Designing for durability and strength Cracking & durability Local studies on corrosion & cracking ‘Other aspects’ Presented by: Professor Mark Alexander Concrete Materials and Structural Integrity Research Unit (CoMSIRU) University of Cape Town
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 over 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.
Uncracked vs cracked concrete ▪ 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 of the input parameters
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 Level of damage Corrosion with minor Macro-cracking and concrete Ingress of contaminants: cracking CO 2 , Cl - cover-cracking, Spalling No corrosion signs Loss of steel cross-section Initiation period Propagation period Acceleration period
Corrosion stages ▪ 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
Influence of cracking on durability ▪ 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
▪ 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
Cracking, and corrosion initiation and propagation ▪ 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 • Width, depth and geometry ❑ concrete quality • Frequency/density ❑ resistivity • Activity or dormancy ❑ corrosion-inducing agents • Orientation (transverse or ❑ crack characteristics 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 .
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 on chloride-induced corrosion rate. Field specimens - marine tidal zone (natural corrosion) Laboratory specimens (accelerated corrosion)
Marine exposure in the marine tidal zone M10 nut and bolt Robben Island Notch Location of field specimens Table Bay ( tidal/splash zone ) Crack Uncoated (exposed) top surface of the beam Hout Bay Noordhek Beach Muizenberg Kalk Bay N N 3 mm thick plate 20 mm diam steel rod Simon's Town Atlantic Beams were pre-cracked using 3-point ◼ ocean flexural loading Indian ocean Epoxy coa Epo oatin ing on n the he The 0.4 and 0.7 mm crack widths maintained ◼ sides of si f the he be beam am Buffels Bay open using a loading rig (A) (B)
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 In-built reservoir ◼ Concrete quality i.e. penetrability to corrosion-sustaining agents quantified using long-term chloride diffusion coefficient Notch Anodic impressed current used initially to induce active corrosion (~ 0.1 µA/cm 2 ) in the specimens >> intention was to eliminate the initiation phase Demec studs
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) 3.5 PC: CEM 1 42.5N GGBS: 50/50 PC/GGBS 3.0 Corrosion rate (µA/cm 2 ) FA: 70/30 PC/FA CSF: 93/7 PC/CSF 2.5 w/b ratio: 0.58 2.0 1.5 1.0 0.5 0.0 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
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