2.1-i

Table of Contents

Chapter 2 ..................................................................................................................................................

Bridge Materials 

 2.1 Timber ............................................................................................ 2.1.1 

 

 2.1.1 Introduction............................................................................... 2.1.1 

 

 2.1.2 Basic Shapes Used in Bridge Construction............................... 2.1.2 

 Round ................................................................................. 2.1.2 

 Rectangular......................................................................... 2.1.2 

 Built-up Shapes .................................................................. 2.1.2 

 

 2.1.3 Properties of Timber ................................................................. 2.1.3 

 Physical Properties ............................................................. 2.1.4 

 Timber Classification................................................... 2.1.4 

 Timber Anatomy.......................................................... 2.1.4 

 Growth Features........................................................... 2.1.5 

 Moisture Content ......................................................... 2.1.5 

 Mechanical Properties ........................................................ 2.1.5 

 Orthotropic Behavior ................................................... 2.1.6 

 Fatigue Characteristics................................................. 2.1.6 

 Impact Resistance ........................................................ 2.1.6 

 Creep Characteristics ................................................... 2.1.7 

 2.1.4 Timber Grading......................................................................... 2.1.7 

 Sawn Lumber ..................................................................... 2.1.7 

 Visual Grading............................................................. 2.1.7 

 Mechanical Stress Grading .......................................... 2.1.8 

 Glued-Laminated Lumber .................................................. 2.1.8 

 

 2.1.5 Types and Causes of Timber Deterioration .............................. 2.1.8 

 Natural Defects................................................................... 2.1.8 

 Fungi................................................................................... 2.1.9 

 Insects............................................................................... 2.1.13 

 Termites ..................................................................... 2.1.13 

 Powder-post Beetles or Lyctus Beetles...................... 2.1.14 
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1-ii

 Carpenter Ants ........................................................... 2.1.15 

 Caddisflies.................................................................. 2.1.16 

 Marine Borers................................................................... 2.1.16 

 Chemical Attack............................................................... 2.1.18 

 Acids .......................................................................... 2.1.18 

 Bases or Alkalis ......................................................... 2.1.18 

 Other Types and Sources of Deterioration ....................... 2.1.18 

 Delaminations ............................................................ 2.1.18 

 Loose Connections..................................................... 2.1.18 

 Surface Depressions................................................... 2.1.19 

 Fire............................................................................. 2.1.19 

 Impact or Collisions................................................... 2.1.21 

 Wear, Abrasion and Mechanical Wear ...................... 2.1.21 

 Overstress................................................................... 2.1.22 

 Weathering or Warping.............................................. 2.1.22 

 Protective Coating Failure................................................ 2.1.23 

 2.1.6 Protective Systems .................................................................. 2.1.23 

 Types and Characteristics of Wood Protectants............... 2.1.24 

 Water Repellents........................................................ 2.1.24 

 Preservatives .............................................................. 2.1.24 

 Fire Retardants........................................................... 2.1.25 

 Paint ........................................................................... 2.1.26 

 

 2.1.7 Inspection Procedures for Timber........................................... 2.1.27 

 Visual Examination .......................................................... 2.1.27 

 Physical Examination ....................................................... 2.1.27 

 Pick or Penetration Test............................................. 2.1.27 

 Timber Boring and Drilling Locations....................... 2.1.28 

 Protective Coatings .................................................... 2.1.29 

 Paint Adhesion........................................................... 2.1.29 

 Paint Dry Film Thickness .......................................... 2.1.29 

 Repainting.................................................................. 2.1.30 

 Advanced Inspection Techniques..................................... 2.1.30 
2.1.iii 
2.1.iv 
2.1.v 
2.1.vi 

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2.1.1

Section 2 

Bridge Materials 

Topic 2.1 Timber 

2.1.1 

Introduction Approximately 7% of the bridges listed in the National Bridge Inventory (NBI) are 

classified as timber bridges. Another 7% of the total have a timber deck supported 

by a steel superstructure. Many of these bridges are very old, but the use of timber 

structures is gaining new popularity with the use of engineered wood products. 

(see Figure 2.1.1). To preserve and maintain them, it is important that the bridge 

inspector understand the basic characteristics of wood. Timber Bridges Design, 

Construction, Inspection and Maintenance August 1992 manual published by the 

United States Department of Agriculture, Forest Service is an excellent reference

to supplement timber information in this manual. To obtain information on this 

manual, visit the National Wood in Transportation website: www.fs.fed.us/na/wit. 

To download publications in PDF format go to 

http://spfnic.fs.fed.us/wit/WITpubs/PubSearch.cfm. 

 

Figure 2.1.1 Glued-laminated Modern Timber Bridge 
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1.2 

2.1.2 

Basic Shapes Used 

in Bridge 

Construction 

Depending on the required structural capacities and geometric constraints, wood 

can be cut into various shapes. 

Round Because sawmills were not created yet, most early timber bridge members were 

made out of solid round logs. Logs were generally used as beams, or stacked and 

used as abutments and foundations. In some parks, bridges that utilize logs as 

structural members are still in service. Round timber members have been used as 

piles driven into the ground or waterway bed. Logs have also been used as 

retaining devices for embankment material. 

Rectangular Once sawmill operations gained prominence, rectangular timber members became 

commonplace. Rectangular timber members were easier to connect together due 

to the flat sides and can be used for decking, superstructure beams, arches and 

truss elements, curbs or railings, and retaining devices (see Figure 2.1.2). 

Plank

(12" dia.)

300 mm (6" - 10")

150 mm – 250 mm

(10" - 14")

250 mm - 350 mm

Pile Beam

(12")

300 mm

(2")

50 mm

 

Figure 2.1.2 Timber Shapes 

Built-up Shapes Modern timber bridge members are fabricated from basic rectangular shapes to 

create built-up shapes, which perform at high capacities. A fundamental example 

of this is the deck-shaped beam. Two other common examples are T-shaped and 

box-shaped beams (see Figure 2.1.3). Using glue-laminate technology and stress 

timber design, these shapes enable modern timber bridges to carry current legal 

loads. 

Refer to Section 6 for further information on timber superstructures and Topic 5.1 

for timber decks. 
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1.3 

T-Shaped Beam Box-Shaped Beam

Rectangle Shaped Beam Deck-Shaped Beam

 

Figure 2.1.3 Built-up Timber Shapes 

2.1.3 

Properties of 

Timber 

Because of its physical characteristics, wood is in many ways an excellent 

engineering material for use in bridges. Perhaps foremost is that it is a renewable 

resource. In addition, wood is: 

3⁄4 Strong, with a high strength to weight ratio 

3⁄4 Economical 

3⁄4 Aesthetically pleasing 

3⁄4 Readily available in many locations 

3⁄4 Easy to fabricate and construct 

3⁄4 Resistant to deicing agents 

3⁄4 Resistant to damage from freezing and thawing 

3⁄4 Able to sustain overloads for short periods of time (shock resistant) 

However, wood also has some negative properties: 

3⁄4 Excessive creep under sustained loads 

3⁄4 Vulnerable to insect attack 

3⁄4 Vulnerable to fire 
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1.4 

These characteristics stem from the unique physical and mechanical properties of 

wood, which vary with the species and grade of the timber. 

Physical Properties There are four basic physical properties that define timber behavior. These 

properties are timber classification, anatomy, growth features, and moisture 

content. 

Timber Classification 

Wood may be classified as hardwood or softwood. Hardwoods have broad leaves 

and lose their leaves at the end of each growing season. Softwoods, or conifers, 

have needle-like or scale-like leaves and are evergreens. The terms "hardwood" 

and "softwood" are misleading because they do not necessarily indicate the 

hardness or softness of the wood. Some hardwoods are softer than certain 

softwoods and vice versa. 

Timber Anatomy 

Wood is a non-homogeneous material. Wood, although an extremely complex 

organic material, has dominant and fundamental patterns to its cell structure. 

Some of the physical properties of this cell structure include (see Figure 2.1.4): 

3⁄4 Hollow cell composition - cell walls consist of cellulose and lignin, and 

are formed in an oval or rectangular shape which accounts for the high 

strength-to-weight ratio of wood; wood with thick cell walls is dense and 

strong; lignin bonds the cells together 

3⁄4 Growth rings - revealed in the cross section of a tree are distinct rings of 

wood produced during a tree’s growing season. One annual ring is 

composed of a ring of earlywood or springwood (light in color, cells have 

thin walls and large diameter) and a ring of latewood or summerwood 

(dark in color, cells have thick walls and small diameter). The rings can 

be easily seen in some trees (Douglas fir and southern pine) and exhibit 

little color difference in other species (spruces and true firs). 

3⁄4 Sapwood - the active, outer part of the tree that conducts sap and stores 

food throughout the tree; is generally permeable and easier to treat with 

preservatives; sapwood is of lighter color than heartwood 

3⁄4 Heartwood - the inactive, inner part of the tree which serves to support the 

tree; may be resistant to decay due to toxic materials deposited in the 

heartwood cells; usually of darker color than sapwood 

3⁄4 Wood rays - groups of cells, running from the center of the tree 

horizontally to the bark, which are responsible for cross grain strength 

3⁄4 Grain - the wood fibers oriented along the long axis of logs and timbers; 

the direction of greatest strength 
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1.5 

 

Figure 2.1.4 Anatomy of Timber 

 Growth Features 

A variety of growth features adversely affect the strength of wood. Some of these 

features include: 

3⁄4 Knots and knot holes - due to intergrown limbs and associated grain 

deviation 

3⁄4 Sloping grain - caused by the normal taper of a tree or by sawing in a 

direction other than parallel to the grain 

3⁄4 Splits, checks, and shakes - separation of the cells along the grain, 

primarily due to rapid or uneven drying and differential shrinkage in the 

radial and tangential directions during seasoning; checks and splits occur 

across the growth rings; a shake is a type of check which occurs between 

the growth rings, peculiar to a few species 

3⁄4 Reaction wood - a type of abnormal wood that is formed in leaning trees; 

the pith is off center; the wood is gelatinous and displays cross grain 

shrinkage checks when seasoned 

Moisture Content 

Moisture content affects wood. It causes dimensional instability and fluctuations 

of weight and affects the strength and decay resistance of wood. It is most 

desirable for wood to have the least moisture content as is possible. This is done 

naturally over time (seasoning) or using kiln drying. 

Mechanical Properties There are four basic mechanical properties that define timber behavior. These 

Earlywood 

Resin 

Canal 

Latewood

Ray 

Sapwood 

Heartwood 

Pith 

Bark
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1.6 

properties are orthotropic behavior, fatigue characteristics, impact resistance and 

creep characteristics. 

Orthotropic Behavior 

Wood is considered a non-homogeneous and an orthotropic material. It is nonhomogeneous because of the random occurrences of knots, splits, checks, and the 

variance in cell size and shape. It is orthotropic because wood has mechanical 

properties that are unique to its three principal axes of anatomical symmetry 

(longitudinal, radial, and tangential). This orthotropic behavior is due to the 

orientation of the cell fibers in wood (see Figure 2.1.5). 

Figure 2.1.5 Three Principal Axes of Wood 

As a result of its orthotropy, wood has three distinct sets of strength properties. 

Because timber members are longitudinal sections of wood, strength properties are 

commonly defined for the longitudinal axis. American Society for Testing and 

Materials (ASTM) Standards and American Forest and Paper Association

(AF&PA) Standards are issued which present strength properties for various types 

of wood. 

Fatigue Characteristics 

Because wood is a fibrous material, it tends to be less sensitive than steel or iron to 

repeated loads. Therefore, it is somewhat fatigue resistant. The presence of knots 

and sloping grain reduces the strength of wood considerably more than does 

fatigue; therefore, fatigue is generally not a limiting factor in timber design. 

Impact Resistance 

Wood is able to sustain short-term loads of approximately twice the level it can 

bear on a permanent basis, provided the cumulative duration of such loads is 

limited. 
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1.7 

Creep Characteristics 

Creep occurs when a load is maintained on wood. That is, the initial deflection of 

the member increases with time. Green timbers may sag appreciably, if allowed to 

season under load. Initial deflection of unseasoned wood under permanent loading 

can be expected to double with the passage of time. Therefore, to accommodate 

creep, twice the initial elastic deformation is often assumed for design. Partially 

seasoned material may also creep to some extent. However, thoroughly seasoned 

wood members will exhibit little permanent increase in deflection with time. 

2.1.4 

Timber Grading Douglas fir and southern pines are the most widely used species of wood for 

bridge construction. The southern pines include several species graded and 

marketed under identical grading rules. Other species, such as western hemlock 

and eastern spruce, are suitable for bridge construction if appropriate allowable 

stresses are used. Some hardwoods are also used for bridge construction. 

Timber is given a grading so that the following can be established: 

3⁄4 Modulus of elasticity 

3⁄4 Tensile stress parallel to grain 

3⁄4 Compressive stress parallel to grain 

3⁄4 Compressive stress perpendicular to grain 

3⁄4 Shear stress parallel to grain (horizontal shear) 

3⁄4 Bending stress 

Timber used for outdoor applications needs to be designed for wet service 

condition. Refer to Timber Bridges: Design, Construction, Inspection, and 

Maintenance, Forest Service, United States Department of Agriculture. 

The ultimate strength properties of wood in the tables at the beginning of this topic 

are for air-dried wood, which is clear, straight grained, and free of strengthreducing defects. Reduction factors need to be applied to these values based on 

use. 

Preservative treatment for decay resistance does not alter the allowable stresses for 

design, provided any moisture associated with the treatment process is removed. 

Unlike steel, the elastic modulus of wood varies with the grades and species. 

Sawn Lumber The grading of sawn timber is accomplished by either a visual grading or a 

mechanical stress grading (MSR). Refer to the tables at the beginning of this 

topic. 

Visual Grading 

This type of grading is the most common and is performed by a certified lumber 

grader. The lumber grader inspects each sawn and surfaced piece of lumber. The 

individual pieces of lumber must meet particular grade description requirements in 
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1.8 

order to be classified at a certain grade. If the requirements are not met, the piece 

of sawn and surfaced lumber is compared to lower grade description requirements 

until the piece of lumber fits into the appropriate grade. Mechanical properties are 

predetermined for each grade. Therefore, once the piece of lumber has been 

graded, the mechanical properties are known. 

Mechanical Stress Grading 

Mechanical stress grading or mechanical stress rating (MSR) grades lumber by the 

relationship between the modulus of elasticity and the bending strength of lumber. 

A machine measures the bending strength and then assigns an elastic modulus. 

The grading mainly depends on the elastic modulus but can be changed by visual 

observance of edge knots, checks, shakes, splits, and warps. Mechanical stress 

grading has a different set of grading symbols than visual grading. 

Glued-Laminated 

Lumber 

Glued-laminated lumber or glulam is not graded in the same way as sawn lumber 

(see the tables at the beginning of this topic). Members have a combination 

symbol that represents the combination of lamination grades used to manufacture 

the member. The symbols are divided into two general classifications which are 

bending combinations or axial (tension or compression) combinations. The 

classifications are based in the anticipated use of the member, either in bending as 

a beam or axial combination as a column or tension member. 

Bending combinations are used for resisting bending stress caused by loads 

applied perpendicular to the wide faces of the laminations. In this case, a lower 

grade lamination is used for the center portion of the member (near the neutral 

axis) while a higher grade lamination is placed on the outside faces where bending 

stresses are higher. 

Axial combinations are used for resisting axial forces and bending stress applied 

parallel to the wide faces of the laminations. In this case, the same grade 

lamination is used throughout the member. 

2.1.5 

Types and Causes 

of Timber 

Deterioration 

Although wood is an excellent material for use in bridges, untreated wood is 

vulnerable to damage from fungi, parasites, and other sources. The untreated inner 

cores of surface treated timbers and poles are vulnerable to these predators if they 

can gain access through the outer treated shell. The degree of vulnerability varies 

with the species and grade of the timber. Bridge inspectors must be able to 

recognize the signs of the various types of damage and be able to evaluate their 

effect on the structure. 

Natural Defects Defects that form from abnormal growth or from the lumber drying process 

include (see Figure 2.1.6): 

3⁄4 Checks - separations of the wood fibers, normally occurring across or 

through the annual growth rings, and generally parallel to the grain 

direction 

3⁄4 Splits - similar to checks except the separations of the wood fibers extend 

completely through the piece of wood; a split is also known as a through 

check 
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1.9 

3⁄4 Shakes - separations along the grain which occur between the annual 

growth rings 

3⁄4 Knots – inter grown limbs which cause the separation of wood fibers 

These four defects provide openings for decay to begin and in some cases indicate 

reduced strength in the member when the defect is in an advanced state. 

surface

check

end check

split (thru)

check

shake

check

(heart)

knot

Figure 2.1.6 Natural Timber Defects 

Fungi Decay is the primary cause of timber bridge replacement. Decay is the process of 

living fungi, which are plants feeding on the cell walls of wood (see Figure 2.1.7). 

The initial process is started by the deposition of spores or microscopic seeds. 

Fruiting bodies (e.g., mushrooms and conks) produce these spores by the billions. 

The spores are distributed by wind, water, or insects. 
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1.10 

 

Figure 2.1.7 Decay of Wood by Fungi 

Spores that survive and experience favorable growth conditions can penetrate 

timber members in a few weeks. Favorable conditions for fungi to grow can only 

occur when these four requirements exist: 

3⁄4 Oxygen - Sufficient oxygen must be available for the fungi to breathe. A 

minimal amount of free oxygen can sustain them in a dormant state, but at 

least 20 percent of the volume of wood must be occupied by air for fungi 

to become active. The air we breathe contains about 21 percent oxygen. 

Absence of oxygen in bridge members would only occur in piling or bents 

placed below the permanent low water elevation or water table, or buried 

in the ground. 

3⁄4 Temperature - A favorable temperature range must be available for the 

growth of fungi to occur. Below freezing, 0oC (32oF), the fungi become 

dormant but resumes its growth as the temperature rises above freezing to 

the 24oC to 29oC (75oF to 85oF) range, where growth is at its maximum. 

Above 32oC (90oF), growth tapers off rapidly, and temperatures in excess 

of 49oC (120oF) become lethal to the fungi. These killing temperatures 

could only occur in bridge members during kiln drying or preservative 

treating. 

3⁄4 Food - An adequate food supply must be available for the fungus to feed 

on. As the entire bridge serves as the food supply, the only prevention is 

to poison the wood supply with preservatives. 

3⁄4 Moisture - The fourth and probably the most controlling requirement is an 

adequate supply of moisture. The term "dry-rot" is misleading because 

dry wood will not rot. Wood must have a moisture content of 20 percent 

or greater for the growth of fungi to become active. Rain or snow is the 

main source of wood wetting. Secondary sources are condensation, 

ground water, and stream water. Exposed surfaces allow moisture to 
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1.11 

evaporate harmlessly. However seasoning shakes, checks and splits, 

interfaces between timber members, and fastener holes are ideal for 

localized moisture accumulation which allow fungi to grow. 

Although there are numerous types and species of fungi, only a few cause decay in 

timber bridge members. Some fungi types that do not cause damage include: 

3⁄4 Molds - cottony or powdery circular growths varying from white or light 

colors to black; molds themselves do not cause decay but their presence is 

an indication that conditions favorable to the growth of fungi exist (see 

Figure 2.1.8) 

3⁄4 Stains - specks, spots, streaks, or patches, varying in color, which 

penetrate the sap wood; sapstain is harmless to wood; it is usually a 

surface phenomenon and, like molds, implies conditions where harmful 

fungi can flourish 

3⁄4 Soft rot - attacks the wood, making it soft and spongy; only the surface 

wood is affected, and thus it does not significantly weaken the member; 

occurs mostly in wood of high water content and high nitrogen content 

Some fungi types that weaken the wood include: 

3⁄4 Brown rot - degrades the cellulose and hemi-cellulose leaving the lignin as 

a framework which makes the wood dark brown and crumbly (see Figure 

2.1.9) 

3⁄4 White rot - feeds upon the cellulose, hemi-cellulose, and the lignin and 

makes the wood white and stringy (see Figure 2.1.9). 

 

Figure 2.1.8 Mold and Stain on Underside of Timber Bridge 
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1.12 

 

Figure 2.1.9 Brown and White Rot 

Brown and white rots are responsible for structural damage to wood. 

The natural decay resistance of wood exposed under conditions favorable for 

decay is distinctly variable, and it can be an important factor in the service life of 

timber bridges. 

The heartwood of many tree species possesses a considerable degree of natural 

decay resistance, while the sapwood of all commercial species is vulnerable to 

decay. 

