CM 323: QUIZ 1
Hydration Recations
2C3S + 6H -> C-S-H + 3CH cement + water -> calcium-silicate-hydrate (glue) + calcium hydroxide 2C2S + 4H -> C-S-H + CH C3A + 3CSH2 + 26H -> C6A/S3H32 (300 cal/g) --> heat gos up when adding aluminum. The more heat, the increased linelyhood of cracks Tricalcium Aluminate + thing + water -> ettringite 2C3A + C6AS3H32 + 4H -> 3C4A/SH12 Tricalcium Aluminate + ettringite + water-> monosulfate C4AF + 10H + 2CH -> C6AFH12 Tetracacium aluminate ferrite + water + calcium hydroxide
coarse aggregate (CA)
4.75 mm to 50 mm (No. 4 sieve) - less cracking
Concrete proportions
6% air 11% portland cement 41% gravel or crushed stone (coarse aggregate) 26% sand (fine aggregate) 16% water
aggregate in portland cement
70%
Fine aggregate (FA)
<4.75mm (No. 200 sieve)
Reactive aggregates
alkali-aggregate reaction - alkali hydroxides react w reactive silicates to form alkali silica gels - absorb water from surroundings through osmosis leads to internal stresses in hardened concrete until its tensile strength is reached and it cracks - if you must have reactive aggregates, alkali content of pc must be limited to < 0.6% ---- or admixture: finely ground pozzolanic materials, reduces water access as much as possible ASR: cancer of concrete Water comes in, paste expands, puts concrete in tension until cracking
oven-dry condition
all free moisture, whether external surface moisture or internal moisture, driven off by heat - will absorb water from the surrounding cement paste
Puget sound
best aggregate in the United States
asphalt concrete
binding agent: tar, not cement
Raw materials in portland cement
calcium: limestone, chalk, etc. Silica: clays and shales 2/3 Calcium + 1/3 Clay --> raw mix should be well homogenized before heat treatment
cement + water
cement paste
hydration
chemical reaction, sets concrete, which is initially plastic. Produces strength and stiffness.
ASTM C294
classifies aggregates - silica minerals (quartz, opal, chalcedony, tridymite, cristobalite) - feldspars - micaceous minerals - carbonite mienrals - sulphides - ferromagnesian minerals - zeolites - tron oxides - clay minerals
mortar + coarse aggregate (CA)
concrete
capillary porosity
created by the originally-water-filled space within the cement paste - act as stress concentrations - reduce strength significantly - decrease in porosity = increased strength - controlled by wc ratio -- high wc ratio: low density paste -- low wc ratio: high density paste
Reclaimed aggregate
crushed portland cement, clay bricks, etc. - can be used for road bases, railroad ballasts - Problem: lots of noise to make, expensive to separate from rebar - Some municipalities demand this for environmental reasons
Surface texture
degree to which the aggregate surface is smooth or rough
apparent specific gravity
density of the material, including internal pores
Aggregate grading
distribution of particles of granular materials among various sizes - depends on proportions of coarse and fine aggregates - greater diversity reduces number of voids - well-graded is better
bulk density
dry-rodded unit weight - weight of aggregate that would fill a unit volume
Porosity and Density
dry-rodded unit weight affects mix design, workability and unit weight
C-A-S-H
ettringite (H32): long, well crystalized needles monosulfate (H12-18): hexagonal, small crystals - 15-20% created during hydration
Igneous rocks
formed on cooling of the magma - granite, basalt: hard, tough, strong - excellent aggregates (road base, railroads)
Bulk Specific Gravity
how many times heavier than water
portland cement
hydraulic cement capable of setting, hardening and remain stable under water. COMPOSITION: calcium silicates, some amount of gypsum
costs associated with aggregates
- economical to put as much aggregate into concrete mix as possible-- cement is very expensive
C-S-H
- glue - poorly crystalline - high surface area: higher bonding energy - 50-60% created during hydration
Normal Weight Crushed stone aggregate
- gravel - sands - normal crushed stone - bulk specific gravity: 2.4-2.9 - bulk density (of bulk unit weight): 1520-1680 kg/m3 (95-105 pcf) - most commonly used
soundness
- aggregate is considered unsound when volume changes in the aggregate induced by weather, such as alternate cycles of wetting and drying or freezing and thawing, result in concrete deterioration depends on: porosity, flaws and contaminants pumics (10% absorption) - no problem w freezing/thawing limestone- breaks: use smaller aggregates than critical size
benefits of aggregate
- confers greater volume stability, better durability than cement paste alone - influences dimensional stability, elastic modulus, durability, workability and cost
fineness modulus
- index of fineness of an aggregate - computed by adding the cumulative percentages of aggregate retained in each of the specified series of sieves and dividing the sum by 100 - the higher the fineness modulus, the coarser the aggregate - the weighted average size of a sieve on which the material is retained, keeping in mind that sieves are counted from the finest (from bottom up) e.g. FM 4.0 = the fourth sieve. US series = No. 16. This would be the average size - different gradings may have the same fineness modulus, as FM only characterizes average size of the aggregate
