CONCRETE DURABILITY

Seminar #2

Physical Properties and Causes of Deterioration of Construction Materials

90-553

by:

Alvin Olar

OVERVIEW

Define durability Exposure conditions

Chemical and Physical

Durability factors w/c ratio and degree of hydration

Components and testing of mass transfer Mechanisms and tests of deterioration Recommendations and Q&A

WHAT IS DURABILITY?

No material itself is durable or non-durable: it is the interaction of the material with its in-service environment that determines its durability. L. Masters

Durable concrete is concrete that will withstand the conditions for which it has been designed, without deterioration, over a period of years

EXPOSURE CONDITIONS

Chemical attack: Leaching and efflorescence (P)

Sulfate attack (P)

Alkali-aggregate reaction (A)

Acids (P)

Corrosion of reinforcement (R)

A = Aggregate, P = Paste, R = Reinforcement

EXPOSURE CONDITIONS

Physical attack: Freezing and thawing (P,A)

Wetting and drying (P)

Temperature Changes (P,A)

Wear and abrasion (P,A)

A = Aggregate, P = Paste, R = Reinforcement

DETERIORATION

Can be caused by the adverse performance of the A, P, or R and can be due to chemical and/or physical causes listed

Common defects; Scaling / spalling / popouts

Cracking / disintegration

Efflorescence / incrustation

DURABILITY FACTORS

Single parameter having the largest influence on durability is the W/C ratio W/C ratio dictates the porosity and permeability

of the paste Permeability determines the vulnerability of

concrete to external agencies,

in order to be durable, concrete must be relatively impervious

Lower W/C ratio increases the strength of concrete and resistance to internal stresses

w/c = 0.475

CAPILLARY PORES

W/C ratios > 0.38, all the cement can hydrate but capillary pores will also be present

Capillary pores represent that part of the gross volume not filled by the products of hydration

The volume of the capillary system is reduced with the progress of hydration

CAPILLARY PORES

Interconnected Pores mainly responsible for the permeability of the

hardened cement paste and its vulnerability to deterioration

Discontinuous Pores as paste matures, the additional products of

hydration fill the water filled capillary voids producing discontinuity

Cementitious Materials

MASS TRANSFER

The permeability of concrete to liquids, gases, and ions is of direct relevance to durability

The rate at which water and aggressive agents penetrate into the pores of concrete is directly related to its resistance to both chemical and physical attack

Higher permeability results in quicker and more severe deterioration

MASS TRANSFER

Three stages of moisture front propagation in concrete can be defined;

for partial vapor pressures below 0.45, before a meniscus is formed, the moisture movement is controlled by adsorption and surface diffusion

for partial vapor pressures between 0.45 and close to 1, moisture transfer is achieved through vapor diffusion and capillary tension

in saturated or nearly saturated material, moisture transfer is mainly due to laminar flow, controlled by viscosity and defined by Darcys Law

ADSORPTION

At low relative vapor pressures (p/ps < 0.45) mass transfer of water vapor occurs by molecular migration rather than coherent flow

This type of flow does not contribute significantly to mass transfer of water through cement paste

SURFACE DIFFUSION

Loosely bound top molecular layers of the adsorbed water may slide over the underlying layer, thus wetting bounding surfaces and hence causing mass transfer

VAPOR DIFFUSION

At p/ps > 0.45 when a meniscus can form, vapor diffusion occurs in accordance with Ficks Law;

Q = Dp dp/dx

where Q = mass transport rateDp = diffusion coefficient

dp/dx = vapor pressure potential gradient

VAPOR DIFFUSION

The continuous film of moisture in a partially saturated sample of hardened cement paste produces a short circuit allowing rapid moisture transfer without coherent flow

BULK FLOW

Movement of liquid through an unsaturated of saturated matrix, or both

In unsaturated material with one end exposed to water the other to the atmosphere, capillary tension (absorption) creates mass transfer

Once pores are saturated, the curvature of the meniscus is zero and a pressure head is required to drive water through the matrix

BULK FLOW

Pressure driven flow is governed by Darcys Law;

< = Kp *h/*lwhere < is the velocity of flow, *h is the head loss

over a flow path of length *l and Kp is the permeability coefficient

PERMEABILITY TESTING

Gas Flow pressure induced gas flow follows Darcys Law

directly measure gas permeability through a field or laboratory specimen

Gas Diffusion measures the concentration of a chosen gas

(usually oxygen) across the sample, due to exposure at the upstream face

PERMEABILITY TESTING

Water-Vapor Diffusion measures the movement of water vapor across

the sample driven by a difference in partial pressure between the upstream and downstream faces

placing a sample on a saturated sand bed with a desiccant on the downstream (top) face and periodically weighing the sample and desiccant

the diffusion coefficient is based on Ficks law

PERMEABILITY TESTING

Absorption and Rate of Absorption measure the weight gain of a sample, volume of

water entering the sample, depth of penetration, or a combination thereof

ASTM C642 - Test Method for Specific Gravity, Absorption, and Voids in Hardened Concrete, is a popular test that gives reliable results

r = (4::/Po) .d/%%tr = mean pore radius (m)

