P.3-i

Table of Contents

Basic Concepts 

Primer ..................................................................................................................................................

 

P.3 Culvert Characteristics...........................................................................P.3.1 

 P.3.1 Introduction...............................................................................P.3.1 

 Purpose of Culvert Inspection ............................................P.3.2 

 Safety..................................................................................P.3.3 

 Maintenance Needs ............................................................P.3.3 

 Outcomes............................................................................P.3.3 

 

 P.3.2 Differentiation Between Culverts and Bridges .........................P.3.4 

 Hydraulic............................................................................P.3.4 

 Structural ............................................................................P.3.4 

 Maintenance .......................................................................P.3.5 

 Traffic Safety......................................................................P.3.5 

 Construction .......................................................................P.3.5 

 Durability............................................................................P.3.5 

 Inspection ..........................................................................P.3.5 

 

 P.3.3 Structural Characteristics of Culverts .......................................P.3.5 

 Loads on Culverts...............................................................P.3.5 

 Categories of Structural Materials......................................P.3.8 

 Construction and Installation Requirements.......................P.3.8 

 

 P.3.4 Culvert Shapes ........................................................................P.3.10 

 Circular.............................................................................P.3.10 

 Pipe Arch and Elliptical Shapes .......................................P.3.11 

 Arches...............................................................................P.3.11 

 Box Sections.....................................................................P.3.12 

 Multiple Barrels................................................................P.3.12 

 Frame Culverts .................................................................P.3.13 

 

 P.3.5 Culvert Materials ....................................................................P.3.13 

 Precast Concrete...............................................................P.3.13 

 Cast-In-Place Concrete.....................................................P.3.15 

 Metal Culverts ..................................................................P.3.15 

 Masonry............................................................................P.3.15 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3-ii

 Timber ..............................................................................P.3.15 

 Other Materials.................................................................P.3.17 

 

 P.3.6 Culvert End Treatments ..........................................................P.3.17 

 

 P.3.7 Hydraulics of Culverts ............................................................P.3.20 

 Hydrologic Analysis.........................................................P.3.21 

 Hydraulic Capacity...........................................................P.3.22 

 Inlet Control .....................................................................P.3.22 

 Outlet Control...................................................................P.3.23 

 Special Hydraulic Considerations ....................................P.3.23 

 

 P.3.8 Factors Affecting Culvert Performance ..................................P.3.23 

 P.3.9 Types and Locations of Culvert Distress ................................P.3.24 

 Types of Distress..............................................................P.3.24 

 Inspection Locations.........................................................P.3.26 

 Overall Conditions............................................................P.3.27 

 Approach Roadway & Embankment................................P.3.27 

 Embankment.....................................................................P.3.28 

 End Treatments.................................................................P.3.29 

 Appurtenance Structure....................................................P.3.32 

 Culvert Barrel...................................................................P.3.33 

 

 P.3.10 Durability ................................................................................P.3.33 

 Corrosion .........................................................................P.3.33 

 Abrasion ...........................................................................P.3.34 

 

 P.3.11 Soil and Water Conditions that Affect Culverts .....................P.3.34 

 pH Extremes .....................................................................P.3.34 

 Electrical Resistivity.........................................................P.3.35 

 Soil Characteristics...........................................................P.3.35 

 

 P.3.12 Culvert Protective Systems .....................................................P.3.35 

 Extra Thickness ................................................................P.3.35 

 Bituminous Coating..........................................................P.3.35 

 Bituminous Paved Inverts.................................................P.3.35 

 Other Coatings..................................................................P.3.36 
P.3.1 

Topic P.3 Culvert Characteristics 

P.3.1 

Introduction A culvert is a structure designed hydraulically to take advantage of submergence 

to increase water carrying capacity. Culverts, as distinguished from bridges, are 

usually covered with embankment and are composed of structural material around 

the entire perimeter, although some are supported on spread footings with the 

streambed serving as the bottom of the culvert. Culverts may qualify to be 

considered “bridge” length. 

Over the years, culverts have traditionally received less attention than bridges. 

Since culverts are less visible it is easy to put them out of mind, particularly when 

they are performing adequately. Additionally, a culvert usually represents a 

significantly smaller investment than a bridge and in the event of a failure usually 

represents much less of a safety hazard. 

Since 1967 there has been an increased emphasis on bridge safety and on bridge 

rehabilitation and replacement programs. In many cases small bridges have been 

replaced with multiple barrel culverts, box culverts, or long span culverts (see 

Figure P.3.1). There have also been recent advances in culvert design and analysis 

techniques. Long span corrugated metal culverts with spans in excess of 12.2 m 

(40 feet) were introduced in the late 1960's. 

 

Figure P.3.1 Culvert Structure 

As a result of these developments, the number, size, complexity, and cost of 

culvert installations have increased. The failure of a culvert may be more than a 

mere driving inconvenience. Failure of a major culvert may be both costly and 

hazardous. 

Like bridges, culverts should be inspected regularly to identify potential safety 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.2 

problems and maintenance needs or other actions required to preserve the 

investment in the structure and to minimize property damage due to improper 

hydraulic functioning. 

Purpose of Culvert 

Inspection 

The National Bridge Inspection Program was designed to insure the safe passage 

of vehicles and other traffic. The inspection program provides a uniform database 

from which nationwide statistics on the structural and functional safety of bridges 

and large culvert-type structures are derived. Although these bridge inspections are 

essentially for safety purposes, the data collected is also used to develop 

rehabilitation and replacement priorities. 

Bridges with spans over 6.1 m (20 ft) in length are inspected on a two-year cycle 

in accordance with the National Bridge Inspection Standards (NBIS). According to 

the American Association of State Highway and Transportation Officials 

(AASHTO) the definition of bridges includes culverts with openings measuring 

more than 6.1 m (20 ft) along the centerline of the road and also includes multiple 

pipes where the distance between openings is less than or equal to half of the pipe 

opening. 

