This study investigates the mechanical properties of lightweight polymer concrete (LWPC) containing four different polymer ratios (10%, 12%, 14%, and 16%) tested at three different temperatures ( /C015 /C176C, +5/C176C, and +25 /C176C) using destructive and non-destructive tests. In addition, a series of expressions are sug- gested to predict the splitting-tensile, ﬂexural and impact strength of LWPC based on the main parame-ters and compressive strength. The analysis of variance (ANOVA) method was also used to determinerelative contributions of the experimental parameters. The results of the destructive tests show that increasing the polymer ratio caused an increase in the compressive, splitting-tensile, and impact strengths, and energy absorption of LWPC. With decreasing the temperature from +25 /C176Ct o /C015 /C176C, the compressive, splitting-tensile, ﬂexural strengths, and elastic modulus (EM) increased, whereas the energy absorption, impact energy, and ductility decreased. The ﬁndings of this research provide beneﬁcial information toward understanding the behavior of LWPC and its safe use in engineering applicationswhere a material of high strength-to-weight ratio is required. Published by Elsevier Ltd. 1. Introduction Over the last decades, ﬁnding novel efﬁcient, cost-effective, and eco-friendly solutions on the development of new infrastructures has become of prime importance [1,2] . The requirement of having energy efﬁcient buildings necessitates the development of new materials and improvement of existing materials and lightweight structures [3–5] . Weight reduction is a key factor in the develop- ment of materials and components to use in many structural and construction practices such as bridge components, parking struc- tures, piers, and beams. Lightweight concretes (LWC) are widely used for such purposes. However, design of these structures pre- sents challenges, as they need to be light whilst being safe, durable and easy to maintain [6,7] .Due to the primary characteristics of LWC (i.e., adequate com- pressive strength, acceptable durability, low density and improved properties of thermal conductivity) its applications in structural applications allow the use of smaller size foundations and beam and column cross-sections. Likewise, the use of LWC in place of conventional concrete reduces the structural dead load, which leads to a reduction in seismic forces [8,9] . Structural LWC also beneﬁts from energy savings, as it possesses improved thermal conductivity [10–12] and sound insulation properties [13–16] . Other advantages of using LWC in structures are the ease of trans- portation and handling of precast units on site, leading to faster construction [17]. LWC with natural lightweight or artiﬁcially calcined aggregates is usually chosen for structural applications where its use will lead to a lower overall cost of construction than that would be expected with the use of normal-weight concrete. LWCs are typically weaker than normal weight concretes; however, several researches have https://doi.org/10.1016/j.conbuildmat.2020.119853 0950-0618/Published by Elsevier Ltd.⇑Corresponding author. E-mail address: kuj76@psu.edu (K. Jafari).Construction and Building Materials 260 (2020) 119853 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/lo cate/conbuildmat shown that the advantages of light-weight aggregates far outweigh the advantages of normal-weight aggregates such as high strength- to-density ratio, durability, ﬁre resistance, and good thermal per- formance [18]. The low compressive strength of LWC is mostly due to the porous nature of the lightweight aggregate and the resulting high void ratio of the concrete mix. Therefore, more cement paste is required for LWC to produce desired workability and strength, and reduction in capillary absorption, chloride con- tent, and sulfate expansion. The porous aggregate reduces the den- sity of the lightweight aggregate concrete; however, it easily fractures under stress. Hence, understanding the behavior of LWC under various environmental conditions would enable engineers to use it in structural applications. Many investigations have been conducted on the mechanical properties, thermal conductivity and sound insulation of light-weight cement concretes [3,19,20] . Sengul et al. [3]investigated the mechanical properties and thermal conductivity of LWC using expanded perlite. The results of the experimental program revealed that the compressive strength and elastic modulus (EM) decreased as the perlite content increased. Also, the thermal con- ductivity was improved with the use of perlite in mixes. Tian et al. [21] investigated the mechanical properties and dynamic modulus of high strength concrete with partial replacement of coarse aggregates instead of lightweight aggregate presaturated water and polymer emulsion (i.e. 15%, 30%, 45% and 70% by vol.). According to their study, as the percentage of replacement increased, EM, dynamic modulus of elasticity, and mechanical properties of high-strength concrete decreased. In the lower amount of replacement, the reduction in the properties were not signiﬁcant; however, in the 45% and 70% level of replacement, the reduction in the properties were obvious. Shaﬁgh et al. [22] studied the possibility of