1. objective
It is very important that the cement after setting shall not undergo any appreciable
change of volume. Certain cements have been found to undergo a large expansion after
setting causing disruption of the set and hardened mass. This will cause serious
difficulties for the durability of structures when such cement is used. The unsoundness
in cement is due to the presence of excess of free lime than that could be combined with acidic
oxide at the kiln. It is also likely that too high a proportion of magnesium content or
calcium sulphate content may cause unsoundness in cement. Soundness of cement may be
determined by two methods, namely Le-Chatelier method and autoclave method.
In the soundness test a specimen of hardened cement paste is boiled for a fixed time
so that any tendency to expand is speeded up and can be detected. Soundness means the ability
to resist volume expansion.
2. apparatus required
|
Fig. 1: Le-Chateliers Apparatus
Le- Chatelier apparatus conforming to IS : 5514-1996
|
|
Fig. 2: Schematic of Le-Chateliers Apparatus
Consist of a small split cylinder of spring brass to other non-corrodible metal
of 0.5mm thickness forming a mould of 30mm internal diameter and 30mm high. On
either side of the split, two indicators are brazed suitably with pointed ends made
of 2mm diameter brass wire in such a way that the distance of these ends to the centre
of the cylinder is 165mm. The split cylinder will be kept between two glass plates.
The temperature of the moulding room, dry materials and water shall be maintained at 27±2°C
The relative humidity of the laboratory shall be 65 ± 5 percent. The moist closet or moist room shall be
maintained at 27 ± 2°C and at a relative humidity of not less than 90 percent.
|
|
Fig. 3: Water bath
Water bath capable of containing immersed Le-Chatelier
moulds with specimens and of raising their temperature
from 27±2°C to boiling in 27±3 minutes.
|
|
Fig. 4: Vernier Calliper
Vernier Calliper should be able to measure upto 30 mm with least count of 0.1 mm
|
|
Fig. 5: Balance
On balance in use, the permissible variation at a load of 1000 g shall
be ± 1.0 g. The permissible variation on new balance shall be one-half
of this value. The sensibility reciprocal shall be not greater than
twice the permissible variation.
|
Weights
The permissible variations on weights in use in weighing the cement shall be as prescribed
in Table 1.
Table 1 : Permissible Variations on Weights
Weight (g) |
Permissible Variation on Weights in use Plus or Minus (g) |
500 |
0.35 |
300 |
0.30 |
250 |
0.25 |
200 |
0.20 |
100 |
0.15 |
50 |
0.10 |
20 |
0.05 |
10 |
0.04 |
5 |
0.03 |
2 |
0.02 |
1 |
0.01 |
3. reference
IS 4031(Part 3):1988 Methods of Physical Test for Hydraulic Cement: Determination of Soundness.
(First Revision) Reaffirmed- 2019
4. procedure
-
Place the lightly oiled mould on a lightly oiled glass sheet and fill it with
cement paste formed by gauging cement with 0.78 times the water required to
give a paste of standard consistency. [refer IS : 4031 (Part 4 )-1988].
-
Cover the mould with another piece of lightly oiled glass sheet, place a small
weight on this covering glass sheet and immediately submerge the whole assembly
in water at a temperature of 27 ± 2°C and keep there for 24 hours.
-
Measure the distance separating the indicator points to the nearest 0.5 mm.
Submerge the mould again in water at the temperature prescribed above.
-
Bring the water to boiling, with the mould kept submerged, in 25 to 30 minutes,
and keep it boiling for three hours. Remove the mould from the water, allow it
to cool and measure the distance between the indicator points.
-
The difference between these two measurements indicates the expansion of the
cement. This must not exceed 10 mm for ordinary, rapid hardening and low heat
Portland cements. If in case the expansion is more than 10 mm as tested above,
the cement is said to be unsound.
5. observation and recording
Soundness/expansion of cement = L1-L2
L1=Measurement taken after 24 hours of immersion in water at a temp. of 27 ± 20C
L2=Measurement taken after 3 hours of immersion in water at boiling temperature.
Calculate the mean of two values to the nearest 0.5 mm.
6. Discussions
-
In the event of cement failing to comply with the specified requirements, a further
test should be made from another portion of the same sample in manner described above,
but after aeration (done by spreading out to a depth of 75 mm and store it for 7 days
in an atmosphere maintained at 27±2°C and relative humidity of 50 to 80 percent).
-
Volume expansion in cement mortar or in cement concrete is caused by the presence of unburnt
lime (CaO), dead burnt MgO and also CaSO4.
-
By Le-chatelier method we can only find out presence of unburnt lime (CaO).
-
Presence of unburnt lime may develop cracks in the cement because of increase in volume.
-
Free lime (CaO) and Magnesia (MgO) are known to react with water very slowly and increase
in volume considerably, which result in cracking, distortion and disintegration.