- Email: [email protected]
Thermal characteristics of a conductive cement-based composite for a snow-melting heated pavement system
Composite Structures 118 (2014) 106–111
Contents lists available at ScienceDirect
Composite Structures journal homepage: www.elsevier.com/locate/compstruct
Thermal characteristics of a conductive cement-based composite for a snow-melting heated pavement system Jong-Pil Won ⇑, Cheol-Keun Kim, Su-Jin Lee, Jae-Ho Lee, Ryang-Woo Kim Department of Civil & Environmental System Engineering, Konkuk University, Seoul 143-701, Republic of Korea
a r t i c l e
i n f o
Article history: Available online 22 July 2014 Keywords: Conductive heat cement-based composites Heat transfer analysis Snow melting Thermal conductivity
a b s t r a c t This study evaluated the thermal properties of an early-opening conductive heated pavement system with snow-melting functionality. The pavement system consisted of several layers: a base layer with copper plates, a conductive cement-based composite layer, and a protective layer of concrete. The conductive cement-based composite was placed over the copper-containing base concrete layer, followed by a concrete protective layer. The surface temperature of the protective concrete layer and the internal temperature of the conductive cement-based composite were measured to determine the thermal conductivity of the pavement system. Our results indicated that the thermal conductivity of the protective concrete was in the range of 1.8075–2.0534 kcal (m h °C)1. Based on the thermal characteristics of the earlyopening conductive heated pavement system and the experimental thermal conductivity results for the protective concrete, heat transfer analysis was performed on a full-scale sample; here, the separation distance between the copper plates in the base concrete layer (1000, 1250, and 1500 mm) was used as a variable. Our results indicated that separation distances of 1000 and 1250 mm provided a fairly uniform temperature distribution; however, with a separation distance of 1500 mm, the temperature between the copper plates in the central area was low. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Losses and damage caused by abnormal climate are steadily increasing worldwide, and one contributing factor is heavy snow. Severe heavy snow paralyses national logistics systems, limiting snow removal efforts. To overcome this, many snow-melting systems have been developed; however, in practice, their application is restricted, due to economic reasons, environmental contamination, and problems and cost associated with construction technology. Currently, due to their effectiveness and convenience, spreadable de-icing materials are commonly used to pretreat the roads and melt existing snow accumulation [1]. Repeated cycles of freezing and melting from chloride de-icing spray causes concrete scaling, in which the concrete surface exfoliates in a misshapen manner, unlike conventional deterioration by frost damage [2,8]. In general, scaling is limited to small areas, but it expands gradually to a larger scale over time. Insignificant scaling does not expose coarse aggregates. If the scaling area expands, however, the aggregates become exposed, and the cement paste peels away from the concrete surface by 3–10 mm. ⇑ Corresponding author. Tel.: +82 2 450 3750; fax: +82 2 2201 0907. E-mail address: [email protected] (J.-P. Won). http://dx.doi.org/10.1016/j.compstruct.2014.07.021 0263-8223/Ó 2014 Elsevier Ltd. All rights reserved.
Another approach for snow melting applications involves placing electrically heated cables inside the pavement to prevent the road from freezing during heavy snow [3]. This method is environmentally friendly because no chloride is used in the process. However, the heating cables are difficult to repair after their incorporation into the cement; thus, the entire cable must be exchanged and the entire pavement section reconstructed. The other drawback to this technique is the cost of supplying electricity to the heated cables. One possible solution is to use conductive concrete [4]. The typical resistance of concrete is 106 X cm [1]; however, the addition of a conductive powder, such as carbon, to the concrete can lower the resistance to <10 X cm [4]. In low-resistance conductive concrete, electric current is generated via electron or electrolyte flow. A current supply is used to create a flow of electrons in the concrete. In contrast, electrolyte flow can occur by interior water flow, such as that associated with residual water, free water, bound water, and/or gravitational water, after concrete hardening [5]. Therefore, enhanced heating performance of the conductive concrete can be obtained through increasing the current flow by lowering the concrete’s resistance or increasing the water content. Excessive water, however, may damage the concrete, in addition to presenting safety issues. Also, despite the concrete’s resistance
J.-P. Won et al. / Composite Structures 118 (2014) 106–111
being lowered with water or conductive additives, concrete still maintains high resistance as a material property; thus, efficient heating is difficult to achieve. Therefore, conductive materials (e.g. carbon materials or metallic materials) must be added to optimise the heating capability of conductive concrete [5]. Conductive concrete can be produced by mixing conductive powder, cement, fine aggregates, and coarse aggregates and pouring the mixture into a uniform thickness to form the slab. Another approach is to divide the concrete into layers [6]. In the layered method, concrete is mixed with a conductive powder (e.g. carbon-based or metallic powder) to form the first layer. The second layer, a heating layer with low resistance, is placed over the first layer, followed by a protective concrete layer. In the former case (the mixed single concrete layer), the entire pavement behaves as a conductor that carries a low current over its surface. The second approach consisting of several layers is electrically safer because a protective concrete layer with relatively high resistance is placed over the current-carrying heated concrete [5]. Conductive concrete exhibits very different heat tendencies due to the materials it contains. Additionally, the thermal properties of conductive concrete depend on the ambient environment and the water content. The majority of previous studies used conductive powders (e.g. graphite, carbon materials, slag aggregates, and metallic materials) in normal concrete to lower the concrete’s resistance [4,5,11]; however, snow melting proceeded at a slow pace with these earlier attempts. In the present study, conductive cement-based composites with very low resistance were used to improve the thermal conductivity of the concrete. An early-opening, conductive, heated pavement system was used to further improve snow-melting performance. The thermal properties of the developed system were evaluated. Based on the experimental results, heat transfer analysis was performed on a full-scale specimen. The internal temperature of the conductive cement-based composite and the surface temperature of the protective concrete layer were measured. Heat generation in this pavement system was attributed mostly to Joule heating with current flow, generated by supplying current to the copper plates. Because it is difficult to express Joule heating analytically, the actual experimental temperature was used as the temperature of the conductive cement-based composite for the heat transfer analysis. The surface temperature of the protective concrete corresponded to the temperature transferred via thermal conductivity. Accurate heat transfer analysis requires determination of the thermal coefficients, including the thermal conductivity, specific heat, unit weight, and outside convection coefficient [7]. The thermal conductivity and outside convection coefficient of concrete affect the temperature difference between the interior and exterior of a structure, whereas specific heat and unit weight affect mostly the temperature increase. Therefore, in this study, the installation distance of copper plates was taken as a variable, and the thermal conductivity was measured experimentally and verified using heat transfer analysis.
