Delatte Failure, Distress and Repair of Concrete Structures

1. Auflage 2009
ISBN: 978-1-84569-703-7
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 352 Seiten

Reihe: Woodhead Publishing Series in Civil and Structural Engineering

Understanding and recognising failure mechanisms in concrete is a fundamental pre-requisite to determining the type of repair, or whether a repair is feasible. This title provides a review of concrete deterioration and damage, as well as looking at the problem of defects in concrete. It also discusses condition assessment and repair techniques.Part one discusses failure mechanisms in concrete and covers topics such as causes and mechanisms of deterioration in reinforced concrete, types of damage in concrete structures, types and causes of cracking and condition assessment of concrete structures. Part two reviews the repair of concrete structures with coverage of themes such as standards and guidelines for repairing concrete structures, methods of crack repair, repair materials, bonded concrete overlays, repairing and retrofitting concrete structures with fiber-reinforced polymers, patching deteriorated concrete structures and durability of repaired concrete.With its distinguished editor and international team of contributors, Failure and repair of concrete structures is a standard reference for civil engineers, architects and anyone working in the construction sector, as well as those concerned with ensuring the safety of concrete structures. - Provides a review of concrete deterioration and damage - Discusses condition assessment and repair techniques, standards and guidelines

Autoren/Hrsg.

Delatte, Norb

Leseproben

2 Types of damage in concrete structures
K. Kovler    National Building Research Institute, Israel V. Chernov    Ashdod Port Company Ltd, Israel Abstract
Different types and sources of damage in concrete structures are dealt with and the classification of damage into different categories is discussed. The simple and convenient classification of damage by sources, which includes chemical attack, fire, overloading by static and dynamic (impact and earthquakes) loads, and, finally, malicious damage, is suggested for the practicing engineer. Examples of different kinds of damage are given and discussed. Finally, two case studies, which help the practicing engineer in making the damage investigation and developing recommendations for rehabilitation of structures and prevention of damage in future, are reported. Key words reinforced concrete damage chemical attack fire overloading impact earthquakes malicious damage 2.1 Introduction
When one starts investigating the damage or failure of a given concrete structure, the reasons for such damage are not always clear. The documentation on the structure and the history of its manufacture and maintenance are not always available. At the same time, the visual inspection and appearance of the structure can provide much valuable information about the type of loading that resulted in the damage and help to understand what happened. Within the same type, each loading situation is unique. Like a detective trying to gather an overall picture of the crime by analyzing fingerprints of persons involved and other evidence, the experienced engineer can recreate and tell the story of the given damage/failure with a certain degree of probability. The present chapter deals with different types and sources of damage in concrete structures. The classification of damage into different categories is discussed. The simple and convenient classification of damage by sources, which includes chemical attack, fire, overloading by static and dynamic (impact and earthquakes) loads and, finally, malicious damage, is suggested for the practicing engineer. Examples of different kinds of damage are given and discussed. Finally, two case studies, which help the practicing engineer in carrying out the damage investigation and developing recommendations for rehabilitation of structures and preventing damage in future, are reported. 2.2 Types of concrete damage
Damage to concrete structures can be categorized in different ways. The classification can be made in terms of damage types, causes, mechanisms of attack, frequency of defects, kinds of deficient structures, financial loss due to different defects, amount and extent of repair measures, etc. Let us consider first the type of concrete damage, and then its causes and mechanisms. Concrete damage can be of the following main types: 1. scaling; 2. spalling; 3. curling; 4. cracking. This classification is based on the simple visual appearance of concrete defects. At the same time, different types of damage described shortly hereafter correspond to causes or mechanisms of concrete damage, and can therefore can be considered as their ‘fingerprints.’ For example, scaling and spalling of concrete are characteristic of freezing–thawing cycles and fires, respectively. Scaling is one of the leading complaints of many homeowners and probably the easiest defect of a concrete surface to avoid. When concrete scales, for example, as a result of freezing and thawing cycles, the finished surface flakes or peels off. Generally it starts as localized small patches, which later may merge and extend to expose large areas. Light scaling does not expose the coarse aggregate. Moderate scaling exposes the aggregate and may involve loss of up to 2.5–10 mm of the surface mortar. In severe scaling, more surface material is lost and the aggregate is clearly exposed and stands out. Scaling is primarily found in outside concrete flat work such as sidewalks, patios, and driveways. In most cases it is blamed on de-icing salts used on the concrete during the winter months. Despite these assertions, the truth is that properly specified, placed, and cured concrete should be able to endure the effects of typical de-icing agents. Spalling can be described as the breaking of layers or pieces of concrete from the surface of a structural element. Spalling occurs often when concrete is exposed to the high and rapidly rising temperatures experienced in fires; however, it can be also a result of other mechanisms, such as steel corrosion or improper design/technology of pretensioning steel for prestressed concrete members. There are four main types of concrete spalling: • surface spalling – affects aggregate on the concrete surface, whereby concrete fragments typically up to 20 mm in diameter become detached; • corner break-off or sloughing off – this tends to occur in the later stages of a fire and affects more vulnerable concrete on wall corners where it is heated on two planes; • explosive spalling – early rapid heat rise forcibly separates pieces of concrete at high pressure, with an ‘explosive’ effect; this form of spalling is considered as the most dangerous one; • corrosion spalling – when reinforcing bars rust, their volume increases and steel cover becomes detached. Curling is the upward or downward bending of the edges of a concrete element (usually slab or beam), giving the concrete member a cupped shape. The curled edges are unsupported by the base, making them susceptible to cracking under heavy loads. Cracking is a path of the (local) separation of a structural element or material into two, or more, pieces under the action of stress. Cracking, like corrosion of reinforcing steel, is not commonly a cause of damage to concrete. Instead, cracking is a symptom of damage created by some other cause. For example, cracking can be the result of one or a combination of factors, such as drying shrinkage, thermal contraction, restraint (external or internal) to shortening, subgrade settlement, and applied loads. While cracks may develop in concrete for a variety of causes, the underlying principle is the relatively low tensile strength of concrete. Cracking can be considered as a separate type of damage, but it also accompanies the rest of the damage types listed before: scaling, spalling, and curling. A comprehensive state-of-the-art report1 reviews the causes of cracking, discusses various tests that can be performed to assess the susceptibility of a material to cracking, and provides several case studies. In particular, the following classification of cracks in concrete is suggested (Table 2.1). Table 2.1 Classification of cracking types 1 Plastic settlement Over and aligned with reinforcement, subsidence under reinforcing bars Poor mixture design leading to excessive bleeding, excessive vibrations 10 min to 3 h Plastic shrinkage Diagonal or random Excessive early evaporation 30 min to 6 h Thermal expansion and contraction Transverse Excessive heat generation, excessive temperature gradients 1 day to 2–3 weeks Drying shrinkage Transverse, pattern or map cracking Excessive mixture water, inefficient joints, large joint spacings Weeks to months Freezing and thawing Parallel to the surface of concrete Lack of proper air void system, non- durable coarse aggregate After one or more winters Corrosion of reinforcement Over reinforcement Inadequate cover, ingress of sufficient chloride More than 2 years Alkali–aggregate reaction material Pattern and longitudinal cracks parallel to the least restrained side Reactive aggregate plus alkali hydroxides plus moisture Typically more than 5 years, but weeks with a highly reactive material Sulfate attack Pattern Internal or external sulfates promoting the formation of ettringite 1–5 years Cracks in concrete are discussed in detail in the next chapter of the book. Another classification of damage types of concrete is available in the work by Al-Mandil et al.,2 which is based on detailed in-situ and laboratory investigations of girder-slab and slab-type decks. This paper distinguished between two main types of damage of concrete: 1. structural damage, such as that resulting from overloading of the bridges; and 2. material damage, such as that resulting from lack of quality control and poor...