THE SELECTION OF MATERIALS FOR MACADAM ROADS[7]
No one rock can be said to be a universally excellent road material. The
climatic conditions vary so much in different localities, and the volume
and character of traffic vary so much on different roads, that the
properties necessary to meet all the requirements can be found in no one
rock. If the best macadam road be desired, that material should be
selected which best meets the conditions of the particular road for
which it is intended.
The movement for better country roads which has received such an impetus
from the bicycle organizations is still felt, and is gaining force from
the rapid introduction of horseless vehicles. To this demand, which
comes in a large measure from the urban population, is to be added that
of the farmer, who is wakening to the fact that good roads greatly
increase the profits from his farm produce, and thus materially better
his condition; and to the farmer, indeed, we must look for any real
improvement in our country roads.
In considering the comparative values of different rocks for
road-building, it must be taken for granted in all cases that the road
is properly laid out, constructed, and maintained. For if this is not
the case, only inferior results can be expected, no matter how good the
material may be.
In most cases the selection of a material for road-making is determined
more by its cheapness and convenience of location than by any properties
it may possess. But when we consider the number of roads all over our
country which are bad from neglect and from obsolete methods of
maintenance that would be much improved by the use of any rock, this
regard for economy is not to be entirely deprecated. At the same time,
as a careless selection leads to costly and inferior results, too much
care cannot be used in selecting the proper material when good roads
are desired at the lowest cost. When macadam roads are first introduced
into a district they are at worst so far superior to the old earth roads
that the question is rarely asked, whether, if another material had been
used, better roads would not have been obtained, and this at a smaller
cost. When mistakes are made they are not generally discovered until
much time and money have been expended on inferior roads. Such errors
can in a great measure be avoided if reasonable care is taken in the
selection of a suitable material. To select a material in a haphazard
way, without considering the needs of the particular road on which it is
to be used, is not unlike an ill person taking the nearest medicine at
hand, without reference to the nature of the malady or the properties of
the drug. If a road is bad, the exact trouble must first be ascertained
before the proper remedy can be applied. If the surface of a macadam
road continues to be too muddy or dusty after the necessary drainage
precautions have been followed, then the rock of which it is constructed
lacks sufficient hardness or toughness to meet the traffic to which it
is subjected. If, on the contrary, the fine binding material of the
surface is carried off by wind and rain and is not replaced by the wear
of the coarser fragments, the surface stones will soon loosen and allow
water to make its way freely to the foundation and bring about the
destruction of the road. Such conditions are brought about by an excess
of hardness or toughness of the rock for the traffic. Under all
conditions a rock of high cementing value is desirable; for, other
things being equal, such a rock better resists the wear of traffic and
the action of wind and rain. This subject, however, will be referred to
again.
Until comparatively recent years but little was known of the relative
values of the different varieties of rock as road material, and good
results were obtained more by chance and general observation than
through any special knowledge of the subject. These conditions, however,
do not obtain at present, for the subject has received a great deal of
careful study, and a fairly accurate estimate can be made of the
fitness of a rock for any conditions of climate and traffic.
In road-building the attempt should be made to get a perfectly smooth
surface, not too hard, too slippery, or too noisy, and as free as
possible from mud and dust, and these results are to be attained and
maintained as cheaply as possible. Such results, however, can only be
had by selecting the material and methods of construction best suited to
the conditions.
In selecting a road material it is well to consider the agencies of
destruction to roads that have to be met. Among the most important are
the wearing action of wheels and horses' feet, frost, rain, and wind. To
find materials that can best withstand these agencies under all
conditions is the great problem that confronts the road-builder.
Before going further, it will be well to consider some of the physical
properties of rock which are important in road-building, for the value
of a road material is dependent in a large measure on the degree to
which it possesses these properties. There are many such properties that
affect road-building, but only three need be mentioned here. They are
hardness, toughness, and cementing or binding power.
