Thermal Conductivity

The thermal conductivity of tire shreds is lower than typical soils and varies depending on the size of the tire shreds.  One project completed by Shao, et al. (1995) used tire shred samples with maximum particle sizes of 0.04 to 1 in.  The thermal conductivities ranged: 0.0563 Btu/hr-ft-°F (0.0838 Cal/m-hr-°C) for 0.04 in. particles tested in a thawed state with a water content less than 1% and with low compaction 0.0988 Btu/hr-ft-°F (0.147 Cal/m-hr-°C) for 1 in. tire shreds tested in a frozen state with a water content of 5% and high compaction     Thermal conductivity was back calculated by Humphrey, Chen, and Eaton (1997) for tire shreds with a maximum size of 2 inches.  A value of 0.12 Btu/hr-ft-°F (0.20 W/m-°C) was obtained.  Table 1 below gives further thermal conductivity values for gravel and five types of tire shreds.     As particle size increases, and more air can circulate in the voids, thermal conductivity increases, and the tire shreds become less effective as insulators.  For insulation projects, tire shreds with a maximum size of 3 inches should be used.  Figure 1 shows that thermal conductivity also tends to decrease as density increases.

Table 1. Summary of apparent thermal conductivities of air dried tire shreds (Lawrence, et al., 1998)

Sample	Density		Void Ratio	Apparent thermal conductivity		Surcharge
	(pcf)	(Mg/m3)		(Btu/hr-ft-°F)	(W/m-°C)
	117.6	1.88	0.41	0.295	0.510	none
gravel	121.6	1.95	0.36	0.326	0.563	half
	123.0	1.97	0.34	0.345	0.596	full
	38.5	0.62	0.85	0.120	0.207	none
F&B-g	43.3	0.69	0.64	0.113	0.195	half
	45.4	0.73	0.56	0.114	0.197	full
	39.1	0.63	0.85	0.145	0.251	none
F&B-s	42.8	0.69	0.69	0.130	0.225	half
	45.3	0.73	0.60	0.134	0.232	full
	39.7	0.64	0.998	0.159	0.275	none
Palmer	45.1	0.72	0.76	0.119	0.206	half
	48.5	0.78	0.63	0.125	0.216	full
	39.2	0.63	0.97	0.158	0.273	none
Pine State	45.4	0.73	0.7	0.139	0.240	half
	49.6	0.79	0.56	0.114	0.197	full
	36.0	0.58	1.13	0.184	0.318	none
Sawyer	41.0	0.66	0.87	0.148	0.256	half
	43.7	0.70	0.76	0.156	0.270	full

Figure 1. Apparent thermal conductivity of tire shreds vs. density (Lawrence, et al., 1998)

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