Experience seems to have proved that cement grout is, everything considered, the best and cheapest filling for the joints in brick pavement. If the filling is properly done, the edges of the brick are supported and the corners do not chip off. With the expansion joints provided at intervals by the bituminous-filled joints, the curbs will not be forced out of line, nor will the pavement be raised from its sand bed by expansion, causing the rumbling sound sometimes noticed.
Where grout filling is used there seems to be no necessity for covering the surface of the pavement with sand, as is usually done, provided the grout is kept damp.
[51]. Since immunity from early natural decay is secured by preservative treatment, the important requisite for wood paving blocks is capacity to withstand the wear and tear of the travel on the street. We have as yet no very satisfactory data as to the ability of the various species of wood to endure the somewhat peculiar and special duty to which paving blocks are subjected.
The test which seems to most nearly approach to what is wanted is that of crushing strength, when the force is applied to the end of the sample, parallel to its fibers; but this does not embrace the effect of impact to which paving blocks are subjected under street travel. Whether this may be considered a function of the end-crushing strength or not is an open question, though there seems good reason to believe that it will prove to be so; and if so, there is no good reason why woods of substantially equal strength under the end-crushing test should not show about the same endurance under street travel, independent of the element of natural durability, which is practically eliminated by preservative treatment.
The end-crushing strength per square inch of some of the kinds of timber named as acceptable is about as follows:
| Southern long-leaved yellow pine | 6900 lbs. |
| Lob-lolly pine | 6500 lbs. |
| Short-leaved pine | 5900 lbs. |
| Cuban pine | 7900 lbs. |
| Norway pine | 6700 lbs. |
| Red gum | 7100 lbs. |
It was formerly very customary to specify that only Southern long-leaved yellow pine might be used for paving blocks, though this requirement was seldom strictly enforced. The fact is, that with the exception of the test based upon the number of growth rings per inch, it requires an expert knowledge, acquired only by long experience, to distinguish with certainty the species of Southern pine from the appearance of the lumber alone. It is now almost impossible to obtain in the market shipments of strictly long-leaved yellow pine, and while that wood is undoubtedly superior to the other pine timbers for paving blocks it seems useless to specify its exclusive use, or to propose specifications designed to exclude lumber made from other species of pine. It was doubtless the recognition of this situation that influenced the Association for Standardizing Paving Specifications, at its last (New Orleans) convention to adopt a specification which practically admits everything known in the market as “Southern yellow pine” having annual growth rings averaging less than eight to the inch and excluding all timber having less than six rings to the inch. Within these limits these specifications practically admit all pine lumber shipped from Southern mills.
The specifications here proposed, by limiting the number of growth rings to nine per inch, would not confine the lumber to true long-leaved yellow pine, but would secure a more mature and solid quality of lumber. It is true, however, that both these specifications and those adopted by the A. S. P. C. exclude most “Cuban pine” lumber which is very rapid growing, the growth rings often numbering but three or four to the inch, though the strength of the wood from this species indicates that it may safely be used for pavement.
[52]. The question of the most economical depth for wood paving blocks is as yet unsettled. In New York City, blocks 3½ inches in depth are adopted as the standard and are being used on streets of the heaviest travel, the practice of Berlin, Paris and other foreign cities being thus followed. The arguments in favor of these short blocks are lower first cost, and that, with much deeper blocks, the usual uneven wear of a wood pavement will make it so rough as to require removal before the blocks are worn down so as to be split up and dislodged from their places. While reliable data on these points are wanting, it seems to the writer very unwise to use such short blocks on streets of heavy travel, and he would recommend that the minimum length for use on such streets be 4½ inches, and he would prefer 5 inches.
On streets of light travel a length of 3½ inches should be satisfactory.