Concrete Technology
Air Entraining
- Add and stabilize microscopic air bubbles; bubbles are uniform and distributed throughout the paste, admixture or cement - Improve durability of concrete - Improve workability - made of various salts and synthetic detergents
Fibers
- Added to concrete during mixing - Take the place of conventional reinforcing system - Fibers are made from: Steel, Plastic, Glass, Natural Materials
Workability Agents
- Air entrainment is the best lubricator of harsh mixes - Can increase cement and add water to increase slump - Can use water reducers to increase slump - Can use finely divided mineral admixtures
Glass Fibers
- Alkali resistant glass fibers - Used to make exterior building facade panels - Very Versatile: Color/Shapes
Factors Affecting Air Content: Cement
- As cement increases - air decreases - Fineness - decreases air - (Type III may need a double dose of air entrainment) - High alkali will entrain more air than low alkali
Permeability-Reducing Agents
- Best way to increase the moist-curing period and reduce W/C ratio to .5 or less - Silica Fumes
Corrosion Inhibitors
- Can add calcium nitrate admixture to block corrosion - Can coat rebar with epoxy - Can use surface treatments or overlays
Factors Affecting Air Content
- Cement - Aggregate - Water & Slump - Slump & Vibration - Concrete Temperature - Admixtures - Mixing - Transportation - Pumping (all important in maintaing air)
Properties of Air Entrained
- Decreases: bleeding, scaling, strength, unit weight - Increases: freeze/thaw resistance, slump, workability
Admixture Effectiveness
- Depends on the quality of the mix and mix ingredients - Governed by ASTM specifications to control content, in turn providing consistency - Amount of admixture per cy is determined by manufacturer; or independent testing can be used.
Air Entrainment
- Developed in 1930's - Air entrained cement or admixture - Creates microscopic bubbles that cling to cement hydrostatically - FA holds them in place - Size 0.01 mm to 0.1 mm - Entrapped air is bigger than 1 mm (difference between entrained & entrapped)
Advantages of Fiber
- Distributed throughout the concrete cross-section - Can have a weight savings - Easy to add to concrete - Reduce plastic shrinkage factors
Freeze/Thaw Resistance
- Entrained air voids act as empty chambers for freeze pressure - Spacing & size of air voids are important - Measurement of air volume does not give full representation of air void system - Also increased with: good aggregate, low W/C ratio (0.45), minimum cement (564 lbs/cy), Proper finishing & curing, and compressive strength of 4000 psi or greater
Difference Between Fiber and Conventional Reinforcing (Rebar)
- Fibers are generally distributed throughout a given cross-section. -Most fibers are relatively short and closely spaced. - Do not achieve the same area of reinforcing
Factors Affecting Air Content: Admixtures
- Fly ash decreases air - Water reducers increase air - Super P, goes either way, depends on chemical makeup
Factors Affecting Air Content: Slump and Vibration
- For constant air entrainment dosage - Air increases with slump to 6-7" then decreases - Vibration decreases air
Super Plasticizers
- High-Range water reducers; make slump concrete high slump or flowing concrete (7" or greater) - Used for the following applications: - Placement of thin sections - Where rebar is tight - for tremie applications underwater - for pumped concrete - where concrete can not be vibrated
Factors Affecting Air Content: Concrete Temperature
- Hot concrete reduces air - Hot water in cold weather reduces air
Sulfate Resistance
- Improved by air entrainment; due to lower W/C ratio that may be used with Air Entrainment and have workability
Factors Affecting Air Content: Water and Slump
- Increase in water increases slump which in turn increases air content
Limits of Super P
- Increased bleeding with same W/C ratio; but less than that of an equal slump due to water addition - Drying shrinkage is greater than low slump low W/C ratio; but less than shrinkage due to high slump due to high W/C ratio
Gas Forming Agents
- Make concrete expand slightly
Damp Proofing Admixture
- Making sure water can not go through
Disadvantages of Fiber
- May not have the required embedded amount to make a difference - Efficiency factor of about .4 to .25 - Difficult to finish concrete with fibers - Only useful in thin concrete sections
Retarders
- May reduce early strength gain (< 7 day) - Most retarders act as weather reducers - Retarders may also entrain air
Why Retard?
