Download Alternatives to Conventional Concrete

CEE 395- Materials for Constructed Facilities
Civil Engineering Materials
Alternatives to Conventional Concrete
Week 6, Lecture 12
I. Lightweight Concrete
 28-day compressive strength of 17 MPa (2500 psi)
and an air-dried unit weight of less than 1850
kg/m3
 Decision to use is based on economics of structure
- tradeoff between extra cost for lightweight
concrete and reduced dead weight
 Lightweight aggregates are used.
o Highly absorptive and can make determination
of a water-cement ratio problematical
o Tend to segregate by floating to the surface.
o Therefore a minimum slump mix, with air
entraining, is used to mitigate this effect
o Nonstructural applications include Styrofoam
"aggregates" used for insulation
II. Heavyweight Concrete
 Uses:
Radiation shielding in nuclear power plants, medical
units, and atomic research test facilities
 Produced by the use of heavy aggregates
o Natural or man-made
o Examples include barite, magnetite, hematite,
geothite, illmenite, and ferrophophorus
o Specific gravity ranges from 3.4 to 6.5
 Workability can be problematic due to heavier
mass.
Use of a higher proportion of sand can help.
Problem can also be avoided by pre-placing aggregate,
then filling the voids with cement grout.
III. High-Strength Concrete
 Contains normal-weight aggregate, fc' > 6,000 psi,
up to 12,000 psi
 Produced by using low w/c ratio.
Superplasticizers is used to regain workability
 Primary use: skyscrapers
 Benefits: Reduced member size give more space
and less weight
 Compressive strength have been documented at
12,000 psi and greater
 Costs in general, is determined by the amount of
cement used in the mix
Notes:
Alternatives to Conventional Concrete
Week 6, Lecture 12
According to a local source:
 7 bag/yard3 mix costs around $66/yard3 (fc'
approximately 6,000 psi)
 Each additional bag costs $4/yard3
 Approximately 1,000 psi gained per bag added,
with addition of plasticizers (to a limit)
 Going to 8 bag mix: 6% Additional cost with 16%
gain in strength
IV. High Workability Concrete
 Accomplished with High Range Water Reducers
(Superplasticizers or HRWR's)
 Must meet or exceed ASTM 494 "Specifications
for Chemical Admixtures for Concrete"
 Benefits:
1. Rapid strength gain due to reduced W/C ratio
o Earlier stripping and reuse of formwork
o Earlier structural use of concrete
2. Improved workability
o Flows around reinforcing steel better
o Reduce "Honeycomb" in structure
o Reduce "Rub out" finishing, which is very
labor intensive
o Improves appearance
3. Also due to the reduced W/C ratio
o Shrinkage cracking reduced
o Lower permeability, which increase
durability
V. Fiber-Reinforced Concrete
Use of Reinforcing Fibers
 Secondary reinforcement of PC concrete
 Steel reinforcing fibers were predominantly used
since the 80's
o Nylon
o Polypropylene
o Other materials
Polypropylene predominant today due to it is resistant
to corrosion
 Benefits:
o Reduce cracking due to intrinsic stress
o Reduce permeability
o Greater impact, abrasion, shatter, and fatigue
resistance
o Provide support and cohesiveness on inclines
- Tested and approved by regulatory agencies
(Con’t)
Notes:
Alternatives to Conventional Concrete
Week 6, Lecture 12
VI. Shrinkage Compensating Concrete
Shrinkage can also be compensated with specific
cements:
 Type K
 Type M
 Type S
 Contain some sort of aluminate
 Contain additional calcium sulfate
These two chemicals react to form the mineral
"entringite" or
(Ca6[Al(OH)6]2*24H20[(SO4)3*1-1/2 H20).
This is expansive and creates internal compressive
stresses.
VII. Roller-Compacted Concrete (RCC)
Properties:
 Developed based on the unique requirements for
mass concrete used for dam construction
 No slump concrete
 Relaxed gradation requirements
 Water content selected for construction
considerations rather than strength
Handling and uses:
 Hauled by dump trucks, spread by bulldozers, and
compacted with vibration compactors
 Dams and large paved areas
Advantages:
1. Economical (low cement content)
2. Minimal form work (layer construction method)
3. Reduced need for external cooling of the structure
(low cement factor limits the heat of hydration)
4. Lower placement costs (high capacity equipment
and rapid placement rates)
5. Shorter construction period
6. Material costs are 1/3 that of conventional
concrete
VIII. High-Performance Concrete (HPC)
Properties:
 Ease of placement and compaction
 Long-term mechanical properties
 Early-age strength
 Toughness
 Volume stability
 Extended life in severe environments
(Con’t)
Notes:
Alternatives to Conventional Concrete
Week 6, Lecture 12
Accomplished by:
 Altering aggregate gradation
 Admixtures
 Improving mixing and placement practices
As more knowledge is gained, HPC will be used more
frequently.
Conclusion
The engineer must be aware of all of the types of
concrete available. A project may have specific
requirements where the use of a special mix may reduce
costs considerably. Full knowledge of any mix must be
attained before implementation.
ACI Manual of Concrete excellent source for complete
information on all these alternate concrete types.
(Con’t)
Notes: