Ch. 6 Concrete & Portland Cement Manufacturing
Portland Cement Concrete Mix design
Selection of the suitable ingredients (cement, aggregate, water and admixtures). • Determining their relative quantities ("proportioning") to produce as economically as possible, concrete of appropriate workability, strength and durability
Self Consolidating Concrete (SCC)?
Self leveling and non-segregating flowable concrete • No or minimal vibration is needed • Uses a very high dosage of Superplasticizer • Higher sand and cement contents • Relatively new in the United States
Maturity Concepts are?
based on the fact some function go the product of the curing time,t, and the concrete curing temp, T. is an equivalent age for concrete cured at some specified time.
Box test for SCC (self consolidating concrete)
box with grate, -lift the grate- -compare heigth ration h1/h2 -very fluid ratio=1
Air entraining admixtures structures and dosage?
-surface active agents or "surfactants" e.g., salts of wood resin (neutralized vinsol resin), synthetic detergents, etc. - approx. 1- 4 oz. /sack of cement for 3-5% air content 4-8 % of concrete volume is air entrained. 12 to 20% of paste volume
What is Ground Granulated Blast Furnace Slag (ggbf)
by-product of the iron industry during production of iron from ore
Two basic raw ingredience
calcareous- calcium oxide material, argillaceous- silica & alumina
Flow of cement pastes?
higher the diameter the more flowable the mortar is using a flow mold, a specific volume of material is molded on the table. The mold is removed and the table is subjected to a specific # of .5 drops using the crank handle.
Before the Design
1. Sieve analysis for coarse and fine aggregates 2. Unit weight and bulk specific gravities 3. Aggregate absorption 4. Required strength 5. Exposure conditions the concrete will be subjected
Process of fly ash
1. chucks of coal 2. pulverized coal 3. furnace or boiler 4. pulverized coal is burned and inorganic materials are melted. 5.motten ash particles are solified on the air 6. fly ash is collected 7. transported to concrete plants
Components of Concrete
1. water 2. cement 3. aggregates ( course & fine) 4. Admixtures
Types of Admixtures
Air entraining admixtures (C 260) Water reducing (Type A) Set retarding (Type B) Set accelerating (Type C) Water reducing and Retarding (Type D) Water reducing and Accelerating (Type E) Water reducing, high range (Superplasticizers Type F) Water reducing, high range and retarding (Superplasticizers Type G) Finely divided mineral admixtures Miscellaneous
what does Alkalies in Portland cement do?
Alkalies may react with some siliceous aggregate, especially in river gravel from blue republic, platte, and laramine river in kansas, nebraska, and whyoming. expressed as eqivalent alkalis = Na2O + 0.658*(K2O) 0.6% limit on Alkalis according to ASTM C150
what do they do to prevent cracking
longitudinal cuts in the concrete slab
Volume of Components that contributed to shrinkage
low or no shrinkage-aggregates high shrinkage-paste dry shrinkage-water chemical shrinkage-cementitious material Ultimate shrinkage= 1/8 to 1/4 i`nches in 20ft.
Factors affecting admixture behavior of the 1. water mix
- Chemistry - Impurities - Hardness - Water content
Factors affecting admixture behavior of the Mixing Procedure:?
- Equipment - How admixture added - When admixture added - Sequence of admixture addition - Concentration of admixture
ASTM C94 Water for mixing has to meet two criteria?
-7-day compressive strength of mortar cubes made with the questionable water should be 90% or more the strength of the portable water. -Neither setting time should be adversly affected.
What is the Gilmore Test?
the needle is lowered on the surface of the cement patty paste w/ normal consistency until it holds the needle. small weight-1.06mm-.25 weight large weight- 2.12mm-1 lb weight initial set time final set time
what does the w/c ration determine?
the porosity of the cement paste, therefore the durability.
Maturity Index is?
the relationship between the tempurtue history of the concrete and the strength. m(t)= sum(Ta-To)*t m(t)-maturity index t-interval in days or hours Ta-is the average concrete temp during each interval time To- is the datum temp (ref temp).
