Architectural Concrete and Chert Content

[jheidt2543]

Anyone have an idea or reference as to the amount of chert allowed in a naturally occurring aggregate for concrete? I believe state DOT's limit chert to 5% of the aggregate for highway construction, true? My question actually has to do with architectural concrete flat work.

 

[Ron]

It depends on the type of chert.

Flinty chert is limited for two reasons: Its tendency for alkali-silica reactivity (ASR) and because of the particle shape (usually elongated, with odd angularity) with limitations for elongated particles being in the 3% to 5% range.

ASR is limited by the record of service of the aggregate. If aggregate sources are prone to ASR, then the aggregate is either not used at all or if the reactivity is controllable, the alkalies in the cement are limited.

Chalcedony chert is particularly susceptible to ASR.

 

[jheidt2543]

Thanks for the comments Ron. I'm not aware of any real problems in our area with ASR, but our freeze/thaw cycles cause a lot of pop-outs from chert content. I've got a copy of a WIDOT report that says the local ready-mix plant's aggregate pit tests out at 2.5% chert. That seems relatively small, but I'm not sure how that would affect pop-outs in a flat slab.

 

[Ron]

If the pop-outs are freeze-thaw induced only, then that's related to the particle shape and the lack of bond to the flinty chert particles. I would limit as the local DOT recommends, since your application is essentially a pavement.

 

[dik]

I don't know if a higher flyash can be used for Architectural concrete in your area. If it can, it will reduce ASR a tad... Are you sure the 'popouts' are caused by freeze thaw and not ASR?

Dik

 

[fearghusq]

ASR characterised by Map or star cracking (Grid like formations). Take a core or nuse feeler plates to determine depth of cracking. ASR usually 25-50mm cracks. Characterised by a whitish apearance and friable (crumbly) nature of spalled pieces.

 

[scone38]

beware...iron oxide can ruin Architectural concrete finishes with staining effects and are not usually noticed until after placement


LIMITS OF DELETERIOUS SUBSTANCES FOR AGGREGATE CONCRETE

Coarse Particles
key for below:
1st #
ASTM* C33-03 Class 4S
2nd #
DOT General Use
3rd #
DOT Bridge/Superstructure

1. Clay lumps and friable
particles Max. 3.0% Max. 3.5% Max. 3.0%
2. Chert (less than 2.4 SpGr SSD) total spall
Max. 5.0% Max. 1.0% Max. 0.5%
3. Combined 1 and 2
Max. 5.0% Max. 3.5% Max. 5.0%
4. Coal and lignite
Max. 0.5% — —
5. Shale
On 1/2” sieve — Max. 0.4% Max. 0.2%
Total sample — Max. 0.7% Max. 0.5%
6. Soft iron oxide —
Max. 0.3% Max. 0.2%
Potential density of popouts per sq. yd. with maximum deleterious limits**
20-30 15-20 5-10

Fine Aggregate
Fine Particles
1st #
ASTM* C33-03 Class 4S
2nd #
DOT General Use

1. Clay lumps and friable
particles Max. 3.0% —
2. Coal and lignite
Max. 0.5% Max. 0.3%
3. Other deleterious substances (shale, mica, soft & flaky particles)
— Max. 2.5%

NOTE: The most troublesome deleterious particle in the Midwest, shale, Is not Identified in the national ASTM specification.
*S Category, severe weathering region: Areas with greater than 500 day-inches weathering index (product of the average annual number of freezing cycle days and the average annual winter rainfall in inches). Meets uniform building codes for commercial and residential construction.
**Due to unusual circumstances or weather conditions during placing, finishing, or curing, the number of popouts may vary from those predicted above.