The best example of a case study of the damage caused to concrete as a result of a fire is the study triggered by the Channel Tunnel fire in November, Two major papers have been published which present the main findings of an investigation of the fire contracted to LCPC, Paris which destroyed a part of the concrete tunnel rings by thermal spalling.
The first paper by reports on the development of a material model for the in situ behavior of rapidly heated concrete, which can account explicitly for the dehydration of concrete and its cross-effects with deformation chemomechanical couplings and temperature chemothermal couplings. In the second paper by , the model was used in a finite element analysis of the tunnel rings which had been exposed to the fire.
The second paper by describes the fire in the tunnel as follows: The fire caused severe damage to the tunnel rings by thermal spalling on a length of a few hundred meters.
Non-Destructive Testing Methods , 4. In the finite element simulation, instead of following this relation directly, the value of D cr for a given w was allowed to vary within a certain range. The factors affecting carbonization are: Buy and read on Free Kindle Apps. The reason for freeze-thaw damage is that: Feedback If you need help or have a question for Customer Service, contact us.
In some parts, the entire tunnel ring up to the chalk substratum was destroyed by spalling. Furthermore, the fact that the spalling occurred over some few hundred meters with regular spalling depth means—from a statistical point of view—that the observed damage patterns may be considered as representative of the in situ concrete behavior. Full details of this case study are reported in the papers and space does not allow further discussion here.
During the s concern was expressed regarding the performance of HSC in a fire. It was considered by some that the steam formed during heating would be contained by the impermeability of the concrete, leading to tensile stresses that would cause spalling of the concrete. Lennon and Clayton carried out fire tests at the BRE on high- strength, unreinforced concrete columns. It was concluded that the inclusion of the polypropylene fibers did not have a significant effect on the maximum temperature obtained before collapse, but did substantially increase the temperature at which first cracking occurred and reduced or eliminated spalling.
Further research is required in this area to evaluate the benefit of adding polypropylene fibers into NSC.
Non-Destructive Testing Methods , Concrete structures have vast dimensions and, usually, numerous sensors are required to monitor an extended structural component. Fibre-optic sensors are very well suited for this measurement task because several sensors or sensor sections can be designed along the naturally extended optical fibre. In addition, the fibre itself can act as a sensor at any point.
This special feature enables extended distributed sensors to be made using only one optical fibre with one lightweight optical lead cable. Different sensor types, such as short-gauge-length or long-gauge-length Fig. Distributed fibre sensors have the very desirable feature of being able to measure not only a physical quantity affecting the fibre e. The scan frequency of such sensors is limited to a few Hz or less. The complement to long sensor fibres is short-gauge-length sensors.
Such local fibre-optic sensors sometimes known as point sensors are, in the case of strain sensors, similar to resistive strain gauges, or to pressure or inclination sensors tilt meters. Such local fibre sensors can also be used as pH sensors or humidity sensors see next section. This group of sensors can be divided into three categories: Also, if components such as connectors or cables have to be exchanged, zero-point data loss does not occur.
This type of sensor provides referenced measurement data, can be embedded or surface-mounted, and is used for static and dynamic measurements with gauge lengths of up to several metres. One example of these SOFO sensors as an integral part of a complex monitoring system is their use in the new Berlin main station, Lehrter Bahnhof. When light propagates through an optical fibre, it is scattered by spontaneous fluctuations in the dielectric constant of the light-guiding material. There are molecular vibrations called Raman scattering , propagating density fluctuations stimulated Brillouin scattering , and non-propagating entropy fluctuations Rayleigh scattering.
Brillouin scattering depends on both temperature and strain, whereas Raman scattering only depends on temperature. Referring to these effects, external perturbations the measurand can then be scanned and evaluated by determining the intensity and the spectral composition of small parts of a backscattered light pulse.
In order to measure only temperature influences distribution along a fibre, the Raman backscattering method is recommended.
Further explanations can be found in the COST fibre-optic sensor guideline. Because the Raman scattering depends only on physical constants and on parameters of the light transmitted into the fibre, absolute temperature values can be obtained directly from the backscattered signal.
