Failed mortar compressive strength

[Lutfi]

The 28 day compressive strength for S type mortar came near 700 PSI. This is well short of the 1700 psi specified strength. The walls do not have much vertical loads on them. However, they are shear walls. WE have cubes that will be tested at 56 days. I do not think they will come any were near specified strength based on the 7 and 28 day breaks. I have two questions as follows:

1.What options available to remedy this situation. (I am thinking of asking the contractor to fill all cells solid with grout as one option).
2.Will taking prisms from the wall for testing add any value?


Regards,
Lutfi

 

[henri2]

Lutfi, a few points/questions

1. Is this wall partially grouted?

2. What was compliance of f'm based on..unit strength method, prism testing...or was it not applicable?

3. Was the mortar based on property or proportion specifications? Proportion specs do not require strength tests of mortar specimens.

4. The water-cementitious materials ratio of the cube will be higher than that of the insitu mortar joints....water is absorbed from the mortar joint due to suction from masonry and also evaporation. Due to the aspect ratio of the mortar joint compared to cube and lower W/C ratio it will be stronger in compressive terms than the cube. Consider testing a 1/2 inch slice of mortar cube and testing whole 2 inch cube...the 1/2 inch slice will certainly be much stronger. Commentary in ASTM C 270 X1.6.3.2 throws some more light on this.

5. You stated this was a Type S mortar. Was it cement-lime based, masonry cement based or mortar cement based/...air-entrained or non-air-entrained?

6. The compressive strength obtained almost meets the compressive strength requirement for Type N based on C 270 property specs. See IBC Table 2103.7(2) which is based on C270 Table 2. Will use of Type N values work?

7. Prims (sawn from wall) and tested in accordance with the provisions of IBC 2105.3 and ASTM C1314 will probably come up to strength because mortar strength does not influence masonry prism strength that much..and this is demonstrated in IBC Table 2105.2.2.1.2 where the requirements for N mortar are only slightly more sringent than M and S mortars. Also check Figure 2 on page SC-6 of the MSJC Spec.

 

[Ron]

henri2...good comments

Lutfi...age old problem, particularly down here in our great state!

Bagged mortar cement suppliers will tell you that their product meets the proportion specifications but don't count on compressive strength compliance. They blame it on the testing labs, which in this case sometimes has validity.

I do not like to use cubes for compressive strength in the field for the reasons henri2 notes as well as the fact that the cube mold mass is so much greater than the specimen mass that a small amount of disturbance to the cube mold can cause problems with the specimen. I usually specify that they leave the specimens in the field for 48 hours, undisturbed, before transporting if using cube molds. I prefer 3x6 cylindrical molds, also left protected in the field for 48 hours instead of 24 hours.

Further, specify that the contractor use a consistent proportional measuring device for all mix materials. I specify that they use a 1 cubic foot plywood box to measure the sand and mortar cement going into the mixer. Measuring by shovels is a poor method of volumetric control and results in oversanding in the morning when the sand is damp and undersanding in the afternoon when it is dry (assuming no rain! which as we know in Florida is a poor assumption!)

 

[UcfSE]

Is the mortar requi4red to have the 1700 psi or is that your f'm? If it is f'm, I would have them test the masonry with the block and mortar and see if you achieve your f'm. That's the actual value we use in design anyway. Your other options are to grout the wall solid, or tear it down and do it over correctly.