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Winter Weather: Optimizing the Use of Fly Ash in
Concrete
 The
use of fly ash in the concrete industry continues to grow. Over
15 million metric tons of fly ash were added to portland cement
concrete in 2006. Its use is beneficial for a number of reasons.
Since it is a by-product material, using it as a concrete ingredient
provides economic advantages and frees up landfill space. In addition,
fly ash promotes concrete workability, especially useful for pumped
concrete mixtures. Fly ash reduces concretes’ permeability
and increases its resistance to sulfate attack and alkali-silica
reaction.
Cold weather and winter conditions can be challenging if concrete
contains fly ash. Especially when fly ash is used at higher levels,
problems are frequently encountered with extended setting times
and slow strength development, leading to low early-age strengths
and delays in the rate of construction. In addition, concretes containing
fly ash are often reported to be more susceptible to surface scaling
when exposed to deicing chemicals than portland cement concrete.
It is therefore important to know how to adjust the amount of fly
ash to minimize the drawbacks, while maximizing the benefits.
Optimizing the Amount of Fly Ash in Concrete—Bayview
High-Rise Apartment
 |
Bayview High-Rise Apartment in Vancouver
(Courtesy:
Busby and Associates) |
The Bayview high-rise apartment complex was constructed in Vancouver
between 1999 and 2001 and consists of a 30-story residential tower
and approximately 3000 m2 (3500 yd2) of commercial
space. The architect for this project worked with EcoSmart (a government-industry
consortium promoting the use of high-volume fly ash concrete) to increase
the level of fly ash used in this project. The owner and contractor
were both willing to use higher volumes of fly ash provided this did
not increase the cost or require changes in construction practices
(for example, changing the construction schedule). The Table shows
the different types of concrete and levels of fly ash used for this
project. The amount of fly ash was optimized on the basis of the requirements
of the concrete specification, the construction schedule and the temperature.
For example, the amount of fly ash was limited to 20% in the slabs
on grade because they were placed in the winter. A 3-day tower cycle
schedule was called for instead of the typical 5-day cycle and, because
of stripping and finishing delays often associated with concrete with
high levels of fly ash, the contractor limited the amount of fly ash
used in the suspended slabs. The project was considered a great success.
The amount of fly ash used was increased on average by 13% over the
contractor's standard practice for this type of construction (Busby
and Associates).
| Table: Concrete Requirements and Fly Ash
Levels Used in the Bayview High-Rise Apartment |
| Element |
Min. 28-day strength,
MPa (psi) |
Fly ash content (%) |
w/cm |
| Parking slabs and slab bands |
35 (5000) |
33 |
0.40 |
| Slab on grade—interior parking |
25 (3600) |
20 |
0.50 |
| Slab on grade—exterioir |
32 (4600) |
20 |
0.45 |
| Core footing |
30 (4350) |
45 |
0.50 |
| Other footings |
25 (3600) |
45 |
0.50 |
| Sheer walls and columns
Foundation to 8th floor
8th to 12th floor
12th to 16th floor
16th floor to roof and other walls |
40 (5800)
35 (5000)
30 (4350)
25 (3600)
|
33
33
33
33
|
0.45
0.45
0.45
0.45
|
| Tower slabs |
25 (3600) |
15 to 25 |
|
| Toppings and housekeeping pads |
20 (2900) |
45 |
|
(Busby
and Associates 2004) |
Optimize Fly Ash Content for Cyclic Freezing
and Thawing and Deicer Salt Scaling
Concrete containing fly ash is generally less resistant to scaling
when subjected to freezing and thawing in the presence of deicer
salts. The lower scaling resistance of fly ash concrete is more
pronounced in lean concretes (low cementitious material content)
or concretes with high levels of cement replaced with fly ash. Thomas
(2007) summarized the following observations:
- Scaling of concrete containing fly ash increases as the water-to-cementitious
materials ratio increases
- Scaling generally increases with increasing fly ash content,
especially at high levels of replacement (~40 to 50%)
- The use of curing compounds (membranes) reduces scaling; especially
that of fly ash concrete
Fly ash concrete is more likely to provide satisfactory scaling
performance if the water-cementitious materials ratio does not exceed
0.45 and the level of fly ash does not exceed about 25%. This, of
course, assumes an adequate air-void system is present in the concrete
and that proper construction practices are adhered to.
Reference:
Thomas, M. D. A., Optimizing
the Use of Fly Ash in Concrete, IS548, Portland Cement
Association, Skokie, IL, 2007, 24 pages.
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