Abstract
Urban heat island (UHI) effect is built environmental issue related to pavements. It is desired to reduce pavement high surface temperature in summer to mitigate UHI effect. High surface temperature also affects slab temperature difference (the top surface temperature minus the bottom surface temperature of the slab). The increased slab temperature difference induces a high possibility of cracking in concrete roads. The prime aim of this study was to reduce the slab surface temperature by using recycled glass as a fine aggregate and zeolite as cement in concrete. Recycled glass was used to replace fine aggregate in proportions of 10%, 20%, and 30% by total weight of aggregate. Zeolite replaced Portland cement in proportions of 10% and 30% for three different proportions of recycled glass concrete mixtures. Optimum proportions were determined by examining mechanical properties of samples and alkali-silica reactions. It was noticed that using recycled glass and zeolite together in concrete reduces pavement surface temperature and temperature gradient in summer.
1. Introduction
Built environmental issue related to pavements is the urban heat island (UHI) effect. Heat islands can be considered as surface and atmospheric heat islands [1, 2]. Hot pavements contribute to the heat island effect. Surface heat islands can affect human thermal comfort and air quality. 70–80°C surface temperatures on pavements have been measured on hot summer days in Phoenix, Arizona [3]. As one major thermal characteristic, solar reflectivity or albedo (1-solar absorptivity) is an indicator of the reflecting power of a surface. It is defined as the ratio of the reflected solar radiation to the incident solar radiation at the surface. It is dimensionless fraction and is measured on a scale from 0 to 1. Albedo of 0 means no reflecting power and albedo of 1 means perfect reflection [4]. Increasing the solar reflectivity of a pavement surface by using surfacing materials of light colour or applying light colour coating on dark surfacing materials can lower the pavement surface temperature [5–8]. Solar reflective materials can be identified as one of the most promising solutions to counteract urban heat island [9].
Portland cement concrete (PCC) pavements are affected by temperature differences. The temperature at the top surface of the slab is higher than that of the bottom during the day. The top tends to expand while the bottom tends to contract. The temperature at the top surface of the slab is lower than that of the bottom at night and the top tends to contract. Kuo [10] recommended using loading at the centre of the slab during the daytime and loading at the edge of the slab at nighttime in analysing of concrete pavement. Negative temperature gradient (nighttime) is not taken into account especially at concrete road design in Germany. This is because negative gradient is less than positive gradient (daytime) [11].
Thermal conductivity is used to estimate the temperature gradient of concrete pavement [12]. Temperature gradient is the uniform flow of heat in a specific sample from one side to the other. Temperature gradients throughout the slab thickness play a key role in calculating thermal stresses in PCC pavements, known as curling [13]. Decreasing the thermal conductivity of concrete also decreases the temperature gradient of concrete road and reduces thermal cracks in concrete slab. It is necessary to decrease the curling stresses resulting from the fluctuation of temperature gradients.
Aggregate type, percent of cement paste, coarse aggregate, fine aggregate, porosity, supplementary cementitious materials (SCMs), moisture, and temperature of local environment are some of the factors that affect the thermal conductivity of concrete [13–16]. Researches done before investigated that mixes with a high fine aggregate proportion had a higher thermal conductivity compared to a high proportion of coarse aggregate [14, 17]. The use of structures with high thermal resistance has become of great importance in hot weather countries where temperature can reach high levels especially in summer [18]. The low value of thermal conductivity is desirable due to the associated ability to provide thermal insulation [19].
This study mainly focuses on reducing the temperature gradient that causes thermal cracks in concrete road and also mitigates heat island. The effects of different proportions of recycled glass and zeolite used as a fine aggregate and Portland cement, respectively, were investigated. Compressive strength, flexural strength, and alkali-silica reaction (ASR) for the determined proportion of materials were investigated. The slab temperatures were measured at various depths of three different types of pavements to control the effects of recycled glass and zeolite on temperature gradient of concrete road. Pavement surface temperatures were measured in summer for different types of concrete mixtures. Ideal concrete mixture which had lower surface temperature was determined.
