Transport mechanisms in concrete and durability test methods
Dhanya B. S. Department of Civil Engineering, IIT Madras
National Workshop on Achieving Durable Concrete Construction Through Performance Testing
Common durability problems in concrete
Corrosion of steel in reinforced concrete
• Chloride induced corrosion
• Carbonation induced corrosion
Sulphate and other chemical attack
Alkali-aggregate reaction
Freezing and thawing
Why durability problems?
Typical section through an RC member
Cover-creteHeart-crete
Strength
Durability!!
Ballim, 2008
Why durability problems?
Concrete• Hydrated Cement Phase (HCP)• Aggregate phase• Interstitial Transition Zone (ITZ)
Gel pores, Capillary pores, Air voids Permeability and porosity
Aggressive species • Moisture, Sulphates, Chlorides, Carbon
dioxide, Oxygen, Alkalies etc.
TRANSPORT MECHANISMS IN CONCRETE
Diffusion Flow under concentration gradient
Gaseous diffusion• Through unsaturated concrete
Ionic diffusion• Through saturated and partially saturated concrete
Molecular diffusion • If the pores are relatively large
Equations of diffusion
C1
C2x
Equations of diffusion
C1
C2
x
+ Δt
Measuring diffusion coefficient
For steady state: measurement of chloride concentrations upstream and downstream
Unsteady state: Chloride ponding or immersion, measurement of chloride content along the depth of concrete and application of error function to determine diffusion coefficient
Permeation
Saturated liquid transfer controlled by a pressure gradient across the concrete
Ionic species dissolved in water also can move by permeation of water
Permeation higher when cracks and defects are present
Equations of Permeation
Water permeability
True indicator of concrete permeability to waterMeasures the steady state permeability
coefficient for water flow under a constant pressure head
Attainment of steady state conditions takes a long time
Can be modified to check the depth of water penetration by splitting the specimen
Measurement of flow rate
or
Measurement of Depth of penetrationaftersplitting
d
Measuring water permeability
Measuring Gas permeability
By measuring the pressure rise in a vacuum cell, placed on the concrete surface
Measuring Gas permeability
By applying a pressure gradient across the test specimen and monitoring the pressure decay over
Measuring Gas permeability
By measuring the gas flow though the concrete sample under a constant pressure gradient
Sorption/ Capillary water absorption
Uptake of liquid into unsaturated or partially saturated solids
Influenced by large capillaries and degree of continuity, compaction, aggregate orientation and distribution, mix composition etc.
Create conditions of capillary rise in concrete Measure the mass of water absorbed by the
concrete over time Test is done for 24 hours, although most of the
readings are taken within the first hour, as it is critical to get many data points in that period
Equation of Sorption
Measuring Sorption
Measure the mass of water absorbed by the concrete over time
Ma
√t
Curve through actual data points
Regression line through initial data pointsSlope = capillarity index, or sorptivity
Epoxy coatedsurface
Absorption
Refers to the bulk uptake of water unsaturated or partially saturated concrete
subjected to complete or partial immersion Easy to measureDifficult to penetrate all pores of concrete cannot get a true measure of porosity
Water absorption measurement
Simplest measurement• Measure oven dry mass, and saturated mass• Saturation generally done for at least 18 hours
Boiling water absorption test Vacuum saturation method
Migration/ Conduction
Movement of ionic species driven by difference in electrical potential
Nernst Planck Equation
Measurement of Migration
Electrical field is introduced into the test container by two electrodes which are connected to a potentiostat
Generally Cathode is the up-stream cell while the anode is the down-stream cell.
