Cement Permeability

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Permeability Of concrete

Transcript of Cement Permeability

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PermeabilityOf concrete

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Permeability

P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials

Durability of concrete: ability to resistweathering action, chemical attack,abrasion, or any process ofdeterioration

Permeability

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Permeability

P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials

chemical processes of degradation

physical processes of degradation

Water Causes

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Permeability

P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials

Water Structure

The H-O-H molecule is covalent bonded. Due todifferences in the charge centers of hydrogen and oxygen,the positively charged proton of the hydrogen ionbelonging to a water molecule attracts the negativelycharged electrons of the neighboring water molecules.

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Permeability

P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials

Water Structure

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Permeability

P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials

Water in Concrete

In concrete, water is a necessary ingredient for thecement hydration reactions and an agent that facilitatesthe mixing of the components of concrete, water ispresent from the beginning.

Gradually, most of the evaporable water in concrete islost, leaving the pores empty or unsaturated.

Concrete will not be vulnerable to water-relateddestructive phenomena if there is a little or no evaporablewater left after drying and provided that the subsequentexposure of the concrete to the environment did not causeto resaturation of the pores. The latter, to a large extent,depends on the hydraulic conductivity, which is alsoknown as the coefficient of permeability (K).

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Permeability

P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials

Definition

Permeability is defined as the property thatgoverns the rate of flow of a fluid into a poroussolid.

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Permeability

P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials

Darcy’s law

For steady-state flow, the coefficient ofpermeability K is determined from Darcy'sexpression:

where Dq/dt is the rate of fluid flow, ν the viscosity of the fluid, ∆ Hthe pressure gradient, A the surface area, and L the thickness of the solid.

Dq/dt = K ( ∆ H A)/(Lν )

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Permeability

P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials

Permeability of the paste

the mixing water is indirectly responsible for permeabilityof the hydrated cement paste because its contentdetermines first the total space and subsequently theunfilled space after the water is consumed by eithercement hydration reactions or evaporation to theenvironment.

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Permeability

P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials

Age (days) Permeability (cm/s 10 -11 )

Fresh 20,000,000

5 4,000

6 1,000

8 400

13 50

24 10ultimate 6

Permeability of Cement Paste

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b l

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Permeability

P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials

Permeability of Aggregate

Compared to 30 to 40 percent capillary porosity of typicalcement pastes in hardened concrete, the volume of poresin most natural aggregates is usually under 3 percent, andit rarely exceeds 10 percent.

P bili

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Permeability

P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials

Permeability of Aggregate

However, the coefficient of permeability of aggregates areas variable as those of hydrated cement pastes ofwater/cement ratios in the range 0.38 to 0.71

P bili

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Permeability

P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials

Reason

The reason some aggregates give much higherpermeability than the cement paste is that the size ofcapillary pores in aggregate is usually much larger.

Most of the capillary porosity in a mature cement paste

lies in the range 10 to 100 nm, while pore size inaggregates are, on the average, larger than 10 microns.

P bili

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Permeability

P K Mehta and P J M Monteiro Concrete: Microstructure Properties and Materials

Permeability of Aggregates

Type of Rock permeability (cm/sec )

dense trap 2.47 x 10 -12

quartz diorite 8.24 x 10 -12

marble 2.39 x 10 -10

granite 5.35 x 10 -9

sandstone 1.23 x 10 -8