Soundness Test
Introduction
It is essential that the cement concrete does not undergo large change in volume after setting.
This is ensured by limiting the quantities of free lime and magnesia which slake slowly causing change in volume of cement (known as unsound).
Soundness of cement may be tested by LeChatelier method or by autoclave method. For OPC, RHC, LHC and PPC it is limited to 10 mm, whereas for HAC and SSC it should not exceed 5 mm.
Importance.
It is a very important test to assure the quality of cement since an unsound cement produces cracks, distortion and disintegration, ultimately leading to failure.
Condition affecting soundness.
The main cause for unsoundness in Portland cement is the hydration of the uncombined lime encased within the cement particles.
Condition affecting soundness.
The main cause for unsoundness in Portland cement is the hydration of the uncombined lime encased within the cement particles.
Exposed, finely ground, free lime in small percentages, hydrates before the cement sets and produces no injurious effect. The uncombined lime in cement is a result of either underburning the clinker or of excess lime in the raw materials.
Freshly ground cement is often unsound due to the presence of uncombined lime. Cement is thus allowed to aerate for two to three weeks, allowing the lime to hydrate, to overcome unsoundness.
Fine grinding of the raw material and clinker help to produce a sound cement. By grinding fine the raw materials, it is possible to produce a homogeneous mixture before burning where the lime is uniformly distributed.
Freshly ground cement is often unsound due to the presence of uncombined lime. Cement is thus allowed to aerate for two to three weeks, allowing the lime to hydrate, to overcome unsoundness.
Fine grinding of the raw material and clinker help to produce a sound cement. By grinding fine the raw materials, it is possible to produce a homogeneous mixture before burning where the lime is uniformly distributed.
The coarse grains of cement may imprison minute particles of uncombined lime which do not hydrate. These lime particles on hydralion produce disintegration.
1. Le Chetelier Method
The mould is placed on a glass sheet and is filled with neat cement paste formed by gauging 100 g cement with 0.78 times the water required to give a paste of standard consistency.
The mould is covered with a glass sheet and a small weight is placed on the covering glass sheet. The mould is then submerged in the water at temperature of 27°-32°C. After 24 hours, the mould is taken out and the distance separating the indicator points is measured. The mould is again submerged in water.
The mould is covered with a glass sheet and a small weight is placed on the covering glass sheet. The mould is then submerged in the water at temperature of 27°-32°C. After 24 hours, the mould is taken out and the distance separating the indicator points is measured. The mould is again submerged in water.
The water is now boiled for 3 hours.
The mould is removed from water and is cooled down. The distance between the indicator points is measured again. The difference between the two measurements represents the unsoundness of cement
The mould is removed from water and is cooled down. The distance between the indicator points is measured again. The difference between the two measurements represents the unsoundness of cement
2. Autoclave Test.
The 25 × 25 × 250 mm specimen is made with neat cement paste. After 24 hours the moulded specimen is removed from the moist atmosphere, measured for length, and so placed in an autoclave at room temperature that the four tides of each specimen are at least exposed to saturated steam.
The temperature of the autoclave is raised at such a rate that the gauge pressure of the steam rises to 2.1 N/mm2 in 1 to 1 ¼ hours from the time the heat is turned on.
The pressure is maintained for 3 hours. Then the heat supply is shut off and the autoclave is cooled at such a rate that the pressure is less than 0.1N/mm2 at the end of the hour.
The pressure is maintained for 3 hours. Then the heat supply is shut off and the autoclave is cooled at such a rate that the pressure is less than 0.1N/mm2 at the end of the hour.
The autoclave is then opened and the test specimens are placed in water at temperature of 90°C.The temperature is gradually brought down to 27±2°C in 15 minutes.
The specimens are maintained at this temperature for next 15 minutes and are then taken out. The length of the specimen is measured again. The difference in the two measurements gives the unsoundness of the cement.
0 comments:
Post a Comment