Shear Capacity of 1916 Mortar 1

[Evank]

In an existing brick wall of some 14 inches thick minimum,
is there a rational guide to determine the probable shear capacity of the mortar present? Is there a small compression test that can be applied to the mortar in place?
This is in an urban environment that probably had reasonably up to date practice as established at that time. So can information be developed that would be satisfactory to a Building Department?

 

[PEinc]

I am currently having the same problem with a stone railroad abutment. The railroad says to assume the old mortar has no strength. If so, my calculations show that, with the bridge deck partially removed for staged reconstruction and with a Cooper E-80 surcharge behind the stone wall where the deck has been removed, the wall should have fallen apart due to both sliding and overturning.

I've checked Amrhein's 4th Edition of "Reinforced Masonry Engineering Handbook" and "Masonry Structures, Behavior and Design" by Drysdale, Hamid, & Baker. Amrhein gives a few allowable shear and tension values for different mortar types. The other authors give a range for ultimate tensile strength of "mortar." However, neither book addresses old mortar.

Anyone have any applicable information?

 

[EPSTR]

I currently have a project with masonry in a similar condition. I used the values from Amrhein's book for unreinforced masonry. Fortunately with new structural additions that I have added to the building I have been able to reduce the shear stress in my calculations to below 9 psi. I have used the dimensions of the wall to come up with a moment of inertia in some other areas of concern. A check for the deflection can allow you to work back to the bending stress. The engineer at the city was ok with this.
I do recommend some form of strength test however. (We have cut sections of demo'd wall to do so) Progressive demolition of areas within the building have shown that the interior wythes of the wall were laid with a different mortar than that of the outer wythe. Depending on how your floors and roof are connected to the walls, this could pose a problem for you.
Assume the worst because most of the mortar becomes way too brittle. Also, reduce the dead load in your calculations accordingly.

 

[wiff]

Consider having a lab do a Windsor test to establish compressive strength of the mortar. You may find the mortar to be a limestone and sand mix of very low compressive strength. Once you know a bit more about the compressive strength, you may be able to extrapolate, either from the local code or from tables, what the recommended shear strength would be for the Type of mortar you are evaluating.

 

[Evank]

Thank you ecastle and wiff. I will investigate both answers

 

[Ingenuity]

Evank and PEinc,

Maybe consider an insitu test using a small flat jack in the mortar joints as per this link:

http://www.ana-usa.com/flatjack.htm

HTH

 

[jheidt2543]

In the book "Structural Analysis of Historic Buildings" by J. Stanley Rabun, John Wiley & Sons, 2000, pages 138 - 151 they discuss Masonry Load-Bearing Walls. They show a 1905 graphical solution to gravity and lateral loads on a brick wall of that period. Then analyze the same wall under the SBC 1991 Wind Criteria. It is an interesting discussion. There are also tables of data on brick and mortar strength. Table 3-13 calls out Allowable Compressive Strength of Brick Masonry as 275 psi for good brick in good lime mortar and 415 psi for best brick in Portland cement mortar.

I've only just received the book, so I have only skimmed it so far, but it seems worth the money to me!

 

[Evank]

Thanks one and all for many ideas. In the meantime I find the dead load seismic shear of a 13 inch wall of brick one story high to pencil out at a measily 579 p.sqr.ft. That may be more than any allowance that I can get close to with any kind of maneuvering!!

 

[dik]

Mortars used at that time are during the transition from those using limes with impurities to act as a hydraulic agent and those using Portland cement as one.

The pure limes used today have no impurities to act as a hydraulic agent and require masonry cement or Portland Cement to achieve this. In early times, it is the impurities in the base material used for making the lime that provide the hydraulic properties required for a lasting mortar.

In early times (generally predating the 1900's), the skilled mason generally knew the sources of the limes used and which provided a better finished product. Although he may not have known about what specific impurities performed better than others, he was trained in the use of certain products. Because the sources of lime and the skills of the mason varied, there is a real 'scatter' in the strengths of mortar, how well it bonded, and how well it lasted.

The lime in some mortars lacking hydraulic impurities has dissolved leaving only a weak sand matrix while others have developed nearly the strength of the stone material used.

There is no manner to determine the strength, except by testing. The actual strengths vary from approximately 20psi to several hundred psi.

I've been involved in a fair amount of restoration work. If you post your eMail address to me: dikcoates@alpha.to, I'll send you a paper I've prepared on Historic Brickwork (not complete, yet, I want to include Terra Cotta and some other stuff). If you're re-doing a brick wall you may have to concern yourself with the porosity of the mortar (to wick moisture from within the building because of no vapour barrier) as well as ensuring that the strength of patching mortar is less than that of the brick (for durability).

 

[Evank]

dik
My e-mail is revank@msn.com
Cannot seem to get your address to work with me.