unskip
${\rmDelta}U$~$=$~change in transmittance

\noindent ${C}_{\mathrm{el}}$~$=$~cost of electricity (\dollar/kWh)

\noindent ${C}_{\mathrm{ng}}$~$=$~cost of natural gas (\dollar/kWh).

The amount of time needed to recover the cost of the original building
supplies is referred to as the payback period.  Equation (\ref{eq15})
shows the relation between total A/C cost reduction and the insulation
cost.
{\begin{equation}
\label{eq15}
\mathrm{PBP}=\frac{C_{\mathrm{i}}}{C_{\mathrm{s}}}
\end{equation}}\unskip
where ${C}_{\mathrm{i}}$ is the insulating material cost (\dollar/kg) and
$C_{\mathrm{s}}$ is the air conditioning cost reduction (\dollar).

Yearly carbon reduction, which is calculated using Equations
(\ref{eq16}) and (\ref{eq17}), represents the total amount of carbon
reduction caused by heating and cooling energy savings (COP $=$ 2.5,
$\eta  = 0.8$).
{\begin{eqnarray}
\mathrm{HCM}=\frac{0.001\cdot \rmDelta U\cdot \mathrm{HDH}\cdot m_{\mathrm{c}}}{\mathrm{COP}}\label{eq16}\Seqnsplit
\mathrm{CCM}=\frac{0.001\cdot \rmDelta U\cdot \mathrm{CDH}\cdot m_{\mathrm{n}}}{\eta}\label{eq17}
\end{eqnarray}}\unskip
$m_{\mathrm{c}}$: amount of CO$_{2}$ released for each unit of electricity
(kg/kWh)

\noindent $m_{\mathrm{n}}$: amount of CO$_{2}$ released for each unit of natural
gas (kg/kWh).

\subsection{Experimental methodology}

{After the completion of water curing and air-drying procedures, a
series of tests were conducted on both the standard and the 12 modified
concrete samples. The primary focus was on evaluating the
thermophysical attributes and compressive strength of these samples. A
compression testing machine, conforming to IS 516 standards, was
utilized for this assessment. The thermophysical properties, including
thermal conductivity and specific heat, were measured using the hot
wire probe technique via a KD2 Pro device (Figure~\ref{fig4}). Two
holes, each 1.3~mm in diameter and 30~mm in length, were drilled to
accommodate probes specifically designed for measuring these
attributes. The thermophysical properties of the tested materials are
summarized in Table~\ref{tab3}.} Thermal conductivities of the modified
concrete samples were measured at room temperature, as shown in
Figure~\ref{fig4}. The measurements were conducted using the KD2 Pro
thermal property analyzer, which offers a precision range of ${\pm}$5
to ${\pm}$10\% for thermal conductivity and ${\pm}$10\% for specific
heat.

\begin{figure*}
\includegraphics{fig04}
\caption{\label{fig4}Evaluation of thermal properties using the KD2 Pro
thermal property analyzer.}
\vspace*{2.5pt}
\end{figure*}

\begin{table*}
\caption{\label{tab3} Thermophysical properties of materials}
\begin{tabular}{cccccc}  
\thead
S. No &amp; Material &amp; 
{$K$} (W${\cdot}$m$^{-1}{\cdot}$K$^{-1}$) &amp; $C_{p}$ (J${\cdot}$kg$^{-1}{\cdot}$K$^{-1}$) &amp;
${\uprho}$ (kg/m$^{3}$) &amp; \parbox[t]{2.5cm}{\centering Compressive
strength (MPa)}\vspace*{2pt}\\ 
\endthead
\01 &amp; Low-density polyethylene 10\% &amp; 0.568~${\pm}$~0.002 &amp; 1096.79~${\pm}$~3 &amp; 2126.2~${\pm}$~2 &amp; 18.36~${\pm}$~0.01 \\ 
\02 &amp; Low-density polyethylene 20\% &amp; 0.523~${\pm}$~0.003 &amp; 1227.47~${\pm}$~5 &amp; 2031~${\pm}$~4 &amp; 16.21~${\pm}$~0.04 \\ 
\03 &amp; Low-density polyethylene 30\% &amp; 0.48~${\pm}$~0.002 &amp; 1474.37~${\pm}$~4 &amp; 1943.2~${\pm}$~3 &amp; 12.4~${\pm}$~0.02 \\ 
\04 &amp; Polypropylene 10\% &amp; 0.956~${\pm}$~0.005 &amp; 881.97~${\pm}$~3 &amp; 2330~${\pm}$~4 &amp; 26.35~${\pm}$~0.01 \\ 
\05 &amp; Polypropylene 20\% &amp; 0.823~${\pm}$~0.004 &amp; 1034.12~${\pm}$~4 &amp; 2110~${\pm}$~5 &amp; 22.54~${\pm}$~0.02 \\ 
\06 &amp; Polypropylene 30\% &amp; 0.712~${\pm}$~0.002 &amp; 1070.50~${\pm}$~3 &amp; 2085~${\pm}$~2 &amp; 21.88~${\pm}$~0.03 \\ 
\07 &amp; High-density polyethylene 10\% &amp; 0.612~${\pm}$~0.005 &amp; 1106.16~${\pm}$~5 &amp; 2336~${\pm}$~4 &amp; 22.46~${\pm}$~0.02 \\ 
\08 &amp; High-density polyethylene 20\% &amp; 0.584~${\pm}$~0.004 &amp; 1281.18~${\pm}$~4 &amp; 2201.1~${\pm}$~3 &amp; 18.35~${\pm}$~0.01 \\ 
\09 &amp; High-density polyethylene 30\% &amp; 0.592~${\pm}$~0.003 &amp; 1331.81~${\pm}$~2 &amp; 2185~${\pm}$~2 &amp; 14.12~${\pm}$~0.04 \\ 
10 &amp; Polyester 10\% &amp; 0.991~${\pm}$~0.005 &amp; 942.02~${\pm}$~5 &amp; 2349.2~${\pm}$~5 &amp; 34.2~${\pm}$~0.05 \\ 
11 &amp; Polyester 20\% &amp; 0.949~${\pm}$~0.004 &amp; 985.96~${\pm}$~3 &amp; 2422~${\pm}$~3 &amp; 32.1~${\pm}$~0.03 \\ 
12 &amp; Polyester 30\% &amp; 0.931~${\pm}$~0.005 &amp; 1045.63~${\pm}$~5 &amp; 2458.8~${\pm}$~2 &amp; 28.6~${\pm}$~0.04 \\ 
13 &amp; Conventional concrete &amp; 1.42~${\pm}$~0.003 &amp; 1126~${\pm}$~6 &amp; 2482~${\pm}$~6 &amp; 36.1~${\pm}$~0.02
\botline
\end{tabular}
{\vspace*{-2pt}}
\end{table*}

\subsection{Measurement of compressive strength}

Compressive strength tests were conducted on specimens containing
varying proportions of plastic materials. These tests were performed
using a digital compression testing machine that conforms to IS 14858,
as shown in Figure~\ref{fig5}(a). 
Following the testing procedures outlined in
IS 516~\cite{38}, both conventional concrete samples and those
containing plastic additives were evaluated.  The assessment revealed
that conventional concrete exhibits a compressive strength of 36.1 MPa,
which serves as a benchmark for comparing the plastic-admixed samples.
The inclusion of plastic materials generally led to a reduction in
compressive strength, with values ranging from 12.4 to 34.2 MPa
depending on the type and proportion of plastic used.


\begin{figure*}
{\vspace*{-1pt}}
\includegraphics{fig05}
{\vspace*{-2pt}}
\caption{\label{fig5}(a) Digital compression testing machine, (b) 
low-density polyethylene, (c) polypropylene, (d)~high-density
polyethylene, and (e) polyester.}
{\vspace*{-3pt}}
\end{figure*}


The failure pattern of cubes of different mixes after the compression test
can be classified as nonexplosive failure. Such a pattern of failure
satisfied the failure modes in accordance with \mbox{BS EN 12390-3.} Through
visual inspection, it can also be seen that most of the cube specimens
showed the cracking to be at approximately 45\textdegree\ to the axis
near the ends.

