Concrete Behavior

Table of Contents

Table of Contents. 1

Structural Cracking. 2

Process. 2

Contributing Factors. 2

Results. 3

Remedies. 3

Investigations of Structural Cracking. 3

Further References. 3

Shrinkage Cracking. 4

Process. 4

Contributing Factors. 4

High water-cement ratio. 4

Temperature extremes. 4

Lack of adequate reinforcement 4

Lack of adequate curing of concrete at initial placement 4

Results. 4

Remedies. 5

Investigations of Shrinkage Cracking. 5

Further References. 5

Freeze-Thaw Effects. 5

Process. 6

Contributing Factors. 6

Freeze-thaw cycles. 6

Moisture pathways. 6

High water-cement ratio. 6

Lack of entrained air 6

Poorly consolidated concrete. 6

Results. 6

Remedies. 7

Investigations of Freeze-Thaw Cracking. 7

Further References. 7

Reinforcement Corrosion. 7

Process. 7

Contributing Factors. 8

Moisture Pathways. 8

High water-cement ratio. 8

Presence of Chloride Ions. 8

Low concrete tensile strength. 8

Electrical contact with dissimilar metals. 8

Remedies. 9

Investigations of Corrosion Damage. 9

Structural Failure. 10

Process. 10

Contributing Factors. 10

Foundation Settlement 10

Poor or uncertain concrete quality. 10

Placement of reinforcement 11

Corrosion of reinforcement 11

Remedies. 11

Investigations of Structural Failure. 11

Other Effects. 11

Thermal Cracking. 11

Alkali-Aggregate Reactivity. 11

D-Cracking. 11

Glossary. 12

 

 

Structural Cracking

 

Process

 

Concrete, when subjected to tensile beyond its tensile stress limit, develops cracks. The tensile stress at which concrete cracks is variable, even within the same batch of concrete, and depends on the total state of stress, the type of stress, the location within the concrete cross-section of the maximum stress, the amount and depth of the reinforcement, and a variety of other factors. The stress at which concrete cracks is in the range of 300-1000 lbs/in2. Cracks that can be unequivocally identified as structural cracks in reinforced concrete are generally innocuous, especially when they are smaller than 1/100 of an inch in width. Concrete cracks can also be a useful diagnostic tool, as they give some indication of the type of stresses that the concrete is sustaining.

 

It is also important to understand that in reinforced concrete, the tensile stresses are not fully transferred to the reinforcement until the concrete has cracked. Before cracking, the concrete is carrying most of the tensile stresses, and the reinforcement is contributing very little to the resistance to the loads. The reinforcement is designed under the assumption that the concrete will have cracked under service loads.

 

Cracking patterns may be different depending on the type of stresses producing the cracks. Tensile stresses due to bending tend to produce cracks that propagate from the edge of the beam, slab, or shell, in a direction parallel to the supports and perpendicular to the face of the beam, slab, or shell. Shear cracks are located close to the support, and take a more diagonal direction. This is because a shearing stress resolves into a tensile stress in a diagonal direction.

 

The v-shaped cracks in this figure result from unintended stresses in the flange of the Tee shapes. 

 

Contributing Factors

 

Low-strength concrete: Concrete is designed for compressive strength, and improved or increased tensile strength is generally incidental to improved compressive strength. Lower strength concrete will have lower tensile strength and be more likely to crack, given equal stress magnitudes.

 

Poorly consolidated concrete: Poor consolidation causes loss of cross sectional area, and provides opportunities for cracks to initiate.

 

Unanticipated stresses: Stresses of magnitudes, and especially directions that were not anticipated in the initial design of the structure may cause unanticipated cracks. Examples of this kind of structural cracking include diagonal cracks at corners of slabs, or at re-entrant corners in building slabs or building facades.

