You can't-lb per cubic foot is a measurement of pressure, tonnes is measure of weight. You could convert Ib per cubic foot to tonnes per cubic foot or more usually, gram per cubic centimetre but not lb per cubic foot to tonnes
80 lbs per cubic foot
The weight of concrete totally depends on the weight of the rock aggregate that is in it. In my area, the rock is hard basalt, which is a little heavier than some other rock. The weight per cubic foot is around 156 pounds.
Granite is figured at 168 pounds per cubic foot. If you know the thickness, you can determine the weight per square foot.
About 100 pounds per cubic foot
Calculate the volume of mix needed by multiplying 20x20x9. This gives cubic inches. Then divide that answer by 144 cubic inches per cubic foot, and you have the volume in cubic feet. A cubic foot of asphalt is a different weight depending on the weight of the rock in contains, but let's 158 pound of asphalt hotmix per cubic foot. Multiply the volume in cubic feet, above, by 158lbs, and you have the weight of mix per 20x20x9. Divide the answer by 2000 to convert to tons. Divide the tonnage by the weight per cubic feet calculated above in the second paragraph.
Granite is figured at 168 pounds per cubic foot. If you know the thickness, you can determine the weight per square foot.
Typical window glass weighs 156 pounds per cubic foot. For comparison, a cubic foot of aluminum weighs about 168 pounds, and a cubic foot of oak is around 50 pounds.
1 cubic yard = 27 cubic feet so 100.00 per cubic yard equates to 100/27 = 3.7 per cubic foot. 3.00 per 5 cubic ft equates to 0.6 per cubic foot - which is definitely less than 3.7 per cubic ft.
Specific weight is the weight of the material per unit volume. Water has the specific weight of 62.43 pounds per cubic foot or 9.807 Newtons per cubic meter.
64 lbs
50 pounds