15 PRESERVATION 

BRIEFS 

Preservation of Historic Concrete 

Paul Gaudette and Deborah Slaton 

National Park Service 

U.S. Department of the Interior 

Introduction to Historic Concrete 

Concrete is an extraordinarily versatile building material 

used for utilitarian, ornamental, and monumental 

structures since ancient times. Composed of a mixture 

of sand, gravel, crushed stone, or other coarse material, 

bound together with lime or cement, concrete undergoes 

a chemical reaction and hardens when water is added. 

Inserting reinforcement adds tensile strength to 

structural concrete elements. The use of reinforcement 

contributes significantly to the range and size of 

building and structure types that can be constructed 

with concrete. 

While early twentieth century proponents of modern 

concrete often considered it to be permanent, it is, 

like all materials, subject to deterioration. This Brief 

provides an overview of the history of concrete and 

its popularization in the United States, surveys the 

principal causes and modes of concrete deterioration, 

and outlines approaches to repair and protection that 

are appropriate to historic concrete. In the context of this 

Brief, historic concrete is considered to be concrete used 

in construction of structures of historical, architectural, 

or engineering interest, whether those structures are old 

or relatively new. 

Brief History of Use and Manufacture 

The ancient Romans found that a mixture of lime putty 

and pozzolana, a fine volcanic ash, would harden 

under water. The resulting hydraulic cement became 

a major feature of Roman building practice, and was 

used in many buildings and engineering projects 

such as bridges and aqueducts. Concrete technology 

was kept alive during the Middle Ages in Spain and 

Africa. The Spanish introduced a form of concrete to 

the New World in the first decades of the sixteenth 

century, referred to as "tapia" or "tabby." This material, 

a mixture of lime, sand, and shell or stone aggregate 

mixed with water, was placed between wooden forms, 

tamped, and allowed to dry in successive layers. Tabby 

was later used by the English settlers in the coastal 

southeastern United States. 

The early history of concrete was fragmented, 

with developments in materials and construction 

techniques occurring on different continents and in 

various countries. In the United States, concrete was 

slow in achieving widespread acceptance in building 

construction and did not begin to gain popularity until 

the late nineteenth century. It was more readily accepted 

for use in transportation and infrastructure systems. 

The Erie Canal in New York is an example of the 

early use of concrete in transportation in the United 

States. The natural hydraulic cement used in the canal 

construction was processed from a deposit of limestone 

found in 1818 near Chittenango, southeast of Syracuse. 

The use of concrete in residential construction was 

Figure 1. The Sebastopol House in Seguin, Texas, is an 1856 Greek 

Revival-style house constructed of lime concrete. Lime concrete 

or "limecrete" was a popular construction material, as it could be 

made inexpensively from local materials. By 1900, the town had 

approximately ninety limecrete structures, twenty of which remain. 

Photo: Texas Parks and Wildlife Department. 

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Figure 2. Chatterton House was the home of the post trader at Fort 

Fred Steel in Wyoming, one of several forts established in the 1860s 

to protect the Union Pacific Railroad. The walls of the post trader's 

house were built using stone aggregate and lime, without cement. 

The use of this material presents special preservation challenges. 

publicized in the second edition of Orson S. Fowler's A 

Home for All (1853) which described the advantages of 

"gravel wall" construction to a wide audience. The town 

of Seguin, Texas, thirty-five miles east of San Antonio, 

already had a number of concrete buildings by the 1850s 

and came to be called "The Mother of Concrete Cities," 

with approximately ninety concrete buildings made 

from local "lime water" and gravel (Fig. 1). 

Impressed by the economic advantages of poured gravel 

wall or "lime-grout" construction, the Quartermaster 

General's Office of the War Department embarked on a 

campaign to improve the quality of building for frontier 

military posts. As a result, lime-grout structures were 

constructed at several western posts soon after the Civil 

War, including Fort Fred Steele and Fort Laramie, both 

in Wyoming (Fig. 2). By the 1880s, sufficient experience 

had been gained with unreinforced concrete to permit 

construction of much larger buildings. A notable 

example from this period is the Ponce de Leon Hotel in 

St. Augustine, Florida. 

Figure 3. The Lincoln Highway Association promoted construction of 

a high quality continuous hard surface roadway across the country. 

The Boys Scouts of America installed concrete road markers along the 

Lincoln Highway in 1928. 

Extensive construction in concrete also occurred through 

the system of coastal fortifications commissioned by the 

federal government in the 1890s for the Atlantic, Pacific, 

and Gulf coasts. Unlike most concrete construction 

to that time, the special requirements of coastal 

fortifications called for concrete walls as much as 20 feet 

thick, often at sites that were difficult to access. Major 

structures in the coastal defenses of the 1890s were built 

of mass concrete with no internal reinforcing, a practice 

that was replaced by the use of reinforcing bars in 

fortifications constructed after about 1905. 

The use of reinforced concrete in the United States dates 

from 1860, when S.T. Fowler obtained a patent for a 

reinforced concrete wall. In the early 1870s, William E. 

Ward built his own house in Port Chester, New York, 

using concrete reinforced with iron rods for all structural 

elements. Despite these developments, such construction 

remained a novelty until after 1880, when innovations 

introduced by Ernest L. Ransome made the use of 

reinforced concrete more practicable. Ransome made 

many contributions to the development of concrete 

construction technology, including the use of twisted 

reinforcing bars to improve bond between the concrete 

and the steel, which he patented in 1884. Two years later, 

Ransome introduced the rotary kiln to United States 

cement production. The new kiln had greater capacity 

and burned more thoroughly and uniformly, allowing 

development of a less expensive, more uniform, and 

more reliable manufactured cement. Improvements in 

concrete production initiated by Ransom led to a much 

greater acceptance of concrete after 1900. 

The Lincoln Highway Association, incorporated in 

1913, promoted the use of concrete in construction of a 

coast-to-coast roadway system. The goal of the Lincoln 

Highway Association and highway advocate Henry 

B. Joy was to educate the country in the need for good 

roads made of concrete, with an improved Lincoln 

Figure 4. The highly ornamental concrete panels on the exterior 

facade of the Baha'i House of Worship in Wilmette, Illinois, illustrate 

the work of fabricator John J. Earley, known as "the man who made 

concrete beautiful. " 
Figure 5. Following World War II, architects and engineers took 

advantage of improvements in concrete production, quality control, 

and advances in precast concrete to design structures such as the Police 

Headquarters building in Philadelphia, Pennsylvania, constructed in 

1961. Photo: Courtesy of the Philadelphia Police Department. 

Highway as an example. Concrete "seedling miles" 

were constructed in remote areas to emphasize the 

superiority of concrete over unimproved dirt. The 

Association believed that as people learned about 

concrete, they would press the government to construct 

good roads throughout their states. Americans' 

enthusiasm for good roads led to the involvement 

of the federal government in road-building and the 

creation of numbered U.S. routes in the 1920s (Fig. 3). 

During the early twentieth century, Ernest Ransome 

in Beverly, Massachusetts, Albert Kahn in Detroit, and 

Richard E. Schmidt in Chicago, promoted concrete 

for use in "Factory Style" utilitarian buildings with 

an exposed concrete frame infilled with expanses 

of glass. Thomas Edison's cast-in-place reinforced 

concrete homes in Union Township, New Jersey 

(1908), proclaimed a similarly functional emphasis 

in residential construction. From the 1920s onward, 

concrete began to be used with spectacular design 

results: examples include John J. Earley's Meridian 

Hill Park in Washington, D.C.; Louis Bourgeois' 

exuberant, graceful Baha'i Temple in Wilmette, Illinois 

(1920-1953), for which Earley fabricated the concrete 

(Fig. 4); and Frank Lloyd Wright's Fallingwater 

near Bear Run, Pennsylvania (1934). Continuing 

improvements in quality control and development 

of innovative fabrication processes, such as the 

Shockbeton method for precast concrete, provided 

increasing opportunities for architects and engineers. 

