Inadequate Grout Strength Test 3

[H57]

We have a project with reinforced concrete masonry specified with a strength of f'm = 1500 psi. The grout has a minimum required strength of 2000 psi. Testing of the grout came back with strengths of about 1500 psi.

What options are avaialble for the contractor at this point? The wall in question is for a single story building, lightly loaded with reinforcing at 32" o.c.

 

[JAE]

ACI 530 doesn't seem to address low strength response procedures like ACI 318 does in its Chapter 5.
The closest you get in ACI 530 is the commentary of section 1.18.6. which doesn't say much.

Having grout less than 2000 psi means you don't know, as the EOR, whether you have masonry that provides the f'm required.

I don't think there is any data on the relationship between C90 blocks and 1500 psi grout like there is for C90 block with 2000 psi grout.
So f'm would be an unknown.

You might be able to cut out some prism samples and test those but I don't know if there is an ASTM method/procedure/acceptance for sampling of built masonry for prism testing purposes.

 

[EngineeringEric]

I would talk to your testing agency first. I believe the prism tests used in Special Inspections often leads to an overly low value. This (plus other reasons) is why the seniors at my firm have chosen to stop requiring the testing of mortar all together. I believe that the composite area for the block + grout has to meet your design assumption.... how it gets there is all in the fun of construction. I may be wrong but typically design of masonry is done with an f'm of say 1900 psi which is: 1500 block + 2000 grout + some mortar capacity from a bag.

And like concrete I believe the capacity of grout increases with time beyond the 28 day design strength so you may reach the 2000 at 112 days?

 

[concretemasonry]

For an f'm, a test is run on a hollow two high block prism to determine the strength of the masonry units, where the highest compressive stresses are. The purpose of the grout is to transfer the wall loads into the reinforcement and not to increase the compressive wall strength when acting as a system.

The specified strength (f'm) on this project seems extremely low and looks like a minimum assumed value.

Many engineers impose a maximum allowable strength on the grout to create a wall that behaves as assumed in the normal masonry wall action and distribution of loads. This is why arbitrary filling of cores changes the loads on a wall by attracting loads and reducing loads on other parts of the wall.

Normally, for engineered masonry, the actual strength of the masonry units control the f'm of a wall system. As an example, mortar strengths have little effect and I have seen 4800 psi hollow prisms (f'm) made with 2200 psi mortar because of failure method in testing mortar compared to the actual performance is a thin confined mortar layer in confined compression. The purpose of the grout is to fill the cores and transmit loads from the wall to the reinforcement and can only work if the slump is 8" to 11" (wet) to insure complete filling and bonding and the CMUs absorb the excess water quickly and create a good bond to both the steel and CMUs. This is why lap distances are important and are based on actual laboratory wall tests over time.

Dick

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

 

[racookpe1978]

This is just a guess, but would he recover some of the strength of the wall if he drilled down from the top of the wall (through the CME bricks down to the foundation), then inserted rebars vertically and filled in sections of the CME with concrete?

Seems like that would "lock together" much more firmly the blocks into a single masses that now are held together only loosely with the "bad" mortar joints? With concrete going up and down you'd get more resistance to racking and block-to-block movement over time

 

[Ron]

As JAE noted, f'm is not known with certainty. The grout within the cells does affect the compressive strength of THAT influenced system (the filled cells and adjacent masonry), thus causing varying stress distributions in the wall. Otherwise, for allowable stresses in the system, only the mortar type is relevant and is the same for both M and S mortar.

The mortar strength is less relevant. In fact, if not inspected, you can only consider about 800 psi for the mortar compressive strength in your design, using Type S mortar.

 

[H57]

Thanks for all the responses.

I was verbally told that the grout did not achieve the required 2000 psi strength. After receiving the actual test report it was the mortar that was insufficient (1580 psi actual vs. 1800 psi design). Additionally the wall with the questionable mortar was a non-load bearing interior partition wall. Needless to say what was presented as a problem quickly went away.

 

[concretemasonry]

Compression tests for mortar are only done on laboratory prepared samples and there is no sampling and testing procedure for site mixed mortar. The ASTM C270 mortar spec, in appendix 1(?) states that lowest strength possible to carry the structural loads since there are more important benefits that are not produced with higher strengths. That is why there is such a high percentage of Types N and S mortar specified in comparison to Type M mortar.

Dick

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

 

[jayrod12]

In place mortar is always stronger than the mortar cubes. The blocks extract water out of the mortar increasing the strength. So low testing numbers don't necessarily mean low in place mortar strength.

 

[Ron]

Field evaluation of mortar is done by ASTM C780, including compressive strength.

From your last post I'm now confused as to whether you are concerned about low mortar strength or low grout strength. Which is it? They are different.

 

[concretemasonry]

jayrod12 -

What testing field procedures can you field for field testing and sampling of mortar? ASTM C270 controlled mortar designs and properties.

The ASTM C270 specification clearly states that it is not a specification to determine the mortar strength.(Section 3.1 relating to Specification Limitatations).

There are two different methods to determine the type of mortar: proportions specifications or properties specifications and two cannot be interchangeably - just one or the other. The property specifications are used to determine proportions a material proposed to be used when mixed to controlled flow, which is does not produce a workable mortar that is desired for a quality wall.

The two tables in C270 (Table 1 and Table 2) are used differently. It clearly states that the compressive strength applies only to laboratory prepared samples and not to field samples.

If you look at Table 2, you will see very little difference in the properties except air content for different cementitious materials and it just a table to define the mortar type from the broad range of a gray scale.