 

Results

 

The result of structural cracking in properly designed concrete is simply a visual problem: owners and building users often find cracks in concrete to be unsightly, or even threatening. However, cracks can be an indication of more serious shortcomings in the structural design. They also form a pathway for moisture and air to reach the reinforcement, and may hasten the corrosion of the reinforcement (although this is the topic of a vigorous debate in the engineering literature). They do allow penetration of moisture and promotion of freeze-thaw effects, and can eventually develop into larger cracks and spalled or damaged areas.

 

Remedies

 

Usually, this is a condition that does not require remedies. Where the cracking becomes unsightly, or admits too much moisture to the interior of the structure, some sort of repair may be warranted. If the structure is deficient, external reinforcement, or external post tensioning may be undertaken. If the problem is superficial, the cracks may be repaired by epoxy injection.

 

Investigations of Structural Cracking

 

Structural cracking is investigated by visual inspection, and by analysis of the structure that attempts to reproduce the conditions that produced the cracks.  The analysis of the structure can involve simply some understanding of the nature of the stresses in the zone where cracking is observed, such as stresses due to edge restraint.  The investigation may also involve more complex computer models, to determine the direction, location, and magnitude of maximum tensile stresses.   The visual inspection may be accompanied by laboratory tests, such as petrographic analysis of the concrete (ASTM C856).  The petrographic analysis would be specifically looking for reasons for the weakness of the cement matrix: lack of cement, chemical attack of the cement matrix, etc.  The petrographic analysis may also be useful in estimate the age of the cracks, as in the case study linked below.  Descriptions of a program of investigation of structural cracking is available in the Miami Marine Stadium case study.  Investigations of structural cracking, and in some cases, incipient structural failure are described in the Kresge Arena case study. 

 

Further References

 


Shrinkage Cracking

 

Process

 

Concrete shrinks as it dries out after initial placement. This is primarily due to the change in volume after excess water is removed from the material by drying. The length change for ordinary concrete can vary from about .01% to .1%, depending on a number of factors listed below. Since most concrete structures are not free to shrink, but are restrained at the ends, the results of the tendency to shrink is to develop tensile stresses in the concrete, which cause the development of cracks, approximately spaced at some interval varying from about 5 feet to about 20 feet. Although it is not possible to control the tendency of the concrete to shrink, the size and severity of the cracks can be controlled by the addition of reinforcement. Shrinkage reinforcement must be continuous and uniformly distributed throughout the structure.

 

Contributing Factors

 

High water-cement ratio

 

The larger the proportion of water in the concrete, the greater the volume change on drying, and the greater the tendency to shrink. As stated above, a water/cement ratio of about .25 is chemically sufficient for hydration of the cement, but additional water must be added to make the concrete workable. Large amounts of excess water are undesirable from the point of view of concrete strength, and dimensional stability, but do improve the workability and the economy of the concrete.

 

Temperature extremes

 

Temperature extremes, especially just after placement of the concrete, may promote more rapid drying and hasten the development of shrinkage cracks.

 

Lack of adequate reinforcement

 

Reinforcement of the concrete cannot prevent shrinkage cracking, but can control the severity and extent of the development of cracks.

 

Lack of adequate curing of concrete at initial placement

 

Curing concrete properly means maintaining the material in a moist condition as it gains its early strength. If drying and subsequent shrinkage develop early, the material has much lower tensile strength and is much more susceptible to the development of cracks.

 

Results

 

The result of shrinkage cracking, like structural cracking in properly designed concrete is simply a visual problem: owners and building users often find cracks in concrete to be unsightly, or even threatening. Shrinkage cracks also form a pathway for moisture and air to reach the reinforcement, and may hasten the corrosion of the reinforcement (although this is the topic of a vigorous debate in the engineering literature). They do allow penetration of moisture and promotion of freeze-thaw effects, and can eventually develop into larger cracks and spalled or damaged areas.

 

Severe shrinkage cracking of a concrete patch: the patching concrete mix was probably overwatered and/or insufficiently cured.

