Role of conductive spreader layer in reducing surface temperature of HMA pavements

Figures & data

Figure 1 Sketch of asphalt solar collector embedded in highway lane. Note: Pipes spaced away from the wheel path.

Table 1 Design variables and their respective ranges.

Variables	Range
General dimensions
Pipe spacing (W)	0.1–2.0 m
Depth (D)	25–100 mm
Spreader layer properties
Conductivity (k s)	1.8–1000 W/m K
Thickness (t s)	0–12.8 mm
Shape	Sloped, lipped shapes
Boundary conditions/inputs
Absorbed solar flux (Q s)	500–1100 W/m^2
Cooling fluid (T f)	12–30°C

Table 2 Default material properties used, unless otherwise noted.

Material	Thermal conductivity (W/m K)	Heat capacity (J/kg K)	Density (kg/m^3)
Pavement	1.8	1050	2350
Spreader	160	900	2700
Note: All bodies assumed to have an initial temperature of 26.8°C.

Table 3 Governing equations and boundary conditions used (unless otherwise noted).

Location and mechanism	Governing equation	Constants
Subdomain – pavement, spreader conduction	∂T/∂t = kΔT/ρC p	k – thermal conductivity
		ρ – density
		C p – heat capacity
		t – time
Top surface boundary condition convection with ambient air	q = h(T sur − T amb)	h – convection coefficient h = 6 W/m^2 K T amb  = 26.8°C
Top surface boundary condition radiation to ambient objects	q = ϵσ(T sur ^4 − T amb ^4)	ϵ – surface emissivity – ϵ = 0.9 σ – Stefan–Boltzmann const. – 5.67 × 10^− 8 J m^− 2 s^− 1 K^− 4
Top surface boundary condition absorbed solar flux	q = q s	q s = 700 W/m^2
Pipe wall boundary condition isothermal	T = T f	T f = 16.8°C
All other boundary conditions insulated	q = 0

Figure 2 Top and side temperature distribution of (a) unmodified pavement, (b) pavement with pipe, (c) pavement with pipe and spreader layer. Note W = 40 cm, t = 8 h; Q _s = 700 W/m².

Figure 3 Top and side-cut temperature distribution with varied pipe spacing (W). Note: t = 8 h.

Figure 4 Effects of pipe spacing (W) on maximum temperature (T _max). Note: t = 8 h, no spreader layer present.

Figure 5 Top and side temperature distribution of pavement with spreader layer depth (a) D = 25 mm, (b) D = 50 mm, (c) D = 75 mm. Note: W = 40 cm, t = 8 h.

Figure 6 Effects of pipe depth (D) on maximum temperature (T _max), with and without spreader layer (t _s = 6.4 mm). Note: W = 40 cm, t = 8 h.

Figure 7 Top and side temperature distribution of pavement with spreader layer thickness, (a) t _s = 1.6 mm, (b) t _s = 3.2 mm, (c) t _s = 6.4 mm. Note: W = 40 cm, t = 8 h.

Figure 8 Effects of spreader layer thickness (t _s) on maximum temperature (T _max), varying pipe spacing (W). Note: t = 8 h.

Figure 9 Sketch of (a) ‘sloped’ and (b) ‘lipped’ spreader layer designs.

Figure 10 Heat flux distribution of pavement with spreader layer shape: (a) flat, t _s = 6.4 mm, (b) sloped, (c) lipped. Note: W = 40 cm, t = 8 h.

Figure 11 Top and side temperature distribution of pavement with spreader layer shape: (a) flat, t _s = 6.4 mm, (b) flat, t _s = 9.6 mm, (c) sloped, (d) lipped. Note: W = 40 cm, t = 8 h.

Figure 12 Top and side temperature distribution of pavement with spreader layer conductivity k _s  =  (a) 10 W/m K, (b) 160 W/m K, (c) 500 W/m K. Note: W = 40 cm, t = 8 h, t _s = 6.4 mm.

Figure 13 Effects of thermal conductivity of spreader layer (k _s) on maximum surface temperature (T _max), comparing x-component (k _sx ) to y-component (k _sy ) of anisotropic materials. Note: W = 40 cm, t = 8 h, t _s = 6.4 mm.

Figure 14 Effects of absorbed solar flux (Q _s) on maximum surface temperature (T _max), varying spreader thickness (t _s). Note: W = 40 cm, t = 8 h.

Figure 15 Effect of coolant temperature (T _f) on maximum temperature (T _max), varying spreader layer thickness (t _s). Note: W = 40 cm, t = 8 h.

Figure 16 Maximum temperature (T _max) of samples subjected to sinusoidal solar flux and varied ambient temperature over 7 days. Note: W = 61 cm.

Figure 17 Maximum temperature distribution of pavement with spreader layer, using sinusoidal solar flux and ambient temperature. Note: W = 61 cm, pipes lie outside wheel path (position of pipes projected onto surface with dashed lines, T _f varies linearly from 16.8 to 26.8°C (front-to-back).