using by-products (i.e., ﬂy ash and lime- stone powder) in lightweight aggregate concrete and determined the properties of the mixes. Tests results showed that using high-volume ﬂy ash signiﬁcantly reduced the short-term mechan- ical properties; however, using limestone powder improved the compressive strength in the majority of mixes. It is worth noting that the chemical characteristics of by-products can also affect the mechanical properties of concrete [23]. In the past few decades, researchers introduced a new type of concrete, polymer concrete (PC), which shows superior mechanical properties compared to those of cement concrete. PC exhibits a higher compressive, tensile, and ﬂexural strength, fast curing, and it provides better protection in corrosive environments [24] and shows better vibration damping properties [25–27] as com- pared to conventional cement concrete. PC is also used in high- performance applications to repair and rehabilitate structures as an alternative to ﬁber reinforced polymers and steel reinforcedpiles [28]. Agavriloaie et al. [29] investigated the mechanical behavior of new type of PC using epoxy polyurethane acryl matrix. They concluded that PC is a lightweight concrete while possessing high durability performance against chemical aggression and freeze–thaw cycles. These advantages make PC suitable for use as insulation and waterprooﬁng material, and in industrial construc- tion where thermal and chemical protective cover is needed. Due to the higher cost of PC, researchers have been searching for a way to reduce the cost and enhance the properties of PC by adding different ﬁllers. Ferdous et al. [30] evaluated the mechani- cal properties of PC by mixing three types of ﬁller materials (ﬂy ash, ﬁre retardant ﬁller, and hollow microsphere). The physical, mechanical properties and durability were evaluated with the change in the resin-to-ﬁller ratio. The results revealed that by increasing the ﬁller ratio, the applications became more cost- effective and the thermal and durability properties improved; however, this resulted in a reduction in mechanical properties. Fer- dous et al. [31] also studied the effect of resin-to-ﬁller ratio andmatrix-to-aggregate ratio on mechanical properties and durability of epoxy-based polymer concrete to determine the proper mixdesign. They reported that the resin-to-ﬁller ratio has signiﬁcanteffect on the distribution of aggregates and a decrease in thematrix-to-aggregate ratio causes a reduction in mechanical anddurability properties. Past researches have shown that tempera-ture is a signiﬁcant parameter that can affect the performance ofengineering materials [32,33] . Khotbehsara et al. [32] investigated the effect of in-service elevated temperatures on the mechanicalproperties and microstructure of particulate-ﬁlled epoxy polymers,which contained a ﬁre retardant and ﬂy ash ﬁllers. They concludedthat the particulate-ﬁlled epoxy resin exhibits good performance inthe retention of mechanical properties at elevated temperatureswhich is beneﬁcial for civil engineering applications. Although there are a few standards, such as ACI 211.2 [34], that provide methods for designing LWC mixes, currently thereare no such reference documents to design LWPC, as the use ofpolymer concrete in construction is relatively new. To achieveserviceable, durable, and economic construction with LWPC,understanding the mechanical properties of LWPC is needed.The primary aim of this study was to investigate the quality,integrity, durability, and mechanical properties of LWPC and theirvariation by temperature to understand the short- and long-termbehavior. To study all these parameters, over 170 specimens wereprepared and tested. 2. Experimental program 2.1. Materials and specimen preparation The maximum size and speciﬁc gravity of porous lightweight aggregates were 19 mm and 1.4, respectively. Natural river sand with speciﬁc gravity of 2.65 and the nominal maximum size of4.75 mm, meeting the requirement of ASTM C33 [35], was used as the ﬁne aggregate. Previous studies [36–38] reported that the coarse to ﬁne aggregate ratio was in the range of 0.4–1.0 (byweight) in the mix design of LWC according to ACI 211.2 code[34]. Because there is currently no design code for lightweight polymer concrete, a reasonable assumption has made. In the cur-rent study, the speciﬁc gravity ratio of the porous lightweight aggregate to ﬁne aggregate was approximately 0.5. As such, in the mix design the weight ratio of the porous lightweight aggre-gate to ﬁne aggregate was selected as 0.5:1. The epoxy resin was used as the adhesive material with a ratio of base/hardener mix 1:0.58 by weight [39]. The 7-day compres- sive and splitting-tensile strength of polymer specimens cured atroom temperature was approximately 70 MPa and 35 MPa, respec- tively. Four different polymer contents -10%, 12%, 14%, and 16%- were used to study the inﬂuence of the binder content. For each polymer content, the speciﬁc amount of base and hardener were initially mixed together. Following this, the resin was blended with the aggregates to prepare the LWPC mix. The mix was ﬁlled in molds in three layers and each layer was pounded 25 times. After the specimens were demolded, they were cured at room tempera- ture (25 ± 3 /C176C) for