Specific Heat Capacity of Ferrocement Using Inverse Thermal

Transcription

1 Specific Heat Capacity of Ferrocement Using Inverse Thermal Analysis RSID6- STR-10 Specific Heat Capacity of Ferrocement Using Inverse Thermal Analysis V. Greepala, Kasetsart University Chalermphrakiat Sakonnakhon Province Campus, Sakonnakhon, Thailand, R. Parichatprecha, Naresuan University, Phisanulok, Thailand, T. Tanchaisawat, Kasetsart University Chalermphrakiat Sakonnakhon Province Campus, Sakonnakhon, Thailand, P. Nimityongskul, Asian Institute of Technology, Pathumthani, Thailand, Abstract The specific heat capacity of ferrocement at elevated temperatures of up to 800 C was analytically determined based on time-varying surface temperature during fire exposure and its temperature-dependent thermal conductivity using inverse thermal analysis approach. The main parameter investigated was the volume fraction of wire mesh. The inverse thermal calculation using finite element method is employed to solve the temperaturedependent specific heat capacity via ANSYS and the one dimensional transient heat transfer problem was solved using a first order 2-node element. The results revealed that specific heat capacity of ferrocement were in the range of J/kg-K however its consisted of two peaks of J/kg-K and J/kg-K at temperatures of 230 C and 600 C, respectively. These values were slightly higher than those of concrete cover given by Euro Code hence ferrocement can be used as fire protection material because it can absorb more heat than concrete cover. 1. Introduction Fire remains one of the most serious potential risks to most buildings and structures. Most structural materials which are weakened when exposed to high temperatures cause buildings to collapse. Therefore, the use of fire protection materials to reduce thermal damage of structural members is important and necessary. Many types of fire protection material were developed to protect structural members. The main classes of material used are cementitious, intumescent, fibrous and composite materials. Ferrocement is one of the cementitious composite materials, which is constructed of hydraulic cement mortar reinforced with close spaced layers of continuous and relatively small sized wire mesh [1]. Since mortar is a good insulator and the reinforcing wire mesh could reduce surface spalling, consequently using ferrocement jacketing for strengthening of structural components like reinforced concrete, prestressed concrete, or steel could enhance the fire resistance of the composite elements. Previous studies showed that a ferrocement jacket was a satisfactory fire protection material due to its structural fire integrity compared with that of plain mortar [2-5]. The use of ferrocement as a fire protection material needs a full understanding not only of the structural fire 1

2 integrity but also the insulation property of this material. The specific heat capacity is one of important value for determining the insulation property. The specific heat of a material is the amount of heat required to heat up 1 g of the material by one degree Celsius or Kelvin. A high specific heat means high ability for retaining heats an ability that is desirable for energy absorption in fire protection materials. It has been previously reported that a use of silica fume and fiber reinforcement increase the specific heat of cement paste[6, 7]. Nevertheless, there is still a lack of knowledge and experimental data on the specific heat capacity of ferrocement. The purpose of this study is to investigate the specific heat capacity of ferrocement. In this study, the specific heat capacity of ferrocement is determined by surface temperatures of specimen obtained from fire resistance test and the temperature-dependent thermal conductivity obtained from previous study [8, 9]. Inverse thermal calculation using finite element method is employed to solve the temperature-dependent specific heat capacity via ANSYS, a multipurpose finite element program. The one dimensional transient heat transfer problem was solved by using a first order 2-node LINK32 from the ANSYS finite element library [10]. The investigated parameter is volume fraction of wire mesh. The ferrocement specimen had dimensions of 20x240x25 mm (width x length x thickness). Ordinary steel bar with a diameter of 6 mm spaced at 10 mm center to center was used as skeletal steel in the longitudinal and transverse directions. The longitudinal and transverse skeletal steel were welded together in the same plane; in other words, there was no overlapping of skeletal steel. Galvanized hexagonal steel wire meshes were used as mesh reinforcement and the number of mesh layers was 0, 2, 6 and 16 layers corresponding to volume fractions of 0%, 0.54%, 1.63% and 4.36% respectively. The layers of wire mesh were overlapped in order to minimize the mesh opening. The mortar had compressive strength and covering of 57 MPa and 1.5 mm respectively, kept constant throughout. A sandwich-sample configuration of ferrocement which consists of a 3-mm air gap and edge insulation was subjected to high temperature which reached a maximum of 1060 C within 3 hours in an electrical furnace. Thermocouple type K was used to measure the temperature. All measurement data were recorded by computerized data logger. 2. Experimental program In this study, the effect of the volume fraction of wire mesh on the specific heat capacity of ferrocement was investigated. The study was carried out in three phases: specimen preparation, fire exposure and determination of specific heat capacity. Altogether, five series of ferrocement specimens were produced and divided into two groups, namely Group A and Group B. Experiments on Group A were conducted in order to observe the specific heat of plain mortar, steel reinforced mortar and ferrocement while Group B was investigated to determine the influence of wire mesh content by varying volume fraction of wire mesh from 0%, 0.54%, 1.63% and 4.36%, which are equivalent to 0, 2, 6 and 16 layers of hexagonal wire mesh respectively. Series 3, which was used as the control series for the two groups, consisted of skeletal steel and had volume fraction of wire mesh and mortar covering of 1.63% and 1.5 mm, respectively. The details of the specimen series and their groups are summarized in Table 1. For each series, four identical ferrocement specimens were cast in order to repeat the fire exposure twice. The specimen configurations were subjected to a temperature envelope where the temperature was gradually increased to a maximum of 1060 C within a duration of 3 hours. 2