107
Fig. 1. Schematic diagram of the early-opening conductive heated pavement system.
2.2. Current supply and temperature measurement The current supply for the pavement system supplied 220 VAC via connection wiring between the supply and fixed copper plates for quick heating of the pavement. Note that this configuration minimised substation costs for future application sites. The surface temperature and internal temperature of the conductive cementbased composite were measured using a digital thermometer over a period of 60 min in 10-min intervals. The internal temperature was measured over a total distance of 100 mm in the sample, as shown in Fig. 2, for application to heat transfer analysis. Unlike the internal temperature, the surface temperature was not measured over the 100-mm distance, but instead was measured at specific points on the copper plates (points ML, MR, and C). The protective concrete outer layer was responsible not for generating heat but for protecting the heating layer to preserve proper operation in terms of the current supply and external load. Hence, the temperature was measured only at the points on the copper plates where heat was generated by contact resistance and at the centre region between the copper plates most affected by the separation distance. 3. Materials and mix proportion 3.1. Cement and aggregates When the conductive, heated pavement system was constructed, early-strength cement (Union Co., Republic of Korea) was used for fast-opening pavement for traffic; the chemical properties of this cement are listed in Table 1. Crushed stones (specific gravity: 2.69; maximum size: 10 mm) were used as the coarse
2. Early-opening conductive heated pavement system 2.1. Composition of the early-opening conductive heated pavement system Fig. 1 shows the composition of the early-opening conductive heated pavement system. The first layer consisted of copper plates set at specific separation distances within the concrete base. A conductive cement-based composite second layer was poured over the first. The copper plates served as a current supply to the conductive cement-based composite. Finally, a protective concrete layer was placed over the conductive composite layer to complete the pavement system.
Fig. 2. Measuring points for a copper-plate separation distance of 1000 mm.
108
J.-P. Won et al. / Composite Structures 118 (2014) 106–111
Table 1 Chemical properties of cement. Chemical properties (%)
Table 3 Properties of the metallic materials.
SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
13 ± 3
17.5 ± 3
63
50 ± 3
62.5
14 ± 3
aggregate, and washed sand (specific gravity: 2.59; fineness modulus: 2.92) was used as the fine aggregate.
Type
Specific gravity
Specific heat (Cal (g °C)1)
Thermal conductivity (kcal (m h °C)1)
Steel fiber Recycled aluminum powder Noncorrosive metallic fiber
7.85 2.69
0.11 0.214
64.8 194
7.20
–
157
3.2. Conductive powder Normal concrete has insulating properties and high electrical resistance [5]. If conductive powder made of carbon is applied, the resistance is lowered to <10 X cm, allowing heat generation in the concrete when current is applied [6]. In this study, graphite and coke powder were added to cement to create a conductive cement-based composite second layer; the physical and chemical properties of these powders are listed in Table 2. 3.3. Metallic materials In addition to the conductive powder, materials with a high electric conductivity, such as steel slag, stainless steel fibre, and carbon fibre, may also be added to the composite to improve heating performance [8]. This study used noncorrosive metal fibre, recycled aluminium powder, and steel fibre. Table 3 lists the properties of these metallic materials. The length, width, and thickness of the noncorrosive metal fibre were 30, 1.2, and 0.02 mm, respectively. The recycled aluminium powder (maximum diameter: 5 mm, with a round shape) had the highest thermal conductivity, whereas the steel fibre (length: 15 mm; diameter: 0.49 mm) had the lowest thermal conductivity. 3.4. Mix proportion The early-opening conductive heated pavement system comprised a base concrete layer that housed the copper plates, a conductive cement-based composite layer, and a protective concrete layer. The conductive cement-based composite consisted of cement and fine aggregate in a ratio of 1:2.45, and the water–binder ratio was 55%. The mix proportion of the metallic materials was 40 kg/m3, with 5 wt% graphite used as the conductive powder additive. Note that the mix proportion of metallic materials (40 kg/m3) was determined based on stable reheating measurements, as verified in preliminary heating performance tests. The mix proportion used for the protective concrete layer is listed in Table 4. 4. Experimental and heat transfer analysis 4.1. Thermal conductivity The thermal conductivity of the concrete depends not only on the composition materials and environmental factors but also on the measurement method. The moisture content, density, and temperature of the concrete are known to significantly affect the thermal conductivity, in addition to the amount and type of aggregate and the external humidity. Thermal conductivity measurements can differ considerably for the same specimen, depending on the
test conditions and the method used. Thus, various techniques have been used to evaluate thermal conductivity to improve the reliability of the measurement. This study measured thermal conductivity using a calibrated Quick thermal conductivity meter (QTM, QTM-500, Kyoto Electronics Mfg. Co. LTD, Japan) by placing the probe in contact with the polished flat specimen surface. Heat was transmitted from a line source in a certain direction with respect to the sensor’s placement on the specimen surface. The thermal conductivity was calculated from the change in temperature and heat over a certain time frame. The general formula to obtain thermal conductivity using a QTM is given in Eq. (1):
k¼
q lnðt2 t1 Þ 4pðT 2 T 1 Þ
ð1Þ
where q = given heat (W/m), k = thermal conductivity of the test specimen (W/m K), t = measurement time (s), T = measurement temperature (K). The thermal conductivity measurement was limited to the protective concrete layer and the conductive cement-based composite. Square test specimens (size: 100 100 20 mm) were made for the measurement. Fig. 3 shows a photograph of the experimental set-up for the thermal conductivity measurements. 4.2. Heating temperature A specific evaluation method for measuring the temperature of early-opening, conductive, heated pavement has yet to be established; thus, in this study, testing was performed based on the methods used in previous studies [6,9,10]. The size of the full-scale specimen used in the evaluation of heating temperature was 70 1000 3000 mm. The pavement body was constructed by fixing the copper plates to the floor and layering a 30 mm-thick layer of the conductive cement-based composite, followed by a 40 mm-thick layer of protective concrete. The thickness of each layer was derived in preliminary testing to find the optimal thickness for generating the most heat. The copper plates that were used to transfer current to the pavement (thickness: 1 mm; width: 50 mm; length: 1200 mm) were separated by 1000, 1250, and 1500 mm for testing. 4.3. Heat transfer analysis To verify the surface temperature distribution of the protective concrete caused by the heating layer, heat transfer analysis was performed on the early-opening conductive heated pavement system. In general, the copper plates are current transfer devices. Hence, heat transfer analysis should be performed by changing
Table 2 Properties of amorphous graphite and coke powder. Type
Specific gravity
Size (mm)
Fixed carbon (%)
Volatile matter (%)
Moisture (%)
Amorphous graphite Coke powder
2.23 0.85
0.044 3
80 85.3
5 0.42
1 0.83
109
J.-P. Won et al. / Composite Structures 118 (2014) 106–111 Table 4 Mix proportions.
