CMU Removal and Testing for a Parapet 1

[maxwolf]

To verify some calc's for an existing composite parapet wall, we need to take some samples of cmu block from the wall and have prism tests done. There's roughly 150 ft of the known parapet wall type - and about 400 ft of an unknown parapet wall type. Any suggestions on min. number of sampling locations? I figure at least two for each type of parapet at random locations. Any book reference suggestions for sampling masonry would be much appreciated.

Thanks.

 

[Ron]

You won't find much in the way of test frequency guidelines in the literature. My rule of thumb is to always require at least 3 tests to represent any area. My reasoning is that 1 test is too isolated and won't necessarily represent an average condition. Two tests will often just confuse you if the results are disparate. Three tests will help you referee the results and provide some reasonable interpretive value.

 

[concretemasonry]

Prism tests are only used to determine the compressive strength of the masonry (f'm). The controlling factor is the compressive strength of the masonry units with the mortar having a very, very minor influence.

The ASTM C270 mortar specifications have a major portion of the appendix relating to the desired strength of the mortar. It recommends to use "the lowest strength mortar possible to carry the loads" (paraphrased since a copy was not readily available). This is because the higher strength mortars do not have some of the desireable properties that the more workable mortars have.

Making prisms from removed units will give you a more realistic idea of the prism strength in comparison to cutting prisms and then capping and testing them. The reason for the differences are the difficulty of cutting out representative prisms out of a wall built in running bond, handing the heavy samples, capping them and then having them tested. Cutting and handling the heavy prisms from a wall add many variables that can affect the results of the simple compression test.

You will also have to find a section to sample without grout, or reinforcement (horizontal or vertical)if you want a true prism strength.

Exactly what type of properties are you trying to determine? - There are other tests that can be done for other purposes.

Dick

 

[hokie66]

Could you describe further your reason for requiring tests of the masonry strength? I can't conceive of a masonry parapet design which would be controlled by the block strength.

 

[maxwolf]

Thanks for the comments.

I agree that 3 samples seems reasonable as a minimum. The reason for doing the testing is that the wall is not to code (NYC) and barely works according to strength design (aci 530-02). The original engineer should have designed with 8" block. Instead it's 4" cmu, supposedly grouted with #4 vert's at 32" oc. It's also hopefully tied to an exterior 4" veneer wall. I initially suggested to the peer review engineer that we consider a windsor probe test, even if it has high variability, since cost is always an issue, but peer review wants blocks removed and tested. We also need to verify quality of grouting. If there are voids so the bars are not well bonded to the block, peer review says the deal is off on approving the parapet. We can test mortar as well.

Let me know if you need more details.

 

[concretemasonry]

The cores on a 4" hollow CMU are not large enough to permit grouting under the current codes. There is a minimum grout space requirement and a block meeting ASTM C90 specs would not qualify because of the required face shell and web thicknesses.

I doubt if the connection to the veneer is sufficient to permit the two units to act as a composite wall, but there is a possibility it may be sufficient. - Check amount of mortar between the block and brick.

Testing the mortar strength may not reveal anything definitive since mortar has such a small effect on the strength of a wall. The odds cutting out a few block prisms (hollow, unreinforced) suitable for prism testing is quite low because of the cutting, handling and testing problems (4" thick & 16" high).

Tear out a section of the wall to examine the condition and the effectiveness of the grouting and use the units for testing purposes. With the net compressive strength of the block, you can determine an assumed f'm from the tables in ACI 530.

A parapet wall is a very high liability item.

Dick

 

[maxwolf]

Concretemasonry,

Can you cite the code section regarding min grout area. I see in Aci 530-02, Table 1.15.1, Grout Space Req's, that for Fine grout with pour ht < 5' that:

Min width of grout space = 2"
Min grout space dim's for grouting cells of hollow units = 2"x3" for Fine and 2.5"x3" for Coarse.

