Grout Strength

[BigH]

Spec says:

"Cement grout shall have a minimum compressive strength of 20MPa at 3 days and 40 MPa at 28 days."

Question: Does this mean . . . ?

P + P = P
P + F = F
F + P = F
F + F = F

The first and last are obvious, but what is your take on the middle two? This is from a QA point of view - not an "engineering" one.

 

[GeoPaveTraffic]

I would read it as, P + P = P. Any other result would be failure.

 

[Panars]

Since the conjunction "and" is used, both requirements must be met for the grout to meet specification.

 

[BigHarvey]

I would say that it depends on the goal of the 3 days result, it might just be informative and in this case I would consider that F+P = P . If we talk quality ( ISO ), if there is no problem with the end product in relation with what the final client expects then it's definitely F+P =P
P+F = F without any doubt.

 

[DRC1]

I tend to agree with Big Harvey. Unless the shotcrete must carry some temporary load, such as soil nailing, the 3 day strength probably is not relevent. If it is, then why is the 28 day strength relevent?

 

[SlideRuleEra]

We would use dual strength requirements, such as this, to grout equipment in place during the construction of electric generating stations. The 3-day number allows timely setting of equipment, say installing electrical devices inside grouted-in-place electrical cabinets. The 28-day number ensures that the grout meets traditional strength requirements, that have a proven record of good performance.

http://www.SlideRuleEra.net

 

[BigH]

Good point SRE - spec writers are "power oriented" - so maybe they couldn't quite adapt to the use of the grout in this particular case - which is for passive rockbolting. I tend to agree with BigHarvey and have played it as such on a few cases. Interesting that in some cases, the installation of the anchors was more than a month or so after the holes were drilled!

p.s. - I'll probably have several more of these QA items to seek opinions on - due to nature of my position on this project. Thanks to all!!

 

[PEinc]

The spec says P AND P. Therefore, according to the spec, both conditions must be met. Sounds like a poorly written spec to me.

However, no information was given on the application. In real life, if the 3 day strength is not met, there is not much choice other than to wait to see if the 28 day strength is achieved. If it is achieved, the problem usually goes away. If the 3 day strength is not met, the contractor may not be allowed to strip forms or erect something on top of the grout as quickly as desired. It is not very common to see someone have to rip out concrete or grout because it didn't hit the 3 day strength specification.

If we are talking about grouted ground anchors, grout strength, although frequently checked, is not very important if the anchors test after approximately 5 days.

So, my final answer is that "it depends" - on the spec writer's intention, on the application, on the size of the problem, on the job schedule, on relationships, and sometimes on which way the wind blows.

 

[BigH]

Thanks PEinc - it is for passive rockbolts and the testing is done well after 14 days - usually after 28 days. On specs, I am . A lot of problems occur when specs give something like a dual acceptance criteria - but do not define if one if for one particular condition and the second for another. Say, 3-day to be met if bolt is to be tested before 7-days have elapsed or 28-day if bolt is tested after 28 days.

 

[PEinc]

BigH,

Are you testing the grout samples or the installed rock bolts after 14 days?

 

[BigH]

Grout cubes are tested at 3, 7 and 28 days. (three cubes for each day). Rockbolt pull tests are pulled, at least to date, well after 14-days and really after 28-days or much longer. We were geting some cubes okay at 3-day (specified strength met) while the cubes of same sample were less than specified at 28-day. Also the other way about. All of rockbolts tested show good behaviour with no movement during the 10minute hold at 1.25%DL. While the realness was that there were some issues on the manner of testing, my point was the clarity of a spec that gives "two" acceptance strengths and how it would be interpreted from a QA point of view. In my 'real' view, if the 28-day was okay and we saw no problems with rockbolt pull tests carried out on bolts from the appropriate "low" result, the rockbolts - as a performance basis - would be considered good. But still, . . .