Designing for durability and strength Cracking & durability - - PowerPoint PPT Presentation

Presented by:

Professor Mark Alexander

Concrete Materials and Structural Integrity Research Unit (CoMSIRU) University of Cape Town

Designing for durability and strength

Cracking & durability Local studies on corrosion & cracking ‘Other aspects’

Contents

▪ Cracking and durability

❑ Overview

• Ingress of corrosive agents in cracked concrete
• Influence of cracking on corrosion initiation and propagation
• Role of uncracked concrete ligaments
• Influence of concrete resistivity

❑ Studies on cracking and corrosion at UCT (Dr Mike Otieno)

• Influence of cover cracking, cover depth and concrete quality on

corrosion rate

• Results (past and present UCT results)

▪ Further aspects

❑ Rebar detailing ❑ Corrosion-resistant steels ❑ Influence of creep and shrinkage on corrosion

Cracking: Possible causes

Identify the cause of cracking

Main causes Crack pattern Crack width Shrinkage random < 0.5 mm ASR random* < 3 mm (or more) Corrosion

• ver reinforcement

small to large… Structural stress concentration small to large… *Or, in members under significant stress, cracks follow path of least resistance (parallel to stress flow)

◼ Also look for other evidence: Silica gel (ASR), rust

stains (corrosion) etc.

▪ Much progress has been made in understanding:

❑ Transport processes of aggressive agents (e.g. chloride ions) into concrete, and ❑ Subsequent corrosion mechanisms that cause damage

▪ Time to corrosion initiation in uncracked concrete can now be ‘reasonably’ predicted (at least

deterministically)

❑ e.g. service life models such as LIFE 365, DuraCrete (we also have South African models).

▪ However:

❑ These models are based mainly on uncracked concrete even though cracking is pervasive in RC

structures by default and design

❑ The prediction accuracy is still limited by inadequate data ❑ The development of full probabilistic methods is limited by lack of reliable data on the variability

• f the input parameters

Uncracked vs cracked concrete

Cracked concrete

▪ In contrast, in cracked RC, it is very difficult to carry out realistic service life modelling:

❑ in the presence of cracks, existing models - largely formulated assuming uncracked

concrete - are inaccurate or not relevant!

▪ Consider the service life of a corrosion-affected RC structure (Tuutti diagram below)

Failure Initiation period Propagation period Acceleration period Level of damage

Ingress of contaminants: CO2, Cl- No corrosion signs

Corrosion with minor cracking Macro-cracking and concrete cover-cracking, Spalling Loss of steel cross-section

▪ The three phases:

corrosion initiation, propagation and acceleration, are reasonably well understood

▪ BUT: most current service life models use the initiation phase, the so-called Initiation Limit State,

as the end-of-life criterion

❑ The models consider the resistance of an uncracked concrete cover layer to ingress of

aggressive agents

▪ For cracked concrete, this approach is inadequate since corrosion can be initiated almost

instantly in the presence of corrosion inducing and sustaining species e.g. chlorides

▪ Therefore, a realistic service life model for cracked RC structures must consider the

propagation period of corrosion Corrosion stages

▪ The quality and thickness of concrete cover are the most influential material parameters affecting

steel corrosion in RC structures, BUT

❑ Protective potential of cover is reduced due to cover cracking - permitting rapid ingress by

aggressive species such as chlorides, carbon dioxide, oxygen and moisture

▪ Despite conflicting results on the influence of cracking on corrosion, the general consensus is that:

Cover cracking leads to increased penetrability, hence shorter time to corrosion initiation, and increased corrosion rates thereafter

Influence of cracking on durability

▪ Thus, we need a paradigm shift in service life definition for corrosion-affected RC structures:

to consider and quantify the propagation phase in addition to the initiation phase

▪ Corrosion initiation needs to be seen more as

a step towards the definition of a serviceability limit state rather than a limit state in itself

Ingress of aggressive agents

▪ Transport properties of cracked concrete cannot be correlated with those measured on uncracked

concrete because more complex transport mechanisms are involved

▪ Ingress of aggressive agents into cracked concrete is a complex phenomenon, still not clearly

understood

❑ Cannot therefore be accurately estimated

from uncracked concrete, and further studies are still required

▪ The current consensus is that:

cover cracking affects both corrosion initiation and propagation, but its effect is modified by crack characteristics, concrete quality and resistivity

▪ Initiation phase: cover cracking can lead to complete elimination of the corrosion initiation phase

if there exists a detrimental combination of

❑ concrete quality ❑ resistivity ❑ corrosion-inducing agents ❑ crack characteristics

• Width, depth and geometry
• Frequency/density
• Activity or dormancy
• Orientation (transverse or

longitudinal w.r.t. steel)

• Healing potential

Cracking, and corrosion initiation and propagation

▪ Propagation phase:

❑ 0.2 - 0.4 mm often considered as a universal threshold crack width, below which corrosion may be

considered to be similar to that in uncracked concrete. However: the adoption of a universal crack width, regardless of the type of crack, may not be valid, particularly since concrete type and quality need to be taken into consideration Role of uncracked concrete ligaments

▪ The role of the concrete ligaments between the cracks is

important since:

❑The quality of the uncracked concrete largely governs the

ingress of corrosion agents to the cathodic areas of the steel

❑This is critical particularly in cathodically controlled corrosion.