Owing to different qualities of the fibre material used, a calibration function must be determined before measurement starts. This method of distributed temperature measurement is used for continuous detection of leakage at pipelines and vessels as well as for mass concrete structures such as concrete gravity dams to monitor the temperature development after pouring and owing to the hydration reaction. Examples for quasi-distributed and distributed strain sensors based on Rayleigh and Brillouin backscattering are given in the next section.
The best-known short-gauge-length fibre-optic sensor is the fibre Bragg grating FBG sensor. They are commercially available; however, for some specific applications, modified types are used. Such sensors can be embedded into materials to measure deformations inside the material. After a short technical description of FBG and FPI sensors, examples of application in the next sections will make these differences clear.
Cases of general oxygen depletion are therefore unlikely to occur except in circumstances where the material is fully immersed in water, buried in saturated soil or surrounded by similar waterlogged media. The effects of progressive reduction in the general availability of oxygen on the cathodic polarisation curves for the reaction:.
Also reproduced in this figure, known as an Evans diagram, is a portion of the anodic polarisation curve for steel in alkaline concrete cf. Assuming no external electrical power source is being used to supply current to a steel reinforcing bar i. The latter usage avoids creating any misimpression about the practical significance of the condition, which can be readily converted to passivity in the event that the oxygen availability at the steel surface increases.
It should be emphasised that the relatively benign effects of general oxygen depletion described above must be distinguished from the more damaging consequences of local oxygen depletion in corrosion macrocells , which can give rise to a form of localised corrosion known as black or green rust.
Under these circumstances, the normal formation of expansive brown hydrated ferric oxide is suppressed and the anodic dissolution of the metal can yield ferrous compounds, which are green initially but darken rapidly when exposed to air. This can cause substantial loss of metal cross section to occur without showing the usual signs of surface cracking and rust staining of the concrete.
Particularly in structurally sensitive locations, it is therefore important to avoid creating inappropriate construction details that are likely to encourage localised ponding of oxygen-denuded saline water in contact with reinforcing bars. Examples of such undesirable details include badly designed and executed joints or failed waterproofing membranes. In Building Materials in Civil Engineering , Concrete structures require not only the design strength to guarantee buildings to bare safe loads, but also durability to insure that concrete can maintain its properties in the natural environment and using conditions for a long term.
Here are some common issues concerning durability. The impermeability of concrete refers to the property of concrete that cannot be pervaded by water, oil and other liquids with pressures. It plays an important role in the durability of concrete. Moreover, it also directly affects the frost-resistance and anti-corrosion of concrete. Impermeability is expressed by the anti-permeability level, P, which is determined by the maximum water pressure MPa borne by standard specimen at 28d age through required tests when it is impermeable. The impermeability levels include P6, P8, P10, P12…, which means that concrete can resist the water pressure of 0.
The key to improve the impermeability of concrete is to enhance density and improve the inner pores structures of concrete. There are specific measures to reduce the water-cement ratio, adopt water-reducing agent, add air-entraining agent, select dense, clean and good aggregates, and strengthen curing. The frost resistance of concrete is the property that concrete resist the role of freeze-thaw cycles without damage when it suffers frost in the saturated state. The reason for freeze-thaw damage is that: Frost resistance is indicated by frost-resistance level.
Berkeley-- surface treatments, J. Part 2 Case studies: Gardner-- highway bridges, K. Boam-- railway structures, J.
Dixon-- concrete pavements, G. Northcott-- marine structures, M. Nielsen Book Data Publisher's Summary In the rapid expansion of the construction industry during the s basic advice regarding cover to reinforcement and impermeability of concrete to moisture for subsequent durability often went unheeded.
Today the UK is seeing the result - a 6 million per annum programme of concrete repair - and similar effects are being observed worldwide.
The book draws together experts from the fields of concrete durability, repair and protection to provide a state-of-the-art review on current thinking, materials and techniques. The basic deterioration mechanisms and methods for the site investigation of distressed concrete structures are initially considered. Find a library where document is available. Dixon, J F Publication Date: Reinforced concrete ; Reinforcement in mater ; Test method ;