As before mentioned, the materials are generally mixed in a dry state, not upon the bare ground, but upon a clean timber or stone platform. The proportions decided upon are measured out either roughly by barrow-loads, or in a more precise manner by means of boxes made of sizes to suit the relative proportions of the ingredients to be used.

Such boxes, in which the quantities to be mixed together can be accurately gauged, should always be used in mixing cement or other concretes intended for important work.

Table Showing The Proportions Of The Concrete Used In Various Works

Where used.

Proportions.

For what used.

1. Peterhead Breakwater

1 Portland cement .

6 sand, shingle, and broken stone, with granite rubble incorporated therein

Concrete blocks.

Do.

1 Portland cement . 5 sand, shingle, and broken stones

Cement in bags.

Do.

1 Portland cement . 4 sand and shingle

Concrete joggles.

2. Newhaven Harbour .

1 Portland cement .

2 sand

5 shingle

Western sea-wall.

3. Wicklow Harbour

1 Portland cement . 7 gravel and sand

In breakwater.

4. Colombo Breakwater

1 Portland cement . 3 stone

2 sand

In ordinary rings of cylinder foundations.

Do.

1 Portland cement . 4 stone and sand

In cutting rings of ditto.

Do.

1 Portland cement .

2 sharp sea sand

4 hand-broken stone (31/2") 2 machine-crushed (11/2") screened stone

Blocks.

5. Greenock Harbour . Do.

2 Portland cement . 7 sand and ballast

1 Portland cement . 6 sand and ballast

3 granite chips

Facing to quay wall.

Plastic concrete behind sheet piling. Backing to quay wall.

Plastic concrete.

6. Chatham Dockyard .

1 Portland cement .

2 sand 4 slag

Dock walls -

For face 9 to 10 inches thick.

Do.

1 Portland cement . 12 gravel

For backing of dock walls.

7. Cork Harbour Fortifications

1 Portland cement . 8 broken stone and sand

The bulk of the sand 1/3 that of the broken stone.

Do.

1 Portland cement . 4 to 6 of broken stone and sand

Under water, more cement to make up for scour.

8. Metropolitan Main Drainage Works

1 Portland cement . 51/2 ballast

For sewers.

Do.

1 Portland cement . 6 gravel

For roofs, floors, etc.

9. For ordinary buildings

1 Portland cement . 8 gravel

For walls.

Do.

1 Portland cement . 6 gravel

For floors, roofs, etc.

1. Sir John Coode. 2. M.P.I.C.E., vol. lxxxvii. p. 99. 3. P. 118. 4. M.P.I.C.E., vol. lxxxvii. p. 186. 5. M.P.I.C.E., vol. lxxxvii. pp. 66, 67. 6. M.P.I.C.E., vol. lxii. p. 94. 7. R.E. Corps Papers, vol. xi. 8. M.P.I.C.E., vol. xxv. 9. Building News.

The measured materials are then heaped up together, and turned over at least twice, better three times, so as to be most thoroughly incorporated.

The dry mixture should then be sprinkled, not drenched, the water being added gradually through a rose, no more being used than is necessary to mix the whole very thoroughly. If too much water be added, it is apt to wash the lime or cement away; at the same time due allowance must be made where the water is liable to soak away or to evaporate quickly.

The moist mixture should then again be turned over twice or three times.

When lime is used it should be in a fine powder.

If a fat lime (which is almost useless for concrete in most positions), it should be slaked and screened.

If a hydraulic lime, it should be finely ground, or, in the absence of machinery for grinding, it should be carefully slaked, and all unslaked particles carefully removed by passing it through a sieve or fine screen.

The lime is often used fresh from the kiln, piled on to the other ingredients during the mixing. This is apt to leave unslaked portions in the lime, and is a dangerous practice.

When Portland cement is used for concrete, it must be thoroughly cooled before mixing. Cements of the Roman class should be fresh.