2. Glass and Zeolite in Concrete
Water absorption capacity of glass is almost zero and when it is used as an aggregate in concrete, it decreases the water absorption and drying shrinkage values which is a desired property for concrete [20]. ASR occurs in concrete when alkalis from cement react with free silica presented in certain aggregates to form alkali-silica gels [21]. This phenomenon reduces the service life of the affected structure. Glass has a higher value of SiO2 and Lam et al. [20] observed whether ASR expansion would be seen or not in concrete with glass in its mix design when there is enough moisture. It was observed that if the proportion of the glass is lower than 25% weight of the aggregate in concrete, ASR expansions are in negligible level. Byars et al. [22] determined that the reactivity of glass particles generally increases with particle size from around 1-2 mm. Glass particles below this size appear to reduce the propensity for ASR in larger glass particles. Ready-mixed concrete made with glass pozzolan and/or glass sand showed increasing in strength development to 1 year, indicating a pozzolanic contribution from the fine glass particles. Krishnamoorthy and Zujip [23] determined that using recycled glass as fine aggregate decreases thermal conductivity of concrete.
Zeolite is a natural or synthetic hydrated aluminosilicate mineral of alkali and alkaline earth metal with an open three-dimensional crystal structure. Zeolite concrete is much less frequent subject of investigation as compared with fly ash, silica fume, or ground granulated blast furnace slag as SCMs. The most significant effects of using zeolite as cement in concrete are reduction in expansion due to alkali-silica reaction, resistance to acid and sulfate attacks, and pozzolanic consumption of calcium hydroxide component of Portland cement hydration in the paste. The performance of natural zeolite in mortar/concrete has been compared with performance of other pozzolanic materials [24–26]. Poon et al. [25] determined that the degree of reaction of natural zeolite in a paste with a higher percentage of replacement is lower than in a paste with a lower percentage of replacement. Chan and Ji [26] noticed that pozzolanic reactivity of natural zeolite is between pulverized fuel ash and silica fume. Kılınçarslan [27] determined that using zeolite as Portland cement decreases the thermal conductivity of concrete.
3. Pavement Surface Temperature
Li et al. [4] investigated the effect of albedo on concrete pavement surface temperature and they determined that surface temperatures were 44°C, 50°C, and 59°C for concrete mixtures with an albedo of 0.29, 0.26, and 0.18, respectively. The effects of thermal conductivity and heat capacity were assumed identical in that study. Qin and Hiller determined that maximum thermal tensile stress of a slab can increase up to 0.3 MPa when the pavement surface’s solar absorptivity rises from 0.65 to 0.85 [28]. Although this stress value can be assumed as small, after combining with external loadings from traffic, it makes the total stress of a slab close to the flexure tensile strength.
When the albedo of the concrete surface is high and the thermal conductivity of the concrete is low, the temperature fluctuations of the pavement surface will be in minimum during a day and temperature gradient will be reduced. This will also reduce thermal cracks on concrete road and lower surface temperature will mitigate heat island.
Sand as a fine aggregate has a good thermal conductivity and if the material used as sand in concrete does not cause ASR and if that material has a lower thermal conductivity than the sand, curling stress will reduce at the concrete pavement slab. Also, this reduction can be obtained by replacing Portland cement with a material which has a lower thermal conductivity. Pancar and Akpınar [29] used glass beads as a fine aggregate in concrete road and they obtained lower surface temperature in summer while the albedo was the same. The concrete pavement surface temperature decreased 12.5°C by using glass bead in their study. If the alternative materials used as fine aggregate and Portland cement have lighter colour than fine aggregate and Portland cement used in control mixture, the albedo will also increase and this property will also help to decrease the pavement surface temperature in summer.
4. Methods
Ten different types of mixtures were designed to study the effects of recycled glass and natural zeolite on compressive strength, flexural strength, and ASR of the concrete. One of these mixtures was a control mixture and three of the mixtures had 10%, 20%, and 30% proportions of recycled glass by total weight of aggregates. Six different mixtures with different proportions (10% and 30%) of zeolite for each recycled glass sample proportion were also prepared. All coarse and fine aggregates were limestone in control mix design. Fine aggregates are more effective in thermal conductivity than coarse aggregates. Due to this reason, recycled glass was used as a fine aggregate to obtain lower slab temperatures in this study. Aggregates used in this study were limestone and Portland cement 42.5 R was used in all mixtures. The mixtures were named using the type and percentage of recycled glass and zeolite in the concrete mixtures. For example, G10Z0 represents the concrete mixture that consists of 10% proportion of recycled glass by total weight of aggregate and 0% proportion of zeolite in cement. Water/cement ratio was 0.50 in all blends. Standard 15 cm × 15 cm × 15 cm cube C30/37 strength class concrete specimens were prepared for the testing program. Mixture designs of the concrete samples are in Table 1. Sieve analysis of recycled glass, used as a fine aggregate, and standard fine aggregate are in Tables 2 and 3, respectively. Chemical properties of fine aggregate, recycled glass, zeolite, and Portland cement are in Table 4.
