f'm vs. grout strength

[PEinVA]

We have a job where the grout didn't reach specified strength.

I am looking to ACI 530 to determine if minimum grout strength is acceptable at some percentage of f'm or how grout affects f'm. We don't have any prism data to go off of.

Any help is appreciated.

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[rrmiv]

Did you specify that the grout must be tested? If so, was the grout tested in accordance with ASTM C1019? If some other method was used, the results are probably not valid if the mold does not match the absorption rate of the masonry units.

We typically do not require grout compression tests as long as it is proportioned according to ASTM C476.

In matters like this, the National Concrete Masonry Association TEK notes are usually helpful.

 

[BigH]

What was/is the purpose of the grout?

 

[dik]

For serious masonry work, I often require that field samples of grout be prepared using absorptive containment. This is done by shaping the specimen using 4 of the CMU blocks and paper lining. The strength of in situ grout is generally very high due to the absorption of the water from the grout mix into the masonry unit.

Dik

 

[PEinVA]

Well a little background
The grout is for partially grouted CMU exterior wall.

We specified 3000 psi grout and required f'm = 2500 psi.

We didn't require grout breaks but the contractor did them anyway and came up low. Like 60% of the required compressive strength.

The testing agency used a cardboard box that was specifically made for the ASTM test. The contractor complained that it was not valid that way so a few more tests were performed the contractors way (4 blocks with the paper as described above) and the results were very comparable.

We got 56 day breaks back and they are now over 2200 psi. We went back and reviewed our calcs and we can live with f'm of 2000 psi.



RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[miecz]

The ultimate net compressive strength of the masonry, f'm, is related to the strength of the block and the strength of the mortar, not to the strength of the grout. If your grout is understrength, it would effect the splice lengths of the rebars, and perhaps the bending strength of the composite wall.

 

[rrmiv]

The way I understand it is you either:
1) specify that the grout must meet the proportions of ASTM C476; or
2) specify the compressive strength.

You shouldn't do both. If you choose the latter, you must test it. Tests must be in accordance with C1019 (4 blocks with paper). We have often seen poor testing methods, for instance using a paper liner that is not very permeable. The only purpose for the paper is to prevent bond with the masonry. If it does not allow the water to escape the grout, the strength results may be very low.

If they proportion the grout per C476, compression tests are not required.

 

[PEinVA]

Except if the strength is below 2000 psi.

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[boffintech]

"The testing agency used a cardboard box that was specifically made for the ASTM test."

Test results are highly dependent on the water content of the sample. So although these cardboard boxes are made for this test be advised:

ASTM-C1019 NOTE 6 reads in part:

Since test results vary with methods of forming and specimen geometry, comparative test results between the specimen described in this test method and the proposed specimen should be required and confined to a single specimen shape and method of forming.

If you have samples out of these cardboard boxes and you don't have comparative test results then you don't have jack.

The cardboard box is unable to absorb the same amount of water as the dry block so I doubt that your test results accurately indicate strength of grout; your test results more accurately indicate m'tll (magnitude of the testing lab's laziness).

The testing lab in this case appears to be lazy or incompetent or both! You didn't specify grout breaks but in order to determine f'm they did them anyway in order to get out of doing the correct albeit more difficult and more expensive unit prism test C-1314 and to top things off they did the C-1019 half-assed generating half-baked BS that caused a big headache over nothing.

Just for fun you should ask the lab to email or fax over a copy of the testing tech's certification.

 

[rrmiv]

miecz, I'm not so sure about your statement that f'm is unrelated to the grout strength.

If the unit strength method is used (typical for my firm), then you are correct. As long as all the criteria is met regarding masonry unit strength, bed joint thickness, and grout and mortar proportions.

However, if the prism test method is used, and the CMU walls are designed to contain grout, then the prism test should include grout. NCMA TEK Note 18-1A states "Where the corresponding construction is to be grouted solid, grout solid each prism using grout representative of that being used in the corresponding construction.....If the corresponding construction is to be partially grouted, construct two sets of prisms – grouting one set while leaving the other set ungrouted."

 

[concretemasonry]

Fortunately, the old concept of testing solidly grouted prisms and using the results as a quality control means is going out of existence. There a number of reasons why it is bad to rely on grout to increase the compressive strength of a wall.

1. A grouted prism can be very misleading when you look at the big picture if the grout is appreciably higher in compressive strength. If the grout is too much stronger than the units masonry strength, the average f'm is increased. Unfortunately, for most walls the outer fibers are the most highly stressed, but the grouted prism give a false reading on the strength of the outer fibers. That is the reason many designers put an upper limit on the grout strength (usually a % of the f'm for ungrouted prism), to make the wall act as it is designed.

2. It is far cheaper to increase the strength of the masonry units than to add grout or high strength grout. It is well known that the mortar strength has a minor effect on the strength of a prism. A plus to using the higher strength units is that they are cured before being used and not subject to the vagueness associated. I had some 4750 psi prisms (f'm) made using 2500 psi mortar.

3. The cost and problems of creating, curing, handling and testing solidly grouted prisms can be excessive and does not always yield reliable results. There are very few labs that have the testing machine with the proper platen thickness to test a fully grouted 8x8x16 unit, so they have to be cut down ($ and time) and then the correction factors are subject to criticism.

There are many that subscribe to the keep it simple philosophy. Test the units (automateted plant production)in advance of use and compare to the results of similar units used in hollow prisms. I was involved in a project of about 10 - 15 to 20 story 6" loadbearing block buildings where there were NO prisms made for the 5 different strengths of block used. Te same mortar was used on all levels of the buildings and there were grout samples taken at the mixer daily. There were virtually no clean-outs used (random camera investigation of the cores). That is keeping it simple and still providing reliability.

Dick

 

[PEinVA]

boffintech,
We did have compartive testing done with the CMU and paper method, there was no appreciable difference in the strength of the grout.

In the end it was resolved but for next time I know to just design at the lowest f'm and specify a higher f'm.

Then I don't have to worry.

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke