low grout prism strength

[msucog]

had some low grout prism strengths show up on a job. the prisms were made from the first set of onsite mixed grout. we checked the proportions and it looks like they were mixing grout with sand:grout of 2.7:1 (type I cement). this falls within ASTM's table of proportions for grout (2.25-3 ratio) if i'm correct...HOWEVER the grout is spec'd out at 3000psi not 2000psi. i'm pretty sure this is the culprit. the strength came up at ~2500psi. does anyone have a cubic foot measurement of what the sand:cement ratio should approximately be for 3000+psi grout. i'm estimating 1.5-1.7:1 sand:cement ratio to get up above 3000psi but this is very ballpark. i'm not proposing to give the contractor the correct proportioning but i'm more curious than anything so that i can have a heads up for next time this comes up.

any help would be appreciated...

 

[concretemasonry]

Are you testing fine grout or coarse grout? This effects the ratio of total aggregate to cement according ASTM C476.

What did the project specifications say about grout subittals? ACI 530 requires written acceptance of of grout mix submittals.

ASTM C476 allows grout mixtures in accordance with the proportions listed in the proportion table from C476. This is for the 2000 psi minimum. For higher strengths, a mix design should be submitted along with the proposed proportions and test reports.

Grout is similar to mortar, in that either proportions or performance must be required, but obviously not both.

I suspect the contractor is trying to use a fine grout, probably because of the amount and placement of reinforcement. - A possible case of more not being better. Coarse grout is usually more economical for any strength.

It also sounds like a possible lack of close control of submittals that makes specifications almost worthless or unenforceable.

 

[dirtsqueezer]

I don't know about masonary grout, but do you think the water/cement ratio might have something to do with it? Could they just be adding too much water? Do you have full time inspections and/or inspection reports that show the amount of water added? If it's added with a hose, you could just use slump to get a better, lower water-content consistency.

Also, if you're mixing with bags on site, it could be expired cement. Moisure in the air will get into those cement bags over time and cause their strenth to be compromised. I've seen mixed some base plate grout past the due date that came up weak. The date should be just stamped right on the bag.

 

[concretemasonry]

dirtsqueezer -

Take a look at ASTM C476 specification for grout and ACI 530. They explain the desired properties of grout and high strength is not a desireable atribute.

Gout must have a high water content and 8" slump is usually too low. The water is needed to insure complete filling of the voids into all joints and around the rebar.

Masonry grout is placed in an area with absorptive "forms" (masonry units) that absorb the excess water as the grout consolidates. The specifications require controlled placement techniques and topping off after all consolidation is acheived. The maximum allowed strength is 5000 psi.

Grout specifications have been fine tuned for many years and the material acceptance is a necessary part of the engineering, inspection and testing process. Grout is not like concrete and is not intended to be. It just uses the same raw materials (water, cement and aggregate).

 

[msucog]

the contractor started out using grout from the plant but decided to use some on site mixed grout too. the grout is fine grout and it appears that their proportion were correct...for 2000psi grout. 3000 psi grout is specified. the architect was fine with using onsite mixed grout as long as the strength came up. since this is the first batch made that came up low, they haven't placed much. the architect/engineer are fine with 2500psi in this instance but the contractor has elected to go back to batch plant grout to avoid any trouble. the slump has been in the 9-10" range and been used in 30-45 minutes after batching. i'm certain they were just mixing it with too little cement for to satisfy the strength requirements.

i'm curious more for my own self. i don't have the submittals in front of me but i can pull the designs on my other jobs. i was wondering what some of you suggest or typically see for 3000psi fine grout. i always like to have a number in my head so that i'm never behind the learning curve when it really matters.

 

[concretemasonry]

msugog -

The NCMA Tek Note 9-4A (ncma.org) give a very good summary of grout for masonry construction. This refers to ASTM 476 (and others) and ACI 530.