\section{Results and discussion}

{In this section, we present a detailed analysis of the outcomes from
the experimental investigations into concrete samples modified with
recycled \mbox{plastics.} The discussion explores the impacts of these
modifications on the mechanical and thermophysical properties of
concrete, as well as the economic and environmental implications of
their use in sustainable construction. Initially, the morphological
characteristics of the modified concrete are examined to understand the
microstructural changes imparted by the inclusion of plastic additives.
This analysis is critical for interpreting the variations in
compressive strength and thermal conductivity observed. Subsequent
discussions focus on assessing the potential benefits in terms of
energy efficiency, specifically through reductions in air conditioning
costs and carbon emissions. These results are contextualized within the
broader framework of advancing sustainable building practices.}

\subsection{Scanning electron microscope}

The morphological characteristics of the concrete with various plastic
admixtures were scrutinized\break utilizing scanning electron microscope
(SEM) images at differing magnification levels as shown in Figure~\ref{fig6}.
The added admixtures, ranging in size from 0.7 to 0.9~mm, were
predominantly uniformly distributed throughout the concrete specimen.
The SEM images revealed that these plastic admixtures formed a seamless
layer within the concrete matrix, a phenomenon consistently observed
across all samples. Consequently, it can be concluded that the
incorporation of plastic admixtures did not exert a detrimental impact
on the morphological properties of the concrete. {The morphologies of the
admixture-based concrete samples and conventional concrete seem to be
similar when observed under an SEM.
The reduction in compressive strength of the samples having more
admixture content is attributed to the variation in  density when
compared to conventional concrete.} 

\begin{figure*}
{\vspace*{-2pt}}
\includegraphics{fig06}
{\vspace*{-3pt}}
\caption{\label{fig6}SEM images of various admixture-based concrete
samples with 10~wt\% of sand replaced by: (a)~LDPE, (b) polypropylene, 
(c) HDPE, and (d) polyester.}
\end{figure*}


\subsection{Yearly A/C cost savings}

The total air conditioning cost savings are derived from reductions in
both heating and cooling expenses. These savings have been achieved by
incorporating various types of waste plastics into concrete roof
structures. To assess the efficacy of these modifications, the air
conditioning costs for roofs made with the modified concrete are
compared against those for conventional concrete roofs. Figure~\ref{fig7}
illustrates the annual savings in air conditioning costs for two
distinct climatic zones: Muscat, characterized by a hot and dry
climate, and New Delhi, which experiences a composite climate. Among
all the range of modified concrete samples studied, the concrete roof
containing 30~wt\% LDPE exhibited the highest annual air
conditioning cost savings, registering values of 0.9946 \dollar/m$^{2}$ for
Muscat and 0.9928 \dollar/m$^{2}$ for New Delhi, respectively.

\begin{figure*}
\includegraphics{fig07}
\vspace*{-4pt}
\caption{\label{fig7}Annual air conditioning cost savings
(\protect\dollar/m$^{2}$).}
\vspace*{-2pt}
\end{figure*}

\begin{figure*}
\vspace*{-2pt}
\includegraphics{fig08}
\vspace*{-2pt}
\caption{\label{fig8}Annual carbon emission mitigation (kg/kWh).}
\vspace*{-2pt}
\end{figure*}