 

Remedies

 

Usually, this is a condition that does not require remedies. Where the cracking becomes unsightly, or admits too much moisture to the interior of the structure, some sort of repair may be warranted. If the structure is deficient, external reinforcement, or external post tensioning may be undertaken. If the problem is superficial, the cracks may be repaired by epoxy injection.

 

Investigations of Shrinkage Cracking

 

Investigations of shrinkage cracking may include investigations of the conditions under which the concrete was placed--hot and dry weather promote early age shrinkage cracking; the addition of water to the concrete during placement makes the material more susceptible to shrinkage cracking.  Investigations should also be undertaken to discern shrinkage cracking from structural cracking. 

 

Further References

 

Freeze-Thaw Effects

 

 

Process

Concrete is a porous material and will absorb water, either into pores, which always exist within the cement matrix, or into previously formed structural or shrinkage cracks.  As is well-known, the volume of water increases as it freezes, and freezing water contained within the concrete can cause stresses to develop in the concrete.  When these stresses exceed the tensile capacity of the concrete, they may cause a number of effects: spalling of the concrete, development of further cracks, 'popouts' of the surface of the concrete

Contributing Factors

 

Freeze-thaw cycles

 

The number of freeze-thaw cycles in a winter season is an important factor in producing damage to concrete.  This quantity varies not only with the coldness of the winter climate, but also with the daily variations in temperature.  The masonry industry defines a weathering index as the product of the average annual number of freezing cycles times the average annual winter rainfall.  The weathering index contours are shown below. 

 

Moisture pathways

 

The pores in concrete, during freezing, must be nearly saturated with water (more than 90 percent of saturation) (Bureau of Reclamation 1997).

 

High water-cement ratio

 

A higher than necessary water-cement ratio in the initial concrete placement contributes to freeze-thaw problems in two ways.  First, more water in the mix reduces the strength of the concrete, and so reduces its resistance to the stresses produced by freezing water.  The reduced strength also makes the concrete more susceptible to structural, shrinkage and thermal cracking.   Second, excess water in the concrete mix dries eventually on aging of the concrete and results in voids in the micro-structure of the concrete.  These voids admit water readily, and if the water freezes, damage to the concrete may result. 

 

Lack of entrained air

 

Entrained air is introduced into concrete by means of a chemical admixture that produces small air bubbles in the concrete matrix that provide space for water expansion during freezing.  If the proper air entraining admixture (AEA), at the correct concentration, is properly mixed into high quality concrete, there should be very little damage resulting from cyclic freezing and thawing except in very severe climates.  (Bureau of Reclamation 1996)  The mechanism of entrained air's contribution to resistance to freeze-thaw cycles appear to be to provide a relief pathway for the expansion of the water due to freezing.  The use of AEA's in exterior exposed concrete did not begin until the mid-1940's and was not widespread in the building industry until well into the 1960's. 

 

Poorly consolidated concrete

 

Poorly consolidated concrete produces voids in the concrete that cannot control freeze thaw action as air entraining does, admits moisture into the concrete, and also weakens the concrete. 

 

Results

Freeze-thaw damage may manifest itself as cracking, delamination, spalling, or popouts.  Cracking may develop or become visible as a result of the enlargement of existing hairline cracks by freeze-thaw action.  Delamination refers specifically to the cover over the concrete reinforcement losing connection to the concrete below the reinforcement.  Zones of delamination are identified by sounding with a  hammer or a chain drag.  When concrete spalls, the corners, or the concrete cover over the reinforcement  lose their connection to the main body of the concrete member by the development of widespread internal cracking or delamination.  Popouts of a concrete surface usually have a further underlying cause, such as overworking during finishing, or improper curing procedures. 

Remedies

Freeze-thaw damage to concrete is not generally repairable, except by removal and replacement of the affected part of the material.  Freeze-thaw action can be arrested by denying access to moisture.  In the case of concrete under a roof membrane which has been wetted and frozen, this may be accomplished by replacement of the roofing membrane with a more suitable material.  Often, simple improvements in drainage can direct water away from the affected zones.  The application of sealers to historic exposed concrete is not generally recommended, as it may alter the appearance of the concrete, or it may entrap mositure within the concrete and cause further problems (Coney, undated).  When the source of the moisture has been removed or controlled, repairs to the cracks or spalls may be undertaken by the methods outlined below. 

Investigations of Freeze-Thaw Cracking

Visual inspection can locate areas of damaged concrete and make an initial determination that the freeze-thaw mechanism is the source of the damage by investigation of moisture pathways, and the pattern of cracking   The resistance of concrete to freezing and thawing by extracting a core of the concrete under investigation and subject the specimen to cycles of wetting drying and freezing according to ASTM C666.  This test does not give any absolute measure of the resistance of the concrete, but does give a relative measure for comparison with other cases.  Petrographic analysis by ASTM C856 can also be very useful: the presence or absence of entrained air can be detected by examination of the concrete.  

Further References

Emmons, Peter H. Concrete Repair and Maintenance Illustrated.  R.S. Means Company 1993.  p. 23.

Reinforcement Corrosion

 

Process

 

Corrosion of embedded steel in concrete, including reinforcement, is a complex electrochemical process that can result in very severe damage to a concrete structure.  In order to corrode, the reinforcement must have access to moisture, oxygen, and electrolyte.  Because concrete is a porous material, permeable to air and water, these three elements are nearly always available in concrete.  However, the alkalinity of the environment within a concrete member tends to suppress the corrosion reaction, and other conditions are necessary for the development of damaging corrosion of reinforcement. 

 

A full corrosion cell consists of two components, a cathode, where free electrons combine with oxygen and water to form hydroxide (OH)ˉ ions and an anode, where iron ionizes by the loss of electrons, and combines with the hydroxide ions to form products of corrosion, commonly known as rust.  The electrons migrate from the anode to the cathode through the steel, while the negatively charged hydroxide ions migrate through a medium, which consists of water and dissolved ions, or electrolytes.  So, the reactions in the corrosion process are

 

anode (oxidation) Fe → Fe++ + 2eˉ

XFe(OH)2 + H2O→  iron oxide products of corrosion (rust) 

 

cathode (reduction) O2 + 2H2O + 4eˉ→ 2(OH)ˉ

 

Generally, because of the high pH (low acidity) within the concrete environment, the corrosion reaction is suppressed, and the reinforcement does not corrode.  However, certain conditions can cause the concrete to become active, either by changing the pH of the environment in the concrete, or by changing the environment.  Examples of these conditions are:

 

    Exposure to chloride ions (de-icing salts, chloride used as an accelerator, seawater)

    Carbonation of the concrete

    Exposure to the atmosphere by development of large cracks. 

 

When a corrosion cell develops within the concrete both half-cells (cathode and anode) are within the steel reinforcement.  Two basic types of mechanisms have been recognized

 

    Micro-cell: anodes and cathodes are distributed over the same area of reinforcement--this mechanism usually results from general diffusion of water and electrolytes through the concrete to the level of the reinforcement

 

    Macro-cell: anodes and cathodes are remote from each other within portions of the reinforcement in electrical contact.  This mechanism usually results from penetration of water and electrolytes to the level of the reinforcement  through a crack.  The anode is approximately at the crack location within the reinforcing steel, and the cathode dispersed along the reinforcing bar at some distance from the crack. 

 

The products of corrosion occupy a much greater volume than the steel.  As  a result of continued deposition of  products of corrosion on the surface of the steel, tensile stresses develop in the concrete, which cause splitting cracks along the surface of the concrete along the length of the reinforcement, localized spalling of the concrete cover over the area of corroding reinforcement, or delamination of large areas of the concrete cover. 

