Improving Thermal Properties of Asphalt Concretes

Transcription

1 Improving Thermal Properties of Asphalt Concretes Dr. A.T.Papagiannakis, P.E. F.ASCE Professor and McDermott Chair, Dept. of Civil and Enviro. Engineering University of Texas at San Antonio Rajesh Kaphle M.Sc. Graduate Srudent Dept. of Civil and Enviro. Engineering University of Texas at San Antonio 1

2 Cool Pavement: 2

3 Not as Cool Pavement: 3

4 Motivation Part of an energy harvesting project financed by CPS Energy a public utility company for the City of San Antonio, Texas. Basic question: Can material selection reduce pavement surface temperatures? 4

5 The Urban Heat Environment 5

6 UHIs UHI effect: Elevated ambient temperatures in urban environments, caused by pavements and buildings acting as heat sinks during hot daytime weather and heat sources during the nighttime (EPA, 2003), DT = T urban T rural The air temperature in large cities during day time may be 1-3 C higher than that in nearby rural areas (EPA, 2008). The reason for the higher ambient temperatures in the evening is that structures (i.e., pavements and buildings) emit the heat they stored during the day. U.S. Environmental Protection Agency. (2003). Reducing urban heat islands: Compendium of strategies U.S. Environmental Protection Agency. (2008). Reducing urban heat islands: Compendium of strategies. 6

7 A case in point Data from Krehbiel and Henebry (2016) 7

8 Summer Day-time Urban vs Rural DT 8

9 Summer Night-time Urban vs Rural DT 9

10 Objectives Analyze the distribution of temperatures in asphalt concrete (AC) pavements for a range of thermal properties and environmental conditions. Evaluate whether incorporating alternative materials can effectively reduce summer time pavement surface temperatures of ACs-mitigate UHI effects. Test the mechanical properties of the modified AC materials. 10

11 Pavement Heat Flow Components q s = direct radiation from the sum q a = diffused solar radiation from clouds q t = terrestrial radiation from buildings etc.. q r = radiation emitted by body q c = heat loss/gain by convection with air q k = heat loss/gain by conduction with lower layers Solaimanian, M., and Kennedy, T. W. (1993). Predicting maximum pavement surface temperature using maximum air temperature and hourly solar radiation. Transportation Research Record Washington DC. 11

12 Thermal Equilibrium q net = q s + q a ± q c ± q k q r q s = 1 α R o τ a m cos i q a = ρ 4 σ T air q c = h e (T s T air ) q k = k T d T s d 4 q r = ε σ T s 2 T z 2 = ρ c κ T t 12

13 Challenge The technical challenges of mitigating the UHI effect are many. Focus is on one of them, namely the cooling of AC pavement surfaces by considering alternative innovative materials as a substitute to conventional aggregates. 13

14 Implementation Algorithm: Finite Element Finite Difference Software: EICM TEMPS FEM or FD commercial software 14

15 Input-Output Layer Properties: Surface Albedo Thicknesses Specific Heat Capacity Heat Permeability Environmental Input: Hourly Air Temperature Hourly Wind Speed Hourly Solar Radiation Input Sources: NOAA s temperature and wind database National solar radiation database ta/nsrdb/ Output: Daily mean, max, min and SD temperatures at specified location 15

16 Typical Thermal Properties of AC Material Heat Permeability W/m/⁰K Heat Capacity J/kg/⁰K Asphalt Binder Limestone Air

17 Estimating Thermal Properties of the Pavement Layers Heat Capacity c: Scalar property (simple weighted sum of c p of constituents) Heat Permeability k: Vector property (defined by the arrangement of constituents) 17

18 Modelling Heat Permeability k = 1 Ø i /k i k = Ø i k i FEM Solution 18

19 Estimating k through FEM q y = k T y 19

20 Parametric Study Heat Permeability Heat Capacity Albedo W/m/⁰K J/kg/⁰K - Baseline Range +20% +10% +67% -20% -10% -67% Two locations: San Antonio, TX International Falls, MN Data Input/Output: Hourly weather data for 1 year (2015) Output average daily average, min, max and SD 1 cm below surface. 20