\subsection{Annual carbon emission mitigation}

Figure~\ref{fig8} exhibits the yearly reduction in carbon emissions
achieved by
different admixture-based concrete roofs, assessed in two separate
climate regions, namely Muscat and New Delhi. The annual mitigation of
carbon emissions represents the overall reduction, stemming from
combined savings in cooling and heating energy. This includes reduced
electricity and natural gas consumption, resulting in the total carbon
emission mitigation measured in kg/kWh. A modified concrete roof that
yields higher air conditioning savings also results in greater carbon
emission mitigation, thus affirming its environmental sustainability.
For the climate of Muscat, the addition of 
10~wt\%  polyester or PP
shows a reduction in carbon emissions of
3.05 kg/kWh and 3.79 kg/kWh, respectively. Likewise, the inclusion of
30~wt\% LDPE shows the highest carbon emission
reduction, measured at 19 kg/kWh. In the climatic conditions of New
Delhi, roofs with  10 wt\% PP and
polyester result in carbon emission reductions of 3.63 kg/kWh and 2.92
kg/kWh, \mbox{respectively.} Moreover,  30~wt\% LDPE shows
the highest carbon emission reduction, recorded at 18.2~kg/kWh.

\subsection{Payback period}

Figure~\ref{fig9} illustrates the PBPs associated with different
admixture-based concrete roofs, showing shorter durations for composite
climates such as New Delhi compared to Muscat's hot and dry conditions.
In Muscat, the roofs incorporating varying ratios of LDPE,
PP, polyester, and HDPE exhibited PBPs of 3.14,
5.3, 6.57, 7.33, 11.30, 10.32, 6.54, 9.9, 10.13, 1.36, 2.35, and 3.47
years, respectively. Remarkably, the roof with 10 wt\%
HDPE showed the shortest PBP of 1.37 years. Similarly, in
the composite climate of New Delhi, roofs with different proportions of
LDPE, PP, polyester, and HDPE had PBPs of 3.14,
5.32, 6.58, 7.34, 11.32, 10.33, 6.55, 9.92, 10.15, 1.37, 2.37, and 3.49
years, respectively. Again, the roof with  10~wt\% 
HDPE displayed the shortest PBP, measured at 1.37 years.
These observations underscore that the hierarchy of energy efficiency
remains consistent, irrespective of the specific climatic conditions. 

\begin{figure*}
\vspace*{-2pt}
\includegraphics{fig09}
\vspace*{-5pt}
\caption{\label{fig9}Payback period (years).}
\end{figure*}

\begin{figure*}
\includegraphics{fig10}
\vspace*{-5pt}
\caption{\label{fig10}\fontsize{9.8}{12}\selectfont
Compressive strength of the modified concrete
samples compared with conventional concrete.}
\end{figure*}

\subsection{Compressive strength}

Figure~\ref{fig10} shows the mean compressive strength of the modified concrete
specimens compared to the conventional block specimens. Conventional
\mbox{concrete} has a mean compressive strength of 36.1 MPa. It is observed
that the addition of LDPE, 
PP, polyester, and HDPE as a
percentage of sand replacement results in a decline in compressive
strength. However, with the addition of 
10~wt\% polyester, the compressive strength is nearly equal to that of
conventional concrete. 

In our analysis, it was observed that the inclusion of  30~wt\% LDPE
led to the most significant decrease in compressive
strength. This phenomenon can be attributed to the inherent properties
of LDPE, which has a lower density (915~kg/m$^{3}$) compared to
traditional sand and aggregate materials. The substitution of a
significant proportion of these heavier materials with lighter LDPE
results in a lower overall density of the concrete mix, which
consequently leads to reduced compressive strength. For reference, the
densities of other plastics used were as follows: polyester at 1500
kg/m$^{3}$, PP at 930 kg/m$^{3}$, and HDPE at 925
kg/m$^{3}$. The varying {densities} of these materials correlate with
their differing impacts on the structural integrity of the concrete.
\looseness=-1

\section{Conclusion}\label{mark-4.}

This study conducted a comprehensive thermoeconomic assessment of
concrete specimens that incorporated four distinct types of recycled
plastic materials---LDPE, PP, polyester, and HDPE. It was found that
these materials, when used as partial substitutes for sand,
significantly enhanced the thermal performance of the concrete compared
to conventional samples. The assessments were performed under the
specific climatic conditions of Muscat and New Delhi, showcasing
regional adaptability. The research explored various impacts of using
plastic in concrete, focusing on mechanical properties and
environmental implications. It was observed that incorporating plastics
influenced the compressive strength and density of the concrete blocks.