Wick action
Transport of liquid from a face in contact with liquid to drying face, where the liquid evaporates and dissolved ions precipitate as salts in the pores or on the surface of the medium
Wetting and drying cycles
DURABILITY TEST METHODS
Test methods to assess chloride penetration
Chloride Conductivity Test (SA DI Manual)
Rapid Chloride Permeability Test (ASTM C 1202)
Accelerated Chloride Migration Test (NORD 492)
Bulk diffusion Test (ASTM C 1556)
Test methods to assess gas penetrability
Oxygen Permeability Test (SA DI Manual)
Torrent Air Permeability Test (SIA 262/1)
Accelerated Carbonation Test
Test methods to assess water penetrability
Water Permeability (DIN 1048 part 5)
Water sorptivity Test (SA DI Manual)
Water Permeability(Germann)
Test method to assess concrete resistivity
Wenner four probe resistivity Test
Category Test method Parameter
Test methods to assess chloride penetration
Rapid Chloride Permeability Total charge passedAccelerated Chloride Migration Non-steady state diffusion coefficient
Chloride Conductivity Chloride ConductivityBulk diffusion Chloride concentration profile
Durability test methods and Parameters
Test methods to assess gas penetrability
Oxygen Permeability Oxygen Permeability IndexTorrent Air Permeability Coefficient of PermeabilityAccelerated Carbonation Carbonation depth
Test method to assess concrete resistivity
Wenner 4 Probe Resistivity Surface Resistivity
Test methods to assess water penetrability
Sorptivity Sorptivity indexGermann water Permeability Surface PermeabilityGerman water Permeability Water penetration depth
Transport mechanisms in concrete and test methods
Mechanism Definition Test methodSorption Capillary action SorptivityPermeation Flow under pressure Oxygen permeability, Torrent air
permeability (Gas permeability)Germann water Permeability, DIN 1048water penetration (Water permeability)
Diffusion Flow under concentrationgradient
Bulk diffusion, RCPT, Accelerated ChlorideMigration, Accelerated carbonation
Migration/Conduction
Movement due to appliedelectric field
RCPT, Accelerated Chloride Migration,Chloride conductivity, Wenner resistivity
Wick action Transport of ions or water froma face in contact with water todrying face
Sorptivity
Absorption bulk intake of water SorptivityAdsorption process of attachment of
molecules on the surfaceSorptivity
Convection Flow due to temperaturedifference
RCPT
Thank You…..
Wenner 4 Probe Resistivity Test
Method Merits Demerits
•Consists of four probes•Through two outer probes current is applied while the inner two probes measure the potential
• Quick method• Resistivity is used to calculate the corrosion rates in concretes•Avoids heating• Very low voltage (10V or lower) is applied for a short time
•Depends on pore solutionconductivity and moisture condition of the specimen•Inclusion of conductive materials affect the results
Resistivity, kΩ cm Concrete quality
> 100 Good50 - 100 Normal10 – 50 Poor< 10 Very poor
Classification Criteria (Andrade et al., 2004)
Rapid Chloride Permeability Test (ASTM C 1202)
Classification Criteria (ASTM C 1202)
Method Merits Demerits
•Specimen subjected to a 60V potential, for 6 hours• -ve side filled with 3% NaClsolution and +veside with 0.3M NaOH solution•Current is measured in each half hour, from which the total charge passed is calculated
•Widely used test method•Gives qualitative classification of concrete
•Current passed is related to all ions in the pore solution•Increase in temperature•Presence of conductive materials affects the test results•Many transport mechanisms act together
60V DC
#20 brass mesh
screen
0.3 N NaOH3% NaCl
Vacuum saturated specimens(100mm dia. , 50 mm thickness)
#20 brass mesh
screen
Accelerated Chloride Migration Test (NT Build 492)
Method Merits Demerits
•30 V potential is applied initially and after measuring the initial current, voltage is adjusted.•After specified duration the specimen is split and silver nitrate solution is sprayed•Depth of chloride penetration is used to calculate the non-steady state diffusion coefficient
•Capable of addressing some of the criticisms of RCPT related to the examination of actual chloride ion movement and temperature rise.
