Masonry Grout Compressive Strength Test Results

[jropert]

I am working on Army Corps project that has both contract drawings and specifications for masonry.

The drawings indicate:
"Masonry System Compressive Strength, f'm = 1500psi"
"Mortar Shall be Type S"
"Grout Compressive Strength, 3000psi"

The specifications state that the mortar is to follow ASTM C 270, Type S and strength (f'm) as indicated [indicated on drawings again to be 1500psi]. The specs also call for grout to conform to ASTM C 476, fine. Grout slump shall be between 8 and 10 inches. Minimum grout strength shall be 3000 psi in 28 days, as tested by ASTM C 1019.

We have completed the masonry construction of one of the small buildings in 3 separate grout pours. We are required to obtain a minimum of 3 grout specimens per each day poured. We did this in accordance with ASTM C 1019 (the test used the masonry block and paper method).

Of the 3 separate tests performed, the 28 day breaks for one set were all below the specified 3000 psi (results were 2750psi, 2570psi, and 2690psi). The other two sets of tests averaged above the 3000psi specified (averages 3620psi, and 3090psi). All slumps were within specified requirements (between 8 to 10 inches).

Are there any allowable tolerances below the "minimum" specified grout strength in this case? We have seen concrete specifications that allow for minus 500psi tolerances in most cases. Are there any such tolerances for masonry. I did not appear to find any in the ASTM C 476, ACI 530, or IBC codes that would state so specifically.

Being that this is an Army Corps project, the Corps QA's will likely simply look at the cold hard numbers of the test reports that don't match the 3000psi grout specified and not consider any practical construction tolerances (i.e. the "big picture").

 

[Ron]

jropert...I am not a fan of the C1019 method for making grout cubes. Because of the variables involved, the grout strength will vary more.

Yes, there are those who feel that this method better represents the "in-place" strength of the grout as noted in the standard. Unfortunately that's not what you should be testing. You should be testing the mix design of the grout, not the in-place strength. Typical tests for concrete in a structure are not intended to test the in-place strength of the concrete, but the mix design. Grout should be no different; however, ASTM C780 (which used to include grout) used the proper premise, was ultimately changed to remove grout from its scope, leaving only the C1019 method as the standard for grout testing.

In early versions of C1019, the variability of the method was recognized and even in laboratory specimens approached an average of about 14%.

 

[concretemasonry]

The ASTM C476 is a standard for masonry grout that is based on proportions and a slump range (8" to 11"). It does not contain a strength requirement, but refers to ASTM C1019 as a standard for the sample preparation and testing.

Neither standard has a strength requirement, so I assume the 3000 psi was picked out of a hat for some reason (a common strength minimum for concrete and not grout). The drawing may have an improper strength requirement.

I have seen engineers impose a maximum strength limit of drawing to prevent grout that is not compatible with the other materials in the masonry. On one project the engineer rejected some grout that was 3500 psi when he specified a maximum of 2500 psi. It is very similar to the lack of importance of mortar strength in high strength masonry (see appendix note 1 of ASTM C270). The reason is that masonry walls are designed as a structural element with the materials in balance to to perform as a unit (stress distribution) based on decades of testing, despite the ASTM testing requirements that produce results that cannot automatically and arbitrarily transferred because each component of masonry (units, mortar, grout) perform in unison. that is the reason you can make a masonry prism of 4800 psi with 2200 psi mortar.

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

 

[jropert]

The ASTM C476 is a standard for masonry grout that is based on proportions and a slump range (8" to 11"). It does not contain a strength requirement, but refers to ASTM C1019 as a standard for the sample preparation and testing.

ASTM C 476-02 paragraph 4.2.2 does state that grout shall be mixed to a slump of 8 to 11 in. as determined by Test Method C 143 and shall have a minimum compressive strength of 2000 psi at 28 days. Also, IBC 2006 section 2105.2.2.1.2 Concrete Masonry states that the grout should either conform to ASTM C 476 or have a minimum grout compressive strength to equal or exceed f'm but not less than 2000 psi. (f'm for this project is again specified to be 1500psi).

We are not surprised that the contract specs have been "pieced together" as we have been running into these types of issues on many other items in this project (the designers pulled specs from two or more different items and thought they could spec a superior product by piecing them together...problem is that this one combined specified item does not exist...but i digress).

Our main concern is that since the ASTM C 476 states "minimum" of 2000 psi and the contract specs state "minimum" 3000 psi, that the Corps will hold us to their "higher" standard. We have not found any codes that appear to allow for variation in grout psi. ASTM C 1019 gives stats for field samples, but those are just random stats.

Would the ASTM C 476 minimum 2000 psi be enough to back us up in this case since is is specified to be followed for this project (with the unfortunate conflicting addition of the 3000psi requirement)? Wouldn't a lower grout strength also be more desirable as the f'm is specified to be 1500 psi (to perform better as a unit)?

 

[TomDOT]

Unfortunately it sounds like the contract you signed called for 3000 PSI grout per ASTM C1019, and you failed to provide it.

Does the contract spell out a remediation for not meeting requirements? Pay reduction for getting out of spec, but not too far? Tear it out and start over?

A 2000 ASTM minimum and 3000 Corps minimum are NOT "conflicting" requirements - you can readily satisfy both conditions by providing 3000 PSI grout. If you had a 2000 ASTM maximum and 3000 Corps minimum, those would be conflicting.

It looks like your mix design targeted maybe a 3100 PSI strength. That's not a lot of margin. Given a 3000 minimum and known variability in mixes and the test method, I would have targeted something more like 3500 PSI.

 

[jropert]

TomDOT, I guess you're correct in that the additional specified Corps requirement is not a "conflict" as stated.

The target strength was much higher than 3100psi, however. Our ready-mix plant had provided lab data for 16 specimens and when averaged was 4340psi at 28 days, with even an average 7 day result of 3098psi.

The slump that day was within the specified limits of 8 to 10 inches (was 9.25 inches). The testing was performed (to the best of our knowledge when witnessing on site) as per ASTM C 1019. The problem is that even with an indicator on site during the pour such as slump meeting the specs, we cannot wait 28 days for results before continuing construction. No one waits and this is not practicable of course.

I guess at this point, it's up to the designers and Corps to direct us if they feel that this is an issue. No remediation was specified.

 

[theCorkster]

If the results failed to meet specification, the structural designer should be notified to see if his design provided sufficient excess to handle the deficient grout compressive strength. If the structure as constructed will perform to design/code requirements, then perhaps the structure won't have to be demolished and rebuilt. It may not be necessary to throw the baby out with the bath water!