3 Table 1 Experimental program and details of test specimens Series Mortar No. of Volume Sectional ID Covering Wire mesh Fraction Geometry (mm) Layers % A. Control specimens and the absence of mesh reinforcement FA NA (Plain Mortar) NA 0 0 Plain Mortar FA NA (Mortar & skeletal steel) NA 0 0 Mortar + Skeletal Steel FA (Ferrocement) B.Test to study the effect of volume fraction of wire mesh FA NA (0% Volume fraction) NA 0 0 FB (0.54% Volume fraction) FA (1.63% Volume fraction) FB (4.36% Volume fraction) Specimen Preparation The ferrocement reinforcement cage consisted of skeletal steel and wire mesh. Ordinary steel bar with a diameter of 6 mm spaced at 10 mm center to center was used as skeletal steel in the longitudinal and transverse directions. The volume fraction of skeletal steel was 2.14%. Galvanized hexagonal wire mesh, which had wire diameter of 0.78 mm, mesh opening of 19 mm and weight per unit area of kg/m 2, was use as mesh reinforcement. The positions of the mesh layers were controlled using galvanized plate spacers, as shown in Figure 1, in order to 3

4 obtain accurate mortar covering. The ferrocement mortar consisted of Ordinary Portland Cement (OPC) Type I and natural river sand passing sieve No.16 [11] mixed at a ratio of 1:2 by weight and the water-cement ratio was 0.48 by weight. After mixing, the mortar was cast in steel molds over the reinforcement cage and compacted using a vibrating table. The dimensions of all specimen were 200x240x25 mm (width x length x thickness). All ferrocement specimens were cured for a period of 7 days and subsequently allowed to air-dry until the time of testing. a) Position of skeletal steel and galvanized plate spacer b) Skeletal steel with galvanized plate spacer before placing wire mesh c) Installation of wire mesh d) Ferrocement specimen Figure 1 Steps in the fabrication of ferrocement 2.2 Fire Exposure A sandwich-sample configuration of ferrocement which consisted of 3-mm middle air gap and edge insulation was put in the temperature controlled electrical furnace, as shown in Figure 2. It should be note that the temperature envelope based on ASTM standard and this study, were maintained area under the curve to be equal even though the two envelopes were not exactly identical due to the performance of the furnace used [2]. A 60-mm ceramic fiber insulation edge was used to control the direction of the heat flow through the ferrocement specimen so that the heat transmission could be considered as a onedimensional heat flow from the hot side to the cold side. The change in temperature during the test which was measured by using thermocouple type K was recorded every 30 seconds by using a computerized data logger. There were three sets of thermocouples installed T1, T2 and T3. Thermocouples T1 were used to monitor the temperature of exposed ferrocement surfaces whereas thermocouple T3 was used to measure the temperature inside the furnace. Another thermocouple (T2) which was provided between the two specimens was used to monitor the unexposed surface temperature. 4