Each year, when an inner layer or ring of sapwood dies and becomes heartwood, 

fungi-toxic compounds are deposited. These compounds provide natural decay 

resistance and are not present in living sapwood. 

 

Most existing timber bridges in this country have been constructed from either 

Douglas fir or southern pine. Older bridges may contain such additional species as 

larch, various pines, and red oak. The above named species are classified as 

moderately decay resistant. Western red cedar and white oak are considered very 

decay resistant. 

In the last 25 years, bridge materials have been obtained increasingly from smaller 

trees in young-growth timber stands. As a result, recent supplies of lumber and 

timbers have contained increased percentages of decay-susceptible sapwood. 
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1.13 

Insects Insects tunnel in and hollow out the insides of timber members for food and 

shelter. Some common types of insects include: 

3⁄4 Termites 

3⁄4 Powder-post beetles or lyctus beetles 

3⁄4 Carpenter ants 

3⁄4 Caddisflies 

Termites 

Termites are pale-colored, soft-bodied insects that feed on wood (see Figure 

2.1.10). All damage is inside the surface of the wood; hence, it is not visible. The 

only visible signs of infestation are white mud shelter tubes or runways extending 

up from the earth to the wood and on the sides of masonry substructures. Termite 

attack of bridge members, however, is rare or nonexistent in bridges throughout 

most of the country due to the constant vibration caused by traffic travelling over 

timber bridges. 

 

Figure 2.1.10 Termites 
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1.14 

 Powder-post Beetles or Lyctus Beetles 

Powder-post beetle larvae also hollow out the insides of timber members and leave 

the outer surface pocked with small holes (see Figure 2.1.11). Often a powdery 

dust is dislodged from the holes. The inside may be completely excavated as the 

larvae of these beetles bore through the wood for food and shelter. 

Figure 2.1.11 Powder Post Beetle 
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1.15 

 Carpenter Ants 

Carpenter ants are large, black ants up to 19 mm (3/4 inches) long that gnaw 

galleries in soft or decayed wood (see Figure 2.1.12). The ants may be seen in the 

vicinity of the infested wood, but the accumulation of sawdust on the ground at the 

base of the timber is also an indicator of their presence. The ants do not use the 

wood for food but build their galleries in the moist and soft or partially decayed 

wood. 

 

Figure 2.1.12 Carpenter Ants 
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1.16 

 Caddisflies 

The caddisfly is another insect that can damage timber piles. It is generally found 

in fresh water but can also be found in brackish water. Bacterial and fungal decay 

make the timber attractive to the caddisfly. 

The caddisfly is an aquatic insect that is closely related to the moth and butterfly 

(see Figure 2.1.13). During the larva and pupa stage of their life cycle, they can 

dig small holes in the timber for protection. The larvae do not feed on the timber, 

but rather use it as a foundation for their silken shelters. This explains why 

caddisfly larvae have been known to exist on creosote treated timber. 

 

Figure 2.1.13 Caddisfly Larva 

Marine Borers Marine borers are found in sea water and brackish water only and cause severe 

damage to timber members in the area between high and low water, although 

damage may extend to the mud line (see Figure 2.1.14). They can be very 

destructive to wood and have been known to consume piles and framing in just a 

few months. 

One type of marine borer is the mollusk borer, or shipworm (see Figure 2.1.15). 

The shipworm is one of the most serious enemies of marine timber installations. 

The most common species of shipworm is the teredo. This shipworm enters the 

timber in an early stage of life and remains there for the rest of its life. Teredos are 

gray and slimy and can typically reach a length of 380 mm (15 inches) and a 

diameter of 10 mm (3/8 inch). Some species of shipworm have been known to 

grow to a length of 1.8 m (6 feet) and up to 25 mm (1 inch) in diameter. The 

teredo maintains a small opening in the surface of the wood to obtain nourishment 

from the sea water. 
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1.17 

 

Figure 2.1.14 Marine Borer Damage to Wood Piling 

 

Figure 2.1.15 Shipworms (Mollusks) 

Another type of marine borer is the crustacean borer. The most commonly 

encountered crustacean borer is the limnoria or wood louse (see Figure 2.1.16). It 

bores into the surface of the wood to a shallow depth. Wave action or floating 

debris breaks down the thin shell of timber outside the borers’ burrows, causing 

the limnoria to burrow deeper. The continuous burrowing results in a progressive 

deterioration of the timber pile cross section, which will be noticeable by an 

hourglass shape developed between the tide levels. These borers are about 3 to 6 

mm (1/8 to 1/4 inches) long and 2 to 3 mm (1/16 to 1/8 inches) wide.
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1.18 

 

Figure 2.1.16 Limnoria (Wood Louse) 

Chemical Attack Most petroleum based products and chemicals do not cause structural degradation 

to wood. However, animal waste can cause some damage, and strong alkalis will 

destroy wood fairly rapidly. Highway bridges are seldom exposed to these 

substances. Timber structures normally do not come in contact with damaging 

chemicals unless an accidental spill occurs. 

Acids 

Wood resists the effects of certain acids better than many materials and is often 

used for acid storage tanks. However, strong acids that have oxidizing properties, 

such as sulphuric and sulphureous acid, are able to slowly remove a timber 

structure’s fiber by attacking the cellulose and hemi-cellulose. Acid damaged 

wood has weight and strength losses and looks as if it has been burned by fire. 

Bases or Alkalis 

Strong bases or alkalis attack and weaken the hemi-cellulose and lignin in the 

timber structure. Attack by strong bases leaves the wood a bleached white color. 

Mild alkalis do little harm to wood. 

Other Types and Sources 

of Deterioration 

Delaminations 

Delaminations occur in glued-laminated members when the layers separate due to 

failure within the adhesive or at the bond between the adhesive and the laminate. 

They provide openings for decay to begin and may cause a reduction in strength 

(see Figure 2.1.17). 

Loose connections

Loose connections may be due to shrinkage of the wood, crushing of the wood 

around the fastener, or from repetitive impact loading (working) of the connection. 

Loose connections can reduce the bridge’s load-carrying capacity (see Figure 
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1.19 

2.1.18). 

Surface depressions

Surface depressions indicate internal collapse, which could be caused by decay. 

Fire 

Fire consumes wood at a rate of about 1 mm (0.05 inches) per minute during the 

first 30 minutes of exposure, and 0.5 mm (0.021 inches) per minute thereafter (see 

Figure 2.1.19). Large timbers build a protective coating of char (carbon) after the 

first 30 minutes of exposure. Small size timbers do not have enough volume to do 

this before they are, for all practical purposes, consumed by fire. Preservative 

treatments are available to retard fire damage. 

 

Figure 2.1.17 Delamination in a Glue Laminated Timber Member 
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1.20 

Figure 2.1.18 Loose Hanger Connection Between the Timber Truss and 

Floorbeam 

Figure 2.1.19 Fire Damaged Timber Member
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1.21 

 Impact or Collisions 

Severe damage can occur to truss members, railings, and columns when an errant 

vehicle strikes them (see Figure 2.1.20). 

Figure 2.1.20 Impact/Collision Damage to a Timber Member 

 Wear, Abrasion and Mechanical Wear 

Vehicular traffic is the main source of wear on timber decks (see Figure 2.1.21). 

Abrasion occurs on timber piles that are subjected to tidal flows. Mechanical wear 

of timber members sometimes occurs due to movement of the fasteners against 

their holes when connections become loose. 

Figure 2.1.21 Wear of a Timber Deck 
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1.22 

 Overstress 

Each timber member has a certain ultimate load capacity. If this load capacity is 

exceeded, the member will fail (see Figures 2.1.22 and 2.1.23). 

 

Figure 2.1.22 Horizontal Shear Failure in Timber Member 

 

Figure 2.1.23 Failed Timber Floor Beam due to Excessive Bending Moment 

 Weathering or Warping 

Weathering is the affect of sunlight, water, and heat. Weathering can change the 
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1.23 

equilibrium moisture content in the wood in a non-uniform fashion, thereby 

resulting in changes in the strength and dimensions of the wood. Uneven 

reduction in moisture content causes localized shrinkage, which can lead to 

warping, checking, splitting, or loosening of connectors (see Figure 2.1.24). 

 

Figure 2.1.24 Weathering on Timber Deck 

Protective Coating 

Failure 

The following paint failures are common on wood: 

3⁄4 Cracking and peeling extend with the grain of the wood. They are caused 

by different shrinkage and swell rates of expansion and contraction 

between springwood and denser summerwood. 

3⁄4 Decay fungi penetrate through cracks in the paint to cause wood to decay. 

3⁄4 Blistering is caused by paint applied over an improperly cleaned surface. 

Water, oil, or grease typically are responsible for blistering. 

3⁄4 Chalking is a degradation of the paint, usually by the ultraviolet rays of 

sunlight, leaving a powdery residue. 

3⁄4 Erosion is general thinning of the paint due to chalking, weathering, or 

abrasion. 

3⁄4 Mold fungi and stain fungi grow on the surface of paint, usually in warm, 

humid, shaded areas with low air flow. They appear as small green or 

black spots.

2.1.6 

Protective Systems Protective systems are a necessity when using timber for bridge construction. 

Proper preparation of the timber surface is required for the protective system to 

penetrate the wood surface and perform adequately. Untreated timber generally 

has a unit weight of about 640 to 800 kilograms per cubic meter (40 to 50 pounds 

per cubic foot (pcf)). 
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1.24 

 

Types and 

Characteristics of Wood 

Protectants 

Water Repellents 

Water repellents retard water absorption and maintain low moisture content in 

wood. This helps to prevent decay by molds and to slow the weathering process. 

Laminated wood (plywood) is particularly susceptible to moisture variations, 

which cause stress between plies due to swelling and shrinkage. 

Preservatives 

Wood preservatives prevent biological deterioration that can penetrate into timber. 

To be effective, the preservatives have to be applied to wood by vacuum-pressure 

treatment. This is done by placing the timber to be treated in a sealed chamber up 

to 2.4 m (8 feet) in diameter and 43 m (140 feet) long. The chamber is evacuated, 

drawing the air from the wood pores and cells. The treatment chemical is then fed 

into the chamber and pressure up to 1380 kPa (200 psi) is applied, forcing the 

chemical into the wood (see Figure 2.1.25). Preservatives are the best means to 

prevent decay but do not prevent weathering. A paint or water repellent coating is 

required for this. Treated timber generally has a unit weight of about 800 

kilograms per cubic meter (50 pounds per cubic foot (pcf)). 

 

Figure 2.1.25 Bridge Timber Member Showing Penetration Depth of 

Preservative Treatment 

Coal tar-creosote is a dark, oily protectant used in structural timber such as pilings 

and beams. Coal tar-creosote treated timber has a dark, oily appearance (see 

Figure 2.1.26). Unless it has weathered for several years, it cannot be painted, 

since paint adheres poorly to the oily surface, and the oils bleed through paint.
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1.25 

 

Figure 2.1.26 Coal-Tar Creosote Treated Timber Beams (Source: Barry 

Dickson, West Virginia University) 

Pentachlorophenol (in a light oil solvent) is an organic solvent solution used as a 

decay inhibitor. It also leaves an oily surface, like creosote, but can be painted 

after all of the solvent has evaporated, usually in one or two years of normal 

service. 

Chromated copper arsenate (CCA) is the most common waterborne salt decay 

inhibitor and is also applied by vacuum-pressure treatment. Timber treated with 

CCA has a green appearance. It is the only pressure-applied preservative that 

readily accepts painting. CCA also provides limited protection against the 

ultraviolet rays in sunlight. 

Pole-fuming is used to kill decay fungi in timber pilings which are already inservice. The treatment chemical, injected through bore holes drilled into the 

piling, spreads along wood fibers for up to 2.7 m (9 feet) from the injection site. It 

stops existing decay and prevents further decay for up to nine years. 

Fire Retardants 

Fire retardants will not indefinitely prevent wood from burning but will retard the 

spread of fire and prolong the time to ignite wood. The two main classes of fire 

retardants are pressure impregnated fire retardant salts and intumescent coatings 

(paints). The intumescent paints expand upon intense heat exposure, forming a 

thick, puffy, charred coating which insulates the wood from the intense heat.

Application of fire retardants may change some wood properties of gluedlaminated timber. 
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1.26 

 Paint 

Wood must be sufficiently dry to permit painting. A few months of seasoning will 

satisfactorily dry new wood. The wood surface must be free of dirt and debris 

prior to painting. Old, poorly adherent paint must be removed and the edges of 

intact paint feathered for a smooth finish. Mildew shows up as green or black spots 

on bare wood or paint. It is a fungus which typically grows in warm, humid, 

shaded areas with low air movement. Mildew must be removed with a solution of 

sodium hypochlorite (bleach) and water. 

There are several common methods to prepare wood for painting: 

3⁄4 Hand tool cleaning is the simplest but slowest method. Sandpaper, 

scrapers, and wire brushes are used to clean small areas. 

3⁄4 Power tool cleaning utilizes powerized versions of the hand tools. They 

are faster than hand tools, but care must be exercised not to damage the 

wood substrate. 

3⁄4 Heat application with an electric heat gun softens old paint for easier 

removal to bare wood. 

3⁄4 Solvent-based and caustic chemical paint removers can efficiently clean 

large areas quickly. Some of the chemicals may, however, present serious 

fire or exposure hazards. Extreme caution must be exercised when 

working around chemical paint removers. 

3⁄4 Open nozzle abrasive blast cleaning and water blast cleaning remove old 

paint and foreign material, leaving bare wood. However, they can easily 

damage wood unless used carefully. 

Paint protects wood from both moisture and weathering. By precluding moisture 

from wood, paint prevents decay. However, paint applied over unseasoned wood 

seals in moisture, accelerating, rather than retarding, decay. Oil-based paint and 

latex paint are both commonly used on wood bridges. 

Oil-based paint provides the best shield from moisture. It is not, however, the most 

durable. It does not expand and contract as well as latex, and it is more prone to 

cracking. Oil/alkyd paints cure by air oxidation. These paints are low cost, with 

good durability, flexibility, and gloss retention. They are resistant to heat and 

solvents. Alkyd paints often contain lead pigments, known to cause numerous 

health hazards. The removal and disposal of lead paint is a regulated activity in all 

states. 

Latex paint consists of a latex emulsion in water. Latex paint is often referred to 

as water-based paint. There are many types of latex paint, each formulated for a 

different application. They have excellent flexibility and color retention, with 

good adhesion, hardness, and resistance to chemicals. 
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1.27 

2.1.7 

Inspection 

Procedures for 

Timber 

There are three basic procedures used to inspect a timber member. Depending on 

the type of inspection, the inspector may be required to use only one individual 

procedure or all procedures. They include: 

3⁄4 Visual 

3⁄4 Physical 

3⁄4 Advanced inspection techniques 

Visual Examination There are two types of visual inspections that may be required of an inspector. 

The first, called a routine inspection, involves reviewing the previous inspection 

report and visually examining the members from beneath the bridge. A routine

inspection involves a visual assessment to identify obvious defects. 

The second type of visual inspection is called an in-depth inspection. An in-depth 

inspection is an inspection of one or more members above or below the water level 

to identify any deficiencies not readily detectable using routine procedures. 

Hands-in inspection may be necessary at some locations. This type of visual 

inspection requires the inspector to visually assess all defective timber surfaces at 

a distance no further than an arm’s length. The timber surfaces are given close 

visual attention to quantify and qualify any defects. The hands-in inspection 

technique may be supplemented by non-destructive testing. 

For timber members, visual inspections reveal areas that need further investigation 

such as checks, splits, shakes, fungus decay, deflection, or loose fasteners. 

Physical Examination Once the defects are identified visually, physical procedures must be used to verify 

the extent of the defect. Most physical inspection procedures for timber members 

involve destructive methods. An inspection hammer, on the other hand, does not 

damage timber and can be used to tap on areas and determine the extent of internal 

decay. This is done by listening to the sound the hammer makes. If it sounds 

hollow, internal decay may be present. Some methods or areas of physical 

examination include: 

Pick or Penetration Test 

A pick or penetration test involves lifting a small sliver of wood with a pick or 

pocketknife and observing whether or not it splinters or breaks abruptly. Sound 

wood splinters, while decayed wood breaks abruptly (see Figure 2.1.27). 
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1.28 

 

Figure 2.1.27 Inspector Performing a Pick Test 

 Timber Boring and Drilling Locations

The following are common timber boring and drilling locations (see Figure 

2.1.28): 

3⁄4 Deck planks - in the bottom, next to a beam. 

3⁄4 Beams - in sides near the deck and in the bottom over the bent cap. 

3⁄4 Cap - under the beams and over posts and piles. 

3⁄4 Post/pile - top under cap and bottom just above ground or water line.

An inspector may be required to take samples to determine the condition of the 

wood. When drilling or boring vertical faces, always drill at a slight upward angle 

so that any drainage will flow away from the plugged hole. 
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1.29 

Deck

Beam

Cap

Deck Nail Deck

Beam

Cap

Drift Pin 

(typ.)

Pile

Ground Line

(or Water Line)

A Section A-A

A

*

*

* *

 

Figure 2.1.28 Timber Boring and Drilling Locations 

 Protective Coatings 

When inspecting timber bridges, keep in mind the environment surrounding the 

bridge and how this can cause failures leading to rapid decay of the underlying 

wood members. 

Paint Adhesion 

Probe the paint with the point of a knife to test paint adhesion to wood. Attempt to 

lift the paint. Adhesion failure may occur between wood and paint or between 

layers of paint. 

A more quantitative paint adhesion assessment is performed in accordance with 

American Society for Testing and Materials (ASTM) D-3359 "Measuring 

Adhesion by Tape Test". An "X" is cut through the paint to the wood surface. 

Adhesive test tape is applied over the "X" and removed in a continuous motion. 

The amount of paint (if any) removed is noted. Adhesion is rated on a scale of 0 to 

5. Refer to ASTM D-3359 for the rating criteria. 

Paint Dry Film Thickness 

Paint dry film thickness is measured with a Tooke Gage (see Figure 2.1.29). With 

this instrument, a groove is cut at a known angle with the grain through the paint to 

expose the wood substrate. The thickness of each layer of paint is measured 

through a 50-power microscope built into the gage. 
SECTION 2: Bridge Materials 

TOPIC 2.1: Timber 

2.1.30 

Figure 2.1.29 Tooke Gage Used to Measure Coating Dry Film Thickness 

 Repainting 

If the coating is to be repainted, the type of paint in the existing topcoat must be 

known, since paints of different type may not adhere well to each other. Two 

methods to determine the type of existing paint are: 

3⁄4 Check historical records of previous painting 

3⁄4 Obtain paint samples from the bridge for laboratory analysis 

Alternately, a test patch may be coated with new paint over intact existing paint. 

After the paint thoroughly dries in accordance with the manufacturer's 

specification, inspect the appearance and adhesion of the new paint. 

Advanced Inspection 

Techniques 

In addition, several advanced techniques are available for timber inspection. 

Nondestructive methods, described in Topic 13.1.1, include: 

3⁄4 Pol-Tek 

3⁄4 Spectral analysis 

3⁄4 Ultrasonic testing 

3⁄4 Vibration 

Other methods, described in Topic 13.1.3, include: 

3⁄4 Boring or drilling 

3⁄4 Moisture content 

3⁄4 Probing 

3⁄4 Shigometer 
2.2-i

Table of Contents

Chapter 2 ..................................................................................................................................................

