368. Making the Concrete. - Materials and Proportion

Concrete for monolithic construction should be made of a good quality of Portland cement, mixed with clean, sharp sand and a proper proportion of aggregates. As previously stated, almost any natural stone, when broken up, ordinary gravel, or even broken bricks or pottery, may be used for the aggregates. Quartzite rock and granite make the best concrete, but the other materials will answer. Shells were used for the aggregate in the Hotel Ponce de Leon.

* Mr. E. L. Ransome.

The proportions may vary from 1 to 4 to 1 to 8. The proportions used in the buildings mentioned are given in the description.

Mixing. - For small buildings the concrete may be mixed by hand, as described in Section 142, but if very much concrete is required, it will be found much more economical to mix it by a regular mixing machine.

Concrete can also be much more thoroughly mixed in a machine properly constructed for the purpose than is possible by hand, and the strength of the concrete is increased in proportion.

The following results on the resistance to crushing of concrete cubes, furnished the author by Mr. E. L. Ransome, are interesting as showing the increased strength obtained by thorough mixing of the materials. The crushing strength is given in tons per cubic foot.

FIRST SERIES.

Age.

1 part

Portland cement to 12 of limestone

1,

2,

3.

4

weeks.

No.

1.

Mixed skillfully and thoroughly by hand

18,

25.

25,

25

tons.

No.

2.

Mixed in mill and turned 56 times

36,

36,

36,

43

"

No.

3.

Mixed in mill and turned 343 times

50,

44.

55,

45

"

SECOND SERIES.

1 part

Portland cement, 1 Rosendale, 12 limestone

1,

2,

3.

4.

8

weeks.

No.

1.

Mixed by hand very thoroughly.................

36,

-

36,

54.

68

tons.

No.

2.

Mixed in mill and turned 500 times ................

54,

81,

90.

90,

117

"

THIRD SERIES.

1 part

Portland cement to 6 of fine granite

1,

2,

3.

4

weeks.

No.

1.

Mixed by hand very thoroughly..

63,

81,

76,

90

tons.

No.

2.

Mixed in mill and turned 300 times

81,

99.

108,

99

"

Series No. 1 was exposed in a cold atmosphere, and does not show the increase in strength from week to week that it should. The third series does not show as much increase in strength from machine mixing as would probably have been the case had coarser aggregates been used.

Inspection. - Concrete work of all kinds requires the most rigid inspection (see Section 85), as almost everything depends upon the quality of the cement and proper mixing. Unless thorough confidence can be placed in the honesty of the contractor to use the proportion of cement specified, it will be necessary to keep an inspector constantly on the ground to see that the full proportion of cement is used. The quality of the cement furnished should also be tested from time to time as the work progresses.