Text 7. Tests of Portland Cement

Test for soundness (i. e. assurance that the cement will not expand after setting is complete). The cement is made into & plastic mass with a specified water content, put into a small brass cylinder cut through on one side and provided with long wires or needles either side of the cut and boiled for 3 hours after being kept in water at 61 °F for 24 hours. The movement at the end of the needles shall not exceed 10 mm.

Test for setting time. The cement is gauged into a paste with a specified amount of water, put into a shallow circular mould and struck off level, and a needle 1 mm. square weighted to about half a pound is applied repeatedly. When this fails to penetrate completely, the time since gauged is known as the initial set. A needle with an annular attachment is then substituted for the plain needle, so arranged that the needle projects half a millimetre below the annular attachment. When the needle makes an indentation but the annular attachment does not, the time since gauging is known as the final set. The initial set should belong enough to enable mixing, transporting, placing and tamping of the concrete to be completed before setting begins. It should not be less than half an hour for slow-setting cement. (Rapid-setting cements can be supplied for special purposes, but these should not be used for reinforced concrete.) The final set should be not more than 10 hours.

Test for tensile strength. The cement is gauged with a standard sand in the proportion of 1 cement to 3 sand, and with 'a carefully specified quantity of water.

This paste is moulded into moulds of special shape to form a test specimen which is readily held in the jaws of a small testing machine. The central cross-section where fracture occurs is 1 in. square.

The specimens are kept in air (temperature about 61 °F and relative humidity at least 90 per cent) for 24 hours, then removed from the moulds and kept in water (temperature about 61 °F) till tested by breaking. The specimens are held in jaws of specified shape and tension applied at the rate of 100 lb./sq. in. in 12 seconds.

Test for compressive strength. Cubes are compacted by vibration in a special machine for 2 minutes, then kept in air at about 61 °F and 90 per cent relative humidity for 24 hours, and then in water at about 61 °F till tested. They are tested on their sides in a machine applying load at the rate of 5,000 lb./sq. in. per minute. No packing is used.

Some people hold that if the cement passes all the mechanical and soundness tests it is to comply with the chemical requirements, for which the tests are more difficult, especially in the field. The principal object of this test is to guard against an excess or inadequacy of lime in relation to silica, alumina and oxide of iron.

Vocabulary notes:

test for soundness	випробування на доброякісність
is made into a plastic mass	його перетворюють на пластичну масу
test for setting time	випробування на час тужавлення
the cement is gauged into a paste	цемент перетворюється на тісто
weighted to about half a pound	навантажена до 1/2 фунта ваги
fails to penetrate completely	не зможе проникнути повніше
the Initial set	початкове тужавлення
slow- (rapid-) setting cement	цемент, що повільно (швидко) тужавіє
test for tensile strength	випробування на опір розтягу
a test specimen	зразок для випробування, дослідний зразок
the central cross-section	центральний попереч ний переріз
relative humidity	відносна вологість
pass ... soundness tests	пройти випробування на доброякісність
guard against an excess or inadequacy	захист від надлишку чи непридатності

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<== попередня сторінка	\|	наступна сторінка ==>
Text 6. Portland Cement	\|	Text 8. Properties And Manufacture of Concrete

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