Transition Zone
- large crystals of ettringite and CH with preferred orientation - porous structure - imperfect zone, fixed w admixtures - higher water content
recycled aggregates
- made from municipal wastes and recycled concrete from demolished buildings and pagements - problems: cost of crushing, grading, dust control, and separation of undesirable constituents Washington doesn't require rebar in pavement --> easier here to reclaim
light weight crushed stone aggregate
- manufactured or natural - bulk density less than 1120kg/m3 (70pcf) - used in lightweight concrete - must be screened to get the desired size distribution - some must be crushed
Sands
- naturally occurring, water or wind born pieces of rock in buried or current stream beds or dunes - often rounded with smooth surfaces, other properties dependent on parent rock - may be washed to remove undesirable material - may be screened to divide into desired size groupings
Gravel
- naturally occurring, water-born pieces of rock, in buried or current stream beds - normally rounded with smooth surfaces, other properties dependent on parent rock - crushed gravel is larger gravel particles that have been reduced in size by a crusher - may be washed to remove undesirable material - may be screened to divide into desired size groupings
Moisture conditions of aggregates
- oven-dry condition - air dry - saturated-surface dry condition - damp/wet condition
Aggregate characteristics affecting concrete behavior
- porosity controls density and moisture absorption, soundness - absorption and surface moisture affects mix-design, strength, abrasion resistance - aggregate size affects water demand, cement content, microcracking (strength) - aggregate grading affects paste content (cost economy), workability - shape and surface texture affects workability, paste demand, initial strength
PC Manufacturing Proecss
- raw mill feed - kiln line Raw (limestone + clay) -> rotary kiln (T=1400C) -> Clinker + Gypsum Grind clinker + gypsum = portland cement
specific gravity
- relative density - ratio of aggregate weight to the weight of an equal volume of water - most natural aggregates have specific gravities bw 2.4-3.0
Crushed stone/manufactured mineral aggregate
- rock layers quarried and processed through a crushing and screen plant to reduce to desired size and divided into desired size groupings--> must have variety of sizes - limestone + dolomites (70% of mix, hard to soft) - granite (15%, hard) - sandstone (2%, soft
Shape and Surface Texture
- rough-textured and elongated particles require more cement paste to produce workable concrete mixtures, thus increasing the cost - shapes: round, angular, elongated, flaky/flat - depends on: rock hardness, grain size, porosity, previous exposure
Granular materials
- sand and gravel - crushed stone - iron ore blast furnace + other slags - manufactured (lightweight and heavyweight) - reclaimed (crushed portland cement concrete, clay bricks, etc.)
deleterious substances in aggregates
- substances causing chemical reaction - substances which undergo disruptive expansion - clay and other surface coatings - aggregate particles with flat or elongated shape - structurally soft/weak particles reactive aggregates lose strength Ideal: having the same properties in aggregate as you have in cement paste -> make harmonious material
Free Moisture and Absorption of Aggregates
- the moisture content and absorption of aggregates are important in calculating the proportions of concrete mixes since any excess water in the aggregates will be incorporated in the cement paste and give it a higher wc ratio than expected - all moisture conditions are expressed in terms of oven dry unit weight
Influence of aggregate on concrete strength
- the rougher the surface of the aggregate, the greater the greater the area in contact with cement paste, the stronger a concrete will be - rounded particles result in lower strength than crushed aggregates - larger size aggregates lead to lower strength in concrete - in lean mixes, larger aggregates give the best value of strength - in rich mixes, smaller aggregates result in higher strength
Synthetic Aggregates
- thermally processed materials, i.e. expanded clays and shale-- manufactured through thermal processes - aggregates made from industrial by-products, i.e. blast-furnace slag & fly ash
heavyweight concrete
1 cy = 200 lbs - good for nuclear shielding, bank walls
Types of Portland Cement
1: normal 2: sulfate resistance, less heat during hydration (place underwater) 3: high early strength,limit source of heat, C3A, to max 15% (emergency repairs) 4: low heat hydration (dams) 5: high sulfate resistance, max 5% C3A (sewage treatment tanks)
classifications of natural aggregates
1. natural mineral aggregates 2. synthetic aggregates 3. recycled aggregates
lightweight aggregate concrete
1/3 lighter - 1 cy of concrete = 100lbs - good for high-rises, less deep foundations required
Concrete: Compressive to Tensile Strength
10:1
1 cy of concrete
150lbs
limestone and dolomites as crushed rock coarse aggregate
very good, but not very strong. Makes a strong transition zone. As long as it's used smaller than critical size. If at critical size--> will be unsound- breaks in freeze/thaw cycles
Alumina
A, AI2O3
dry-rodded unit weight
weight required to fill a contained or specified unit volume, after it has been rodded to attain maximum packing
Angular
well defined edges and corners - crushed rock, best w cement--> develops the best bond
Flaky/flat
when thickness is small relative to two other dimensions - bad for concrete--> water accumulates underneath, weak
ASTM
American Society for Testing and Materials
ASTM related to aggregates
C 125, D 8
Calcium Oxide
C, CaO
Compounds of Portland Cement
C3S = tricalcium silicate C2S = dicalcium silicate C3A = tricalcium aluminate C4AF = tetracalcium aluminate ferrite
Products of hydration reaction
C3S2H3 = C-S-H gel CH = calcium hydroxide C6A/S3H32 = ettringite C4ASH12 = monosulfate
Reactive compounds in portland cement
C3S: tricalcium silicate C2S: dicalcium silicate C3A: tricalcium aluminate C/SH2: gypsum C4AF: tetra-calcium alumino ferrite
CH
Calcium Hydroxide - large hexagonal crystals - low surface area - poor bonding energy - 20-25% created during hydration
Water
H, H20
Microstructure of concrete
Hydrated Cement paste: - products: C-S-H, ettriginite, monsulfate - porosity: gel, capillary pores, entrained/entrapped air voids Transition Zone bw aggregate and cement paste (TZ)
Sedimentary Rocks
Stratified rocks - cost effective: near the surface - about 80% of aggregates - natural sand and gravel - limestone = excellent - sandstone = poor
Sulfate
S (with a line on top), SO3
Silica
S, SiO2
Chemical Composition and PC strength
Tricalcium silicate and dicalcium silicate impact strength the most-- tricalcium silicate more so in the first year, and dicalcium silicate more so in the following year
Elongated
When length is considerably larger than the other two dimensions
hydraulic cement
stable under water harden by reaction with water and form a water-resistant product e.g. portland cement
entrapped air
accidental voids. Poor consolidation
aggregate in base materials for roads
aggregate base --> doesn't compress, very strong
dam or wet condition
aggregate containing moisture in excess of the SSD condition - the Free Water, which will become part of the mixing water, is in excess of the SSD condition of the aggregate - too wet, leaks moisture - will give up water to the surrounding cement paste
Aggregate size and strength
the smaller the aggregate, the stronger the concrete
Saturated-Surface dry condition
ideal moisture condition -> you know exactly how much water there is: - enough for hydration - enough so no additional water absorbed - not too much, so it doesn't leak and get wet on the outside moisture states are such that during mixing they will neither absorb any of the mixing water added; nor will they contribute any of their contained water to the mix. Aggregates in the SSD condition may posses "bound water" (water held by physical-chemical bonds at the surface) on their surfaces since this water cannot be easily removed from the aggregate - equilibrium: will not give up or absorb water from surrounding cement paste
metamorphic rock
igneous or sedimentary rocks that have changed their original texture, crystal structure, or mineralogy composition due to physical and chemical conditions below the earth's surface - marble, schist, slate - Marble = excellent - Slate = poor
entrained air
intentional voids. Allows for expansion/contraction in temperature variances
Round
loosing edges and corners - riverbed - doesn't bond well w cement paste
Hydrated Cement Paste
lots of C-S-H + CH + erringite
MSA
maximum size of aggregate MSA must be <1/5 of the narrowest dimension of the form in which concrete is to be placed MSA must be < 3/4 of the maximum clear distance between the rebar (so it can correctly consolidate) MSA must be <1/3 thickness of unreinforced slabs
cement paste + fine aggregate (FA)
mortar
air dry
no surface moisture, but some internal moisture remains - aggregate is dry, not enough water for hydration - will absorb water from the surrounding cement paste
cement
powder pulverized material that develops binding forces due to a reaction with water
impact crusher
produces finer crushed product than cone crushers
nonhydraulic cement
products of hydration are not resistant to water (i.e. limestone)
modulus of elasticity
ratio of stress over strain, strain curve - concrete is non-linear bc it's a composite material
fineness
reactivity of cement with water - the finer a cement, the more rapidly it will react, and the strength development will be enhanced (expensive)
gypsum
regulates heat during hydration
aggregate
rock-like material - prevents shrinkage (cement paste is what shrinks)
natural mineral aggregates
sand + gravel - bulk density 95-105lbs/ft3 - produce Normal Weight Concrete (150lbs/ft3) - aggregates w bulk densities >70lbs/ft3 called lightweight - aggregates weighing more than 130lbs/ft3 called heavy weight
Natural Mineral Aggregates
sand, gravel, crushed rock derived from natural sources - igneous, sedimentary, metamorphic
critical aggregate size
size below which high internal stresses of cracking the particle will not occur