The potential ASR expansion of the prepared mortar bars (25 × 25 × 285 mm) with a water to cement ratio of 0.47 was assessed in accordance with ASTM C1260 [30]. Ten series of mortar bars were prepared in total. In these series, three different proportions (10%, 20%, and 30%) of recycled glass and two different proportions (10% and 30%) of zeolite for each recycled glass sample were prepared with control mixture. The test period was 28 days. The result of mortar bars test is shown in Figure 1. ASR expansion needs to meet the requirements prescribed in ASTM C1260 (<0.1% within 14 days). Mixture proportions of mortar bars are in Table 5.
Compressive and flexural strength measurements were done for all samples at 28 and 90 days. The compressive strength values need to be at least 37 MPa at 28-day measurement for C30/37 type of concrete. After ASR and compressive strength measurements, some mixture samples were eliminated and temperature measurements of concrete road were made for ideal mixtures. In order to monitor the temperature profiles during the day, temperature sensors were installed at every 5 cm depth of 3 different types of 25 cm thick concrete slabs. The length and width of the slabs were 1 m and 1.5 m, respectively.
5. Results
It was observed that all samples used in this study were able to meet the ASR expansion requirements. Using recycled glass, smaller than 1 mm, as fine aggregate increases the expansions after 14 days and using zeolite as Portland cement significantly decreases the expansions of mixtures with recycled glass. ASR test results for ten different types of mixtures are in Figure 1.
Compressive and flexural strength of ten different types of mixtures for 28 and 90 days are in Table 6. G30Z0, G30Z10, and G30Z30 mixtures did not meet the compressive strength requirements of C30/37 type of concrete and these mixtures were eliminated for the temperature gradient measurements.
After ASR and compressive strength measurements, it was observed that the standard mixture, G20Z0 mixture, and G20Z30 mixture were suitable for temperature gradient measurements and these measurements were done on these samples. Before measuring temperature gradients on the site, thermal conductivities of standard mixture, G20Z0 mixture, and G20Z30 mixture concrete were measured by using the Testo-635 product. Thermal conductivities were determined as 2.57 W/m·K, 2.2 W/m·K, and 2.08 W/m·K for standard mixture, G20Z0 mixture, and G20Z30 mixture, respectively. One of the slabs was standard concrete mixture type and the others were G20Z0 and G20Z30 types of concrete. The sensors were installed at the middle of the slabs and the slabs were on 25 cm thick crushed limestone base course layer. The slab temperatures at various depths were measured between 08:00 AM and 6:00 PM in August. Temperature measurements are in Figures 2, 3, and 4 for three different types of concrete slabs.
It is observed from Figure 2 that the highest temperature measurement of the top surface of the control mixture slab was at 2:00 PM and it was 50.0°C. The biggest temperature difference on top surface of the slab was 20.0°C during the testing time. The biggest temperature difference on bottom of the slab was 4.0°C during the testing time. The biggest temperature difference between the top and bottom surface of the slab was 14.0°C at 2:00 PM. The bottom surface temperatures changed in the range of 34°C–38°C during the measurement time.
It is observed from Figure 3 that the highest temperature measurement of the top surface of the G20Z0 mixture slab was at 2:00 PM and it was 41.0°C. The biggest temperature difference on top surface of the slab was 13.0°C during the testing time. The biggest temperature difference on bottom of the slab was 4.0°C during the testing time. The biggest temperature difference between the top and bottom surface of the slab was 7.0°C at 2:00 PM. The bottom surface temperatures changed in the range of 30°C–34°C during the measurement time.
It is observed from Figure 4 that the highest temperature measurement of the top surface of the G20Z30 mixture slab was at 2:00 PM and it was 38.0°C. The biggest temperature difference on top surface of the slab was 10.0°C during the testing time. The biggest temperature difference on bottom of the slab was 3.0°C during the testing time. The biggest temperature difference between the top and bottom surface of the slab was 5.0°C at 2:00 PM. The bottom surface temperatures changed in the range of 30°C–33°C during the measurement time.