*
Gmax (mm)
Slump (mm)
Air (%)
S/a (%)
W/C (%)
Unit weight (kg/m3) Water
Cement
Sand
Gravel
10
180 ± 20
4±1
55
51.3
200
390
908
743
AD*(%)
0.8
Polycarboxylate superplasticizer.
Fig. 3. Experimental set-up for thermal conductivity measurements.
Table 5 Thermal conductivity results. Type
Protective concrete
*
Thermal conductivity (kcal (m h °C)1)
Batch
1
2
3
#1
1.8075
1.8602
1.8848
#2
2.0413
1.9997
2.0534
Mean (SD)*
1.9412 (0.09)
Standard deviation.
the temperature of the copper plates only (as an input variable). In this case, if current I is supplied through the copper plates of resistance R, then the energy consumed in the conductor I2R becomes the Joule heat. However, in this study, the temperature distribution in accordance with the thermal conductivity of each layer was derived by measuring the internal temperature of the conductive cement-based composite over a constant distance and using these measured values in the heat transfer analysis. The surface temperature of the protective concrete layer was obtained through numerical analysis and compared with the surface temperature obtained experimentally. 5. Results and discussion This study evaluated the thermal properties of the early-opening conductive, heated pavement system with snow-melting functionality. For this, the surface and internal temperatures of the pavement system were measured. The thermal conductivity measurement was limited to the layer of protective concrete. Heat transfer analysis was performed using the thermal property coefficients and the heating temperature obtained by testing. The temperature of the conductive cement-based composite applied to the heat transfer analysis was measured in a full-scale test sample of the early-opening conductive heated pavement system. The internal temperature of the pavement system was determined after a duration of 60 min, the time necessary for sufficient snow melting with application of a heating current.
Fig. 4. Temperature of the early-opening conductive heated pavement system for a copper-plate separation distance of 1000, 1250, and 1500 mm: (a) conductive cement-based composite, and (b) protective concrete.
5.1. Thermal conductivity Our results indicated that the thermal conductivity of the protective concrete was 1.8075–2.0534 kcal (m h °C)1; the test values are listed in Table 5. In general, the thermal conductivity of concrete depends on the aggregate type used. However, our experimental results were in good agreement with those from a previous study (1.7–2.53 kcal (m h °C)1) for a concrete temperature of 38 °C [7]. 5.2. Heating temperature Fig. 4 shows the results from the heating test. The temperature of the conductive cement-based composite was highest when the distance between the copper plates was 1000 mm, and decreased in the order of 1500 and 1250 mm. However, although the surface temperature of the protective concrete exhibited differences in the initial temperature rise, which appeared to depend on the temperature difference in the conductive cement-based composite,
110
J.-P. Won et al. / Composite Structures 118 (2014) 106–111
Fig. 5. Current flow rate for a copper plate separation distance of 1000, 1250, and 1500 mm.
similar temperature distributions ranging from 4.1 to 4.5 °C were obtained by the end of the experiment. In the case of conductive cement-based composites, the resistance was lowered by adding conductive powder or metallic materials. If an electric current is applied, Joule heat is generated. In this study, the applied current passed through the copper plates and current transfer devices and flowed to the low-resistance region. Hence, the region of least resistance, that is, where the metallic materials had become concentrated, played an important role. In concrete, the added metallic material is not evenly distributed in the concrete when the materials are mixed and set; consequently, regions of lower and higher resistance are produced. Current flows more readily where metallic materials are more concentrated, and the temperature rise in these regions is much higher compared with regions of lower resistance. Therefore, as the distance between the copper plates increases, the temperature distribution should be lower due to a higher specific resistance. Thus, the greater heat generated at a 1500 mm separation, as opposed to a 1250 mm separation, was attributed to the inhomogeneous dispersion of the metallic material additive. Additionally, more heat was generated (as contact resistance) between the copper plates and the conductive cement-based composite. Hence, the temperature should be highest at the copper plates and should decrease towards the centre of the separation distance; this is supported by the specific resistance’s being proportional to the distance between the copper plates. However, full-scale tests showed results contrary to this for separation distances of 1250 and 1500 mm. When the distance was 1250 mm, the measured temperature was highest between the plates (at
the separation centre) and lowest on the copper plates. When the distance was 1500 mm, the temperature measured at the left copper plate was lower than that measured at the centre of the separation between the plates. As mentioned previously, this result was attributed to the difference in heat due to the contact resistance between the current supply and copper plates, as well as to the uneven dispersion of metallic materials. When the distance between the copper plates was 1000 mm, the internal temperature measured was very high, in contrast to that measured for separation distances of 1250 and 1500 mm; this difference was related to current values, as shown in Fig. 5. The current was highest when the distance between the copper plates was 1000 mm. This is because this was the shortest distance tested, and as the contact area between the copper plates and the conductive heat cement-based composites was constant, the current flow was higher due to the reduced specific resistance at this distance. Thus, it is considered that relatively large heat generation occurred because Joule heat, which greatly affects heat generation, is proportional to the square of current and resistance. The surface temperature of the protective concrete was maintained over a constant temperature range of 4.1–4.5 °C, despite the temperature difference in the conductive cement-based composite. In this case, the difference in the surface temperature was not large because the protective concrete had constant thermal conductivity, unlike the conductive cement-based composite, which exhibited larger differences in heat generation, depending on the resistance. 