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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: