The Handyman's Pocket Book: Concrete.

CONCRETE

We are indebted to the Cement ad Concrete Association for much of the information in these notes

Portland cement is generally put in paper bags of 1cwt., though smaller quantities can often be obtained.  As it absorbs moisture readily it is essential that it is stored in a dry place - not merely on a dry floor, but out of a damp atmosphere.

SAND.  To make concrete a mixture of cement, sand, and aggregate is needed.  The sand is generally specified as "sharp," that is, with angular surfaces, though tests in recent years have shown that sand with rounded grains makes concrete which is just as strong.  Sea sand, however, contains a lot of salt and this retards setting. though it is just as strong once hardening has been completed.  Sand containing organic matter should be avoided.  Sand obtained from a builders' merchant is sold by the cubic yard.  That is, a yard of sand is the quantity that would fill a box 3 ft. high, 3 ft. wide, and 3 ft. long.  It is generally possible to obtain half or quarter of a yard.

Whatever the sand, it must be clean, and if there is any doubt it should be washed in a trough with running water.  Any dirt will be carried away, leaving the sand at the bottom.  The best sand consists of a mixture of both fine and coarse grains, because the fine particles fill in the voids between the large grains.

AGGREGATE.  Various materials can be used - granite, gravel, clinker, and broken brick.  It should be clean because dirt prevents the cement from adhering to it.  The size depends on the job.  For a foundation quite large pieces up to 2.5 in. can be used, whilst for walls and floors, etc, the largest pieces should not be more than, say, 0.75 in.

WATER.  Should be pure.  Tap drinking water is suitable.  If sea water is used the setting rate will be slowed up, but ultimately the strength is not affected.

PROPORTIONS.  These vary according to the job, and the following table gives a few typical mixes.  Note that the quantity of water is specified as well as those of the solids, because it has been found that the quantity of water affects the resulting concrete considerably, both the strength and resistance to water penetration.  If experience shows a mix to be too wet, add more sand and aggregate.  If it is too dry, use less aggregate in the next mix; do not add more water.

EXAMPLE.  Foundation for a garden work-shed: I bucket cement, 3.25 buckets of sand, 5 buckets aggregate, just over half a bucket of water.

MIXING.  Thorough mixing is essential, and is first done dry.  An existing concrete floor can be used, or a mixing board can be made up as in Fig. 1.  Put on to one end of mixing board about one-half of the 

Fig. 1 Board for mixing concrete

coarse aggregate, add the whole of the sand, and then the remaining half of the aggregate.  Finally pour on the cement.  Now mix the whole thoroughly, shovelling the mixture to form a fresh pile, back again to the first position, and thence once more to the second position.  This time shovelling three times will effectively mix the whole.

Water is now added.  The method generally recommended today is to apply the water with a watering can fitted with a rose, little at a time.  When all the water has been used the mixture should be turned over thoroughly three times.

PLACING.  This should follow mixing immediately, and it should be in position within half an hour.  It follows then that the amount prepared in any one mix depends upon the speed with which it can be placed.  If it will take more than half an hour to place a certain quantity, the amount prepared should be cut down.  Account has thus to be taken of whether one is working single-handed or with assistance.

If possible, all the concrete should be laid in one operation, but if this is impossible and the work has to be resumed the following day the surface should be scrubbed with a wire brush and washed over to remove all loose particles.  The new concrete is then well tamped against the old.

SETTING.  Newly laid concrete should not be exposed to the sun, especially in hot summer weather.  It should be protected by hanging cloths - old sacking, etc., over it.  Alternatively if this cannot be done a layer of damp sand about 1 in. thick can be spread over it once the initial set has, taken place - say after six of seven hours.  If neither can be done the work should be sprinkled with water from time to time - once again after the initial set has taken place.

Concrete begins to set within half an hour of being laid, but full strength is not attained until several weeks - indeed the hardening process continues for a long time afterwards.  A working degree of hardness is reached after four or five days, however, enabling any shuttering to be removed, and any brickwork, etc., to be laid on top.  It should be treated with respect, however, for a week of so.

QUANTITIES.  Estimating the amount of materials required is largely a matter of arithmetic, but it should be realised that it is not a matter of adding together the quantities of sand, aggregate, and cement.  The volume of finished concrete is practically that of coarse aggregate, because the sand and the cement merely fill in the voids between the pieces of aggregate and therefore "disappear" so to speak.  If then you reckon on a cubic foot of aggregate for every cubic foot of concrete, and add the proportionate amounts of sand and cement you will not be far out.  Cement is sold by weight (cwt. bags) but if you reckon on 1 cubic foot of loose cement weighing 90 lb. you will not be far out.

The first calculation is that of the cubic content of the finished concrete.  Supposing you are laying a foundation 9 ft. long by 6 ft. wide by 9 in. deep.  The volume will be:

9 x 6 x 0.75 = 40.5 cubic ft. or 1.5 cubic yards approx.

This, then, is the quantity of coarse aggregate required, and, assuming that you are using the mix for the better class foundations (see chart on above), you will need proportions of : 

1 part cement,  3.25 parts sand,  5 parts coarse aggregate,  0.625 water.