By hardness is meant the power possessed by a rock to resist the wearing
action caused by the abrasion of wheels and horses' feet. Toughness, as
understood by road-builders, is the adhesion between the crystal and
fine particles of a rock, which gives it power to resist fracture when
subjected to the blows of traffic. This important property, while
distinct from hardness, is yet intimately associated with it, and can in
a measure make up for a deficiency in hardness. Hardness, for instance,
would be the resistance offered by a rock to the grinding of an emery
wheel; toughness, the resistance to fracture when struck with a hammer.
Cementing or binding power is the property possessed by the dust of a
rock to act, after wetting, as a cement to the coarser fragments
composing the road, binding them together and forming a smooth,
impervious shell over the surface. Such a shell, formed by a rock of
high cementing value, protects the underlying material from wear and
acts as a cushion to the blows from horses' feet, and at the same time
resists the waste of material caused by wind and rain, and preserves the
foundation by shedding the surface water. Binding power is thus,
probably, the most important property to be sought for in a
road-building rock, as its presence is always necessary for the best
results. The hardness and toughness of the binder surface more than of
the rock itself represents the hardness and toughness of the road, for
if the weight of traffic is sufficient to destroy the bond of
cementation of the surface, the stones below are soon loosened and
forced out of place. When there is an absence of binding material, which
often occurs when the rock is too hard for the traffic to which it is
subjected, the road soon loosens or ravels.
Experience shows that a rock possessing all three of the properties
mentioned in a high degree does not under all conditions make a good
road material; on the contrary, under certain conditions it may be
altogether unsuitable. As an illustration of this, if a country road or
city park way, where only a light traffic prevails, were built of a
very hard and tough rock with a high cementing value, neither the best,
nor, if a softer rock were available, would the cheapest results be
obtained. Such a rock would so effectively resist the wear of a light
traffic that the amount of fine dust worn off would be carried away by
wind and rain faster than it would be supplied by wear. Consequently the
binder supplied by wear would be insufficient, and if not supplied from
some other source the road would soon go to pieces. The first cost of
such a rock would in most instances be greater than that of a softer one
and the necessary repairs resulting from its use would also be very
expensive.
A very good illustration of this point is the first road built by the
Massachusetts Highway Commission. This road is on the island of
Nantucket and was subjected to a very light traffic. The commission
desired to build the best possible road, and consequently ordered a very
hard and tough trap rock from Salem, considered then to be the best
macadam rock in the state. Delivered on the road this rock cost $3.50
per ton, the excessive price being due to the cost of transportation.
The road was in every way properly constructed, and thoroughly rolled
with a steam roller; but in spite of every precaution it soon began to
ravel, and repeated rolling was only of temporary benefit, for the rock
was too hard and tough for the traffic. Subsequently, when the road was
resurfaced with limestone, which was much softer than the trap, it
became excellent. Since then all roads built on the island have been
constructed of native granite bowlders with good results, and at a much
lower cost.
If, however, this hard and tough rock, which gave such poor results at
Nantucket, were used on a road where the traffic was sufficient to wear
off an ample supply of binder, very much better results would be
obtained than if a rock lacking both hardness and toughness were used;
for, in the latter case, the wear would be so great that ruts would be
formed which would prevent rain water draining from the surface. The
water thus collecting on the surface would soon make its way to the
foundation and destroy the road. The dust in dry weather would also be
excessive.
Only two examples of the misuse of a road material have been given, but,
as they represent extreme conditions, it is easy to see the large number
of intermediate mistakes that can be made, for there are few rocks even
of the same variety that possess the same physical properties in a like
degree. The climatic and physical conditions to which roads are
subjected are equally varied. The excellence of a road material may,
therefore, be said to depend entirely on the conditions which it is
intended to meet.
It may be well to mention a few other properties of rock that bear on
road-building, though they will not be discussed here. There are some
rocks, such as limestones, that are hygroscopic, or possess the power of
absorbing moisture from the air, and in dry climates such rocks are
distinctly valuable, as the cementation of rock dust is in a large
measure dependent for its full development on the presence of water. The
degree to which a rock absorbs water may also be important, for in cold
climates this to some extent determines the liability of a rock to
fracture by freezing. It is not so important, however, as the
absorptive power of the road itself, for if a road holds much water the
destruction wrought by frost is very great. This trouble is generally
due to faulty construction rather than to the material. The density or
weight of a rock is also considered of importance, as the heavier the
rock the better it stays in place and the better it resists the action
of wind and rain.