Wright's Guggenheim Museum in New York City 

(1959); Geddes Brecher Qualls & Cunningham'S Police 

Headquarters building in Philadelphia, Pennsylvania 

(1961); and Eero Saarinen's soaring terminal building at 

Dulles International Airport outside Washington, D.C., 

and the TWA terminal at Kennedy Airport in New 

York (1962), exemplify the masterful use of concrete 

achieved in the modern era (Fig. 5). 

Figure 6. The Bailey Magnet School in Jackson, Mississippi, was 

designed as the Jackson Junior High School by the firm of N. W. 

Overstreet & Town in 1936. The streamlined building exemplifies the 

applicability of concrete to creating a modern architectural aesthetic. 

Photo: Bill Burris, Burris/Wagnon Architects, P.A. 

Figure 7. Detailed bas reliefs as well as sculptures, such as this lion at 

the Bailey Magnet School, could be used as ornamentation on concrete 

buildings. Sculptural concrete elements were typically cast in molds. 

Throughout the twentieth century, a wide range of 

architectural and engineering structures were built using 

concrete as a practical and cost-effective choice-and 

concrete also became valued for its aesthetic qualities. 

Cast in place and precast concrete were readily adapted 

to the Streamlined Moderne style, as exemplified by the 

Bailey Magnet School in Jackson, Mississippi, designed 

as the Jackson Junior High School by N.W. Overstreet 

& Town in 1936 (Figs. 6 and 7). The school is one of 

many concrete buildings designed and constructed 

under the auspices of the Public Works Administration. 

Recreational structures and landscape features also 

utilized the structural range and unique character of 

exposed concrete to advantage, as seen in Chicago'S 

Lincoln Park Chess Pavilion, designed by Morris 

Webster in 1956 (Fig. 8), and the Ira C. Keller Fountain 

in Portland Oregon, designed by Lawrence Halprin in 

1969 (Fig. 9). Concrete was also popular for building 

interiors, with ornamental features and exposed 

structural elements recognized as part of the design 

aesthetic (See Figs. 10 and 11 in sidebar). 

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Historic Interiors 

The expanded use of concrete provided new opportunities to 

create dramatic spaces and ornate architectural detail on the 

interiors of buildings, at a significant cost savings over traditional 

construction practices. The architectural design of the Berkeley 

City Club in Berkeley, California, expressed Moorish and Gothic 

elements in concrete on the interior of the building (Fig. 10). Used 

as a woman's social club, the building was designed by noted 

California architect Julia Morgan and constructed in 1929. The 

vaulted ceilings, columns, and ornamental capitals of the lobby 

and the ornamental arches and beamed ceiling of the "plunge" are 

all constructed of concrete. 

Figure 10. The Berkeley City Club has significant interior spaces alld features of 

concrete construction, including the lobby and pool. Photos: Una Gilmartin (left) 

and Brian Kehoe (right), Wiss, Janney, Elstner Associates, Illc. 

The historic character of a building's interior can also be conveyed 

in a more utilitarian manner in terms of concrete features and 

finishes (Fig. 11). The exposed concrete structure-columns, 

capitals, and drop panels-is an integral part of the character 

of this old commercial building in Minneapolis. In concrete 

warehouse and factory buildings of the early twentieth century, 

exposed concrete columns and formboard finish concrete slab 

ceilings are common features as seen in this warehouse, now 

converted for use as a parking garage and shops. 

Figure 11. Whether in a circa 1925 office (left) or in a parking garage 

and retail facility (right), exposed concrete structures help characterize 

these building interiors. Photo: Minnesota Historical Society (left). 

Concrete Characteristics 

Concrete is composed of fine (sand) and 

coarse (crushed stone or gravel) aggregates 

and paste made of portland cement and water. 

The predominant material in terms of bulk is 

the aggregate. Portland cement is the binder 

most commonly used in modern concrete. It 

is commercially manufactured by blending 

limestone or chalk with clays that contain 

alumina, silica, lime, iron oxide and magnesia, 

and heating the compounds together to high 

temperatures. The hydration process that 

occurs between the portland cement and water 

results in formation of an alkali paste that 

surrounds and binds the aggregate together as 

a solid mass. 

The quality of the concrete is dependent on 

the ratio of water to the binder; binder content; 

sound, durable, and well-graded aggregates; 

compaction during placement; and proper 

curing. The amount of water used in the mix 

affects the concrete permeability and strength. 

The use of excess water beyond that required 

in the hydration process results in more 

permeable concrete, which is more susceptible 

to weathering and deterioration. Admixtures 

are commonly added to concrete to adjust 

concrete properties such as setting or hardening 

time, requirements for water, workability, and 

other characteristics. For example, the advent 

of air entraining agents in the 1930s provided 

enhanced durability for concrete. 

During the twentieth century, there was 

a steady rise in the strength of ordinary 

concrete as chemical processes became better 

understood and quality control measures 

improved. In addition, the need to protect 

embedded reinforcement against corrosion 

was acknowledged. Requirements for concrete 

cover over reinforcing steel, increased cement 

content, decreased water-cement ratio, and air 

entrainment all contributed to greater concrete 

strength and improved durability. 

Mechanisms and Modes of 

Deterioration 

Causes of Deterioration 

Concrete deterioration occurs primarily because 

of corrosion of the embedded steel, degradation 

of the concrete itself, use of improper techniques 

or materials in construction, or structural 

problems. The causes of concrete deterioration 

must be understood in order to select an 

appropriate repair and protection system. 
While reinforcing steel has played a pivotal role in 

expanding the applications of concrete in twentieth 

century architecture, corrosion of this steel has also 

caused deterioration in many historic structures. 

Reinforcing steel embedded in the concrete is normally 

surrounded by a passivating 

oxide layer that, when present, 

protects the steel from corrosion 

and aids in bonding the 

steel and concrete. When the 

concrete's normal alkaline 

environment (above a pH 

of 10) is compromised and 

the steel is exposed to water, 

water vapor, or high relative 

humidity, corrosion of the 

Lack of proper maintenance of building elements 

such as roofs and drainage systems can contribute to 

water-related deterioration of the adjacent concrete, 

particularly when concrete is saturated with water 

and then exposed to freezing temperatures. As water 

within the concrete freezes, it 

expands and exerts forces on 

the adjacent concrete. Repeated 

freezing and thawing can result 

in the concrete cracking and 

delaminating. Such damage 

appears as surface degradation, 

including severe scaling and 

micro-cracking that extends 

into the concrete. The condition 

is most often observed near 

steel reinforcing takes place. A 

reduction in alkalinity results 

from carbonation, a process that 

occurs when the carbon dioxide 

in the atmosphere reacts with 

calcium hydroxide and moisture 

in the concrete. Carbonation 

starts at the concrete's exposed 

surface but may extend to the 

reinforcing steel over time. 

