Masonry Grout Compression Test Samples Moulded in Cylinders 5

[wannabeSE]

I am curious how the compression test results vary when masonry grout is molded in cylinders designed for concrete versus absorptive masonry molds per ASTM 1019 Standard Test Method for Sampling and Testing Grout. My guess is the cylinder strengths would be less because the masonry mold removes moisture and lower the w/c ratio. I think it is a bad idea to test grout in cylinders, but I am wondering if the cylinder test will always be lower and if there was a ball park idea of how much lower.

 

[concretemasonry]

I can't see how it would be as high or and as representative of as the the method of making a form out of the masonry units used, which represent the composite properties of the materials working together.

Really, the strength of the grout is not that critical, but the cylinders are cheaper to prepare, handle and move. Some engineers actually have set limits on the maximum strength of grout in order to maintain compatibility with the design for a project.

Masonry design and construction is based on decades of test history and correlation and composite construction (masonry units, mortar and grout) and chasing individual the precise individual material properties is ineffective. It has been shown that even the mortar strength has little, if any effect on the strength of a masonry wall.

Dick

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

 

[a2mfk]

It has been shown that even the mortar strength has little, if any effect on the strength of a masonry wall.

You mean compressive strength correct? Mortar bond strength is very important in partially reinforced or unreinforced CMU in getting lateral loads into the filled cell "influence zone strips".

 

[wannabeSE]

Concretemasonry,
I am not talking about testing composite properties with masonry prisms. ASTM C 1019 specifies grout molds formed by setting masonry units next to each other to form a 3" min square mold (in plan) and lining it with a permeable material (paper towels) so the grout does not stick to the masonry. The grout is subject to the compression test without the masonry unit molds. C 1019 also mentions alternative molds (cardboard) but provides no specific guidelines.

The reason I ask is because ASTM 476 describes two methods of proportioning grout. One method involves using Table 1 with prescribed volumes of materials. The other method involves designing the grout for a specific strength and testing strength per ASTM C 1019. I recently received a submittal for a grout mix where non permeable (concrete)cylinders where used for grout molds rather than following ASTM C 1019. The National Redi Mixed Concrete Association's CIP22 - Grout and the National Concrete Masonry Associations TEK 18-8B - Grout Quality Assurance both say that concrete molds should not be used for masonry grout testing. I am wondering if this is because the test results will be too low or is not recommended for other reasons?

By the way, I think the reason for testing the mix design rather than using the prescriptive methods is because Redi Mixed companies batch by weight and the ASTM 476's Table 1 cookbook uses volumes. Not that it makes any difference, most of my work is in high seismic areas where fully-grouted walls and double open end units are the norm.

 

[Ron]

I have been against ASTM C1019 since it became popular. The grout used to fill cells is concrete. It is soupy concrete, but nevertheless, it is concrete. As CM noted, there is an attempt to replicate the field conditions for a compressive strength test. That doesn't work. Secondly, the testing process is to test the MIX DESIGN not the in-situ conditions. If you want to test the in-situ condition, do a prism test.

The design is done based on an expectation of compressive strength from the grout mix. It is not done based on some nebulus value of compressive strength obtained from variously adulterated samples prepared as required in C1019. In my not so humble opinion, the only way to properly test the compressive strength of the mix design is to sample, cure and test just as you would for any concrete mix.

 

[dik]

We do the standard testing for masonry grout (not mortar) and use the results. I've also had 'informal' tests done using 4 CMU's arranged in a swastika type pattern giving a 4"x4" void that is filled with the grout. This is separated from the CMU with a paper towel so no bonding occurs. The 4x4x8 'cubes' have been tested and the strength of these is better than twice the strength of the regular tests.

When filling a masonry wall with grout, you can see the line of the grout as the CMU's dampen. I've done 20 storey buildings with CMU's...

Dik

 

[Ron]

dik...your description is similar to ASTM C1019.

