长江三峡导游词Durability of marine concrete structures – field investigations and modelling
Rob B. Polder
TNO Building and Construction Rearch, Delft, The Netherlands
Mario R. de Rooij
测量实习报告TNO Building and Construction Rearch, Delft, The Netherlands,
Delft University of Technology, Delft, The Netherlands
This article prents a ries of investigations on six concrete structures along the North Sea coast in The Netherlands. They had ages between 18 and 41 years and most of them were made using Blast Furnace Slag cement. Visual inspections showed corrosion damage in only one structure, related to relatively low cover depths. All structures showed considerable chloride ingress with a large scatter within the relatively small tested areas. The interpretation was bad on the DuraCrete model for chloride ingress. Curve fitting to chloride profiles produced chloride surface contents and apparent diffusion coefficients. Comparison was made to previously published data on chloride ingress and electrical resistivity of similar concretes. It was found that a single mean value and standard deviation a
pplied to all concrete up to 7 m above mean a level for the chloride surface content. Above 7 m, the local microclimate had a decisive influence, either increasing or reducing the chloride surface content. Apparent chloride diffusion coefficients did not depend on height above a level. Their age dependency was expresd in a single value for the exponential aging coefficient. A simplified environmental factor was adopted from literature.
A probabilistic model for corrosion initiation in Blast Furnace Slag Cement concrete in marine environment was propod, the DuMaCon version of the DuraCrete model. Its application is explained for design of new structures and for asssment of existing structures. Issues for further rearch are the critical chloride content and the target failure probability for corrosion initiation, the effect of drying out on chloride transport in the marine splash zone and the nature and influence of spatial variation of chloride ingress.
Key words: Concrete, marine environment, chloride, blast furnace slag cement, reinforcement corro-sion, rvice life, probabilistic model
1 Introduction
Durability of Concrete structures in marine environment has been an issue for many decades, due to
the perception of a water as aggressive to concrete and reinforcement and the long
rvice life that is expected for marine infrastructure such as harbour and coastal defence
structures. In particular in the 1970s, a lot of work has been done due to increasing construction for the oil and gas offshore industry, e.g., in Concrete in the Oceans [Leeming 1989]. In The
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Netherlands, a collective rearch programme was carried out under supervision of CUR
committee B23, resulting in [CUR 100, 1981]. In that study, sixty structures of various ages were visually inspected and five were investigated in more depth. Generally the durability was
found to be “good”; relatively little deterioration was obrved. The most important threat to durability was found to be corrosion of reinforcement due to chloride ingress, mainly in older structures with relatively low concrete cover to the reinforcement [Wiebenga 1980]. In view of the young age of the investigated structures relative to the slow rate of degradation, it was
recommended to carry out a similar study in about 15 years time.
In the 1990's, a group of European rearchers developed a methodology for quantitative
rvice life design of concrete structures, entitled “DuraCrete”, which was bad on the
approach propod in the 1980's [Siemes et al. 1983]. Explanations of the principles and
examples of its application have been published [CEB 1997, Siemes et al. 1998]. DuraCrete's
final report includes models for predicting corrosion initiation due to chloride ingress and due to carbonation as well as models for propagation of corrosion and subquent cracking and
spalling [DuraCrete R17, 2000]. Using the DuraCrete methodology it is possible to quantify the reliability of a structure with respect to predefined limit states that concern durability
[Vrouwenvelder & Schiessl 1999].
This new quantitative approach and the availability of new investigative techniques such as electrochemical methods and concrete microscopy prompted CUR and TNO to start an
investigation into durability in marine environment in 2000 under the title “Durability of
喝酒以后多久可以开车Marine Concrete structures” (DuMaCon). Its objectives were to quantify the durability of
marine structures in The Netherlands and to provide degradation models and associated failure probabilities for existing marine structures. This article describes the original DuraCrete model, the fieldwork carried out to collect data, an overview of the results, some comparison to data from other sources, subquently propod modifications to the DuraCrete model and
recommendations for its application.
2 DuraCrete model for corrosion initiation due to chloride ingress
2.1 Basic concept
The DuraCrete degradation model for chloride induced corrosion is bad on the concept of chloride transport into concrete by diffusion and initiation of reinforcement corrosion when a critical chloride content is exceeded at the steel surface. Diffusion modelling of chloride ingress into concrete was propod in the 1970s [Collepardi et al. 1972] and further developed in the following decades [e.g. Bamforth & Price, 1993, Maage et al. 1996]. After the critical “threshold”
chloride content has reached the steel and has broken down its normal passivation, the steel starts to dissolve. Dissolved iron ions react to form corrosion products, at some point in time
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causing expansive stress and cracking of the concrete cover. Eventually, the loss of steel cross ction may become critical with respect to structural capacity [Vrouwenvelder & Schiessl 1999]. The two-stage concept of initiation and propagation of corrosion was developed by
[Bazant 1979 and Tuutti 1982]. In most rvice life design approaches, however, the initiation period is considered dominant and the propagation period is neglected. For more information on corrosion of steel in concrete the reader is referred to [Bertolini et al. 2004].
