RUMBLING - BRICK If all the joints of a brick pavement are filled with cement grout, vehicles in going over the pavement are likely to produce a considerable rumble or roar, due apparently to a hollow space between the bricks and the foundation. At times this rumbling is very pronounced. It is most common in hot weather, but occurs also in cold weather. The fact of this noise and its remedy are more clearly established than its cause. The hollow space under the pavement which gives rise to the rum bling may be due to any one of three distinct causes, viz.: 1, ex pansion of the pavement due to an increase of temperature; 2, crowding inward of the curbs due to expansive action of the freez ing water in the soil outside of the curbs; and 3, shrinkage of the sand cushion due to its drying out.
In hot weather, the crown of the pavement may be lifted from the foundation by the expansion of the bricks due to an increase of temperature. This can occur only when the curbs are firmly enough supported to serve as the abutments of the layer of brick when acting as an arch. To this explanation it has been objected (a) that the expansion of the brick would be insufficient to produce the effect and (b) that the single course of brick could not act as an arch and carry a loaded vehicle. • The amount and force of expansion of a brick pavement having cement-filled joints is illustrated by the fact that not infrequently the thrust of the pavement is sufficient to force the curb outward except where it is supported by the walk from the curb to the adjoining property, at which places the break in the curb makes manifest its movement. Again, in pavements having all of the joints filled with cement, a considerable number of joints, both transverse and longitudinal, can be found in which the cement mortar has been crushed until it is practically only so much sand. Further, the expansion of the top course of brick is frequently sufficient to lift the brick from the foundation and form a ridge in the surface. This ridge sometimes runs crosswise of the street and sometimes lengthwise, according to whether it is due to longi tudinal or to transverse expansion. If the ridge is only an inch or two high, the pavement will be likely to settle gradually nearly or quite back to its former position, without material damage; but occasionally the ridge rises to a height of a foot or more. in
which case a crack usually opens at the highest point and finally a considerable strip of the pavement breaks up, some of the bricks not infrequently being thrown several feet into the air.
What will be the effect of a difference of temperature of 30° F. on a brick pavement 40 feet wide having a crown of 6 inches? The co-efficient of expansion of brick is 0.000.003,4 for 1° F.,* and 40 feet for a difference of temperature of 30° F. would be 0.000,003,4 X 30 X 40 X 12=0.049 inch. If the pavement lifts from its foundation and acts as an arch, its own weight will compress the bricks and in part neutralize the expansion. Owing to the lack of the necessary data the amount of this compression can not be computed accurately. The pavement if 4 inches thick will weigh about 45 pounds per square foot. The co-efficient of elasticity of ordinary brick is about 3,500,000 pounds per square inch,f and of paving brick from 3,500,000 to 7,000,000 pounds per square inch, I the co-efficient of elasticity of ordinary Portland cement mortar is about 1,500,000 pounds per square inch.§ It will be assumed that the co-efficient of elasticity of a brick pavement is 4.000.000 pounds per square inch. which is exact enough for the purpose of this illus tration. Considering a strip of the pavement 1 foot wide and equating the moment of the crown thrust and the moment of the weight of half the pavement. we find that the compression in the arch is approximately 18.000 pounds. If the thrust is uniformly distributed it is equal to a compression of practically 400 pounds per square inch, and will produce a shortening of 0.048 inch in the width of the pavement. This shortening is almost exactly the expansion for a change of 30° F., and consequently a rise of temperature of 30° would be required to neutralize the compression of the brick due to the arch action of the pavement, and any con ditional increase of temperature would lift the brick from the foundation.