What is Flash setting?
too little gypsum is cement (reacts to fast) too much C3A too much surface area Gypsum conversion to plaster (hot cement) hot cement desiccates gypsum can mix through false set
Tests on Fresh Concrete
• 1. Slump • 2. Air content • 2. Unit weight • 4. Temperature of fresh concrete
Mechanical Properties of fresh and hard concrete
• 1. Strength • 2. Modulus of Elasticity • 3. Poisson's Ratio • 4. Coefficient of Thermal Expansion • 5. Shrinkage • 6. Creep Coefficient • 7. Fracture Toughness • 8. Fatigue Behavior
Hard Concrete
6% air 11% portland cement 41% gravel or crushed stone 26% sand 16% water
Key Ingredients for Hardened Concrete:
C- calcium oxide = "Lime" CaO S- Silicon Dioxide = "Silica" =SiO2 F- iron oxide = "Fe2O3 A - Aluminum Dioxide = Alumina = Al2O3 -aluminate
Active Ingredients in Portland cement? Type I/II
Lime (Calcium Oxide) Iron Oxide Silica (Silicon Dioxide) Alumina (Al. Oxide)
Water Reducing Admixtures?
Uses: - reduces mixing water for the same slump; - increases strength; and - reduces cost • Structure: - water-soluble organic compounds like Lignosulfonates, Hydroxycarboxylic acids, Carbohydrates. • Dosage: - Variable
Plasticizing admixtures (ASTM C 494 &1017)
Water-soluble polymers designed to enhance workability by increasing slump, particularly in excess of 7in.(flowing concrete, C 1018) or to reduce the water requirements for a desired workability (C 494)
Impact of slag on fresh concrete?
Workability • Pumpability / Finishability • Water Demand
Beneficial Effects of Admixtures
Workability:Water-reducing admixtures Plasticizers Air-entraining admixtures Pumping aids Anti-washout admixtures set-control:Set-accelerating admixtures Set-retarding admixtures Extended-set admixtures Water reducing admixtures strength durability:Water reducing admixtures Air-entraining admixtures Corrosion inhibators
Applications to Structural Behavior
a. Flexural cracking in reinforced concrete: Cracking initiates when concrete reaches limiting failure strain. b. Role of reinforcing steel: Reinforcing steel confines concrete and inhibits concrete expansion. c. Flexural cracking in un-reinforced concrete members: Cracking sudden with little warning, if any. Failure is sensitive to surface flaws.
Hardened concrete carbonation factors & prevention
carbonation increase with: *high water-cement ratio *low cement content *short curing spread *low strength *high permeable paste Protection: Adequate concrete covering
Terminology
cement + water = cement paste+sand=mortar mortar + coarse aggregate/rock = concrete
Stretching modes of concrete
flexure, direct tension
Whats the purpose of the Maturity index?
it to reference the strength of the concrete that is collected from data in the lab and compare with the sample data in the field.
Air entering admixtures?
To improve the resistance of concrete to freezing and thawing
Portland Cement Composition Type
Typical Kiln Output C3S 49% C2S 25% C3A 12% C4AF 8%
Tests for physical properties of concrete
shear, modulus of elasticity, flexure, tension, compression,
Basic Considerations Strength and Durability
• Concrete specifications may require: - Minimum Compressive strength - Limits on the W/C ratio - Minimum cement content • Additional requirements related to durability and freeze/thaw resistance (affect W/C and admixtures) • Need to compromise between all requirements • Durability and performance greatly affected by the mixing, transport, placing and finishing, and curing
Mineral Admixtures?
• Pozzolanic • Cementitious • Non-reactive
Proportioning when designing silica fume?
• Proportioning is an important factor to consider when designing silica fume concrete. Some things to remember are: - Control the weights of all ingredients - Monitor the moisture contents of aggregates - Control air content - Don't use chloride or chloride bearing ingredients as an accelerator - Pay attention to hot and cold weather conditions
Water Reduction Mode
• Reduce mix water only - Reduces Water/Cement • Reduce mix water and cement content • Increase workability
SET ACCELERATING ADMIXTURES?
• Uses: - to accelerate setting - to increase rate of strength gain in cold-weather concreting • Structure: - mostly calcium chloride-based or "calcium nitrite" • Dosage: - about 1-2% by wt. of cement • Potential problems: - reduced sulfate resistance, increased alkali- aggregate reaction, and corrosion
Miscellaneous Admixtures?
• ViscosityModifiers-Toraiseviscosityofwater - Pumping aids & antiwashout admixtures • Anti-freezing admixtures - To lower the freezing point of water • Expansion Producing admixtures - Gas forming - Expansive hydrates • Corrosion Inhibators • Alkali-Agregate Reaction Inhibiting Admixt. • Shrinkage-reducing Admixtures • Damp-Proofing Admixtures (hydrophobic compounds)
Basic Concepts - W/C ratio
• W/C determines the porosity of the cement paste, and therefore the durability • Duff Abrams (1918) σc = 14,000 psi / B 1.5(w/c) B depends on the cement type, typical B=4
Concerns with water reduction mode?
• Temporary effect • Retardation - Minimize dosage Operate at low end of range of product effectiveness • Stickiness, elasticity • Air bubble stability
Factors affecting admixture behavior of the? 3...
temperature
Superplasticizers?
(High range water reducers, Types F & G) • Dosage: 12 oz /sack of cement for about 30% water reduction • Uses: - Reduction of w/c ratio - Pumping concrete to increase slump or underwater concreting - Thin or densely reinforced sections (consolidation is difficult) - SCC • Structure: - long chain organic polymers or modified lignosulfates • e.g. sulfonated melamine-fomaldehyde condensate (SMF) • sulfonated nepthalene-formaldehyde condensate (SNF) • modified lignosulfonates (MLS) etc.
Other Important cement types
*White Portland cement *blended Hydraulic cement *masonary cement *Expansive cement (type k) hydration chemical reaction *Specialty cement
SET RETARDING admixtures?
- (i) increases setting time thereby offseting the effect of hot weather; - (ii) allows for unusual placement or long haul distance; and - (iii) allows time for special finishes • Structure: - organic compounds like salts of hydroxy carboxylic acids, carbohydrates, etc. • dosage: - approx. 3-6 oz./sack of cement
Properties Chemical Properties of silica fume?
- Amorphous (not a crystalline material), silicon dioxide, other trace elements • The amorphous property this enables the silica to dissolve in the concrete mix and allows for a chemical reaction • The silicon dioxide causes the silica fume to react in the concrete mix.
What are the Physical Properties of silica fume?
- Particle Size- less than 1 micron - Bulk Density- very very light - Specific Gravity- very low (2.2) - Specific Surface- large due the small particle size
Factors affecting admixture behavior of the Mix characteristics:
- Proportions - Water-cement ratio - Slump
Factors affecting admixture behavior of the 2. Aggregate characteristics:
- Type - Chemistry - Gradation
Benefits of Maturity Testing?
-Early application of prestressed -Early opening of the faculty to the traffic -early removal of forms -curing times are usually cut dramatically, expecially during winter -non-distructive & inexpensive
Admixtures?
-Materials other than water, aggregates, hydraulic cement or fiber reinforcement (ASTM C494) -Can be liquid or solid • Approx. 70% concrete in the U.S. contains one or more admixtures • Added to the batch before or during mixing
Effects of the impurities in the mix water?
-efflorescence -corrosion -volume instability -reduced durability
Hardened Concrete Manufacturing Process:
1. Extracting: limestone, shale, clay: from the quarry 2. Crushing sequence 3. Raw material storage 4. Take raw material storage 5. grinding mill w/ dust collector 6. feed it into a kiln (back material) 7. Clinker: rock combo is cooled 8.Clinker Gypsum ----> Portland Cement 9. grinding silos 10. storage silos:bulk, rail, truck, car 11. Bagging Facility
Hardened Concrete Rotary Kiln
1. Temp up to 1650 degree C/ 3000 degree F 2. Few degree inclined 3. Slowly rotating: 1-2 revs/min 4.Approx. 250 ft long 5. Feed residence time 30 min 6. Grinding w/ 1-5 % gypsum
Hydration Features?
1.Fineness: reaction occurs at the surface; so the finer the cement the higher the reaction rate 2.Moisture: continued presense of water is required, i.e. curing is neccessary for strength gain of concrete.
Soil-Cement
4 steps of soil cement: 1. spreading 2. mixing 3.compacting 4.curing
What is a fluxing agent and why is it used?
Added to lower the formation temp of C3S to about 2500 degree F 1. Aluminum oxide ="alumina" Al2O3 "A" 2. Iron oxide = Fe2O3 ="F"
Factors affecting admixture behavior?
Admixture behavior is very sensitive to many variables. • Cement Characteristics - Type - Chemistry - Fineness - Impurities
Macro-Behavior
Behavior of a cylinder in compression • By studying how and why a typical 6x12 cylinder breaks, we learn a great deal about the mechanical behavior of concrete • Cylinder failures can generally be characterized into two types (with various combinations): a. Diagonal b. Columnar
What is Carbonation and what are the effects?
CO2 from the air penetrates the concrete & reacts w/ calcium hydroxide to form calcium carbonate. Effects: fresh concrete-surface durability hardened concrete- carbonation shrinkage, corrosion of steel.
Hardened Concrete Typical Input Feed:
CaO = 63% SiO2 = 21% Al2O3 = 6% Fe2O3= 3%
Steps to shrinkage cracking?
Cement and water admixture-->> Chemical shrinkage..depending on the material Mositure loss than mositure gain causes drying. drying causes shrinkage depending on the mix
Hardened Concrete Cost
Cement approx. $76-$82 ton Ready Mix concrete: 3000 psi approx $68-yd^3
Creep Test?
Concrete compression
Mid-Range Water Reducers?
NoASTMstandard • Reduces water 6-12% • Used in production of concrete with slump between 5 - 8 inch • Used in concrete with moderate w/c ratio • Controlled setting characteristics • Dosages up to 15 fl. oz/cwt (=100 lbs of cement)
Wagner Turbidimeter?
Determines fineness of cement, using the photoelectric cell to measure light passing through suspended pulverized material. Microammeter measures current generated in the cell; indicated reading is a measure of the turbidity of the suspension.
Plastic shrinkage?
Evaportaiton->>Bleeding->>Drying->>shrinkage depending on mix shrinkage-->>stress->>cracking
Water reduction effectiveness?
Normal Range-5-8% Mid range- 8-15% High range- 5-30% (Superplasticizer)
Hardened Concrete Use:
Foundation, columns, beams, slabs, roads, cooling towers
Water-Cement Ratio
Given proper consolidation, compressive strength is inversely proportional to the water-cement ratio
Why is gypsum added?
Gypsum is added to retard the early setting & hardening if not the reaction would happen quickly
Five Major types of Cement -> Pure Portland Cement
I- Normal (general use) -Beams/columns II-Moderate Sulfate Resistance _speed up Roadways III- High early strength- it is type I but finer ground IV-Low Heat of Hydration - Large aggregate V- High Sulfate Resistance
Blended Hydraulic Cement (ASTM C595)
IS- Portland blast furnace slag cement IP- Portland pozzolan cement P- Portland pozzolan cement I(PM) -Pozzolan modified portland cement I(SM)- Slag modified portland cement S- slag cement
What is Hydration?
Is the chemical and physical process that takes place between the cement and the water resulting in the hardening of the cement.
Sulfate resistance?
Is the sulfates in the soil, air, water, or deicing salts react w/ the C3A to form expansive mineral-ettringite =C3A*3CS*H32
WHAT IS THE BLAINE FINENESS TEST?
It is when you test the air permeability of the cement sample and compare that to air permiability of the standard material.
Active ingredients in Fly ash? Type F
Lime (Calcium Oxide) Iron Oxide Silica (Silicon Dioxide) Alumina (Al. Oxide)
Hardened Concrete Raw Material & Source
Lime, iron, silica, alumina Calcareous (CaCO3 > 75%) Limestone, shalk, shells) Argillaeous (CaCO3 <40%) slate, shale, clay
Expansion of ettringite effects?