- Off set hot weather accelerated set - Delay initial set; prevent cold joints - Delay set for special finishes; exposed aggregate
Synthetic Fibers
- Polyester, Polyethylene, Polypropylene, etc. - Used at 0.1% by volume - Reduce shrinkage - Used in ultra-thin white topping
Tests for Air Content
- Pressure Method - Volumetric - Gravimetric Method
Other Admixtures
- Pumping Aids - Grouting Admixtures - Fungicidal, Germicidal, and Insecticidal Admixtures
Air Detainers
- Rarely Used; eliminate air - Used when the mix cannot be re-proportioned or the dosage of air-entraining admixture cannot be adjusted
Admixtures are used to:
- Reduce cost of the mix - Achieve certain concrete properties more effectively - Insure quality of concrete during all phases and differing weather conditions - To overcome emergencies during concreting
Resistance to Alkali-Silica Reactivity
- Reduced expansion caused by alkali-silica reactivity
Steel Fibers
- Short discrete lengths of steel - Volume used in concrete .25% - 2% (more than 2% affect workability) - Use of steel fibers: airport pavements, bridge decks, highway pavements, precast concrete
Natural Fibers
- Sisal, bamboo, jute, wood - Typically have too much volume to change - Are better when they can be completely sealed (ie. plastic w/ wood fibers)
Powdered Admixtures
- Special storage bins - Pre-measured plastic bags
Factors Affecting Air Content: Mixing
- Type of mixer - Condition of mixing blades - Time of mixing - Mixing speed
Deicing Chemicals
- Used for snow and ice removal - Cause surface scaling in concrete without air entrainment - Low concentrations of deicers actually cause more scaling than high concentrations
Transporting and Handling: Pumps
- Used for: conveying concrete from central discharge point to formwork - Advantages: pipelines take up little space and can be readily extended. Delivers concrete in continuous stream. pump can move
Mobile Batcher Mixer
- Used for: intermittent production of concrete at job site, or small quantities - Advantages: combined materials transporter and batching and mixing system. (one-man operation)
Ready Mixed Concrete
- ASTM C94 - Truck-mixed concrete - Mixed completely in a truck mixer
Workability
Improves workability: - With more water - With more sand - Reduces segregation (too much air entrainment causes the mix to be sticky)
Flowing Concrete
- 7" or greater slump with little segregation; Usually excessive slump causes segregation, but super plasticizer limits segregation - Effect is short lived 30-60 minutes; Usually super plasticizer is added at the job site
Truck Mixed Concrete
- 70 to 100 revolutions required for mixing at a speed of 6 to 8 rpm - After mixing, drum revolves at agitating speed of 2 to 6 rpm - Discharge before exceeding 300 drum revolutions - Discharge before 1 1/2 hours
Transporting and Handling: Wheel Barrows and Georgia Buggies
- Used for: short flat hauls on types of onsite concrete construction - Advantages: versatile - ideal inside and on job sites with changing placing conditions - Disadvantages: slow and labor intensive
Transporting and Handling: Screw Spreaders
- Used for: spreading concrete over large flat areas - Advantages: concrete can be quickly spread over a wide area to a uniform depth - Watch for: they are normally used as part of a paving train. They should be used for spreading before vibration is applied
Agitator Truck
- Used for: transporting concrete for all uses. haul distances must allow discharge of concrete within 1 1/2 hours - Advantages: operate usually from central mixing plants - Description: open top and agitating arm
Accelerators
- Used to accelerate strength development not set - Can add additional bag or two of cement/cy - Can elevate temperature of mix (materials) - Can use high early cement (Type III) - Calcium Chloride - most common; Not the best side effects: - Causes increased drying/shrinkage - Can cause rebar corrosion - Can cause discoloration - Can cause scaling
Water Reducers
- Used to reduce the quantity of mixing water (and in turn cement) for a certain slump - Normal water reducers reduce water 5-10% (rare) - Mid-Range water reducers reduce water 6-12% (most common) - High-Range water reducers reduce water 12-30% (added at job site) - Usually increase strength by lowering W/C ratio - Even though less water is used, a water reducer can cause increased drying/shrinkage - Can impact bleeding depending on type used - Can retard setting - Can entrain air - If water reducers are used and no reduction in mixing water occurs, slump is much higher
Retarding Admixtures
- Used to slow the setting rate of concrete; cold water is best - Materials: Sugars, Lighin, Borax
Transporting and Handling: Buckets
- Used with: cranes, cableways, and helicopters - Advantages: Enables full versatility of cranes, cableways, and helicopters to be exploited clean discharge
Factors Affecting Air Content: Transportation
1-2% air is lost
Factors Affecting Air Content: Pumping
1-3% air is lost
What is an Admixture
Any ingredient added to the mix other than: portland cement, water, & aggregate.
Bonding Admixtures & Bonding Agents
Bonding Admixture - Added to increase bond strength between old and new concrete Bonding Agent - Applied to existing concrete surfaces immediately before new concrete is placed
Air Entraining Materials
Cement - Chemicals added at the cement mill Admixtures - Chemicals added at the batch plant (usually liquid)
Stationary/Central Mixing
Central-mixed concrete: - Mixed completely in a stationary mixer Delivered in: - A truck agitator - A truck mixer operating at agitating speed - A non agitating truck
Factors Affecting Air Content: Aggregate
Coarse - For 1 1/2" or greater - air entrainment has little effect Fine - Increasing fine aggregate increases air - Fineness < # 100 increases - air decreases
Storing and Dispensing Admixtures
Liquid Admixtures: - Stored in barrels or bulk tankers - Should not be allowed to freeze (Heated environments) - Should be properly labeled - Should not combined with other admixtures prior to dispensing
Option B - Prescription Based
Purchaser specifies mixture proportions including: - Cement content - Water content - Admixture type and content
Option C - Mixed Option
Purchaser specifies: - Compressive strength - Cement content - Admixture type and content Concrete producer selects mix proportions
Option A - Performance Based
Purchaser specifies: - Compressive strength Concrete producer selects mix proportions
Coloring Admixtures
Red - used around electrical or gas lines Yellow - marks safety curbs
Fiber Size and Shape
Variety of sizes: - Length 6 mm to 150 mm - Thickness 0.005 mm to .75 mm - Shape: Round, Flat, Crimped, Deformed
Remixing Concrete
Water may be added at job site provided: - Spec. W/C ratio is not exceeded - Measured slump less than specified - Allowable mixing and agitating time (or drum revolutions) are not exceeded - Concrete is remixed - Minimum 30 revolutions at mixing speed - Until uniformity is within limits
Stength
When air content is constant: - Strength is inversely related to W/C ratio - Higher W/C = lower strength When CA, cement, and slump are constant: - Increase air entrainment reduces fine aggregate and water required for workability; therefore reduces W/C ratio which minimizes the reduction of strength
Non-Agitating Truck
Why would you use a non-agitating equipment? - Cheap - Used for: transport concrete on short hauls over smooth roads - Advantages: cost of non-agitating equipment is lower than that of trucks of agitators or mixers and you can haul bigger quantities
Admixture are:
air entraining, water reducer, retarder
Admixtures DO NOT
relieve the importance of wall designed mix
