34 research outputs found
Investigation on the influence of fibre reinforcement on chloride induced corrosion of RC structures
Corrosion of reinforcement is the main cause of deterioration of RC structures located in marine environments or subjected to de-icing salts. In order to delay the ingress of chlorides, such structures require
the use of thick, dense concrete covers and strict crack width limitations. Given the crack limiting effect
provided by fibres, it would be of interest to incorporate fibre reinforcement to conventionally reinforced concrete structures. Nevertheless, whereas fibre reinforcement might delay the penetration of detrimental agents into the concrete through crack control, there are other aspects which need to be addressed to determine whether fibres can improve the overall durability of RC structures.
Several experiments were conducted within the present project, parts of them still ongoing, to investigate
some of these aspects, namely: (i) the effect of fibres on the diffusivity of uncracked concrete; (ii)
whether fibres may affect the corrosion onset of rebars for any crack width; (iii) whether steel fibres,
due to their conductive nature, might influence the resistivity and consequently the corrosion rate of
embedded rebars; and (iv) whether there is a risk of galvanic corrosion between steel fibres and rebars.
Obtained results from chloride migration and bulk diffusion tests showed that fibres had no significant influence on the diffusion coefficient of concrete. Results from experiments in which RC beams were subjected to different loading conditions and thereafter naturally corroded through exposure to highly
concentrated salt solution, showed a trend for earlier corrosion initiation with increasing crack width.
Concrete mixes incorporating fibres exhibited similar or delayed corrosion onset compared to their plain concrete counterparts. While resistivity of FRC was consistently lower than resistivity of plain concrete measured under AC at 1 kHz, corrosion rate measurements based on the galvanostatic pulse technique showed no clear correlation between the presence of fibres and the corrosion rate
Corrosion initiation in cracked fibre reinforced concrete: Influence of crack width, fibre type and loading conditions
This paper reports results from an ongoing project aimed at investigating the influence of fibre reinforcement on corrosion of rebar in chloride environments. Material tests showed that the resistivity of concrete decreased with the addition of fibres, whereas the chloride migration coefficient remained unaffected. Fibres at low dosages (<1.0% vol.) did not significantly affect the compressive and flexural strength of concrete but greatly enhanced its toughness. The results from corrosion tests showed a tendency of an earlier initiation of corrosion with increasing crack widths, while a small improvement was observed by the addition of fibres in terms of delayed corrosion initiation
Characterisation of bending cracks in R/FRC using image analysis
In this study, experiments were conducted to induce bending cracks of specific surface crack width to reinforced concrete beams made of plain concrete (RC) and reinforced concrete beams made of fibre reinforced concrete (R/FRC). After injecting and impregnating the cracks with dyed epoxy resin, image processing and analysis were employed to investigate the internal crack morphology. Several crack features including crack width (accumulated, effective and maximum), branching and tortuosity were defined and quantified. The results showed that in addition to arrested crack development, the presence of fibres yielded a distinctive change in the internal crack pattern, including increased branching and tortuosity, both of which have positive implications regarding concrete permeation. Likewise, specimens with fibres exhibited reduced maximum individual crack widths near the rebar, potentially increasing the ability of autogenous crack healing and reducing the risk of corrosion initiation
PREDICTING CHLORIDE INDUCED DEPASSIVIATION AND MINIMUM CONCRETE COVER WITH DIFFERENT BINDERS
Abstract Corrosion of steel reinforcement represents the major cause affecting durability of reinforced concrete structures in road and marine environments. To assure durability, standards attempt to provide specifications for long-term performance by simple deemed-to-satisfy rules for approximate environmental classification. This paper presents results from a study of modelling of chloride ingress in concrete with fly ash and ground granulated blast-furnace slag. Chloride threshold values for corrosion initiation are discussed. A physical model, ClinConc, was employed to calculate the chloride ingress profiles after exposure under marine (submerged) and road environments for 100 years. The model was validated using field data after exposure in the Swedish seawater for about 20 years. The results show that the addition of mineral additions in general increases the resistance of concrete to chloride ingress and allows smaller concrete cover thicknesses. However, one critical parameter is the chloride threshold value. In consideration of both the chloride resistance and the alkalinity, which influence the critical chloride threshold value, the concrete with mineral additions still reveals sufficient margin to allow a significantly lower chloride threshold for initiation of corrosion of reinforcement steel in concrete
Lattice Girder Elements in Bending: Pilot Experiment.