•Presence of conductive materials affects the test results•Many transport mechanisms acts together
Nordtest Method BUILD 492,Migration coefficient (m2/s)
Concrete quality
< 2 × 10-12 Very good2 – 8 × 10-12 Good8 – 16 × 10-12 Normal> 16 × 10-12 Poor
Classification Criteria (RILEM TC PSC –230 Draft)
Chloride Conductivity Test (DI Manual, SA)
Method Merits Demerits
•Consists of a two cell conduction rig in which concrete core specimens are exposed on either side to a 5M NaCl solution •Chloride conductivity is determined by measuringthe current flowing through the specimen due to the application of 10V potential difference
•Specimenspreconditioned before testing to standardize the pore water solution•Nullify the effect of other ions in the pore solution
•Destructive•Can not be applied on site
Chloride conductivity (mS/cm) Concrete quality
< 0.75 Very good0.75 – 1.50 Good1.50 – 2.50 Poor
> 2.50 Very poor
Classification Criteria:Alexander et al., 1999
Bulk diffusion test (ASTM C 1556)
Method Merits Demerits
•Natural diffusion under a very high concentration gradient •Specimens saturated with saturated lime water is immersed in 3% NaClsolution for 35 days•Uni-directional diffusion•Chloride profiling with profile grinder•Chloride ion concentration determined
•Identical to natural diffusion process•Other transport mechanisms are avoided
•Slow process•Powder can be lost during profiling•Effect of aggregates is unavoidable
Apparent diffusion coefficient (x 10-12 m2/s)
Resistance to chloride penetration
> 15 Low10 – 15 Moderate5 – 10 High2.5 – 5 Very High< 2.5 Extremely High
Classification criteria – Nilsson et al.1998
Oxygen Permeability Test (DI Manual, SA)
Method Merits Demerits
•Measures the pressure decay of oxygen passed through an oven dried, 30 mm thick slice of a 70mm diameter core placed in a falling head permeameter•The oxygen permeability index is defined as the negative log of the coefficient of permeability
•Good correlation with accelerated carbonation test
•Sensitive to macro- voids and cracks•Sensitive to the edges of the specimen
Oxygen permeability indexOPI
Concrete quality
> 10 Very good9.5 - 10 Good9.0 – 9.5 Poor
< 9 Very poor
Classification Criteria:Alexander et al., 1999
Torrent Air Permeability Test Swiss Standard SIA 262/1-E:2003
Method Merits Demerits•Method is based on creating a vacuum on the surface of the concrete and monitoring the rate at which the pressure is raising in the test chamber after the vacuum pump has been disconnected
•Simple•Non destructive•Both lab and site application
•Sensitive to the moisturecondition of the specimen•Test is more sensitive to the surface condition of the specimen
Classification criteria:Torrent and Jacobs, 2009
Torrent kT×10-16m2 Concrete quality
< 0.01 Very good0.01 - 0.1 Good0.1 - 1.0 Normal1.0 - 10 Poor
> 10 Very poor
Accelerated Carbonation Test
Method Merits Demerits•Measuring the depth of carbonation at different sections of the prismatic samples at different time intervals (70,98, 112 and 154 days)•Phenolphthalein used as indicator – colourless implies carbonated
•Mechanism is diffusion, which is similar to the natural phenomena
•High degree of carbonation may lead to microstructuralalterations at high CO2concentrations
Sorptivity Test (DI Manual, SA)
Method Merits Demerits
•Measures the rate of movement of a water front through the concrete under capillary suction
•Simulates natural phenomena•Applicable to drilled cores
•Sensitive to macro-voids and cracks•Sensitive to the micro-structural properties of the near-surface zone of concrete•Destructive
Water sorptivitytest, mm/√h Concrete quality
< 6 Very good6 - 10 Good10 - 15 Poor
> 15 Very poor
Classification Criteria:Alexander et al., 1999
Germann Water Permeability Test (GWT)
Method Merits Demerits
•A sealed pressure chamber is attached to the concrete surface.•Water is filled into the pressure chamber and a specified water pressure is applied to the surface•Pressure is kept constant using a micro-meter gauge with an attached pin that reaches into the chamber
•Non-destructive•Both field and lab test
•Pressure is not sufficient for impermeable concretes•Water flow may not be parallel to the gasket if pores are present
Coefficient of water permeability,
m2
Concrete quality
< 10-12 Good10-12 - 10-10 Normal
> 10-10 Poor
Classification Criteria (RILEM TC PSC –230 Draft)
Method Merits Demerits•Measure of the resistance of concrete against the penetration of water exerting pressure. •The test be done when the age of concrete is between 28 and 35 days•A water pressure of 0.5 N/mm2 is applied for a period of 3 days•After the pressure is released, the specimen is split into two and the depth of water penetration is noted.
•Depth of water penetration is measured
•Destructive•Air compressor is needed to keep the pressure constant
Water Permeability Test (DIN 1048 part 5)