Durability of Reinforced Concrete Structures

Paul Chess, Warren Green

• Publisher: CRC Press
• Year: 2019
• Print ISBN: 9781032176086
• eBook ISBN: 9781000767261
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

Reinforced concrete structures corrode as they age, with significant financial implications, but it is not immediately clear why some are more durable than others. This book looks at the mechanisms for corrosion and how corrosion engineering can be used for these problems to be minimized in future projects. Several different examples of reinforced concrete structures with corrosion problems are described and the various life enhancement solutions considered and applied are discussed. The book includes a chapter on the effectiveness of corrosion monitoring techniques and questions why the reality is at odds with current theory and standards.

Specialist contractors, consultants and owners of corrosion damaged structures will find this an extremely useful resource. It will also be a valuable reference for students at postgraduate level.

1

The Problem

1.1 Reinforced Concrete

Imagine the world without steel-reinforced concrete structures and buildings. Imagine our primitive existence without the wonder and delight of steel-reinforced concrete.

Concrete itself is not a modern material. It has been in use for thousands of years. Calcined impure gypsum was used as a cementing material by the ancient Egyptians (Neville, 2011). Since Roman times lime-based hydraulic cements (those set by chemical reaction with water) have been in use and such ancient mortars still survive today, including examples in the Pantheon in Rome and aqueducts in France. The word concrete comes from the Latin concretus, which means ‘mixed together’ or compounded (Macdonald, 2003).

The Romans ground together lime and volcanic ash or finely ground burnt clay tiles. The active silica and alumina in the ash or tiles combined with the lime to produce what became known as ‘pozzolanic cement’ from the name of the village of Pozzuoli, near Vesuvius, where the volcanic ash was first found (Neville, 2011). The name ‘pozzolanic cement’ is used to this day to describe cements obtained simply by the grinding of natural materials at normal temperature (Neville, 2011).

The Middle Ages brought a decline in the use of hydraulic mortars until the 18th century when John Smeaton in 1756 used a mortar produced from pozzolana mixed with a limestone containing a considerable proportion of clayey material to rebuild Eddystone Lighthouse, off the Cornish coast of England (Neville, 2011).

Lime concrete was the precursor to modern concrete and was made with natural hydraulic cements (Macdonald, 2003). The first modern concrete was made with Portland cement as patented by Joseph Aspdin in 1824, a Leeds bricklayer, stonemason and builder (Neville, 2011). The name ‘Portland cement’ is because of its resemblance to stone quarried near Portland in Dorset, England.

The name ‘Portland cement’ has remained throughout the world to this day to describe a cement obtained by mixing together calcareous and argillaceous, or other silica-, alumina- and iron oxide-bearing materials in appropriate proportions, burning them at clinkering temperature (around 1,500°C), cooling, before grinding with about 5% gypsum (calcium sulphate).

Concrete then basically consists of mineral aggregate held together by a cement paste, so if we consider a normal mix it will consist of cement, sand (fine aggregate), coarse aggregate and water (and often other admixtures) which are mixed together to form eventually a hard, strong material.

The principles of reinforcing concrete to provide both tensile and compressive strength were understood in ancient times. However, it was not until the 19th century that a number of European and North American inventors developed and patented reinforcing methods. This resulted in the widespread introduction of a fully fledged concrete industry (Macdonald, 2003).

Concrete is strong in compression but weak in tension and shear and so much of the concrete is reinforced, usually with steel. The steel reinforcement can take the form of conventional carbon steel (black steel), prestressing steel, post-tensioned steel and steel fibres, and its widespread utility is primarily due to the fact it combines the best features of concrete and steel. The properties of these two materials are compared in Table 1.1.