A test was conducted on two Lattice Girder Elements in order to evaluate the mechanical behaviour in bending. Lattice Girder Elements are used as permanent formwork in in-situ concrete construction. However, the present design method for the elements is based on empirical expressions and is believed to be over conservative and, most important, not sufficient to describe the mechanical behaviour. In the conducted test it was found that the peak-load was limited by buckling of the top chord in the lattice girder. Furthermore, cracking of the concrete started at a low load level (typical at 20 percent of the peak-load). However, the crack widths were small even at the peak-load. The suggested analytical method seems to be able to predict the behaviour rather well but the study indicates some parameters that are crucial and which need to be investigated further
Effektiva Betongkonstruktioner: Funktionella materialkrav
De viktigaste resultaten frÄn projektet Àr: 1.Den nedbrytningsmekanism som huvudsakligen begrÀnsar livslÀngden hos anlÀggningskonstruktioner Àr armeringskorrosion initierad av kloridintrÀngning. Vad gÀller nedbrytning av frostangrepp finns det inget som tyder pÄ att detta har begrÀnsat livslÀngden hos dessa konstruktioner i nÄgon större utstrÀckning. Armeringskorrosion initierad av karbonatisering Àr heller normalt inget problem för problem för anlÀggningskonstruktioner eftersom dessa Àr uppförda med betong som har lÄga vct. 2.Genomförda litteraturstudier och undersökningar i laboratorium visar att betong med tillsatsmaterial i de flesta fall fÄr egenskaper som Àr likvÀrdiga eller bÀttre jÀmfört med motsvarande betong med enbart Portlandcement (CEM I). Vad gÀller bestÀndighet blir denna i mÄnga fall bÀttre för betong med tillsatsmaterial, t.ex. vad gÀller motstÄnd mot kloridintrÀngning. FrostbestÀndigheten (salt-frost avskalning) blir vid lÄga mÀngder tillsatsmaterial (†20 vikt-% av total bindemedelsmÀngd) likvÀrdig eller nÄgot sÀmre om inverkan av karbonatisering beaktas. Vid halter upp till 35 vikt-% (vilket motsvarar ett CEM II/B) blir frostbestÀndigheten nÄgot sÀmre, men klarar fortfarande god frostbestÀndighet enligt SS 137244. 3. Genomförda LCA visar att betong med tillsatsmaterial fÄr lÀgre klimatpÄverkan Àn motsvarande betong med enbart Portlandcement. Detta Àr bÄde ett resultat av en lÀgre klimatpÄverkan frÄn betongsammansÀttning men ocksÄ av en bÀttre bestÀndighet vid anvÀndning av tillsatsmaterial (speciellt med avseende pÄ konstruktioner exponerade för klorider). Det rekommenderas att de krav pÄ betongsammansÀttningar och minsta erforderliga tÀckande betongskikt i olika exponeringsmiljöer som finns regelverk för anlÀggningskonstruktioner, frÀmst SS 137003, EKS och AMA AnlÀggning ses över.Effektivare konstruktioner - krav pÄ cement och beton
BestÀndighet hos anlÀggningskonstruktioner. Etapp II -Funktionella materialkrav, Betong med flygaska eller GGBS, BestÀndighetsegenskaper (delprojekt 1c)
I Sverige Ă€r de huvudsakliga nedbrytningsmekanismerna för armerade betongkonstruktioner i vĂ€gmiljö armeringskorrosion initierad av kloridintrĂ€ngning samt frostangrepp med tösalter. Det kan ocksĂ„ vara korrosion av karbonatisering och frost med sötvatten, i miljöer utan tösalter (lĂ€ngs jĂ€rnvĂ€gar). Nedbrytningsmekanismerna Ă€r olika allvarliga för konstruktionens funktion, dĂ€r vissa kan resultera i att konstruktionens bĂ€rförmĂ„ga reduceras (t.ex. armeringskorrosion av klorider) medan andra frĂ€mst resulterar i problem av estetisk karaktĂ€r (t.ex. avflagning av frost). Med inblandning av tillsatsmaterial som flygaska och slagg (GGBS) ökar bl.a. betongens tĂ€thet. Detta verkar positivt för betongkonstruktioners bestĂ€ndighet, som exempelvis minskad kloridintrĂ€ngning, men