Multiple barrel culvert installations with relatively small pipes can therefore meet 

the definition of a bridge. Structures included in the NBIS are evaluated by 

utilizing a standardized inventory appraisal process that is based on rating certain 

structural and functional features. The data obtained is recorded on standardized 

inspection forms. The minimum data required for bridge length culverts is shown 

on the Structure Inventory and Appraisal Sheet (SI&A). Procedures for coding 

these items are provided in the FHWA Recording and Coding Guide for the 

Structure Inventory and Appraisal of the Nation's Bridges (Coding Guide) 

While the importance of the NBIS inspection program cannot be overemphasized, 

the SI&A data sheets are oriented toward bridges rather than culverts; thus, they 

do not allow an inspector to collect either detailed condition data or maintenance 

data. Additionally, the NBIS program does not specifically address structures 

where the total opening length is less than 6.1 m (20 feet). However, some type of 

formal inventory and inspection is needed for culverts that are not bridge length. In 

many cases, the failure of a culvert or other structure with openings less than 6.1 m 

(20 ft) long can present a life threatening hazard. Although the primary purpose of 

this, and other sections relating to culverts is to provide inspection guidelines for 

culverts included in the NBIS program, the guidelines should also be generally 

applicable to culverts with openings which are less than 6.1 m (20 feet) long. For 

culverts (and bridges) less than 6.1 m (20 ft) in length, the state in which the 

structure is located must incorporate it into their “Local Bridge Inventory”. In this 

case, the state defines the minimum structure opening to be included in the “Local 

Bridge Inventory”. 

Ideally, all culverts should be inventoried and periodically inspected. Some 

limitations may be necessary because a considerable effort is required to establish 

a current and complete culvert inventory. Small culverts may not warrant the same 

rigorous level of inspection as large culverts. Each agency should define its culvert 

inspection program in terms of inspection frequency, size, and type of culverts to 

be inventoried and inspected, and the information to be collected. Culverts larger 

than 6.1 m (20 ft) must be inspected every two years under the NBIS program. If 

possible all culverts should be inventoried and inspected to establish a structural 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.3 

adequacy and to evaluate the potential for roadway overtopping or flooding. 

The types and amount of condition information to be collected should be based on 

the purpose for which the information will be used. For example, if small pipes are 

not repaired but are replaced after failures occur, then the periodic collection of 

detailed condition data may not be warranted. Documentation of failures as well as 

the causes of failures, may be all the condition data that is needed. However, the 

inventory should be updated whenever a replacement is accomplished. 

Safety Safety is the most important reason why culverts should be inspected. To insure 

that a culvert is functioning safely, the inspector should evaluate structural 

integrity, hydraulic performance, and roadside compatibility. 

3⁄4 Structural Integrity - The failure of major culverts can present a life 

threatening safety hazard. The identification of potential structural and 

material problems requires a careful evaluation of indirect evidence of 

structural distress as well as actual deterioration and distress in the culvert 

material. 

3⁄4 Hydraulic Performance - When a culvert's hydraulic performance is 

inadequate, potential safety hazards may result. The flooding of adjacent 

properties from unexpected headwater depth may occur. Downstream 

areas may be flooded by failure of the embankment. The roadway 

embankment or culvert may be damaged because of erosion. 

3⁄4 Roadside Compatibility - Many culverts, like older bridges, present 

roadside hazards. Headwalls and wingwalls higher than the road or 

embankment surface may constitute a fixed obstacle hazard. Abrupt dropoffs over the end of a culvert or steep embankments may represent 

rollover hazards to vehicles that leave the roadway. 

3⁄4 Hazards of Culvert Inspection – Discussed in Topic 3.2, Safety Practices. 

Maintenance Needs Lack of maintenance is a prime cause of improper functioning in culverts and 

other drainage structures. Regular periodic inspections allow minor problems to be 

spotted and corrected before they become serious. 

Outcomes The primary outcome of this topic as well as Topics 3.1, 3.2, 4.2, 4.3, 5.5, 7.5, 

7.12, 11.2, 12.3, and 12.4 is to provide information that will enable bridge 

inspectors to do the following tasks: 

3⁄4 Properly inspect an existing culvert. 

3⁄4 Evaluate structural adequacy. 

3⁄4 Evaluate hydraulic adequacy and recognize potential flood hazards. 

3⁄4 Rate the condition of the culvert. 

3⁄4 Correctly document and rate the findings of a culvert inspection using the 

appropriate coding items. 

3⁄4 Recognize and document traffic safety conditions. 

3⁄4 Recommend corrective actions/maintenance needs. 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.4 

To meet the primary outcome, the topics provide general procedures for 

conducting, reporting, and documenting a culvert inspection, and guidelines for 

inspecting and rating specific hydraulic and structural culvert components. 

A second outcome of these sections is to provide users with the information 

necessary to understand and evaluate the significance of defects found during an 

inspection of an existing culvert. To meet this outcome, a review of how culverts 

should function structurally and hydraulically is provided briefly in this section, 

and covered in more detail in Topics 7.12, 12.3, and 12.4. Durability concepts are 

also reviewed. 

P.3.2 

Differentiation 

Between Culverts 

and Bridges 

Traditional definitions of culverts are based on the span length rather than function 

or structure type. For example, part of the culvert definition included in the Bridge 

Inspector's Training Manual 70 states: 

“...structures over 20 feet in span parallel to the roadway are usually called 

bridges; and structures less than 20 feet in span are called culverts even though 

they support traffic loads directly.” 

Many structures that measure more than 6.1 m (20 feet) along the centerline of the 

roadway have been designed hydraulically and structurally as culverts. The 

structural and hydraulic design of culverts is substantially different from bridges, 

as are construction methods, maintenance requirements, and inspection 

procedures. A few of the more significant differences between bridges and culverts 

are: 

Hydraulic Culverts are usually designed to operate at peak flows with a submerged inlet to 

improve hydraulic efficiency. The culvert constricts the flow of the stream to cause 

ponding at the upstream or inlet end. The resulting rise in elevation of the water 

surface produces a head at the inlet that increases the hydraulic capacity of the 

culvert. Bridges may constrict flow to increase hydraulic efficiency or be designed 

to permit water to flow over the bridge or approach roadways during peak flows. 

However, bridges are generally not designed to take advantage of inlet 

submergence to the degree that is commonly used for culverts. The effects of 

localized flooding on appurtenant structures, embankments, and abutting 

properties are important considerations in the design and inspection of culverts. 

Structural Culverts are usually covered by embankment material. Culverts must be designed 

to support the dead load of the soil over the culvert as well as live loads of traffic. 

Either live loads or dead loads may be the most significant load element depending 

on the type of culvert, type and depth of cover, and amount of live load. However, 

live loads on culverts are generally not as significant as the dead load unless the 

cover is shallow. Box culverts with shallow cover are examples of the type of 

installation where live loads may be significant. 