When carbonation reaches 

Figure 8. The Chess Pavilion in Chicago'S Lincoln Park 

was designed by architect Morris Webster and constructed 

in 1956. The pavilion is a distinctive landscape feature, 

with its reinforced concrete cantilevered slab that provides 

cover for chess players. 

the surface of the concrete but 

can also eventually occur deep 

within the concrete. This type 

of deterioration is usually most 

severe at joints, architectural 

details, and other areas with 

more surface exposure to 

weather. In the second half of 

the twentieth century, concrete 

has utilized entrained air (the 

incorporation of microscopic 

air bubbles) to provide 

enhanced protection against 

damage due to cyclic freezing 

of saturated concrete. 

the metal reinforcement, the 

concrete no longer protects the 

steel from corrosion. 

Corrosion of embedded 

reinforcing steel may be 

initiated and accelerated if 

calcium chloride was added to 

the concrete as a set accelerator 

during original construction 

to promote more rapid curing. 

It may also take place if the 

concrete is later exposed to 

deicing salts, as may occur 

during the winter in northern 

climates. Seawater or other 

marine environments can 

Figure 9. The Ira C. Keller Fountain in Portland, Oregon, 

was designed by Lawrence Halprin and constructed in 

1969. The fountain is constructed primarily of concrete 

pillars with formboard textures and surrounding elements, 

patterned with geometric lines, which fa cilitate the path 

The use of certain aggregates 

can also result in deterioration 

of the concrete. Alkaliaggregate reactions-in some 

cases alkali-silica reaction 

(ASR)-occur when alkalis 

normally present in cement 

react with certain aggregates, 

leading to the development of 

an expansive crystalline gel. 

When this gel is exposed to 

moisture, it expands and causes of water. Photo: Anita Washko, Wiss, Janney, Elstner 

Associates, Inc. cracking of the aggregate and 

concrete matrix. Deleterious also provide large amounts 

of chloride, either from 

inadequately washed original aggregate or from 

exposure of the concrete to seawater. 

Corrosion-related damage to reinforced concrete is 

the result of rust, a product of the corrosion process of 

steel, which expands and thus requires more space in 

the concrete than the steel did at the time of installation. 

This change in volume of the steel results in expansive 

forces, which cause cracking and spalling of the 

adjacent concrete (Fig. 12). Other signs of corrosion of 

embedded steel include delamination of the concrete 

(planar separations parallel to the surface) and rust 

staining (often a precursor to spalling) on the concrete 

near the steel. 

aggregates are typically found only in certain areas of 

the country and can be detected through analysis by an 

experienced petrographer. Low-alkali cements as well 

as fly ash are used today in new construction to prevent 

such reactions where this problem may occur. 

Problems Specifically Encountered with 

Historic Concrete 

Materials and workmanship used in the construction 

of historic concrete structures, particularly those built 

before the First World War, sometimes present potential 

sources of problems. For example, where the aggregate 

consisted of cinder from burned coal or crushed brick, 

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Figure 12. The concrete lighthouse at the Kilauea Point 

Light Station, Kilauea, Kauai, Hawaii, was constructed 

circa 1913. The concrete, which was a good quality, high 

strength mix for its day, is in good condition after almost 

one hundred years in service. Deterioration in the form of 

spalling related to corrosion of embedded reinforcing steel 

has occurred primarily in areas of higher ornamentation 

such as projecting bands and brackets (see close-up photo). 

the concrete tends to be weak and porous 

because these aggregates absorb water. Some of 

these aggregates can be extremely susceptible 

to deterioration when exposed to moisture 

and cyclic freezing and thawing. Concrete 

was sometimes compromised by inclusion of 

seawater or beach sand that was not thoroughly 

washed with fresh water, a condition more 

common with coastal fortifications built prior to 

1900. The sodium chloride present in seawater 

and beach sand accelerates the rate of corrosion 

of the reinforced concrete. 

Another problem encountered with historic 

concrete is related to poor consolidation of the 

concrete during its placement in forms, or in molds in the case 

of precasting. This problem is especially prevalent in highly 

ornamental units. Early twentieth century concrete was often 

tamped or rodded into place, similar to techniques used in 

forming cast stone. Poorly consolidated concrete often contains 

voids (lfbugholes" or "honeycombs"), which can reduce the 

protective concrete cover over the embedded reinforcing 

bars, entrap water, and, if sufficiently large and strategically 

numerous, reduce localized concrete strength. Vibration 

technology has improved over time and flowability agents are 

also used today to address this problem. 

A common type of deterioration observed in concrete is the 

effect of weathering from exposure to wind, rain, snow, and 

salt water or spray. Weathering appears as erosion of the 

cement paste, a condition more prevalent in northern regions 

where precipitation can be highly acidic. This results in the 

exposure of the aggregate particles on the exposed concrete 

surface. Variations may occur in the aggregate exposure due 

to differential erosion or dissolution of exposed cement paste. 

Erosion can also be caused by the mechanical action of water 

channeled over concrete, such as by the lack of drip grooves in 

belt courses and sills, and by inadequate drainage. In addition, 

high-pressure water when used for cleaning can also erode the 

concrete surface. 

In concrete structures built prior to the First World War, 

concrete was often placed into forms in relatively short 

vertical lifts due to limitations in lifting and pouring 

techniques available at the time. Joints between different 

concrete placements (often termed cold joints or lift lines) may 

sometimes be considered an important part of the character of 

a concrete element (Fig. 13). However, wide joints may permit 

water to infiltrate the concrete, resulting in more rapid paste 

erosion or freeze-thaw deterioration of adjacent concrete in 

cold climates. 

In the early twentieth century, concrete was sometimes placed 

in several layers parallel to the exterior surface. A base concrete 

was first created with form work and then a more cement rich 

mortar layer was applied to the exposed vertical face of the 

Figure 13. Fort Casey on Admiralty Head, Fort Casey, Washington, was 

constructed in 1898. The lift lines from placement of concrete are clearly 

visible on the exterior walls and characterize the finished appearance. 
base concrete. The higher cement content in the facing 

concrete provided a more water-resistant outer layer 

and finished surface. The application of a cement-rich 

top layer, referred to in some early concrete publications 

as "waterproofing," was also used on top surfaces of 

concrete walls, or as the top layer in sidewalks. With this 

type of concrete construction, deterioration can occur 

over time as a result of debonding between layers, and 

can proceed very rapidly once the protective cement-rich 

layer begins to break down. 

It is common for historic concrete to have a highly 

variable appearance, including color and finish texture. 

Different levels of aggregate exposure due to paste 

erosion are often found in exposed aggregate concrete. 

This variability in the appearance of historic concrete 

increases the level of difficulty in assessing and repairing 

weathered concrete. 

Signs of Distress and Deterioration 

Characteristic signs of failure in concrete include 

cracking, spalling, staining, and deflection. Cracking 

occurs in most concrete but will vary in depth, width, 

direction, pattern, and location, and can be either active 

or dormant (inactive). Active cracks can widen, deepen, 

or migrate through the concrete, while dormant cracks 

remain relatively unchanged in size. Some dormant 

cracks, such as those caused by early age shrinkage of 

the concrete during curing, are not a structural concern 

but when left unrepaired, can provide convenient 

channels for moisture penetration and subsequent 

damage. Random surface cracks, also called map cracks 

due to their resemblance to lines on a map, are usually 

related to early-age shrinkage but may also indicate 

other types of deterioration such as alkali-silica reaction. 

Structural cracks can be caused by temporary or 

continued overloads, uneven foundation settling, seismic 

forces, or original design inadequacies. Structural cracks 

are active if excessive loads are applied to a structure, if 

the overload is continuing, or if settlement is ongoing. 