 

Contributing Factors

 

Moisture Pathways

 

If the surface of the concrete is subject to long-term wetting, the water will eventually reach the level of the reinforcement, either through diffusion through the porous structure of the concrete, or by traveling along cracks in the concrete. Concrete roof decks, by their nature, are meant to be protected from moisture.  However, the presence of moisture on roofing systems may result from failure of the roofing membrane, poor detailing of drainage facilities, or lack of maintenance of drainage facilities. 

 

High water-cement ratio

 

Concrete placed with a high water-cement ratio, as seen under Freeze-thaw cycles, is more porous due to the presence of excess water in the plastic concrete.  The porosity increases the rte of diffusion of water and electrolytes through the concrete and makes the concrete more susceptible to cracking.

 

Presence of Chloride Ions

 

This is clearly the most important risk factor for bridge decks, which are continually exposed to chloride ions by the use of de-icing salts,  but is less of a factor for roof structures.  However, calcium chloride was frequently used as a set accelerator for concrete placed in the 1940's through 1960's, and chloride ions may be present in the original construction of a thin-shell concrete roof. 

 

Carbonation of Concrete

 

Carbonation refers to the chemical process where free  calcium hydroxide in the porewater of the concrete combines with atmospheric carbon dioxide to form calcium carbonate.  The chemical reaction is

 

Ca(OH)2 + CO2 → CaCO3 + H2O

 

As the calcium hydroxide is alkaline and the calcium carbonate is not, the pH of the environment of the reinforcing may be significantly lowered by this reaction.  This causes 'depassivation' of the reinforcement, and allows the corrosion process to initiate.  This process is more active in concrete which is subject to wetting and drying, and in which pathways for atmospheric carbon dioxide exist through cracks or pores in the concrete. 

 

Low concrete tensile strength

 

Concrete with low tensile strength facilitates corrosion damage in two ways.  First, the concrete develops tension or shrinkage cracks more easily, admitting moisture and oxygen, and in some cases chlorides, to the level of the reinforcement.  Second, the concrete is more susceptible to developing cracks at the point that the reinforcement begins to corrode. 

 

Electrical contact with dissimilar metals

 

Dissimilar metals in contact initiate a flow of electrons that promotes the corrosion of one or the other, by a process known as galvanic corrosion.  The following list of metals commonly used in construction is in order of their reactivity potential, from least reactive to most reactive. 

 

zinc

aluminum

steel

iron

lead

brass

copper

 

When two dissimilar metals are in contact with each other the more active metal (lower on the list) will induce corrosion on the less active.  The products of corrosion are larger in volume than the base metal, so that embedded metals subject to such corrosion may induce cracking and damage in the concrete. 

 

Corrosion of the reinforcing in this column, primarily due to inadequate concrete cover on the reinforcement, has caused spalling of the concrete cover.

 

Remedies

 

The initial remedy for corrosion of concrete reinforcement is always to remove the source of the water entering the concrete.  If the water is penetrating a roofing membrane, the roof must be repaired or replaced.  If the water is entering the concrete because of improper drainage patterns, a more favorable drainage scheme must be implemented.  Usually, though, by the time that corrosion damage is detected, portions of the concrete will also require repair.  Remedies for corrosion-damaged concrete include removal of all delaminated concrete, cleaning of the reinforcement by abrasive blast cleaning, high pressure water, or needle scaling, and use of a concrete patching material.  If the steel has lost a large part of its cross section, it may also need to be replaced to restore the original capacity of the structure.  The reinforcement may be further protected by encapsulation by coating with epoxy. 

 

Cathodic protection is also occasionally used to alter the direction of the corrosion current, by installing a sacrificial anode electrically connected to the reinforcement at a near location. 