2.2 The DuraCrete model
The DuraCrete model involves a limit state formulation for chloride induced corrosion initiation which can be simplified by stating that failure (that is, corrosion initiation) occurs when C > C crit , with C the chloride content at the reinforcement surface and C crit the critical
(threshold) content. The critical chloride content is a complex function of concrete properties, in parti
cular of the physics and chemistry (pH, water, oxygen, prence of voids) at the
steel/concrete interface. Nowadays it is realid that there is no single general value for it, but rather a gradual increa of the probability of corrosion with increasing chloride content [Vassie 1984, Gaal 2004]. For real structures (as oppod to laboratory specimens) a value of 0.5%chloride ion by mass of cement is considered to be the best mean value for Portland cement concrete. No well-established value for Blast Furnace Slag cement is available. Further treatment of this subject is outside the scope of this article.
According to the DuraCrete model for chloride transport, the chloride content at the steel C(x.t)is a time dependent function described by:
(1)With C s chloride surface content (% by mass of concrete or cement); C i initial chloride content (%); x depth of the steel (m); D 0diffusion coefficient (m 2/s) at t 0(s, usually 28 days); K environmental coefficient (-); n Cl ageing exponent (-); erf errror function, stemming from solving Fick’s cond law of diffusion.
2.3 Application of the model
The DuraCrete model can be applied in the design stage of new structures by inrting into equation (1) values for the design cover depth, the expected surface content (bad on experience, e.g. from DuraCrete tables) and experimentally determined chloride diffusion coefficients of trial concrete mixes, to calculate the point in time when C(x,t) reaches C crit
, who
C x t C C C erf s s i (,)()=−−⎛⎜⎜
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value also is taken from experience or tables. The input values for cover depth and chloride diffusion coefficient are varied until the calculation indicates that corrosion initiation is
postponed until the end of the desired rvice life period. In a later stage, concrete producers must prove that their product can meet the required diffusion coefficient. However, this type of calculation is deterministic and conquently produces the mean time-to-corrosion initiation.
The DuraCrete rvice life design method includes taking into account the scatter and
distribution type of the input variables and the required reliability (or probability of failure) of the result. Generally, the accepted probability of failure at the end of the rvice life will be a few percent. Taking into account the stochastic character of the variables, full probabilistic and mi-probabilistic calculations are possible, either using statistical parameters for all variables or by using (partial) safety factors. Various DuraCrete reports provide all such information. In this paper, we will not go into detail of the probabilistic calculations.
In the past few years, (mi-)probabilistic calculations have been made taking into account that exceeding the limit state “corrosion initiation” is a rviceability limit state (SLS), with an
associated target failure probability of a few percent (reliability index β=1.8). This approach has been ud for the rvice life design of the Westerschelde Tunnel [Siemes et al. 1998, Gehlen 2000], the Groene Hart Tunnel and other structures in the High Speed railway Line (HSL) in The Netherlands.如何自学cad
DuraCrete explicitly states that the same methodology can be ud for asssment of existing structures [DuraCrete R17, 2000]. Application of the model to practical cas, however, is
relatively new. For existing structures some of the input parameters are different. For example, one of the input parameters for the model is the measured value of the diffusion coefficient of 28-days old
concrete. It is not possible to measure this value on concrete that is already 20 years old. On the other hand, the chloride surface content and the cover depth and their statistical distribution can be established experimentally; hence their values can be assd
experimentally and do not have to be assumed like in the design stage.
The DuMaCon study aimed to collect data on existing marine structures, carry out model
calculations and to validate and/or modify the model.
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3 Field investigations
3.1 Structures
Six structures were lected for field investigations that were thought reprentative for a larger group of structures. Criteria were age, cement type, production (cast in situ or prefabricated),availability and interest of the owner in the study. Some characteristics of the structures are described in Table 1. On each of the structures, one to six test areas were investigated in detail.Table 1: Characteristics of the investigated structures
All structures are located on the coast or the estuaries and harbours of the Southwestern part of The Netherlands. Historical records were studied in preparation of the inspections. The amount of information available regarding the composition and production of concrete was quite low.In most cas, relevant information such as cement content, water-to-cement ratio and curing was not well documented.
The Pier at Scheveningen is a bridge type structure, with a promenade deck compod of
precast cross beams and precast slabs, supported by precast piles (not investigated), with some parts of the deck cast in situ. Marine exposure due to waves splashing occurs on the underside of the deck, which is between 5 and 11 m above mean a level; the top surface of the deck is protected by a (later) building and was not investigated. Two test areas were located on slabs,two on beams and two on the cast in situ deck. A schematic is shown in Figure 1.Structure
Year of construction Cement type Production Pier Scheveningen 1960Portland cement,
Precast Blast furnace slag
Cast in situ cement
Discharge sluice 1960Blast furnace slag
毫不可惜Cast in situ Haringvliet cement
Quay wall 1968Blast furnace slag
Cast in situ Calandkanaal cement
Quay wall 1973Blast furnace slag
Cast in situ Hartelhaven cement
Quay wall 1982Blast furnace slag
Cast in situ Europahaven cement
Eastern Scheldt 1980-1984Blast furnace slag关于母亲的故事
Precast in field plant,Storm Surge Barrier cement cast in situ
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