Bridge Materials 

 2.2 Concrete ............................................................................................ 2.2.1 

 2.2.1 Introduction............................................................................... 2.2.1 

 Portland Cement ................................................................. 2.2.1 

 Water ............................................................................... 2.2.1 

 Air ............................................................................... 2.2.2 

 Aggregates.......................................................................... 2.2.2 

 Admixtures ......................................................................... 2.2.2 

 

 2.2.2 Properties of Concrete............................................................... 2.2.3 

 Physical Properties ............................................................. 2.2.4 

 Mechanical Properties ........................................................ 2.2.4 

 High Performance Concrete ............................................... 2.2.5 

 2.2.3 Reinforced Concrete ................................................................. 2.2.6 

 

 2.2.4 Prestressed Concrete ................................................................. 2.2.9 

 

 2.2.5 Types of Concrete Deterioration............................................. 2.2.13 

 Structural Cracks .............................................................. 2.2.13 

 Flexure Cracks ........................................................... 2.2.13 

 Shear Cracks .............................................................. 2.2.14 

 Crack Size .................................................................. 2.2.16 

 Nonstructural Cracks........................................................ 2.2.17 

 Crack Orientation ............................................................. 2.2.19 

 Scaling.............................................................................. 2.2.21 

 Delamination .................................................................... 2.2.23 

 Spalling............................................................................. 2.2.23 

 Chloride Contamination ................................................... 2.2.24 

 Efflorescence.................................................................... 2.2.24 

 Ettringite Formation ......................................................... 2.2.24 

 Honeycombs .................................................................... 2.2.25 

 Pop-outs............................................................................ 2.2.25 

 Wear ............................................................................. 2.2.25 

 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2-ii

 Collision Damage ............................................................. 2.2.26 

 Abrasion ........................................................................... 2.2.27 

 Overload Damage............................................................. 2.2.27 

 Reinforcing Steel Corrosion............................................. 2.2.28 

 Prestressed Concrete Deterioration .................................. 2.2.29 

 2.2.6 Other Causes of Concrete Deterioration ................................. 2.2.30 

 Temperature Changes....................................................... 2.2.30 

 Chemical Attack............................................................... 2.2.30 

 Moisture Absorption......................................................... 2.2.31 

 Differential Foundation Movement.................................. 2.2.31 

 Design and Construction Deficiencies ............................. 2.2.31 

 Fire Damage ..................................................................... 2.2.32 

 

 2.2.7 Protective Systems .................................................................. 2.2.32 

 Types and Characteristics of Concrete Coatings.............. 2.2.32 

 Paint ........................................................................... 2.2.32 

 Oil-based Paint .................................................... 2.2.32 

 Latex Paint........................................................... 2.2.32 

 Epoxy Paint ......................................................... 2.2.33 

 Urethanes............................................................. 2.2.33 

 Water Repellent Membranes...................................... 2.2.33 

 Surface Preparation.................................................... 2.2.34 

 Blast Cleaning............................................................ 2.2.34 

 Acid Etching .............................................................. 2.2.34 

 Types and Characteristics of Reinforcement Coatings..... 2.2.34 

 Epoxy Coating ........................................................... 2.2.34 

 Galvanizing................................................................ 2.2.35 

 Cathodic Protection.................................................... 2.2.35 

 

 2.2.8 Inspection Procedures for Concrete and Protective Coatings . 2.2.35 

 Visual Examination .......................................................... 2.2.36 

 Physical Examination ....................................................... 2.2.36 

 Advanced Inspection Techniques..................................... 2.2.37 

 Physical Examination of Protective Coatings .................. 2.2.38 

 Areas to Inspect.......................................................... 2.2.38 

 Coating Failures......................................................... 2.2.39 

 
2.2.1

Topic 2.2 Concrete 

2.2.1 

Introduction A large percentage of the bridge structures in the nation’s highway network are 

constructed of reinforced concrete or prestressed concrete. It is important that the 

bridge inspector understand the basic characteristics of concrete in order to 

efficiently inspect and evaluate a concrete bridge structure. 

Concrete, commonly mislabeled as "cement", is a mixture of various components 

that, when mixed together in the proper proportions, chemically react to form a 

strong durable construction material ideally suited for certain bridge components. 

Cement is only one of the basic ingredients of concrete. It is the “glue” that binds 

the other components together. Concrete is made up of the following basic 

ingredients: 

3⁄4 Portland cement 

3⁄4 Water 

3⁄4 Air 

3⁄4 Aggregates 

3⁄4 Admixtures (reducers, plasticizers, retarders) 

Portland Cement The first ingredient, Portland Cement, is one of the most common types of cement, 

and it is made with the following raw materials: 

3⁄4 Limestone - provides lime 

3⁄4 Quartz or cement rock - provides silica 

3⁄4 Claystone - provides aluminum oxide 

3⁄4 Iron ore - provides iron oxide 

The cement is produced by placing the above materials through a three process 

high temperature kiln system. During the three process kiln system, the 

temperature can range from 100° C to 1510° C (212° F to 2750° F). The first zone 

in the kiln process is known as the drying process. During this process, the 

materials are dehydrated due to the high temperature. The calcining zone is the 

next step and results in the production of lime and magnesia. The final step, called 

the burning zone or clinkering zone, produces clinkers or nodules of the sintered 

materials. Upon cooling, the clinkers are ground into a powder and finish the 

Portland cement production process. 

Water The second ingredient, water, can be almost any potable water. Impurities in 

water, such as dissolved chemicals, salt, sugar, or algae, produce a variety of 

undesirable effects on the quality of the concrete mix. Therefore, water with a 

noticeable taste or odor may be suspect. 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.2

Air The third ingredient of concrete is air. Small evenly distributed amounts of 

entrained air provide: 

3⁄4 Increased durability against freeze/thaw effects 

3⁄4 Reduced cracking 

3⁄4 Improved workability 

3⁄4 Reduced water segregation 

Air entrainment also reduces the weight of concrete slightly. Many tiny air 

bubbles introduced into the plastic concrete naturally create lighter weight 

concrete. The typical air entrainment additive is a vinsol resin. Air entrainment 

additives act like dishwashing liquids. When mixed with water, they create 

bubbles. These bubbles become part of the concrete mix, creating tiny air voids. 

Through extensive lab testing, it has been proven that when exposed to freeze/thaw 

conditions, the voids prevent excess pressure buildup in the concrete. 

Aggregates The fourth ingredient, aggregates, comprise approximately 75% of a typical 

concrete mix by volume. Some aggregate qualities which result in a strong and 

durable concrete are: 

3⁄4 Abrasion resistance 

3⁄4 Weather resistance 

3⁄4 Chemical stability 

3⁄4 Chunky compact shape 

3⁄4 Smooth, non-porous surface texture 

3⁄4 Cleanliness and even gradation 

Normal weight concrete has a unit weight of about 2240 to 2400 kg/m3 (140 to 150 

pcf). Typical aggregate materials for normal weight concrete are sand, gravel, 

crushed stone, and air-cooled, blast-furnace slag. 

Lightweight concrete normally has a unit weight of 1200 to 1840 kg/m3 (75 to 115 

pcf). The weight reduction comes from the aggregates and air entrainment. 

Lightweight aggregates differ depending on the location where the lightweight 

concrete is being produced. The common factor in lightweight aggregates is that 

they all have many tiny air voids in them that make them lightweight with a low 

specific gravity. 

Admixtures The fifth ingredient of most concrete mixes is one or more admixtures to change 

the consistency, setting time, or concrete strength. Pozzolans are a common type 

of admixture used to reduce permeability. There are natural pozzolans such as 

diatomite and pumicite, along with artificial pozzolans which include admixtures 

such as fly ash. 

Admixtures can either be minerals or chemicals. The mineral admixtures include 

fly ash, silica fume, and ground granulated blast-furnace slag. Chemical 

admixtures can include water reducers, plasticizers, retarders, high range water 

reducers, and superplasticizers. 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.3

Fly ash is a by-product from the burning of ground or powdered coal. Fly ash was 

added to concrete mixes as early as the 1930’s. This turned out to be a viable way 

to dispose of fly ash while positively affecting the concrete. The use of fly ash in 

concrete mixes improves concrete workability, reduces segregation, bleeding, heat 

evolution and permeability, inhibits alkali-aggregate reaction, and enhances sulfate 

resistance. 

The use of fly ash in concrete mixes also has some drawbacks, however, such as 

increased set time and reduced rate of strength gain in colder temperatures. 

Admixture effects are also reduced when fly ash is used in concrete mixes. This 

means, for example, that a higher percentage of air entrainment admixture is 

needed for concrete mixes using fly ash. 

Silica fume (microsilica) results from the reduction of high purity quartz with coal 

in electric furnaces while producing silicon and ferrosilicon alloys. It affects 

concrete by improving compressive strength, bond strength, and abrasion 

resistance. Microsilica also reduces permeability. Concrete with a low 

permeability minimizes steel reinforcement corrosion, which is of major concern 

in areas where deicing agents are used. These properties have contributed to the 

increased use of high performance concrete in recent bridge design and 

construction. 

Some disadvantages that result from the use of silica fume include a higher water 

demand in the concrete mix, a larger amount of air entraining admixture, and a 

decrease in workability. 

Ground granulated blast-furnace slag is created when molten iron blast furnace 

slag is quickly cooled with water. This admixture can be substituted for cement on 

a 1:1 basis. However, it is usually limited to 25% in areas where the concrete will 

be exposed to deicing salts and to 50% in areas that do not need to use deicing 

salts. 

Water reducing admixtures and plasticizers are used to aid workability at lower 

water/cement ratios, improve concrete quality and strength using less cement 

content, and help in placing concrete in adverse conditions. These admixtures can 

be salts and modifications of hydroxylized carboxylic acids, or modifications of 

lignosulfonic acids, and polymeric materials. Some of the potentially negative 

effects that are encountered when using water reducers and plasticizers include 

loss of slump and excess setting time. 

Retarding admixtures are used to slow down the hydration process while not 

changing the long-term mechanical properties. This type of admixture is needed 

when high temperatures are expected during placing and curing. Retarders slow 

down the setting time to reduce unwanted temperature and shrinkage cracks which

result from a fast curing mix. 

2.2.2 

Properties of 

Concrete 

It is necessary for the bridge inspector to understand the different physical and 

mechanical properties of concrete and how they relate to concrete bridges in 

service today. 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.4

Physical Properties The major physical properties of concrete are: 

3⁄4 Thermal expansion - concrete expands as temperature increases and 

contracts as temperature decreases 

3⁄4 Porosity - because of entrapped air, the cement paste never completely 

fills the spaces between the aggregate particles, permitting absorption of 

water and the passage of water under pressure 

3⁄4 Volume changes due to moisture - concrete expands with an increase in 

moisture and contracts with a decrease in moisture 

3⁄4 Fire resistance - quality concrete is highly resistant to the effects of heat; 

however, temperatures over 370°C (700°F) may cause damage 

3⁄4 Formability - concrete can be cast to any shape prior to curing 

Mechanical Properties The major mechanical properties of concrete are: 

3⁄4 Strength - Plain, unreinforced concrete has a 28-day compressive strength 

ranging from about 17 MPa (2500 psi) to about 41 MPa (6000 psi). Higher 

strength concrete, with compressive strengths ranging from 41 MPa (6000 

psi) to about 76 MPa (11,000 psi), is also available and becoming more 

commonly used. However, its tensile strength is only about 10% of its 

compressive strength, its shear strength is about 12% to 13% of its 

compressive strength, and its flexural strength is about 14% of its 

compressive strength (see Table 2.2.1). 

 Six principal factors that increase concrete strength are: 

- Increased cement content 

- Increased aggregate strength 

- Decreased water-to-cement ratio 

- Decreased entrapped air 

- Increased curing time (extent of hydration) 

- Use of pozzolanic admixtures and slag 

3⁄4 Elasticity - Within the range of normal use, concrete is able to deform a 

limited amount under load and still return to its original orientation when 

the load is removed (elastic deformation). Elasticity varies as the square 

root of compressive strength. See Topic P.2 for modulus of elasticity and 

how it affects elastic deformation. 

3⁄4 Creep - In addition to elastic deformation, concrete exhibits long-term, 

irreversible, continuing deformation under application of a sustained load. 

Creep (plastic deformation) ranges from 100% to 200% of initial elastic 

deformation, depending on time. 

3⁄4 Isotropy - Plain, unreinforced concrete has the same mechanical properties 

regardless of which direction it is loaded. 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.5

3500

490 438 350

0

500

1000

1500

2000

2500

3000

3500

4000

Compressive

Strength (100%)

Flexural Strength

(14%)

Shear Strength 

(12-13%)

Tensile Strength

(10%)

Note: Percentages represent a comparison of various strength properties 

with the compressive strength of concrete.

PSI

(24 MPa)

(3.4 MPa) (3.0 MPa) (2.4 MPa)

 

Table 2.2.1 Strength Properties of Concrete (24 Mpa) (3500 psi Concrete) 

High Performance 

Concrete 

High performance concrete (HPC) has been used for more than 25 years in the 

building industry. Under the FHWA’s Strategic Highway Research Program 

(SHRP) Implementation Program, four types of high performance concrete mix 

designs were developed (see Table 2.2.2). High performance concrete is 

distinguished from regular concrete by its curing conditions and proportions of the 

ingredients in the mix design. The use of fly ash and high range water reducers 

play an important role in the design of HPC, as well as optimizing all components 

of the mix. Due to the increased strength and reduced permeability of HPC, bridge 

decks using HPC are expected to have double the life of conventional concrete 

bridge decks. The type and strength characteristics of concrete used to construct 

bridge components can be found in the bridge file under design specifications. 

HPC Type

Minimum Strength 

Criteria

Water-Cementitious 

Ratio

Minimum 

Durability Factor

Very Early Strength 

(VES) 13.8 MPa (2,000 PSI)/ 6 hours 80%

High Early Strength 

(HES) 34.5 MPa (5,000 PSI)/ 24 hours 80%

Very High Strength 

(VHS) 69 MPa (10,000 PSI)/ 28 hours 80%

Fiber Reinforced HES + (steel or poly) 80%

Additional information on the definition of HPC:

 May 1996.

 - "HPC Defined for Highway Structures," Charles Goodspeed, Suneel Vanikar, an

 Cook; Concrete International , February 1996, The American Concrete Institute

 - "Workshop Showcases High-Performance Concrete Bridges," Focus Newslette

≤ 0 4.

≤ 0 35 .

≤ 0 35 .

≤ 0 35 .

 

Table 2.2.2 FHWA’s SHRP Implemented HPC Mix Designs 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.6

2.2.3 

Reinforced 

Concrete 

Concrete is commonly used in bridge applications due to its compressive strength 

properties. However, in order to supplement the limited tensile, shear and flexural

strengths of concrete, steel reinforcement is used. 

Steel reinforcement has a tensile yield strength of 276 MPa (40 ksi) or 414 MPa 

(60 ksi) and therefore has approximately 100 times the tensile strength of 

commonly used concrete. Therefore, in reinforced concrete members, the concrete 

resists the compressive forces and the steel reinforcement primarily resists the 

tensile forces. The type of steel reinforcement used in reinforced concrete is "mild 

steel", which is a term used for low carbon steels. The steel reinforcement is 

located close to the tension face of a structural member to maximize its efficiency 

(see Figure 2.2.1). 

 

Figure 2.2.1 Concrete Member with Tensile Steel Reinforcement Showing 

Shear reinforcement is also needed to resist diagonal tension (refer to Topic P.1). 

Shear cracks start at the bottom of concrete members near the support and 

propagate upward and away from the support at approximately a 45° angle. 

Vertical or diagonal shear reinforcement is provided in this area to intercept the 

cracks and to stop the cracks from opening wider. 

Reinforcing bars are also placed uniformly around the perimeter of a member to 

resist stresses resulting from temperature changes and volumetric changes of 

concrete. This steel is referred to as temperature and shrinkage steel. 

Steel reinforcing bars can be "plain" or smooth surfaced, or they can be 

"deformed" with a raised gripping pattern protruding from the surface of the bar 

(see Figure 2.2.2). The gripping pattern improves bond with the surrounding 

concrete. Modern reinforced concrete bridges are generally constructed with 

"deformed" reinforcing steel.
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.7

INCHPOUND 

BAR SIZE

DIAMETER 

(in.) 

AREA 

(in.2

) 

METRIC 

BAR SIZE 

DIAMETER 

(mm) 

AREA 

(mm2) 

#3 0.375 0.11 #10 9.5 71.0 

#4 0.500 0.20 #13 12.7 129.0 

#5 0.625 0.31 #16 15.9 200.0 

#6 0.750 0.44 #19 19.1 284.0 

#7 0.875 0.60 #22 22.2 387.0 

#8 1.000 0.79 #25 25.4 510.0 

#9 1.128 1.00 #29 28.7 645.0 

#10 1.270 1.27 #32 32.3 819.0 

#11 1.410 1.56 #36 35.8 1006.0 

#14 1.693 2.25 #43 43.0 1452.0 

#18 2.257 4.00 #57 57.3 2581.0 

 

Figure 2.2.2 Standard Deformed Reinforcing Bars (Source: Concrete 

Reinforcing Steel Institute) 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.8

 

Figure 2.2.2 Standard Deformed Reinforcing Bars (Source: Concrete 

Reinforcing Steel Institute) (Continued) 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.9

In US units, reinforcing bars up to 1” nominal diameter are identified by numbers 

that correspond to their nominal diameter in eighths of an inch. For example, a #4 

bar has a 1/2 inch nominal diameter (or 4 times 1/8 inch). For the remaining bar 

sizes (#9, #10, #11, #14, and #18), the area is equivalent to the old 1”, 1 1/8”, 1 

1/4”, 1 1/2”, and 2” square bars, respectively. 

Reinforcing bars can also be used to increase the compressive strength of a 

concrete member. When reinforcing bars are properly incorporated into a concrete 

member, the steel and concrete acting together provide a strong, durable 

construction material. 

Reinforcing bars can be protected or unprotected from corrosion. Unprotected 

reinforcement is referred to as “black” steel because only mill scale is present on 

the surface. 

The deformed epoxy coated bar is the most common type of protected reinforcing 

bar used. It is commonly specified when a concrete member may be exposed to an 

adverse environment. The epoxy provides a protective coating against corrosion 

agents such as de-icing chemicals and brackish water, and is inexpensive 

compared to other protective coatings. Another type of protected reinforcing bar is 

the galvanized bar. Unprotected bars are given a zinc coating, which slows down 

or stops the corrosion process. See Topic 2.2.9 for a detailed description of 

reinforcement protective coatings. 

In the near future, tensile reinforcement made of Fiber Reinforced Polymer (FRP) 

composites may become common. Currently the FHWA and other government 

agencies and universities are performing research to better understand the 

properties and appropriate methods for use of this material in new construction. 

FRP is lighter weight than traditional steel reinforcement, can be designed with a 

wide range of mechanical properties including tensile, flexural, impact and 

compressive strengths and provides a viable alternative in areas where deicing 

salts are used due to the fact that the deck does not deteriorate due to steel 

reinforcement corrosion. Current holdbacks to widespread use of FRP 

reinforcement are the relative cost when compared to steel, the limited amount of 

contractors with experience, and not having much performance data. 

2.2.4 

Prestressed 

Concrete 

Another type of concrete used in bridge applications is prestressed concrete, which 

uses high tensile strength steel strands as reinforcement. To reduce the tensile 

forces in a concrete member, internal compressive forces are induced through 

prestressing steel tendons or strands. When loads are applied to the member, any 

tensile forces developed are counterbalanced by the internal compressive forces 

induced by the prestressing steel. By prestressing the concrete in this manner, the 

final tensile forces under primary live loads are typically within the tensile strength 

limits of plain concrete. Therefore, properly designed prestressed concrete 

members do not develop flexure cracks under service loads (see Figure 2.2.3). 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.10

C

T

T

C

C

C

 - Deflection

T - Tension

 - Compression

C C

T T

Live load on a bridge

Deflection due to live load 

(Note: Tension on bottom 

face of beam) 

Prestressing

the strands

Prestressing strands create 

compressive force on bottom 

of beam

Final condition of prestressed 

concrete beam under live load. 

(Note: Compression is now on 

the bottom face of the beam)

 

Figure 2.2.3 Prestressed Concrete Beam 

There are three methods of prestressing concrete: 

3⁄4 Pretensioning - during fabrication of the member, prestressing steel is 

placed and tensioned prior to casting and curing of the concrete (see 

Figure 2.2.4) 

3⁄4 Post-tensioning - during fabrication of the member, ducts are cast-in-place 

so that after curing, the prestressing steel can be passed through the ducts 

and tensioned (see Figure 2.2.5) 

3⁄4 Combination method - this is used for long members for which the 

required prestressing force cannot safely be applied using pretensioning 

only
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.11

 

Figure 2.2.4 Pretensioned Concrete I-Beams 

 

Figure 2.2.5 Post-tensioned Concrete Box Girder 

Steel for prestressing, which is named high tensile strength steel, comes in three 

basic forms: 

3⁄4 Wires (ASTM A421) - single wires or parallel wire cables; the parallel 

wire cables are commonly used in post-tensioning operations; the most 

popular wire size is 6 mm (1/4 inch) diameter and the most common grade 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.12

of steel is the 1860 Mpa (270 ksi) grade. 

3⁄4 Strands (ASTM A416) - fabricated by twisting wires together; the seven 

wire strand is the most common type of prestressing steel used in the 

United States, and the 1860 Mpa (270 ksi) grade is most commonly used 

today 

3⁄4 Bars (ASTM A322 and A29) - high tensile strength bars typically have a 

minimum ultimate stress of 1000 Mpa (145 ksi); the bars have full length 

deformations that also serve as threads to receive couplers and anchorage 

hardware 

Epoxy coated prestressing strand is a newer alternative to help minimize the 

amount of corrosion that occurs to otherwise unprotected strands. The epoxy is 

applied to the ordinary seven wire low relaxation prestressing strand through a 

process called “fusion bonding”. Once the epoxy is applied, the strand has very 

little bond capacity and an aluminum oxide grit has to be applied to aid in the 

bonding. From recent testing by the FHWA, the epoxy coated strands have a 

tendency to slip when advanced curing temperatures are 63°C (145°F) and above. 

This slip occurs because the epoxy material begins to melt at these temperatures. 

Since the epoxy coating has a tendency to melt, this type of alternative is not used 

unless protection of the prestressing strand is critical. 

In pretensioned members, transfer of tendon tensile stress occurs through bonding, 

which is the secure interaction of the prestressing steel with the surrounding 

concrete. This is accomplished by casting the concrete in direct contact with the 

prestressed steel. 

For purposes of crack control in end sections of pretensioned members, the 

prestressing steel is sometimes debonded. This is accomplished by providing a 

protective cover on the steel, preventing it from contacting the concrete. Crack 

control at the beam ends may also be obtained by using draped strands. A number 

of strands are draped from both ends of the beam to the beam’s third points 

resulting in end strand patterns with center of gravities near the beam center of 

gravity. 

In post-tensioned members, transfer of tendon tensile stress is accomplished by 

mechanical end anchorages and locking devices. If bonding is also desired, 

special ducts are used which are pressure injected with grout after the tendons are 

tensioned and locked off. 

For post-tensioned members, when bonding is not desirable, grouting of tendon 

ducts is not performed and corrosion protection in the form of galvanizing, 

greasing, or some other means must be provided. 

In prestressed concrete beams, shear strength is enhanced by the local compressive 

stress present. However, mild shear reinforcement is still required. Similar to 

reinforced concrete, prestressed concrete also requires mild steel temperature and 

shrinkage reinforcement. 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.13

2.2.5 

Types of Concrete 

Deterioration 

In order to properly inspect a concrete bridge, the inspector must be able to 

recognize the various types of defects or deterioration associated with concrete. 

The inspector must also understand the causes of the defects or deterioration and 

how to examine them. There are many common defects or deterioration that 

occur on reinforced concrete bridges: 

3⁄4 Cracking (flexure, shear, temperature, shrinkage, mass concrete) 

3⁄4 Scaling 

3⁄4 Delamination 

3⁄4 Spalling 

3⁄4 Chloride contamination 

3⁄4 Efflorescence 

3⁄4 Ettringite formation 

3⁄4 Honeycombs 

3⁄4 Pop-outs 

3⁄4 Wear 

3⁄4 Collision damage 

3⁄4 Abrasion 

3⁄4 Overload damage 

3⁄4 Reinforcing steel corrosion 

3⁄4 Prestressed concrete deterioration 

Structural Cracks A crack is a linear fracture in concrete. It may extend partially or completely 

through the member. There are two basic types of cracks: structural and nonstructural cracks. Structural cracks are caused by dead load and live load stresses. 

Cracking is considered normal for mildly reinforced concrete (e.g., in cast-in-place 

tee-beams) as long as the cracks are small and there are no rust stains or other 

signs of deterioration present. Larger structural cracks indicate potentially serious 

problems, because they are directly related to the structural capacity of the 

member. When cracks can be observed opening and closing under load, they are 

referred to as “working” cracks. See Table 2.2.3 for crack width guidelines. 

There are two types of structural cracks: flexure and shear (see Figure 2.2.6). 

Flexure Cracks 

Flexure cracks are caused by tensile forces and therefore develop in the tension 

zones. Tension zones occur either on the bottom or the top of a member, 

depending on the span configuration. Tension zones can also occur in substructure 

components. Tension cracks terminate when they approach the neutral axis of the 

member. If a beam is a simple span structure (refer to Topic P.2.9), flexure cracks 

can often be found at the mid-span at the bottom of the member where bending or 

flexure stress is greatest (see Figure 2.2.7). If the beams are continuous span 

structures (refer to Topic P.2.9), flexure cracks can also occur at the top of 

members at or near their interior supports. 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.14

Shear Cracks

Shear cracks are caused by diagonal tensile forces that typically occur in the web 

of a member near the supports where shear stress is the greatest. Normally, these 

cracks initiate near the bearing area, beginning at the bottom of the member, and 

extending diagonally upward toward the center of the member (see Figure 2.2.8). 

Shear cracks also occur in abutment backwalls, stems and footings, pier caps, 

columns, and footings. 

Although structural cracks are typically caused by dead load and live load forces, 

they can also be caused by overstresses in members resulting from unexpected 

secondary forces. Restricted thermal expansion or contraction such as caused by 

frozen bearings, or forces due to the expansion of an approach slab or failure of a 

backwall can induce significant forces which result in cracks (see Figure 2.2.9). 

Flexure Cracks

Shear Cracks

 

Figure 2.2.6 Structural Cracks 

 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.15

 

Figure 2.2.7 Flexural Crack on a Tee Beam 

 

Figure 2.2.8 Shear Crack on a Slab Beam 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.16

Cold

Thermal Forces Hot

 

Figure 2.2.9 Thermal Forces 

 

 Crack Size

Crack size is very important in assessing the condition of an in-service bridge. 

Cracks may extend partially or completely through the concrete member. On 

reinforced concrete, cracking will usually be large enough to be seen with the 

naked eye. A crack comparitor card can be used to measure and differentiate 

cracks (see Figure 2.2.10 and Table 2.2.3). 

 

Figure 2.2.10 Crack Comparitor Card 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.17

CRACK WIDTH GUIDELINES 

REINFORCED CONCRETE PRESTRESSED CONCRETE

English Metric English Metric 

HAIRLINE (HL) < 1/16" < 1.6mm < 0.004" < 0.1mm 

 (0.0625) 

NARROW (N) 1/16" to 1/8" 1.6 to 3.2mm 0.004 to 0.009" 0.1 to 0.23mm

 0.0625” – 0.125” 

MEDIUM (M) 1/8" to 3/16" 3.2 to 4.8mm 0.010 to 0.030" 0.25 to 0.76mm

 0.125” – 0.1875” 

WIDE (W) >3/16" > 4.8mm > 0.030" > 0.76mm 

 >0.1875” 

 

Table 2.2.3 Crack Width Guidelines 

Cracks can be classified as hairline, narrow, medium, or wide. Hairline cracks are 

small and cannot be measured with normal equipment, such as a six-foot rule. 

Medium and wide cracks are cracks that can be measured by simple means or a 

crack comparitor card. On conventionally reinforced structures, hairline cracks are 

usually insignificant. All other crack widths may be significant and should be 

monitored and recorded in the inspection notes. 

On prestressed structures, all cracks are significant and an optical crack gauge is 

the proper instrument needed to measure and differentiate cracks. 

When reporting cracks, the length, width, location, and orientation (horizontal, 

vertical, or diagonal) should be noted. Both large and small cracks in main 

members, especially in prestressed members, should be carefully recorded. The 

presence of rust stains or efflorescence or evidence of differential movement on 

either side of the crack should be indicated. 

Nonstructural Cracks Nonstructural cracks result from internal stresses due to dimensional changes. 

Nonstructural cracks are divided into three categories: 

3⁄4 Temperature cracks (see Figure 2.2.11) 

3⁄4 Shrinkage cracks (see Figure 2.2.12) 

3⁄4 Mass concrete cracks 

Though these cracks are nonstructural and relatively small in size, they provide 

openings for water and contaminants, which can lead to serious problems. 

Temperature cracks are caused by the thermal expansion and contraction of the 

concrete. Concrete expands or contracts as its temperature rises or falls. If the 

concrete is prevented from contracting, due to friction or because it is being held in 

place, it will crack under tension. Inoperative bearing devices and clogged 

expansion joints can also cause this to occur. Shrinkage cracks are due to the 

shrinkage of concrete caused by the curing process. Volume reduction due to 

curing is also referred to as plastic shrinkage. Plastic shrinkage cracks occur while 

the concrete is still plastic and are usually short, irregular shapes and do not extend 

the full depth into the member. Mass concrete cracks occur due to thermal 

gradients (differences between interior and exterior) in massive sections 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.18

immediately after placement and for a period of time thereafter. Temperature, 

shrinkage, and mass concrete cracks typically do not significantly affect the 

structural strength of a concrete member. 

 

Figure 2.2.11 Temperature Cracks 

 

Figure 2.2.12 Shrinkage Cracks 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.19

In concrete bridge decks, temperature and shrinkage cracks can occur in both the 

transverse and longitudinal directions. In retaining walls and abutments, these 

cracks are usually vertical, and in concrete beams, these cracks occur vertically or 

transversely on the member. However, since temperature and shrinkage stresses 

exist in all directions, the cracks could have other orientations. 

Inspectors must exercise care in distinguishing between nonstructural cracks and 

structural cracks. However, regardless of the crack type, water seeps in and causes 

the reinforcement to corrode. The corroded reinforcement expands and exerts 

pressure on the concrete. This pressure can cause delaminations and spalls. 

Crack Orientation In addition to classifying cracks as either structural or nonstructural and recording 

their lengths and widths, inspectors must also describe the orientation of the 

cracks. The orientation of the crack with respect to the loads and supporting 

members is an important feature that must be recorded accurately to ensure the 

proper evaluation of the crack. The orientation of cracks may generally be 

described by one of the following five categories: 

3⁄4 Transverse cracks – These are fairly straight cracks that are roughly 

perpendicular to the centerline of the bridge or a bridge member (see 

Figure 2.2.13). 

3⁄4 Longitudinal cracks - These are fairly straight cracks that run parallel to 

the centerline of the bridge or a bridge member (see Figure 2.2.14). 

3⁄4 Diagonal cracks - These cracks are skewed (at an angle) to the centerline 

of the bridge or a bridge member, either vertically or horizontally. 

3⁄4 Pattern or map cracking - These are inter-connected cracks that form 

networks of varying size. They vary in width from barely visible, fine 

cracks to cracks with a well defined opening. Map cracking resembles the 

lines on a road map (see Figure 2.2.15). 

3⁄4 Random cracks - These are meandering, irregular cracks. They have no 

particular form and do not logically fall into any of the types described 

above. 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.20

 

Figure 2.2.13 Transverse Cracks 

 

Figure 2.2.14 Longitudinal Cracks 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.21

 

Figure 2.2.15 Pattern or Map Cracks 

Scaling Scaling is the gradual and continuing loss of surface mortar and aggregate over an 

area due to the chemical breakdown of the cement bond. Scaling is accelerated 

when the member is exposed to a harsh environment. Scaling is classified in the 

following four categories: 

3⁄4 Light or minor scale - loss of surface mortar up to 6 mm (1⁄4 inch) deep, 

with surface exposure of coarse aggregates (see Figure 2.2.16) 

3⁄4 Medium or moderate scale - loss of surface mortar from 6 to 13 mm (1⁄4 

inch to 1⁄2 inch) deep, with mortar loss between the coarse aggregates (see 

Figure 2.2.17) 

3⁄4 Heavy scale - loss of surface mortar from 13 to 25 mm (1⁄2 inch to 1 inch) 

deep; coarse aggregates are clearly exposed (see Figure 2.2.18) 

3⁄4 Severe scale - loss of coarse aggregate particles, as well as surface mortar 

and the mortar surrounding the aggregates; depth of the loss exceeds 25 

mm (1 inch); reinforcing steel is usually exposed (see Figure 2.2.19)
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.22

Figure 2.2.16 Light or Minor Scaling 

Figure 2.2.17 Medium or Moderate Scaling

Figure 2.2.18 Heavy Scaling 

Figure 2.2.19 Severe Scaling 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.23

When reporting scaling, the inspector should note the location of the defect, the 

size of the affected area, and the scaling classification. For severe scale, the depth 

of penetration of the defect should also be recorded. 

Delamination Delamination occurs when layers of concrete separate at or near the level of the 

top or outermost layer of reinforcing steel. The major cause of delamination is 

expansion of corroding reinforcing steel. This is commonly caused by intrusion of 

chlorides or salt. Another cause of delamination is severe overstress in a member. 

Delaminated areas give off a hollow “clacking” sound when tapped with a 

hammer. When a delaminated area completely separates from the member, the 

resulting depression is called a spall. 

When reporting delamination, the inspector should note the location and the size of 

the defect. 

Spalling A spall is a depression in the concrete (see Figure 2.2.20). Spalls result from the 

separation and removal of a portion of the surface concrete, revealing a fracture 

roughly parallel to the surface. Spalls can be caused by corroding reinforcement, 

friction from thermal movement, and overstress. Reinforcing steel is often 

exposed in a spall, and the common shallow pothole in a concrete deck is 

considered a spall. Spalls are classified as follows: 

3⁄4 Small spalls - not more than 25 mm (1 inch) deep or approximately 150 

mm (6 inches) in diameter 

3⁄4 Large spalls - more than 25 mm (1 inch) deep or greater than 150 mm (6 

inches) in diameter 

Figure 2.2.20 Spalling on a Concrete Deck 

When concrete is overstressed, it gives or fractures. Over time, the fracture opens 

wider from debris, freeze/thaw cycles, or more overstress. This cycle continues 

until a spall is formed. Spalls caused from overstress are very serious and should 

be brought to the attention of the Chief Bridge Engineer. Most spalls are caused 

from corroding reinforcement, but if the spall is located at or near a high moment 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.24

region, overstress may be the cause. Examples that might indicate a spall was 

caused by overstress include: 

3⁄4 A spall that is at or near flexure cracks in the lower portion of a beam at 

mid-span 

3⁄4 A spall that is at or near flexure cracks in the top of a continuous member 

over a support 

Similarly, when concrete is overstressed in compression, it is common for the 

surface to crush and then spall. 

When reporting spalls, the inspector should note the location of the defect, the size 

of the area, and the depth of the defect. 

Chloride Contamination Chloride contamination in concrete is the presence of recrystalized soluble salts. 

Concrete is exposed to chlorides in the form of deicing salts, acid rain, and in some

cases, contaminated water used in the concrete mix. It causes accelerated 

reinforcement corrosion that leads to cracking of the concrete. 

Efflorescence The presence of cracks permits moisture absorption and increased flow within the 

concrete that is evidenced by dirty-white surface deposits called efflorescence. 

Efflorescence is a combination of calcium carbonate leached out of the cement 

paste and other recrystalized carbonate and chloride compounds (see Figure 

2.2.21). In order to estimate the percent of concrete contaminated by chloride, 

nondestructive testing is required (refer to Topic 13.2.2). 

Figure 2.2.21 Efflorescence 

Ettringite Formation Ettringite formation is an internal defect that occurs in concrete from the reaction 

of sulfates, calcium aluminates, and water. From this reaction, ettringite, which is 

a crystalline mineral, expands up to eight times in volume compared to the volume 

of the tricalcium nitrates (C3A). Ettringite formation is initially formed when water 

is added to the cement but prior to the concrete’s initial set. The initial formation 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.25

does not harm the concrete. A secondary or delayed ettringite formation occurs 

after the concrete has hardened. This formation creates very high forces in 

hardened concrete and is the cause of the deterioration. The only way to identify 

ettringite formation as a cause of premature concrete deterioration is through 

advanced inspection techniques such as petrographic analysis. Recent studies have 

shown that ettringite formation is linked to alkali-silica reaction (ASR), but further 

research is still needed. 

Honeycombs Honeycombs or construction voids are hollow spaces or voids that may be present 

within the concrete. Honeycombs are construction defects caused by improper

vibration during concrete placement, resulting in the segregation of the coarse 

aggregates from the fine aggregates and cement paste. In some cases, honeycombs 

are the result of insufficient vibration, where the entire concrete mix does not 

physically reach the formwork surface (see Figure 2.2.22). 

Figure 2.2.22 Honeycomb 

Pop-outs Pop-outs are conical fragments that break out of the surface of the concrete, 

leaving small holes. Generally, a shattered aggregate particle will be found at the 

bottom of the hole, with a part of the fragment still adhering to the small end of the 

pop-out cone. Pop-outs are caused by aggregates which expand with absorption of 

moisture. Other causes of pop-outs include use of reactive aggregates and high 

alkali cement. 

Wear Wear is the gradual removal of surface mortar due to friction and occurs to 

concrete surfaces, like a bridge deck, when exposed to traffic. Advanced wear 

exhibits polished aggregate, which is potentially a safety hazard when the deck is 

wet. The scraping action of snowplows and street sweepers also wears the deck 

surface and damages curbs, parapets, and pier faces. 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.26

Collision Damage Trucks, derailed railroad cars, or marine traffic may strike and damage concrete 

bridge components (see Figures 2.2.23 and 2.2.24). The damage is generally in 

the form of cracking or spalling, with exposed reinforcement. Prestressed beams 

are particularly sensitive to collision damage, as exposed tendons undergo stress 

corrosion and fail prematurely. 

Figure 2.2.23 Concrete Column Collision Damage 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.27

Figure 2.2.24 Collision Damage to Prestressed Concrete I-Beam

Abrasion Abrasion damage is the result of external forces acting on the surface of the 

concrete member and is similar to wear. Erosive action of silt-laden water running 

over a concrete surface and ice flow in rivers and streams can cause considerable 

abrasion damage to concrete piers and pilings. In addition, concrete surfaces in 

surf zones may be damaged by the abrasive action of sand and silt in the water. 

Abrasion damage can be accelerated by freeze-thaw cycles. This will usually occur 

near the water line on concrete piers. The use of the term "scour" to indicate 

"abrasion" is incorrect. The term scour is used to describe the loss of streambed 

material from around the base of a pier or abutment due to stream flow or tidal 

action (see Topic 11.2). 

Overload Damage Overload damage or serious structural cracking occurs when concrete members are 

sufficiently overstressed. Concrete decks, beams, and girders are all subject to 

damage from such overload conditions. Note any excessive vibration or deflection 

that may occur under traffic, which can indicate overstress. Other visual signs that 

can indicate overstress due to tension include excessive sagging, spalling, and/or 

cracking at the mid-span of simple span structures and at the supports of 

continuous span structures. Diagonal cracks close to support points may be an 

indication of overstress due to shear or torsion. Permanent deformation is another 

visual sign of overstress damage in a member. If overload damage is detected or 

suspected, the Chief Bridge Engineer should be notified immediately (see Figure 

2.2.25). 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.28

Figure 2.2.25 Overload Damage 

Reinforcing Steel 

Corrosion 

Due to the chemistry of the concrete mix, reinforcing steel embedded in concrete 

is normally protected from corrosion. In the high alkaline environment of the 

concrete, a tightly adhering film forms on the steel that protects it from corrosion. 

However, this protection is eliminated by the intrusion of chlorides, which enables 

water and oxygen to attack the reinforcing steel, forming iron oxide (i.e., rust). 

Chloride ions are introduced into the concrete by marine spray, industrial brine, or 

deicing agents. These chloride ions can reach the reinforcing steel by diffusing 

through the concrete or by penetrating cracks in the concrete. An inspector may 

see rebar rust stains on the outer concrete surfaces before a spall occurs. The 

corrosion product (rust) can occupy up to 10 times the volume of the corroded 

steel that it replaces. This expansive action creates internal pressures up to 20.7 

MPa (3000 psi) that will cause the concrete to yield, resulting in wider cracks, 

delaminations, and spalls (see Figure 2.2.26). 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.29

Figure 2.2.26 Loss of Bond: Concrete / Corroded Reinforcing Bar 

Prestressed Concrete 

Deterioration 

Prestressed concrete members deteriorate in a similar fashion to ordinary concrete 

members. However, the effects on Prestressed concrete member performance are 

usually more detrimental. Significant defects include: 

3⁄4 Structural cracks 

3⁄4 Exposed prestressing tendons 

3⁄4 Corrosion of tendons in the bond zone 

3⁄4 Loss of camber due to concrete creep 

3⁄4 Loss of camber due to lost prestress forces 

Structural cracks indicate an overload condition has occurred. These cracks 

expose the tendons to the environment, which can lead to corrosion. 

Exposed steel tendons via cracks or collision damage corrode at an accelerated rate 

due to the high tensile stresses carried and can fail prior to any measurable section 

loss due to environmentally induced cracking (EIC). 

Environmentally induced cracking in steel prestressing strands can occur when the 

steel prestressing strands are subject to high tensile stresses in a corrosive 

environment. Rust stains may be present. The strands, which are normally 

ductile, undergo a brittle failure due to the combination of the corrosive 

environment along with the tensile stresses. 

