CEE300/TAM324Aggregates and concrete

Aggregates

Understand and interpret important physical properties

Concrete and cement

Understand concrete as a composite material

Understand the fundamental concepts about portland

cement cementFrom last lecture:* Aggregates are granular particulates, working together

as a group* For civil engineering applications, aggregates are normally

comprised of naturally occurring rocks and common minerals (silicates, aluminates, carbonates, etc.): ceramics

* Hardness, strength, chemical stability and gradation are important properties

Rock types - Igneous Formed on cooling of lava

(melted rock)

Mainly quartz, feldspar, pyroxene,

silica glass

Extrusive versus intrusive

effect?

Rock examples– Granite (intrusive)

– Diorite (intrusive)

– Rhyolite (extrusive)

– Basalt (extrusive)

Rock types - Sedimentary

Formed by weathering of existing rocks

Deposition: mechanical or chemical Consolidation: physical or chemical

Mainly quartz, calcite, clay

Rock examples- Gravel, sand, silt, clay (mechanical

deposition, physical consolidation)- Sandstone, shale (mechanical deposition, chemical consolidation)- Limestone (chemical deposition,

chemical consolidation)

Often laminar, usually weak and porous

Rock types - Metamorphic

Formed by alteration (pressure and temperature) of existing rock

Mainly quartz, calcite, mica, feldspar, pyroxene

Rock examples– Marble (metamorphosed

limestone)– Slate (low-pressure

metamorphosedshale)

– Phyllite, schist, gneiss (high- pressure metamorphosed shale)

– Quartzite (metamorphosedsandstone)

Often laminar, usually dense and strong

Recall gradation curves

50 mm

Size (log)

% p

assi

ng

0.15 mm

100

0

Lack of particles within this size range

Well graded

(combined coarse and fine aggs)

Uniformly graded

Gap graded

Aggregate particle shape

(Gravel and natural sand)Increasing water demand and compaction effort

(Crushed stone & manufactured sand)

Aggregate surface texture also has an effect on bond and water demand“glassy” to “rough”

Physical properties ofaggregatesStrength

Aggregates should not be weaker than design load or the strength of strength of concrete

Generally not a problem:Crushing strength of aggs = 100 to 200+ MPa

Compressive strength of concrete = 25 to 50 MPa

Porosity

The nature and amount of pores and voids affect certain properties

Impermeable pores (Vi)

Solid (Vs)

Absorption

Moisture content

Specific gravity

Density of a single aggregate piece!

Used in mix design computations

Permeable pores (Vp)

Expressed as a ratio of density of solid to that of water (1000 kg/m3)

Pores, voids and moisture absorption

drySaturated surface-dry(permeable pores wet)

wet

pore

void

Draws in water

Gives out water

Inter-relation between gradation, voids and bulk density

Containers with volume

V

Uniformly graded Well graded

* High void content within aggregate sample.

* Lower bulk density of sample (m1/V)

m1 mass of

agg. sample

m2

* Lower void content within aggregate sample.

* Higher bulk density of sample (m2/V)

“Density” of the entire aggregate sample (many pieces), as opposed to specific gravity of a single piece

Void content = volume of voids (non-solids) / total volume

What is concrete?Graded mineralaggregates serve as inert and stable filler for cement matrix

The cement “paste” matrix is a mixture portland cement (pc) and water

Chemical or mineral admixtures may be added to obtain needed properties

Constituents:

Cement paste (22-30% by vol.)

Aggregate* (70% by vol.)

air (1-8% by vol.)other

A view inside pc concrete

PC concrete made with siliceous rounded gravel

PC concrete made with crushed limestone

How are these two materials different?

Concrete as a composite material

strength permeability

shrinkage creep

Properties of concrete

Properties ofcement matrix

Thus cement properties are important!

Portland cement -- definitioncement: cohesive or adhesive material that bonds fragments together

inorganic cements: typically comprised of mineral oxides

hydraulic cements: set and harden, even under water, by virtue of a chemical reaction with water (hydration)

portland cement