6. Discussion
It is observed from Figure 5 that the surface temperature of standard mixture is higher than G20Z0 mixture between 8:00 AM and 6:00 PM and the surface temperature of G20Z0 mixture is higher than G20Z30 mixture between 10:00 AM and 6:00 PM. Using recycled glass in proportion of 20% by total weight of aggregate decreases the biggest surface temperature of concrete pavement from 50°C to 41°C at 2:00 PM. Replacing zeolite with Portland cement in proportion of 30% in this type of recycled concrete mixture decreases the biggest surface temperature 3°C more.
Top and bottom surface temperature differences measured for three concrete samples are in Figure 6. It is observed that these differences are very close to 0°C at 10:00 AM and 5:00 PM. Top and bottom surface temperature differences are increasing between these hours and come to the highest point at 2:00 PM for three samples. The biggest top and bottom surface temperature difference is 14°C, 7°C, and 5°C for standard sample and G20Z0 and G20Z30 samples, respectively.
The reflection coefficient or albedo was not measured by a device in this study. However based on visual observation it was determined that G20Z0 mixture and standard mixture have almost the same surface colour and G20Z30 mixture had a slightly lighter colour than these mixtures (Figure 7). At the end of this study, it was determined that the concrete pavement surface temperature can decrease from 50°C to 38°C by using G20Z30 type of concrete mixture instead of control mixture and the biggest top and bottom surface temperature difference may decrease from 14°C to 5°C by this method.
7. Conclusion
If the temperature gradient value is high, it is probable to see cracks in concrete roads. Temperature gradients also need to be decreased. This can be done by reducing the pavement surface temperature and thermal conductivity of concrete. Reducing the surface temperature also helps to mitigate heat island effect. The first concern of this study was to reduce concrete pavement surface temperature and temperature gradients in concrete while maintaining the concrete strength in the necessary limit during the day in summer.
Aggregates used in this study were limestone. Recycled glass (smaller than 1 mm) was used to replace fine aggregate in proportions of 10%, 20%, and 30% by total weight of aggregate. Zeolite replaced Portland cement in proportions of 10% and 30% for three different proportions of recycled glass concrete mixtures. ASR measurements were done and all samples met the ASR expansion requirements. Using recycled glass increased the ASR expansions but replacing Portland cement with zeolite in proportion of 30% decreased these expansions significantly. Compressive strength and flexural strength of all samples were measured at 28 and 90 days. The sample in proportion of 30% recycled glass as a fine aggregate did not meet the compressive strength requirements whether zeolite was added or not. G30Z0, G30Z10, and G30Z30 mixtures were eliminated for temperature gradient measurements. Temperature gradient measurements were done on concrete slabs for standard mixture, G20Z0 mixture, and G20Z30 mixture between 08:00 AM and 6:00 PM in summer. At the end of this study it was observed that recycled glass (smaller than 1 mm) can be used as a fine aggregate up to proportion of 20% by total weight of aggregate in concrete mixtures. Using recycled glass in proportion of 20% by total weight of aggregate decreases the biggest surface temperature of concrete pavement from 50°C to 41°C. Replacing zeolite with Portland cement in proportion of 30% in this type of recycled concrete mixture decreases the biggest surface temperature 3°C more. Using G20Z0 and G20Z30 mixtures mitigates heat island effect by decreasing surface temperature. The biggest temperature differences between top and bottom surfaces of concrete slab occur at 2:00 PM in summer. These differences are 14°C, 7°C, and 5°C for standard mixture, G20Z0 mixture, and G20Z30 mixture, respectively. G20Z0 and G20Z30 mixtures have smaller temperature gradients in summer and they are good alternative mixtures to prevent thermal cracks.
Recycled glass (smaller than 1 mm), which is needed to be used in sustainability, as a fine aggregate and zeolite, which decreases the emission of carbon dioxide released during the production of cement, as Portland cement in concrete, can be used with zeolite in concrete mixture and this mixture is very effective in mitigating heat island and decreasing temperature gradient of concrete pavement.
Competing Interests
The author declares that there are no competing interests.
Acknowledgments
This research has been supported by Ondokuz Mayıs University, under Project no. PYO.MİM.1902.15.001.