Compressive strength of mortar

[SCGeo]

Hello all,

I work for a small firm and help manage our geotechnical and materials (concrete, mortar, grout, structural steel) testing among other things. I am an EIT, with just about a year's worth of experience.

We are handling the materials testing for a large retail center. Type S (1800 psi) mortar is being placed between CMU's. 2''x 4'' cylinders are being constructed with sample mortar and cured for 28 days at 100% RH in a water/lime bath. Initially we were getting breaks well in excess of 3000 psi but I have just completed 2 sets that broke under strength (1550 psi and 1500, respectively.) I have checked the calibration of the break machine, observed the mixing process on site, used different capping mechanisms, etc..
I wasn't able to discern any difference in the mixing process on site, nor the temperature of the samples.

My question is, could the handling of the samples between the site and lab, or doing a sloppy job of constructing the cylinders or other factors account for such a difference?

With low concrete breaks I have always been able to pinpoint a factor such as temperature, adding too much water onsite, problems with the mix design etc. that would be a likely cause. I want to have done everything in my power to ensure that things were being done correctly on our end before we have to recommend remediation measures.

Anybody experienced this before?

 

[jdonville]

SCGeo,

Talk with your supervisor and the firm's principal engineer. They will want to know about your results and will be able to offer you guidance on how to communicate the results.

My firm is involved with testing on a very high-profile job involving very high strength concrete (in the ballpark of 10000 psi, I think). Initially, the breaks were fine. As time went on, however, the numbers went below the target strength and the contractor was forced to remove quite a bit of concrete and change their design mix. I don't know if they just got lucky with the initial mix design or if some other factor was involved (like a change in aggregate, cement or admixture, or possibly an ambient environmental influence), but sometimes these things happen.

The owner of the retail project will want to know about the potential problem sooner rather than later.

Also, are you casting any spare samples? You may want to cast some extra, break some early and plot the breaks as a function of age to give yourself a better handle on what's happening - and possible earlier warning of samples that aren't meeting the design strength (for whatever reason).

Good Luck!

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[MotorCity]

SCGeo,

Variation in test results of any material are to be expected. Consider the number of cylinders that failed below 1800 psi. For example, if you cast a total of 4 cylinders and 2 failed at 1500 psi, that may raise a few eyebrows. However, if your project is quite large and 2 cylinders of 100 failed at 1500 psi, there may be less concern. Another thing to consider is the intended use of the mortar. Is its function critical to the safety of the structure or is it being used as fill or patch material? Also, for the cylinder that broke at 1500 psi, I would not consider it to be unusable. ACI does accept mortar and concrete that fall within a certain percentage below the stated strength. I have seen 4000 psi concrete cylinders break anywhere from 2500 to 8000 psi. Try and look at the overall picture. If you believe that it is not acceptable, don't be afraid to speak up. Good luck.

 

[SCGeo]

Thanks to both of you for your replies. I have been casting spares and will break those as well to get some idea of deviation. In the meantime I have spoken to my supervisor and he expressed similar sentiments. We have already constructed more than 40 sets and have had only the two fail so far and they were within 300 psi of design strength. We informed the GC that we are willing to accept this if it proves to be an anomaly or one time occurrence.

Thanks for the replies.

 

[samdamon]

Geo-

A couple of comments. It sounds like the project mortar was specified via property specification of the mortar components rather than via the proportions of the constitutents. AS you are probably aware both ways of spec'cing mortar are allowed by ASTM C270.

It sounds like you are using breaks of sample mortar taken from the field as a method of verifying as-built mortar strength. Normally this would only be done if the constitutents of the mortar have changed during the project.

ASTM C270 standard's intent is that mortar properties, mixing proportions and mix materials are established as satisfactory by testing at the START of the job, and then those identical proportions and materials are used throughout the project. Quality control in the field would normally consist of verifying the proportions and types of materials used, rather than taking field cubes and breaking them. If you read the ASTM C270 spec carefully you will see that there is no direct connection between the published compressive strength derived from lab tests at the start of the job, and field-cured compressive strength. So you have to be careful when using lab cube breaks of field mixed mortar during a job as an acceptance criteria for the mortar.

Quite a few engineers think of quality control criteria for mortar and concrete as similar, in that you have to get certain minimum ranges of strengths from field breaks to be certain of the as-built quality. Mortar acceptance is actually very different. At least thats my understanding!

Good luck.

 

[Zambo]

what is a CMU? I am surprised that you were getting mortar cylinder breaks in excess of 3000psi which seems very high.By the way I'm British so perhaps we use a different term...

Zambo