These cracks are dormant if the temporary overloads 

have been removed or if differential settlement has 

stabilized. Thermally-induced cracks result from 

stresses produced by the expansion and contraction 

of the concrete during temperature changes. These 

cracks frequently occur at the ends or re-entrant corners 

of older concrete structures that were built without 

expansion joints to relieve such stress. 

Spalling (the loss of surface material) is often associated 

with freezing and thawing as well as cracking and 

delamination of the concrete cover over embedded 

reinforcing steel. Spalling occurs when reinforcing 

bars corrode and the corrosion by-products expand, 

creating high stresses on the adjacent concrete, which 

cracks and is displaced. Spalling can also occur when 

water absorbed by the concrete freezes and thaws (Fig. 

14). In addition, surface spalling or scaling may result 

from the improper finishing, forming, or other surface 

Figures 14. Layers of architectural concrete that have debonded 

(spaUed) from the surface were removed from a historic water tank 

during the investigation performed to assess existing conditions. 

Photos: Anita Washko, Wiss, Janney, Elstner Associates, Inc. 

phenomena when water-rich cement paste (laitance) 

rises to the surface. The resulting weak material is 

vulnerable to spalling of thin layers, or scaling. In some 

cases, spalling of the concrete can diminish the loadcarrying capacity of the structure. 

Deflection is the bending or sagging of structural beams, 

joists, or slabs, and can be an indication of deficiencies in 

the strength and structural soundness of concrete. This 

condition can be produced by overloading, corrosion 

of embedded reinforcing, or inadequate design or 

construction, such as use of low-strength concrete or 

undersized reinforcing bars. 

Staining of the concrete surface can be related to soiling 

from atmospheric pollutants or other contaminants, 

dirt accumulation, and the presence of organic growth. 

However, stains can also indicate more serious 

underlying problems, such as corrosion of embedded 

reinforcing steel, improper previous surface treatments, 

alkali-aggregate reaction, or efflorescence, the deposition 

of soluble salts on the surface of the concrete as a result 

of water migration (Fig. 15). 

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Planning for Concrete Preservation 

The significance of a historic concrete building or 

structure-including whether it is important for its 

architectural or engineering design, for its materials 

and construction techniques, or both-guides decision 

making about repair and, if needed, replacement 

methods. Determining the causes of deterioration is also 

central to the development of a conservation and repair 

plan. With historic concrete buildings, one of the more 

difficult challenges is allowing for sufficient time during 

the planning phase to analyze the concrete, develop 

mixes, and provide time for adequate aging of mock-ups 

for matching to the original concrete. 

An understanding of the original construction 

techniques (cement characteristics, mix design, original 

intent of assembly, type of placement, precast versus 

cast in place, etc.) and previous repair work performed 

on the concrete is important in determining causes 

of existing deterioration and the susceptibility of the 

structure to potential other types of deterioration. 

For example, concrete placed in short lifts (individual 

concrete placements) or constructed in precast segments 

will have numerous joints that can provide entry points 

for water infiltration. Inappropriate prior repairs, such 

as installation of patches using an incompatible material, 

can affect the future performance of the concrete. Such 

prior repairs may require corrective work. 

As with other preservation projects, three primary 

approaches are usually considered for historic concrete 

structures: maintenance, repair, or replacement. 

Maintenance and repair best achieve the preservation 

goal of minimal intervention and the greatest retention 

of existing historic fabric. However, where elements of 

the building are severely deteriorated or where inherent 

problems with the material lead to ongoing failures, 

replacement may be necessary. 

During planning, information is gathered through 

research, visual survey, inspection openings, and 

laboratory studies. The material should then be 

reviewed by professionals experienced in concrete 

deterioration to help evaluate the nature and causes of 

the concrete problems, to assess both the short-term and 

long-term effects of the deterioration, and to formulate 

proper repair approaches. 

Condition Assessment 

A condition assessment of a concrete building or 

structure should begin with a review of all available 

documents related to original construction and prior 

repairs. While plans and specifications for older 

concrete buildings are not always available, they can 

be an invaluable resource and every attempt should be 

made to find them. They may provide information on 

the composition of the concrete mix or on the type and 

location of reinforcing bars. If available, documents 

related to past repairs should also be reviewed to 

Figure 15. Evidence of 

moisture movement through 

concrete is apparent 

in the form of mineral 

deposits on the concrete 

surface. Cyclic freezing 

and thawing of entrapped 

moisture, and corrosion of 

embedded reinforcement, 

have also contributed to 

deterioration of the concrete 

column on this fence at 

Crocker Field in Fitchburg, 

Massachusetts, designed by 

the Olmsted Brothers. 

understand how the repairs were made and to help 

evaluate their anticipated performance and service life. 

Archival photographs can also provide a valuable source 

of information about original construction. 

A visual condition survey will help identify and 

evaluate the extent, types, and patterns of distress 

and deterioration. The American Concrete Institute 

offers several useful guides on how to perform a visual 

condition survey of concrete. Generally, the condition 

assessment begins with an overall visual survey, 

followed by a close-up investigation of representative 

areas to obtain more detailed information about modes 

of deterioration. 

A number of nondestructive testing methods can be 

used in the field to evaluate concealed conditions. Basic 

techniques include sounding with a hand-held hammer 

(or for horizontal surfaces, a chain) to help identify areas 

of delamination. More sophisticated techniques include 

impact-echo testing (Fig. 16), ground penetrating radar, 

pulse velocity, and other methods that characterize 

concrete thickness and locate voids or delaminations. 

Magnetic detection instruments are used to locate 

embedded reinforcing steel and can be calibrated to 

identify the size and depth of reinforcement. Corrosion 

measurements can be taken using copper-copper 

sulfate half-cell tests or linear polarization techniques to 

determine the probability or rate of active corrosion of 

the reinforcing steel. 

To further evaluate the condition of the concrete, 

samples may be removed for laboratory study to 

determine material components and composition, 

and causes of deterioration. Samples need to be 

representative of existing conditions but should be taken 

from unobtrusive locations. Laboratory studies of the 

concrete may include petrographic evaluation following 

ASTM C856, Practice for Petrographic Examination 

of Hardened Concrete. Petrographic examination, 

consisting of microscopical studies performed by a 

geologist specializing in the evaluation of construction 

materials, is performed to determine air content, watercement ratio, cement content, and general aggregate 

characteristics. Laboratory studies can also include 
chemical analyses to determine chloride content, sulfate 

content, and alkali levels of the concrete; identification 

of deleterious aggregates; and determination of depth 

of carbonation. Compressive strength studies can 

be conducted to evaluate the strength of the existing 

concrete and provide information for repair work. The 

laboratory studies provide a general identification of 

the original concrete's components and aggregates, 

and evidence of damage due to various mechanisms 

including cyclic freezing and thawing, alkali-aggregate 

reactivity, or sulfate attack. Information gathered 

through laboratory studies can also be used to help 

develop a mix design for the repair concrete. 

Cleaning 

As with other historic structures, concrete structures are 

cleaned for several reasons: to improve the appearance 

of the concrete, as a cyclical maintenance measure, or 

in preparation for repairs. Consideration should first be 

given to whether the historic concrete structure needs to 

be cleaned at all. If cleaning is required, then the gentlest 

system that will be effective should be selected. 