5 Figure 2: Schematic diagram of sandwich-sample configuration and the arrangement of ferrocement jacket simulation in experimental furnace 2.3 Determination of Specific Heat In this study, the specific heat capacity of ferrocement at elevated temperatures of up to 800 C are determined by surface temperatures of specimen obtained from fire exposure in Section 2.2 and the temperature-dependent thermal conductivity obtained from previous study [8] as shown in Figure 3. Figure 3: Thermal conductivity of ferrocement at elevated temperatures [8] Inverse thermal calculation using finite element method is employed to solve the temperature-dependent specific heat capacity via ANSYS, a multipurpose finite element program. A finite element model of a ferrocement specimen subjected to fire resistance test is illustrated in Figure 4. 5

6 Figure 4: Finite element modeling for determining specific heat of ferrocement The one dimensional transient heat transfer problem was solved by using a first order 2- node LINK32 from the ANSYS finite element library [10]. The thickness of ferrocement specimen was divided into 25 equal elements and 26 nodes. The material properties of ferrocement were considered unique, although it consists of mortar and wire mesh layers, which is the so-called effective material properties. From the sandwich-sample configuration of the ferrocement specimens, the boundary conditions for the simulation of fire resistance test can be written as: 0 { } x { } { q } { q } { T} = T1,{ T} = T2 (1) x= = = = 0 x= Since the temperature-dependent thermal properties, material density ( ρ ( T )) conductivity ( kt ( )), and boundary conditions, { T} = x= 0 { T1} and { q } { q } 2 the unknown temperature-dependent specific heat, ( ) p (2) and thermal = = are known, x= c T, can be assumed to be a trial specific heat, cp( T ) trial, in order to calculate unexposed surface temperatures at x=0.025, { T } calculated unexposed surface temperatures, { T 2} cal surface temperatures, { T 2}. A new value of trial specific heat, cp( T ) trial way that the computed unexposed temperatures { T 2} cal { T 2}. The iterative procedure is shown in Figure 5. 2 cal. The, must be satisfied the measure unexposed, was assigned in such a matched the measured temperatures 6

7 Figure 5: Flow chart for specific heat determination by trial and error 3. Results and Discussion The objective of this part is to determine the specific heat capacity of ferrocement specimens, which is determined by the temperature-dependent thermal conductivity obtained from previous experimental study [8] and surface temperatures of specimen obtained from fire exposure. The inverse thermal calculation using finite element method is employed to solve the temperature-dependent specific heat capacity via ANSYS, the multi-purpose finite element program. The one dimensional transient heat transfer problem was solved using a first order 2- node LINK32 from the ANSYS finite element library [10]. The thickness of ferrocement specimen was divided into 25 equal elements and 26 nodes. Materials properties of ferrocement were considered as unique. The results, which are averaged from 4 specimens, are summarized and shown in Table 2 and Figure 6-7. It was found that specific heat-temperature relationship of ferrocement consists mainly of two peaks when temperatures approximately reached 230 C and 600 C, respectively. The first peak indicated that heat from fire was absorbed by evaporation of free water in specimen [12-14] and the second peak could be explained by phase change in steel wire mesh [15] and a transformation of microstructures of cement paste [14, 16-18]. Moreover it is of interest to note that specific heat of ferrocement tends to increase as temperature was increased. However at high temperature (> 700 C) the specific heat was slightly lower than that under temperature range of C and no longer dependent on temperature. 7

8 Table 2: Specific heat capacity of ferrocement Specimen ID Average specific heat at elevated temperatures (J/kg-K) Mean Temperatures 20 C 100 C 179 C 230 C 281 C 400 C 500 C 543 C 586 C 629 C 700 C 800 C A. Effect of reinforcement incorporation FA NA (Plain mortar) FA NA (Mortar & St) FA (Ferrocement) B. Effect of the volume fraction of wire mesh FA NA (0%) FB (0.54%) FA (1.63%) FB (4.36%) Figure 6: Effect of reinforcement incorporation on specific heat capacity of ferrocement at elevated temperatures Compared with plain mortar, the incorporation of skeletal steel and wire mesh increased specific heat capacity of ferrocement especially at the two peaks (230 C and 600 C) as shown in Figure 6. It was also found that the specific heat capacity of ferrocement consisting of wire mesh was slightly higher than that consisting only skeletal steel and significantly higher than the specific heat of plain mortar. Regarding the effects of volume fraction of wire mesh on the specific heat capacity of ferrocement, the ferrocement specimen having a mortar covering of 1.5 mm (except for the case 8