There are two types of environmentally induced cracking. The first is called stress 

corrosion cracking (SCC). This type of cracking grows at a slow rate and has a 

branched cracking pattern. The corrosion of prestressing steel along with the 

tensile stress in the steel causes a cracking pattern perpendicular to the stress 

direction. 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.30

The second type is called hydrogen-induced cracking (HIC) and occurs due to 

hydrogen diffusing into the prestressing steel. Once in the steel, hydrogen gas is 

formed. The hydrogen gas applies an internal pressure to the prestressing steel. 

This internal pressure, in conjunction with the tensile stress due to prestressing, 

has the ability to create very brittle, non-branching, fast growing cracks in the 

prestressing steel strands. The specific type of environmentally induced cracking 

can only be positively identified after failure through the use of advanced 

inspection techniques. 

When deteriorated concrete cover allows corrosion of the tendons in the bond zone 

(the end thirds of the beam), loss of development occurs which reduces prestress 

force. This can sometimes be evidenced by reduced positive camber and 

ultimately structural cracking. Prestress force can also be reduced through a beamshortening phenomenon called creep, which relaxes the steel tendons. Loss of 

prestress force is followed by structural cracking at normal loads due to reduced 

live load capacity. 

2.2.6 

Other Causes of 

Concrete 

Deterioration 

Temperature Changes Freezing and thawing are common causes of concrete deterioration (see Figure 

2.2.27). Porous concrete absorbs water, and when this water freezes, high 

expansive pressures are created due to the larger volume created by ice formation. 

These pressures often produce cracking and light spalling. Freeze/thaw damage 

should not be confused with scaling. 

Figure 2.2.27 Freeze-Thaw Damage on a River Pier 

Chemical Attack Aside from accelerated rebar corrosion, the use of salt or chemical deicing agents 

contributes to weathering through recrystallization. This is quite similar to the 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.31

effects of freezing and thawing. 

Sulfate compounds in soil and water are also a problem. Sodium, magnesium, and 

calcium sulfates react with compounds in cement paste and cause rapid 

deterioration of the concrete. 

Alkali-silica reaction (ASR) is a reaction between the alkalis in cement with the 

silica molecules of various aggregates. When the reaction takes place, a gel-like 

substance is formed. Once exposed to moisture, the gel expands and causes 

cracking in the concrete. 

Moisture Absorption All concrete is porous and will absorb water to some degree. As water is absorbed, 

the concrete will swell. If restrained, the material will burst or the concrete will 

crack. This type of deterioration is limited to concrete members that are 

continuously submerged in water. 

Differential Foundation 

Movement 

Foundation movement can also cause serious cracking in concrete substructures. 

Differential settlement induces stresses in the supported superstructure and can 

lead to concrete deterioration. 

Design and Construction 

Deficiencies 

Some conditions or improper construction methods that can cause concrete to 

deteriorate are: 

3⁄4 Insufficient reinforcement bar cover - Insufficient concrete cover over 

rebars may lead to early corrosion of the steel reinforcement. 

3⁄4 Weep holes and scuppers - Improper placement or inadequate sizing of 

scuppers and weep holes can cause an accumulation of water with its 

damaging effects. 

3⁄4 Leaking deck joints 

3⁄4 Improper curing - A primary cause of concrete deterioration (loss of 

strength). 

3⁄4 Soft spots - Soft spots in the subgrade of an approach slab will cause the 

slab to settle and crack. 

3⁄4 Premature form removal - If the formwork is removed between the time 

the concrete begins to harden and the specified time for formwork 

removal, cracks will probably occur. 

3⁄4 Improper vibration - If the concrete is not properly vibrated, internal 

settling of the concrete mix can cause surface cracking above the 

reinforcing bars as the mix settles around the bars. Excessive vibration 

may cause segregation (separation of water, aggregate, and cement) of the 

concrete mix. 

3⁄4 Impurities - The inclusion of clay or soft shale particles in the concrete 

mix will cause small holes to appear in the surface of the concrete as these 

particles dissolve. These holes are known as mudballs. 

3⁄4 Internal voids - If reinforcing bars are too closely spaced, voids, which 

collect water, can occur under the reinforcing mat if the mix is not 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.32

properly vibrated. 

Fire Damage 3⁄4 Extreme heat will damage concrete. High temperatures (above 370°C 

(700°F)) will cause a weakening in the cement paste and lead to cracking. 

2.2.7 

Protective Systems 

Types and 

Characteristics of 

Concrete Coatings 

Coatings form a protective barrier film on the surface of concrete to preclude entry 

of water and chlorides into the porous concrete. The practice of coating the 

concrete surface varies with each agency. Two primary concrete coatings are paint 

and water repellent membranes. 

 Paint 

Paint is applied in one or two layers. The first layer fills the voids in a rough 

concrete surface. The second layer forms a protective film over the first. On 

smooth concrete surfaces, only one layer may be necessary. 

Several classes of paint are used to coat concrete: 

3⁄4 Oil-based paint 

3⁄4 Latex paint 

3⁄4 Epoxy paint 

3⁄4 Urethanes 

Oil-based Paint 

Oil-based paint is declining in use but is still found on some older concrete 

structures. Oil paint is subject to saponification failure in wet areas. Saponification 

is a chemical attack on the coating caused by the inherent alkalinity of the 

concrete. The moisture may be from humidity in the atmosphere, rain runoff, or 

ground water entering the porous concrete from below. Saponification does not 

occur over dry concrete (or occurs at a greatly reduced rate). 

Latex Paint 

Latex paint consists of a resin emulsion. Latexes can contain a variety of synthetic 

polymer binding agents. Latex paint resists attack by the alkaline concrete. 

Acrylic or vinyl latexes provide better overall performance, in that they are more 

resistant to alkaline attack than oil-based paint. Latex paints, however, are 

susceptible to efflorescence. Efflorescing is a process in which water-soluble salts 

pass outward through concrete and are deposited at the concrete/paint interface. 

This can cause loss of coating adhesion. If the paint is also permeable to water, the 

salts are deposited on the paint surface as the water evaporates. 

Acrylics do not chalk as rapidly as other latexes and have good resistance to 

ultraviolet rays in sunlight. Polyvinyl acetate latexes are the most sensitive to 

attack by alkalis. 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.33

Epoxy Paint 

Epoxy paint uses a cross-linking polymer binder, in which the epoxy resin in the 

paint undergoes a chemical reaction as the paint cures, forming a tough, crosslinked paint layer. Epoxies have excellent resistance to chemicals, water, and 

atmospheric moisture. Most epoxies are sensitive to the concrete's moisture 

content during painting. Polyamide-cured and water-base epoxy systems, however, 

have substantially overcome the moisture intolerance problem. For other epoxy 

systems, the concrete moisture should be measured prior to painting. 

Urethanes 

Urethanes are usually applied over an epoxy primer. They provide excellent 

adhesion, hardness, flexibility, and resistance to sunlight, water, harmful 

chemicals, and abrasion. They are, however, sensitive to temperature and humidity 

during application. The urethanes used on concrete require moisture to cure. In 

high humidity, the paint cures too quickly, leaving a bubbly appearance. 

Many states now apply moisture-cured urethane anti-graffiti coatings on accessible 

concrete structures (see Figure 2.2.28). These are smooth, clear coatings applied 

without a primer coat. Spray paint and indelible marker ink adhere poorly to the 

smooth urethane, permitting easier cleaning than if they were applied to porous 

concrete. 

Figure 2.2.28 Anti-Graffiti Coating on Lower Area of Bridge Piers 

 Water Repellent Membranes 

Water repellent membranes (sealers) applied to concrete bridge decks, piers, 

abutments, columns, barriers, or aprons form a tight barrier to water and chlorides. 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.34

The membrane penetrates up to 10 mm (3/8 inch) into the concrete to give strong 

adhesion. Membranes have good resistance to abrasion from weathering and 

traffic. Methyl methacrylate, silane, and silicone are three common water repellent 

coatings. 

 Surface Preparation 

Concrete, as with any other surface, must be properly cleaned prior to coating. 

The surface may also require roughening to improve coating adhesion, as the 

forms used to mold concrete leave a surface that is too smooth for good coating 

adhesion. In addition, the oils applied to wooden forms to facilitate removal may 

impede coating adhesion. 

 Blast Cleaning

Blast cleaning with dry abrasives, high pressure water (up to 380 Mpa (55,000 

psi)), or a water/abrasive mix is used to remove dirt, old paint, grease, and 

deteriorated concrete. It is also the best method to roughen the surface. 

Open nozzle blast cleaning uses compressed air free of oil and moisture to propel 

the abrasive at speeds up to 645 kilometers per hour (400 miles per hour). 

Centrifugal wheel blast cleaning uses a rotating wheel to propel abrasive. The 

most common abrasive is sand, although many others, such as steel shot and grit, 

silica, aluminum oxide, and silicon carbide, are also available. Unlike dry abrasive 

blast cleaning, high pressure water can penetrate deep into concrete. The concrete 

must be allowed to dry thoroughly before a coating is applied. 

 Acid Etching

Acid etching is an efficient method of cleaning concrete. Hydrochloric acid (also 

called muriatic acid) reacts with the alkaline concrete surface, allowing surface 

contaminants to easily wash away. It leaves a roughened surface profile for good 

coating adhesion. All acid must be removed prior to coating application. 

Types and 

Characteristics of 

Reinforcement Coatings 

Because unprotected steel reinforcement corrodes and has adverse effects on 

concrete, some type of protective coating should typically be used on all steel 

reinforcement placed in concrete structures to ensure minimal steel corrosion. 

Steel reinforcement can be protected by the following methods: 

3⁄4 Epoxy coating 

3⁄4 Galvanizing 

3⁄4 Cathodic protection 

Epoxy Coating 

Epoxy coating is resistant to chemicals, water, and atmospheric moisture. Epoxies 

utilize an epoxy polymer binder, which forms a tough, resilient film upon drying 

and curing. Drying is by solvent evaporation, while curing entails a chemical 

reaction between the coating components. Epoxy coatings have excellent 

atmospheric exposure characteristics, as well as resistance to chemicals and water. 

They are often used as the intermediate coat in a three layer paint system. There 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.35

are also two- and three-layer systems, which use only epoxies. One disadvantage 

of epoxies is that they chalk when exposed to sunlight. This chalking must be 

removed prior to topcoating with another layer of epoxy or another material. If not 

removed, the chalking will compromise subsequent adhesion. 

 Galvanizing 

Another method of protecting steel reinforcement is by galvanizing the steel. This 

also slows down the corrosion process and lengthens the life of the reinforced 

concrete. This occurs by coating the bare steel reinforcement with zinc. The two 

unlike metals form an electrical current between them and one metal virtually 

stops its corrosion process while the other’s accelerates due to the electrical 

current. In this situation, the steel stops corroding, while the zinc has accelerated 

corrosion. 

 Cathodic Protection 

Steel reinforcement corrosion can also be slowed down by cathodic protection. 

Corrosion of steel reinforcing bars in concrete occurs by an electrical process in a 

moist environment at the steel surface. During corrosion, a voltage difference 

(less than 1 volt) develops between rebars or between different areas on the same 

rebar. Electrons from the iron in the rebar are repelled by the negative anode area 

of the rebar and attracted to the positive cathode area. This electron flow 

constitutes an electrical current, which is necessary for the corrosion process. 

Corrosion occurs only at the anode, where the electrons from the iron are given up.

By cathodic protection, this electrical current is reversed, which slows or stops 

corrosion. By the impressed current method, an electrical DC rectifier supplies 

electrical current from local electrical power lines to a separate anode embedded in 

the concrete. The anode is usually a wire mesh embedded just under the concrete 

surface. Another type of anode consists of an electrically conductive coating 

applied to the concrete surface. The wires from the rectifier are embedded in the 

coating at regular intervals. 

When the impressed current enters the mesh or coating anode, the voltage on the 

rebars is reversed, turning the entire rebar network into a giant cathode. Since 

natural corrosion occurs only at the anode, the rebars are protected. 

The natural corrosion process is allowed to proceed with electrons leaving the iron

atoms in the anode. With impressed current cathodic protection, however, the 

electrons are supplied from an external source, the DC rectifier. Thus, the 

artificial anode mesh or coating is also spared from corrosion. 

During the bridge inspection, check that all visible electrical connections and 

wiring from the rectifier to the concrete structure are intact. 

2.2.8 

Inspection 

Procedures for 

Concrete and 

Protective Coatings 

There are three basic procedures used to inspect prestressed and reinforced 

concrete members. Depending on the type of inspection, the inspector may be 

required to use only one individual procedure or all procedures. They include: 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.36

3⁄4 Visual 

3⁄4 Physical 

3⁄4 Advanced inspection techniques 

Visual Examination There are two types of visual inspections that may be required of an inspector. 

The first, called a routine inspection, involves reviewing the previous inspection 

report and visually examining the members from beneath the bridge. All concrete 

surfaces should receive a thorough visual assessment to identify obvious defects 

during a routine inspection. 

The second type of visual inspection is called an in-depth inspection. An in-depth 

inspection is an inspection of one or more members above or below the water level 

to identify any deficiencies not readily detectable using routine procedures. 

Hands-on inspection may be necessary at some locations. This type of visual 

inspection requires the inspector to visually assess all defective concrete surfaces 

at a distance no further than an arm’s length. The concrete surfaces are given 

close visual attention to quantify and qualify any defects. The hands-on inspection 

technique may be supplemented by non-destructive testing. 

Physical Examination Areas of concrete or rebar deterioration identified visually should also be 

examined physically using an inspection hammer. This hands-on effort verifies 

the extent of the defect and its severity. 

High stress areas should be sounded for defects using an inspection hammer. 

Hammer sounding is commonly used to detect areas of delamination and unsound 

concrete. For large horizontal surfaces such as bridge decks, a chain drag may be 

used. A chain drag is made of several sections of chain attached to a handle (see 

Figure 2.2.29). The inspector drags this across a deck and makes note of the 

resonating sounds. A delaminated area will have a distinctive hollow “clacking” 

sound when tapped with a hammer or revealed with a chain drag. A hammer 

hitting sound concrete will result in a solid "pinging" type sound. 

The location, length and width of cracks found during the visual inspection and 

sounding procedures should be given special attention. For typical reinforced 

concrete members, a crack comparitor card can be used to measure the width of 

cracks. This type of crack width measuring device is a transparent card about the 

size of an identification card. The card has lines on it that represent crack widths. 

The line on the card that best matches the width of the crack lets the inspector 

know the measured width of the crack. For prestressed members, crack widths are 

usually narrower in width. For this reason, a crack gauge, which is a more 

accurate crack width-measuring device, should be used. For crack width 

guidelines, see Table 2.2.3. 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.37

Figure 2.2.29: Inspector Using a Chain Drag 

Advanced Inspection 

Techniques

If the extent of the concrete defect cannot be determined by the visual and/or 

physical inspection procedures described above, advanced inspection techniques 

should be used. Nondestructive methods, described in Topic 13.2.2, include: 

3⁄4 Acoustic wave sonic/ultrasonic velocity measurements 

3⁄4 Delamination detection machinery 

3⁄4 Electrical methods 

3⁄4 Electromagnetic methods 

3⁄4 Pulse velocity 

3⁄4 Flat jack testing 

3⁄4 Ground-penetrating radar 

3⁄4 Impact-echo testing 

3⁄4 Infrared thermography 

3⁄4 Laser ultrasonic testing 

3⁄4 Magnetic field disturbance 

3⁄4 Neutron probe for detection of chlorides 

3⁄4 Nuclear methods 

3⁄4 Pachometer 

3⁄4 Rebound and penetration methods 

3⁄4 Ultrasonic testing 

3⁄4 Smart concrete which is a coating that allows sensing of strain similar to a 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.38

strain gauge 

Other methods, described in Topic 13.2.3, include: 

3⁄4 Core sampling 

3⁄4 Carbonation 

3⁄4 Concrete permeability 

3⁄4 Concrete strength 

3⁄4 Endoscopes and videoscopes 

3⁄4 Moisture content 

3⁄4 Reinforcing steel strength 

3⁄4 Petrographic examination 

Other methods for protective coatings include: 

3⁄4 Paint dry film thickness (Tooke Gauge) 

Physical Examination of 

Protective Coatings 

 Areas to Inspect 

While inspecting protective coatings, pay close attention to the following areas: 

3⁄4 Areas open to direct weathering by wind, rain, hail, or seawater spray. 

3⁄4 Roadway splash zones along curbs, parapets, and expansion dams. These 

areas are subject to impact abrasion by debris from passing vehicles. 

3⁄4 Inaccessible or hard-to-reach areas where coatings may be missing or 

improperly applied. 

3⁄4 All concrete joints. 

3⁄4 Areas that retain moisture or salt. Horizontal surfaces of concrete beams 

and piers are common examples. Also inspect areas where drainage 

systems deposit salt and water, such as beneath catch basins, scuppers, 

downspouts, and bearing areas. 

3⁄4 Impact areas on bridge decks and parapets where snowplows or vehicle 

accidents damage coatings.
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.39

 Coating Failures 

The following failures are characteristic of paint on concrete: 

3⁄4 Lack of adhesion/peeling can be caused by poor adhesion of the primer 

layer to the concrete or by poor bonding between coating layers. 

Waterborne salts depositing under a water-impermeable coating 

(efflorescence) will also cause a coating to peel. 

3⁄4 Chalking is a powdery residue left on paint as ultraviolet light degrades 

the paint. 

3⁄4 Erosion is a gradual wearing away of a coating. It is caused by abrasion 

from wind-blown sand, soil and debris, rain, hail, or debris propelled by 

motor vehicles. 

3⁄4 Checking is composed of short, irregular breaks in the top layer of paint, 

exposing the undercoat. 

3⁄4 Cracking is similar to checking, but with cracking, the breaks extend 

completely through all layers of paint to the concrete substrate. 

3⁄4 Microorganism failure occurs as bacteria and fungi feed on paint 

containing biodegradable components. The damp nature of concrete 

makes it susceptible to this type of paint failure. 

3⁄4 Saponification results from a chemical reaction between concrete, which is 

alkaline, and oil-based paint. It destroys the paint, leaving a soft residue. 

3⁄4 Wrinkling is a rough, crinkled paint surface due to excessive paint 

thickness or high temperature during painting. It is caused by the surface 

of the paint film at the air interface solidifying before solvents have had a 

chance to escape from the interior of the paint film. 
SECTION 2: Bridge Materials 

TOPIC 2.2: Concrete 

2.2.40

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2.3-i

Table of Contents

Section 2 ..................................................................................................................................................