seven days. The mix designs and notations for the four types of LWPCs are presented in Table 1 . Cylindrical specimens with dimensions of 76 /C2152 mm, 76/C276 mm, and 152 /C240 mm were made for the compression, splitting-tensile, and impact tests, according to ASTM C39 [40], ASTM C496 [41], and ACI 544 [42], respectively. For ﬂexural test, the prismatic specimens with the size of 75 /C275/C2500 mm were cast according to ASTM C78 [43]. For ultrasonic and electrical con- ductivity tests, the cubic specimens with the dimension of 100 mm were prepared.2 F. Heidarnezhad et al. / Construction and Building Materials 260 (2020) 119853 2.2. Test setup The compressive strength of specimens was determined using a 3000-kN uniaxial testing machine (UTM) in accordance with ASTM C39 [40]. The axial strains were measured by a linear variable dif- ferential transducer (LVDT) during the test. According to ASTM C496 [41], splitting-tensile strength tests were conducted on cylin- drical specimens. The prismatic specimens were tested to deter- mine the ﬂexural strength under three-point bending according to ASTM C78 [43]. The impact energy of specimens was deter- mined with the use of drop weight impact test which is suggested by ACI Committee 544 [42]. A 20.4 N.m energy was absorbed by the specimen in each drop and this cycle was repeated until the complete failure occurred. The total number of drops indicated the impact energy of the specimen. An ultrasonic pulse velocity (UPV) apparatus was used to gener- ate the longitudinal elastic wave pulses to measure the transit time through the specimens according to ASTM C597 [44]. To evaluate the durability of steel bars inside concrete against the attack of chloride ion and corrosion, an electrical resistivity apparatus was used to perform AC-Impedance spectroscopy as illustrated in Fig. 1 . This instrument applies an alternating current at an intended frequency without causing any disruption and measures the voltage between the two sides of the concrete specimen and the impedance (Z) can be calculated [45]. The resistivity of LWPC specimens was measured with the frequency of 10 kHz. 3. Results and discussions 3.1. Hardened density According to ASTM C125 [46], density is explained as the mass of a material divided by the volume of a skeleton plus permeable and impermeable pores. The density of LWPC specimens was mea-sured and compared with that of polymer concrete (PC) specimens formerly investigated [47], which are presented in Table 2 .I ti s worth noting that there is no available data from previous studies on the density of PC mix with 16% polymer content. The results of this comparison show that, for a given polymer content, the den- sity of LWPC was almost 20% lower than that of PC. This weight reduction is quite signiﬁcant and can be beneﬁcial in reducing both the damage sustained by structures under certain types of loading (e.g. earthquake) and overall cost of construction. 3.2. Failure modes Fig. 2 shows the specimens tested at +25 /C176C after peak stress to illustrate the trends of crack propagation and lateral expansion. The image shown in Fig. 2 (a) was taken at the peak stress and it illustrates that the initial crack occurred at the top of the specimen. As the displacement increased, this crack propagated ( Fig. 2 (b)) and with further increase in displacement ( Fig. 2 (c)) additional cracks were formed at various parts of the specimen and the middle-height region started to show signs of bulging. As the test continued, the cracks connected to each other and this resulted in the failure of the specimen ( Fig. 2 (d)). It was observed that for specimens tested at lower temperatures (+5 /C176C, and /C015 /C176C), more brittle failure and a sudden interface debonding between aggre- gates and polymer occurred and as such, no lateral deformation was visually observed. However, specimens tested at +25 /C176C showed visible lateral deformations and exhibited a prolonged softening behavior in their stress–strain curves. 3.3. Compressive behavior The compression test results of LWPC with 10%, 12%, 14%, and 16% polymer contents tested at various temperatures ( /C015 /C176C, +5/C176C, and +25 /C176C) are shown in Fig. 3 . To ﬁnd out the failure point of the stress–strain curves of the specimens, after the cracks com- pletely propagated, the compressive strength drastically decreased and the failure point is deﬁned as the point just before this sudden reduction occurred in the curves. The compressive strength increased with decreasing the temperature. Decreasing the tem- perature from +25 /C176Ct o+ 5 /C176C and +5 /C176Ct o/C015 /C176C resulted in anTable 1 Notation and mix design of various types of LWPC tested at different temperatures. Notation Lightweight coarse aggregate (by weight) (%) Fine aggregate (by weight) (%) Polymer ratio (%) Temperature ( /C176C) LWPC10/ /C015 30.0 60.0 10 /C015 LWPC10/+5 30.0 60.0 10 +5LWPC10/+25 30.0 60.0 10 +25LWPC12/ /C015 29.3 58.7 12 /C015 LWPC12/+5 29.3 58.7 12 +5LWPC12/+25 29.3 58.7 12 +25LWPC14/ /C015 28.7 57.3 14 /C015 LWPC14/+5 28.7 57.3 14 +5 LWPC14/+25 28.7 57.3 14 +25 LWPC16/ /C015 28.0 56.0 16 /C015 LWPC16/+5 28.0 56.0 16 +5LWPC16/+25 28.0 56.0 16 +25 Electrical resistivity device Constant load LWPC sample Conductive paste