Test For Tensile Strength

This is the most important test in most cases, and it should be made with the aid of a proper machine, as hereinafter described (see p. 182).

Seven Days' Test

The tensile strength of Portland cement, as required by the original specification for the Metropolitan Main Drainage Works, was 400 lbs. on the briquette area of 21/4 inches' after six days' immersion.

Shortly after this the specified breaking weight was raised to 500 lbs. per area of 21/4 square inches.

The average strength of the cement supplied under this specification during five years was 806.63 lbs. on the briquette area.

The standard breaking weight specified on these works was afterwards raised to 787 lbs. on the briquette area, or 350 lbs. per square inch after seven days' immersion, the specification being as follows : -

"The whole of the cement shall be Portland cement of the very best quality, ground extremely fine, weighing not less than 112 lbs. to the striked bushel, and capable of maintaining a breaking weight of 350 lbs. per square inch seven days after being made in a mould and immersed in water during the interval of seven days."

The rigid testing on these and other engineering works has raised the tensile strength of the best cement manufactured since that date.

The breaking weights specified on the works mentioned below are shown in the following Table (page 169).

The tensile strength of Portland has been increased of late years, besides which improvements in the methods of testing, and the increased care with which they are carried out, cause higher results to be shown with cements similar to those of former years.

Cements can be obtained which will stand a tensile stress of 550 lbs. per square inch, even higher.

Particulars required by various Public Departments and others in their Specifications for Portland Cement.

Fineness.

Tenslie Strength

NEAT.

Rate at which weight is applied.

Department or Work.

Number of meshes per square inch in sieve.

Residue. Per cent.

Weight per bushel in lbs.

On 21/4 sq. inches.

On 1 sq. inch.

Age when tested.

Remarks.

India Office . .

5800

20

Not less than 116, not more than 120, average 118

lbs. 800,

lbs. 355

1 day in air,

6 days in water

100 lbs. in

12 seconds and hang at rest 15 seconds

Admiralty. . .

625 and 2500

0 10

114 to 116, average 115

787

350

7 days

...

Sieve 35 BWG. ,, 37 „

War Office . .

2500

15

110

900

400

7 days

...

For ordinary buildings.

Metropolitan Board of Works

5800

10

Specific gravity not less than 3.1

None specified

..

..

With three times its weight of dry sand, which has passed through a sieve of 400, and been retained upon one of 900 meshes to the square inch. Briquettes to be put into water 24 hours after made, and to remain in water 27 days more. Then to have 250 lbs. per square inch applied at the rate of 200 lbs. per minute.

Clyde Trust1 . .

2500

15

Not less than 115

787

350

7 days

...

Trinity House 2 .

1600

10

Not less than 116

350 500 750

155 222 333

2 days 4 days 7 clays

"Slowly"

Peterhead Harbour3

2500

10

Average at least

116 lbs., not less than 114

350

500 800

156

222 355

48hoursafter gauging

4 days

7 days

"Slowly"

1 Mr. Deas.

2 Sir J. Douglass (Stoney).

3 Sir John Coode.

There is, however, great danger in raising this test too high. A cement having great tensile strength after a short interval of setting can only be obtained by using a maximum amount of lime in the manufacture, and an excess of lime in the cement may cause it to expand in the work months or even years after it is used.

It is better, therefore, to require a moderate tensile strength, such as from 300 to 350 lbs. per square inch.