9 0% volume fraction) was conducted to investigate the influence of the volume fraction by varying the volume fraction of wire mesh from 0, 0.54, 1.63 and 4.36%. The results are summarized and shown in Figure 7. It was found that an increase in wire mesh content cause slightly decrease in specific heat capacity of ferrocement at low temperature, however at the first peak the specimen which has volume fraction of 1.63% show the highest specific heat. It is of interest to observe that the value of specific heat at the second peak increased as the volume fraction was increased because at this elevated temperature steel has a chemical transformation, so the higher volume fraction of steel mesh need more energy to transform steel phase. Figure 7: Effect of volume fraction of wire mesh on specific heat capacity of ferrocement at elevated temperatures 4. Conclusions The influence of the volume fraction of wire mesh on specific heat capacity of ferrocement was investigated. The following conclusions can be drawn: 1. The values of specific heat capacity of ferrocement were in the range of J/kg-K however its consisted of two peaks of J/kg-K and J/kg- K at temperatures of 230 C and 600 C, respectively. 2. The specific heat capacity of ferrocement were slightly higher than those of concrete cover given by Euro Code [12, 15], hence ferrocement can be used as fire protection material because it can absorb more heat than concrete cover. 5. Acknowledgements The authors would like to sincerely thank the Royal Thai Government of Thailand and Kasetsart University Chalermphrakiat Sakonnakhon Province Campus for providing the financial support to carry out this research work. 9

10 6. References [1] Naaman, A.E., Ferrocement and Laminated Cementitious Composites Techno Press 3000, Michigan, USA, 2000 [2] Greepala, V. and P. Nimityongskul, Structural integrity of ferrocement panels exposed to fire. Cement and Concrete Composites, (5): p [3] Greepala, V. and P. Nimityongskul, Influence of Heating Envelope on Structural Fire Integrity of Ferrocement Jackets. Fire Technology, in Press. [4] Greepala, V. and P. Nimityongskul, Influence of heating envelope on post-fire mechanical properties of ferrocement jackets. Thammasat International Journal of Science and Technology, (3): p [5] Greepala, V. and P. Nimityongskul. Structural integrity and insulation property of Ferrocement exposed to fire. in Eighth International Symposium and Workshop on Ferrocement and Thin Reinforced Cement Composites Bangkok, Thailand. [6] Xu, Y. and D.D.L. Chung, Increasing the specific heat of cement paste by admixture surface treatments. Cement and Concrete Research, : p [7] Chung, D.D.L., Cement reinforced with short carbon fibers: a multifunctional material. Composites: Part B, : p [8] Greepala, V., et al., Thermal conductivity of ferrocement in Eleventh East Asia-Pacific Conference on Structural Engineering & Construction (EASEC-11) Building a Sustainable Environment. 2008: Taipei, Taiwan. [9] Greepala, V., Fire Resistance of Ferrocement, in School of Engineering and Technology. 2007, Asian Institute of Technology. [10] ANSYS, ANSYS Analysis Guides, Release , Ansys Inc: Canonsburg, PA 15317, USA. [11] ASTM, Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates, ASTM C136-95a, in Annual Book of ASTM Standards. 2004, American Society for Testing and Materials: West Conshohocken, United States. [12] EC2, ENV : General Rules Structural Fire Design, in Eurocode 2: Design of Concrete Structures. 1993, European Committee for Standardization: Brussels. [13] Asako, Y., T. Otaka, and Y. Yamaguchi, Fire resistance characteristics of materials with polymer gels which absorb aqueous solution of calcium chloride. Numerical Heat Transfer: Part A, : p [14] Leiva, C., et al., Influence of the type of ash on the fire resistance characteristics of ash-enriched mortars. Feul, : p [15] EC3, ENV : General Rules Structural Fire Design, in Eurocode 3: Design of steel Structures. 1995, European Committee for Standardization: Brussels. [16] Georgali, B. and P.E. Tsakiridis, Microstructure of fire-damaged concrete. A case study. Cement and Concrete Composites, : p [17] Paya, J., et al., Thermogravimetric methods for determining carbon content in fly ashes. Cement and Concrete Research, (5): p [18] Heikal, M., Effect of temperature on the physico-mechanical and mineralogical peoperties of Homra pozzolanic cement pastes. Cement and Concrete Research, : p