When I drew up a block in cad with 3/4" walls per ASTM C90, Table 1, I get two 2"x6.5" cells. (3.625"W block - 2(.75" walls)=2.125" cell width) So I think if they used fine grout, it might just work, but not with coarse.

Does the mortar (or grout) filled gap betwn the wythes (cmu to veneer brick) need to be at least 3/4", per Table 1.15.1 if gouted? Or is it permitted to fill in this gap with mortar as the mason builds up the wall?

Also, you're saying if we cut out portions of the wall for testing, we can test the grouted units and then use a table in 530 to determine f'm? What table is that?

Thanks for all the comments.

 

[concretemasonry]

Max -

When you are looking at Table 1.15.1, you are looking at the correct requirements for permitted grout space. This is based on the only version of ACI 530 (actually ACI 530-02)that I have handy here right now.

Unfortunately, you can throw out your cad drawing because you assumed the block were made to the ASTM minimums, the block were 2 core and that the face shell thicknesses were the minimum. Most block, especially 4" are not made to the minimums (not economical to save on material because of the manufacturing methods), so the actual grout space is different. All bock have a face shell that varies in thickness and the minimum grout space (especially 4") is less than what you would calculate. The tapered cores are a factor in 4" block, but not a big factor in large size block, like 6", 8", 10", 12", 14" and 16" thick units. In reality, the minimum face shell produced for a 4" block is commonly 1" and the maximum face shell dimension may increase to as much as 1 1/4". Many block producers, especially in the east, make a traditional 3 core unit because of the ease of manufacturing. That is why I suggested you remove a small section to see how the wall is actually constructed and configuration of the units.

The table I mentioned referring to the relationship between the CMU unit strength and mortar type to determine the f'm for design is in the "Specifications for Masonry Structures" (ACI 530.1-02 is what I have here). It is Table 2 that is referred to in Section 1.4 B.2.b. This, again, is to determine the f'm by the units strength method.

I think that when you get some 4" units out of the wall, I assume you will find unit strengths way beyond the ASTM C90 requirements since very few block producers manufacture to just meet the minimums because the minimum standards have not been increased but the manufacturing methods and controls have improved to where the savings on materials are very small in comparison to the manufacturing savings by making a better block since handling and contractor. The material cost is a small percentage of the real cost and selling price. The cheapest way to make a stronger block is to add more water, but contractors will not accept that approach. When I was in the block business, we typically were about 30% to 50% over the ASTM minimum strength and 300% over minimums were possible. It was just too costly to make a low strength block because all other costs go up with low strength.

This will hopefully give some additional insight to the compressive strength and the f'm. Flexural strength is a different matter because the steel may be required depending on your conditions. In some cases, the steel may not be necessary for calculation purposes for low wall heights and lower wind pressures.

Dick

 

[maxwolf]

Concretemasonry,

One more question: since the grouting width is probably less than code minimum, then it sounds like the 4" cmu cannot be grouted - or at least not to code. Assuming for argument that we take out a block or two and they are fully gouted with the bars pretty much in the center of the cells, can the peer review engineer still say that the cell dim's are not to code for grouting and therefore reject the parapet?

Thanks again for all the help.

 

[concretemasonry]

You will not be able to get a representative block out with rebar in it, but you could get a good look at the present condition of the wall and evaluate it from a potential performance standpoint and see how effective the "grouting" was. The mason probably used mortar instead of grout, but he may have been smart enough to add water to increase the slump and provide a "grout" to fill the voids.

A prism or block test with rebar in it would be worthless. It is the same thing as testing a concrete cylinder with a rebar in it. - In college, we used to embed (and hide) a short rebar in cylinders as a prank and change the failure mode to something meaningless. We only did it when there were som cylinders that were not sabotaged.

You have to evaluate the condition of the present wall construction and the materials in the wall. It may or may not meet the current construction codes and specifications, but it may be adequate for the proposed situation. How knows? - you may find a unique block that could technically meet the current codes for grouting. The grout is only used to bond the steel to the masonry units.

Dick