5.3. Heat transfer analysis Heat transfer analysis was performed experimentally using the internal temperature of the early-opening conductive heated pavement system measured. Thermal conductivity, an input variable used in the heat transfer analysis, was measured using a conductive pavement test sample. The conductive, heated pavement system used in the analysis consisted of a 30 mm-thick conductive cement-based composite and a 40 mm-thick layer of protective concrete. Table 6 lists the measured temperature of the heated layer derived by full-scale tests of the early-opening conductive heated pavement system and by heat transfer analysis, the measured surface temperature of the protective concrete, and the surface temperature of the protective concrete obtained by analysis. The comparisons showed that when the distance between the copper plates was 1250 and 1500 mm, the surface temperature obtained by analysis and that obtained by actual full-scale tests were very similar. However, the two temperatures were significantly different at a separation distance of 1000 mm between the copper
Table 6 Surface and internal temperature of early-opening conductive heated pavement system. Interval (mm)
Temperature (°C) ML
L6
L5
L4
L3
L2
L1
C
R1
R2
R3
R4
R5
R6
MR
1000
A B C
11.7 4.1 9.3
– – –
– – –
12.3 – 8.9
9.6 – 7.5
9.6 – 6.9
9.4 – 6.7
9.8 3.7 6.7
8.6 – 6.6
9.5 – 7.2
12.0 – 8.7
14.0 – 10.8
– – –
– – –
15.0 4.5 11.5
1250
A B C
5.6 4.6 4.5
– – –
5.9 – 4.3
5.7 – 4.3
6.1 – 4.5
6.9 – 4.8
7.2 – 5.1
7.5 2.5 5.1
6.9 – 5.0
6.6 – 4.8
6.3 – 4.6
7.0 – 4.5
5.2 – 3.9
– – –
3.6 4.1 3.3
1500
A B C
2.1 2.3 2.1
3.4 – 2.8
5.1 – 3.6
6.6 – 4.2
5.7 – 4.1
5.6 – 3.9
4.2 – 3.5
4.7 1.9 3.4
4.4 – 3.3
4.6 – 3.2
3.7 – 3.1
3.9 – 3.2
4.6 – 3.7
6.1 – 4.6
7.4 4.3 5.9
A: internal temperature by experiment. B: surface temperature by experiment. C: surface temperature by heat transfer analysis.
J.-P. Won et al. / Composite Structures 118 (2014) 106–111
plates. This likely occurred because the temperature obtained from actual full-scale test was applied as the temperature of the conductive heat cement-based composites for heat transfer analysis, and when the distance was 1000 mm, the current was greater, resulting in greater heating and higher temperatures. 6. Conclusion This study evaluated the thermal properties of an early-opening conductive heated pavement system for melting snow. For this, the surface and internal temperatures of the pavement system were measured. Tests were performed on a full-scale specimen using the thermal property coefficients obtained from experiments, which were verified through heat transfer analysis. Conclusions of this study may be summarised as follows: (1) The internal temperature of the conductive cement-based composites was highest when the separation distance between the copper plates was 1000 mm; it decreased in the order of 1500 and 1250 mm. For the concrete protective layer, the surface temperature of all specimens was measured as 4.1–4.5 °C for a 60-min period after the application of a current. This result was attributed to the constant thermal conductivity of the protective concrete layer, as opposed to the conductive cement-based composite, which depends on the dispersion of metallic materials, the contact resistance, and the distance between the copper plates. (2) Joule heating is difficult to simulate using heat transfer analysis. Hence, in this study, the internal temperature of the conductive cement-based composite was measured over a constant distance, and the values were applied to the analysis. In the case of the protective concrete, real thermal conductivity was measured and used as an input variable. The surface temperature distribution of protective concrete was analysed in accordance with the thermal conductivity. (3) According to the temperature distribution of the early-opening conductive heated pavement system through heat transfer analysis, when the distance between the copper plates was 1000 and 1250 mm, the temperature distribution was uniform. However, for a separation distance of 1500 mm, the temperature was lowest at the centre point between the two plates. This result was attributed to the increase in the specific resistance resulting from greater separation between the two copper plates.
111
(4) The surface temperature of the protective concrete layer obtained by analysis and the surface temperature obtained experimentally with a full-scale test were compared. When the distance between the copper plates was 1250 and 1500 mm, the two temperatures were very similar. However, for a plate separation of 1000 mm, the two temperatures showed different tendencies. In this case, the temperature from the full-scale tests was applied as the temperature of the heating layer in the heat transfer analysis process. When the distance was 1000 mm, a relatively higher current was used; the temperature difference was large due to the difference in heat generation.
Acknowledgment This paper was supported by Konkuk University in 2013. References [1] Wang Huajun, Liu Linbo, Chen Zhihao. Experimental investigation of hydronic snow melting process on the inclined pavement. Cold Reg Sci Technol 2010;63:44–9. [2] Zhao Hongming, Wu Zhimin, Wang Songgen, Zheng Jianjun, Che Guangjie. Concrete pavement deicing with carbon fiber heating cables. Cold Reg Sci Technol 2011;65:413–20. [3] Jo DH. A study on the durability of electrically conductive concrete with coarse aggregate. Seoul national university of science and technology [Master’s thesis]; 2010. [4] Hu Hu, Tian Xin. Test and study on electrical property of conductive concrete. Proc Earth Planet Sci 2012;5:83–7. [5] Tuan CY. Implementation of conductive concrete for deicing (Roca bridge): Nebraska department of roads Project No. SPR-P1(04) P565. University of Nebraska-Lincoln; 2008. [6] Tumidajski PJ, Xie P, Arnott M, Beaudoin JJ. Overlay current in a conductive concrete snow melting system. Cem Concr Res 2003;33:1807–9. [7] Kim Kook-Han, Jeon Sang-Eun, Kim Jin-Keun, Yang Sungchul. An experimental study on thermal conductivity of concrete. Cem Concr Res 2003;33:363–71. [8] Giergiczny Zbigniew, Glinicki Michal A, Sokołowski Marcin, Zielinski Marek. Air void system and frost-salt scaling of concrete containing slag-blended cement. Constr Build Mater 2009;23:2451–6. [9] Lee Chun-Yao, Wang Siang-Ren. Analysis of resistance characteristics of conductive concrete using press-electrode method. World Acad Sci Eng Technol 2010;72:91–4. [10] Wang Xiufeng, Wang Yonglan, Jin Zhihao. Electrical conductivity characterization and variation of carbon fiber reinforced cement composite. Mater Sci 2002;37:223–7. [11] Hu Hu, Tian Xin. Test and study on electrical property of conductive concrete. Proc Earth Planet Sci 2012;5:83–7.