Three primary methods are used for cleaning concrete: 

water methods, abrasive surface treatments, and 

chemical surface treatments. Low-pressure water (less 

than 200 psi) or steam cleaning can effectively remove 

surface soiling from sound concrete; however, care is 

required on fragile or deteriorated surfaces. In addition, 

water and steam methods are typically not effective in 

removing staining or severe soiling. Power washing 

with high-pressure water is sometimes used to clean or 

remove coatings from sound, high-strength concrete, but 

high-pressure water washing is generally damaging to 

and not appropriate for concrete on historic structures. 

When used with proper controls and at very low 

pressures (typically 35 to 75 psi), microabrasive 

Figure 16. Impact echo testing is performed on a concrete structural 

slab to help determine depth of deterioration. In this method, a short 

pulse of energy is introduced into the structure and a transducer 

mounted on the impacted surface of the structure receives the 

reflected input waves or echoes. These waves are analyzed to help 

identify flaws and deterioration within the concrete. 

surface treatments using very fine particulates, such 

as dolomitic limestone powder, can sometimes clean 

effectively. However, micro abrasive cleaning may alter 

the texture and surface reflectivity of concrete. Some 

concrete can be damaged even by fine particulates 

applied at very low pressures. 

Chemical surface treatments can clean effectively 

but may also alter the appearance of the concrete by 

bleaching the concrete, removing the paste, etching 

the aggregate, or otherwise altering the surface. 

Detergent cleaners or mild, diluted acid cleaners may 

be appropriate for removal of staining or severe soiling. 

Cleaning products that contain strong acids such as 

hydrochloric (muriatic) or hydrofluoric acid, which will 

damage concrete and are harmful to persons, animals, 

site features, and the environment, should not be used. 

For any cleaning process, trial samples should be 

performed prior to full-scale implementation. The 

intent of the cleaning program should not be to return 

the structure to a like new appearance. Concrete can 

age gracefully, and as long as soiling is not severe or 

deleterious, many structures can still be appreciated 

without extensive cleaning. 

Methods of Maintenance and Repair 

The maintenance of historic concrete often is thought of 

in terms of appropriate cleaning to remove unattractive 

dirt or soiling materials. However, the implementation 

of an overall maintenance plan for a historic structure is 

the most effective way to help protect historic concrete. 

For examples, the lack of maintenance to roofs and 

drainage systems can promote water related damage 

to adjacent concrete features. The repeated use of 

deicing salts in winter climates can pit the surface of old 

concrete and also may promote decay in embedded steel 

reinforcements. Inadequate protection of concrete walls 

adjacent to driveways and parking areas can result in the 

need for repair work later on. 

The maintenance of historic concrete involves the regular 

inspection of concrete to establish baseline conditions 

and identify needed repairs. Inspection tasks involve 

monitoring protection systems, including sealant joints, 

expansion joints, and protective coatings; reviewing 

existing conditions for development of distress such as 

cracking and delaminations; documenting conditions 

observed; and developing and implementing a cyclical 

repair program. 

Sealants are an important part of maintenance of historic 

concrete structures. Elastomeric sealants, which have 

replaced traditional oil-resin based caulks for many 

applications, are used to seal cracks and joints to keep 

out moisture and reduce air infiltration. Sealants are 

commonly used at windows and door perimeters, 

at interfaces between concrete and other materials, 

and at attachments to or through walls or roofs, such 

as with lamps, signs, or exterior plumbing fixtures. 

9 
10 

Figure 17. (a) The 63rd Street Beach House was constructed on the shoreline of Chicago in 1919. The highly exposed aggregate concrete of the 

exterior walls of the beach house was used for many buildings in the Chicago parks as an alternative to more expensive stone construction. Photo: 

Leslie Schwartz Photography. (b) Concrete deterioration included cracking, spalling, and delamination caused by corrosion of embedded reinforcing 

steel and concrete damage due to cyclic freezing and thawing. (c) Various sizes and types of aggregates were reviewed for matching to the original 

concrete materials. (d) Mock-ups of the concrete repair mix were prepared for comparison to the original concrete. Considerations included aggregate 

type and size, cement color, proportions, aggregate exposure, and surface finish. (e) The craftsman finished the surface to replicate the original 

appearance in a mock-up on the structure. Here, he used a nylon bristle brush to remove loose paste and expose the aggregate, creating a variable 

surface to match the adjacent original concrete. 

Where used for crack repairs on historic facades, the 

finished appearance of the sealant application must 

be considered, as it may be visually intrusive. In some 

cases, sand can be broadcast onto the surface of the 

sealant to help conceal the repair. 

Urethane and polyurethane sealants are often used to 

seal joints and cracks in concrete structures, paving, 

and walkways; these sealants provide a service life of 

up to ten years. High-performance silicone sealants 

also are often used with concrete, as they provide a 

range of movement capabilities and a service life of 

twenty years or more. Some silicone sealants may stain 

adjacent materials, which may be a problem with more 

porous concrete, and may also tend to accumulate 

dust and dirt. The effectiveness of sealants for sealing 

joints and cracks depends on numerous factors 

including proper surface preparation and application. 

Sealants should be examined as part of routine 

maintenance inspections, as these materials deteriorate 

faster than their substrates and must be replaced 

periodically as a part of cyclical maintenance. 

Repair of historic concrete may be required to 

address deterioration because the original design and 

construction did not provide for long-term durability, 

or to facilitate a change in use of the structure. 

Examples include increasing concrete cover to protect 

reinforcing steel and reducing water infiltration into the 

structure by repair of joints. Any such improvements 

must be thoroughly evaluated for compatibility with 

the original design and appearance. Care is required in 

all aspects of historic concrete repair, including surface 

preparation; installation of form work; development 

of the concrete mix design; and concrete placement, 

consolidation, and curing. 

An appropriate repair program addresses existing 

distress and reduces the rate of future deterioration, 

which in many cases involves moisture-related issues. 

The repair program should incorporate materials and 

methods that are sympathetic to the existing materials 

in character and appearance, and which provide good 

long-term performance. In addition, repair materials 

should age and weather similarly to the original 

materials. In order to best achieve these goals, concrete 

repair projects should be divided into three phases: 

development of trial repair procedures, trial repairs and 

evaluation, and production repair work. 
For any concrete repair project, the process of investigation, 

laboratory analysis, trial samples, mock-ups, and full-scale 

repairs allows ongoing refinement of the repair work as 

well as implementation of quality-control measures. The 

trial repair process provides an opportunity for the owner, 

architect, engineer, and contractor to evaluate the concrete 

mix design and the installation and finishing techniques for 

the repairs from both technical and aesthetic standpoints. 

The final repair materials and procedures should match 

the original concrete in appearance while meeting the 

established criteria for durability. Information gathered 

through trial repairs and mock-ups is invaluable in refining 

the construction documents prior to the start of the overall 

repair project (Fig. 17). 

Surface Preparation 

In undertaking surface preparation for historic concrete 

repair, care must be taken to limit removal of existing 

material while still providing an appropriate substrate for 

repairs. This is particularly important where ornamentation 

and fine details are involved. Preparation for localized 

repairs usually begins with removal of the loose concrete 

to determine the general extent of the repair, followed by 

saw-cutting the perimeter of the repair area. The repair area 

should extend beyond the area of concrete deterioration 

to a sufficient extent to provide a sound substrate. When 

repairing concrete with an exposed aggregate or other 

special surface texture, a sawcut edge may be too visually 

evident. To hide the repair edge, techniques such as lightly 

hand-chipping the edge of the patch may be used to 

conceal the joint between the original concrete and the new 

repair material. The depth to which the concrete needs to 

be removed may be difficult to determine without invasive 

probing in the repair area. Removal of concrete should 

typically extend beyond the level of the reinforcing steel, if 

present, so that the patch encapsulates the reinforcing steel, 

which provides mechanical attachment for the repair. 

