| Author |
Message |
Anonymous
| | Posted on Wednesday, June 29, 2005 - 07:22 pm: |    |
Has anyone encountered quantifying (from a specifications standpoint) shrinkage cracking in architectural concrete paving (or any other type of "architectural" concrete), especially in terms of or in relation to mix designs? |
Ronald L. Geren, RA, CSI, CCS, CCCA
Senior Member
Username: specman
Post Number: 176
Registered: 03-2003
| | Posted on Wednesday, June 29, 2005 - 07:47 pm: | |
The URL below may provide some insight. It's a PhD Dissertation regarding shrinkage in concrete and it does address composition. However, this document pertains more to concrete in civil projects, but its application to architectural concrete may still be appropriate.
http://scholar.lib.vt.edu/theses/available/etd-05092002-143014/unrestricted/MokaremDissertation.pdf |
Mark Gilligan SE, CSI
Senior Member
Username: markgilligan
Post Number: 13
Registered: 05-2005
| | Posted on Thursday, June 30, 2005 - 01:52 am: | |
Two key concepts to minimize shrinkage cracking are control of restraint and minimizing the total water in the mix design. Type and amount of cement and type of aggregate can also help minimize shrinkage. Specifying a maximum water cement ratio is counter productive since this normally results in the use of more cement and water. Similarly minimum cement contents are counter productive.
There is an ASTM test procedure to measure concrete shrinkage. You could specify a maximum allowable shrinkage ratio and perform tests on the mix design but because of the time to run these tests (aprox 56 days) this approach is not used a lot.
You can minimize shrinkage cracks by installing crack control joints no farther apart than 15 feet. If you go this route do not allow the reinforcing to be continuous through the crack control joint since that prevents the joint from opening.
I have seen projects where the Contractor is required to inject epoxy in cracks over a certain size.
If the reinforcing must be continuous then the best thing is to put in enough reinforcing so that the cracks will be well distributed and tight. In this case I start at aprox 0.5% of reinforcing steel.
Given that it is very difficult to achieve concrete with no cracks it would be prudent to select a finish that either can be repaired or which is tolerant of small cracks. Precast concrete is another story. |
Anonymous
| | Posted on Thursday, June 30, 2005 - 11:05 am: | |
Gentlemen,
Thanks for your responses. I don't think that I'm (architect-specifier) up to reading the more scientific aspects of the subject in a 100+ page doctoral dissertation, but, to Mr. Gilligan, I would like to know more about what the specific ASTM standard is, even though it would be measuring long after-the-fact (I could find several, but it's not clear which are applicable), and what magnitude/starting point for specifying max. shrinkage ratios should be considered; Is 0.03%, per the doctoral dissertation, a reasonable req't?). |
Phil Kabza
Senior Member
Username: phil_kabza
Post Number: 118
Registered: 12-2002
| | Posted on Thursday, June 30, 2005 - 02:55 pm: | |
I'd like to add a comment to Mark's note concerning minimizing shrinkage cracking by installing crack control joints. It's important to design a slab aware of the context in which it will be used. We occasionally struggle with structural engineers who want to design crack control joints into slabs that will receive resilient floor coverings. These joints then require special treatment to keep the joints from telegraphing through or otherwise affecting the applied floor finish. Our conclusion is that in such circumstances, we're often better off allowing widely-distributed random cracking that will later be concealed by the applied finish, then we are by inducing the shrinkage cracking to concentrate in a few locations, where curling becomes more likely and epoxy joint sealants or other fillers are required.
We often have to resist engineering efforts to design in far more strength than is needed in interior slabs on ground; the additional strength means additional cement, which increases overall water content, thereby increasing shrinkage. |
Mark Gilligan SE, CSI
Senior Member
Username: markgilligan
Post Number: 15
Registered: 05-2005
| | Posted on Friday, July 01, 2005 - 03:20 am: | |
This is in response to the request for more information. Follows are provisions pulled from my master specification.
1.05 SUBMITTALS
1. Where shrinkage limit is specified submit shrinkage test results.
2.02 CONCRETE MIXES
G. Mixes:
1. Class “B”: Typical concrete, normal weight aggregate, f’c=3000 psi, 1 inch aggregate, 4 inch maximum slump with water reducing admixture or 8 inch maximum slump with High Range Water Reducing Admixture, water/cement ratio 0.55 maximum, [4 percent air entrainment] [Air entrainment optional], [drying shrinkage limit of 0.045 percent].
H. Clarification of Mix Properties:
1. Drying shrinkage limit is percentage change in length when tested as per ASTM C157 with 4 inches x 4 inches x 11 inches specimen. The specimens shall be stored using the air store option. Measurements shall be taken at the times required by the standard with the measurement taken after eight weeks of air storage to be used to determine compliance with the specified limits.
3.16 FIELD QUALITY ASSURANCE
3. When shrinkage limit is specified take a shrinkage sample [at the first pour for the] [from each days pour for each] mix design.
As to whether the 0.03% is achievable, you need information about what can be done locally. As you see my default is slightly higher. Note that 0.045% at 4 weeks is not the same as 0.045% at 5 years. As a rough approximation the shrinkage at 4 weeks is half of the long term shrinkage.
Shrinkage is an involved topic, so if you want to predict the actual shrinkage and the impact on your structure I suggest that you contacct an expert. |
Phil Kabza
Senior Member
Username: phil_kabza
Post Number: 119
Registered: 12-2002
| | Posted on Saturday, July 02, 2005 - 07:23 am: | |
Mark - This is a great help toward controlling a real problem. I haven't seen an approach to defining and limiting shrinkage in standard specifications before.
I'd find this more applicable if it were specified as a concrete mix design testing requirement rather than as a field test. We'll never be in a position to reject non-complying pours, only fight about the damages that might result from the excess shrinkage. Have you taken any shots at defining shrinkage limits as part of preconstruction testing? |
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