Masonry Magazine June 1981 Page. 16
The extent of firing affects brick density. Generally, the longer the firing and the higher the temperature, the more dense the brick. Typical densities for various bricks are provided in Table 1. These values are average densities. The density for grouted hollow brick assumes 130 lb per cu ft density brick and 120 lb per cu ft density grout. Hollow brick may range from 75% to 60% solid. This may significantly change the density, however the values provided in Table 1 are for grouted hollow brick assuming 60% solid.
The density, as the specific heat, of brick masonry is slightly less than that of brick, however for simplified effective thermal storage calculations these differences are usually insignificant. Typically, the maximum amount of mortar in solid brick or grouted hollow brick walls constructed with full collar joints would be about 30% of the wall volume: however, considering that mortar has a density of about 120 lb per cu ft, this results in a less than 3% reduction in the density of the wall as compared to the density of the brick.
When considering the use of a grouted hollow wall, two wythes of masonry constructed with a grouted space in between, the density of the wall should be approximated by calculation using Equation 2:
Pe=[(P1xt1) + (P2xt2) + (Poxto)]/(t1+t2+to)
For hollow brick which is grouted, there are two paths for heat flow, one path is through the webs of the hollow brick and the other path is through the face shells and grout. The average thermal resistivity of grouted hollow brick may be determined by using Equation 3:
r=[(r1xt1)/l] + [[(r1 × 2tf) + r2 × (l - 2tf)] × [(l-t1)/(lxl)]]
where:
r = Average thermal resistivity of grouted hollow brick, in (°F ft² hr)/Btu in.
r1 = Thermal resistivity of brick, in (°F. ft² ⚫hr)/Btu in.
r2 = Thermal resistivity of grout, in (°F. ft² hr)/Btu in.
t1 = Thickness of the brick, in inches.
tf = Thickness of the face shell, in inches.
tw = Total thickness of the webs, in inches.
l = Length of the brick, in inches.
Considering the thermal resistivities and thickness of a 6 x 4 x 12 grouted hollow brick:
r1 = 0.11 (°F ft² hr)/Btu in.
r2 = 0.08 (°F ft² hr)/Btu in.
l = 12 in.
For a 14-in. thick brick thermal storage wall, constructed of a wythe of 4-in. face brick, a 4-in. grouted space and a 6-in. grouted hollow brick wythe, the densities may be selected from Table 1 where:
t1 = 4 in., P1 = 130 lb/cu ft
t2 = 6 in., P2 = 126 lb/cu ft
to = 4 in., Po = 120 lb/cu ft
By substituting these values into Equation 2, the average density of the wall, p, is found to be:
P = [(4x130)+(6x126)+(4x120)]/(4+6+4)
P = 125 lb/cu ft
Thermal Conductivity. The thermal conductivity, k, of brick is discussed in Technical Notes 4 Revised. Typical values of the thermal conductivity of brick are provided in Table 1. The thermal conductivity of brick varies with density. The denser the brick, generally the greater the thermal conductivity. The thermal conductivity of grouted brick should be determined by the dual path procedure described in Technical Notes 4 Revised.
t1 = 4 in.
tf = 1.25 in.
l = 6 in.
and substituting these values into Equation 3, the average thermal resistivity of a 6 x 4x 12 grouted hollow brick would be:
r= [(0.11x4)/12] + [[(0.11 × 2.50) + 0.08 × (6-2.50)] × [(12-4)/(12×6)]]
= 0.037 + [[0.275 +0.280] × 0.11]
= 0.098 (°F. ft² hr)/Btu in. or approximately 0.10 (°F. ft² hr)/Btu in.
The average thermal conductivity is the inverse of the average thermal resistivity. The average thermal conductivity, k, for a 6 x 4 x 12 grouted hollow brick would be:
k=1/r
k=1/0.10 °F ft² hr)/Btu in.
k=10.00 Btu/hr/F/ft² per inch of thickness
The average thermal resistivity of a storage media is the summation of the thermal resistivity times the thickness of the materials divided by the total thickness. This is expressed in Equation 5: