Grade indicates compressive strength of cement after 28 days under water curing.
It means 1 millimeter square (1mm* 1mm) of cement concrete can withstand upto a pressure of so much value without cracking. ie 33 grade cement can withstand upto a pressure of 33 newtons per millimeter square or 33 mega pascals.
Similarly 43 grade and 53 grade cements can withstand upto a pressure of 43 newtons per millimeter square or 43 mega pascals and 53 newtons per millimeter square or 53 mega pascals respectively.
1 newton per millimeter square is equal to 1 mega pascal (1N/mm2 = 1MPa)
(images of compression followed by cracking .courtesy : www.sensorprod.com)
here F is compressive force.
The specimen subjected to compression force tends to crack after the compression pressure crosses the withstanding limit of the specimen. So the maximum withstanding pressure limit of the specimen without cracking indicates the compression strength of the specimen.
(apply the same concept for cement) The maximum withstanding compression pressure of a cement specimen without cracking indicates the compressive strength or grade of the cement.
So, if a cement specimen cracks between a applied compressive pressure of 33 MPa and below 43 MPa, it is 33 Grade cement. Similarly, if a specimen cracks between 43MPa and below 53MPa, it is 43 Grade cement. Also if a specimen cracks 53MPa and above its 53 Grade cement.
