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