Only a few of the properties of rock important to road builders have
been considered, but if these are borne in mind when a material is to be
selected better results are sure to be obtained. In selecting a road
material the conditions to which it is to be subjected should first be
considered. These are principally the annual rainfall, the average
winter temperature, the character of prevailing winds, the grades, and
the volume and character of the traffic that is to pass over the road.
The climatic conditions are readily obtained from the Weather Bureau,
and a satisfactory record of the volume and character of the traffic can
be made by any competent person living in view of the road.
In France the measuring of traffic has received a great deal of
attention, and a census is kept for all the national highways. The
traffic there is rated and reduced to units in the following manner: A
horse hauling a public vehicle or cart loaded with produce or
merchandise is considered as the unit of traffic. Each horse hauling an
empty cart or private carriage counts as one-half unit; each horse, cow,
or ox, unharnessed, and each saddle horse, one-fifth unit; each small
animal (sheep, goat, or hog), one-thirtieth unit.
A record is made of the traffic every thirteenth day throughout the
year, and an average taken to determine its mean amount. Some such
general method of classifying traffic in units is desirable, as it
permits the traffic of a road to be expressed in one number.
Before this French method can be applied to the traffic of our country
it will be necessary to modify considerably the mode of rating. This,
however, is a matter which can be studied and properly adjusted by the
Office of Public Road Inquiries. It is most important to obtain a record
of the average number of horses and vehicles and kind of vehicles that
pass over an earth road in a day before the macadam road is built. The
small cost of such a record is trifling when compared with the cost of a
macadam road (from $4,000 to $10,000 per mile for a fifteen-foot road),
in view of the fact that an error in the selection of material may cost
a much larger sum of money. After a record of the traffic is obtained,
if the road is to be built of crushed rock for the first time, an
allowance for an immediate increase in traffic amounting at least to ten
or fifteen per cent had best be made, for the improved road generally
brings traffic from adjoining roads.
To simplify the matter somewhat, the different classes of traffic to
which roads are subjected may be divided into five groups, which may be
called city, urban, suburban, highway, and country road traffic,
respectively. City traffic is a traffic so great that no macadam road
can withstand it, and is such as exists on the business streets of large
cities. For such a traffic stone and wood blocks, asphalt, brick, or
some such materials are necessary. Urban traffic is such as exists on
city streets which are not subjected to continuous heavy teaming, but
which have to withstand very heavy wear, and need the hardest and
toughest macadam rock. Suburban traffic is such as is common in the
suburbs of a city and the main streets of country towns. Highway traffic
is a traffic equal to that of the main country roads. Country road
traffic is a traffic equal to that of the less frequented country roads.
The city traffic will not be considered here. For an urban traffic, the
hardest and toughest rock, or in other words, a rock of the highest
wearing quality that can be found, is best. For a suburban traffic the
best rock would be one of high toughness but of less hardness than one
for urban traffic. For highway traffic a rock of medium hardness and
toughness is best. For country road traffic it is best to use a
comparatively soft rock of medium toughness. In all cases high cementing
value should be sought, and especially if the locality is very wet or
windy.
