Particulate Composites PDF
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Summary
This document provides a summary of particulate composites, including examples such as concrete and asphalt. Models for calculating elastic modulus are discussed, specifically the parallel, series, and Hirsch models. Various aspects of aggregates are also described, such as classification and physical properties.
Full Transcript
Particulate Composites Examples of particulate composite materials: - Portland cement concrete (concrete) - Asphalt concrete (asphalt) - Aerospace materials (TiC particles in aluminum matrix Cement Paste = Cement + Water - Low E - Not durable -...
Particulate Composites Examples of particulate composite materials: - Portland cement concrete (concrete) - Asphalt concrete (asphalt) - Aerospace materials (TiC particles in aluminum matrix Cement Paste = Cement + Water - Low E - Not durable - Expensive Mortar - Cement + Fine Aggregate + water - Properties imbetween paste and concrete - Ex. stucco Concrete - Cement + Fine Aggregate + coarse + water - Rigid - Durable - Inexpensive - Dimensionally more stable then paste = elastic modulus E V = volume P = particulate phase (aggregate filler etc. ) M = matrix phase (binder, cement paste, asphalt…) Models for elastic modulus of matrices 1. Parallel Model - Ecomp = EpVp + EmVm - Known as law of mixtures - Equal strain in both components - Different stress in each component - Assumes perfect bond between phases - Similar to soft particles in a hard matrix - ‘Upper bound’ solution 2. Series model - Both components have same stress - Components have unequal strains - Assumes no bond between layes - Applies best to hard particles in a soft matrix - ‘Lower bound’ 3. Hirsch model - Combination of parallel and series models - “X” is ameasure of the contribution of each model to the actual behaviour - can also be interpreted as the degree of bonding between the two X components and is usually 0.5 for portland cement Both series and hirsch models predict Ec = 0 for porous materials since Ep = 0 Parallel over predicts Ec for porous materials: Ec = (1-Vp)^n where n~3 4. Counto: 5. Aggregate Model: odular Ratio M m = Ep/Em - Where Ep is elastic modulus of particulate phase and Em is elastic modulus of matrix phase Failure in Particulate Composites - Depends on nature of composite - Concrete: crack develops and propagates through matrix. Can be stopped by particle in its path - Thus concrete can have many cracks before it actually fails - Influencing parameters: - Particulate content - Elastic modulus mismatch between particle and matrix - Interfacial zone around particle - Bond between particle and matrix - Strength of matrix and particulate ee Charts on Slides 22-26 of Particulate Composites S ggregates A Granular, inert, inorganic materials that can be classed as either natural or synthetic Natural: - Sand, gravel, crushed rock - No change in composition during production Synthetic: - Industrial by-products - Manufactured aggregates with special properties - Recycled construction materials 1. Classification a. By origin i. Igneous ii. Sedimentary iii. Metamorphic b. By specific gravity (ratio of density of substance to water) i. Heavy Weight: S.G. > 3.0 - Used in radiation shielding ii. Normal weight, 2.6-3.0 iii. Light Weight < 2.3 - Produces structural lightweight concrete - Used in lightweight insulating concrete c. By particle size i. Coarse aggregate: Retained on No.4 Sieve, 4.75mm 1. Gravel and crushed stone 2. For concrete, Typically 9.5 to 25 mm, sometimes 37.5 ii. Fine aggregate (sand): Passing No.4 sieve, 4.75 mm 1. CANADA USES 5 mm SIEVE 2. Fine aggregate is usually 35 to 45% of total aggregate in concrete 3. Ps = Sand Content = MFA/(MFA = MCA) 2. Physical Properties a. Porosity: i. Vol of pores inside aggregate / Total Vol of aggregate ii. Low porosity is generally desirable iii. As Porosity increases 1. Bulk density decreases 2. E decreases 3. Strength decreases 4. Abrasion resistance decreases . Durability decreases 5 b. Voids content i. Total Vol of voids in sample / Total Vol of sample including voids c. Unit Weight / Bulk Density / Unit Mass i. Mass of aggregate (compacted) / Vol of aggregate + voids (compacted) 3. Interactions with Water a. Moisture states: i. Oven dry - heated to 105 C ii. Air Dry iii. Saturated Surface Dry iv. Wet v. “AGG” Natural or in-situ condition in stockpile b. Absorption Capacity % i. (WSSD - WOD)/WOD c. Effective Absorption % i. (WSSD-WAD)/WSSD d. Surface Moisture % i. (WWET - WSSD) /WSSD e. Moisture Content % i. (WAGG - WOD) / WOD Where W is weight Wet or Dry Aggregate: - If moisture content > absorption capacity - Aggregate is wet - Moisture content < absorption capacity - Aggregate is dry and will absorb more Bulking of Sand - Menisci formed between particles which results in repulsive forces - Can increase bulk volume of snad - Dry and fully sat. sand will occupy less vol. Than sand at 10% moisture content - Therefore, difficult to vol. Batch sand accurately - Gb = Bulk Specific Gravity (OD) = WOD / (WSSD - WSW) - GbSSD = Bulk Specific Grabity (SSD) = WSSD / (WSSD - WSW) - Ga = Apparent Specific Gravity = WOD / (WOD - WSW) - Wsw = weight of a saturated sample in water - Gb < GbSSD < Ga 4. Geometric Properties a. Particle Size and Grading: i. MSA: maximum size aggregate is smallest sieve through which entire aggregate sample passes ii. If all particles pass through x size, then material is called x size b. Grading Specifications i. Common method: 1. M ax and min cumulative percentages of material passing each sieve (give them a range basically) ii. Performance Method: 1. Very broad max/min range initially, but once gradation is proposed, supplier must produce within very tight allowable deviation iii. Max vs Nominal Max: 1. Max Size: all pass through 2. Nominal Max Size: First Sieve size that catches some c. Types of Grading: i. Continuous: contains all size fractions between max and min sizes ii. Open: Only one size fraction is present iii. Gap: Missing one or more sizes between max and min sizes d. Reduction of Voids: i. Ideally minimise void spaces between aggregate particles ii. Must be modified in practice iii. Modify amount of sand so that a workable concrete is made iv. If concrete is pumped -> increase sand content more e. Particle shape and surface texture: i. Shape: 1. Angularity 2. Sphericity ii. Texture: 1. Smooth 2. Rough - Round, smooth = better workability - Rough, angular = better cement-aggregate bond, more aggregate interlock 5. Mechanical Properties a. Strength and Toughness b. Bond between paste and aggregate i. Influences concrete strength c. Modulus of elasticity i. Low modulus may be better for durability since hygral or thermal vol. Changes lead to lower stress when aggregate is more compressible. ii. Low modulus may be better for seismic conditions to increase ductility and energy absorption iii. Higher modulus may be required for tuning of machinery foundations to required frequency d. Surface Chemistry i. Hydrophobic 1. Positive surface charge therefore more easily wetted by asphalt cement than by water ii. Hydrophilic 1. More easily wetted by water (negative surface charge) . Durability 6 a. Unstable vol. Changes b. Freeze-thaw deterioration c. M echanical degradation or breakdown d. Chemical degradation e. Deleterious (harmful) substances i. Absorbent particles - shale, chert, flint, weathered rock ii. Clay lumps, friable particles - easily broken up iii. Coal or wood particles - weak iv. Organic impurities - contaminants from soil etc v. Flat or elongated particles - difficult to compact and place f. Harmful materials: i. Organic impurities - affects setting and strength ii. Materials finer than 75 um - affects bond, increases water requirement (more SA) iii. Clay coatings - affects bond and thus strength iv. Coal, lignite, or other lightweight mat’ls - affects durability, can cause stains and popouts v. Soft particles - affects durability and strength vi. Clay lumps and friable particles - affects workability and durability, may cause popouts vii. Chert of less than 2.40 relative density - affects durability, popouts viii. Alkali-reactive aggregates - causes abnormal expansion, map cracking, popouts ix. Chlorides and sulphates - causes rebar corrosion, efflorescence, more rapid set g. Alkali-silica reaction (ASR) i. Caused by reaction of silica in aggregates with alkalis in cement (Na and K) ii. Controlling ASR: 1. Non-reactive aggregates 2. Sup. cementing materials 3. Minimising water exposure 4. Limit alkali loading -> low alkali cement Cement 1. Cement Production a. Limestone Quarrying (CaCO3) b. Crushing and Grinding (SiO2) c. Dry Process -> preheating or Wet process d. Rotary Kiln (produces CO2) i. Dehydration ii. Calcination iii. Clinkering iv. Cooling e. Cooling f. Clinker Storage g. Cement Grinding (Add Gypsum) h. Cement silos dispatching 2. 1 ton cement generates approx 1 ton CO2 3. Cement materials: Calcium, Iron, Silica, Alumina, Sulfate 4. Role of Gypsum: a. Gypsum = Ca(SO)4 H20 b. ~2% added c. Interground with clinker to avoid flash set d. Flash set is early reaction of C3A causing premature stiff/loss of workability 5. Hydration of Cement a. Chemical reaction of cement and water to become bonding agent b. Not a drying out process c. Solid mat’ls will not dissolve under water d. Chemistry of hydration i. Oxides combine to form FOUR primary cementing mat’ls 1. C2S -> Di-Calcium Silicate 2. C3S -> Tri-Calcium Silicate 3. C3A -> Tri-Calcium Aluminate 4. C4AF -> Tetra-calcium Alumnino-ferrite ii. In cement notation: 1. C = CaO 2. S = SiO2 3. F = Fe2O3 4. A = Al2O3 6. Hydration of C3S and C2S a. CSH - Calcium Silicate Hydrate C3S > C4AF > C2S b. Fineness i. How fineness controls the rate of reaction: ii. The finer, the faster reaction and thus rate of strength gain (SA) c. Heat Evolved i. How much heat is evolved in the process 11.Types of Portland Cement: a. Type 10: Normal Portland b. Type 20: Moderate Portland: moderate sulphate resistance / heat of hydration i. Reduced C3A c. Type 30: High early strength i. Reduced C2S, Increased C3S, increased fineness d. Type 40: Low-heat i. Reduced C3S, Increased C2S e. Type 50: Sulphate resistant i. Further reduced C3A 12.Blended Hydraulic Cements: a. A mixture of portland cement and one of: i. Fly ash ii. Slag iii. Silica Fume iv. Ternary Blend v. Calcined Clay b. Can be interground or blended after grinding Supplementary Cementing Materials - Material added with cement which contributes to hardened concrete properties through cementitious (hydraulic) or pozzolanic activity - Commonly used for economy - 85% of concrete in Canada has SCM in it 1. What are Pozzolans? iliceous or alumino-siliceous material that, in finely divided form, and in presence of S moisture, chemically reacts with calcium hydroxide released by the hydration of portland cement to form compounds possessing cementing properties. E.g. fly ash, volcanic ash, silica fume 2. How do they work? a. Portland Cement + water -> CSH and Ca(OH)2 b. Ca(OH)2 + Amorphous SiO2 in Pozzolan -> CSH
