Compressive strength of Microsilica mortar IS 1727

DETERMINATION OF COMPRESSIVE STRENGTH OF MICROSILICA-CEMENTMORTAR

TEST METHOD: IS 1727:1967, Reaffirmed 2008

OBJECTIVE: To determine the compressive strength of Microsilica-cement mortar as obtained in standard mortar test cubes prepared and tested under specified conditions.

APPARATUS: Balance of accuracy 0.01gm, Standard vibration machine, gauging trowels, Standard sand, Graduated measuring jar, cube mould of size 50x50x50mm, Planetary mixer with paddle, mixing bowl and scrapper, flow table and its accessories, tamping rod, mixing trowel, 33 grade ordinary Portland cement.

Procedure for Compressive strength of Microsilica
Laboratory shall be maintained at a temperature and relative humidity of 27±2oC and 65±5%. Prepare a standard test mortar by mixing, microsilica, cement and standard sand in the proportion 0.2: 0.8: 3 by weight

Three test specimens of size 50x50x50mm shall be casted for the test with estimated quantity of materials suggested below:

Microsilica: 50 gms
Cement: 200gms
Standard sand: 750gms

All appliances shall be cleaned prior to mixing & temperature of water and the test room at the time of mixing shall be between 27± 2oC.The amount of water required for gauging shall be equal to that required to give a flow of 105± 5% with 25 drops in 15 seconds determined as:

With weighed quantity of materials as mentioned in equation (1), prepare trial test mortar with different percentage of water until specified flow is obtained. Mixing of the mortar shall be done mechanically by means of planetary mixer. Mixing paddle and bowl shall be dried and placed in the mixing position in the mixer, then introduce the materials into the bowl and mix in the following manner

Place all the mixing water in the bowl.
Add the flyash mixture (flyash and cement) to the water and start the mixer & mix at a slow speed of 140± 5 rev/min for 30 seconds.
Add the entire quantity of sand slowly over a period of 30 seconds while mixing at slow speed.Stop the mixer, change to medium speed (285± 10 rev/min) & mix for 30 seconds.

Stop the mixer and let the mortar stand for one and half minutes. During the first 15 seconds of this interval quickly scrap down into the batch any mortar that may have been collected on side of the bowl. Then for the remainder of this interval cover the bowl with lid.

Finish by mixing for one minute at medium speed 285± 10 rev/min

In case requiring a remixing interval, any mortar adhering to the sides of the bowl shall be quickly scrapped down into the batch with the scraper prior to remixing.

Upon the completion of mixing, mixing paddle shall be shaken to remove excess mortar into the mixing bowl. Wipe the flow table top, clean, dry and place the mould at the centre. Place a layer of mortar about 25mm thick into the mould & tamp 20 times with tamping rod.
The tamping shall be sufficient to ensure uniform filling of the mould, then fill the mould to overflow & tamp as specified for the first layer. Cut off the mortar with the plane surface flush with the top of the mould by drawing the straight edge of the trowel (held nearly perpendicular to the mould) with a sawing motion across the top of the mould. Wipe the table top and remove any water around the edge of the flow mould. Lift the mould away from the mortar one minute after the mixing operation. Immediately drop the table to height of 12.5mm, 25drops in 15 seconds.

Flow is the resulting increase in the average base diameter of the mortar, measured at least four diameters of approximately equispaced intervals expressed as percentage of original base diameter.

If the measured flow is equal to the required flow of 105±5, then proceed for casting of test specimens of size 50x50x50mm with the test mortar, otherwise repeat the procedure by preparing fresh trial batch of mortar with increase or decrease in water percentage until required flow is obtained.

Place the mortar in 50x50x50mm size cube mould in layer of 25mm thickness and tamp 25 times with tamping rod. The tamping shall be sufficient to ensure uniform filling of the mould, then fill the mould to overflow & tamp as specified for the first layer. Cut off the mortar with the plane surface flush with the top of the mould by drawing the straight edge of the trowel (held nearly perpendicular to the mould) with a sawing motion across the top of the mould. Keep the specimens with moulds at a temperature of 27±2oC and relative humidity of 90% for a period of 24 hours after casting of specimens. Remove the specimen from the moulds and keep them in water at a temperature of 27±2oC for the entire curing period. Water in the curing tank or chamber shall be renewed every seventh day. Minimum of 3 specimens shall be tested at 7 days. The cubes shall be tested on their sides without any packing between the cube and steel platens of the testing machine. Load shall be steadily and uniformly applied at the rate of 140kg/cm2/min

Calculate the compressive strength by dividing the maximum load with the average area over which the load is applied. The average compressive strength of 3 specimens shall be calculated and reported to nearest 1.0kg/cm2

Corresponding test on plain cement sand mortar cubes:

For the purpose of comparison, similar tests with cubes from mortar containing no microsilica shall be carried out as follows:

Not less than three mortar cubes shall be prepared out of a mix with one part of 33 grade ordinary Portland cement used in the test and three part of standard sand, stored, cured and tested in the same manner as specified above and average compressive strength shall be calculated and reported to nearest to 1.0kg/cm2

Read more :

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