According to ASTM C476:

4.2 Grout proportions shall be determined by ONE of the following methods:

4.2.1 Requirements of Table 1.
<This will provide the proportions for a grout meeting the minimum 2000 psi strength requirements It generally states the fine aggregate should be be between 2 1/4 to 3 times the VOLUME of the cementitious materials. For coarse grout, coarse aggregate of 1 to 2 times the VOLUME of the cementitious materials may be added. Like most ASTM aggregate requirements, variations from gradation requirements may be permitted if a history of performance can be shown>

4.2.2 Specified Compressive Strength.
Proportions established by 28 day compressive strength tests in accordance with test method C1019 that obtained the specified compressive strength. The grout shall be mixed to a 8 to 11 inch slump.
<This is for verification of the 2000 psi or higher strengths if speified>

The specifications should require only one of the methods. It is the classic case of either requiring the proportions or requireing the performance.

The first is a conservative and simple "bullet-proof method to acheive the minimum 2000 psi strength with out excessive testing and complications. The second method is similar to a concrete mix design that is verified by site testing for compliance when higher strengths are required.

According to ACI, it is best to balance the specified grout strength with the specified concrete masonry assembly strength so one element is not considerably stronger than the other. ACI 530 also has a maximum limit of 5000 psi as the specified strength for grout, but stronger grout may be used if it tests above that strength. - This all is a means to provide a balanced design where one portion of the wall does not absorb an excessive amount of the load.

From a practical standpoint, 3000 psi f'm and grout is a very workable system. I have had 4500+ psi f'm hollow prisms made (8000 psi block), but you would have to have some very unique loads and conditions to require this level of strength. Also, you may not be able to find a testing laboratory with a testing machine with thick platens capable of testing a 16" high sample with that much area.

Just as a starting point, a fine aggregate equal to 2 times the cement volume would give you about 3000+ psi grout, depending on the fineness.

 

[msucog]

ok thanks. that helps me narrow down the proportions. as i said, i like to keep a number in the back of mind because i know the subject will eventually come back around...doesn't mean i'll give the answer, but at least i'll have a ballpark idea of what it should be.

as far as testing, i always test what the specs say (the specs are usually horrible anyway since they're copied from the last 20 jobs so there's always contradictory language that leaves the test method open). we can test nearly anything out there with our equipment, so it'd have to be a very special situation that we wouldn't be able to test (i don't recall seeing one yet on any of my jobs). however, i'll usually try to test masonry prisms ungrouted for ease of moving the things and then test grout prisms seperately. plus this gives me the opportunity to test the grout a little more frequently if needed. for onsite grout, i test much more frequently because the stuff if so variable since jose might mix it one day and juan might mix it the next. even with volumetric measuring, you still catch them just throwing stuff in there as soon as you turn your back. i'm glad to see that most architects are not fond of grout made on site and get rather picky about it simply because there's no practical controls in place to keep the mix fairly consistent.

 

[henri2]

msucog, I have a gut feeling (based on testing done about 2 decades ago)that a cement to fine aggregate ratio of 1 cement to 1.5-1.7 fine aggregate will produce an fg value easily in excess of 3000 psi.

 

[msucog]

i figured that range would give a nice fluff factor for 3000psi. 2 makes sense and is easier to remember. as i said, these guys were mixed closer to 3 and getting in the 2500psi range. i don't have a lot of personal knowledge or data (ready mixed is usually used for everything around here and i don't dive head first in to their mix designs) as a basis across such a wide spectrum of ratios for fine grout, but this definitely helps get some round figures in my head. thanks.

 

[DarthSoilsGuy]

throwing out a balloon here to shoot down.

my initial guess was 2:1 by figuring a 4:2:1 mix without stone filler. sounds too easy, comments?

 

[concretemasonry]

Just a reminder - Do not overshoot the strength. It results in poor construction.

The grout strength should be as much as the f'm of the masonry with a maximum of 5000 psi. Most authorities rcommend it to be as close to the masonry strength as possible. - Very much like mortar where you should use the weakest mortar possible, but still carry the intended loads.