Cracking, and corrosion initiation and propagation

Influence of concrete resistivity

▪ Resistivity has a major influence on initiation and propagation of corrosion

❑ As a general rule resistivity is inversely proportional to corrosion rate in

concrete

▪ Resistivity affects corrosion rate even when other corrosion-governing

factors such as cover cracking occur

▪ On the other hand, measurements of surface resistivity are not sensitive to

the presence of cracking

▪ Blended cement concretes generally have higher resistivities compared to plain Portland cement

• concretes. Thus, corrosion rate in:

❑ blended cement concretes tends to be more resistivity-controlled ❑ plain concretes tends to be more cathodically controlled

• The current guidelines are inadequate in regard to cracked concrete

Studies on cracking and corrosion at UCT (with acknowledgments to Dr Mike Otieno)

Influence of crack width, cover depth & concrete quality on corrosion rate

▪ Three separate studies have been carried out on:

❑ Influence of cracking, cover depth and concrete quality on chloride-induced corrosion

▪ Experimental variables (summary for the studies):

❑ Flexural surface crack width: 0, ‘incipient’, 0.2, 0.4, 0.7 mm ❑ Cover depth: 20, 40 mm ❑ Concrete quality: function of binder type and w/b ratio

• Plain PC and blended cements (GGBS, FA, CSF)
• w/b ratios: 0.40, 0.55 and 0.58

❑ Specimens (prisms with single bar) exposed to:

• cyclic wetting (5% NaCl) and air-drying in the lab – accelerated corrosion
• marine tidal zone in Cape Town, South Africa – natural corrosion

▪ Objective is to quantify the combined influence of crack width, cover depth and concrete quality

• n chloride-induced corrosion rate.

Field specimens - marine tidal zone (natural corrosion) Laboratory specimens (accelerated corrosion)

M10 nut and bolt 3 mm thick plate 20 mm diam steel rod Crack Notch Uncoated (exposed) top surface of the beam Epo Epoxy coa

• atin

ing on n the he si sides of f the he be beam am ◼

Beams were pre-cracked using 3-point flexural loading

◼

The 0.4 and 0.7 mm crack widths maintained

• pen using a loading rig

Atlantic

• cean

Indian

• cean

(A)

N

Hout Bay Kalk Bay

Location of field specimens

(tidal/splash zone)

Robben Island Muizenberg Buffels Bay Simon's Town Noordhek Beach Table Bay

(B)

N

Marine exposure in the marine tidal zone

Demec studs Notch In-built reservoir

Data collection

◼ Corrosion rate (coulostatic LPR technique) ◼ Half-cell potential (vs. Ag/AgCl) ◼ Concrete resistivity (4-point Wenner probe) ◼ Crack width monitoring every 2 weeks (mechanical gauge)

Other tests

◼ Concrete quality i.e. penetrability to corrosion-sustaining agents quantified using long-term

chloride diffusion coefficient Anodic impressed current used initially to induce active corrosion (~ 0.1 µA/cm2) in the specimens >> intention was to eliminate the initiation phase

Cracking and corrosion – early results (Scott)

Influence of crack width, cover depth and quality on corrosion rate (PC, and blends of GGBS, FA and CSF)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

PC GGBS FA CSF PC GGBS FA CSF PC GGBS FA CSF PC GGBS FA CSF 20 mm cover 40 mm cover 20 mm cover 40 mm cover 0.2 mm crack width 0.7 mm crack width

Corrosion rate (µA/cm2) PC: CEM 1 42.5N GGBS: 50/50 PC/GGBS FA: 70/30 PC/FA CSF: 93/7 PC/CSF w/b ratio: 0.58

Influence of crack width and quality on corrosion rate (constant cover) (Plain PC and GGBS (SL) blends; two w/c)

0.2 0.4 0.6 0.8 1

PC-40 PC-55 SL-40 SL-55 PC-40 PC-55 SL-40 SL-55 PC-40 PC-55 SL-40 SL-55 PC-40 PC-55 SL-40 SL-55 Uncracked Incipient-cracked 0.4 mm cracked 0.7 mm cracked