 

[concretemasonry]

dik -

I agree totally about the results and predictability of using the 4 block to form a grout sample because it uses same grout as the project and the paper towels minimize the bond, so the grout initially cures just as it would in a wall in contact with the masonry units and moisture is drawn out. After a short period the block masonry units can be removed and the samples moved easily (without damage) for controlled curing and testing. Most of the masonry wall testing done in the last 50 years (what the codes are based on) used 4x4x8 (or similar grout samples.

The "neanderthal" method of job site prisms is not practical to get reasonable results. It is virtually impossible to make, handle and deliver samples for testing without having extra samples. The prisms also have too many factors affecting the results (unit strength, mortar strength) so the results are really a "mish-mash" of factors and not a real measure of the grout. It is not as simplistic as dealing with concrete cylinders.

This is similar to all the points I have seen through sitting in on ASTM, NCMA and MSJC committees for about 30 years when we wrote the standards that ended up being ACI 530 about 15 years ago.

It is not that different from mortar where the strengths are in a range and the strengths are made for lab samples and tested in a lab to determine is the materials to fall within the the very wide spectrum of strengths for classification into a type. How many engineers out there actually think a mortar in a 3/8" thickness (confined compression) can actually be compared to a 2x2x2 lab compression test except someone doing a research project. - How else you explain a 4800 psi hollow 2 block high prisms(ungrouted)to break at 4850 psi with mortar that tested at 2200 psi in a 2x2x2 lab cube?

Of course, the compressive strength of the grout is rarely a factor. On a building complex in Brazil all buildings used the same grout (7 or 22 stories made no difference). They were a little more advanced and eliminated the clean-out in the 6" loadbearing walls walls by using a video camera to inspect a random sampling of the cores to be grouted. The block were also pre-approved at the plant and color coded for the different strengths required since there were many different buildings.

Dick

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

 

[darthsoilsguy2]

Grout testing is one of the biggest wastes of time and money out there. check the ticket, see the batch process, and look at the grout. 95% of the useful things an inspector can do with masonry is: reinforcing steel and embeds are right, cells are fully grouted, weather protection is right. the industry is full of people that transition from testing soil and concrete who need to unlearn. I suppose if i had a f'm=2500 psi or more, i would be on board with testing but that hasn't happened yet for me. as for mortar, i intend to prevent field mortar testing from ever occurring on any job i'm involved on.... again watch it batch.

 

[dik]

First time I did that was over 40 years ago... and I wasn't aware of any standards... first attempt was to test a couple grouted CMU's and couldn't find a testing unit big enough... I was doing unusual things with grouted masonry walls and just wanted to have an idea of what the actual strength was... there was nothing in the codes at the time...

Dik

 

[dik]

darthsoilsguy... I would agree that testing that doesn't provide any meaningful results is meaningless... the moisture drawn out of the grout by the CMU's increases the strength significantly... I agree with the CM guy to some extent... but, with the uncertainty and scatter of results, the Neanderthal method gives a better representation of the grout strength than the refined standard tests...

Dik

 

[darthsoilsguy2]

my bias against masonry "testing" is more of a professional services related one.... There are a lot of people in my region who WANT masonry inspection to be all about these material tests because it is easier to train people to do a test, easier to report a test, easier to look like you are doing work, and easier to justify camping a useless troll on-site and sending invoices for it..... When someone is on-site and is selling ONLY VISUAL INSPECTIONS, the person has to bring REAL value or is quickly subject to emperor-no-clothes criticism by the client. My region is full of people who make $10-$15 per hour performing masonry inspections from commercial to critical facilities, and the people who hire them. Masonry construction is important. Batching control and QA of that is important. i don't see any value in mortar cubes or grout testing any more than on a pre-construction or submittal basis, regardless of method. 10 times out of 10 i would rather hear that the mortar and grout were prescriptively batched to spec, than have a performance test.

 

[wannabeSE]

To all who replied, thank you very much.