In most culvert designs the soil or embankment material surrounding the culvert 

plays an important structural role. Lateral soil pressures enhance the culverts 

ability to support vertical loads. The stability of the surrounding soil is important 

to the structural performance of most culverts. 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.5 

Maintenance Because culverts usually constrict flow there is an increased potential for 

waterway blockage by debris and sediment, especially for culverts subject to 

seasonal flow. Multiple barrel culverts may also be particularly susceptible to 

debris accumulation. Scour caused by high outlet velocity and turbulence at inlet 

end is a concern. As a result of these factors, routine maintenance for culverts 

primarily involves the removal of obstructions and the repair of erosion and scour. 

Prevention of joint leakage may be critical in culverts bedded in pipeable soils to 

prevent undermining and loss of support. 

Traffic Safety A significant safety advantage of many culverts is the elimination of bridge 

parapets and railings. Culverts can usually be extended so that the standard 

roadway cross section can be carried over the culvert to provide a vehicle recovery 

area. However, when ends are located near traffic lanes or adjacent to shoulders, 

guardrails may be used to protect the traffic. Another safety advantage of culverts 

is that less differential icing occurs. Differential icing is the tendency of water on 

the bridge deck to freeze prior to water on the approaching roadway. Since 

culverts are under fill material and do not have a bridge deck, the temperature of 

the roadway over the culvert is at or near the temperature of the roadway 

approaching the culvert. 

Construction Careful attention to construction details such as bedding, compaction, and trench 

width during installation is important to the structural integrity of the culvert. Poor 

compaction or poor quality backfill around culverts may result in uneven 

settlement over the culvert and possibly structural distress of the culvert. 

Durability Durability of material is a significant problem in culverts and other drainage 

structures. In very hostile environments such as acid mine drainage and chemical 

discharge, corrosion and abrasion can cause deterioration of all commonly 

available culvert materials. 

Inspection The inspection and assessment of the structural condition of culverts requires an 

evaluation of not only actual distress but circumstantial evidence such as roadway 

settlement, pavement patches, and embankment condition. 

P.3.3 

Structural 

Characteristics of 

Culverts 

Loads on Culverts In addition to their hydraulic functions, culverts must also support the weight of 

the embankment or fill covering the culvert and any load on the embankment. 

There are two general types of loads that must be carried by culverts: dead loads 

and live loads. 

Dead Loads 

Dead loads include the earth load or weight of the soil over the culvert and any 

added surcharge loads such as buildings or additional earth fill placed over an 

existing culvert. If the actual weight of earth is not known, 1920 kilograms per 

cubic meter (120 pounds per cubic foot) is generally assumed. 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.6 

Live Loads 

The live loads on a culvert include the loads and forces, which act upon the culvert 

due to vehicular or pedestrian traffic. The highway wheel loads generally used for 

analysis are shown in Figure P.3.2. The effect of live loads decreases as the height 

of cover over the culvert increases. When the cover is more than two feet, 

concentrated loads may be considered as being spread uniformly over a square 

with sides 1.75 times the depth of cover. This concept is illustrated in Figure P.3.3 

and P.3.4. In fact, for single spans, if the height of earth fill is more than 2.4

meters (8 feet) and exceeds the span length, the effects of live loads can be ignored 

all together. (see AASHTO) 

Figure P.3.2 AASHTO Live Load Spacing for Highway Structures 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.7 

Figure P.3.3 Surface Contact Area for Single Dual Wheel 

Figure P.3.4 Distribution of Live Load (Single Dual Wheel) for Depth of Cover
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.8 

Categories of Structural 

Materials 

Based upon material type, culverts can be divided into two broad structural 

categories: rigid and flexible. 

3⁄4 Rigid Culverts 

Culverts made from materials such as reinforced concrete and stone masonry are 

very stiff and do not deflect appreciably. The culvert material itself provides the 

needed stiffness to resist loads. In doing this, zones of tension and compression 

are created. The culvert material is designed to resist the corresponding stresses. 

Rigid Culverts are discussed in detail in Sections 7.5, 7.12, and 12.3. 

3⁄4 Flexible culverts 

Flexible culverts are commonly made from steel or aluminum. In some states 

composite materials are used. As stated earlier, flexible culverts rely on the 

surrounding backfill material to maintain their structural shape. Since they are 

flexible, they can be deformed significantly with no cracks occurring. 

As vertical loads are applied, a flexible culvert will deflect if the surrounding fill 

material is loose. The vertical diameter decreases while the horizontal diameter 

increases. Soil pressures resist the increase in horizontal diameter. 

For flexible culverts with large openings, sometimes longitudinal and/or 

circumferential stiffeners are used to prevent excessive deflection. 

Circumferential stiffeners are usually metal ribs bolted around the circumference 

of the culvert. Longitudinal stiffeners may be metal or reinforced concrete. This 

type of stiffener is sometimes called a thrust beam. 

Construction and 

Installation 

Requirements 

The structural behavior of flexible and rigid culverts is often dependent on 

construction practices during installation (see Figure P.3.5). Items, which require 

particular attention during construction, are discussed briefly in the following text. 

This information is provided so that the bridge inspector may gain insight on why 

certain structural defects are found when inspecting a culvert. 

3⁄4 Compaction and Side Support - Good backfill material and adequate 

compaction are of critical importance to flexible culverts. A wellcompacted soil envelope is needed to develop the lateral pressures 

required to maintain the shape of flexible culverts. Well-compacted 

backfill is also important to the performance of rigid culverts. Poorly 

compacted soils do not provide the intended lateral support. 

3⁄4 Trench Width - Trench width can significantly affect the earth loads on 

rigid culverts. It is therefore important that trench widths be specified on 

the plans and that the specified width not be exceeded without 

authorization from the design engineer. 

3⁄4 Foundations and Bedding - A foundation capable of providing uniform 

and stable support is important for both flexible and rigid culverts. The 

foundation must be able to support the structure at the proposed grade and 

elevation without concentration of foundation pressures. Foundations 

should be relatively yielding when compared to side fill. Establishing a 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.9 

suitable foundation requires removal and replacement of any hard spots or 

soft spots. Bedding is needed to level out any irregularities in the 

foundation and to insure uniform support. When using flexible culverts, 

bedding should be shaped to a sufficient width to permit compaction of the 

remainder of the backfill, and enough loose material should be placed on 

top of the bedding to fill the corrugations. When using rigid culverts, the 

bedding should conform to the bedding conditions specified in the plans 

and should be shaped to allow compaction and to provide clearance for the 

bell ends on bell and spigot type rigid pipes. Adequate support is critical in 

rigid pipe installations, or shear stress may become a problem. 

3⁄4 Construction Loads - Culverts are generally designed for the loads they 

must carry after construction is completed. Construction loads may exceed 

design loads. These heavy loads can cause damage if construction 

equipment crosses over the culvert installation before adequate fill has 

been placed or moves too close to the walls, creating unbalanced loading. 

Additional protective fill may be needed for equipment crossing points. 

3⁄4 Camber - In high fills the center of the embankment tends to settle more 

than the areas under the embankment side slopes. In such cases it may be 

necessary to camber the foundation slightly. This should be accomplished 

by using a flat grade on the upstream half of the culvert and a steeper 

grade on the downstream half of the culvert. The initial grades should not 

cause water to pond or pocket. 

Backfill

Bedding

Foundation

Culvert

Trench Width

Figure P.3.5 Culvert Construction and Installation Requirements 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.10

P.3.4 

Culvert Shapes A wide variety of standard shapes and sizes are available for most culvert 

materials. Since equivalent openings can be provided by a number of standard 

shapes, the selection of shape may not be critical in terms of hydraulic 

performance. Shape selection is often governed by factors such as depth of cover 

or limited headwater elevation. In such cases a low profile shape may be needed. 

Other factors such as the potential for clogging by debris, the need for a natural 

stream bottom, or structural and hydraulic requirements may influence the 

selection of culvert shape. Each of the common culvert shapes are discussed in the 

following paragraphs. 

Circular The circular shape is the most common shape manufactured for pipe culverts (see 

Figure P.3.6). It is hydraulically and structurally efficient under most conditions. 

Possible hydraulic drawbacks are that circular pipe generally causes some

reduction in stream width during low flows. It may also be more prone to clogging 

than some other shapes due to the diminishing free surface as the pipe fills beyond 

the midpoint. With very large diameter corrugated metal pipes, the flexibility of 

the sidewalls dictates that special care be taken during backfill construction to 

maintain uniform curvature. 

Figure P.3.6 Circular Culvert Structure 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.11

Pipe Arch and Elliptical 

Shapes 

Pipe arch and elliptical shapes are often used instead of circular pipe when the 

distance from channel invert to pavement surface is limited or when a wider 

section is desirable for low flow levels (see Figure P.3.7). These shapes may also 

be prone to clogging as the depth of flow increases and the free surface diminishes. 

Pipe arch and elliptical shapes are not as structurally efficient as a circular shape. 

Figure P.3.7 Pipe Arch Culvert 

Arches Arch culverts offer less of an obstruction to the waterway than pipe arches and can 

be used to provide a natural stream bottom where the stream bottom is naturally 

erosion resistant (see Figure P.3.8). Foundation conditions must be adequate to 

support the footings. Riprap is frequently used for scour protection. 

Figure P.3.8 Arch Culvert 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.12

Box Sections Rectangular cross-section culverts are easily adaptable to a wide range of site 

conditions including sites that require low profile structures. Due to the flat sides 

and top, rectangular shapes are not as structurally efficient as other culvert shapes 

(see Figure P.3.9). In addition, box sections have an integral floor. 

Figure P.3.9 Concrete Box Culvert 

Multiple Barrels Multiple barrels are used to obtain adequate hydraulic capacity under low 

embankments or for wide waterways (see Figure P.3.10). In some locations they 

may be prone to clogging as the area between the barrels tends to catch debris and 

sediment. When a channel is artificially widened, multiple barrels placed beyond 

the dominant channel are subject to excessive sedimentation. The span or opening 

length of multiple barrel culverts includes the distance between barrels as long as 

that distance is less than half the opening length of the adjacent barrels.

Figure P.3.10 Multiple Cell Concrete Culvert 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.13

Frame Culverts Frame culverts are constructed of cast-in-place (see Figure P.3.11) or precast 

reinforced concrete. This type of culvert has no floor (concrete bottom) and fill 

material is placed over the structure. 

 

Figure P.3.11 Frame Culvert 

P.3.5 

Culvert Materials 

Precast Concrete Precast concrete culverts are manufactured in six standard shapes: 

3⁄4 Circular 

3⁄4 Pipe arch 

3⁄4 Horizontal elliptical 

3⁄4 Vertical elliptical 

3⁄4 Rectangular 

3⁄4 Arch 

With the exception of box culverts, concrete culvert pipe is manufactured in up to 

five standard strength classifications. The higher the classification number, the 

higher the strength. Box culverts are designed for various depths of cover and live 

loads. All of the standard shapes are manufactured in a wide range of sizes. 

Circular and elliptical pipes are available with standard sizes as large as 3.7 m (144 

inches) in diameter, with larger sizes available as special designs. Standard box 

sections are also available with spans as large as 3.7 m (144 inches). Precast 

concrete arches on cast-in-place footings are available with spans up to 12.5 m (41

feet). A listing of standard sizes is provided at the end of Topics 7.12, 12.3, and 

12.4.
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.14

Figure P.3.12 Standard Concrete Pipe Shapes 

(Source: FHWA Culvert Inspection Manual, Supplement to the BIRM, July 1986) 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.15

Cast-in-Place Concrete Culverts that are reinforced cast-in-place concrete are typically either rectangular 

or arch-shaped. The rectangular shape is more common and is usually constructed 

with multiple cells (barrels) to accommodate longer spans. One advantage of castin-place construction is that the culvert can be designed to meet the specific 

requirements of a site. Due to the long construction time of cast-in-place culverts, 

precast concrete or corrugated metal culverts are sometimes selected. However, in 

many areas, cast-in-place culverts are more practical and represent a significant 

number of installations. 

Metal Culverts Flexible culverts are typically either steel or aluminum and are constructed from 

factory-made corrugated metal pipe or field assembled from structural plates. 

Structural plate products are available as plate pipes, box culverts, or long span 

structures (see Figure 12.4.4). Several factors such as span length, vertical and 

horizontal clearance, peak stream flow and terrain determine which flexible culvert 

shape is used. 

Masonry Stone and brick are durable, low maintenance materials. Prior to the 1920's, both 

stone and brick were used frequently in railroad and road construction projects 

because they were readily available from rock cuts or local brickyards. Currently 

stone and brick are seldom used for constructing culvert barrels. Stone is used 

occasionally for this purpose in locations which have very acidic runoff, but the 

most common use of stone is for headwalls where a rustic or scenic appearance is 

desired. A stone masonry arch culvert is shown in Figure P.3.13. Refer to Topic 

2.4 for a detailed discussion of stone masonry. 

 

Figure P.3.13 Typical Stone Masonry Arch Culvert 

Timber There are a limited amount of timber culverts throughout the nation. 

Timber culverts are generally box culverts and are constructed from individual 

timbers similar to railroad ties. Timber culverts are also analagous to a short span 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.16

timber bridge on timber abutments (see Figure P.3.14). 

An inspection of a timber culvert should be conducted in the same manner as a 

timber bridge, including sounding and drilling to determine the extent of decay. 

The inspector should accurately describe the construction of the timber culvert and 

make note of the following timber defects and their location and extent: 

3⁄4 Defects from Checks, Splits, and Shakes 

3⁄4 Decay by Fungi 

3⁄4 Damage by Parasites 

3⁄4 Damage from chemical attack 

3⁄4 Damage from fire 

3⁄4 Damage from Impact/Collisions 

3⁄4 Damage from Abrasion/Wear 

3⁄4 Damage from Overstress 

3⁄4 Damage from Weathering/Warping 

Refer to Topic 2.1.5 for a more detailed presentation of the types and causes of 

timber deterioration. 

Bulging of the walls and any shape deformations may indicate unstable soil 

conditions. These problems and their location and extent should be recorded. 

The vast majority of these culverts do not have floors. The inspector should check 

carefully at the footings for any scour or undermining. A probing rod should be 

used since scour holes can and do fill up with sediment. 

 

Figure P.3.14 Timber Box 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.17

Other Materials Aluminum, steel, concrete, and stone masonry are the most commonly found 

materials for existing culverts. There are several other materials which may be 

encountered during culvert inspections, including cast iron, stainless steel, terra 

cotta, asbestos cement, and plastic. These materials are not commonly found 

because they are either relatively new (plastic), labor intensive (terra cotta), or 

used for specialized situations (stainless steel and cast iron). 

P.3.6 

Culvert End 

Treatments 

Culverts may have end treatments or end structures. End structures are used to 

control scour, support backfill, retain the embankment, improve hydraulic 

efficiency, protect the culvert barrel, and provide additional stability to the culvert 

ends. 

The most common types of end treatments are: 

3⁄4 Projecting - The barrel simply extends beyond the embankment. No 

additional support is used (see Figure P.3.15). 

3⁄4 Mitered - The end of the culvert is cut to match the slope of the 

embankment. This type of treatment is also referred to as beveling and is 

commonly used when the embankment has some sort of slope paving (see 

Figure P.3.16). 

3⁄4 Skewed - Culverts, which are not perpendicular to the roadway, may have 

their ends cut parallel to the roadway (see Figure P.3.17). 

3⁄4 Pipe end section - A section of pipe is added to the ends of the culvert 

barrel. These are typically used on relatively smaller culverts. 

3⁄4 Headwalls - Used along with wingwalls to retain the fill, resist scour, and 

improve the hydraulic capacity of the culvert. Headwalls are usually 

reinforced concrete (see Figure P.3.18), but can be constructed of timber 

or masonry. Metal headwalls are usually found on metal box culverts. 

Figure P.3.15 Culvert End Projection 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.18

 

Figure P.3.16 Culvert Mitered End 

Figure P.3.17 Culvert Skewed End 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.19

 

Figure P.3.18 Culvert Headwall 

Miscellaneous Appurtenance Structures may also be used with end treatments to 

improve hydraulic efficiency and reduce scour. Typical appurtenances are: 

3⁄4 Aprons - Used to reduce streambed scour at the inlets and outlets of 

culverts. Aprons are typically concrete slabs, but they may also be riprap

(see Figure P.3.19). Most aprons include an upstream cutoff wall to 

protect against undermining. 

3⁄4 Energy Dissipators - Used when outlet velocities are likely to cause 

streambed scour downstream from the culvert. Stilling basins, riprap or 

other devices that reduce flow velocity can be considered energy 

dissipators (see Figure P.3.20). 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.20

 

Figure P.3.19 Apron 

Figure P.3.20 Energy Dissipator 

P.3.7 

Hydraulics of 

Culverts 

Culverts are primarily constructed to convey water under a highway, railroad, or 

other embankment. A culvert which does not perform this function properly may 

jeopardize the throughway, cause excessive property damage, or even loss of life. 

The hydraulic requirements of a culvert usually determine the size, shape, slope, 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.21

and inlet and outlet treatments. Culvert hydraulics can be divided into two general 

design elements: 

3⁄4 Hydrologic Analysis 

3⁄4 Hydraulic Analysis 

A hydrologic analysis is the evaluation of the watershed area for a stream and is 

used to determine the design discharge or the amount of runoff the culvert should 

be designed to convey. 

A hydraulic analysis is used to select a culvert, or evaluate whether an existing 

culvert is capable of adequately conveying the design discharge. To recognize 

whether a culvert is performing adequately the inspector should understand the 

factors that influence the amount of runoff to be handled by the culvert as well as 

the factors which influence the culvert's hydraulic capacity. 

Hydrologic Analysis Most culverts are designed to carry the surface runoff from a specific drainage 

area. While the selection and use of appropriate methods of estimating runoff 

requires a person experienced in hydrologic analysis and would usually not be 

performed by the inspector, the inspector should understand how changes in the 

topography of the drainage area can cause major changes in runoff. Climatic and 

topographic factors are briefly discussed in the following sections. 

Climatic Factors 

Climatic factors that may influence the amount of runoff include: 

3⁄4 Rainfall intensity 

3⁄4 Storm duration 

3⁄4 Rainfall distribution within the drainage area 

3⁄4 Soil moisture 

3⁄4 Snow melt 

3⁄4 Rain-on-snow 

3⁄4 Rain-hail 

3⁄4 Other factors 

Topographic Factors 

Topographic factors that may influence runoff include: 

3⁄4 The land use within the drainage area 

3⁄4 The size, shape, and slope of the drainage area 

3⁄4 Other factors such as the type of soil, elevation, and orientation of the area

Land use is the most likely characteristic to change significantly during the service 

life of a culvert. Changes in land use may have a considerable effect on the amount 

and type of runoff. Some surface types will permit more infiltration than other 

surface types. Practically all of the rain falling on paved surfaces will drain off 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.22

while much less runoff will result from undeveloped land. If changes in land use 

were not planned during the design of a culvert, increased runoff may exceed the 

capacity of an existing culvert when the land use does change. 

The size, shape, and slope of a culvert's drainage area influence the amount of 

runoff that may be collected and the speed with which it will reach the culvert. The 

amount of time required for water to flow to the culvert from the most remote part 

of a drainage area is referred to as the time of concentration. Changes within the 

drainage area may influence the time of concentration. 

Straightening or enclosing streams and eliminating temporary storage by replacing 

undersized upstream pipes are examples of changes which may decrease time of 

concentration. Land use changes may also decrease time of concentration since 

water will flow more quickly over paved surfaces. Since higher rainfall intensities 

occur for shorter storm durations, changes in time of concentration can have a 

significant impact on runoff. Drainage areas are sometimes altered and flow 

diverted from one watershed to another. 

Hydraulic Capacity The factors affecting capacity may include headwater depth, tailwater depth, inlet 

geometry, the slope of the culvert barrel, and the roughness of the culvert barrel. 

The various combinations of the factors affecting flow can be grouped into two 

types of conditions in culverts: 

3⁄4 Inlet control 

3⁄4 Outlet control 

Inlet Control Under inlet control the discharge from the culvert is controlled at the entrance of 

the culvert by headwater depth and inlet geometry (see Figure P.3.21). Inlet 

geometry includes the cross-sectional area, shape, and type of inlet edge. Inlet 

control governs the discharge as long as water can flow out of the culvert faster 

than it can enter the culvert. 

Figure P.3.21 Factors Affecting Culvert Discharge (Source: Adapted from 

Concrete Pipe Handbook, American Concrete Pipe Association) 

Most culverts, except those in flat terrain, are designed to operate under inlet 

control during peak flows. Since the entrance characteristics govern, minor 

modifications at the culvert inlet can significantly effect hydraulic capacity. For 

example, change in the approach alignment of the stream may reduce capacity, 

while the improvement of the inlet edge condition, or addition of properly 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.23

designed headwalls and wingwalls, may increase the capacity. 

Outlet Control Under outlet control water can enter the culvert faster than water can flow through 

the culvert. The discharge is influenced by the same factors as inlet control plus 

the tailwater depth and barrel characteristics (slope, length, and roughness). 

Culverts operating with outlet control usually lie on flat slopes or have high 

tailwater. 

When culverts are operating with outlet control, changes in barrel characteristics 

or tailwater depth may effect capacity. For example, increased tailwater depth or 

debris in the culvert barrel may reduce the capacity. 

Special Hydraulic 

Considerations 

Inlet and Outlet Protection 

The inlets and outlets of culverts may require protection to withstand the hydraulic 

forces exerted during peak flows. Inlet ends of flexible pipe culverts, which are not 

adequately protected or anchored, may be subject to entrance failures due to 

buoyant forces. The outlet may require energy dissipators to control erosion and 

scour and to protect downstream properties. High outlet velocities may cause scour 

which undermines the endwall, wingwalls, and culvert barrel. This erosion can 

cause end-section drop-off in rigid sectional pipe culverts. 

Protection Against Piping 

Seepage along the outside of the culvert barrel may remove supporting material. 

This process is referred to as “piping”, since a hollow cavity similar to a pipe is 

often formed. Piping can also occur through open joints. Piping is controlled by 

reducing the amount and velocity of water seeping along the outside of the culvert 

barrel. This may require watertight joints and in some cases anti-seep collars. 

Good backfill material and adequate compaction of that material are also 

important. 

P.3.8 

Factors Affecting 

Culvert 

Performance 

Some of the common factors that can affect the performance of a culvert include 

the following: 

3⁄4 Construction Techniques - Specifically, how well the foundation was 

prepared, the bedding placed, and the backfill compacted. 

3⁄4 The characteristics of the stream flow - water depth, velocity, turbulence. 

3⁄4 Structural Integrity - how well the structure can withstand the loads to 

which it is subjected, especially after experiencing substantial 

deterioration and section loss. 

3⁄4 Suitability of the Foundation - Can the foundation material provide 

adequate support? 

3⁄4 Stability of the embankment in relationship to other structures on the 

upstream or downstream side. 

3⁄4 Hydraulic capacity - if the culvert cross section is insufficient for flow, 

upstream ponding could result and damage the embankment. 

3⁄4 The presence of vegetation - can greatly affect the means and efficiency of 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.24

the flow through the culvert. 

3⁄4 The possibility of abrasion and corrosion caused by substances in the 

water, the surrounding soil or atmosphere. 

P.3.9 

Types and 

Locations of 

Culvert Distress 

Types of Distress 

The combination of high earth loads, long pipe-like structures and running water 

tends to produce the following types of distress: 

3⁄4 Shear or bending failure - High embankments may impose very high loads 

on all sides of a culvert and can cause shear or bending failure (see Figure 

P.3.22). 

3⁄4 Foundation failure - Either a smooth sag or differential vertical 

displacement at construction or expansion joints (settlement). Tipping of 

wingwalls. Lateral movement of precast or cast-in-place box sections (see 

Figure P.3.23). 

3⁄4 Hydraulic failure - Full flow design conditions result in accelerated scour 

and undermining at culvert ends as well as at any irregularities within the 

culvert due to foundation problems (see Figure P.3.24). 

3⁄4 Debris accumulation - Branches, sediment and trash can often be trapped 

at the culvert entrance restricting the channel flow and causing scour (see 

Figure P.3.25). 

Figure P.3.22 Bending or Shear Failure 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.25

 

Figure P.3.23 Cracking of Culvert End Treatment Due to Foundation Settlement

 

Figure P.3.24 Scour and Undermining at Culvert Inlet 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.26

 

Figure P.3.25 Debris and Sediment Buildup 

Inspection Locations A logical sequence for inspecting culverts helps ensure that a thorough and 

complete inspection will be conducted. In addition to the culvert components, the 

inspector should also look for highwater marks, changes in the drainage area, and 

other indications of potential problems. In this regard, the inspection of culverts is 

similar to the inspection of bridges. 

