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CEE300/TAM324Concrete mechanical properties and degradation

Explore inter-relation between hydration reactions and microstructure development in the cement paste matrix

Understand concrete degradation mechanisms (durability)

From last lecture: * The different cement compounds hydrate differently (varying reaction

rate, hydration products).* Capillary pore volume and inter-connectivity have a significant effect

on concrete strength and permeability, respectively.* Capillary pore characteristics are controlled by mixture w/c and

concrete age

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Hydration kinetics – C3S

Unhydrated C3S

Unhydrated C3S

water

C3S

DissolvedCa++ and OH- ions

from C3S

slow dissolutionfast

dissolution

Hydration products (C-S-H and CH)growing on surface

Critical ion concentration triggers nucleation

and hydration

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Sequence of microstructure development

(a) 3 seconds(b) 1 day(c) 7 days(d) 300 days

Why important?

Note reduction of capillary porosity, the increase in CSH, and the consumption of cement as time goes on and the hydration reaction continues.

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Recall: effect of porosity on strength of ceramics

P oenP

Small amounts of capillary porosity

(defects!) have a huge,

detrimental effect on the strength of brittle materials

like ceramics and concrete.

increasing w/c ratio

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w/c and capillary porosity

As cement hydrates, newproduct (gel) fills up space, thus lowering capillary porosity and restricting pore network

(fully hydrated)

Ultimate porosity is a function of the originalw/c ratio of the cementpaste

“gel” porosity actually increases!

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Factors affecting concrete strength

w/c ratio and porosity

age and cure temperature:“maturity”

Increasing curetemperature

Increasing moistcure duration

moist curing conditions

Abrams’ rule: fc = K1/(K2)w/cm

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Material durability

Definition: ability of a material to resist weathering action, chemical attack, abrasion, or other deterioration mechanism.

Now considered as important as material strength!

Most deterioration mechanisms caused by moisture oraggressive ion ingress into the concrete.

Protection: limit moisture ingress into concrete

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Permeability and diffusivity

w/c ratio also affects permeability anddiffusion properties of concrete

Low (desirable)permeability anddiffusivity achievedby low w/c ratio or addition of silica fume additive

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Freezing & Thawing

Non-connected entrained air entrained bubbles (0.1 mm dia.; mean separation 0.4 mm) act to alleviate stresses. However, strength is reduced with entrained air.

Freezing water in the internal pores and cracks of the concrete (hydraulic pressure)

Non-air

entrained

Airentrained

Prevention:* incorporate appropriate entrained air system* use low permeabaility concrete

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Steel reinforcement corrosion

Following conditions arerequired for embeddedrebar to corrode:1.Provision of anode-

cathode couple

2. Maintenance of anode

3. Maintenance of cathode

Cells exist within a single baror between two dissimilar metals

Disruption of passive layer (rust layer)on bar due to chlorides or carbonation

Presence of oxygen and water nearcathode site

steel bar

concrete

e-

anode cathodeFe++ OH-

In presence of O2andH2O

If passivity layerIs disrupted

rustCarbonationor chlorides

Stresses built up by formation of expansive product (rust) on surface of steel bar, causing surrounding concrete to crack.

(automatic)

Prevention: minimize O2, moisture and chloride content near steel:

* use appropriately large “cover” depths,* restrict application of chlorides,* use low permeability concrete

(air (CO2) or chloride ingress to steel)

(air and water ingress to steel)