Mineral formation causing -fresh concrete pore-filled with no net expansion -in recently set concrete, expansion w/out cracking. -in hard concrete; expansion & cracking delayed ettringite formation (DEF)
What is the Vicat Test?
Needle test in cement using the 1mm needle is allowed to penetrate the paste every 15 min unit penetration reaches 25mm or less is obtained. using interpolation, the time needed for the 25mm to be reached is recorded as start time and the time the needle doesnt penetrate the paste is the final time.
Autoclaved Cellular Concrete
Production: Mortar + gas forming admixture pressure steam cured (autoclaved) block or panel form density: 19-63 ln/ft^3 compressive strength: 300 to 1500 psi
Chlorides Allowed by ACI 318 For Corrosion Protection of Reinforcing Steel
Reinforced Concrete Exposed to Chlorides 0.15% - 564 lbs. cement/CY x 0.0015 = 0.85 lbs/CY chlorides Other Reinforced Concrete, Damp Condition 0.30% - 564 lbs. cement/CY x 0.0030 = 1.69 lbs/CY chlorides
Synthetic Pozzolans material?
Silica fume (micro-silica), fly ash, ground granulated blast furnace (ggbf) slags, calcined clay (metakaolin)
Soundness?
Specially graded sand to retain: - 2% on #30 sieve - 72% on #50 sieve - 98% on #100 sieve Soundness. Ability to retain its volume after setting. ASTM C150 limits autoclave expansion to 0.8%. • Cement paste bars are subjected to heat and high pressure and the expansion is measured
Heat production benefit from fly ash?
The pozzolanic reaction releases less heat than the hydration of an equivalent mass of Portland cement.
Set-control admixtures
To control setting or to induce early hardening
Reasons for using admixtures?
To enhance concrete properties • To modify fresh concrete behavior • To compensate for cement characteristics • To make concrete economic
Compressive Strength Test
Ultimate Compressive Strength, f'c = Pult/ A • Factors affecting compressive strength: • (i)water-cementratio • (ii)cementtype • (iii)admixture • (iv)type&gradingof aggregates • (v)curingand • (vi)temperature&moisture content at the time of testing
Natural Pozzolans material?
Unaltered volcanic ash, weathered volcanic ash, pumice, diatomaceous earth, opaline cherts and shales
Shotcrete
Used for: Concrete to be placed in difficult locations, thin sections, and large areas Advantages: Placing concrete in freeform shapes, repairing structures, thin linings,etc. Watch for: Quality of work depends on skill of those using equipment
Air Entraining Admixtures?
Uses: - to purposefully entrain microscopic air bubbles (10-1000 microns w/ most @ 100 microns) in concrete - air entertainment improves freeze-thaw resistance and resists surface scaling due to deicing salts
Factors affecting flexural testing
a. Flexural tests are sensitive to "flaws" in a specimen: shrinkage cracks nicks, gouges, indentations, surface finish b. Flexural tests are sensitive to pre-existing stresses: thermal stresses shrinkage stresses c. Temperature and moisture conditioning are critical in flexural testing
What is the reaction of the water reducer admixture?
water reducer has both positive and negative charge, it attracts to the cement particle that has both charged and reduces the cement particle to be all one charge.
Structural Light-weight Concrete
• Air-dry densiy: less than 115 lb/ft3 • Compressive strength: 1000 to 5,000 psi • Properties: High Thermal and sound insulation, very absorbent, tend to segregate and float to the surface, lower creep • Rotarykilnexpandedclays,shales,andslates • Pelletizedorextrudedflyash • Expanded slags
Bond Strength of fresh and hardened concrete
• Bond components: • Chemical bonding • Mechanical interlock • Adhesion and friction • Factors affecting bond strength: • Cement chemistry • Aggregate chemistry, shape, surface texture, cleanliness • Mortar strength
Cracking in fresh or hardened concrete
• Bond micro cracks form at ITZ • Mortar micro cracks form within the mortar • Aggregate micro cracks form with in the aggregate particles • Micro cracks ultimately coalesce to form readily visible "macro" cracks
Silica Fume?