For typical installations, it is usually convenient to begin the field inspection with 

general observations of the overall condition of the structure and inspection of the 

approach roadway. The inspector should select one end of the culvert and inspect 

the embankment, waterway, headwalls, wingwalls, and culvert barrel. The 

inspector should then move to the other end of the culvert. The following 

sequence is applicable to all culvert inspections: 

3⁄4 Overall condition 

3⁄4 Approach roadway and embankment settlement 

3⁄4 Waterway (see in Topic 11.2) 

3⁄4 End treatments 

3⁄4 Appurtenance structures 

3⁄4 Culvert barrel 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.27

Overall Condition General observations of the condition of the culvert should be made while 

approaching the culvert area. The purpose of these initial observations is to 

familiarize the inspector with the structure. They may also point out a need to 

modify the inspection sequence or indicate areas requiring special attention. The 

inspector should also be alert for changes in the drainage area that might affect 

runoff characteristics. 

Approach Roadway and 

Embankment 

Inspection of the approach roadway and embankment includes an evaluation of the 

functional adequacy (see Figure P.3.26). 

The approach roadway and embankment should also be inspected for the following 

functional requirements: 

3⁄4 Signing 

3⁄4 Alignment 

3⁄4 Clearances 

3⁄4 Adequate shoulder profile 

3⁄4 Safety features 

 

Figure P.3.26 Approach Roadway at a Culvert Site 

Defects in the approach roadway and embankment may be indicators of possible 

structural or hydraulic problems in the culvert. The approach roadway and 

embankment should be inspected for the following conditions: 

3⁄4 Sag in roadway or guardrail 

3⁄4 Cracks in pavement 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.28

3⁄4 Pavement patches or evidence that roadway has settled 

3⁄4 Erosion or failure of side slopes 

Approach roadways should be examined for sudden dips, cracks, and sags in the 

pavement (see Figure P.3.27). These usually indicate excessive deflection of the 

culvert or inadequate compaction of the backfill material. 

 

Figure P.3.27 Roadway Over a Culvert 

New pavement can temporarily hide approach problems. It is advisable for the 

inspector to have previous inspection reports that may indicate the age of the 

present overlay. 

It is important to note that not all defects in the approach roadways have an 

adverse affect on the culvert. Deterioration of the pavement may be due to 

excessive traffic and no other reason. 

Embankment The embankment around the culvert entrance and exit should be inspected for slide 

failures in the fill around the box (see Figure P.3.28). Check for debris at the inlet 

and outlet and within the culvert. Also note if vegetation is obstructing the ends.
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.29

Figure P.3.28 Slide Failure 

End Treatments The SI&A Inspection Sheet does not specifically address end treatments in terms 

of inventory data or condition. The condition rating of end treatments is part of 

SI&A Item 62, Culvert Condition, and can have an impact on SI&A Item 67, 

Structural Evaluation. 

Inspections of end treatments primarily involve visual inspection, although hand 

tools should be used such as a plumb bob to check for misalignment, a hammer to 

sound for defects, and a probing rod to check for scour and undermining. In 

general, headwalls should be inspected for movement or settlement, cracks, 

deterioration, and traffic hazards. Culvert ends should be checked for 

undermining, scour, and evidence of piping. 

Figure P.3.29 Headwall and Wingwall End Treatment on Box Culvert 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.30

The most common types of box culvert end treatments are: 

3⁄4 Skewed Ends 

3⁄4 Headwalls 

Both end treatment types use wingwalls to retain the embankment around the 

opening. 

Wingwalls should be inspected to ensure they are in proper vertical alignment (see 

Figure P.3.30). Wingwalls may be tilted due to settlement, slides or scour. See 

Topic 10.1 for a detailed description of defects and inspection procedures of 

wingwalls. 

Figure P.3.30 Potential for Tilted Wingwalls 

Skewed Ends - Skewing the end of a culvert has nearly the same effect on 

structural capacity as does mitering (see Figure P.3.31). Stresses increase because 

a full box shape is not present at the end. 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.31

Figure P.3.31 Skewed End 

Headwalls – Headwalls and wingwalls should be inspected for undermining and 

settlement. Cracking, tipping or separation of culvert barrel from the headwall and 

wingwalls is usually good evidence of undermining. (see Figure P.3.32 and 

P.3.33).

Figure P.3.32 Culvert Headwall 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.32

Appurtenance Structures Typical appurtenance structures are: 

3⁄4 Aprons 

3⁄4 Energy Dissipators 

Aprons – should be checked for any undermining or settlement. The joints 

between the apron and headwalls should be inspected to see if it is watertight. (see 

Figure P.3.33) 

Figure P.3.33 Apron 

Energy Dissipaters – are used when outlet velocities are likely to cause streambed 

scour downstream from the culvert (see Figure P.3.34). Energy dissipaters may 

include stilling basins, riprap or other devices. Energy dissipaters should be 

inspected for material defects and overall effectiveness. 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.33

Figure P.3.34 Energy Dissipater 

Culvert Barrel The full length of the culvert should be inspected from the inside. All components 

of the culvert barrel should be visually examined, including walls, floor, top slab, 

and joints. The concrete should be sounded by tapping with a hammer particularly 

around cracks and other defects. It is important to time the inspection so that water 

levels are low. Culverts with small diameters can be inspected by looking through 

the culvert from both ends or by using a small movable camera. The condition of 

the culvert barrel is rated under SI&A Item 62, which covers all structural 

components of a culvert. 

For concrete box culverts, the culvert barrels should be inspected primarily for 

defects such as misalignment, joint defects, cracking, spalling, and other material 

defects. For a detailed description of concrete box culvert inspection, refer to 

Topic 7.12. 

P.3.10 

Durability Although the structural condition is a very important element in the performance 

of culverts, durability problems are probably the most frequent cause of 

replacement. Culverts are more likely to "wear away" than fail structurally. 

Durability is affected by two mechanisms: corrosion and abrasion. 