In summary, the following key observations were made based on the
research conducted:
\begin{itemize}
\item Among all the modified concrete samples incorporating LDPE in
various wt\%, it was found that 30 wt\% 
LDPE yielded the most substantial annual savings in air
conditioning costs. Specifically, the cost savings were calculated to
be \dollar0.9946 per square meter for Muscat and \dollar0.9928 per square meter
for New Delhi.

\item The analysis of the annual carbon emission mitigation indicates that
the samples with a {{30~wt\%}}
LDPE composition exhibited the most remarkable
reduction. The values were quantified as 19 kg/kWh for Muscat and 18.2
kg/kWh for New Delhi.

\item The PBP for each of the concrete samples was
evaluated, revealing that HDPE added at a 
{{10~wt\%}} replacement level for
sand had the shortest payback duration, amounting to 1.37 years for
both the Muscat and New Delhi climate conditions.

\item It was empirically determined that the incorporation of LDPE,
PP, polyester, and HDPE as a percentage of sand replacement
in the concrete samples led to a reduction in compressive strength.

\item Remarkably, the addition of 10~wt\% polyester
to the concrete mixture yielded a compressive strength almost
equivalent to that of conventional concrete.
\end{itemize}

These observations serve to underline the potential for incorporating
specific types of plastic waste materials as partial sand replacements
in concrete mixes to achieve both environmental sustainability and
economic viability.

\section{Limitations and future research}

\subsection{Limitations}

This study highlights the potential of plastic-infused concrete for
sustainable construction but recognizes several limitations. The
scalability of production and the economic feasibility of implementing
such \mbox{materials} on a larger scale were not explored. Additionally, the
environmental impacts, including toxicity and potential leaching
effects of the plastics used, were not thoroughly examined. Assessments
of the thermal stability of materials under various climatic \mbox{conditions}
were also absent, which are critical for validating their use in diverse
environments.

\subsection{Future research}

Future studies should expand the scope by exploring additional types of
waste plastics and conducting long-term performance evaluations under
different environmental conditions. It is also crucial to {investigate}
the environmental safety of these materials, particularly focusing on
their toxicity and leaching potential. Further research should aim to
validate the mechanical and thermal properties of plastic-infused
concrete across varied climatic conditions to enhance its application
in sustainable building practices.\looseness=-1

\section*{Nomenclature}
\mbox{}
\begin{inftab}
\fontsize{9.5}{11}\selectfont
\begin{tabular}{ll}
\tbody
\textit{A}$_{1}$, \textit{A}$_{2}$, \textit{A}$_{3}$ &amp;  \parbox[t]{5cm}{Variables showing input and output heat flow (-)}\\
{\textit{b}} &amp;  Slab admittance (W${\cdot}$m$^{-2}{\cdot}$K$^{-1}$)\\
{\textit{c}} &amp;  Cyclic thickness (m)\\
{\textit{C}}$_{\mathrm{el}}$ &amp;  Cost of electricity (\dollar/kWh)\\
{\textit{C}}$_{\mathrm{i}}$ &amp;  Insulating material cost (\dollar/kg)\\
{\textit{C}}$_{\mathrm{ng}}$ &amp;  Cost of natural gas (\dollar/kWh)\\
{\textit{C}}$_{p}$ &amp;  Specific heat capacity (kJ${\cdot}$kg$^{-1}{\cdot}$K$^{-1}$)\\
{\textit{C}}$_{\mathrm{s}}$ &amp;  Air conditioning cost reduction (\dollar)\\
\textit{h}$_{\mathrm{i}}$ &amp;  \parbox[t]{5.3cm}{Convective heat transfer coefficients for internal surfaces (W${\cdot}$m$^{-2}{\cdot}$K$^{-1}$)}\\
\textit{h}$_{\mathrm{o}}$ &amp;  \parbox[t]{5.3cm}{Convective heat transfer coefficient for the external surface (W${\cdot}$m$^{-2}{\cdot}$K$^{-1}$)}\\
$K$ &amp; Thermal conductivity (W${\cdot}$m$^{-1}{\cdot}$K$^{-1}$) \\
{\textit{l}}  &amp;  Finite thickness (m)\\
{\textit{M}} &amp;  Heat flux (W/m$^{2}$)\\
{\textit{m}}$_{\mathrm{c}}$ &amp;  \parbox[t]{5.3cm}{Amount of CO$_{2}$ {released for each unit of electricity} (kg/kWh)}\\
{\textit{m}}$_{\mathrm{n}}$ &amp;  \parbox[t]{5.3cm}{Amount of CO$_{2}$ {released for each unit of natural gas} (kg/kWh)}\\
{\textit{N}} &amp;  Thermal resistance (m$^{2}{\cdot}$K/W)\\
\textit{N}$_{{\mathrm{CD}}}$ &amp;   Number of cooling hours {(h)}\\
\textit{N}$_{\mathrm{HD}}$ &amp;   Number of heating hours {(h)}\\
{\textit{p}} &amp;  period (s)\\
{\textit{q}} &amp;  Periodic heat flux (W/m$^{2}$)\\
\textit{r}$_{1}$, \textit{r}$_{2}$, \textit{r}$_{3}$, \textit{r}$_{4}$ &amp;  Specific construction material (-)\\
{\textit{R}}$_{\mathrm{si}}$ &amp;  Internal surface resistance (m$^{2}{\cdot}$K)/W\\
{\textit{R}}$_{\mathrm{so}}$ &amp;  External surface resistance (m$^{2}{\cdot}$K)/W\\
{\textit{s}} &amp;  Time span (s)\\
{\textit{T}} &amp;  Periodic temperature ({\textdegree}C)\\
{\textit{T}}$_{\mathrm{b}}$ &amp;  Base temperature ({\textdegree}C)\\
$T_{\mathrm{i}}$ &amp;  Indoor temperature ({\textdegree}C)\\
{\textit{T}}$_{\mathrm{m}}$ &amp;  Mean daily temperature ({\textdegree}C)\\
{\textit{U}} &amp;  \parbox[t]{5cm}{Unsteady thermal transmittance use (kg/kWh)}\\
{\textit{V}} &amp;  Building material thickness (m)\\
$T_{\infty}$ &amp;  Outdoor air temperature ({\textdegree}C)
\end{tabular}
\end{inftab}

\section*{Greek letters}

\begin{inftab}
\begin{tabular}{ll}
\tbody
{${\alpha}$} &amp;  Thermal diffusivity (m$^{2}$/s)\\
{${\rmDelta}$}{\textit{U}} &amp;  Change in transmittance (W${\cdot}$m$^{-2}{\cdot}$K$^{-1}$)\\
{${\eta}$} &amp;  \parbox[t]{5cm}{Efficiency of natural gas power generation for heating (-)}\\
{${\rho}$} &amp;  Density (kg/m$^{3}$)\\
{${\tau}$} &amp;  Time variable (h)
\end{tabular}
\end{inftab}

\section*{Acronyms}

\begin{inftab}
\begin{tabular}{ll}
\tbody
CCM &amp; \parbox[t]{5cm}{yearly carbon reduction by cooling (kg/kWh)}\\
CCS &amp; cooling cost savings (\dollar/m$^{2}$)\\
CDH &amp; cooling degree-hours ({\textdegree}C-h)\\
COP &amp; coefficient of performance (-)\\
HCM &amp; \parbox[t]{5.5cm}{yearly carbon reduction by heating (kg/kWh)}\\
HDH &amp; heating degree-hours ({\textdegree}C-h)\\
HDPE &amp; high-density polyethylene\\
HCS &amp; heating cost savings (\dollar/m$^{2}$)\\
LDPE &amp; low-density polyethylene\\
%OPC &amp; ordinary Portland cement\\
PBP &amp; payback period (years)\\
{{PP}} &amp; polypropylene
\end{tabular}
\end{inftab}

\section*{Declaration of interests}

The authors do not work for, advise, own shares in, or receive funds
from any organization that could \mbox{benefit} from this article, and have
declared no affiliations other than their research organizations.

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