Corrosion rate (µA/cm2) PC: CEM 1 42.5N SL: 50/50 PC/GGBS Cover: 40 mm (constant) w/b ratio: 0.40 and 0.55

Influence of crack width on corrosion rate - lab samples: Uncracked vs. ‘Incipient’ cracked

PC-40 PC-55 SL-40 SL-55

0.025 0.05 0.075 0.1 0.0 0.1 0.2 0.3 0.4

• corr. rate (µA/sq. cm), Incipient-cracked

Line of equality

• corr. rate (µA/sq. cm), Uncracked

Influence of crack width on corrosion rate: ‘Incipient’ cracked vs. 0.4 mm crack

PC-40 PC-55 SL-40 SL-55 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0

• corr. rate (µA/sq. cm), Incipient-cracked
• corr. rate (µA/sq. cm), 0.4 mm cracked

Line of equality

Influence of crack width on corrosion rate:0.4 mm crack vs. 0.7 mm crack

PC-40 PC-55 SL-40 SL-55

0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0

• corr. rate (µA/sq. cm), 0.4 mm cracked
• corr. rate (µA/sq.cm), 0.7 mm cracked

Line of equality

Effect of concrete resistivity - marine tidal zone samples

PC-40 SL-40 FA-55 FA-40 SL-55 R² = 0.88 R² = 0.89 R² = 0.95 R² = 0.89 0.1 0.3 0.5 0.7 0.9 1.1 1.3 20 30 40 50 60 70 80 90 100 110 120 130 140

Average corrosion rate (µA/cm2) Average resistivity (kΩ-cm) Uncracked Incip.-cracked 0.4 mm cracked 0.7 mm cracked 20 mm cover Exposure: marine tidal zone Exposure period: ~ 122 wks

Concept of a ‘threshold crack width’ seems not to be sound

Summary of main findings from corrosion research (‘to date’)

▪ Combined influence of concrete cover, crack width and concrete quality is evident in these results. ▪ Other factors being constant, corrosion rate:

❑ Increases with increase in crack width ❑ Decreases with increase in cover ❑ Decreases with increase in concrete quality (binder type, w/b ratio)

▪ Concrete quality has a dominating role in the effect of either crack width or cover depth on corrosion

rate.

▪ The challenge is to determine a suitable combination of these variables to meet the required

durability performance.

▪ A prediction model incorporating cover depth, concrete quality and crack width is proposed.

▪ Cover cracking has a profound effect on durability, and hence service life, of RC structures prone to

• corrosion. It influences factors such as:

❑ Rates of ingress of corrosion-inducing species ❑ The nature of transport mechanisms, and ❑ Corrosion kinetics

▪ The resistivity of the concrete system is also important in corrosion rate studies (even in cracked

concrete!)

❑ in blended binder concretes, resistivity is the predominant factor in controlling corrosion rates. By

contrast,

❑ plain Portland cement systems are strongly influenced by cover depth (ingress of oxygen and

moisture i.e. cathodic-controlled). Summary of main findings from corrosion research (‘to date’)

▪ Even incipient cracking can have a large influence on corrosion rates (but long-term effects need

to be investigated): …the concept of a universal threshold crack width below which corrosion rate is assumed to be negligible needs to be challenged - important regarding codes

• Performance-based crack widths should be adopted -

▪ The ultimate aim should be to incorporate the influence of cracking into performance-based

design codes and service life prediction models

▪ Corrosion rate prediction models should incorporate (directly or indirectly) the influence on

cracking on corrosion rate Summary of main findings from corrosion research (‘to date’)

▪ Application of corrosion resistant alloys

❑ Typically contain chromium, e.g. stainless steel. ❑ Expensive, only applied in special cases.

▪ Coatings on the reinforcing steel

❑ Provide a barrier between the steel and corrosive environment ❑ Fusion-bonded epoxy has a very variable record – not used in SA ❑ Galvanised steel (sacrificial corrosion) can be effective – mainly in carbonating conditions.

▪ Corrosion inhibitors

❑ Can be applied as penetrating CI, or admixed into the concrete ❑ Have different actions:

• Forming a protective layer on the steel surface
• Providing a buffer action in the surrounding electrolyte

Other corrosion control measures

▪Rebar detailing

❑ Issue of tolerances

➢ Over-tolerance for the bending of the steel, coupled with under-tolerance for the fixing of the

formwork, can have major consequences!

➢ The contaminant-time relationship is highly non-linear – halving the cover can effectively reduce

the time to initiation by 75-80%

▪Influence of creep and shrinkage on corrosion

➢

Creep and shrinkage will influence crack width, by slowly increasing crack width with time.

➢

Very little research done on this – many unknowns.

➢

Corrosion itself can increase the deflections, particularly when occurring under load.

Other aspects

Thank you