URM Testing

[connect2]

A number of the FEMA documents have a test method to determine the bed joint shear strength of the mortar of URM. Basically you remove two bricks from either side of a brick which remains in place undisturbed. On one side you have a 'push' cylinder that loads the brick, on the other side you set a dial gauge to measure movement of the brick as you increase the load on the brick. The FEMA documents say the failure load, if you will, is 'the test load at first movement of a masonry unit'. So our dial gauge reads in 0.001 of an inch increments. If we consider 0.001" the 'first movement' the applied load is small. Generally absolute failure of the bed mortar, ie the brick let go from the mortar, the pump pressure dropped back to zero, was around 0.035". But there is a considerable difference in the applied load between 0.001" and 0.010". The question is what would you consider as the 'failure movement', and would you take it as the 0.001" movement or would you use a different higher movement value?

Thanks.

 

[wiktor]

Instead of measuring the movement, measure and record the max.pressure at failure, i.e. at cracking off the brick.

 

[connect2]

Wiktor,
Yes you do record all the way through, the pressure, the movement, from first movement detected/observed in this case with a dial gauge accurate to 0.001" until finally the mortar bed fails and the brick slides and the jack pressure drops back to zero. Movement of the brick is detected long before this occurs as noted previously. So at first movement you record the pressure, the movement, etc. eventually you use this pressure to convert to applied load which is then divided by the horizontal bed area to get the mortar shear strength. The question is what constitutes sufficient first movement?

 

[wiktor]

Sufficient first movement will be the sudden drop in the pressure increase. This will be your ultimate load at failure. Try to keep pressure increase steady at say 10 psi/sec, but this will depend on the type and diameter of jack used. 5-10 lbs increase per second should be reasonable enough, until specs have something different.
This will be valid for typical cement/lime mortars. I have no idea how elastomer mortars will behave, as some displacement will be expected and the movement recorded.

 

[connect2]

Sounds reasonable re the pressure drop however we have movements of close to 0.030, that's almost 1/32" of movement of the masonry unit with steady and holding pressure for more than a minute before we go to our next pressure increment increase. What I'm trying to sort is the FEMA statement with this test '....the test load at the first movement of a masonry unit'. Sometimes you can see the cracking starting to occur, sometimes you can hear it but you can never see the brick move althought the gauge definetely is telling you the brick is moving. It is a sand-lime mortar from 1914 with variously a clay or sand-lime brick. A sand lime mortar is very soft, plastic almost.

 

[henri2]

The testing program you describe is similar to the one described in LA City's Division 88 (formerly Division 68)"Earthquake Hazard Reduction in Existing Buildings".

 

[connect2]

henri 2;
The testing method to determine inplace mortar shear strength and ultimately the masonry shear strength for URM is pretty well found throughout. I don't have the LA reference you list above. What does it say about '....first movement...' or does it say something/anything other than the FEMA doc's I've referenced above. With sand-lime mortars as Witkor has mentioned above you can get a prety fair amount of movement before an actual, cracking snapping, sudden failure occurs. Cement based mortar will fail much more in a brittle sudden fashion. Not near as much movement as sand lime.