Bridge Materials 

 2.3 Steel........................................................................................................ 2.3.1 

 2.3.1 Introduction............................................................................... 2.3.1 

2.3.2 Common Methods of Steel Member Fabrication...................... 2.3.1 

 Rolled Beams ..................................................................... 2.3.1 

 Plate Girders....................................................................... 2.3.1 

2.3.3 Common Steel Shapes Used in Bridge Construction................ 2.3.1 

 2.3.4 Properties of Steel..................................................................... 2.3.4 

 Physical Properties ............................................................. 2.3.4 

 Mechanical Properties ........................................................ 2.3.6 

 

 2.3.5 Types and Causes of Steel Deterioration .................................. 2.3.6 

 Corrosion ............................................................................ 2.3.6 

 Fatigue Cracking ................................................................ 2.3.7 

 Overloads............................................................................ 2.3.9 

 Collision Damage ............................................................. 2.3.10 

 Heat Damage .................................................................... 2.3.10 

 Paint Failures.................................................................... 2.3.11 

2.3.6 Factors that Influence Fatigue Life ......................................... 2.3.13 

 2.3.7 Protective Systems .................................................................. 2.3.14 

 Corrosion of Steel............................................................. 2.3.14 

 Galvanic Action................................................................ 2.3.15 

 Types and Characteristics of Steel Coatings .................... 2.3.15 

 Surface Preparation for Painting................................ 2.3.15 

 Methods of Surface Preparation................................. 2.3.16 

 Paint.................................................................................. 2.3.16 

 Paint Layers ............................................................... 2.3.17 

 Types of Paint ............................................................ 2.3.18 

 Oil/alkyd Paint..................................................... 2.3.18 

 Vinyl Paint........................................................... 2.3.18 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3-ii

 Epoxies ................................................................ 2.3.18 

 Epoxy Mastics ..................................................... 2.3.18 

 Urethanes............................................................. 2.3.19 

 Zinc-rich Primers................................................. 2.3.19 

 Latex Paint.......................................................... 2.3.19 

 Protection of Suspension Cables ...................................... 2.3.19 

 Weathering Steel .............................................................. 2.3.20 

 Uses of Weathering Steel........................................... 2.3.20 

 2.3.8 Inspection Procedures for Steel............................................... 2.3.21 

 Visual Examination .......................................................... 2.3.21 

 Steel Members .......................................................... 2.3.21 

 Protective Coatings ................................................... 2.3.23 

 Weathering Steel........................................................ 2.3.25 

 Color.................................................................... 2.3.25 

 Texture................................................................. 2.3.28 

 Physical Examination ....................................................... 2.3.29 

 Steel Members ........................................................... 2.3.29 

 Protective Coatings .................................................... 2.3.29 

 Mill Scale ......................................................... 2.3.29 

 Paint Adhesion ................................................. 2.3.30 

 Paint Dry Film Thickness................................. 2.3.30 

 Repainting ........................................................ 2.3.31 

 Weathering Steel .............................................. 2.3.31 

 Advanced Inspection Techniques..................................... 2.3.32 

 

 2.3.9 Other Bridge Materials............................................................ 2.3.32 

 Cast Iron ........................................................................... 2.3.32 

 Properties of Cast Iron ............................................... 2.3.32 

 Types of Cast Iron Deterioration ............................... 2.3.33 

 Wrought Iron .................................................................... 2.3.33 

 Properties of Wrought Iron ........................................ 2.3.33 

 Types of Wrought Iron Deterioration ........................ 2.3.34 

 Aluminum......................................................................... 2.3.34 

 Properties of Aluminum............................................. 2.3.34 

 Types of Aluminum Deterioration............................. 2.3.34 

 

Steel Designation 

Steel 

Description 

 

American Society 

for Testing 

and Materials 

(ASTM) 

American Association of 

State Highway and 

Transportation Officials 

(AASHTO) 

Years 

in Use 

Structural Carbon Steel A7 M94 1900-1967 

Structural Nickel Steel A8 M96 1912-1962 

Structural Steel A36

(A709 Grade 36) 

M183

(M270 Grade 36) 

1960-Present 

(1974-Present) 

Structural Silicon Steel A94 M95 1925-1965 

Structural Steel A140 1932-1933 

Structural Rivet Steel A141 M97 1932-1966 

High-Strength Structural 

Rivet Steel 

A195 M98 1936-1966 

High-Strength Low-Alloy 

Structural Steel 

A242 M161 1941-Present 

Low and Intermediate 

Tensile Strength Carbon 

Steel Plates 

A283 1946-Present 

Low and Intermediate 

Tensile Strength CarbonSilicon Steel Plates 

A284 1946-Present 

Steel Sheet Piling A328 M202 1950-Present 

Structural Steel for 

Welding 

A373 M165 1954-1965 

High-Strength Structural 

Steel

A440 M187 1959-1979 

High-Strength Low-Alloy 

Structural Manganese 

Vanadium Steel 

A441 M188 1954-1989 

High-Yield-Strength, 

Quenched and Tempered

Alloy Steel Plate 

(Suitable for Welding) 

A514 

(A709 Grade 100/100W) 

M244 

(M270 Grade 100/100W) 

1964-Present 

(1974-Present) 

Hi Strength Low-Alloy 

Columbium-Vanadium

Steel of Structural Quality 

A572 

(A709 Grade 50) 

M223 

(M270 Grade 50) 

1966-Present 

(1974-Present) 

Hi-Strength Low-Alloy 

Structural Steel with 50 

ksi Minimum Yield Point 

to 4 inches Thick 

A588 

(A709 Grade 50W) 

M222 

(M270 Grade 50W) 

1968-Present 

(1974-Present) 

High-Strength Low-Alloy 

Steel H-Piles and Sheet 

Piling

A690 1974-Present 

Quenched and Tempered 

Low-Alloy Structural Steel 

Plate with 70 ksi Minimum 

Yield Strength to 4 inches 

Thick

A852 

(A709 Grade 70W) 

M313 

(M270 Grade 70W) 

1985-Present 

(1985-Present) 

Summary of Steel Designations (Primary Source: Beer and Johnston, Mechanics of Materials, 

New York: McGraw-Hill, 1981) 

2.3.iv
2.3.1

Topic 2.3 Steel 

2.3.1 

Introduction Steel is a widely used construction material for bridges due to its strength, relative 

ductility, and reliability. It is found in a variety of members on a large number of 

bridges. Therefore, the bridge inspector should be familiar with the various 

properties and types of steel. 

2.3.2 

Common Methods 

of Steel Member 

Fabrication 

Rolled Beams 

Rolled beams are manufactured in structural rolling mills. The flanges and 

web are one piece of steel. Rolled beams in the past were generally available 

no deeper than 914 mm (36”) in depth but are now available from some mills 

as deep as 1120 mm (44”). 

Rolled beams are generally compact’ sections which satisfy flange to web 

thickness ratios to prevent buckling. 

Rolled beams generally will have bearing stiffeners but no intermediate 

stiffeners since they are compact. 

Plate Girders 

Plate girders are often specified when the design calls for members deeper 

than 914 mm (36”) or 1120 mm (44”). 

Plate girders are built-up shapes composed of any combination of plates, bars, 

and rolled shapes. The term “built-up” describes the way the final shape is 

made. 

Older fabricated multi-girders were constructed of riveted built-up members. 

Today’s fabricated multi-girders are constructed from welded members. 

2.3.3 

Common Steel 

Shapes Used in 

Bridge 

Construction 

Steel as a bridge construction material is available as wire, cable, plates, bars, 

rolled shapes, and built-up shapes. Typical areas of application for the various 

types of steel shapes are listed below: 

3⁄4 Wires are typically used as prestressing strands or tendons in beams and 

girders (See Figure 2.2.4). 

3⁄4 Cable-stay and steel suspension bridges are primarily supported by steel 

cables (see Figure 2.3.1). 

3⁄4 Steel plates have a wide variety of uses. They are primarily used to 

construct built-up shapes (see Figure 2.3.2). 

3⁄4 Steel bars are generally placed in concrete to provide tensile reinforcement 

in the form of deformed round bars. Steel bars can also be used as 

primary or secondary tension members (see Figure 2.3.3). 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.2

3⁄4 Rolled shapes are used as structural beams and columns and are made by 

placing a block of steel through a series of rollers that transform the steel 

into the desired shape. These steel shapes are either hot rolled or cold 

rolled. The typical rolled shape is an “I” shape. The “I” shape comes in 

many sizes and weights (see Figure 2.3.4). They can also be fabricated 

with a straight or tapered flange thickness. Other rolled shapes are 

channel or “C” shapes, angles, and “T” shapes. 

3⁄4 Built-up shapes are also used as structural beams and columns but are 

composed of any combination of plates, bars, and rolled shapes. The term 

“built-up” describes the way the final shape is made. Built-up shapes are 

used when an individual rolled shape cannot carry the required load or 

when a unique shape is desired. Built-up shapes are riveted, bolted, or 

welded together. Common built-up shapes include I-girders, box girders, 

and truss members (see Figure 2.3.5). 

 

Figure 2.3.1 Steel Cables 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.3

Figure 2.3.2 Steel Plates 

Figure 2.3.3 Steel Bars 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.4

Figure 2.3.4 Rolled Shapes 

Figure 2.3.5 Built-up Shapes 

2.3.4 

Properties of Steel 

Physical Properties Many of the nation's largest bridges are constructed primarily of steel. When 

compared with iron, steel has greater strength characteristics, it is more elastic, and 

can withstand the effects of impact and vibration better. 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.5

Iron consists of small amounts of carbon. However, when the carbon content is 

greater than 0.l%, the material is classified as steel. Steel has a unit weight of 

about 7850 kg/m3

 (490 pcf). 

ASTM and AASHTO define the required properties for various steel types. ASTM 

classifies each type with an "A" designation, while AASHTO uses an “M” 

designation. 

Low carbon steel, steel with carbon content less than approximately 0.3%, defines 

some of the most common steel types: 

3⁄4 A7 steel - the most widely used bridge steel up to about 1967; obsolete 

due to poor weldability characteristics 

3⁄4 A373 steel - similar to A7 steel but has improved weldability 

characteristics due to controlled carbon content 

3⁄4 A36 steel - the latest of the low carbon steels, first used in 1960; it features 

good weldability and improved strength 

Structural nickel steel (A8) was used widely prior to the 1960's in bridge 

construction, but welding problems occurred due to relatively high carbon content.

Structural silicon steel (A94) was used extensively in riveted or bolted bridge 

structures prior to the development of low alloy steels in the 1950's. This steel 

also has poor weldability characteristics due to high carbon content. 

Quenched and tempered alloy steel plate (A514) was developed primarily for use 

in welded bridge members. 

High strength, low alloy steel is used where weight reduction is required, where 

increased durability is important, and where atmospheric corrosion resistance is 

desired; examples include: 

3⁄4 A441 steel - manganese vanadium steel 

3⁄4 A572 steel - columbium-vanadium steel (replaced by A441 in 1989) 

3⁄4 A588 & A852 steel - a "weathering steel," was developed to be left 

unpainted, which develops a protective oxide coating upon exposure to the 

atmosphere under proper design and service conditions (refer to Topic 

2.3.5 for a further description of weathering steel) 

These steels are also copper bearing, which provides increased resistance to 

atmospheric corrosion and a slight increase in strength. 

Some of the steel types listed above were used widely in the past but are no longer 

being manufactured. A new ASTM designation (A709) was developed in 1974. 

This designation covers carbon and high-strength low-alloy steel structural shapes, 

plates, and bars, and quenched and tempered alloy steel for structural plates 

intended for use in bridges. Six grades are available in four yield strength levels 

(36, 50, 70, and 100). The steel grade is equivalent to the yield strength in units of 

kips per square inch (ksi). Grades 36, 50, 50W, 70W, and 100/100W are also 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.6

included in ASTM Specifications A36, A572, A588, A852, and A514, 

respectively. Grades 50W, 70W, and 100W have enhanced atmospheric corrosion 

resistance and are labeled with a “W” for weathering steel. 

In 1996, a new steel type, High Performance Steel (HPS), was introduced to bridge 

construction. This type of steel was designed to improve weldability, toughness, 

and atmospheric corrosion resistance. Prior to the new steel designs, a set of “goal 

properties” was implemented and then testing took place to meet the goals. The 

first grade of HPS was HPS-70W, which was produced by Thermo-MechanicalControlled Processing (TMCP). The HPS-70W has improved Charpy V-Notch 

impact properties compared to 70W. Currently the HPS grades available are HPS50W, HPS-70W, and HPS-100W. 

In addition to the ASTM steel designations, the American Association of State 

Highway and Transportation Officials (AASHTO) also publishes its own steel 

designation (M270). For each ASTM steel designation, there is generally a 

corresponding AASHTO steel designation. For a summary of the various ASTM 

and AASHTO steel designations, refer to the table at the beginning of Topic 2.3. 

Mechanical Properties Some of the mechanical properties of steel include: 

3⁄4 Strength - steel is isotropic and possesses great compressive and tensile 

strength, which varies widely with type of steel 

3⁄4 Elasticity - the modulus of elasticity is nearly independent of steel type 

and is commonly assigned as 200,000 MPa (29,000,000 psi) 

3⁄4 Ductility - both the low carbon and low alloy steels normally used in 

bridge construction are quite ductile; however, brittleness may occur 

because of heat treatment, welding, or metal fatigue 

3⁄4 Fire resistance - steel is subject to a loss of strength when exposed to high 

temperatures such as those resulting from fire (see Topic 2.3.5 – for 

specific temperature information) 

3⁄4 Corrosion resistance - unprotected carbon steel corrodes (i.e., rusts) 

readily; however, steel can be protected 

3⁄4 Weldability - steel is weldable, but it is necessary to select a suitable 

welding procedure based on the chemistry of the steel 

3⁄4 Fatigue - fatigue problems in steel members and connections can occur in 

bridges due to numerous live load stress cycles combined with poor weld 

or connection details 

2.3.5 

Types and Causes 

of Steel 

Deterioration 

Corrosion To properly inspect a steel bridge, the inspector must be able to recognize the 

various types of steel defects and deterioration. The inspector must also 

understand the causes of the defects and how to examine them. The most 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.7

recognizable type of steel deterioration is corrosion (see Figure 2.3.6). Bridge 

inspectors should be familiar with corrosion since it can lead to a substantial 

reduction in member capacity. Corrosion is the primary cause of section loss in 

steel members and is most commonly caused by the wet-dry cycles of exposed 

steel. When deicing chemicals are present, the effect of corrosion is accelerated. 

Figure 2.3.6 Steel Corrosion and Complete Section Loss on a Stringer Web 

Some of the common types of corrosion include: 

3⁄4 Environmental corrosion - primarily affects metal in contact with soil or 

water and is caused by formation of a corrosion cell due to deicing salt 

concentrations, moisture content, oxygen content, and accumulated 

foreign matter such as roadway debris and bird droppings 

3⁄4 Stray current corrosion - caused by electric railways, railway signal 

systems, cathodic protection systems for pipelines or foundation pilings, 

DC industrial generators, DC welding equipment, central power stations, 

and large substations 

3⁄4 Bacteriological corrosion - organisms found in swamps, bogs, heavy clay, 

stagnant waters, and contaminated waters can contribute to corrosion of 

metals 

3⁄4 Stress corrosion - occurs when tensile forces expose an increased portion 

of the metal at the grain boundaries, leading to corrosion and ultimately 

fracture 

3⁄4 Fretting corrosion - takes place on closely fitted parts which are under 

vibration, such as machinery and metal fittings, and can be identified by 

pitting and a red deposit of iron oxide at the interface

Fatigue Cracking Another type of deterioration is fatigue cracking (see Figure 2.3.7). Fatigue failure 

occurs at a stress level below the yield stress and is due to repeated loading. 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.8

Fatigue cracking has occurred in several types of bridge structures around the 

nation. This type of cracking can lead to sudden and catastrophic failure on certain 

bridge types. Therefore, the bridge inspector should know where to look and how 

to recognize early stages of fatigue crack development. 

Figure 2.3.7 Fatigue Crack at Coped Top Flange of Riveted Connection 

Some factors leading to the development of fatigue cracks are: 

3⁄4 Frequency of truck traffic 

3⁄4 Age or load history of the bridge 

3⁄4 Magnitude of stress range 

3⁄4 Type of detail 

3⁄4 Quality of the fabricated detail 

3⁄4 Material fracture toughness (base metal and weld metal) 

3⁄4 Weld quality 

3⁄4 Ambient temperature 

There are two basic types of bending in bridge members: in-plane and out-ofplane. When in-plane bending occurs, the cross section of the member resists the 

load according to the design and undergoes nominal elastic deformation. Out-ofplane bending implies that the cross section of the member is loaded in a plane 

other than that for which it was designed and undergoes significant elastic 

deformation or distortion. More correctly, out-of-plane bending should be referred 

to as out-of-plane distortion. Out-of-plane distortion is common in beam webs 

where transverse members, such as floorbeams, connect and can lead to fatigue 

cracking (see Figure 2.3.8). 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.9

There is a distinction between fatigue that is caused from in-plane (as designed) 

bending and out-of-plane distortion. 

Additional information about fatigue and fracture in steel bridges is presented in 

Topic 8.1. 

Figure 2.3.8 Out-of-plane Distortion 

Overloads Overloads are loads that exceed member or structure design loads. Steel is elastic 

(i.e., it returns to the original shape when a load is removed) up to a certain point, 

known as the yield point (see Topic P.2). When yield occurs, steel will bend or 

elongate and remain bent or elongated after the load has been removed. This type 

of permanent deformation of material beyond the elastic range is called plastic 

deformation. Plastic deformations due to overload conditions may be encountered 

in both tension and compression members. 

The symptoms of plastic deformation in tension members are: 

3⁄4 Elongation 

3⁄4 Decrease in cross section, commonly called "necking down" 

The symptoms of plastic deformation in compression members are: 

3⁄4 Buckling in the form of a single bow 

3⁄4 Buckling in the form of a double bow or "S" type, usually occurring where 

the section under compression is pinned or braced at the center point 

An overload can lead to plastic deformation, as well as complete failure of the 

member and structure. This occurs when a tension member breaks or when a 

compression member exhibits buckling distortion at the point of failure. 

Fatigue Crack 

Location 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.10

Collision Damage Components and structural members of a bridge that is adjacent to a roadway or 

waterway are susceptible to impact damage. Indications of impact damage include 

dislocated and distorted members (see Figure 2.3.9). 

Figure 2.3.9 Collision Damage on a Steel Bridge 

Heat Damage Steel members will undergo serious deformation upon exposure to extreme heat 

(see Figure 2.3.10). In addition to sagging, or elongation of the metal, intense heat 

often causes members to buckle and twist; rivets and bolts may fail at connection 

points. Buckling could be expected where the member is under compression, 

particularly in thin sections such as the web of a girder. 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.11

Figure 2.3.10 Heat Damage 

Temperatures affecting steel strength are as follows: 

 

3⁄4 204–260°C (400°-500°F) - starts to affect strength 

3⁄4 482–538°C (900°-1000°F) - major loss of strength

Paint Failures The following paint failures are common on steel: 

3⁄4 Chalking, erosion, checking, cracking, and wrinkling (see Figure 2.3.11), 

as described in ASTM D-3359.

3⁄4 Blisters are caused by painting over oil, grease, water, salt, or by solvent 

retention. Corrosion can occur under blisters. 

3⁄4 Undercutting occurs when surface rust advances under paint. It commonly 

occurs along scratches that expose the steel or along sharp edges (see 

Figure 2.3.12). The corrosion undermines intact paint, causing it to blister 

and peel. 

3⁄4 Pinpoint rusting can occur at pinholes in the paint, which are tiny, deep 

holes in the paint, exposing the steel (see Figure 2.3.13). It can also be 

caused by thin paint coverage. In this case, the "peaks" of the roughened 

steel surface protrude through the paint and corrode. 

3⁄4 Microorganism failure is caused by bacteria or fungi attacking 

biodegradable coatings. Oil/alkyds are the most often affected. 

3⁄4 Alligatoring can be considered a widely spaced checking failure, caused by 

internal stresses set up within the surface of a coating during drying (see 

Figure 2.3.14). The stresses cause the surface of the coating to shrink more 

rapidly to a much greater extent than the body of the coating. This causes 

large surface checks that do not reach the steel substrate.
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.12

Figure 2.3.11 Paint Wrinkling 

Figure 2.3.12 Rust Undercutting at Scratched Area 

Figure 2.3.13 Pinpoint Rusting 

Figure 2.3.14 Paint Peeling from Steel Bridge Members 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.13

3⁄4 Mudcracking can be considered a widely spaced cracking failure, where the 

breaks in the coating extend to the steel substrate, allowing rapid corrosion 

(see Figure 2.3.15). Mudcracking is often a phenomenon of inorganic zincrich primers, which are applied as a very thick layer or are applied on a hot 

surface. Rapid curing causes the shrinkage, which yields the alligatoring, 

and ultimately, mudcracks. 

3⁄4 Bleeding occurs when soluble colored pigment from an undercoat 

penetrates the topcoat, causing discoloration. 

Figure 2.3.15 Mudcracking Paint 

2.3.6 

Factors that 

Influence Fatigue 

Life 

Main factors influencing the development of fatigue cracks are: 

1. Magnitude of Stress Range 

Trucks (not cars or buses) produce stress ranges that may lead to fatigue 

cracks. 

2. Number of Cycles (Frequency)

This is dependent on: 

3⁄4 Frequency of truck traffic 

3⁄4 Age or load history of the bridge 

3. Fatigue Prone Details 

This depends on: 

3⁄4 The type of detail 

3⁄4 Quality of the fabricated detail 

3⁄4 Weld quality 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.14

Other factors influencing the development of fatigue cracks are: 

Material Fracture Toughness 

Toughness of the: 

3⁄4 Base metal 

3⁄4 Weld metal 

Toughness is based on the chemical composition of the steel. 

Ambient Temperature 

3⁄4 Colder – more likely to crack 

Fatigue and fracture are discussed in further detail in Topic 8.1 and the “Fracture 

Critical Inspection Techniques for Steel Bridges” NHI Course number FHWANHI-130078.

2.3.7 

Protective Systems Protective systems, when applied properly, provide protection needed against rust 

or corrosion. A thorough understanding of the steel corrosion process will help in 

the inspection of protective systems on bridge members. 