If the concrete was originally of lower strength and quality, 

the assessment of present soundness is more difficult. 

Deteriorated and unsound concrete is typically removed 

using pneumatic chipping hammers. Removal of concrete 

in historic structures is better controlled by using smaller 

chipping hammers or hand tools. The area of the concrete 

to be repaired and the exposed reinforcing steel are 

then cleaned, usually by careful sandblast and air blast 

procedures applied only within the repair area. Adjacent 

original concrete surfaces should be protected during this 

work. In some cases, project constraints such as dust control 

may limit the ability to thoroughly clean the concrete and 

steel. For example, it may be necessary to use needle scaling 

(a small pneumatic impact device) and wire brushing 

instead of sandblasting. 

Supplemental steel may be needed when existing 

reinforcing steel is severely deteriorated, or if reinforcing 

steel is not present in repair areas. Exposed existing 

reinforcing and other embedded steel elements can be 

cleaned, primed, and painted with a corrosion-inhibiting 

coating. The patching material should be reinforced 

and mechanically attached to the existing concrete. 

Reinforcement materials used in repairs most often 

include mild steel, epoxy-coated steel, or stainless steel, 

depending on existing conditions. 

Formwork and Molds 

Special formwork is needed to recreate ornamental 

concrete features - which may be complex, in high 

relief, or architecturally detailed-and to provide special 

surface finishes such as wood form board textures. 

Construction of the formwork itself requires particular 

skill and craftsmanship. Reusable forms can be used for 

concrete ornamentation that is repeated across a building 

facade, or precast concrete elements may be used to 

replace missing or unrepairable architectural features. 

Formwork for ornamental concrete is often created using 

a four-step process: a casting of the original concrete is 

taken; a plaster replica of the unit is prepared; a mold or 

form is made from the plaster replica; and a new concrete 

unit is cast. Custom formwork and molds are often the 

work of specialty companies, such as precasters and cast 

stone fabricators. 

The process of forming architectural features or special 

surface textures is particularly challenging if early age 

stripping (removal of formwork early in the concrete 

curing process) is needed to perform surface treatment 

on the concrete. Timing for formwork removal is related 

to strength gain, which in turn is partly dependent on 

temperature and weather conditions. Early age removal of 

formwork in highly detailed concrete can lead to damage 

of the new concrete that has not yet gained sufficient 

strength through curing. 

Selection of Repair Materials and Mix Design 

Selection and design of proper repair materials is a 

critical component of the repair project. This process 

requires evaluation of the performance, characteristics, 

and limitations of the repair materials, and may involve 

laboratory testing of proposed materials and trial repairs. 

The materials should be selected to address the specific 

type of repair required and to be compatible with special 

characteristics of the original concrete. Some modern 

repair materials are designed to have a high compressive 

strength and to be impermeable. Even though inherently 

durable, these newer materials may not be appropriate for 

use in repairing a low strength historic concrete. 

The concrete's durability, or resistance to deterioration, 

and the materials and methods selected for repair 

depend on its composition, design, and quality of 

workmanship. In most cases, a mix design for durable 

replacement concrete should use materials similar to 

those of the original concrete mix. Prepackaged materials 

are often not appropriate for repair of historic concrete. 

The concrete patching material can be air entrained or 

polymer-modified if subject to exterior exposure, and 

should incorporate an appropriate selection of aggregate 

and cement type, and proper water content and water 

11 
12 

• • I '{\ I ., • 

• , -II t, . \ ',~,' j ~ ;.\ '\ -.: " \: .... ·~·~I"I I "'".",'. , .. . ..r.,,. ,T, . " I.... • •. ~ •. .. ~ • -: • \ I·, ... ~ t' I'!, ' ;. ' "w --. I '.' 

. I, .',: "i '" 1 ~ ) .' '. j I I - • , , 

b .• I 

Figure 18. (a) Exposed aggregate precast concrete is 

sounded with a hammer to detect areas of deterioration. 

Corrosion of the exposed reinforcing steel bar has led to 

spalling of the adjacent concrete. (b) Samples of aggregate 

considered for use in repair concrete are compared to the 

original concrete materials in terms of size, color, texture, 

and reflectance. (c) Various sample panels are made using 

the selected concrete repair mix design for comparison to 

the original concrete on the building, and the mix design is 

adjusted based on review of the samples. (d) After removal 

of the spall, the concrete surface is prepared for installation 

of a formed patch. (e) Prior to placement of the concrete, 

a retarding agent is brush-applied to the inside face of the 

formwork to slow curing at the surface. After the concrete 

is partially cured, the forms are removed and the surface 

of the concrete is rubbed to remove some of the paste and 

expose the aggregate to match the original concrete. 

to cement ratio. Some admixtures, including polymer modifiers, 

may change the appearance of the concrete mix. Design of the 

concrete patching material should address characteristics required 

for durability, workability, strength gain, compressive strength, 

and other performance attributes. During installation of the 

repair, skilled workmanship is required to ensure proper mixing 

procedures, placement, consolidation, and curing. 

Matching and Repair Techniques for Historic Concrete 

Repair measures should be selected that retain as much of the 

original material as possible, while providing for removal of an 

adequate amount of deteriorated concrete to provide a sound 

substrate for a durable repair. The installed repair must visually 

match the existing concrete as closely as possible and should be 

similar in other aspects such as compressive strength, permeability, 

and other characteristics important in the mix design of the 

concrete (Fig. 18). 

Understanding the original construction techniques often provides 

opportunities in the design of repairs. For example, joints between 

the new and old concrete can be hidden in changes in surface 

profile and cold joints. The required patching mix for the concrete 

to be used in the repair will likely need to be specially designed 

to replicate the appearance of the adjacent historic concrete. A 

high level of craftsmanship is required for finishing of historic 

concrete, in particular to create the sometimes inconsistent finish 

and variation in the original concrete in contrast to the more even 

appearance required for most non-historic repairs. 

To match the various characteristics of the original concrete, trial 

mixes should be developed. These mixes need to take into account 

the types and colors of aggregates and paste present in the original 

concrete. Different mixes may be needed because of variations 

in the appearance and composition of the historic concrete. The 

trials should utilize different forming and finishing techniques 

to achieve the best possible match to the original concrete. Initial 

trials should first take place on site but off the structure. The mix 

designs providing the best match are then installed as trial repairs 

on the structure, and assessed after they have cured. 

Achieving compatibility between repair work and original 

concrete may be difficult, especially given the variability often 

present in historic concrete materials and finishes. Formed rather 

than trowel-applied patch repairs are recommended for durability, 

as forming permits better ranges of mix ingredients (such as coarse 

aggregates) and improved consolidation as compared to trowelapplied repairs. Parge coatings usually are not recommended 

as they do not provide as durable repair as formed concrete. 

However, in some cases parge coatings may be appropriate to 

match an original parged surface treatment Proper placement 

and finishing of the repair are important to obtain a match with 

the original concrete. To minimize problems associated with rapid 

curing of concrete, such as surface cracking, it is important to use 

proper curing methods and to allow for sufficient time. 