TABLE 1.1 Properties of Steel and Concrete

Property	Concrete	Steel
Strength in tension	Poor	Good
Strength in compression	Good	Buckling can occur
Strength in shear	Fair	Good
Durability	Good	Corrodes if unprotected
Fire resistance	Good	Poor, low strength at high temperatures

Since its inception in the mid-19th century, steel-reinforced concrete has become the most widely used construction material in the world (and the second most-used material by mankind after water). Reinforced concrete is a wonderful composite material. It combines the best features of concrete and steel. Concrete gives it strength in compression while the steel gives it strength in tension and shear.

1.2 Steel Reinforcement

Rebar (short for reinforcing bar), collectively known as reinforcing steel, reinforcement steel or steel reinforcement, is a steel bar or mesh of steel wires.

The most common type of rebar is carbon steel.

There is also other steel reinforcement including: galvanised steel reinforcement; stainless steel reinforcement; and metallic-clad steel reinforcement for use in reinforced concrete construction.

Reinforced concrete construction using other alternate reinforcement such as bronze, fibre-reinforced polymer (FRP) reinforcement or basalt, is also available in limited quantities.

1.3 Prestressed and Post‑Tensioned Concrete

Prestressed concrete is composed of high-strength concrete and high-strength steel. The concrete is ‘prestressed’ by being placed under compression by the tensioning of high-strength ‘steel tendons’ located within or adjacent to the concrete volume.

Prestressed concrete is used in a wide range of buildings and civil structures where its improved performance can allow longer spans, reduced structural thicknesses, and material savings compared to conventionally reinforced concrete. Applications can include high-rise buildings, residential buildings, foundation systems, bridge and dam structures, silos and tanks, industrial pavements and nuclear containment structures.

Prestressed concrete members can be divided into two basic types: pre-tensioned and post-tensioned.

Pre-tensioned concrete is where the high-strength steel tendons are tensioned prior to the surrounding concrete being cast. The concrete bonds to the tendons as it cures, following which the end-anchoring of the tendons is released, and the tendon tension forces are transferred to the concrete as compression by static friction.

Post-tensioned concrete is where the high-strength steel tendons are tensioned after the surrounding concrete has been cast. The tendons are not placed in direct contact with the concrete, but are encapsulated within a protective duct typically constructed from plastic or galvanised steel materials. There are then two main types of tendon encapsulation systems: those where the tendons are subsequently bonded to the surrounding concrete by internal grouting of the duct after stressing (bonded post-tensioning); and those where the tendons are permanently de-bonded from the surrounding concrete, usually by means of a greased sheath over the tendon strands (unbonded post-tensioning).

1.4 Concerns about Durability of Reinforced Concrete

The environments that we place our reinforced concrete structures in mean that various deterioration processes can affect their in-service durability, leading to loss of functionality, unplanned maintenance/remediation/replacement, and in the worst cases, loss of structural integrity and resultant safety risks.

Deterioration of concrete can be separated into two broad category types: (i) degradation of concrete, and (ii) corrosion of steel reinforcement. Figure 1.1 summarises the causes of (i) and (ii) which can include one or more or mechanical, physical, structural, chemical, biological and reinforcement corrosion mechanisms.

FIGURE 1.1 Summary of concrete deterioration types. (Adapted from Bertolini et al., 2004.)

Amongst these, the most common cause of deterioration of reinforced concrete structures is corrosion of conventional carbon steel (black steel), prestressing steel and post-tensioned steel reinforcement.

1.5 Corrosion of Steel Reinforcement in Concrete

1.5.1 Causes

The passivity (thin protective oxide film) provided to steel reinforcement by the alkaline environment of concrete may be lost or locally compromised ...

Table of contents

• Cover
• Half-Title
• Title
• Copyright
• Contents
• Authors
• 1 The Problem
• 2 The Corrosion Process in Reinforced Concrete: The State of the Art
• 3 Monitoring Corrosion and Why Most of the Current NDT Techniques Are Flawed
• 4 Solutions for New Structures
• 5 Maximising Service Life with Minimal Capital Expenditure
• 6 Advantages and Disadvantages of Different Remediation Procedures
• 7 Examples of Damage and Remediation with Different Structures
• Index