Copper electrodes Fig. 1. Electrical conductivity setup.Table 2 Density of PC and LWPC. Epoxy resin by weight (%)Density (kg/m3) Decrement density ratio (%)LWPC PC 10 1735 2135 18.7 12 1756 2184 19.6 14 1780 2211 19.5 16 1815 – –F. Heidarnezhad et al. / Construction and Building Materials 260 (2020) 119853 3 increase in the compressive strength by approximately 4% and 9%, respectively, in all series. The signiﬁcant reason for high compres- sive strength at lower temperatures was the increase in the inter- face bond strength between the aggregates and the polymer. Viklund et al. [48] showed that increasing temperature can cause a reduction in the size and volume of pores due to increased epoxyresin mobility at higher temperatures, and this leads to a decrease in the compressive strength. As shown in Fig. 3 , increasing the polymer content caused an increase in the compressive strength and corresponding axial strain. The rate of increase in the compres- sive strength and corresponding strain decreased with increasing polymer content, and there was only a slight difference between the compressive strength and strain of specimens with 14% and16% polymer contents. The compressive strength, axial strain cor- responding to maximum strength, and failure strain of all speci- mens are provided in Table 3 . A comparison was made between the compressive strength results of LWPC and those of the polymer concrete with larger coarse aggregate (PCL) manufactured with the same size aggre-gates and polymer content, and tested at the same temperature [39]. The comparison shows that the compressive strength of LWPC was 10% to 20% lower than that of the corresponding PCL depending on the test temperature and polymer content. The comparison also showed that the temperature played more inﬂuential role on the compression test of PCL than that of LWPC. Fig. 2. The crack propagation process of LWPC16/+25 after peak axial stress (a) initial crack (b) opening modes, (c) sliding mode (d) failure mode. 0102030405060 0 0.003 0.006 0.009 0.012Compressive stress (MPa) Strain (a)LWPC10/-15 LWPC10/+5 LWPC10/+25 0102030405060 0 0.003 0.006 0.009 0.012Compressive stress (MPa) Strain (b)LWPC12/-15 LWPC12/+5 LWPC12/+25 0102030405060 0 0.003 0.006 0.009 0.012Compressive stress (MPa) Strain (c)LWPC14/-15 LWPC14/+5 LWPC14/+25 0102030405060 0 0.003 0.006 0.009 0.012Compressive stress (MPa) Strain (d)LWPC16/-15 LWPC16/+5 LWPC16/+25 Fig. 3. Compressive stress–strain curve for LWPC having (a) 10%, (b) 12%, (c) 14%, and (d) 16% epoxy resin.4 F. Heidarnezhad et al. / Construction and Building Materials 260 (2020) 119853 3.4. Elastic modulus (EM) The EM of LWPC specimens was calculated based on the initial slope of the stress–strain curves (approximately 30%-40% of the maximum compressive strength) [39,49] . As shown in Fig. 4 , the EM increased with decreasing temperature from +25 /C176Ct o /C015 /C176C. There was only a slight difference (i.e. approximately 6%) between the EM of various types of LWPC specimens tested at the temperature of /C015 /C176C. On the other hand, this difference for specimens tested at +5 /C176C and +25 /C176C was 14% and 29%, respec- tively. These observations demonstrate that with a decrease in the tested temperature, the difference between the EM of differenttypes of LWPC becomes negligible. This does not agree with the authors’ previous study on polymer concrete [39], where it was found that the difference between EM of PCL and polymer concrete with smaller coarse aggregate (PCS) at higher temperatures (+25 /C176C and +65 /C176C) was negligible. This is attributed to several reasons including signiﬁcant differences in the aggregate size and distribu- tion, volume fraction, surface characteristics, air content, and bond strength with matrix in PC versus LWPC, as well as the differences in the test temperature. Polymer concrete is more rigid at lower temperatures; however, at temperatures higher than +25 /C176C, it shows a softer behavior and it completely loses its internal resis- tance at 65 /C176C[39]. Additional studies are recommended to better understand the inﬂuence of temperature on EM. 3.5. Energy absorption Energy absorption is the amount of absorbed energy per unit volume by the specimen deﬁned as the area under stress–straincurve up to failure strain [50–52] . It is shown in Fig. 3 that the duc- tility of concrete decreased and the softening behavior became more abrupt with decreasing temperature. For specimens tested at +5 /C176Ct o/C015 /C176C, after the peak stress, the specimens lost their strengths rapidly. The energy absorption of all specimens is shown inFig. 5 . At a given temperature, increasing in the polymer content led to increasing the energy absorption of LWPC, as both stress and strain increased. Decreasing the temperature from +25 /C176Ct o+ 5 /C176C, the energy absorption decreased signiﬁcantly (i.e. in the range of 40% to 52% depending on polymer content). In the authors’ previ- ous study on normal weight PC [39], the maximum energy absorp- tion was obtained at a temperature of +25 /C176C, which is in agreement with the current study. As can be seen in Fig. 5 ,a decrease in the temperature from +5 /C176Ct o/C015 /C176C resulted in a smaller decrease, in the range of 10% to 28%, in the energyabsorption. 