Corrosion of Steel Corrosion can be defined as a wearing away of metal by a chemical or 

electrochemical oxidizing process. Corrosion in metals is a form of oxidation 

caused by a flow of electricity from one part of the surface of one piece of metal to 

another part of the same piece. The result is the conversion of metallic iron to iron 

oxide. Once the corrosion process takes place, the steel member has a loss of 

section which results in a loss of structural capacity. Both conduction and soluble 

oxygen are necessary for the corrosion process to occur. 

A conductive solution (water) or electrolyte must be present in order for current to 

flow. Corrosion occurs very slowly in distilled water, but much faster in salty 

water, because the presence of salt (notably sodium chloride) improves the ability 

of water to conduct electricity and contributes to the corrosion process. In the 

absence of chlorides, steel (iron) corrodes slowly in the presence of water. Water is 

both the medium in which corrosion normally occurs and provides the corrosion 

reaction. In addition, oxygen accelerates the corrosion process. Corrosion stops or 

proceeds at a reduced rate when access to water and oxygen is eliminated or 

limited. Water and oxygen are therefore essential for the corrosion process. For 

example, corrosion of steel does not occur in moisture-free air and is negligible 

when the relative humidity of the air is below 30% at normal or lower 

temperatures. The presence of chlorides in the water will accelerate corrosion by 

increasing the conductivity of the water. 

To have corrosion take place in steel, then, one must have: 

3⁄4 Oxygen 

3⁄4 An electrolyte to conduct current 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.15

3⁄4 An area or region on a metallic surface with a negative charge (cathode) 

3⁄4 An area or region on the metallic surface with a positive charge (anode) 

Exposure of steel to the atmosphere provides a plentiful supply of oxygen. The 

presence of oxygen can limit corrosion by the formation of corrosion product films 

that coat the surface and prevent water and oxygen from reaching the uncorroded 

steel. The presence of contaminants such as chlorides accelerates the corrosion rate 

on steel surfaces by disrupting the protective oxide film. 

Galvanic Action The term "galvanic action" is generally restricted to the changes in normal 

corrosion behavior that result from the current generated when one metal is in 

contact with a different one. The two metals are in a corrosive solution when one 

metal may become an anode when it contacts a dissimilar metal. In such a 

"galvanic couple," the corrosion of one of the metals (e.g., zinc) will be accelerated, 

and the corrosion of the other (e.g., steel) will be reduced or possibly stopped. 

Galvanized coatings on highway guardrails and zinc-rich paint on structural steel 

are examples of galvanic protection using such a sacrificial (zinc) anode. 

Types and 

Characteristics of Steel 

Coatings 

Surface Preparation for Painting

The steel surface must be properly prepared prior to paint application. All foreign 

material must be removed. The following steps must be taken when preparing the 

surface for painting: 

3⁄4 Dirt and dust particles or spent abrasive from blast cleaning interfere with 

paint adhesion to the steel substrate and prevent application of a smooth, 

uniform film of paint. Debris embedded in the paint can also wick 

moisture and corrosive elements through the film to the substrate. 

3⁄4 Rust cannot be penetrated by most paints. Rust can become poorly 

adherent. In such cases, disbonding of the rust carries away the paint 

layers, permitting accelerated corrosion. 

3⁄4 Flash rust is a light layer of rust, which forms on the cleaned steel soon 

after exposure to the air, particularly in moist or humid environments. This 

layer may not be thoroughly wetted and may impede adhesion. 

3⁄4 Salts trapped in the paint film can cause blistering and disbonding. 

3⁄4 Oil and grease prevent good paint adhesion and must be completely 

removed. Welding smoke and inspection markings leave an oily residue 

that must also be completely removed. 

3⁄4 Dead paint that is loose, cracking, or flaking will eventually lift from the 

surface, carrying any new paint with it. 

3⁄4 Mill scale is a layer of iron oxide on the surface of steel. It forms when the 

steel is heated at high temperatures in a furnace. Mill scale has a bluish, 

somewhat shiny appearance, which may be difficult to see on partially blast

cleaned steel. It must be completely removed when using most coating 

materials, as it may disbond upon expansion and contraction, carrying the 

paint with it. 

3⁄4 Weld spatter may also dislodge, leaving a bare exposed steel surface. 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.16

The surface should also be roughened to promote paint adhesion, as paint will not 

adhere well to a smooth surface. 

Methods of Surface Preparation

The Society of Protective Coatings publishes a set of standards and specifications 

describing the following methods of surface preparation: 

3⁄4 Solvent cleaning 

3⁄4 Hand tool cleaning 

3⁄4 Power tool cleaning 

3⁄4 Abrasive blast cleaning 

3⁄4 Water blast cleaning 

Solvent cleaning removes oil and grease. It is usually used in conjunction with or 

prior to the mechanical preparation methods. Common solvents include petroleum 

and coal tar solvents, turpentine, mineral spirits, alkaline cleaners, and emulsion 

cleaners, which contain oil soaps mixed with kerosene or mineral spirits. 

Hand tool cleaning is used for removing loosely adhering paint, rust, or mill scale. 

It will not remove tightly adhering mill scale, or dirt and oils in crevices. Due to its 

slow speed, hand tool cleaning is used mostly for small area spot cleaning. 

Common hand tools include scrapers, wire brushes, chipping hammers, knives, 

chisels, and abrasive pads. 

Power tool cleaning is effective on both plane and contoured steel surfaces. Power 

tool cleaning devices remove loose paint, rust, and scale. Power tools do not leave 

the residue common with blast cleaning. Also, power tools are used on small areas 

and where the abrasive could damage sensitive surroundings. 

Abrasive blast cleaning is the preferred surface preparation method for coatings, 

which require a high degree of cleanliness and a uniformly roughened surface 

profile. Blast cleaning is a production method, which can remove mill scale. A 

water collar is sometimes used with abrasive blast cleaning to prevent abrasive rust 

and paint particles from becoming airborne. 

Water blast cleaning (hydroblasting) may be high or low pressure, hot or cold, with 

or without detergent, depending upon the type of cleaning desired. Water does not 

etch a steel surface and may not remove tight paint, rust, or mill scale. Abrasives 

may be injected into the water stream to remove tightly adhering material for faster 

clearing or to produce a roughened surface profile. Sand is the most common 

abrasive. The process can remove old paint, rust, and mill scale. It yields a degree 

of cleanliness equivalent to open nozzle abrasive blast cleaning. High water 

pressure may cause flash rusting. Water blast cleaned areas must be either cleaned 

by dry abrasive blast cleaning or a corrosion inhibiting chemical must be added to 

prevent flash rusting. 

Paint Once the steel surface is properly prepared, the appropriate type and application of 

paint must be chosen based on the paint characteristics. 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.17

Paint is by far the most common coating used to protect steel bridges. Paint is 

composed of four basic compounds: pigments, vehicle (also called binder), solvents 

(also called thinners), and additives (such as thickeners and mildewoides). The 

pigments contribute such properties as inhibition of corrosion of the metal surface 

(e.g., zinc, zinc oxide, red lead, and zinc chromate), reinforcement of the dry paint 

film, stabilization against deterioration by sunlight, color, and hardness. Pigments 

are generally powders before being mixed into paint. The vehicle also remains in 

the dry-cured paint layer. It binds the pigment particles together and provides 

adhesion to the steel substrate and to other paint layers. Thus, the strength of the 

binder contributes to the useful life of the coating. Paint can be classified as 

inorganic or organic, depending on the vehicle. Inorganic paint uses a water soluble 

silicate binder which reacts with water during paint curing. Most types of paint 

contain one of a variety of available organic binders. The organic binders cure 

(harden) by one or more of the following mechanisms: 

3⁄4 Evaporation of solvents 

3⁄4 Reaction with oxygen in the air 

3⁄4 Polymerization through the action of heat or a catalyst 

3⁄4 Combination of reactive components in the binder 

Solvents, which are liquids (such as water and mineral spirits), are included in paint 

to transport the pigment-binder combination to the substrate, to lower paint 

viscosity for easier application, to help the coating penetrate the surface, and to wet 

the substrate. Since the solvent is volatile, it eventually evaporates from the dry 

paint film. Additives are special purpose ingredients that give the product extra 

performance features. For example, mildewoides reduce mildew problems, and 

thickeners lengthen the drying time for application in hot weather. 

Paint used on steel bridges acts as a physical barrier to moisture, oxygen, and 

chlorides, all of which promote corrosion. While water and oxygen are important 

to corrosion, chlorides from deicing road salts or seawater spray accelerate the 

corrosion process significantly. 

Paint Layers

Paint on steel is usually applied in up to three layers, or coats: 

3⁄4 Primer coat 

3⁄4 Intermediate coat 

3⁄4 Topcoat 

The primer coat is in direct contact with the steel substrate. It is formulated to have 

good wetting and bonding properties and may or may not contain passivating 

(corrosion-inhibiting) pigments. 

The intermediate coat must strongly adhere to the primer. It provides increased 

thickness of the total coating system, abrasion and impact resistance, and a barrier 

to chemical attack. 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.18

The topcoat (also called the finish coat) is typically a tough, resilient layer, 

providing a seal to environmental attack, water, impact, and abrasion. It is also 

formulated for an aesthetic appearance. 

Types of Paint

A wide variety of paints are applied to steel bridges. All of them except some zincrich primers use an organic binder. 

Oil/alkyd Paint 

Oil/alkyd paints use an oil such as linseed oil and an alkyd resin as the binding 

agent. Alkyd resin is synthetically produced by reacting a drying oil acid with an 

alcohol. Alkyd paints are low cost, with good durability, flexibility, and gloss 

retention. They are also tough, with moderate heat and solvent resistance. They 

should not be used in water immersion service or in alkaline environments. 

A disadvantage is their offensive odor. They are also slow drying, difficult to clean 

up, and have poor exterior exposure. Alkyd paints often contain lead pigments, 

which are known to cause numerous health problems. The removal and disposal of 

lead-based paints is a regulated activity in all states. 

Vinyl Paint 

Vinyl paints are based on various vinyl polymer binding agents dissolved in a 

strong solvent. These paints cure by solvent evaporation. Vinyls have excellent 

chemical, water, salt, acid, and alkali resistance, good gloss retention, and are 

applicable at low temperatures. Conversely, their disadvantages include poor heat 

and solvent resistance, and poor adhesion. Vinyls are usually not used with other 

types of paint in a paint system. Vinyl coatings can be formulated to serve as 

primer, intermediate, and topcoat in paint systems. 

Epoxies 

Epoxies utilize an epoxy polymer binder, which forms a tough, resilient film upon 

drying and curing. Drying is by solvent evaporation, while curing entails a 

chemical reaction between the coating components. Epoxy coatings have excellent 

atmospheric exposure characteristics, as well as resistance to chemicals and water. 

They are often used as the intermediate coat in a three-layer paint system. There 

are also two- and three-layer systems, which use only epoxies. One disadvantage of 

epoxies is that they chalk when exposed to sunlight. This chalking must be 

removed prior to topcoating with another layer of epoxy or another material. If not 

removed, the chalking will compromise subsequent adhesion. 

Epoxy Mastics 

Epoxy mastics are heavy, high solid content epoxy paints, often formulated with 

flaking aluminum pigment. The mastics are useful in applications where a heavy 

paint layer is required in one application. They can be formulated with wetting and 

penetrating agents, which permit application on minimally prepared steel surfaces. 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.19

Urethanes 

Urethanes are commonly used as the topcoat layer. They provide excellent sunlight 

resistance, hardness, flexibility (i.e., resistance to cracking), gloss retention, and 

resistance to water, harmful chemicals, and abrasion. All-urethane systems are also 

available which utilize urethane paints as primer, intermediate, and topcoat. 

Zinc-rich Primers 

Zinc-rich primers contain finely divided zinc powder (75% to 95%) and either an 

organic or inorganic binder. They protect the steel substrate by galvanic action, 

wherein the metallic zinc corrodes in preference to the steel. The materials have 

excellent adhesion and resist rust undercutting when applied over a properly 

prepared surface. The zinc-rich primers must be well mixed prior to application, or 

some coated areas will be deficient in zinc, lowering the substrate protection. 

Latex Paint 

Latex paint consists of a resin emulsion. The term covers a wide range of 

materials, each formulated for a different application. Latex on steel has excellent 

flexibility (allowing it to expand and contract with the steel as the temperature 

changes) and color retention, with good adhesion, hardness, and resistance to 

chemicals. Latex paint has low odor, faster drying time, and easier clean up. 

The disadvantages of latex paint include sacrificed durability, and it must be 

applied at temperatures over 10°C (50°F). 

It is important to document the existing paint system on a bridge. The paint type 

may be shown on the bridge drawings or specifications. Some agencies list the 

paint type and application date on the bridge. Once the existing paint is 

determined, a compatible paint for any required rehabilitation can be chosen. 

Protection of 

Suspension Cables 

Suspension cables of steel suspension bridges are particularly difficult to protect 

from corrosion. One method is to wrap the cables with a neoprene elastomeric 

cable wrap system or with a glass-fabric-reinforced plastic shell. In some cases, the 

elastomeric cable wrap has retained water and accelerated corrosion. Another 

method is to pour or inject paints into the spaces between the cable strands. 

Commonly, inhibitive pigments, such as zinc oxide, in an oil medium are used. Red 

lead pigment was commonly used in the past. Lead constitutes a significant health 

hazard, and care must be exercised care when inspecting cables. Do not inhale or 

ingest old paint. The paint on the exterior surface of a suspension cable dries, but 

the paint on the interior, surrounding individual strands, stays in the liquid, uncured 

state for years. The exterior of the cable is often topcoated with a different paint, 

such as an aluminum pigmented oil-based paint. Another option to protect 

suspension cables is to wrap tightly with small diameter wires. This allows the 

cable to “breath” while still providing a protective cover. 

A newer technique used to resist the corrosion process of suspension cables is 

forced air dehumidification. On larger structures (such as the Kobe Bridge in Japan 

and the Ben Franklin Bridge in Pennsylvania), dry air is passed through the cables, 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.20

which does not allow the steel to be exposed to moisture. For this protection 

system to work, the relative humidity of the forced air should be less than 

approximately 60%. 

Weathering Steel In the proper environments, weathering steel does not require painting but produces 

its own protective coating. When exposed to the atmosphere, weathering steel 

develops a protective oxide film, which seals and protects the steel from further 

corrosion. This oxide film is actually an intended layer of surface rust, which 

protects the member from further corrosion and loss of material thickness. 

Weathering steel was first used for bridges in 1964 in Michigan. Since then, 

thousands of bridges have been constructed of weathering steel in the United 

States. The early successes of weathering steel in bridges led to the use of this steel 

in locations where the steel could not attain a protective oxide layer and where 

corrosion progressed beyond the intended layer of surface rust. Therefore, it is 

important for the inspector to distinguish between the protective layer of rust and 

advanced corrosion that can lead to section loss. It is also important to note that 

fatigue cracks can initiate in rust pitted areas of weathering steel. 

The frequency of surface wetting and drying cycles determines the oxide film’s 

texture and protective nature. The wetting cycle includes the accumulation of 

moisture from rainfall, dew, humidity, and fog, in addition to the spray of water 

from traffic. The drying cycle involves drying by sun and wind. Alternate cycles 

of wetting and drying are essential to the formation of the protective oxide coating. 

The protective film will not form if weathering steels remain wet for long periods 

of time. 

Uses of Weathering Steel

Weathering steels may be unsuitable in the following environments: 

3⁄4 Areas with frequent high rainfall, high humidity, or persistent fog 

3⁄4 Marine coastal areas where the salt-laden air may deposit salt on the steel, 

which leads to moisture retention and corrosion 

3⁄4 Industrial areas where chemical fumes may drift directly onto the steel and 

cause corrosion 

3⁄4 Areas subject to “acid rain” which has a sulfuric acid component 

The location and geometrics of a bridge also influence performance of weathering 

steel. Locations where weathering steel may be unsuitable include: 

3⁄4 Tunnel-like situations which permit concentrated salt-laden road sprays, 

caused by high-speed traffic passing under the bridge, to accumulate on the 

superstructure 

3⁄4 Low level water crossings where insufficient clearance over bodies of 

water exists so that spray and condensation of water vapor result in 

prolonged periods of wetness 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.21

2.3.8 

Inspection 

Procedures for 

Steel 

There are three basic procedures used to inspect a steel member. Depending on the 

type of inspection, the inspector may be required to use only one individual 

procedure or all procedures. They include: 

3⁄4 Visual 

3⁄4 Physical 

3⁄4 Advanced inspection techniques 

Visual Examination Steel Members

Steel members should be inspected for corrosion, section loss, buckling, and 

cracking. 

In the inspection records, identify the location of FCMs and describe the FCM 

inspection frequency and procedures. Inspect FCMs according to the AASHTO 

“Manual for Condition Evaluation and Load and Resistance Factor Rating”. See 

Topic 8.1 for detailed description of inspection procedures of fracture critical 

members. 

Some common inspection locations and signs of fatigue distress include: 

3⁄4 Bent or damaged members - determine the type of damage (e.g., collision, 

overload, or fire), inspect for proper alignment, and check for cracks, tears, 

and gouges near the damaged location 

3⁄4 Corrosion, which could reduce structural capacity through a decrease in 

member section and make the member less resistant to both repetitive and 

static stress conditions; since rust continually flakes off of a member, the 

severity of corrosion cannot always be determined by the amount of rust; 

therefore, corroded members must be examined by physical as well as 

visual means (see Figure 2.3.16) 

3⁄4 Fatigue cracks - fatigue cracks are common at certain locations on a bridge, 

and certain inspection procedures should be followed when fatigue cracks 

are observed (see Figure 2.3.17 and Topic 8.1 for additional information 

about fatigue cracks) 

3⁄4 Other stress-related cracks - determine the length, size, and location of the 

crack 

3⁄4 Points on the structure where a discontinuity or restraint is introduced 

3⁄4 Loose members which could force the member or other members to carry 

unequal or excessive stress 

3⁄4 Damaged members, regardless of damage magnitude, which are 

misaligned, bent, or torn 

3⁄4 Welded details 

3⁄4 Repairs that show indiscriminate welding or cutting procedures 

3⁄4 Areas of excessive vibrations or twisting 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.22

Inspection procedures for in-plane fatigue cracks: 

3⁄4 Report the fatigue crack immediately 

3⁄4 Determine the visual ends of the crack 

3⁄4 Examine other identical details on the bridge for cracks 

3⁄4 Examine other details for breaks in the paint and the formation of oxide 

(rust) 

3⁄4 If a suspect area is located, a more detailed non-destructive examination, 

may be required (see Topic 13.3).

Figure 2.3.16 Corrosion of Steel 

Figure 2.3.17 Fatigue Crack 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.23

 Protective Coatings

Rust typically starts in a few characteristic places, then spreads to larger areas. 

Examine sharp edges and square corners of structural members (see Figure 2.3.18). 

Paint is generally thinner at sharp edges and corners than at rounded edges and 

corners or flat surfaces. Rusting starts at sharp edges, then undercuts intact paint as 

it spreads away from the edge. Inside square corners often receive an extra thick 

layer of paint due to double or triple passes made over them. Extra thick layers are 

prone to cracking, exposing the steel. It is difficult to completely remove dirt and 

spent blast cleaning abrasive from inside corners. Painting over this foreign 

material results in early peeling and corrosion. 

Figure 2.3.18 Paint Failure on Edge of Steel Truss Member 

Examine all areas that retain moisture and salt. Check under scuppers and beneath 

downspouts. Check horizontal surfaces under the edge of bridge decks and under 

expansion dams, where roadway deicing salt runoff collects (see Figure 2.3.19). 

Examine the bottom inside flange of girders. 

Inspect inaccessible or hard-to-reach areas that may have been missed during 

painting. A flashlight and inspection mirror may be needed here. Examine the 

inside surfaces of lattice girders and beams. Examine the top surface of girder 

upper flanges under the bridge deck, if possible. 

Inspect around bolts, rivets, and pins (see Figure 2.3.20). Rust detected around the 

heads may indicate corrosion along the entire length of the bolt, rivet, or pin, 

causing reduced structural integrity. 

Examine roadway splash or spray zones, where debris and corrosive deicing saltladen water are directly deposited on painted members by passing traffic (see 

Figure 2.3.21). On through-truss bridges, this includes some bracing members 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.24

above the roadway. 

Examine areas exposed to wind and rain, seawater spray, and other adverse weather 

conditions. 