Hairline cracks that show no sign of increasing in size may often 

be left unrepaired. The width of the crack and the amount of 

movement usually limits the selection of crack repair techniques 

that are available. Although it is difficult to determine whether 

cracks are moving or non-moving, and therefore most cracks 
should be assumed to be moving, it is possible to repair 

non-moving cracks by installation of a cementitious 

repair mortar matching the adjacent concrete. It is 

generally desirable not to widen cracks prior to the mortar 

application. Repair mortar containing sand in the mix may 

be used for wider cracks; unsanded repair mortar may be 

used for narrower cracks. 

When it is desirable to re-establish the structural integrity 

of a concrete structure involving dormant cracks, epoxy 

injection repair has proven to be an effective procedure. 

Such a repair is made by first sealing the crack on both sides 

of a wall or structural member with epoxy, polyester, wax, 

tape, or cement slurry, and then injecting epoxy through 

small holes or ports drilled in the concrete. Once the epoxy 

in the crack has hardened, the surface sealing material 

may be removed; however, this type of repair is usually 

quite apparent. Although it may be possible to inject epoxy 

without leaving noticeable residue, this process is difficult 

and, in general, the use of epoxy repairs in visible areas of 

concrete on historic structures is not recommended. 

Active structural cracks (which move as loads are 

added or removed) and thermal cracks (which move as 

temperatures fluctuate) must be repaired in a manner that 

will accommodate the anticipated movement. In some more 

extreme cases, expansion joints may have to be introduced 

before crack repairs are undertaken. Active cracks may 

be filled with sealants that will adhere to the sides of 

the cracks and will compress or expand during crack 

movement. The design, detailing, and execution of sealant 

repairs require considerable attention, or they will detract 

from the appearance of the historic building. The routing 

and cleaning of a crack, and installation of an elastomeric 

sealant to prevent water penetration, is used to address 

cracks where movement is anticipated. However, unless 

located in a concealed area of the concrete, this technique 

is often not acceptable for historic structures because the 

repair will be visually intrusive (Fig. 19). Other approaches, 

such as installation of a cementitious crack repair, may need 

to be considered even though this type of repair may be less 

effective or have a shorter service life than a sealant repair. 

Replacement 

If specific components of historic concrete structures are 

beyond repair, replacement components can be cast to 

match historic ones. Replacement of original concrete 

should be carefully considered and viewed as a method of 

last resort. In some cases, such as for repeated ornamental 

units, it may be more cost-effective to fabricate precast 

concrete units to replace missing elements. The forms 

created for precast or cast-in-place units can then be used 

again during future repair projects. 

Careful mix formulation, placement, and finishing are 

required to ensure that replacement concrete units will 

match the historic concrete. There is often a tendency to 

make replacement concrete more consistent in appearance 

than the original concrete. The consistency can be in 

stark contrast with the variability of the original concrete 

Figure 19. A high-speed grinder ia used to widen a crack in 

preparation for installation of a sealant. This process is called 

"routing. " After the crack is prepared, the sealant is installed to 

prevent moisture infiltration through the crack. Although sealant 

repairs can provide a durable, watertight repair for moving cracks, 

they tend to be very visible. 

due to original construction techniques, architectural 

design, or differential exposure to weather. Trial repairs 

and mock-ups are used to evaluate the proposed 

replacement concrete work and to refine construction 

techniques (Fig 20). 

Protection Systems 

Coatings and Penetrating Sealers. Protection systems 

such as a penetrating sealers or film forming coating 

are often used with non-historic structures to protect 

the concrete and increase the length of the service life 

of concrete repairs. However, film-forming coatings 

are often inappropriate for use on a historic structure, 

unless the structure was coated historically. Filmforming coatings will often change the color and 

appearance of a surface, and higher build coatings can 

also mask architectural finishes and ornamental details. 

For example, the application of a coating on concrete 

having a formboard finish may hide the wood texture 

of the surface. Pigmented film-forming coatings are 

also typically not appropriate for use over exposed 

aggregate concrete, where the uncoated exposed surface 

contributes significantly to the historic character of 

13 
14 

Figure 20. (a) The Jefferson Davis Memorial in Fairview, Kentucky, 

constructed from 1917-1924, is 351 feet tall and constructed of 

unreinforced concrete. The walls of the memorial are 8 feet thick at the 

base and 2 feet thick at the top of the wall. Access to the monument 

for investigation was provided by rappelling techniques, while ground 

supported and suspended scaffolding was used to access the exterior during 

repairs. (b) The concrete was severely deteriorated at isolated locations, with 

spalling and damage from cyclic freezing and thawing of entrapped water. 

In addition, previous repairs were at the end of their service life and removal 

of deteriorated concrete and failed previous repairs was required. Light 

duty chipping hammers were used to avoid damage to adjacent material 

when removing deteriorated concrete to the level of sound concrete. (c) 

Field samples were performed to match the color, finish, and texture of the 

original concrete. A challenge in matching of historic concrete is achieving 

variability of appearance. (d) The completed surface after repairs exhibits 

intentional variability of the concrete surface to match the appearance of 

the original concrete. Some formwork imperfections that would normally be 

removed by finishing were intentionally left in place, to replicate the highly 

variable finish of the original concrete. (e) The Jefferson Davis Memorial 

after completion of repairs in 2004. Photo e: Joseph Lenzi, Senler, Campbell 

& Associates, Inc. 
concrete. In cases where the color of a substrate needs to 

be changed, such as to modify the appearance of existing 

repairs, an alternative to pigmented film-forming coatings is 

the use of pigmented stains. 

Many proprietary clear, penetrating sealers are currently 

available to protect concrete substrates. These products 

render fine cracks and pores within the concrete 

hydrophobic; however, they do not bridge or fill cracks. 

Clear sealers may change the appearance of the concrete in 

that treated areas become more visible after rain in contrast 

to the more absorptive areas of original concrete. Once 

applied, penetrating sealers cannot be effectively removed 

and are therefore considered irreversible. They should 

not be used on historic concrete without thorough prior 

consideration. However, clear penetrating sealers provide 

an important means of protection for historic concrete that 

is not of good quality and can help to avoid more extensive 

future repairs or replacement. Thus they are sometimes 

appropriate for use on historic concrete. Once applied, these 

sealers will require periodic re-application. 

Waterproofing membranes are systems used to protect 

concrete surfaces such as roofs, terraces, plazas, or balconies, 

as well as surfaces below grade. Systems range from coal 

tar pitch membranes used on older buildings, to asphalt or 

urethane-based systems. On historic buildings, membrane 

systems are typically used only on surfaces that were 

originally protected by a similar system and surfaces that are 

not visible from grade. Waterproofing membranes may be 

covered by roofing, paving, or other architectural finishes. 

Laboratory and field testing is recommended prior to 

application of a protection system or treatment on any 

concrete structure; testing is even more critical for historic 

structures because many such treatments are not reversible. 

As with other repairs, trial samples are important to 

evaluate the effectiveness of the treatment and to determine 

whether it will harm the concrete or affect its appearance. 

Cathodic Protection. Corrosion is an electrochemical 

process in which electrons flow between cathodic (positively 

charged) and anodic (negatively charged) areas on a 

metal surface; corrosion occurs at the anodes. Cathodic 

protection is a technique used to control the corrosion of 

metal by making the whole metal surface the cathode of 

an electrochemical cell. This technique is used to protect 

metal structures from corrosion and is also sometimes 

used to protect steel reinforcement embedded in concrete. 