Corrosion Corrosion affects all metals and alloys, although the rates can vary widely 

depending both upon the chemical and physical properties of the metal and upon 

the environmental condition to which it is exposed. When a metal corrodes a very 

low voltage electrical current is established between two parts of a metal surface 

that have different voltage potential. The difference in voltage potential may be 

caused by slight variations in the material, changes in surface condition, or the 

presence of foreign materials. The current removes metallic ions from one location 

and deposits them at another location, causing corrosion. The chemicals present in 

the water greatly influence its effectiveness as an electrolyte. 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.34

Corrosion is the deterioration of culvert materials by chemical or electrochemical 

reaction to the environment. Culvert corrosion may occur in many different soils 

and waters. These soils and waters may contain acids, alkalis, dissolved salts, 

organics, industrial wastes or other chemicals, mine drainage, sanitary effluents, 

and dissolved or free gases. However, culvert corrosion is generally related to 

water and the chemicals that have reacted to, become dissolved in, or been 

transported by the water. 

Corrosion can attack the inside or outside of the culvert barrel. The chemicals in 

drainage water can attack the material on the interior of the culvert. Culverts 

subject to continuous flows or standing water with aggressive chemicals are more 

likely to be damaged than those with intermittent flows. The exterior of culverts 

can be attacked by chemicals in the ground water which can originate in the soil, 

be introduced through contaminates in the backfill soil, or be transported by 

subsurface flow. 

Although less common than with metal pipe, corrosion can occur in concrete 

culverts. Metallic corrosion can take place in the reinforcing steel when it is 

exposed by cracking or spalling, when the concrete cover is inadequate or when 

the concrete is porous enough to allow water to contact the reinforcing steel. 

If the steel corrodes, the corrosion products expand and may cause spalling of the 

concrete. Corrosion can also take place in the concrete itself. It is not, however, the 

same type of electrochemical reaction that occurs in metal. Other reactions 

between the concrete materials and the chemicals present in the stream flow or 

ground water are involved and can result in deterioration of the concrete. 

Abrasion Abrasion is the process of wearing down or grinding away surface material as 

water laden with sand, gravel, or stones flows through a culvert. Abrasive forces 

increase as the velocity of the water flowing through a culvert increases; for 

example, doubling the velocity of a stream flow can cause the abrasive power to 

become approximately four-fold. 

Often corrosion and abrasion operate together to produce far greater deterioration 

than would result from either alone. Abrasion can accelerate corrosion by 

removing protective coatings and allowing water-borne chemicals to come into 

contact with corrodible culvert materials. 

P.3.11 

Soil and Water 

Conditions that 

Affect Culverts 

Certain soil and water conditions have been found to have a strong relationship to 

accelerated culvert deterioration. These conditions are referred to as "aggressive" 

or "hostile." The most significant conditions of this type are: 

pH Extremes pH is a measure of the relative acidity or alkalinity of water. A pH of 7.0 is 

neutral; values of less than 7.0 are acid, and values of more than 7.0 are alkaline. 

For culvert purposes, soils or water having a pH of 5.5 or less are strongly acid and 

those of 8.5 or more are strongly alkaline. 

Acid water stems from two sources, mineral and organic. Mineral acidity comes 

from sulfurous wells and springs, and drainage from coal mines. These sources 

contain dissolved sulfur and iron sulfide which may form sulfurous and sulfuric 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.35

acids. Mineral acidity as strong as pH 2.3 has been encountered. Organic acidity 

usually found in swampy land and barnyards rarely produces a pH of less than 4.0. 

Alkalinity in water is caused by strong alkali-forming minerals and from limed and 

fertilized fields. Acid water (low pH) is more common to wet climates and alkaline 

water (high pH) is more common to dry climates. As the pH of water in contact 

with culvert materials, either internally or externally, deviates from neutral, 7.0, it 

generally becomes more hostile. 

Electrical Resistivity This measurement depends largely on the nature and amount of dissolved salts in 

the soil. The greater the resistance the less the flow of electrical current associated 

with corrosion. High moisture content and temperature lower the resistivity and 

increase the potential for corrosion. Soil resistivity generally decreases as the 

depth increases. The use of granular backfill around the entire pipe will increase 

electrical resistivity and will reduce the potential for galvanic corrosion. 

Several states rely on soil and water resistivity measurements as an important 

index of corrosion potential. Some states and the FHWA have published 

guidelines that use a combination of the pH and electrical resistivity of soil and 

water to indicate the corrosion potential at proposed culvert sites. The collection of 

pH and electrical resistivity data during culvert inspections can provide valuable 

information for developing local guidelines. 

Soil Characteristics The chemical and physical characteristics of the soil, which will come into contact 

with a culvert, can be analyzed to determine the potential for corrosion. The 

presence of base-farming and acid-forming chemicals is important. Chlorides and 

other dissolved salts increase electrical conductivity and promote the flow of 

corrosion currents. Sulfate soils and water can be erosive to metals and harmful to 

concrete. The permeability of soil to water and to oxygen is another variable in the 

corrosion process. 

P.3.12 

Culvert Protective 

Systems 

There are several protective measures that can be taken to increase the durability 

of culverts. The more commonly used measures are: 

Extra Thickness For some aggressive environments, it may be economical to provide extra 

thickness of concrete or metal. 

Bituminous Coating This is the most common protective measure used on corrugated steel pipe. This 

procedure can increase the resistance of metal pipe to acidic conditions if the 

coating is properly applied and remains in place. Careful handling during 

transportation, storage, and placement is required to avoid damage to the coating. 

Bituminous coatings can also be damaged by abrasion. Field repairs should be 

made when bare metal has been exposed. Fiber binding is sometimes used to 

improve the adherence of bituminous material to the metallic-coated pipe. 

Bituminous Paved 

Inverts 

Paving the inverts of corrugated metal culverts to provide a smooth flow and to 

protect the metal has sometimes been an effective protection from particularly 

abrasive and corrosive environments. Bituminous paving is usually at least 3 mm 

(1/8-inch) thick over the inner crest of the corrugations. Generally only the lower 

quadrant of the pipe interior is paved. Fiber binding is sometimes used to improve 
Basic Concepts Primer 

TOPIC P.3: Culvert Characteristics 

P.3.36

the adherence of bituminous material to the metallic-coated pipe. 

Other Coatings There are several other coating materials that are being used to some degree 

throughout the country. Polymeric, epoxy, fiberglass, clay, and concrete field 

paving, have all been used as protection against corrosion. Galvanizing is the most 

common of the metallic coatings used for steel. It involves the application of a thin 

layer of zinc on the metal culvert. Other metallic coatings used to protect steel 

culverts are aluminum and aluminum-zinc.