Concrete blocks 12 inches cube, made of Portland cement, weighing 110.56 lbs. per bushel. This cement (neat) broke under a tensile stress of 427 lbs. per square inch after seven days' immersion in water.

The blocks were made in layers 1 inch thick, and compressed by ramming, or in a hydraulic press.

They were kept twelve months before being tested - half of them in air, the others in water.

Composition of Concrete.

Crushed at Tons.

Blocks kept in Air.

Blocks kept in Water.

Cement. Ballast.

Tons.

Tons.

1 to 1

107*

170*

1 „ 2

149

160

1 „ 3

113

115

1 „ 4

103

108

1 „ 5

89

99

1 „ 6

80

91

1 ,, 7

75

80

1 „ 8

61

76

1 „ 9

54

68

1 „ 10

48

48

* Exceptional.

These experiments showed that the blocks made with the larger proportions of cement are the stronger, the strength being nearly in proportion to the quantity of cement.

Further experiments showed that compressed blocks were "apparently stronger than uncompressed blocks in larger proportion than their difference in density."

1 Manufacturers' Circular. 2 Proceedings Institute Civil Engineers, vol. xxxii. Table 1, Appendix.

The relative strength of the concrete cubes made with different kinds of aggregate is shown in the following Table.

Several different proportions between the aggregate and cement were tried, but the following relate to cubes containing eight parts of the aggregate to one of cement.

Blocks ] 2 inches cube (compressed), 8 Aggregate to 1 Cement.1

Material for Aggregate.

Crushed

at Tons.

Blocks in Air.

Blocks in Water.

Ballast ...

61

76

Portland stone..

110

126

Gravel ...

74

85

Pottery ...

97

118

Slag ...

85

70

Flints ...

103

117

Glass..

65

94

These experiments showed that the concrete of pottery or broken stone was stronger than that of gravel, probably because in the latter case a good deal of the cement is taken up in binding the particles of sand together; partly because the gravel was wanting in angularity.

Tar Concrete is made of broken stones and tar.

About 12 gallons tar are used per cubic yard concrete.

If the tar is too thin, pitch is added to bring it to the proper consistency.

Adding 1/2 to 1 bushel of dried and pounded chalk, or dead lime, dried clay, brick dust, or pounded cinders, etc., to every 12 gallons tar, tends to harden the mass.

The materials should be heated, or, at all events, be made perfectly dry, before admixture with the tar.2

Mineral tar or bitumen is better for the purpose than coal tar. The former contains an oil which in coal tar is very volatile - escapes, and leaves the tar brittle.

Iron Concrete is composed of cast-iron turnings, asphalte, bitumen, and pitch.

Gas tar is sometimes substituted for the asphalte.

This material has been tried as a backing for armour plates in iron fortifications.

Concrete consisting of 1 part iron borings to 34 of gravel (by bulk) was used with success at the Stranraer Pier.3

Lead Concrete, made of broken bricks immersed in lead, has also been used in iron forts.

1 M.P.I.C.E. vol. xxxii. Table 5, Appendix.

2 Hurst 3 Stevenson On Harbours.