:= viscosity of water (Pa.s)

Po= atm. pressure (Pa)

d = depth of penetration (m)

t = time (s)

PERMEABILITY TESTING

Water Penetration measuring the depth of penetration or volume

of water entering the sample when one end of the unsaturated sample is subjected to a pressure head In situ test

an in-situ test is the Figgs test a 10mm dia. by 40mm deep hole is drilled into the

surface being tested and the time to disperse a known volume of water is measured

Initial moisture conditions effect results

PERMEABILITY TESTING

Pressure induced water flow through a saturated sample follows Darcys Law

The test is performed on a water-saturated specimen and involves subjecting one end of the sample to a pressure head

The measurement of outflow determines the permeability coefficient

PERMEABILITY TESTING

Ionic Diffusion because of the extent of reinforcement

corrosion damage and freeze thaw deterioration due to de-icing and marine salts, the main concern with ionic diffusion is chloride ion migration into concrete

testing includes establishing chloride ion concentration profiles of in-situ concrete by analyzing cores at successive depths

PERMEABILITY TESTING

use of a concrete slice as a membrane between two salt solutions

the ion concentration change on the lower concentration side is monitored and correlated to a diffusion coefficient using Ficks Law

may take months to establish steady state

ASTM C1202 - Test Method for Electrical Indication of Concretes Ability to Resist Chloride Ion Penetration

measure total charged passed in 6h across a sample

PERMEABILITY TESTING

sandwiched between NaOH and NaCl solutions.

the driving force is a 60-V dc current

measures the effect of all dissolved ions in the concrete, without emphasis on a particular ion

CHEMICAL ATTACK

Leaching and Efflorescence salts left behind on the surface by evaporation

of water or interaction with CO2 in the atmosphere leads to efflorescence

typical salts are sulfates and carbonates of Na, K, and Ca

an aesthetic rather than a durability problem, however, efflorescence indicates substantial leaching is occurring

CHEMICAL ATTACK

Sulfate Attack External - caused by ingress of sulfate from

soils, groundwater, and acid rain

Internal - caused by extensive sulfate content of cement, aggregates, mineral or chemical admixtures, and mix water

CHEMICAL ATTACK

External Sulfate Attack results in microstructural changes leading to

deterioration of mechanical properties

common type is MgSO4

C - S - H

Mg - S - H - has no cementing properties

CHEMICAL ATTACK

Internal Sulfate Attack Delayed Ettringite Formation - DEF

caused by decomposition and reformation of ettringite due to improper curing procedures

reformed ettringite is expansive and results in the formation of peripheral cracks/gaps around the aggregate

CHEMICAL ATTACK

Tests for Sulfate Attack the resistance of concrete to sulfate attack can

be tested in the laboratory by storing specimens in a solution of sodium or magnesium sulfate, or in a mixture of the two

the effects of exposure can be estimated by; the loss in strength

changes in dynamic modulus

its expansion

loss of weight

CHEMICAL ATTACK

Alkali-Aggregate Reaction - AAR reactions taking place in the presence of water

between the alkalis in Portland cement and certain rocks and minerals containing active siliceous or carbonate materials

the reactions result in large volume increases producing cracking, spalling, and popouts

CHEMICAL ATTACK

Tests for AAR ASTM C 227 - Test Method for Potential

Alkali Reactivity of Cement-Aggregate Combinations (Mortar-Bar Method)

measures physical reactivity of aggregate

ASTM C 289 - Test Method for Potential Alkali-Silica Reactivity o f Aggregates (Chemical Method)

measures the silica content of the aggregate

CHEMICAL ATTACK

Acid Attack in damp conditions SO2, C02, and other acid

fumes present in the atmosphere attack concrete by dissolving and removing part of the set cement, leaving behind a soft, mushy mass

various physical and chemical tests on the resistance of concrete to acids have been developed, but there are no standard procedures

results should be carefully interpreted

CHEMICAL ATTACK

Corrosion of Reinforcement for corrosion of steel embedded in concrete to

occur, the following conditions must all be met; the provision of an anode-cathode couple with at

least part of the steel acting as an anode

the maintenance of an electrical current

the presence of moisture

the presence of oxygen

CHEMICAL ATTACK

High alkaline conditions present within concrete (ph of 12 to 12.5) cause a passive oxide film to form on the surface of the iron and prevent corrosion

Atmospheric Carbonation reduces the ph by converting calcium hydroxide in cement paste to calcium carbonate