21 Increasing k; Texas 21

22 Increasing c; Texas 22

23 Increasing a; Texas 23

24 Temp. C Increasing k; Minnesota Perm. k Perm. 1.2k Date 24

25 Temp. C Increasing c; Minnesota Cap. c Cap. 1.1c Date 25

26 Temp. C Increasing a; Minnesota Alb Alb Date 26

27 Parametric Study Observations + The average daily pavement surface temperatures dropped by a 3+ degrees by increasing the albedo a. - The average daily pavement surface temperatures changed very little by increasing, c and k. + The range in daily summer time temperatures decreases by a few degrees by increasing a, c or k (in order of significance). Trend is the same, for hot and cold climates. Increasing the albedo a, may be undesirable in densely-built areas because the radiated heat from pavement surface will heat the adjacent buildings ( canyon effect). Dial-a-color asphalt binders and surface emulsion treatments are becoming available. Increasing the c and k of the asphalt concrete would be ideal. 27

28 Colored Pavements! 28

29 Urban Canyons 29

30 Macroscopic Urban Heat Models Recent literature examples: MIT: Norford et al. (2018), Estimation of the Urban Heat Island Effect UI U-C: Sen and Roesler (2017), An Uncoupled Pavement-Urban Canyon Model for Heat Islands

31 Heat Properties of Alternative Aggregates Heat Perm. k Heat Capacity c Material W/m/⁰K J/kg/⁰K Limestone Granite Glass Silica PP Graphite

32 Unmodified Mix Volumetrics Property Value Binder content 4.33% Air Voids 5.26% VMA 14.91% VFA 64.78% G mm at N ini 85% 32

33 Modifying the AC with Graphite Sieve #8 Sieve #4 33

34 Thermal Property Changes from Graphite Modification Type k (W/m/ K) c (J/Kg/ K) a Unmodified AC Graphite modified AC Change 189.4% increase 4.75% decrease - 34

35 Temp. C Graphite Modified AC; Texas Un Mod Graphite Mod /1 6/2 6/3 6/4 6/5 6/6 6/7 6/8 6/9 6/10 6/11 6/12 6/13 6/14 6/15 Date 35

36 Temp. C Graphite Modified AC; MN Location Un Mod Graphite Mod /1 6/2 6/3 6/4 6/5 6/6 6/7 6/8 6/9 6/10 6/11 6/12 6/13 6/14 6/15 Date 36

37 Comparing Surface Ts.; Texas Max Day Unmod Graphite Mod. D(Temp) 6/ / / / / / / / / / / / / / / Average Drop 3.32 Min Day Unmodified Graphite Mod. D(Temp) 6/ / / / / / / / / / / / / / / Average Rise

38 Comparing Surface Ts.; Minnesota Max Day Un Mod Graphite Mod Delta Tempr 6/ / / / / / / / / / / / / / / Average Drop 2.85 Min Day Un Mod Graphite Mod Delta Tempr 6/ / / / / / / / / / / / / / / Average 38 Rise 1.74

39 Graphite Modification Observations The summer-time average surface temperature reduction from AC graphite modification were marginal. There was a reduction of 3+ degrees in summer-time maximum temperatures. There was a 1.7+degrees increase in summer-time minimum temperatures. 39

40 Material Testing Indirect Tension (IT) Wheel Rutting (APA) 40

41 Sample Close-up Conventional AC Graphite Modified AC 41

42 ITS Max Load kn IT Test Results Sample ID Unmodified AC Graphite Modified AC 42

43 Passes to 12 mm ruts APA Test Results Sample ID Unmodified AC Graphite Modfied AC 43

44 Take-away AC thermal properties affecting surface temperatures are (in decreasing order of significance): a, c and k. Graphite modification of the AC reduced summer time average surface temperatures considerably. Tensile strength and rutting resistance of the graphite modified AC is not quite at par with that conventional AC. Some rutting resistance improvement can be realized from lower AC temperatures. 44

45 Obrigado Questões? 45