208 
purposes, furnishes another problem for 
the engineer. Soft stone, when placed in 
the pavement and rolled, may seemingly be 
treated with bituminous material and thus 
cover up all cracks or fractures which may- 
have been made by the initial rolling, but 
the rolling of the pavement, both after the 
first application of the bituminous material 
and by the final compacting of the same, 
will again fracture more or less of the 
stone aggregate. These fractures, occur- 
ring as they do after the application of the 
bituminous material, will be void of bind- 
ing qualities and the stone will immediately 
start to break down through continued 
rubbing, one piece against the other. This, 
in itself, produces dust and allows the 
moisture to penetrate into the pavement, 
again causing destruction of the binding 
powers of bituminous material. In addi- 
tion, this soft stone is readily worn away 
by passing vehicles and serves to shorten 
the life of a bituminous pavement. 
Stone crushed in quarries, where there 
is an abundance of stripping or where 
there is a stratum of soft stone, or stone 
hauled upon the highway and allowed to 
stand for several days subject to clouds 
of dust from passing vehicles, becomes 
covered with a fine film of dust particles. 
Stone of this character, when placed in the 
road and subjected to the application of 
bituminous material, cannot be properly 
bonded, inasmuch as the bituminous ma- 
terial, while covering each stone, is unable 
to directly bond itself to the stone proper, 
for the dust layer covering the stone will 
act as a barrier, and, while the bitum- 
inous material will, or may, hold the stone 
in place in the pavement, it will not pro- 
duce the proper bonding of one particle of 
the stone with another. Stone with a 
smooth, non-porous surface will prove a 
failure on account of the inability of the 
bituminous material to adhere thereto. 
Wet or damp stones result, likewise, in the 
bituminous material not obtaining any ad- 
hesion. It is well also to avoid those 
classes of stone which do not crush in a 
cubical or near-cubical shape. Stones 
that break or crush in splinters will not 
take the compression and will not inter- 
lock themselves one with the other in the 
manner most desired in accomplishing first- 
class results. 
The selection of a suitable binder is a 
most important matter, requiring skill and 
judgment. There is no one “best” product, 
and the selection must be governed by a 
number of conditions. The principal fea- 
tures to be considered in this connection 
are the characteristics of the stone to be 
used ; the manner of applying the bitum- 
inous cement, whether the material is to 
be applied hot or cold, by means of dis- 
tributors with or without pressure, or by 
pouring pots ; the quantity and character 
of traffic to which the road will be sub- 
jected; climatic conditions; the cost of bi- 
tuminous material, and the probable cost 
of application. 
PARK AND CEMETERY. 
Inasmuch as the results to be obtained 
are the securing of a compact, uniformly 
solid paving surface impervious to water, 
it is obvious that continued and experi- 
enced supervision and exceptional care 
should be employed. Equally important 
with the selection of the bituminous mate- 
rial is the care with which it must be heat-, 
ed, as the slightest overheating will burn 
the product and render it unfit as a binder. 
It has been my experience that no suc- 
cessful results can be accomplished unless 
the inspector keeps continually with him 
on the work a thermometer with which to 
test the temperaure to which the bitumi- 
nous cement is being subjected. Another 
frequent failure occurs through lack of 
uniformity in the quantity of bituminous 
material applied to the stone and is caused 
by crude or careless applications, and can- 
not bring about proper results. The hand 
pouring should be supplanted by suitable 
mechanical distributors which will apply 
the bituminous cements under pressure and 
insure thorough penetration and a more 
even application. 
In enumerating the various mistakes 
which so often occur in penetration work, 
I would say that the cause of failures can 
be readily traced in many instances to 
faulty plans and specifications, lack of 
care and judgment in the selection of ma- 
terials, poor and inadequate equipment 
with which to carry on the work, inex- 
perienced labor for both skilled and un- 
skilled service, continuation of work dur- 
ing unseasonable or inclement weather, 
and insufficient funds to expedite the work 
under way. I might say in passing that 
probably the most noticeable cause of fail- 
ure is that a class of incompetent con- 
tractors, having absolutely no experience 
in bituminous construction, bid on high- 
way work and look to the engineering de- 
partment as a school of instruction for 
their guidance. 
As to what the life of a bituminous 
penetration pavement will be, time alone 
can tell, as the body of such a road con- 
tains a mass of material that is not evenly, 
properly or finely graded. Both coarse and 
fine sections exist, permitting an applica- 
tion of the bituminous binder either in ex- 
cess or deficiency, and, possibly, in some 
instances, only a light seal application is 
secured. While this condition does not ex- 
ist generally, I must acknowledge that it 
is there, and I feel that wherever this 
method of construction has been, or is be- 
ing followed, it is only a question of time 
until the weak places show signs of dis- 
integration. These, of course, can be re- 
paired by an efficient maintenance force, 
and upon this maintenance force the prob- 
able length of life of this type of pave- 
ment depends. Where this class of pave- 
ment is supervised under the most expert 
management, the results at best are only 
those of a surface character and produce 
a road that for the time being seems to 
give satisfactory results. 
Bituminous concrete pavements, con- I 
structed by the mixing together of stone, 1 
sand and bituminous cement, in mechanical 
mixers, reduce the chance of failure to the 
minimum and eliminate many of the causes 
of failure found in penetration construc- 
tion. 
It is not necessary for me to discuss here 
the material, its size, quality or character, 
or to explain in detail how this mixture is 
secured, as the average engineer is well 
acquainted with this type of pavement, in- 
asmuch as it is very similar to the mixture 
used in all the large cities for street pur- 
poses. While the best results from this 
type of pavement are obtained by using a 
cement concrete foundation, it is not abso- 
lutely necessary that such a foundation be 
used, as experience has demonstrated that 
water bound macadam roads, where they 
have been properly drained, can be resur- 
faced with a mechanical mixture, and the 
results therefrom compare favorably with 
those of sheet asphalt streets. This type 
of construction has been a success and I 
feel safe in saying that most of the old 
water bound macadam roads that have been 
torn by the heavy traffic can readily be 
brought to grade, properly crowned, made 
firm, and covered with a bituminous con- 
crete that will be economical and service- 
able for many years. 
I do not believe very much in the use of 
telford foundation for a mechanically mixed 
surface, as it is almost impossible to ob- 
tain, upon a telford foundation, a surface 
that will not be more or less wavy, and 
we all know that bituminously constructed 
pavements which have a wavy surface are 
soon doomed to destruction. In the con- 
struction of this type of pavement there 
are a few points which must be closely 
watched. The first — and to my mind the j 
most important — is the mixture produced \ 
by the majority of mixers at present in use. 
There is a tendency in all revolving mix- J 
ers to bring the large stone together, and, 
unless in the dumping of the mixer this is 
closely watched, the purpose of mixing— ^ 
which is that of having the aggregate of ; 
different sizes so mixed as to give the low- | 
est percentage of voids— will be defeated. 
This condition will again arise if the ma- 
terial has to be transported a very great 
distance from the mixer to the road, as 
there will be a tendency for the larger 
stone to work toward the top of the load 
and the smaller material toward the bot- 
tom. So it is important in placing the ma- 
terial in the pavement that it should first be 
dumped upon a platform and from there 
shoveled into place in as uniform a mix- 
ture as possible. 
I have used two distinct types of mixed 
bituminous construction. In one of these j 
there was used a cut-back bituminous ma- 
terial which remained in a plastic state for 
several days after being placed on the road, 
thereby bringing about a continuous bond- 
ing process, by means of local traffic pass- J 
ing over the road. The result of this was I 