For reiniorced concrete, cathodic protection is typically 

accomplished by connecting an auxiliary anode to the 

reiniorcing so that the entire reiniorcing bar becomes a 

cathode. In sacrificial anode (passive) systems, current flows 

naturally by galvanic action between the less noble anode 

(such as zinc) and the cathode. In impressed-current (active) 

systems, current is impressed between an inert anode 

(such as titanium) and the cathode. Cathodic protection is 

intended to reduce the rate of corrosion of embedded steel 

in concrete, which in turn reduces overall deterioration. 

Protecting embedded steel from corrosion helps to prevent 

concrete cracking and spalling. 

Impressed-current cathodic protection is the most 

effective means of mitigating steel corrosion and has 

been used in practical structural applications since the 

1970s. However, impressed-current cathodic protection 

systems are typically the most costly to install and 

require substantial ongoing monitoring, adjustment, 

and maintenance to ensure a proper voltage output 

(protection current) over time. Sacrificial anode cathodic 

protection dates back to the 1800s, when the hulls of 

ships were protected using this technology. Today 

many industries utilize the concept of sacrificial anode 

cathodic protection for the protection of steel exposed 

to corrosive environments. It is less costly than an 

impressed-current system, but is somewhat less effective 

and requires reapplication of the anode when it becomes 

depleted. 

Re-alkalization. Another technique currently available 

to protect concrete is realkalization, which is a process 

to restore the alkalinity of carbonated concrete. The 

treatment involves soaking the concrete with an alkaline 

solution, in some cases forcing it into the concrete to 

the level of the reiniorcing steel by passage of direct 

current. These actions increase the alkalinity of the 

concrete around the reiniorcement, thus restoring the 

protective alkaline environment for the reiniorcement. 

Like impressed-current cathodic protection methods, it 

is costly. Other corrosion methods are also available but 

have a somewhat shorter history of use. 

Careful evaluation of existing conditions, the causes and 

nature of distress, and environmental factors is essential 

before a protection method is selected and implemented. 

Not every protection system will be effective on each 

structure. In addition, the level of intrusion caused by 

the protection system must be carefully evaluated before 

it is used on a historic concrete structure. 

Summary 

In the United States, concrete has been a popular 

construction material since the late nineteenth century 

and recently has gained greater recognition as a historic 

material. Preservation of historic concrete requires a 

thorough understanding of the causes and types of 

deterioration, as well as of repair and replacement 

materials and methods. It is important that adequate 

time is allotted during the planning phase of a project 

to provide for trial repairs and mock-ups in order 

to evaluate the effectiveness and aesthetics of the 

repairs. Careful design is essential and, as with other 

preservation efforts, the skill of those performing 

the work is critical to the success of the repairs. The 

successful repair of many historic concrete structures 

in recent years demonstrates that the techniques and 

materials now available can extend the life of such 

structures and help ensure their preservation. 

15 
16 

Selected Reading 

American Concrete Institute. Guide for Making a Condition Survey of Concrete in Service. ACI Committee 201, ACI 201.1R-92. 

American Concrete Institute. Guide to Evaluation of Concrete Structures before Rehabilitation. ACI Committee 364, ACI 364.1R-07. 

American Concrete Institute. Concrete Repair Guide. ACI Committee 546, ACI 546R-04. 

American Concrete Institute. Guide for Evaluation of Existing Concrete Buildings. ACI Committee 437, ACI 437R-03. 

Childe, H.L. Manufacture and Uses of Concrete Products and Cast Stone. London: Concrete Publications Limited, 1930. 

Collins, Peter. Concrete: The Vision of a New Architecture. New York, New York: Faber and Faber, 1959. 

Cowden, Adrienne B., compo Historic Concrete: An Annotated Bibliography. Washington, D.C: National Park Service, 1993. 

Komandant, August E. Contemporary Concrete Structures. New York, New York: McGraw Hill, 1972. 

Erlemann, Gustav G. "Steel Reinforcing Bar Specification in Old Structures." Concrete International, April 1999, 49-50. 

Federal Highway Administration. Guide to Nondestructive Testing of Concrete. FHWA Publication Number FHWA-SA-97-105. 

Gaudette, Paul E. "Special Considerations in Repair of Historic Concrete." Concrete Repair Bulletin, January/February 2000,12-13. 

Jester, Thomas C, ed. Twentieth CentunJ Building Materials. New York, New York: McGraw-Hill, 1995. 

Johnson, Arne P., and Seung Kyoung Lee. "Protection Methods for Historic Concrete at Soldier Field." Preserve and Play: Preserving 

Historic Recreation and Entertainment Sites. Washington, D.C: Historic Preservation Education Foundation, National Council for 

Preservation Education, and National Park Service, 2006. 

Macdonald, Susan, ed. Concrete: Building Pathology. Osney Mead, Oxford, u.K.: Blackwell Science, 2003. 

McGovern, Martin S. "A Clear View of Sealers." Concrete Construction, January 2000, 53-58. 

Morton, W. Brown III, Gary L. Hume, Kay D. Weeks, H. Ward Jandl, and Anne E. Grimmer. The Secretary of the Interior's Standards for 

Rehabilitation & Illustrated Guidelines for Rehabilitating Historic Buildings. Washington, D.C: National Park Service, 1983, reprinted 1997. 

"Repairing Cracks." Concrete Repair Digest, August/September 1992, 160-164. Condensed from ACI document 224.1R-93. 

Slaton, Deborah. "Cleaning Historic Concrete." Concrete Repair Bulletin, January/February 2000,14-15. 

Acknowledgements 

Paul Gaudette is an engineer with Wiss, Janney, Elstner Associates, Inc., in Chicago, Illinois. Deborah Slaton is an architectural 

conservator with Wiss, Janney, Elstner Associates, Inc., in Northbrook, Illinois. All photographs by Paul Gaudette unless otherwise 

stated. Front cover image: Kyle Normandin, Wiss, Janney, Elstner Associates, Inc. 

The authors wish to thank William Bing Coney; author of the first edition of this preservation brief, who served as a peer reviewer 

for the current edition. In addition, the authors gratefully acknowledge the assistance of the following individuals as peer reviewers 

of this brief: Arne Johnson and Una Gilmartin, Wiss, Janney, Elstner Associates, Inc.; Robert Joyce, Quality Restorations, Inc.; Susan 

Macdonald, New South Wales Heritage Office; Miles T. Murray; Restruction Corporation; and Jack Pyburn, OJP/Architect, Inc. Anne 

E. Grimmer, Chad Randl, and former staff Sharon C Park, FAIA, of the Technical Preservation Services, National Park Service, offered 

valuable comments during development of the brief. Charles E. Fisher of the National Park Service was the technical editor for this 

publication project. 

This publication has been prepared pursuant to the National Historic Preservation Act of 1966, as amended, which directs the Secretary 

of the Interior to develop and make available information concerning historic properties. Comments about this publication should 

be addressed to: Charles E. Fisher, Technical Preservation Publications Program Manager, Technical Preservation Services-2255, 

National Park Service, 1849 C Street, NW, Washington, DC 20240. This publication is not copyrighted and can be reproduced without 

penalty. Normal procedures for credit to the authors and the National Park Service should be provided. The photographs used in 

this publication may not be used to illustrate other publications without permission of the owners. For more information about the 

programs of the National Park Service's Technical Preservation Services see our website at http://www.nps.govlhistorylhps/tps.htm 

ISBN: 978-0-16-078946-5 

U.S. Government Printing Office Stock Number: 024-005-01253-2 2007