718

THE COLLIERY GUARDIAN.

October 2, 1914.

Concrete Pit Props and Beams.

The use of concrete and reinforced concrete in mines
has for some years been receiving the increased atten-
tion of mining engineers. For underground purposes,
the lining of shafts and galleries, and the construction of
ventilating partitions, offer the principal occasions for
the use of reinforced concrete, and some minor uses,
such as the construction of underground stores for
explosives and of tracks or guides for railways and cages
have also been found.

A further important use is for pit props and beams.

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These can be made of any shape and size and to bear
any weight or pressure with material usually easily pro-
curable close to the pit mouth. The moulding of them
is a simple operation which does not call for special
experience, but requires uniform care. In this con-
nection a short description of concrete pit props and
beams is given in a pamphlet that has been issued by
the Associated Portland Cement Manufacturers (1900)

Limited, with the object of giving sufficient information
to enable mine managers and others to make them on
their works; the company will at all times, it may be
added, be pleased to furnish any further information
in their power.

How to Make Concrete.

Concrete is made by mixing Portland cement with
sand and coarse material. A well-known brand of Eng-
lish cement of “ slow-setting ” quality should be used.
There are many suitable coarse materials, such as gravel



or ballast, broken stone, and the like. For pit props
and beams the best material is gravel, ballast, or hard
stone. The size of the coarse material may vary accord-
ing to the purpose for which it is to be used. For work
in which reinforcement is employed, or for moulded
work, such as pit props and beams, the coarse material
should not be large, otherwise it will not work between
the steel rods or wire, or into all parts of the mould. It

is preferable, therefore, to have the coarse material vary-
ing in size between that which will pass a j in. sieve and
be retained on a | in. sieve. All material that passes
through the | in. sieve would be termed sand. If there
is any considerable quantity of very fine stone dust or
any exceptionally large material; this should be sifted
out. If the material passing the | in. mesh is insuffi-
cient to make up the required proportion of sand, then
a coarse clean sand must be used. All materials used,
both sand and coarse material, must be perfectly clean
if a good job is to be ensured. No material should be
used which contains any considerable amount of loam
or clayey matter unless it is first washed.

The quantity of concrete that can be mixed up or
“ gauged,” as it is termed technically, at a time will
depend upon the number of moulds that can be filled at
one operation. All material should be carefully
measured, so that the right proportions are maintained,
and for this purpose box measures of a suitable capacity
should be used. A rough-and-ready measure some-
times used is either a pail or a barrow, the former for a
small quantity and the latter for a large one. The top
of the measure should always be struck off flush when
filled to ensure uniform quantities.

The proportions of the mixture should be 1 cu. ft.
(901b.) of Portland cement to eu. ft. of sand, and
3 cu. ft. of coarse material. Concrete should be mixed
as near to the place where it is to be used as practicable,
for if left standing for any length of time it may set
and become useless.

Concrete which has begun to set should never be
allowed to be beaten up and re-used under any circum-
stances whatever. Nevertheless, concrete that has
begun to set, or has set, may be used as a coarse
material if broken up and mixed with a fresh proportion
of sand and Portland cement.

The concrete should be mixed on a clean wooden
bench or stage, and the latter may be covered with a
thin piece of sheet iron or zinc if much mixing is to be
done.

The sand should first be measured and spread in a
layer of even thickness. The Portland cement should
then be distributed over the surface of the sand, and
the whole turned over dry with the shovel until the two
materials are seen by the uniform colour to be thoroughlv
mixed. The coarse material should now be thrown over
the mixture, and the whole turned at least three times
drv and three times wet. It is better to add the water a
little at a time until the right consistency is obtained,
than to pour it on all at once. The best manner in
which to apply the water is from a watering-can with a
rose-head.

The amount of water to be added should be such that
a slightlv “ sloppv ” mixture will result, which when
lightlv tamped will adequately fill every corner of the
mould.

How to Make Concrete Pit Props and Beams.

Concrete pit props and beams may be made in any
size or shape, and require to be reinforced by
embedding steel or iron rods in the concrete in order
to secure the maximum strength with a minimum of
weight.

Fig. 1 shows one type of mould in which several props
or‘beams can be cast at one time.. The ends and sides
are loose, and can be held together either by hooks and
eyes or bv wedges placed between blocks on the base-
board. Larger sized props or beams, however', being
very heavy, would be more conveniently made singly
and not in groups as in the mould illustrated.

The spiral reinforcement for the props can be bought
from the wire makers alreadv wound, and the vertical
reinforcement can be readily fixed in position with light
wire ties.

About in. of concrete should be spread evenly over
the bottom of the mould and carefullv tamped. On
top of this laver the reinforcing framework is placed,
and pressed in until within about | in. of the bottom,
core being taken that it occupies a central position in
the mould. Concrete is then filled in, and should be
most carefully tamped into the moulds by poking it with
a small bar or paddle-shaped piece of timber, so as to
make it quite dense and to get it into every corner and
interstice of the mould and between the reinforcement.

To avoid sharp edges, which are easily chipped,
triangular pieces or strips are placed in the bottom of
the mould along the sides, while the top edge may be
similarly bevelled or chamfered with a trowel or by
running an edging tool having a triangular edge at its
bottom.

Figs. 2 and 3 show detail of the reinforcement neces-
sarv for props and beams of equal strength to the best
larch.

The props or beams should be left in the moulds until
thev are thoroughlv set, and it mav be necessary to
allow two da vs or more to elapse before removing the
sides of the mould. They should be left gii the base-
board until strong enough to bear their own weight.

Before re-using the moulds clean off every portion of
concrete clinging to them.

Moulds should be very slightly greased or brushed
with soft soap to prevent the concrete sticking, but care
must bo taken not to allow any of the grease or soap to
mix with the concrete, or to get on to anv steel or iron
rods that mav bo embedded therein, otherwise they will
not properly bond with, or grip into, the concrete.

Pit props and beams should be stacked in a protected
position, sheltered from the sun and wind. Thev
should be wetted daily through a hose for at least 14
da vs after removal from the mould. The props can be
safelv used about two months from time of removal
from mould, but continue to increase in strength for
at least a vear afterwards.

It is possible, however, to mature the props and
beams more rapidlv by stacking them in a chamber
into which a small amount of steam is im’ected, so that
a warm, moist atmosphere is created. This will have
the effect of quickening the hardening of the concrete,