Figure 2.3.19 Water and Salt Runoff Under Expansion Dam Deck Opening 

Figure 2.3.20 Corroding Rivet Head 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.25

Figure 2.3.21 Roadway Spray Zone Damage 

 Weathering Steel 

It is particularly important for weathering steel to be inspected in the following 

locations: 

3⁄4 Where water ponds or the steel remains damp for long periods of time due 

to rain, condensation, leaky joints, or traffic spray 

3⁄4 Where debris is likely to accumulate 

3⁄4 Where the steel is exposed to salts and atmospheric pollutants 

3⁄4 Near defective joints or drainage devices 

Color 

The color of the surface of weathering steel is an indicator of the protective oxide 

film (see Figure 2.3.22). The color changes as the oxide film matures to a fully 

protective coating. 

A yellow-orange, for new steel with initial exposure, is acceptable (see Figures 

2.3.23 and 2.3.24). For bridges that have been in service for several years, purple 

brown color is acceptable (see Figure 2.3.25), while black color indicates failed 

condition (see Figure 2.3.26). 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.26

Figure 2.3.22 Color of Oxide Film is Critical in the Inspection of Weathering 

Steel; Dark Black Color in an Indication of Non-protective Oxide 

Figure 2.3.23 Yellow Orange – Early Stage of Exposure or Active Corrosion 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.27

Figure 2.3.24 Light Brown – Early Stage of Exposure 

Figure 2.3.25 Chocolate Brown to Purple Brown - Boldly Exposed and Good 

Degree of Protection 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.28

Figure 2.3.26 Black – Non-protective Oxide 

An area of steel, which is a different color than the surrounding steel indicates a 

potential problem. The discolored area should be investigated to determine the 

cause of the discoloration. Color photographs are an ideal way to record the color 

of the weathering steel over time. A color coupon should be included in each 

photograph to enable comparison. 

Texture 

The texture of the oxide film also indicates the degree of protection of the film. 

An inspection of the surface by tapping with a hammer and vigorously brushing 

the surface with a wire brush determines the adhesion of the oxide film to the steel 

substrate. Surfaces, which have granules, flakes, or laminar sheets are examples of 

non-adhesion. Table 2.3.1 presents a correlation between the texture of the 

weathering steel and the degree of protection. 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.29

Appearance Degree of Protection 

Tightly adhered, capable of 

withstanding hammering or vigorous 

wire brushing 

Protective oxide 

Dusty Early stages of exposure; should change 

after few years 

Granular Possible indication of problem, 

depending on length of exposure and 

location of member 

Small flakes, 6 mm (1/4 inch) in 

diameter

Initial indication of non-protective oxide

Large flakes, 13 mm (1/2 inch) in 

diameter or greater

Non-protective oxide 

Laminar sheets or nodules Non-protective oxide, severe conditions 

Table 2.3.1 Correlation Between Weathering Steel Texture and Condition 

Physical Examination Steel Members 

Once the defects are identified visually, physical procedures must be used to verify 

the extent of the defect. For steel members, the main physical inspection 

procedures involve the use of an inspection hammer or wire brush. Excessive 

hammering, brushing or grinding may close surface cracks and make the cracks 

difficult to find. Corrosion results in loss of member material. This partial loss of 

cross section due to corrosion is known as section loss. Section loss may be 

measured using a straight edge and a tape measure. However, a more exact method 

of measurement, such as calipers or an ultrasonic thickness gauge (D-meter), 

should be used to measure the remaining section of steel. The inspector must 

remove all corrosion products (rust scale) prior to making measurements. 

The inspector should measure the bridge members to verify that the sizes recorded 

in the plans or inspection report are accurate. If incorrect member sizes are used, 

the load rating analysis for safe load capacity of the bridge will be inaccurate. 

Protective Coatings

The degree of coating corrosion must be assessed during the inspection. Coating 

corrosion is measured differently than structural corrosion. There are a variety of 

proprietary procedures which use a set of photographic standards to evaluate and 

categorize the degree and extent of coating corrosion on composite spans, cross 

frames, exterior fascias, and bearings. A simple method entails evaluation of 

painted surfaces in accordance with SSPC-Vis 2. Vis 2 is a pictorial standard for 

evaluating the degree of rusting on painted steel surfaces. 

Mill Scale 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.30

Incomplete removal of mill scale can provide a starting point for corrosion. When 

mill scale cracks, it allows moisture and oxygen to reach the steel substrate. Mill 

scale accelerates corrosion of the substrate because of its electrochemical 

properties. To check for mill scale corrosion during a paint inspection, use a knife 

to remove a small patch of paint in random spots. Inspect the exposed surface for 

mill scale, either intact or rusted. Probe with a knife or other sharp object at weld 

spatter to check for rusting. Re-coat areas where paint is removed. 

Invisible microscopic chloride deposits from deicing salt or seawater spray may 

permeate a corroding steel surface. Painting over a partially cleaned chloridecontaminated surface simply seals in the contaminant. Salt deposits draw moisture 

through the paint by osmosis, and corrosion will continue. 

Paint Adhesion 

Paint can undergo adhesion failure between paint layers or between the primer and 

steel. Some bridge painting contracts specify minimum acceptable paint adhesion 

strength for new paint. Over time, however, adhesion strength may degrade as the 

paint weathers and is affected by sunlight, or as rusting occurs under the paint. 

The simplest test of adhesion is to probe under paint with the point of a knife. A 

more quantitative evaluation is performed by a tape test, as described in Topic 2.1.

Paint Dry Film Thickness 

There are a variety of instruments to measure the dry film thickness of paint 

applied to steel. Accuracy ranges from 10% +/- to 15% +/-, and they fall into 

three classes: 

3⁄4 Magnetic pull-off 

3⁄4 Fixed probe 

3⁄4 Destructive test 

The magnetic pull-off dry film thickness gages use the attractive force between a 

magnet and the steel substrate to determine the paint thickness. The thicker the 

paint, the lower the magnetic force. These instruments must be calibrated prior to 

and during use with plastic shims of known thickness, or with ferrous plates coated 

with a non-ferrous layer. Such shims are produced by the National Bureau of 

Standards (NBS). 

The fixed probe gages also use a magnet. Measurement of paint thickness is done 

by an electrical measurement of the interaction of the probe's magnetic field with 

the steel rather than by the force to move the magnet. They are normally 

calibrated with plastic shims. Neither the magnetic pull-off nor fixed probe gages 

can be used closer than one inch to edges, as this will distort the reading. SSPCPA2 "Measurement of Dry Paint Thickness With Magnetic Gages" provides a 

detailed description of how to calibrate and take measurements using magnetic 

gages. 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.31

A destructive method for measuring dry film thickness uses the Tooke Gage 

described in Topic 2.1. An advantage of this method is that it can be used at any 

location, including close to edges. While the magnetic gages measure the 

combined thickness of all paint layers, the Tooke Gage measures each layer 

individually. Limitations of the destructive test are that only coatings up to 50 mils 

thick can be measured and multiple layers of the same color cannot be 

distinguished. 

Repainting 

If the coating is to be repainted, the type of in-place paint must be known, since 

different type paints may not adhere to each other. Methods described in Topic 

2.1 can be used to determine the type of in-service paint. 

Weathering Steel 

Weathering steel with any of the following degree of protection should be 

inspected: 

3⁄4 Laminar texture of steel surface, such as slab rust or thin and fragile sheets 

of rust 

3⁄4 Granular and flaky rust texture of steel surface 

3⁄4 A very coarse texture 

3⁄4 Large granular (3 mm (1/8 inch) in diameter) texture 

3⁄4 Flakes (13 mm (1/2 inch) in diameter) 

3⁄4 Surface rubs off by hand or wire brush revealing a black substrate 

3⁄4 Surface is typically covered with deep pits 

If such conditions are discovered, the following steps should be taken to determine 

the adequacy of the oxide film: 

3⁄4 Scrape the surface of the steel to the bare metal 

3⁄4 Check to determine the extent of pitting 

3⁄4 Measure the remaining section thickness with calipers or an ultrasonic 

thickness gauge 

It is important to set a benchmark at the point where the metal thickness 

measurement is taken so that any metal loss may be monitored with future 

measurements. Benchmarks are important since steel rolled sections and steel 

plates often vary within acceptable tolerances in thickness from the nominal 

thickness values. 

Data obtained from the inspection should include visual observations of the steel 

(e.g., color, texture, and flaking), physical measurements with a thickness gauge, 

and observation of environmental conditions. 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.32

Advanced Inspection 

Techniques 

In addition, several advanced techniques are available for steel inspection. 

Nondestructive methods, described in Topic 13.3.2, include: 

3⁄4 Acoustic emissions testing 

3⁄4 Computer programs 

3⁄4 Computer tomography 

3⁄4 Corrosion sensors 

3⁄4 Smart paint 1 is paint with microencapsulated dyes that outline a fatigue 

crack as it forms and propagates 

3⁄4 Smart paint 2 is paint that sends out electrical signals which are picked up 

by electrodes if material begins to vibrate 

3⁄4 Dye penetrant 

3⁄4 Magnetic particle 

3⁄4 Radiographic testing 

3⁄4 Robotic inspection 

3⁄4 Ultrasonic testing 

3⁄4 Eddy current 

Other methods, described in Topic 13.3.3, include: 

3⁄4 Brinell hardness test 

3⁄4 Charpy impact test 

3⁄4 Chemical analysis 

3⁄4 Tensile strength test 

2.3.9 

Other Bridge 

Materials 

Cast Iron Iron is an elemental metal smelted from iron ore. Cast iron is the most widely 

used cast metal. However, it is easily fractured by shocks and has low tensile 

strength due to a large percentage of free carbon and slag. Consequently, it is 

basically a poor bridge construction material and is not used in new bridge 

construction today. It may, however, be found in compression members of old 

bridges. 

Cast iron is gray in color due to the presence of tiny flake-like particles of graphite 

(carbon) on the surface. It has a unit weight of approximately 7210 kg/m3 (450 

pcf). 

Properties of Cast Iron

Some of the mechanical properties of cast iron include: 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.33

3⁄4 Strength - tensile strength varies from 172 MPa (25,000 psi) to 345 MPa 

(50,000 psi), while compressive strength varies from 448 MPa (65,000 

psi) to 1,035 MPa (150,000 psi) 

3⁄4 Elasticity - cast iron has an elastic modulus of 89,635 MPa (13,000,000 

psi) to 206,850 MPa (30,000,000 psi): elasticity increases with a decrease 

in carbon content 

3⁄4 Workability - cast iron possesses good machinability, and casting is 

relatively easy and inexpensive 

3⁄4 Weldability - cast iron can not be effectively welded due to its high free 

carbon content 

3⁄4 Corrosion resistance - cast iron is generally more corrosion resistant than 

the other ferrous metals 

3⁄4 Brittleness - cast iron is very brittle and prone to fatigue-related failure 

when subjected to bending or tension stresses 

 Types of Cast Iron Deterioration

The primary forms of deterioration in cast iron are similar to those in steel. 

Wrought Iron When iron is mechanically worked or rolled into a specific shape, it is classified as 

wrought iron. This process results in slag inclusions that are embedded between 

the microscopic grains of iron. It also results in a fibrous material with properties 

in the worked direction similar to steel. Wrought iron is no longer made in the 

United States. However, wrought iron tension members still exist on some older 

bridges, and it was well-suited for use in the early suspension bridges. 

Properties of Wrought Iron

Some of the mechanical properties of wrought iron include: 

3⁄4 Strength - wrought iron is anisotropic (i.e., its strength varies with the 

orientation of its grain) due to the presence of slag inclusions; 

compressive strength is about 241 MPa (35,000 psi), while tensile strength 

varies between 248 MPa (36,000 psi) and 345 MPa (50,000 psi) 

3⁄4 Elasticity - modulus of elasticity ranges from 165,000 MPa (24,000,000 

psi) to 200,000 MPa (29,000,000 psi), nearly as high as steel 

3⁄4 Impact resistance - wrought iron is tough and is noted for impact and 

shock resistance 

3⁄4 Workability - wrought iron possesses good machinability 

3⁄4 Weldability - wrought iron can be welded, but care should be exercised 

when welding the metal of an existing bridge 

3⁄4 Corrosion resistance - the fibrous nature of wrought iron produces a tight 

rust which is less likely to progress to flaking and scaling than is rust on 

carbon steel 

3⁄4 Ductility - wrought iron is generally ductile; reworking the wrought iron 
SECTION 2: Bridge Materials 

TOPIC 2.3: Steel 

2.3.34

causes a finer and more thread-like distribution of the slag, thereby 

increasing ductility 

Types of Wrought Iron Deterioration

The primary forms of deterioration in wrought iron are similar to those in steel. 

Aluminum Aluminum is widely used for signs, light standards, railings, and sign structures. 

Aluminum is seldom used as a primary material in the construction of vehicular 

bridges. 

Properties of Aluminum

The properties of aluminum are generally similar to those of steel. However, a 

few notable differences exist: 

3⁄4 Weight - aluminum alloy has a unit weight of about 2800 kg/m3

 (175 pcf) 

3⁄4 Strength - aluminum is not as strong as steel, but alloying can increase its 

strength to that of steel 

3⁄4 Corrosion resistance - aluminum is highly resistant to atmospheric 

corrosion 

3⁄4 Workability - aluminum is easily fabricated, but welding of aluminum 

requires special procedures 

3⁄4 Durability - aluminum is durable 

3⁄4 Expense - aluminum is more expensive than steel 

Types of Aluminum Deterioration

The primary forms of deterioration in aluminum are: 

3⁄4 Fatigue cracking - the combination of high stresses and vibration caused 

by wind produces fatigue 

3⁄4 Pitting - aluminum can pit slightly, but this condition rarely becomes 

serious 
2.4-i

Table of Contents

Chapter 2 ..................................................................................................................................................

Bridge Materials 

 2.4 Stone Masonry ....................................................................................... 2.4.1 

 

 2.4.1 Introduction............................................................................... 2.4.1 

 

 2.4.2 Properties of Stone Masonry..................................................... 2.4.1 

 Physical Properties ............................................................. 2.4.1 

 Mechanical Properties ........................................................ 2.4.2 

 Mortar................................................................................. 2.4.2 

 

 2.4.3 Stone Masonry Construction Methods...................................... 2.4.2 

 Rubble Masonry ................................................................. 2.4.2 

 Squared-Stone Masonry ..................................................... 2.4.2 

 Ashlar Masonry .................................................................. 2.4.2 

 2.4.4 Types and Causes of Stone Masonry and Mortar Deterioration2.4.2 

 

 2.4.5 Protective Systems .................................................................... 2.4.4 

 2.4.6 Inspection Procedures for Stone Masonry and Mortar ............. 2.4.4 
SECTION 2: Bridge Materials 

TOPIC 2.4: Stone Masonry 

2.4-ii

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2.4.1

Topic 2.4 Stone Masonry 

2.4.1 

Introduction Stone masonry is seldom used in new bridge construction today except as facing or 

ornamentation. However, many old stone bridges are still in use and require 

inspections (see Figure 2.4.1). Granite, limestone, and sandstone are the most 

common types of stone that were used and are still seen today in bridges. In 

addition, many smaller bridges and culverts were built of locally available stone. 

Stone masonry typically has a unit weight of approximately 170 pcf. 

Figure 2.4.1 Stone Masonry Arch 

2.4.2 

Properties of Stone 

Masonry 

The physical properties of stone masonry in bridge applications are of primary 

concern. Strength, hardness, workability, durability, and porosity properties of 

both the stone and the mortar play important roles in the usage of stone masonry. 

Physical Properties The major physical properties of stone masonry are: 

3⁄4 Hardness – the hardness of stone varies based on the stone type. Some 

types of sandstone are soft enough to scratch easily, while other stones 

may be harder than some grades of steel 

3⁄4 Workability - measures the amount of effort needed to cut or shape the 

stone. Harder stones are not as workable as softer stones stones 

3⁄4 Porosity – porosity in a stone indicates the amount of open or void space 

within that stone. All stones have some degree of porosity. A stone that is 

less porous can resist freeze/thaw action better than a stone with a higher 

degree of porosity. Water absorption is directly related to the degree of 

porosity 
SECTION 2: Bridge Materials 

TOPIC 2.4: Stone Masonry 

2.4.2

Mechanical Properties The major mechanical properties of stone masonry are: 

3⁄4 Strength – a stone generally has sufficient strength to be used as a loadbearing bridge member, even though the strength of an individual stone 

type may vary tremendously. As an example, granite’s compressive 

strength can vary from 7,700 to 60,000 psi (53 to 414 MPa). For the 

typical bridge application, a stone with a compressive strength of 5,000 

psi (34.5 MPa) is acceptable. The mortar is almost always weaker than 

the stone 

3⁄4 Durability – durability of a stone depends on how well it can resist 

exposure to the elements, rain, wind, dust, frost action, heat, fire, and airborne chemicals. Some stone types are so durable that they are able to 

effectively resist the elements for two hundred years, while other stone 

types deteriorate after about ten years 

Mortar Mortar is primarily composed of sand, cement, lime and water. The cement is 

generally Portland cement and provides strength and durability. Lime provides 

workability, water retentivity and elasticity. Sand is filler and contributes to 

economy and strength. The water, as in the case of concrete, can be almost any 

potable water. See Topic 2.2 for more information on mortar. 

2.4.3 

Stone Masonry 

Construction 

Methods 

There are three general methods of stone masonry construction: 

3⁄4 Rubble masonry 

3⁄4 Squared-stone masonry 

3⁄4 Ashlar Masonry 

Rubble Masonry Rubble masonry consists of rough stones which are un-squared and used as they 

come from the quarry. It could be constructed to approximate regular rows or 

courses (coursed rubble) or could be un-coursed (random rubble). Random rubble 

was the least expensive type of stone masonry construction and was considered 

strong and durable for small spans if well constructed. 

Squared-Stone Masonry Squared-stone masonry consists of stones, which are squared and dressed roughly. 

It could be laid randomly or in courses. 

Ashlar Masonry Ashlar consists of stones, which are precisely squared and finely dressed. Like 

squared-stone masonry, it could be laid randomly or in courses. 

2.4.4 

Types and Causes 

of Stone Masonry 

and Mortar 

Deterioration 

The primary types of deterioration in stone masonry are: 

3⁄4 Weathering – hard surfaces degenerate into small granules, giving stones a 

smooth, rounded look; mortar disintegrates 

3⁄4 Spalling – small pieces of rock break out 

3⁄4 Splitting – seams or cracks open up in rocks, eventually breaking them into 

smaller pieces (see Figure 2.4.2) 

3⁄4 Fire – masonry is not flammable but can be damaged by high temperatures 
SECTION 2: Bridge Materials 

TOPIC 2.4: Stone Masonry 

2.4.3

Figure 2.4.2 Splitting in Stone Masonry 

Some of the major causes of these forms of deterioration are: 

3⁄4 Chemicals – gases and solids, such as deicing agents, dissolved in water often 

attack stone and mortar; oxidation and hydration of some compounds found in 

rock can also cause damage 

3⁄4 Volume changes – seasonal expansion and contraction can cause fractures to 

develop, weakening the stone 

3⁄4 Frost and freezing – water freezing in the seams and pores can spall or split 

stone or mortar 

3⁄4 Abrasion – due primarily to wind or waterborne particles 

3⁄4 Plant growth – roots and stems growing in crevices and joints can exert a 

wedging force, and lichen and ivy can chemically attack stone surfaces

3⁄4 Marine growth – chemical secretions from rock-boring mollusks deteriorate 

stone 

Two major factors that affect the durability of stone masonry include: 

3⁄4 The proper curing of mortar 

3⁄4 Correct placement of stones during construction
SECTION 2: Bridge Materials 

TOPIC 2.4: Stone Masonry 

2.4.4

2.4.5 

Protective Systems The different types of protective systems used for concrete can also be used for 

stone masonry. The two most common systems that are used are paints and water 

repellant membranes or sealers. See Topic 2.2 for a complete description of the 

different types of protective systems. 

2.4.6 

Inspection 

Procedures for 

Stone Masonry and 

Mortar 

The examination of stone masonry and mortar is similar to that of concrete. The 

joints should be carefully inspected for cracks, loose or missing mortar, vegetation, 

water seepage and other forms of mortar deterioration. The stones should be 

carefully inspected for cracks, crushing, missing, bulging, and misalignment. 

Inspection techniques are generally the same as for concrete. (See Topic 2.2 for 

the examination of concrete). Masonry arches or masonry-faced concrete arches 

should be checked for mortar cracks, vegetation, water seepage through cracks, 

loose or missing stones or blocks, weathering, and spalled or split blocks and 

stones.