3.6. Splitting-tensile strength The results of splitting-tensile strength for LWPC mixtures are depicted in Fig. 6 . An increase in the polymer content and a decrease in temperature led to increasing in the splitting-tensile strength. There was only a slight difference (i.e. 3–8%) between the splitting-tensile strength of specimens tested at /C015 /C176C and +5/C176C, as in both temperatures, the specimens were frozen and this led to an increased interface bond. At +25 /C176C, with each 2% increase in the polymer content from 10% to 16%, the splitting-tensile strength increased by 21%, 17%, and 10%, respectively. At all tem- peratures, the effect of the change in the polymer content was more signiﬁcant as it changes from 10% to 12% than that from 14% to 16%.Table 3 Properties of different LWPC mixes tested at different temperatures. Specimen Maximum compressivestress (MPa)Strain at maximumstress (10 /C03)Failure strain(10 /C03) LWPC10/ /C015 38.2 3.03 3.72 LWPC10/+5 34.9 3.39 4.50LWPC10/+25 33.0 5.23 8.57LWPC12/ /C015 46.4 3.65 4.60 LWPC12/+5 42.6 4.15 6.43LWPC12/+25 41.5 5.56 10.52 LWPC14/ /C015 54.1 4.06 5.11 LWPC14/+5 49.3 4.48 6.93LWPC14/+25 47.3 6.13 11.17LWPC16/ /C015 55.7 4.21 5.35 LWPC16/+5 51.8 4.52 7.41LWPC16/+25 49.0 6.49 11.72 9121518 -15 5 25Modulus of elasticity (GPa) Temperature ( C)LWPC16 LWPC14 LWPC12 LWPC10 Fig. 4. Modulus of elasticity of LWPC having different polymer content versus temperature.050100150200250300350 -15 5 25Energy absorption (J) Temperature ( C)LWPC16 LWPC14 LWPC12 LWPC10 Fig. 5. Energy absorption of different types of LWPC. 33.544.555.566.5 LWPC10 LWPC12 LWPC14 LWPC16Splitting-tensile strength (MPa)-15C +5C +25C Fig. 6. Splitting-tensile strength of LWPC.F. Heidarnezhad et al. / Construction and Building Materials 260 (2020) 119853 5 Several researchers suggested a relationship between the differ- ent powers of compressive and splitting-tensile strengths. Gho- lampour and Ozbakkaloglu [53] provided a summary of the mechanical properties of conventional concrete considering design codes for the prediction of splitting-tensile based on their com- pressive strength. In ACI 318 [54], for cement concrete fst= 0.56pfcmand for high-strength cement concrete the equation is fst= 0.6–0.7pfcm[55,56] where fstis the mean splitting-tensile strength (MPa) and fcmis the mean compressive strength (MPa). According to Jafari et al. [39], for polymer concrete, the recom- mended equation is fst= 0.8pfcm. However, some other researchers suggested that the relationship between compressive strength and splitting-tensile strength with the power of (2/3) has higher accu- racy than that with the power of (1/2) [57,58] . Neville [57] reported that for conventional cement concrete, the splitting-tensile strength is equal to f st= 0.23 ( fcm)2/3. In this study, the rela- tionship of splitting-tensile strength and compressive strength for LWPC was obtained with the power of (2/3) as shown in Fig. 7 ,a s this led to a higher accuracy than that attained with the power of (1/2). The constant value for LWPC was found to be 0.37, which indicates that LWPC exhibits a higher splitting-tensile strength compared cement concrete. 3.7. Flexural strength Fig. 8 shows the ﬂexural strength of different types of LWPC. The results illustrate that by increasing the polymer content from 10% to 16% at /C015 /C176C, +5 /C176C, and +25 /C176C, the maximum ﬂexural strength increased 63%, 66%, and 53%, respectively. In general, a decrease in temperature at any polymer content caused an increase in the ﬂexural strength. This increase was more signiﬁcant when the temperature dropped from +25 /C176Ct o+ 5 /C176C as from +5 /C176C to/C015 /C176C. In this study, the relationship of ﬂexural strength and compressive strength for LWPC was obtained with the power of (2/3) as shown in Fig. 9 , as this led to a higher accuracy than that attained with the power of (1/2). The ﬂexural strength of LWPC, expressed as a function of compressive strength, was fr=0.40 (fcm)2/3which is slightly lower than the suggested value for cement concrete by Ahmad and Shah [59] asfr=0.44 (fcm)2/3where fris the mean ﬂexural strength (MPa). According to literature [57,59] , for conventional concrete, the ratio of ﬂexural strength to splitting- tensile strength is 1.9; however, for LWPC, this value is around 1.0. The reason is partially because of the size of ﬂexural specimens that prepared for this study and partially because during castingLWPC samples, the segregation occurred as the polymer move toward the top of the specimen due to the lower density of poly- mer compared to the aggregates. For ﬂexural specimens, this phe- nomenon is more obvious as the bottom of the specimen become weak and the cracks propagated rapidly in the interfacial zone due to the lack of binder.3.8. Impact energy The impact energy of LWPC specimens is presented in Fig. 10 . As illustrated in this ﬁgure that the impact energy of specimens increased with an increase in the polymer content. The difference between the impact energy of LWPC12 and LWPC10 was much higher than that between LWPC14 and LWPC12, which was in turn higher than the difference between LWPC16 and LWPC14. For example, at +25 /C176C temperature, increasing the polymer content from 10% to 12% and 12% to 14% led to an increase in the impact energy by 32% and 14%, respectively. However, at the same tem- perature, an additional increase in the polymer content from 14% to 16%, led to only 9% increase in the impact energy. When polymer was added in larger amounts, the bond between the polymer and aggregates increased and the adhesive material was able to com- y = 0.37x R² = 0.81 01234567 10 11 12 13 14 15Splitting-tensile strength (MPa) (fcm)2/3(MPa)Experimental data Linear (Experimental data) Fig. 7. Relationship between compressive strength and splitting-tensile strength.33.544.555.566.57 LWPC10 LWPC12 LWPC14 LWPC16Flexural strength (MPa)-15C +5C +25C Fig. 8. Flexural strength of LWPC. y = 0.40x R² = 0.89 01234567 10 11 12 13 14 15Flexural strength (MPa) (fcm)2/3(MPa)Experimental data Linear (Experimental data) Fig. 9. Relationship between compressive strength and ﬂexural strength. 100150200250 -15 5 25Energy absorption (J) Temperature ( C)LWPC16 LWPC14 LWPC12 LWPC10 Fig. 10. Impact energy of LWPC mixes at different temperatures.6 F. Heidarnezhad et al. / Construction and Building Materials 260 (2020) 119853 pletely coat the aggregates. As a result, more energy was required to break this bond and cause cracks in the specimens. Experimental observations showed that after initial cracks occurred on the sur- face of specimens, they rapidly propagated and connected to each other in the next two or three cycles leading to the complete failure of the specimen. Lower impact energy of LWPC with lower poly- mer content can also be attributed to the presence of large number of voids and their connection inside the specimen. Moreover, increasing the temperature from /C015 /C176C to +25 /C176C caused an increase in the impact energy. This is because, at low temperatures the viscoelastic property of polymer content decreased and hence less energy was required for the initiation and development of cracks, which led to a lower impact energy. 3.9. Ultrasonic pulse velocity Generally, UPV decreases as the voids inside the specimens increase. The ultrasonic pulse velocities of LWPC of this study were determined to be in the range of 3162–3432 m/s. As can be seen in Fig. 11 , UPV increased with increasing polymer content, as the voids were ﬁlled with polymer and the adhesive material ﬁrmly bonded the aggregates together. The comparison between UPV of LWPC samples showed that the low UPV value is partially due to the internal air voids in concrete and partially due to the voids inside porous aggregates. Several researchers have proven that there is a relationship (linear or exponential) between the com- pressive strength and UPV [60,61] .Fig. 11 depicts the linear rela- tionship between the compressive strength and UPV for LWPC obtained in the current study: fcm¼58V/C0149 ð1Þ where fcmis the mean compressive strength (MPa), and Vis the UPV (km/s). For conventional concrete, the UPV value above 4.0 km/s shows excellent and homogenous concrete [60–63] . 3.10. Electrical resistivity The electrical resistivity test was used to measure the ion passed through the LWPC and evaluate the probability of corrosion inside the concrete. The electrical resistance of LWPC specimens is presented in Fig. 12 . The results show that the electrical resistance of LWPC specimens increased with increasing the percentage of polymer content, which led to lower porosity. Each 2% increase in the polymer content from LWPC10 to LWPC16 increased the electrical resistance 16%, 7% and 6%, respectively. It is worth noting that the electrical resistivity of all LWPC mixes were higher than 120Xm, indicating that the corrosion of steel bars inside concrete would not be probable [64].4. Probabilistic models for destructive tests of LWPC 4.1. Variables for prediction Here, the dependency of the mechanical properties obtained through destructive tests to the temperature, polymer content, and compressive strength of LWPC was investigated. Temperature, polymer content, and compressive strength and their effects on LWPC indicated the inﬂuential variables used in this investigation. All possible experimental and material properties were considered in this research; nevertheless, some of them were eliminated because of their statistical insigniﬁcance. In Figs. 14-16 , vertical axis represents the splitting-tensile strength ( ft), ﬂexural strength (fr) and impact energy ( IE). The horizontal axis depicts the experi- mental properties such as polymer content ( P), temperature ( T) and compressive strength ( fcm). Correlation factor (R2) of the trend lines shows the effect of each parameter that should be partici- pated in the empirical equations. It should be noted that these ﬁg- ures provide only an insight into the trends between parameters and variables utilized for modelling in RT software so, they should not be utilized to establish relationships between the model parameters and variables. Therefore, the dependency of the param- eters is discussed in detail in the following sections. 4.2. Dependence of destructive test parameters on polymer content, temperature, and compressive strength An increase in the polymer content is associated with a clear increase in mechanical properties. This is conﬁrmed by Fig. 13 , which represents that the polymer content and a linear regression line for the mechanical properties of LWPC. Moreover, increasing the temperature leads to decrease the splitting-tensile and ﬂexural strength values and increase the impact energy as shown in Fig. 14 . In addition, increasing the compressive strength, the most effective factor compared to other destructive tests, led to an increase in splitting-tensile strength, ﬂexural strength, and impact energy as shown in Fig. 15 (a)–(c) respectively. All of the ﬁgures revealed the great dependence of destructive test parameters on polymer content, temperature, and compressive strength. 4.3. Predictive models To attain the best estimation of destructive test properties of LWPC, empirical equations are presented below. These equations reported by way of regression using Eq. (2). A stepwise regression method was performed to specify the most statistically signiﬁcant variables. In Eq. (2),X1to X nare predictor variables based on the mix design, environmental situation and compressive strength of the LWPC, and a 1to a nare regression coefﬁcients. Eq. (3)is the nat-y = 58x - 149 R² = 0.95 303540455055 3.1 3.2 3.3 3.4 3.5Compressive strength (MPa) UPV (km/s)Experimental data Linear (Experimental data) Fig. 11. Relationship between compressive strength and UPV for LWPC.162 .m188 .m202 .m215 .m 050100150200250 LWPC10 LWPC12 LWPC14 LWPC16Electrical resistivity ( .m)= 120 .m Fig. 12. Results of electrical resistivity of LWPC test at +25 /C176C.F. Heidarnezhad et al. / Construction and Building Materials 260 (2020) 119853 7 ural logarithmic form of Eq. (2)to model a linear relation among the parameters which previously mentioned. fðX1; :::XnÞ¼a1ðX1Þa2ðX2Þa3:::ðXnÞanþ1ð2Þ LnðfðX1; :::XnÞÞ ¼ cþa2LnðX1Þþa3LnðX2Þþ :::þanþ1LnðXnÞð 3Þ Furthermore, the highest value of coefﬁcient of variation was removed from the model. The quality of the model was then con- trolled by way of the prediction versus experimental graphs. Inaddition, the mean of standard deviation of the model error should not have a great change and coefﬁcient of variation of regression should be in an allowable range; if not, the prior model is more valuable [65–67] . The regression coefﬁcients were set to their respective mean estimation in the suggested predictive equations. This regression was predicted according to the effect of various parameters on the splitting-tensile strength, ﬂexural strength and impact energy of the LWPC presented in Eqs. (4)–(6), respectively. The stepwiseR² = 0.91 3456 8 1 01 21 41 61 8ft(MPa) P (%) (a)R² = 0.89 34567 8 1 01 21 41 61 8fr(MPa) P(%) (b)R² = 0.89 100140180220260 8 1 01 21 41 61 8IE(N.m) P(%) (c) Fig. 13. Dependency of (a) splitting-tensile strength, (b) ﬂexural strength, (c) impact energy to the polymer content. R² = 0.07 3456 250 275 300ft(MPa) T(K) (a)R² = 0.08 34567 250 275 300fr(MPa) T(K) (b)R² = 0.08 100140180220260 250 275 300IE(N.m) T(K) (c) Fig. 14. Dependency of (a) splitting-tensile strength, (b) ﬂexural strength, (c) impact energy to the temperature. R² = 0.95 3456 30 40 50 60ft(MPa) fcm(MPa) (a)R² = 0.93 34567 30 40 50 60fr(MPa) fcm(MPa) (b)R² = 0.60 100140180220260 50 150 250 350IE(N.m) EA(N.m) (c) Fig. 15. Dependency of destructive tests to (a) and (b) compressive strength and (c) energy absorption. R² = 0.9742 3456 3456Predicted ft(MPa) Observed ft(MPa) (a)R² = 0.951 34567 357Predicted fr(MPa) Observed fr(MPa) (b)R² = 0.9745 110150190230270 110 150 190 230 270Predicted IE(J) Observed IE(J) (c) Fig. 16. Comparison between the measured and predicted values (a) splitting tensile strength, (b) ﬂexural strength, and (c) impact energy.8 F. Heidarnezhad et al. / Construction and Building Materials 260 (2020) 119853 omission procedure was used to attain an agreement between model simplicity and model accuracy [68,69] . ft¼1:05/C20:14lnðPÞ/C02:7 T/C18/C19 /C2ðfcmÞ2=3ð4Þ fr¼1:15/C20:14lnðPÞ/C06:5 T/C18/C19 /C2ðfcmÞ2=3ð5Þ IE¼ðPÞ0:94/C2ðTÞ0:39/C2ðEAÞ0:12ð6Þ where ftis the splitting-tensile strength (MPa), fris the ﬂexural strength (MPa), fcmis the mean compressive strength (MPa) obtained from the experiments, Pis the ratio of polymer content (i.e., 14), Tis the temperature ( /C23K). The comparison between the measured and predicted values by Eqs. (4)–(6) are shown in Fig. 16 . The values of R factor, coefﬁcient of variation (COV) of stan- dard deviation (sd), and mean ( l) of standard deviation for models are presented in Table 4 . As can be seen, a high correlation between the measured and predicted values was obtained in all tests.4.4. ANOVA analysis The ANOVA was used to determine the optimum conditions of the main test parameters in different experimental tests [39,70] . The ANOVA results for the compressive, splitting-tensile, ﬂexural, and impact strengths for different polymer ratios of LWPC mixes tested at various temperatures are shown in Tables 5–8 , respec- tively. Based on the results, in LWPC, the polymer ratio played a signiﬁcant role on the mechanical properties, with the contribu- tions of 88%, 92%, 90%, and 91% for the compressive, splitting- tensile, ﬂexural strengths, and impact energy, respectively. How- ever, Jafari et al. [39] reported that in PC, temperature had a major inﬂuence on the compressive, splitting-tensile, and ﬂexural strengths with percent contributions of 70%, 85%, and 