Water and Admixtures
Potable and non-potable waters are both suitable for mixing concrete. However, impurities in the mixing water can affect concrete set time, strength, and long-term durability. Chloride ions, for instance, can accelerate corrosion of reinforcing steel.
Acceptable Criteria
(Specified in ASTM C94)
After 7 days, the compressive strength of mortar cubes made with the questionable water should not be less than 90% of the strength of cubes made with potable or distilled water.
The set time of cement paste made with the questionable water should (by the Vicat Test) not be 1 hour less than 1-1/2 hours more than the set time of paste made with potable or distilled water.
Efflorescence: White stains forming on the concrete surface due to the formation of calcium carbonate. Caused by impurities in mixing water.
Disposal and Reuse of Concrete Wash Water
Wastewater is generally generated from truck wash systems, washing of central mixing plant, storm water runoff from the ready-mix plant yard, etc. Wastewater is considered a hazardous substance by the Water Quality Act (it contains caustic soda and potash). Also, the high pH makes concrete wash water hazardous under the EPA’s definition of corrosivity.
Admixtures for Concrete
Admixtures: ingredients other than portland cement, water and aggregates that may be added to concrete to impart a specific quality to either the plastic mix or the hardened concrete.
Classified by the following chemical and functional physical characteristics:
1. Air Entrainers
2. Water Reducers
3. Retarders
4. Hydration Controller Admixtures
5. Accelerators
6. Supplementary Cementitious Admixtures
7. Specialty Admixtures
Four Major Reasons for Using Admixtures:
1. Reduce the cost of concrete construction
2. Achieve certain properties in concrete more effectively than by other means
3. Ensure quality of concrete during the stages of mixing, transporting, placing and curing in adverse weather conditions
4. Overcome certain emergencies during concrete operations
Air Entrainers: Produce tiny air bubbles in the hardened concrete to provide space for water to expand upon freezing.
As moisture within the concrete pore structure freezes, 3 mechanisms contribute to the development of internal stresses:
1. Critical saturation – upon freezing, water expands in volume by 9%.
2. Hydraulic pressure – freezing water draws unfrozen water to it.
3. Osmotic pressure – Water moves from the gel to capillaries to satisfy thermodynamic equilibrium and to equalize alkali concentrations. Voids permit water to flow from the interlayer hydration space and capillaries into the air voids.
In addition to improving durability, air entrainment provides other important benefits to both freshly mixed and hardened concrete, including resistance to freeze-thaw cycles, deicers and salts, sulfates, and alkali-silica reactivity.
Air-entraining admixtures are available from several manufacturers and can be composed of a variety of materials, such as:
1. Salts of wood resins
2. Synthetic detergents
3. Salts of sulfonated lignin
4. Salts of petroleum acids
5. Salts of proteinaceous material
6. Fatty and resinous acids
7. Alkylbenzene sulfonates
8. Salts of sulfonated hydrocarbons
Air-Entrainers are usually liquid and should meet the specifications of ASTM C260. The agents enhance air entrainment by lowering the surface tension of the mixing water.
Water Reducers: increase the mobility of the cement particles in the plastic mix, allowing workability to be achieved at lower water contents.
Workability of fresh or plastic concrete requires more water than is needed for hydration. This is why water reducers are typically needed.
Water Reducers Mechanism: Cement grains develop small electric charges on their surface as a result of the cement-grinding process. Dissimilar charges attract, causing the grains to cluster or “flocculate” which in turn limits the workability. The chemicals in the water-reducing admixtures reduce the static attraction among cement particles.
Water reducing admixtures are used indirectly to gain strength, increase the slump of concrete, which in turn increases workability, while reducing the W-C materials ratio.
Superplasticizers: High-range water reducers that either greatly increase the flow of the fresh concrete or reduce the amount of water required for a given consistency. Used for the following cases:
1. Where a low W-C materials ratio is beneficial
2. When placing thin sections
3. When placing concrete around tightly spaced reinforcing steel
4. When placing cement underwater
5. When placing concrete by pumping
6. Where consolidating the concrete is difficult
Retarders: Used where conditions require that time between mixing and placing or finishing the concrete by increased. Also used for the following reasons:
1. Offsetting the effect of hot weather
2. Allowing the unusual placement of long haul distances
3. Providing time for special finishes (exposed aggregate)
Retarders can reduce the strength of concrete at early ages (one to three days). They can also entrain air and improve workability.
Hydration-Control Admixtures: Have the ability to stop and reactivate the hydration process of concrete. Consist of two parts: a stabilizer and an activator. Very useful admixtures in extending the use of ready-mixed concrete if the work at the job-site has been stalled for various reasons.
Accelerators: Used to develop early strength of concrete at a faster rate than that developed in normal concrete. Also used to:
1. Reduce the amount of time before finishing operations begin
2. Reduce curing time
3. Increase rate of strength gain
4. Plug leaks under hydraulic pressure efficiently
The first three reasons are particularly applicable for concrete work placed during cold temperatures.
Calcium chloride, CaCl2, is the most widely used accelerator. It is used under the following conditions:
1. Concrete is prestressed
2. Concrete contains embedded aluminum such as conduits, particularly if the aluminum is in contact with steel
3. Concrete is subjected to alkali-aggregate reaction
4. Concrete is in contact with water or soils containing sulfates
5. Concrete is placed during hot weather
6. Mass applications of concrete
Supplementary Cementitious Admixtures (byproducts of other industries used for admixtures):
1. Fly Ash
2. Ground Granulated Blast Furnace Slag
3. Silica Fume
4. Natural Pozzolans
Fly Ash: Most commonly used pozzolans in civil engineering. By-product of the coal industry. Combusting pulverized coal in an electric power plant burns off the carbon and most volatile materials. Carbon contents of common coals range from 70% to 100%, while the non-carbon percentages are impurities (clay, feldspar, quartz, shale), which fuse as they pass through the combustion chamber. This fused material is fly ash.
Fly Ash Composition:
1. Silica (SiO2)
2. Alumina (AlO2)
3. Iron Oxide (Fe2O3)
4. Lime (CaO)
Fly Ash Classification:
1. Class N – Raw or calcined natural pozzolans.
2. Class F – Fly ash with pozzolans properties (less than 5% CaO, typically).
3. Class C – Fly ash with pozzolans and cementitious properties (15%-30% CaO).
The spherical shape of fly ash increases its workability with fresh concrete. Fly ash also extends the hydration process, allowing greater strength development and reduced porosity. Studies typically show 20% fly ash by weight is recommended for a smaller pore size distribution. A lower heat of hydration reduces early strength of the concrete, but gains strength through the extended reaction – more than what would be attained by plain portland cement.
Ground Granulated Blast Furnace Slag: GGBF slag is made from iron blast furnace slag. It is nonmetallic hydraulic cement consisting basically of silicates and aluminosilicates of calcium, which develop into a molten condition with iron in a blast furnace. It is then chilled by quenching it in water, which forms a glassy sandlike granulated material. This material is then ground to less than 45 microns, which produces the slag.
Used as a cementitious material in concrete since the beginning of the 1900s. Commonly constitutes between 30% and 45% of the cementing material in the mix.
Silica Fume: A byproduct of the production of silicon metal or ferrosilicon alloys. Used beneficially as a mineral admixture in concrete. A very reactive pozzolan. Concrete containing silica fume has high strength and is very durable. It also reduces concrete corrosion induced by deicing or marine salts.
Silica Fume Composition: Silicon metal and alloys are produced in electric furnaces. The raw materials are quartz, coal and woodchips. The smoke that results from furnace operation is collected and sold as silica fume. The composition contains primarily noncrystalline silicon dioxide.
Natural Pozzolans: A pozzolan is a siliceous and aluminous material that, although it possesses little or no cementitious value, will (in the presence of moisture) react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties.
Some natural pozzolans include fine volcanic ash combined with burnt lime, which was used 2000 years ago for building construction. Up to 15% of the weight of portland cement is hydrated lime. Adding a pozzolan to portland cement generates an opportunity to convert this free lime to a cementitious material.
Specialty Admixtures: Other admixtures to be aware of include:
1. Workability agents
2. Corrosion inhibitors
3. Damp-proofing agents
4. Permeability-reducing agents
5. Fungicidal, germicidal and insecticidal admixtures
6. Pumping aids
7. Bonding agents
8. Grouting agents
9. Gas-forming agents
10. Coloring agents
11. Shrinkage reducing
No comments:
Post a Comment