Contents lists available at ScienceDirect
Composite Structures journal homepage: www.elsevier.com/locate/compstruct
Thermal characteristics of a conductive cement-based composite for a snow-melting heated pavement system Jong-Pil Won ⇑, Cheol-Keun Kim, Su-Jin Lee, Jae-Ho Lee, Ryang-Woo Kim Department of Civil & Environmental System Engineering, Konkuk University, Seoul 143-701, Republic of Korea
a r t i c l e
i n f o
Article history: Available online 22 July 2014 Keywords: Conductive heat cement-based composites Heat transfer analysis Snow melting Thermal conductivity
a b s t r a c t This study evaluated the thermal properties of an early-opening conductive heated pavement system with snow-melting functionality. The pavement system consisted of several layers: a base layer with copper plates, a conductive cement-based composite layer, and a protective layer of concrete. The conductive cement-based composite was placed over the copper-containing base concrete layer, followed by a concrete protective layer. The surface temperature of the protective concrete layer and the internal temperature of the conductive cement-based composite were measured to determine the thermal conductivity of the pavement system. Our results indicated that the thermal conductivity of the protective concrete was in the range of 1.8075–2.0534 kcal (m h °C)1. Based on the thermal characteristics of the earlyopening conductive heated pavement system and the experimental thermal conductivity results for the protective concrete, heat transfer analysis was performed on a full-scale sample; here, the separation distance between the copper plates in the base concrete layer (1000, 1250, and 1500 mm) was used as a variable. Our results indicated that separation distances of 1000 and 1250 mm provided a fairly uniform temperature distribution; however, with a separation distance of 1500 mm, the temperature between the copper plates in the central area was low. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Losses and damage caused by abnormal climate are steadily increasing worldwide, and one contributing factor is heavy snow. Severe heavy snow paralyses national logistics systems, limiting snow removal efforts. To overcome this, many snow-melting systems have been developed; however, in practice, their application is restricted, due to economic reasons, environmental contamination, and problems and cost associated with construction technology. Currently, due to their effectiveness and convenience, spreadable de-icing materials are commonly used to pretreat the roads and melt existing snow accumulation [1]. Repeated cycles of freezing and melting from chloride de-icing spray causes concrete scaling, in which the concrete surface exfoliates in a misshapen manner, unlike conventional deterioration by frost damage [2,8]. In general, scaling is limited to small areas, but it expands gradually to a larger scale over time. Insignificant scaling does not expose coarse aggregates. If the scaling area expands, however, the aggregates become exposed, and the cement paste peels away from the concrete surface by 3–10 mm. ⇑ Corresponding author. Tel.: +82 2 450 3750; fax: +82 2 2201 0907. E-mail address: [email protected] (J.-P. Won). http://dx.doi.org/10.1016/j.compstruct.2014.07.021 0263-8223/Ó 2014 Elsevier Ltd. All rights reserved.
Another approach for snow melting applications involves placing electrically heated cables inside the pavement to prevent the road from freezing during heavy snow [3]. This method is environmentally friendly because no chloride is used in the process. However, the heating cables are difficult to repair after their incorporation into the cement; thus, the entire cable must be exchanged and the entire pavement section reconstructed. The other drawback to this technique is the cost of supplying electricity to the heated cables. One possible solution is to use conductive concrete [4]. The typical resistance of concrete is 106 X cm [1]; however, the addition of a conductive powder, such as carbon, to the concrete can lower the resistance to <10 X cm [4]. In low-resistance conductive concrete, electric current is generated via electron or electrolyte flow. A current supply is used to create a flow of electrons in the concrete. In contrast, electrolyte flow can occur by interior water flow, such as that associated with residual water, free water, bound water, and/or gravitational water, after concrete hardening [5]. Therefore, enhanced heating performance of the conductive concrete can be obtained through increasing the current flow by lowering the concrete’s resistance or increasing the water content. Excessive water, however, may damage the concrete, in addition to presenting safety issues. Also, despite the concrete’s resistance
J.-P. Won et al. / Composite Structures 118 (2014) 106–111
being lowered with water or conductive additives, concrete still maintains high resistance as a material property; thus, efficient heating is difficult to achieve. Therefore, conductive materials (e.g. carbon materials or metallic materials) must be added to optimise the heating capability of conductive concrete [5]. Conductive concrete can be produced by mixing conductive powder, cement, fine aggregates, and coarse aggregates and pouring the mixture into a uniform thickness to form the slab. Another approach is to divide the concrete into layers [6]. In the layered method, concrete is mixed with a conductive powder (e.g. carbon-based or metallic powder) to form the first layer. The second layer, a heating layer with low resistance, is placed over the first layer, followed by a protective concrete layer. In the former case (the mixed single concrete layer), the entire pavement behaves as a conductor that carries a low current over its surface. The second approach consisting of several layers is electrically safer because a protective concrete layer with relatively high resistance is placed over the current-carrying heated concrete [5]. Conductive concrete exhibits very different heat tendencies due to the materials it contains. Additionally, the thermal properties of conductive concrete depend on the ambient environment and the water content. The majority of previous studies used conductive powders (e.g. graphite, carbon materials, slag aggregates, and metallic materials) in normal concrete to lower the concrete’s resistance [4,5,11]; however, snow melting proceeded at a slow pace with these earlier attempts. In the present study, conductive cement-based composites with very low resistance were used to improve the thermal conductivity of the concrete. An early-opening, conductive, heated pavement system was used to further improve snow-melting performance. The thermal properties of the developed system were evaluated. Based on the experimental results, heat transfer analysis was performed on a full-scale specimen. The internal temperature of the conductive cement-based composite and the surface temperature of the protective concrete layer were measured. Heat generation in this pavement system was attributed mostly to Joule heating with current flow, generated by supplying current to the copper plates. Because it is difficult to express Joule heating analytically, the actual experimental temperature was used as the temperature of the conductive cement-based composite for the heat transfer analysis. The surface temperature of the protective concrete corresponded to the temperature transferred via thermal conductivity. Accurate heat transfer analysis requires determination of the thermal coefficients, including the thermal conductivity, specific heat, unit weight, and outside convection coefficient [7]. The thermal conductivity and outside convection coefficient of concrete affect the temperature difference between the interior and exterior of a structure, whereas specific heat and unit weight affect mostly the temperature increase. Therefore, in this study, the installation distance of copper plates was taken as a variable, and the thermal conductivity was measured experimentally and verified using heat transfer analysis.