Chloride ions have the ability to destroy the passive oxide film of steel at high alkalinities

PHYSICAL ATTACK

Freezing and Thawing Porous materials containing moisture are

susceptible to damage under cycles of freezing and thawing (frost attack)

freeze-thaw depends on the moisture content and rate of replenishment

higher porosity means more freezable water

essential that substantial hydration takes place before exposure to frost - lower porosity and permeability

PHYSICAL ATTACK

Mechanisms of frost attack 2 main dilating pressures exist - hydraulic and

diffusion pressures

Hydraulic pressure 9% increase in volume as water freezes

excess water in the cavity is expelled

hydraulic pressure developed depends on the resistance to flow- length of path and permeability of the paste between air void

PHYSICAL ATTACK

if the concrete is at least 91% (critically) saturated hydraulic pressure can exist

PHYSICAL ATTACK

Diffusion pressure caused by osmotic pressure brought about by

local increases in solute concentration due to the separation of frozen (pure) water from the solution

cations from deicing salts cause an increase in the number of pores occupied by adsorbed water and thus becoming osmotically active

deicing salts tend to accelerate deterioration

PHYSICAL ATTACK

When dilating pressure in the concrete exceeds its tensile strength, damage occurs scaling, spalling, to complete disintegration

Freeze thaw deterioration is a function of the amount of adsorbed water relative to free void space

W/C ratio, degree of hydration, and moisture content

PHYSICAL ATTACK

PHYSICAL ATTACK

Tests for freeze thaw and salt scaling resistance ASTM C 666 - Test Method for Resistance of

Concrete to Rapid Freezing and Thawing A) Rapid freezing and thawing in water

B) Rapid freezing in air and thawing in water

cycle - 40oF - 0oF and 0oF to 40oF

PHYSICAL ATTACK

Durability factor

(D.F.) = # of cycles x relative

at end of test dynamic modulus

# of cycles at which

exposure is to be determined

If the surface area of the cement paste can be determined the amount of adsorbed water can be determined

by nitrogen adsorption or X-ray scattering

PHYSICAL ATTACK

ASTM C 457 - Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete

most significant parameter determined is the air void spacing factor (limit = 0.004 - 0.008in or 0.1 -0.2mm)

PHYSICAL ATTACK

ASTM C 672 - Test Method for Scaling Resistance of Concrete Surfaces Exposed to Deicing Chemicals (ponding method)

sample is dried for 14 days at 23oC and 50% RH

test surface is covered with a salt solution and the specimen is submitted to a series of 50 daily freezing and thawing cycles

the deterioration is evaluated visually (drawback)

PHYSICAL ATTACK

CSA CAN3 A23.1.2 - Brine Immersion Test sample is dried to a constant mass at 110oC

specimen is immersed in a 3% by mass NaCl solution for 24 hours

submitted to 50 daily cycles of freezing and thawing

weight loss of sample is monitored

PHYSICAL ATTACK

Wetting and Drying deterioration due to wetting and drying cycling

is caused by volume changes and is an osmotic diffusion pressure dilation

depends on pore size, pore size distribution, and permeability - ions increase critical pore size

tests for total porosity, capillary rise, depth of penetration, and adsorption give indication of wetting and drying resistance

PHYSICAL ATTACK

Temperature Changes; Differential coefficients of thermal expansion

between cement paste and aggregate may lead to cracking

Generally concretes with a higher coefficient of thermal expansion are less resistant to temperature changes

Heated and cooled between 4 and 60oC at 2.2oC per minute

PHYSICAL ATTACK

Wear and Abrasion; Under may circumstances, concrete surfaces are

subjected to wear - (pavements, sidewalks, hydraulic structures)

The test employed should match the cause of wear

Function of compressive strength of concrete and aggregate, and w/c ratio

PHYSICAL ATTACK

ASTM C 779 - Test Method for Abrasion Resistance of Horizontal Concrete Surfaces

A) the revolving disk machine operates by sliding and scuffing of steel disks in conjunction with abrasive grit

B) the dressing wheel machine operates by impact and sliding friction of steel dressing wheels

C) the ball bearing machine operates by high contact stresses, impact, and sliding friction from steel balls

Measures depth of wear with respect to exposure time

CONCLUSION

The mix proportions and degree of hydration play the largest role in concrete durability, irrespective of the type of attack

The rate of which moisture and ions penetrate into concrete give indication of its ability to withstand deterioration

Both chemical and physical attack has been cause for large amounts of damage

RECOMMENDATIONS

Cement, aggregate, water, and admixtures are required to meet normal specifications for average use

W/C ratio should be not exceed 0.45 Good curing to obtain adequate hydration Air entraining should be used for severe

exposure Air drying after curing and before exposure

RECOMMENDATIONS

Adequate drainage and run-off features in the design of concrete elements and structures

Q and A