f'm of concrete masonry unprotected in cold weather

[shaneelliss]

There was a group of storage units constructed of CMU and placed in very cold weather without proper protection (15° to 20°F). Wind shields were not used, blankets were only used on about 1/2 the structure, the blocks were not heated prior to placing as required, etc. So it is likely the mortar and grout froze and will not reach the original design compressive strength of 2000 psi.

Because of this the building authority that I work for would not accept the construction. So the contractor hired an engineer and core samples were taken from grouted sections of the wall and crushed. The compressive strength of the samples was between 500 and 900 psi (this was grout compressive strength, not prism testing). The engineer did some calcs and said the building was ok even with those low grout strengths and without requiring prism testing.

Is there anything in the code that is violated with the previous narrative? The IBC 2104.3 has cold weather construction requirements, but if these weren't met, and the engineer show by calcs that it is still ok, should the building authority back off? I can't find anything in the code requiring a minimum compressive strength of the grout or masonry as a whole. Am I missing something?

 

[shaneelliss]

Forgot to mention that this is reinforced masonry in seismic zone D. Don't know if that matters, but thought I would add it in case it does.

 

[ronstruc]

There might be a problem with anchors, bearing plates, and lap splices. These are a function of the masonry strength. The lap splice length would increase as the strength of the masonry decreases. I would require the engineer to provide justification for all of these.

Sampling the grout only is using the unit strength method to determine the compressive strength of the masonry. If the 2006 IBC is the governing code the quality assurance section 2105 specifies the minimum grout compressive strength for CMU to be 2000 psi (2105.2.2.1.2.3) if the unit strength method is used to determine f'm. Otherwise the prism method would need to be used, if that is the case than you can't test only the grout, you would need to perform a prism test cut from the masonry construction as specified in 2105.3.

 

[shaneelliss]

Thank you for mentioning problems with anchors, etc. I hadn't even considered those items.

As I read 2105.2.2.1.2.3 it says that the grout only has to meet either 2000 psi or conform to ASTM C 476. I don't have ASTM C 476 to verify, but have been told that that spec gives a mix design and that the grout that was used meets that mix design and thus satisfies the code requirement even though it doesn't have the strength.

 

[ishvaaag]

See if ASTM C 476 also mandates specific conditions for placement of the mortar, not only the mix. It would be funny that a gap in the codes would allow spoiled mortars be "legal".

 

[woodman88]

You might also look at IBC Section 1714.1. It talks about reasonable doubt as to stability and a required engineering assessment. See if the Engineer report meets this section requirements.

Garth Dreger PE
AZ Phoenix area

 

[woodman88]

Oh, and you might require the report to state that it meets the section requirements for the overall building design. I have see a few reports stating that something is okay but never stating that the building is okay. See ronstruc comment about the lap splice length.

Garth Dreger PE
AZ Phoenix area

 

[concretemasonry]

Mortar has a minor effect on the f'm. The masonry units are by far the most important factor. I have made 4850 pf'm prisms using high strength block and 2000 psi mortar. If the mortar was not saturated, it will continue to cure again when moisture and warmer temperatures are present. If you look in the appendix of ASTM C270, there is a description of the concept of using the weakest mortar to carry the structural load. There are properties much more important than the strength. There is the classic arguement whether mortar is mean to hold units together or keep the apart. You cannot compare the

The block are essentially fully cured from a strength standpoint since there is normally in-plant curing in addition to curing while in inventory and on the job.to permit economical handling since the normal strength are too low.

You cannot use mortar in place of grout since grout must be 8"-11" slump to perform well - it is not like concrete. The purpose of grout is to bond the reinforcement to the masonry units. It is common to see maximum strength limit set for grout that are only a few percent above the block unit strength, except high strength block when there is usually a maximum set ate 3000 psi. It is poor design that relies on the solid grouted walls to provide compressive strength since higher strength units are far more economical, unless it is unique situation or a very small application.

ASTM 270 is a material standard an engineer uses for selection of mortar type and for specifying either proportion standards or performance stand and the two cannot be used at the same time. The project dictates the type of mortar selected.

ASTM 476 is also material standard and the engineer selects the type of grout and the strength range desired. The selection of fine or coarse grout is dependent on the size of the grout space.

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

 

[JLNJ]

I'm with concretemasonry above in that if the units and the mortar are OK, the lower strength grout probably is not a deal breaker. If the mortar DID freeze it may have expanded and cracked the units. I have seen this happen near door and window jambs. Then you have a whole other set of problems to deal with as the units aren't wholly intact.

The thing that irritates me is that a contractor gets away with NOT doing something he's contractually obligated to do (i.e. temporary protection), he pays no price for his misdeeds, and the Owner gets an inferior product with no cost savings. It hardly seems fair to the Owner.

This can be made even worse if the EOR takes responsibility for the contractor's error, taking on all the liability and getting none of the financial gain.

 

[shaneelliss]

Thanks for the comments. So I guess you are saying that even if the grout is weak, the compressive strength of the wall still might be fine. But I am guessing that because we are in a seismic zone D, when/if the building sees any of the lateral loads that rely on the grout-steel-block interaction, that is where the trouble may arise. Even a strong wind could be trouble if I am understanding correctly.

I have been told that the mortar was placed on very cold days and left unprotected as well. Would a prism test cut from the walls let us know how bad off the building is or should we just visually inspect for cracking at the joints?

As JLJN says, we don't like approving a building that we know is inferior to what it should be. Even with the letter from the engineer, we are still looking for some way to put the responsibility on the contractor, but we are having a hard time finding anything to stand on.

 

[JAE]

I'd take prism tests...these are simply very similar to the concept used by taking cylinders of concrete for f'c.

Grout strength tests: I would agree with the above that development lengths and anchor bolts might be a big deal with this - especially with seismic design category D.

 

[woodman88]

JLNJ, when doing this type of assessment I have made it a requirement to get the owners permission so that they will be aware of the situation and has the option to demand the contractor to redo the work.

Garth Dreger PE
AZ Phoenix area

 

[concretemasonry]

Was to grout sample that was tested sampled from before construction or after it froze.

I hope they used the correct sample preparation method with absorptive molds and not steel molds (see ACI 530). Grout is not the same as concrete.

If they tested the grout after freezing, what procedure did they use and are there any correlations with fresh grout strength.

Without the proper sampling and tests it is impossible to see if the grout was weak (which is difficult to do) or if the construction/protection was bad. - The weather not that bad since you probably referring to the air temperature (high low or average?) and not the wall temperature.

It sound like a possible lack of good specs and a lack of inspection and testing during construction to sort out the reason for the supposed problem and to catch any contractor errors and slopiness, especially since the protection was minimal.

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