107
Fig. 1. Schematic diagram of the early-opening conductive heated pavement system.
2.2. Current supply and temperature measurement The current supply for the pavement system supplied 220 VAC via connection wiring between the supply and fixed copper plates for quick heating of the pavement. Note that this configuration minimised substation costs for future application sites. The surface temperature and internal temperature of the conductive cementbased composite were measured using a digital thermometer over a period of 60 min in 10-min intervals. The internal temperature was measured over a total distance of 100 mm in the sample, as shown in Fig. 2, for application to heat transfer analysis. Unlike the internal temperature, the surface temperature was not measured over the 100-mm distance, but instead was measured at specific points on the copper plates (points ML, MR, and C). The protective concrete outer layer was responsible not for generating heat but for protecting the heating layer to preserve proper operation in terms of the current supply and external load. Hence, the temperature was measured only at the points on the copper plates where heat was generated by contact resistance and at the centre region between the copper plates most affected by the separation distance. 3. Materials and mix proportion 3.1. Cement and aggregates When the conductive, heated pavement system was constructed, early-strength cement (Union Co., Republic of Korea) was used for fast-opening pavement for traffic; the chemical properties of this cement are listed in Table 1. Crushed stones (specific gravity: 2.69; maximum size: 10 mm) were used as the coarse
2. Early-opening conductive heated pavement system 2.1. Composition of the early-opening conductive heated pavement system Fig. 1 shows the composition of the early-opening conductive heated pavement system. The first layer consisted of copper plates set at specific separation distances within the concrete base. A conductive cement-based composite second layer was poured over the first. The copper plates served as a current supply to the conductive cement-based composite. Finally, a protective concrete layer was placed over the conductive composite layer to complete the pavement system.
Fig. 2. Measuring points for a copper-plate separation distance of 1000 mm.
108
J.-P. Won et al. / Composite Structures 118 (2014) 106–111
Table 1 Chemical properties of cement. Chemical properties (%)
Table 3 Properties of the metallic materials.
SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
13 ± 3
17.5 ± 3
63
50 ± 3
62.5
14 ± 3
aggregate, and washed sand (specific gravity: 2.59; fineness modulus: 2.92) was used as the fine aggregate.
Type
Specific gravity
Specific heat (Cal (g °C)1)
Thermal conductivity (kcal (m h °C)1)
Steel fiber Recycled aluminum powder Noncorrosive metallic fiber
7.85 2.69
0.11 0.214
64.8 194
7.20
–
157
3.2. Conductive powder Normal concrete has insulating properties and high electrical resistance [5]. If conductive powder made of carbon is applied, the resistance is lowered to <10 X cm, allowing heat generation in the concrete when current is applied [6]. In this study, graphite and coke powder were added to cement to create a conductive cement-based composite second layer; the physical and chemical properties of these powders are listed in Table 2. 3.3. Metallic materials In addition to the conductive powder, materials with a high electric conductivity, such as steel slag, stainless steel fibre, and carbon fibre, may also be added to the composite to improve heating performance [8]. This study used noncorrosive metal fibre, recycled aluminium powder, and steel fibre. Table 3 lists the properties of these metallic materials. The length, width, and thickness of the noncorrosive metal fibre were 30, 1.2, and 0.02 mm, respectively. The recycled aluminium powder (maximum diameter: 5 mm, with a round shape) had the highest thermal conductivity, whereas the steel fibre (length: 15 mm; diameter: 0.49 mm) had the lowest thermal conductivity. 3.4. Mix proportion The early-opening conductive heated pavement system comprised a base concrete layer that housed the copper plates, a conductive cement-based composite layer, and a protective concrete layer. The conductive cement-based composite consisted of cement and fine aggregate in a ratio of 1:2.45, and the water–binder ratio was 55%. The mix proportion of the metallic materials was 40 kg/m3, with 5 wt% graphite used as the conductive powder additive. Note that the mix proportion of metallic materials (40 kg/m3) was determined based on stable reheating measurements, as verified in preliminary heating performance tests. The mix proportion used for the protective concrete layer is listed in Table 4. 4. Experimental and heat transfer analysis 4.1. Thermal conductivity The thermal conductivity of the concrete depends not only on the composition materials and environmental factors but also on the measurement method. The moisture content, density, and temperature of the concrete are known to significantly affect the thermal conductivity, in addition to the amount and type of aggregate and the external humidity. Thermal conductivity measurements can differ considerably for the same specimen, depending on the
test conditions and the method used. Thus, various techniques have been used to evaluate thermal conductivity to improve the reliability of the measurement. This study measured thermal conductivity using a calibrated Quick thermal conductivity meter (QTM, QTM-500, Kyoto Electronics Mfg. Co. LTD, Japan) by placing the probe in contact with the polished flat specimen surface. Heat was transmitted from a line source in a certain direction with respect to the sensor’s placement on the specimen surface. The thermal conductivity was calculated from the change in temperature and heat over a certain time frame. The general formula to obtain thermal conductivity using a QTM is given in Eq. (1):
k¼
q lnðt2 t1 Þ 4pðT 2 T 1 Þ
ð1Þ
where q = given heat (W/m), k = thermal conductivity of the test specimen (W/m K), t = measurement time (s), T = measurement temperature (K). The thermal conductivity measurement was limited to the protective concrete layer and the conductive cement-based composite. Square test specimens (size: 100 100 20 mm) were made for the measurement. Fig. 3 shows a photograph of the experimental set-up for the thermal conductivity measurements. 4.2. Heating temperature A specific evaluation method for measuring the temperature of early-opening, conductive, heated pavement has yet to be established; thus, in this study, testing was performed based on the methods used in previous studies [6,9,10]. The size of the full-scale specimen used in the evaluation of heating temperature was 70 1000 3000 mm. The pavement body was constructed by fixing the copper plates to the floor and layering a 30 mm-thick layer of the conductive cement-based composite, followed by a 40 mm-thick layer of protective concrete. The thickness of each layer was derived in preliminary testing to find the optimal thickness for generating the most heat. The copper plates that were used to transfer current to the pavement (thickness: 1 mm; width: 50 mm; length: 1200 mm) were separated by 1000, 1250, and 1500 mm for testing. 4.3. Heat transfer analysis To verify the surface temperature distribution of the protective concrete caused by the heating layer, heat transfer analysis was performed on the early-opening conductive heated pavement system. In general, the copper plates are current transfer devices. Hence, heat transfer analysis should be performed by changing
Table 2 Properties of amorphous graphite and coke powder. Type
Specific gravity
Size (mm)
Fixed carbon (%)
Volatile matter (%)
Moisture (%)
Amorphous graphite Coke powder
2.23 0.85
0.044 3
80 85.3
5 0.42
1 0.83
109
J.-P. Won et al. / Composite Structures 118 (2014) 106–111 Table 4 Mix proportions.
*
Gmax (mm)
Slump (mm)
Air (%)
S/a (%)
W/C (%)
Unit weight (kg/m3) Water
Cement
Sand
Gravel
10
180 ± 20
4±1
55
51.3
200
390
908
743
AD*(%)
0.8
Polycarboxylate superplasticizer.
Fig. 3. Experimental set-up for thermal conductivity measurements.
Table 5 Thermal conductivity results. Type
Protective concrete
*
Thermal conductivity (kcal (m h °C)1)
Batch
1
2
3
#1
1.8075
1.8602
1.8848
#2
2.0413
1.9997
2.0534
Mean (SD)*
1.9412 (0.09)
Standard deviation.
the temperature of the copper plates only (as an input variable). In this case, if current I is supplied through the copper plates of resistance R, then the energy consumed in the conductor I2R becomes the Joule heat. However, in this study, the temperature distribution in accordance with the thermal conductivity of each layer was derived by measuring the internal temperature of the conductive cement-based composite over a constant distance and using these measured values in the heat transfer analysis. The surface temperature of the protective concrete layer was obtained through numerical analysis and compared with the surface temperature obtained experimentally. 5. Results and discussion This study evaluated the thermal properties of the early-opening conductive, heated pavement system with snow-melting functionality. For this, the surface and internal temperatures of the pavement system were measured. The thermal conductivity measurement was limited to the layer of protective concrete. Heat transfer analysis was performed using the thermal property coefficients and the heating temperature obtained by testing. The temperature of the conductive cement-based composite applied to the heat transfer analysis was measured in a full-scale test sample of the early-opening conductive heated pavement system. The internal temperature of the pavement system was determined after a duration of 60 min, the time necessary for sufficient snow melting with application of a heating current.
Fig. 4. Temperature of the early-opening conductive heated pavement system for a copper-plate separation distance of 1000, 1250, and 1500 mm: (a) conductive cement-based composite, and (b) protective concrete.
5.1. Thermal conductivity Our results indicated that the thermal conductivity of the protective concrete was 1.8075–2.0534 kcal (m h °C)1; the test values are listed in Table 5. In general, the thermal conductivity of concrete depends on the aggregate type used. However, our experimental results were in good agreement with those from a previous study (1.7–2.53 kcal (m h °C)1) for a concrete temperature of 38 °C [7]. 5.2. Heating temperature Fig. 4 shows the results from the heating test. The temperature of the conductive cement-based composite was highest when the distance between the copper plates was 1000 mm, and decreased in the order of 1500 and 1250 mm. However, although the surface temperature of the protective concrete exhibited differences in the initial temperature rise, which appeared to depend on the temperature difference in the conductive cement-based composite,
110
J.-P. Won et al. / Composite Structures 118 (2014) 106–111
Fig. 5. Current flow rate for a copper plate separation distance of 1000, 1250, and 1500 mm.
similar temperature distributions ranging from 4.1 to 4.5 °C were obtained by the end of the experiment. In the case of conductive cement-based composites, the resistance was lowered by adding conductive powder or metallic materials. If an electric current is applied, Joule heat is generated. In this study, the applied current passed through the copper plates and current transfer devices and flowed to the low-resistance region. Hence, the region of least resistance, that is, where the metallic materials had become concentrated, played an important role. In concrete, the added metallic material is not evenly distributed in the concrete when the materials are mixed and set; consequently, regions of lower and higher resistance are produced. Current flows more readily where metallic materials are more concentrated, and the temperature rise in these regions is much higher compared with regions of lower resistance. Therefore, as the distance between the copper plates increases, the temperature distribution should be lower due to a higher specific resistance. Thus, the greater heat generated at a 1500 mm separation, as opposed to a 1250 mm separation, was attributed to the inhomogeneous dispersion of the metallic material additive. Additionally, more heat was generated (as contact resistance) between the copper plates and the conductive cement-based composite. Hence, the temperature should be highest at the copper plates and should decrease towards the centre of the separation distance; this is supported by the specific resistance’s being proportional to the distance between the copper plates. However, full-scale tests showed results contrary to this for separation distances of 1250 and 1500 mm. When the distance was 1250 mm, the measured temperature was highest between the plates (at
the separation centre) and lowest on the copper plates. When the distance was 1500 mm, the temperature measured at the left copper plate was lower than that measured at the centre of the separation between the plates. As mentioned previously, this result was attributed to the difference in heat due to the contact resistance between the current supply and copper plates, as well as to the uneven dispersion of metallic materials. When the distance between the copper plates was 1000 mm, the internal temperature measured was very high, in contrast to that measured for separation distances of 1250 and 1500 mm; this difference was related to current values, as shown in Fig. 5. The current was highest when the distance between the copper plates was 1000 mm. This is because this was the shortest distance tested, and as the contact area between the copper plates and the conductive heat cement-based composites was constant, the current flow was higher due to the reduced specific resistance at this distance. Thus, it is considered that relatively large heat generation occurred because Joule heat, which greatly affects heat generation, is proportional to the square of current and resistance. The surface temperature of the protective concrete was maintained over a constant temperature range of 4.1–4.5 °C, despite the temperature difference in the conductive cement-based composite. In this case, the difference in the surface temperature was not large because the protective concrete had constant thermal conductivity, unlike the conductive cement-based composite, which exhibited larger differences in heat generation, depending on the resistance. 5.3. Heat transfer analysis Heat transfer analysis was performed experimentally using the internal temperature of the early-opening conductive heated pavement system measured. Thermal conductivity, an input variable used in the heat transfer analysis, was measured using a conductive pavement test sample. The conductive, heated pavement system used in the analysis consisted of a 30 mm-thick conductive cement-based composite and a 40 mm-thick layer of protective concrete. Table 6 lists the measured temperature of the heated layer derived by full-scale tests of the early-opening conductive heated pavement system and by heat transfer analysis, the measured surface temperature of the protective concrete, and the surface temperature of the protective concrete obtained by analysis. The comparisons showed that when the distance between the copper plates was 1250 and 1500 mm, the surface temperature obtained by analysis and that obtained by actual full-scale tests were very similar. However, the two temperatures were significantly different at a separation distance of 1000 mm between the copper
Table 6 Surface and internal temperature of early-opening conductive heated pavement system. Interval (mm)
Temperature (°C) ML
L6
L5
L4
L3
L2
L1
C
R1
R2
R3
R4
R5
R6
MR
1000
A B C
11.7 4.1 9.3
– – –
– – –
12.3 – 8.9
9.6 – 7.5
9.6 – 6.9
9.4 – 6.7
9.8 3.7 6.7
8.6 – 6.6
9.5 – 7.2
12.0 – 8.7
14.0 – 10.8
– – –
– – –
15.0 4.5 11.5
1250
A B C
5.6 4.6 4.5
– – –
5.9 – 4.3
5.7 – 4.3
6.1 – 4.5
6.9 – 4.8
7.2 – 5.1
7.5 2.5 5.1
6.9 – 5.0
6.6 – 4.8
6.3 – 4.6
7.0 – 4.5
5.2 – 3.9
– – –
3.6 4.1 3.3
1500
A B C
2.1 2.3 2.1
3.4 – 2.8
5.1 – 3.6
6.6 – 4.2
5.7 – 4.1
5.6 – 3.9
4.2 – 3.5
4.7 1.9 3.4
4.4 – 3.3
4.6 – 3.2
3.7 – 3.1
3.9 – 3.2
4.6 – 3.7
6.1 – 4.6
7.4 4.3 5.9
A: internal temperature by experiment. B: surface temperature by experiment. C: surface temperature by heat transfer analysis.
J.-P. Won et al. / Composite Structures 118 (2014) 106–111
plates. This likely occurred because the temperature obtained from actual full-scale test was applied as the temperature of the conductive heat cement-based composites for heat transfer analysis, and when the distance was 1000 mm, the current was greater, resulting in greater heating and higher temperatures. 6. Conclusion This study evaluated the thermal properties of an early-opening conductive heated pavement system for melting snow. For this, the surface and internal temperatures of the pavement system were measured. Tests were performed on a full-scale specimen using the thermal property coefficients obtained from experiments, which were verified through heat transfer analysis. Conclusions of this study may be summarised as follows: (1) The internal temperature of the conductive cement-based composites was highest when the separation distance between the copper plates was 1000 mm; it decreased in the order of 1500 and 1250 mm. For the concrete protective layer, the surface temperature of all specimens was measured as 4.1–4.5 °C for a 60-min period after the application of a current. This result was attributed to the constant thermal conductivity of the protective concrete layer, as opposed to the conductive cement-based composite, which depends on the dispersion of metallic materials, the contact resistance, and the distance between the copper plates. (2) Joule heating is difficult to simulate using heat transfer analysis. Hence, in this study, the internal temperature of the conductive cement-based composite was measured over a constant distance, and the values were applied to the analysis. In the case of the protective concrete, real thermal conductivity was measured and used as an input variable. The surface temperature distribution of protective concrete was analysed in accordance with the thermal conductivity. (3) According to the temperature distribution of the early-opening conductive heated pavement system through heat transfer analysis, when the distance between the copper plates was 1000 and 1250 mm, the temperature distribution was uniform. However, for a separation distance of 1500 mm, the temperature was lowest at the centre point between the two plates. This result was attributed to the increase in the specific resistance resulting from greater separation between the two copper plates.
111
(4) The surface temperature of the protective concrete layer obtained by analysis and the surface temperature obtained experimentally with a full-scale test were compared. When the distance between the copper plates was 1250 and 1500 mm, the two temperatures were very similar. However, for a plate separation of 1000 mm, the two temperatures showed different tendencies. In this case, the temperature from the full-scale tests was applied as the temperature of the heating layer in the heat transfer analysis process. When the distance was 1000 mm, a relatively higher current was used; the temperature difference was large due to the difference in heat generation.
Acknowledgment This paper was supported by Konkuk University in 2013. References [1] Wang Huajun, Liu Linbo, Chen Zhihao. Experimental investigation of hydronic snow melting process on the inclined pavement. Cold Reg Sci Technol 2010;63:44–9. [2] Zhao Hongming, Wu Zhimin, Wang Songgen, Zheng Jianjun, Che Guangjie. Concrete pavement deicing with carbon fiber heating cables. Cold Reg Sci Technol 2011;65:413–20. [3] Jo DH. A study on the durability of electrically conductive concrete with coarse aggregate. Seoul national university of science and technology [Master’s thesis]; 2010. [4] Hu Hu, Tian Xin. Test and study on electrical property of conductive concrete. Proc Earth Planet Sci 2012;5:83–7. [5] Tuan CY. Implementation of conductive concrete for deicing (Roca bridge): Nebraska department of roads Project No. SPR-P1(04) P565. University of Nebraska-Lincoln; 2008. [6] Tumidajski PJ, Xie P, Arnott M, Beaudoin JJ. Overlay current in a conductive concrete snow melting system. Cem Concr Res 2003;33:1807–9. [7] Kim Kook-Han, Jeon Sang-Eun, Kim Jin-Keun, Yang Sungchul. An experimental study on thermal conductivity of concrete. Cem Concr Res 2003;33:363–71. [8] Giergiczny Zbigniew, Glinicki Michal A, Sokołowski Marcin, Zielinski Marek. Air void system and frost-salt scaling of concrete containing slag-blended cement. Constr Build Mater 2009;23:2451–6. [9] Lee Chun-Yao, Wang Siang-Ren. Analysis of resistance characteristics of conductive concrete using press-electrode method. World Acad Sci Eng Technol 2010;72:91–4. [10] Wang Xiufeng, Wang Yonglan, Jin Zhihao. Electrical conductivity characterization and variation of carbon fiber reinforced cement composite. Mater Sci 2002;37:223–7. [11] Hu Hu, Tian Xin. Test and study on electrical property of conductive concrete. Proc Earth Planet Sci 2012;5:83–7.