Cement hydrates when water is added to it and cohesion and solidity is exhibited. It binds together the aggregates by adhesion. The strength of mortar and concrete depends upon the type and nature of cement. So, it should develop a minimum specified strength if it is to be used in structures. Cement is tested for compressive and tensile strengths.

**Determination of Cement Strength**

Cement hydrates when water is
added to it and cohesion and solidity is exhibited. It binds together the
aggregates by adhesion. The strength of mortar and concrete depends upon the
type and nature of cement. So, it should develop a minimum specified strength if
it is to be used in structures. Cement is tested for compressive and tensile
strengths.

Conditions Affecting Strength:
Cement is very strong at early ages if a high lime or high alumina content is
there. Gypsum and Plaster of Paris in small percentages also tend to increase
the strength slightly, but when present in quantities larger then 3 per cent,
these substances provide variable effects. The effect of the clinker compounds
on strength have already been discussed. In addition to the effect of composition,
the strength of cement is greatly influenced by the degree of burning, the
fineness of grinding, and the aeration it receives subsequent to final
grinding. An underburnt cement is likely to be deficient in strength.

**Compressive Strength: **Compressive
strength is the basic data required for mix design. By this test, the** **quality
and the quantity of concrete can be cotrolled and the degree of adulteration
can be checked.

The test specimens are 70.6 mm
cubes having face area of about 5000 sq. mm. Large size specimen cubes cannot
be made since cement shrinks and cracks may develop. The temperature of water
and test room should be 27 o ± 2 o C. A mixture of cement and standard sand in the
proportion 1:3 by weight is mixed dry with a trowel for one minute and then
with water until the mixture is of uniform colour. Three specimen cubes are
prepared. The material for each cube is mixed separately. The quantities of
cement, standard sand and water are 185 g, 555 g and (P/4) + 3.5, respectively
where P = percentage of water required to produce a paste of standard
consistency. The mould is filled completely with the cement paste and is placed
on the vibration table. Vibrations are imparted for about 2 minutes at a speed
of 12000±4 00 per minute.

The cubes are then removed from
the moulds and submerged in clean fresh water and are taken out just prior to
testing in a compression testing machine. Compressive strength is taken to be
the average of the results of the three cubes. The load is applied starting
from zero at a rate of 35 N/sq mm/minute. The compressive strength is
calculated from the crushing load divided by the average area over which the
load is applied.The result is expressed in N/mm^{2}. The minimum
specified strength for some of the cements is given in Table 5.4.

**Table 7
Minimum Specified Strength in N/mm ^{2}**

^{}

**Tensile Strength: **The
tensile strength may be determined by Briquette test method or by split tensile**
**strength test.

**Importance: **The
tensile strength of cement affords quicker indications of defects in the cement
than** **any other test. Also, the test is more conveniently made than the
compressive strength test. Moreover, since the flexural strength, is directly
related to the tensile strength this test is ideally fitted to give information
both with regard to tensile and compressive strengths when the supply for
material testing is small.

**Briquette Method: **A mixture
of cement and sand is gauged in the proportion of 1:3 by weight. The** **percentage
of water to be used is calculated from the formula (P/5) + 2.5, where P =
percentage of water required to produce a paste of standard consistency. The
temperature of the water and the test room should be 27 o ± 2 o C. The mix is
filled in the moulds of the shape shown in Fig. 5.11.

After
filling the mould, an additional heap of mix is placed on the mould and is
pushed down with the standard spatula, until the mixture is level with the top
of the mould. This operation is repeated on the other side of the mould also.
The briquettes in the mould are finished by smoothing the surface with the
blade of a trowel. They are then kept for 24 hours at a temperature of 27 o ±
2 o C and in an atmosphere having 90 per cent humidity. The briquettes are then
kept in clean fresh water and are taken out before testing. Six briquettes are
tested and the average tensile strength is calculated. Load is applied steadily
and uniformly, starting from zero and increasing at the rate of 0.7 N/sq mm of
section in 12 seconds.

Ordinary Portland cement should
have a tensile strength of not less than 2.0 N/mm^{2} after 3 days and
not less than 2.5 N/mm^{2} after 7 days.

**Notes: (i) In the tension test of cement the load
on the briquette should be applied centrally. Since briquettes become brittle
with age, the effect of slight eccentricity or any torsional strain is
pronounced in long-time tests.**

**(ii) The strength increases when the loading rate
is increased from that specified.**

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