kan i vissa fall ocksĂ„ ha en negativ pĂ„verkan sĂ„som försĂ€mrad frostbestĂ€ndighet vid karbonatisering eller ett nĂ„got lĂ€gre kloridtröskelvĂ€rde. Ofta redovisas dessa positiva och negativa effekter separat, vilket gör det svĂ„rt att bedöma den samlade effekten pĂ„ konstruktionens livslĂ€ngd. En ytterligare faktor som bör beaktas Ă€r den frĂ„n eventuella klimatförĂ€ndringar, inte minst eftersom anlĂ€ggningskonstruktioner ofta dimensioneras för över 100 Ă„r. Genom att göra livslĂ€ngdbedömningar för betongkonstruktionen kan man fĂ„ en bĂ€ttre uppfattning om hur bestĂ€ndigheten pĂ„verkas av flygaska och GGBS. I denna bedömning ges möjlighet att analysera effekter frĂ„n samtliga pĂ„verkande faktorer och dess variationer. Det finns idag farhĂ„gor kring frostbestĂ€ndigheten hos betong med GGBS, dĂ„ det har visat sig att karbonatiseringen hos slaggbetong kan ha en negativ inverkan pĂ„ frostbestĂ€ndigheten vid saltfrostangrepp1. Det verkar dĂ„ finnas en viss krisrisk nivĂ„ pĂ„ inblandningen av GGBS, uppgifter i nationell och internationell litteratur visar att saltfrostbestĂ€ndigheten hos betong med mer Ă€n ca 30-40 vikt-% GGBS av bindemedlet försĂ€mras betydligt nĂ€r betongen karbonatiserat pĂ„ grund av att porsystemet förĂ€ndras och blir grövre. Exakt var denna kritiska grĂ€ns med avseende pĂ„ frostbestĂ€ndighet ligger, Ă€r dock nĂ„got oklar och behöver utredas ytterligare. Ăven för betong med flygaska sĂ„ pĂ„verkas porsystemet av karbonatisering. Men för betong med flygaska Ă€r farhĂ„gorna mer inriktade pĂ„ en försĂ€mrad frostresistens av att luftporbildande tillsatsmedel pĂ„verkas av vissa flygaskor. Det har visat sig att restkolet absorberar delar av luftporbildaren vilket resulterar i en minskad eller varierande lufthalt i betongen. Genom att anvĂ€nda flygaska som uppfyller kraven i SS-EN 450-1 (kategori A) minimeras eventuella problem av kompatibiliteten med luftporbildare. Ăven för flygaskecement (CEM II/A-V eller CEM II/B-V) kan luftporsystemet pĂ„verkas negativt och högre dosering av luftporbildare behövas. Syftet med denna rapport Ă€r att ta fram bĂ€ttre underlag vid beslut om anvĂ€ndningen av flygaska och/eller GGBS i betong till anlĂ€ggningskonstruktioner. MĂ„let Ă€r att hitta lĂ€mpliga doseringar (och k-faktorer), med fokus pĂ„ GGBS och saltfrostangrepp. Resultaten Ă€r ocksĂ„ tĂ€nkt att kunna anvĂ€ndas för att ge förslag pĂ„ hur nuvarande regelverk kan revideras för att bĂ€ttre kunna utnyttja flygaska och/eller GGBS.Effektivare konstruktioner - krav pĂ„ cement och beton
MATERIAL TESTING AND STRUCTURAL ANALYSIS OF FRC BEAMS - AFRACTURE MECHANICS APPROACH
Abstract
The presented work has been focused on strain-softening FRC and the interrelationship
between material properties and structural behaviour. The main purpose of this work was to
establish a procedure for structural analysis of flexural members with a combination of steel
fibres and conventional reinforcement. A systematic approach for material testing and
structural analysis, based on fracture mechanics, has been used and this covers: (1) material
testing; (2) inverse analysis; (3) adjustment of the s-w relationship for fibre efficiency; and
(4) cross-sectional and structural analysis. The results suggest that the approach used for the
material testing provides the necessary properties to perform analyses based on non-linear
fracture mechanics. The structural behaviour could be predicted with good agreement with
FEM using both bi-linear and multi-linear s-w relationships. When comparing the peak loads
obtained in the experiments with the results from the analyses, the agreement was good, with
a high correlation. This demonstrates the strength of the fracture-mechanics approach for
material testing and structural analysis