• By product of ferro-silicon industry • Approx. 200,000 tons produced annually • Used 5-10% by weight of cement • For 7.5% silica fume, 250,000 SF particles per grain of cement
What is Fly ash?
• By-product of burning finely ground coal in electricity generating power plants • Captured from the exhaust gases of the plants by electrostatic precipitators or bag houses • Approx. 70 million tons were produced in 2003 in the U.S., and only approx. 27 millions were used
Types of Early Volume Change?
• Chemical Shrinkage • Autogenous Shrinkage • Plastic Shrinkage • Drying Shrinkage
ASTM Fly Ash Classes?
• Class F - produced when either anthracite, bituminous or sub bituminous coal is burned - Low in lime (< 7%), high silica • Class C - normally comes from lignite or sub bituminous coal - High Lime (> 15%)
Effects of adding Silica Fume
• Cohesion - Silica Fume increases the cohesive properties of concrete. This enables the concrete to be placed at a higher slump without segregating. • Reduction of Bleeding - This is due to the silica fumes surface area and water adsorptive properties. This makes it more durable to the effects of deicing salts or sea water. • Helpful when finishing flat work • Useful when dealing with fast track projects • Increased Hardness - This results in an increase in the concretes compressive strength. Useful in high rise structures because the size of the concrete members can be reduced.
Splitting Tensile Strength Test
• Commonly done on cylinder or core specimens • 6 in. x 12 in. specimen size • Typical value: 360 psi to 450 psi
Creep on hardened concrete
• Concrete under sustained stress or load undergoes a gradual increase of strain or deflection with time • Causes: - crystalline flow (in the aggregate and hardened cement paste) - plastic flow of the cement paste - exudation of cement gel water - closing of internal voids • Where: - In any concrete structure with sustained load, e.g., columns in tall buildings and bridges.
Factors Affecting Strength
• Consolidation • Water-cementratio • Cementtype • Admixture • Type and grading of aggregates • Curing • Specimensizeeffect • Temperatureandmoisturecontentatthetimeof testing
Slag Composition and Hydration
• Constituent/% by Weight - CaO 35-45 - SiO2 32-38 - Al2O3 8-16 - MgO 5-15 - Fe2O3 <2% - Sulfur 1-2% • Hydration of Slag: Activator water + slag ->CSH +other phases (NO CH)
Self- Consolidating Concrete (SCC)?
• Features: - Highly flowable, nonsegregating - No mechanical consolidation • Important properties: - Flowability (uses a large dosage of Superplasticizer) - Stability (uses a high cement factor)
Temperature Control of Concrete
• Fly ash and GGBFS used as partial replacement for portland cement cause a slower setting and slower strength development • This is desirable in hot weather concreting - it helps improve the handling characteristics
Effect of Aggregate on Strength of fresh and hardened concrete
• For w/c < 0.40, aggregate strength may be similar to paste strength • For w/c > 0.40, aggregate strength does not make much difference • On average, coarse aggregates experience higher stresses than concrete • Bond failure at ITZ is common
Pozzolans are "Reactive Silica"
• Glass • React with Calcium Hydroxide • Given: heat, water, time
Basic Concepts - Aggregate grading
• Grading to produce the densest packing Fuller & Thompson (1907) Pt = (d / D) 0.5 Pt - fraction of solids finer than size d D - maximum aggregate size • Most specifications include gradation limits for aggregates depending on the application
Process Additions of admixtures?
• Ground with clinkers • Fly Ash, ggbf, limestone dust, etc.