85%, respec- tively. ANOVA was also used to statistically evaluate signiﬁcantparameters. The frequency value (F-value) for each parameter was used in statistics, which forms the quality characteristics. The observations indicate that the F-values of two mix design parameters were all higher than F0.05,(2,5) = 5.79 which shows that they had a statistical effect on the mechanical behaviors of LWPC. 5. Summary and conclusions This research has studied the experimental and analytical methods to determine the mechanical properties of different LWPC mix designs. In addition, the optimization method has been used for evaluation of the LWPC mixtures. The following conclusions are drawn:Table 4 Statistics of predictive equations. Predictive model R-factor COV(sd) l(sd) Splitting-tensile strength 0.96 0.289 0.011 Flexural strength 0.94 0.289 0.015Impact energy 0.99 0.316 0.038 Table 5 Results of ANOVA for the compressive strength LWPC. Control factor Degrees of freedom Sum of square Variance Variance ratio (F-value) Contribution (%) Polymer content, P (%) 3 523 174.0 580 87.7 Temperature, T ( /C176C) 2 72 35.9 119 12.0 Error 6 2 0.3 – 0.3Total 11 596 – 100 Table 6 Results of ANOVA for the splitting-tensile strength LWPC. Control factor Degrees of freedom Sum of square Variance Variance ratio (F-value) Contribution (%) Polymer content, P (%) 3 7.88 2.63 210 91.7 Temperature, T ( /C176c) 2 0.63 0.32 25 7.4 Error 6 0.07 0.01 – 0.9 Total 11 8.59 – 100 Table 7 Results of ANOVA for the ﬂexural strength LWPC. Control factor Degrees of freedom Sum of square Variance Variance ratio (F-value) Contribution (%) Polymer content, P (%) 3 8.59 2.86 127 89.7 Temperature, T ( /C176c) 2 0.85 0.43 19 8.9 Error 6 0.13 0.02 – 1.4Total 11 9.58 – 100 Table 8 Results of ANOVA for the impact energy LWPC. Control factor Degrees of freedom Sum of square Variance Variance ratio (F-value) Contribution (%) Polymer content, P (%) 3 15,885 5295 519 91.2 Temperature, T ( /C176c) 2 1472 736 72 8.4 Error 6 61 10 – 0.4Total 11 17,418 – 100F. Heidarnezhad et al. / Construction and Building Materials 260 (2020) 119853 9 /C15The density of LWPC is approximately 20% lower than that of PC. This can help reduce the overall cost of construction and the damage sustained by structures under certain types of load- ing (e.g. earthquake). /C15Increasing the polymer content from 10% to 16% leads to an increase in the mechanical properties, ductility, and energy absorption of LWPC specimens, with observed increases of 32% in compressive strength, 57% in splitting tensile strength, 52% in ﬂexural strength, and 69% in energy absorption. The rate of increase in the compressive strength decreases with increas- ing polymer content, and only a slight difference was observed between the compressive strengths of mixes with 14% and 16% polymer contents. The non-destructive tests reveal that the porosity of specimens reduces as the polymer content increases. /C15The reduction of the testing temperature from +25 /C176Ct o/C015/C176C causes an increase in the mechanical properties of specimens, with observed increases of 12% in compressive strength, 14% in splitting tensile strength, 14% in ﬂexural strength. However, a signiﬁcant decrease in the impact resistance, energy absorp- tion, and ductility is observed with decreasing temperature. /C15The difference between the splitting-tensile and ﬂexural strengths of LWPC is very low and the ratio is higher than 0.9; however, for conventional concrete, this ratio is typically around 0.5–0.6. /C15The results of ANOVA revealed that the polymer ratio had more substantial inﬂuence on the results of destructive test in com- parison with temperature particularly in the ﬂexural strength and splitting-tensile tests. The equations suggested in this research provide accurate estimations of the mechanical prop- erties (i.e., splitting-tensile and ﬂexural strength), and impact energy of LWPC based on experimental parameters and com- pressive strength tests result, which make them applicable in order to use in the pre-design of LWPCs. This paper has provided beneﬁcial information on the mechan- ical properties of LWPC and their variation by temperature. Although the results are quite positive, they also point to the importance of careful design and implementation of LWPC, as the mechanical properties of the material can degrade signiﬁcantly under certain conditions. 6. Future work For future research, the reaction mechanism of polymerization (from the time that epoxy and resin are mixed until the test day) will be studied using the proposed instrument by Akhoundi et al. [71]. In this approach, the proposed materials are tagged with Cy3 and Cy5 ﬂuorescent label dyes. The ﬂuorescent images are taken by the confocal microscope setup to provide valuable infor- mation on polymerization reaction. Moreover, the effect of adding micro-ﬁllers are analyzed with this method to reduce the overall cost of polymer applications. CRediT authorship contribution statement Fatemeh Heidarnezhad: Conceptualization, Investigation, Val- idation, Writing - original draft, Visualization. Khashayar Jafari: Conceptualization, Investigation, Validation, Writing - original draft, Visualization. Togay Ozbakkaloglu: Conceptualization, Supervision, Writing - review & editing.