 

Investigations of Corrosion Damage

 

Delamination is investigated by sounding the concrete with a hammer or a chain drag and listening for a hollow, or drum-like sound.  Areas of delamination may be marked on a plan, after a systematic program of investigation.  A number of electrochemical methods are also available for investigations of corrosion.  The most commonly used procedure is the measurement of half-cell potentials (ASTM C876).  In this test, the anodic electrical potential of the reinforcement is measured at specific locations throughout the structure, and compared to a reference value.  The concrete is wetted at the location of each measurement, and the open-circuit potential is taken between a reference electrode at the measurement location and an electrode attached directly to the reinforcement mat.  The result can be presented as a contour map of  half-cell potentials.  According to the ASTM standard, using a copper-copper sulfate reference electrode, the following potentials give the following indications of corrosion activity. 

 

-50      to         -200 mV        passive

-200    to         -350 mV        uncertain

-350 mV or less                     corrosion is very likely

 

This site shows the use of a commercial system for taking half-cell potentials.  (Requires Adobe Acrobat Reader)

 

The test gives no indication of the rate of corrosion, because it only measures open-circuit potentials.  However, it has been made easy to perform, and is a commonly used, and commonly available means of obtaining some indication of corrosion activity.  The rate of corrosion can be determined using a procedure known as linear polarization resistance.  In this procedure the resistance of the reinforcement is determined by measuring current at a number of voltages slightly higher and lower than the open circuit half-cell potential.  When the area of reinforcement that has been polarized is known, then the rate of corrosion of the material can be determined.  If this is unknown, determinations of relative corrosion rate in different parts of the structure can be made.  This procedure is incorporated into commercially available devices.   This site shows one such device.

Severe corrosion and loss of concrete cover can threaten the structural safety of a building. 

 

Structural Failure

 

Process

 

Structural failure is a distinct process from structural cracking, described above.  Structural failure in concrete is only rarely a total collapse of a part or all of a structure.  Usually, a structural failure is evident as a large deflection or other excessive displacement, or the development of cracks beyond the limit of tolerability.  The types of conditions that usually produce structural failures of reinforced concrete shell structures are inadequate edge support, or unanticipated loading.   These conditions are more fully described in Module II. 

 

Contributing Factors

 

Foundation Settlement

 

Settlement, especially differential settlement of the foundation produces unanticipated stresses in the superstructure, due to loss of support at the locations of the settlement.  Thin-shell concrete structures in particular, often rely on lateral support to resist the thrusts developed in a barrel or dome.  Small lateral movements of the support can dramatically increase the bending moments present in the shell.  Differential vertical movements of individual supports also produce large bending moments in the part of the shell adjacent to that support. 

 

Poor or uncertain concrete quality

 

Concrete of low compressive strength or poorly consolidated concrete is more subject to creep, or long term compressive deformations.  As a low-rise arch, vault or dome creeps, the structure sags, inducing further compressive stresses, which may lead to a creep instability. 

 

Placement of reinforcement

 

Structural failures may result from misplaced or omitted reinforcement in zones where tension occurs.  This may be a design deficiency, or the result of errors or changes during construction. 

 

Corrosion of reinforcement

 

Large-scale corrosion of the reinforcement removes a significant proportion of the cross sectional area of the concrete through delamination of the concrete cover on the reinforcement, and may also remove a significant portion of the cross sectional area of the steel tension reinforcement. 

 

Remedies

 

The repair of a structural failure requires intervention through a program including temporary support of the structure, usually followed by removal and replacement of the affected portions of the structure.  Some alternative repair methods do not require removal of the existing structure.  These include the provision of external prestressing, or repair with bonded fiber reinforced polymer sheets or plates.  Both of the methods are likely to have a substantial visual impact on the structure, and should be used cautiously on historically significant structures. 