Roller-Compacted Concrete (RCC)
• Lean • No-slump • Almost dry • Compactedinplace by vibratory roller or plate compaction equipment
Basic Considerations - Economy
• Materials + Labor + Equipment • Minimize the cement content - Use the lowest slump that will permit adequate placement - Use largest practical maximum size of aggregates - Optimum ratio of coarse to fine aggregates - Use appropriate admixtures • Benefits : reduced shrinkage and heat of hydration • Problems: early strength low, non-uniformity high
Characteristics of concrete made with mineral admixtures
• May affect rate of strength gain. • May affect water requirement for a given slump. • Lower heat of hydration (replaces C3S with C2S) • Behaves like Type II or Type IV Portland cement Increased long-term strength (at the expense of early strength) Reduced permeability May influence susceptibility to sulfate attack and AAR
Classes of Specialty Concrete
• Self- Consolidating Concrete (SCC) • Lightweight • Heavyweight • High-strength • High-workability • Expansive • Polymer-modified • Roller-compacted • Shotcrete • Fiber-reinforced • High-performance
Composition of typical fly ashes?
• Silica (SiO2)40-90% • Alumina (Al2O3) 20-60% • Iron oxide (Fe2O3) 5-25% • Lime (CaO) 1-15%
Pozzolans Differ in:
• Particle Size & Shape • Impurities • Uniformity • "Reactivity"
The Pozzolanic Reaction
• Reactive silica + lime + water → CSH • The reaction will only occur if the silica is reactive (i.e., non-crystalline or glassy) and very finely divided (high surface area))
Cementitious Materials
• Silica Fume - ASTM C1240 • Ground Granulated Blast Furnace Slag (GGBFS) - ASTM C989 - Grade 80 - Low Activity Index - Grade 100 - Moderate Activity Index - Grade 120 - High Activity Index • Fly Ash and Natural Pozzolans - ASTM C618 - Class N - Raw or calcinated natural pozzolans • diatomaceous earths, tuffs, and volcanic ashes - Class F - Fly ash with pozzolanic properties; Low calcium (<10%) - Class C - Fly ash with pozzolanic and cementitious properties; High calcium (10 - 30%)
how does silica fume affect Interfacial Transition Zone (ITZ)
• Silica fume is particularly effective in decreasing the porosity of the transition zone between paste and aggregate • Tensile strength is more influenced than the compressive strength
Basic Considerations - Workability
• The mix must be capable of being placed and compacted properly with the equipment available • For mixes without admixtures, workability depends more on characteristics of the aggregates than on those of the cement • Increase the mortar content NOT THE WATER ALONE to increase workability
Absolute Volume / ACI Method
• The most popular in the U.S. • Some other methods can be used but they give similar results • The design is only a starting point, trial batches are necessary, as well as observation of performance; adjustments must be made afterwards • Use local experience
Mortar Strength of fresh and hardened concrete
• The strength of the mortar phase is intimately related to the strength of the Portland cement itself • Variations in cement strength will lead to variations in concrete strength • Variations in the rate at which the cement gains strength will lead to variations in the rate at which concrete gains strength
Admixtures: Technical Data sheet lists?
• Themaineffectonconcrete • Additional influences - beneficial and detrimental • Physical properties of the material • Concentrationoftheactiveingredients • The presence of any potential detrimental substances (chlorides, sulfates, nitrates) • pH • Potential occupational hazards for the users • Conditions for storage and recommended shelf life • Preparation of admixtures and procedure for introducing it into the concrete • Recommended dosage under identified conditions, recommended maximum dosage and effects of overdosing
Specification of Silica fume?
• There are two specifications that have been published specifically for silica fume concrete. - ASTM C 1240 (Standard Specification for Silica Fume Used in Cementitous Mixtures) - AASHTO M 307 (Standard Specification for Use of Silica Fume as a Mineral Admixture in Hydraulic-Cement Concrete, Mortar, and Grout)
Flexural Strength (Modulus of Rupture) Test
• Third-point loading test is more common • Specimen Size (BS & ASTM): 6 " x 6" x 30" or 4" x 4" x 15" • Third-point modulus of rupture = 0.9 * center-point modulus of rupture
What are pozzolanic material?
•Pozzolans are siliceous or siliceous/aluminous materials which have little or no cementations value alone • When finely divided, pozzolans will react with lime and water at ordinary temperature to form a product similar to that produced by cement hydration