How To Carry Out Impact Test & Acid Test Of Stone Aggregates
Impact test is used to determine the degree to which stone aggregates can withstand impact forces. The test is carried out with the aid of an impact testing machine which consists of a frame with guides in which a hammer weighing between 13.5 to 15kg hangs. This hammer is used for exerting impact forces on the stone aggregates. The hammer is allowed to fall freely from a height of 380mm along the height of the impact testing machine.
The impact testing machine consists of parts such as the lifting handle, vertical guide bar, the circular base, the cup (around 102mm diameter & height 50mm) and the hammer which weighs between 132-137N.
Procedure: How Impact Testing of Stone Aggregates is Done
Aggregates of size 10mm to 12.5mm are filled in a cylinder in three equal layers. Each layer of fill is tamped 25 times. The aggregates are then transferred into the cup of the impact testing machine and tamped 25 times again. The hammer is allowed to fall freely on the specimen 15 consecutive times. This gives rise to fines from the specimen. The specimen is then sieved through sieve size 2.36mm and the impact value is determined thus:
Impact value (I) = W2/W1
W2 = Weight of fines
W1 = Original weight of specimen
This value is found as a percentage by multiplying the result by 100.
Recommended Impact Values for Various Civil Engineering Works
The recommended impact values for various civil engineering works include:
Impact value for wearing course should not be greater than 30%.
Impact value for bituminous mechadam should not be greater than 35%.
Impact value for water bound mechadam should not be greater than 40%.
Importance/Relevance of Acid Testing of Aggregates in Civil Engineering
Acid test is used for determining the presence of calcium carbonate (CaCo3) in stone aggregates. It is a common substance found in rocks as the minerals calcite and aragonite, most notably in chalk and limestone, eggshells, gastropod shells, shellfish skeletons and pearls. Stone materials containing this compound are known as calcareous rocks. For details on the various types of rocks, check out my earlier post on rocks here and this one on the properties of rocks. Calcium carbonate is formed when calcium ions from hard water reacts with carbonate ions, forming limescale. For more details on calcium carbonate, see this wiki page. The presence of calcium carbonate in rocks weakens the weather-resisting properties of rocks and stone aggregates. It is therefore pertinent to determine this using acid testing of stone aggregates.
How to Carry Out Acid Test of Stone Aggregates
The procedure for carrying out the acid testing of stone aggregates is pretty simple. Find the steps below:
- Weigh a sample of stone aggregates of 50 – 100 grammes.
- Make a solution of 1% hydraulic acid with water.
- Add the stone aggregates to the solution.
- Leave the stone aggregates in the solution for 7 days.
- Stir, shake, or agitate the solution at intervals.
- After 7 days, observe the specimens and pass your verdict.
The following points are noteworthy in the determination of stones that have good weather resistance:
A good building stone maintains its sharp edges and keeps its surface intact. Hence, if there are broken edges and powders are formed on the surface of the stone aggregates, it indicates the presence of calcium carbonate, which renders it a poor material for weather resistance.