ASTM C109 vs. C780

[gbkxbb]

Over my young career (3 years) I've always tested mortar following C109, which included placing mortar cubes in a water bath.

Recently, we had some low breaks on a project. The mortar supplier said that C109 was the wrong procedure and that C780 should be followed instead, with the most notable differnce being that cubes should be stored in a moist room, not in a water bath.

Have I been doing it wrong? Will the different curing methods make a huge difference in compressive strength?

Thanks

 

[Ron]

gb...this argument has been around for at least 30 years that I know of.

C109 is a laboratory method, but is applicable to mortar. C780 is appropriate for mortar testing for preconstruction and construction evaluation.

2-inch cube molds may be used for either C109 or C780. As for curing, storing in water is acceptable as is using a moisture room...So no, you are doing nothing wrong in that respect if you are controlling the water temperature.

You need to note that using small diameter cylinders and cube molds will yield different results. ASTM considers that a cylinder test is acceptable if it reaches 85% of the cube strength.

One problem with using cube molds in the field is that the mass of the mold is tremendously greater than the mass of the specimen, so if the mold gets disturbed, it can affect the specimen...perhaps more than other specimen types. In particular, vibration of the mold during the initial 24-hour cure period can be critical.

The different curing methods should have no effect on the results; provided the temperature is stable and maintained at about 73F.

You mention that you are getting "low" breaks. Is this low compared to, say, the stated strength of a Type S mortar? If so, the results of field tests will not likely reach the values stated for the mortar types.

Do a trial batch in the lab to establish the expected strength for comparison to the stated "Type" strength.

Consider using the proportion specification of ASTM C270 and requiring more precise measurement of the volume of the materials going into the mixer. Using a shovel for measurement is not acceptable.

 

[concretemasonry]

gbkxbb-

I suggest you fully read and understand the ASTM 270 specifications that lays out the acceptable procedures for mortar acceptance and testing.

This is a unique standard that contains 2 different concepts for mortar properties and testing AND the different methods cannot be combined or tested at will for obvious reasons.

One method is performance and the other is proportioning. The key is to understand the differences and separation for testing and responsibilities. That is also the reason for different test procedures for compliance.

As an example, if a mortar is mixed precisely according to the specifications and fails, where is the real responsibility? Also what if a mortar is for other proportions is made that exceedes the requirements for proportioning, but performs above the minimums.

After you have had time to contemplate this age-old problem, just look at Note 1 in the appendix to ASTM 270. you will find that this comment that strength is NOT necessarily the most important property of a mortar, but workability and the workability to construct a properly may be more important. This goes beyond specifications unless you you can identify the particle shape of the aggregate, the weather conditions, mixing cycle and allowable re-tempering and many other variables.

After getting confused somewhat, realize the the strength of the mortar has very little do do with the strength of a masonry wall and the masonry unit properties are the deciding factor. I have seen 4800 psi hollow prisms made with 2200-2500 psi mortar. Even for masonry grouting, many engineers place and upper limit on strength of grout to insure a balanced design.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[emmgjld]

Very good discussion. Ron was getting to the point & concretemasonry hit it on the head.