It is hardly necessary to say that when there is a choice the strength and quality of the cementing material should be in proportion to the importance of the part the concrete has to play.

Thus fat lime concretes would be objectionable almost anywhere except as filling in the spandrils of arches.

1 R.E. Corps Papers, vol. xxii.

2 M.P.I.C.E., vol. Ixxxvii. p. 66.

Hydraulic lime, or cement, is advisable for concrete in nearly all situations.

Eminently hydraulic limes should be used for concrete foundations in damp ground, and in the absence of cement for subaqueous work of any kind.

Portland cement concretes are adapted for all positions, especially for work under water, or where great strength is required; also in situations where the concrete has to take the place of stone, as in facing to walls, copings, etc. etc.

For work to be executed between tides, where the concrete is required to set quickly but not to attain any great ultimate strength, Roman or Medina cement may be used with advantage.

When, for the sake of its strength, Portland cement concrete is necessarily used under water, it must be protected by canvas covering or other means from any action which would wash it away before it had time to set.

When concrete is likely to be exposed to great heat, as in fire-proof floors, gypsum has been used as a matrix (see p. 249).

Bulk Of Concrete Produced

The bulk of concrete obtained from a mixture of proper proportions of cement, sand, and aggregate, varies considerably according to the nature and proportions of the materials and method of treatment; but it should in general be a little more than the cubic content of the aggregate before mixing, as the other substances, if in proper proportion, should nearly fit into and disappear in its voids.

The following examples show how the bulk of concrete produced varies according to circumstances : -

Concrete of 1 Portland cementl to 6 shingle (or broken stone) and 2 sand.

27 cubic feet shingle or broken stone,

9 „ sand,

41/2 „ Portland cement (31/2 bushels), 25 gallons water,

Make one cubic yard of concrete.

Concrete of 1 Portland cement to 6 broken brick and 2 sand.

30 cubic feet broken brick 2 " mesh, 10 sand,

5 Portland cement, 12 gallons water,

Made one cubic yard of concrete.

Concrete of 1 Portland cement to 7 Thames ballast 1 (consisting of

2 stone 1 sand).

33 cubic feet ballast,

41/2 cubic feet Portland cement (31/2 bushels), 30 gallons water,

Make one cubic yard of concrete.

Concrete of 1 Portland cement to 12 gravel, used at Chatham dockyard.

322/5 cubic feet gravel (before shrinkage),

23/5 „ Portland cement, 50 gallons water,

Made one cubic yard of concrete in situ.

1 Hurst.

Concrete of 1 Portland cement to 8 stone and sand, used at Cork Harbour works.

27 cubic feet stone broken to l1/2inch gauge, 9 „ sand, 41/2 „ Portland cement,

Made one cubic yard of concrete in situ.

In some concrete landings made with breeze from gasworks and

Portland cement.

29 cubic feet breeze broken to 3/4 guage, 8 „ Portland cement,

Made one cubic yard of concrete in situ.

Concrete used at Portland Breakwater Fort, stone used in two sizes and mortar mixed separately.

14 cubic feet stones broken to 31/2-inch guage, 14 „ do. l1/2 „

10 „ sand,

5 „ Portland cement, 231/2 gallons water,

Make one cubic yard of concrete in situ.

After being rammed the concrete is compressed into about nine-tenths of the volume it occupies when first made.

Selenitic Concrete

Concrete may be made with selenitic cement mortar as the matrix.

Portland cement is sometimes added in small quantities to the selenitic cement.

"Prom a series of experiments made on behalf of the patentees, it appears that a mixture of one part of Portland, four parts of selenitic cement, and twenty-five parts of sand, was if anything superior to the same Portland used with four parts of sand." 1

The patentees' directions for preparing the concrete are as follow : -

For Concrete

4 full-sized pails of water; 2 bushels of prepared selenitic lime; 2 bushels of clean sand.

These ingredients are to be mixed as before in the edge-runner or tub, and then turned over two or three times on the gauging-floor, to ensure thorough mixing with 12 or 14 bushels of ballast. When the tub is used the sand will be first mixed dry with the ballast, and the lime poured into it from the tub and thoroughly mixed on the gauging-floor. An addition of one-sixth of best Portland cement will be found to improve the setting.

Expansion Of Concrete

Concrete, when made with hot lime or cement, swells to an extent amounting to from one-eighth to three-eighths of an inch per foot of its linear dimensions.

This is owing to the imperfect slaking or cooling of the lime or cement.

It is probable that when such expansion takes place there is a slight disintegration throughout the mass of concrete, and that its coherence is destroyed.

It has been ascertained by experiment that when lime or cement is carefully slaked the concrete practically does not expand at all, and concrete should be so carefully prepared that no expansion will take place.

In masses of concrete, thin in proportion to their area - such as concrete laid in situ instead of paving - cracks are sure to occur unless the area is divided into portions by the introduction of laths so as to break up the surface by dry open joints at intervals.

1 Building News, 30th January 1874.

The expansion which occurs in concrete made with hot lime or cement has been taken advantage of in underpinning walls that have settled in parts; hot concrete forced tightly into openings made below the faulty portions, expands and separates, filling the opening, and lifting the superincumbent work into its proper position.