The quantities are therefore:

Cement 90 (lb.) x 27 x 1.5 / 5 / 112 = (say) 6.5 cwt.

Sand 1.5 / 5 x 3.25 = 2.975, say I yard.

You need:

cement: 6.5 cwt.  sand: 1 yd.  coarse aggregate 1.5 yd.

Water can be measured by the bucket in proportion of 0.625 bucket to every 5 buckets aggregate, 3.25 buckets sand, and 1 bucket cement.

THE SITE.  It is important that the foundations are high enough to keep the building dry.  It is also desirable to make the foundation a few inches wider than the building all round.  Indicate the size and position of the foundation on the ground by driving in four small pegs, one at each corner, and surround them with a piece of string tho mark the outline, as shown in Fig. 2.  Test first with a wood square, and then measure the diagonals to see that they are equal.

The highest corner of the site is now ascertained by using a spirit level and long straight-edge.  Four fairly long levelling pegs about 1.5 in. square with their bottom ends pointed are driven into the ground, one at each corner.  They should be long enough to extend into the gourd beyond the depth to be taken out for the foundation, and their outer faces kept level with the side lines of the foundation site.  They stand about 1 in. clear of the end lines of the site (Fig. 2).

Fig. 2. Temporary and levelling pegs in position.

Starting with the peg marking the highest corner site, cut it off about an inch or more above the ground level according to the height the foundation is required to stand above the ground.  With a long straight-edge resting on the end of the levelled peg A, and with a spirit level resting on the straight-edge, the next peg is marked and cut level.  The two remaining pegs are marked and cut similarly.

BOARDING.  The boards should be fitted around the pegs, as shown in Fig. 3, to contain the concrete when it is mixed.  The two side boards are fitted first.  They stand outside the pegs, and need not extend down the whole depth of the foundation, but should run an inch or so below the ground at the lowest point .  Care should be taken not to disturb the soil unduly.  A sharp spade is used to make straight outside cuts, and the soil is scooped out inside to allow the boards to fit down level with the top ends of the levelling pegs, to which they are fixed, preferably with screws.  The two end boards are fitted inside the pegs and are screwed through them.  Test with a straight-edge and spirit level, as shown in Fig. 3.

Fig. 3. Boards to contain concrete levelled and fixed.

For heavy building, it is advisable to have a deep footing round the edges of the site with a thinner layer of concrete in the middle, as in Fig. 4.  Level the site and fit the boards as before, and put another line of boards inside to give the width of footing desired.  Pegs are driven as before and are cut level with the outer boards, while the inner boards should be screwed through the pegs to facilitate their later removal.  For large foundations it may be necessary to drive intermediate pegs and join the boards at them.  In arranging the woodwork it is necessary to consider its easy removal when the concrete has set.

Fig. 4. Arrangement for concrete footing

It is always advisable to use screws in preference to nails for assembling, and not to drive them from the side against which the concrete will be placed.  The wood "form" or "mould" having thus been built up, the inside soil may be removed to the required depth and all is ready to receive the concrete.

LAYING THE CONCRETE.  Mixing the concrete has already been dealt with.  The measured quantity of water is best sprinkled from a watering can fitted with a rose.  It should be added gradually, the heap being turned over until the whole is of even consistency.  Do not use too much water.  Each mixing of concrete should be well tamped down.  This mixture could be used to fill the mould level, but if a 

Fig. 5. Concrete being levelled with straight-edge.

smooth, hard-wearing surface is desired it should be brought up to within about 0.5 in. of the top of the wood mould and finished with a top layer composed of 1 part cement and 2 parts sand.  Care should be taken to mix this quite stiff as it will become considerably thinner with working.

To obtain a clean, level surface the mixture of sand and cement is laid over the coarser concrete, worked fairly even with a trowel or float, and finally levelled with a straight-edge long enough to reach over the side boards of the mould and be worked from side to side, as shown in Fig. 5.  The new concrete should be protected from the sun in hot weather, and should be damped occasionally for the first few days to prevent rapid drying.  The boards forming the mould may then be removed, and this will be facilitated if they are oiled or whitewashed on the side facing the concrete previous to the mixture being placed in the mould.

INSETTING BOLTS.  It is sometimes advisable to bolt down a wood building to its foundations.  The positions of the bolts need careful marking.  The holes could be bored in the bottom rails of the wood sections forming the building first.  The sections could then be placed on the foundation and the holes scribed, or their positions could be marked by measuring.  Holding plates, see Fig. 5, should be fitted on the bolt, holes for their reception are cut in the concrete foundation, and they are cemented in place.  If this work is done before the wood mould is removed it may be possible to mark the positions of the wood sections of the building on the top edges of the boards and line up the bolts with a straight-edge.  The bolts could be fitted before or after a finer cement finish is applied to the rougher concrete.  A cold chisel could be used to cut the holes for the bolts and holding plates, and a stronger hold will be obtained if they are cut larger at the bottom.