Rocks belonging to the same species and having the same name, such as
traps, granites, quartzites, etc., vary almost as much in different
localities in their physical road-building properties as they do from
rocks of distinct species. This variation is also true of the mineral
composition of rocks of the same species, as well as in the size and
arrangement of their crystals. It is impossible, therefore, to classify
rocks for road-building by simply giving their specific names. It can be
said, however, that certain species of rock possess in common some
road-building properties. For instance, the trap[8] rocks as a class are
hard and tough and usually have binding power, and consequently stand
heavy traffic well; and for this reason they are frequently spoken of as
the best rocks for road-building. This, however, is not always true, for
numerous examples can be shown where trap rock having the above
properties in the highest degree has failed to give good results on
light traffic roads. The reason trap rock has gained so much favor with
road-builders is because a large majority of macadam roads in our
country are built to stand an urban traffic, and the traps stand such a
traffic better than any other single class of rocks. There are, however,
other rocks that will stand an urban traffic perfectly well, and there
are traps that are not sufficiently hard and tough for a suburban or
highway traffic. The granites are generally brittle, and many of them do
not bind well, but there are a great many which when used under proper
conditions make excellent roads. The felsites are usually very hard and
brittle, and many have excellent binding power, some varieties being
suitable for the heaviest macadam traffic. Limestones generally bind
well, are soft, and frequently hygroscopic. Quartzites are almost always
very hard, brittle, and have very low binding power. The slates are
usually soft, brittle, and lack binding power.
The above generalizations are of necessity vague, and for practical
purposes are of little value, since rocks of the same variety occurring
in different localities have very wide ranges of character. It
consequently happens in many cases, particularly where there are a
number of rocks to choose from, that the difficulty of making the best
selection is great, and this difficulty is constantly increasing with
the rapidly growing facilities of transportation and the increased range
of choice which this permits. On account of their desirable road
properties some rocks are now shipped several hundred miles for use.
There are but two ways in which the value of a rock as a road material
can be accurately determined. One way, and beyond all doubt the surest,
is to build sample roads of all the rocks available in a locality, to
measure the traffic and wear to which they are subjected, and keep an
accurate account of the cost both of construction and annual repairs for
each. By this method actual results are obtained, but it has grave and
obvious disadvantages. It is very costly (especially so when the results
are negative), and it requires so great a lapse of time before results
are obtained that it cannot be considered a practical method when
macadam roads are first being built in a locality. Further than this,
results thus obtained are not applicable to other roads and materials.
Such a method, while excellent in its results, can only be adopted by
communities which can afford the necessary time and money, and is
entirely inadequate for general use.
The other method is to make laboratory tests of the physical properties
of available rocks in a locality, study the conditions obtaining on the
particular road that is to be built, and then select the material that
best suits the conditions. This method has the advantages of giving
speedy results and of being inexpensive, and as far as the results of
laboratory tests have been compared with the results of actual practice
they have been found to agree.
Laboratory tests on road materials were first adopted in France about
thirty years ago, and their usefulness has been thoroughly established.
The tests for rock there are to determine its degree of hardness,
resistance to abrasion, and resistance to compression. In 1893 the
Massachusetts Highway Commission established a laboratory at Harvard
University for testing road materials. The French abrasion test was
adopted, and tests for determining the cementing power and toughness of
rock were added. Since then similar laboratories have been established
at Johns Hopkins University, Columbia University, Wisconsin Geological
Survey, Cornell University, and the University of California.
The Department of Agriculture has now established a road-material
laboratory in the Division of Chemistry, where any person residing in
the United States may have road materials tested free by applying for
instructions to the Office of Public Road Inquiries. The laboratory is
equipped with the apparatus necessary for carrying on such work, and the
Department intends to carry on general investigations on roads. Part of
the general plan will be to make tests on actual roads for the purpose
of comparing the results with those obtained in the laboratory.
Besides testing road materials for the public, blank forms for recording
traffic will be supplied by the department to any one intending to
build a road. When these forms are filled and returned to the
laboratory, together with the samples of materials available for
building the road, the traffic of the road will be rated in its proper
group, as described above; each property of the materials will be tested
and similarly rated according to its degree, the climatic conditions
will be considered, and expert advice given as to the proper choice to
be made.
FOOTNOTES:
[7] By Logan Waller Page, expert in charge of Road Material Laboratory,
Division of Chemistry.
[8] This term is derived from the Swedish word _trappa_, meaning steps,
and was originally applied to the crystallized basalts of the coast of
Sweden, which much resemble steps in appearance. As now used by road
builders, it embraces a large variety of igneous rocks, chiefly those of
fine crystalline structure and of dark-blue, gray, and green colors.
They are generally diabases, diorites, trachytes, and basalts.--PAGE.