It is definitely not like a simple ready-mix design where the only real site tests are for compression strength (field prepared sample is non-absorbant mold cured under laboratory conditions) to provide and idealistic measure of the in-place concrete and the site-meaningful measures of the air content of the mix itself and the slump, which is a measure of the ability to be placed for the application.

Table 1 is a more practical table because it proportions of the materials to determine the proportions of the approved materials to be used. Again, looking at the table, you see there is a broad range of proportions to the used for different mortar types.

The observation on the in place strength of the mortar strength is very obvious, especially because it is used as a very thin layer between absorbent materials (masonry units)in compression compared to lab samples that are cubic (shear failure) prepared with non-absorbent materials.

Looking at individual materials (mortar, masonry units, grout) in a masonry wall is almost like determining the acceptable materials (cement coarse aggregate, fine aggregate) of concrete before it is placed in concrete forms. - It can be a little confusing to be precise when the site use really dictates the real performance of the structural element (wall or slab).

Dick

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

 

[jayrod12]

I agree with everything you said. My comment is still valid however. You must look at the masonry as a complete unit instead of its individual pieces.

 

[JAE]

Dick (concretemasonry) - I'm always impressed with your zeal in answering posts.

However, I'm also always amazed that your answers rarely zero in on the actual question but rather appear
to provide us all with a lecture on various masonry aspects that have very little to do with the question.

H57's question was SPECIFICALLY what does an engineer do when presented with low grout strengths.

ACI has chapter 5 which provides a method of approaching low concrete strengths. ACI 530 does not.
Nor does there appear to be anything in the IBC.

What would be some steps that an engineer could or should do to respond to this condition? Nothing? Tear it out?
Cut out wall for prism tests? Take cores of the actual grout? This is the question.

H57's question has nothing to do with whether f'm=1500 is low (it isn't) or whether grout filled cores change how loads are
distributed through the wall, or lap lengths, slump of grout, how many engineers specify a maximum f'm, proportion vs. prescription specs, etc.

Please direct your zeal to the questions because I sense you have many years of experience in masonry but seem to mis-apply it here.
Thanks.

 

[a2mfk]

Since the poster already said it was non-load bearing partition wall, it seems mostly academic.

But I can't be the only one that thought to check your calcs to see what you needed since he said it was a lightly loaded single story wall with filled cells at 32" o.c.? There is almost always some breathing room in your allowable compressive stress unless you are really pushing your cell spacing....

 

[H57]

a2mfk,

There is breathing room, but the code dictates a minimum compressive strength of grout to be 2000 psi. So if the test were to come back indicating that the grout does not meet that criteria then what would/could an engineer do?

 

[jayrod12]

You could core a piece of the grout out and test the in place, Not a standard test but it could give you some confidence. I think you'd be surprised at the strength of it in place as opposed to samples taken from the truck.

 

[concretemasonry]

HZ57 -

I do not think you will be able to easily find a correlation between a smaller circular cored sample with the standard grout testing procedures outlined in ASTM C1019, which is used for testing with grout specification (ASTM C476). There might some obscure study that was done somewhere.

Since you selected the "default" minimum value of the ASTM C1019 by not specifying anything different, I would check the grout test report to determine the grout sample was made in a rectangular shape (4"x4"x8") using absorptive forms to simulate the actual absorption and curing. I have seen labs that have an inexperienced technician (especially in the summer since I was one while in college)that make samples in a concrete cylinder or test improper samples and fail to fill out the paperwork properly. If you see errors, notify the supervisor of the lab as a courtesy. A non-absobant mold will always give much lower compressive strengths than a properly prepared cubic sample that is topped off 30 minutes after making (to adjust for the normal consolidation.

If the sample preparation and testing appears to be proper, I would look at how critical the grout strength is for the application. If it is for cores for minimum steel and spacing the 1500 psi might be adequate. The ironic thing is that if there was no steel and grout, the ungrouted wall of f'm of 1500 psi in compression or non-load-bearing would there would be no question of adequacy.

If it is for a situation where the grout was necessary for the grout to be used to transfer loads, then you would have take a close look at the adequacy and if it is not adequate, you would have to look at the proportioning and materials used.

Proportioning and materials. -
The proportioning of the grout is by volume proportions that have a wide latitude of proportions that will probably not be able to check on now since there are probably no site notes to prove the the methods used were acceptable or not.

Materials -
It would be prudent to obtain recent representative tests of the aggregates used to determine compliance with ASTM C404.

Mill tests of the cement or invoices of the cement purchased should met the appropriate ASTM cement requirements. Most admixtures (waterproofers, accelerators or air entraining agents) are not not acceptable in conjunction with reinforcement without approval.

The only other possibility would be damage to the test sample in transportation or stripping from the block that make the form for test specimen. I assume the sample were cured at the job site (between 60F and 80F) kept moist and could possibly be jarred. It is unlikely that the sample was shipped to the testing lab without being stripped in the field since the test specimen and forming block surrounding would weigh in excess of 125#.

I don't have much more to add without making more assumptions on the sampling, testing and materials or seeing the drawings and specifications.

Dick



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

 

[henri2]

The wall in question is non-load bearing. The QA requirement does not include testing but simply verification that material submittals indicate materials conform to applicable standards.

If this had been a load bearing and or shear wall with f'm of 1500 psi and low grout strength results, IBC 2105.3/MSJC Spec Art 1.4 B.4 Testing prisms from constructed masonry should be considered.