 

Investigations of Structural Failure

 

Investigations of structural failure are completed by a coordinated program of observations of the structure and analysis of the structure.  Field investigations look into the patterns of damage and attempt to infer possible causes, while the structural analysis is used to confirm the likelihood of failure due to the possible causes being investigated.  Tools for the investigation of a failure include consulation of the as-built construction plans, and any of the concrete investigation methods described above.  Concrete strength in situ can be measured using a rebound hammer (ASTM  C805) or a Windsor probe (ASTM  C803).  Concrete strength can also be determined by taking core specimens.  Reinforcement can be roughly located using a rebar locator, or by exploratory drilling. 

The case studies give examples of the investigation of structural failures, particularly the Kresge Arena case study. 

Other Effects

 

There are a number of less common deterioration mechanisms in concrete.  The references have further information on these effects. 

 

Thermal Cracking

 

In most cases, shrinkage movements in concrete are larger than the normal thermal expansion and contraction cycles.   However, under highly restrained conditions, cracking due to the development of thermal stresses may appear.  This is most likely to occur in concrete that has a high solar exposure, due to being south-facing or horizontal, and thus undergoes large daily temperature variations.  If not provided with sufficient expansion joints, the top layer of concrete may delaminate and buckle outwards.  This condition is remedied by saw cutting expansion joints into the concrete at appropriate intervals and general repair of the affected concrete. 

 

Alkali-Aggregate Reactivity

 

Certain types of sulfate-containing aggregates, when wetted, react with the alkaline elements in concrete, causing large volume changes around the aggregate.  This process produces large and widespread tensile stresses in the affected zones of the concrete.   Because in this condition, practically the entire volume of the concrete is affected, it is practically incurable, and usually calls for removal and replacement of the affected concrete. 

 

D-Cracking

 

Whereas most freeze-thaw cracking of normal weight concrete occurs in the cement matrix, when freeze-thaw expansion and damage occurs in porous aggregate, it produces a characteristic pattern of roughly parallel cracks exuding calcite.  These cracks most frequently occur at exposed corners and edges in the concrete.  The main defense against this condition is ensuring that the concrete element is not subjected to periodic wetting.  Otherwise, removal and replacement of the concrete may be warranted. 

 

Efflorescence and cracking pattern characteristic of D-cracking

 

Glossary

 

abrasive blast cleaning

Cleaning a hard-surfaced material using grit carried by compressed air, as in sandblasting.  This procedure is rarely recommended for historic structures

aggregate

Solid filler added to concrete.  This normally consists of coarse aggregate (gravel) and fine aggregate (sand). 

air entraining admixture

a chemical added to plastic concrete to generate small air bubbles in the concrete (air entrainment)

alkalinity

 

Solutions with a higher concentration of hydroxyl ions (OH)- than hydrogen ions (H+) are said to be alkaline.     The alkalinity of a solution is measured by its pH, with a pH value greater than 7 indicating alkalinity.  The pH of porewater in concrete usually ranges around 11 to 13.

 

anode

 

In a battery or electrochemical cell, the anode is positively charged, and electrons migrate from the solution to the anode.  Oxidation takes place at the anode.  (see cathode)

 

bending

 

The primary mode of structural response of a beam (or a plate).  In bending, a beam develops tension on one face and compression on the opposite face.  This is described more fully in Module II

 

carbonation

 

 The combination of free calcium hydroxide (calcite) in concrete with carbon dioxide to form calcium carbonate.   The calcium hydroxide is alkaline, while the calcium carbonate is weakly acidic.  The result of carbonation is reduced pH in the porewater, which may depassivate the reinforcement and promote corrosion

 

The chemical reaction of carbonation is

 

        Ca(OH)2 + CO2 → CaCO3 + H2O

 

cathode

 

In a battery or electrochemical cell, the cathode is positively charged, and electrons migrate from the cathode into the solution.  Reduction takes place at the cathode. (see anode)

 

cathodic protection

 

Concrete reinforcement can be protected by providing an electrical connection to a nearby sacrificial anode, made of a material with a higher electrochemical potential than iron, converting the reinforcing steel into a cathode, and inducing galvanic corrosion in the anode. 

 

consolidation

 

The process of removing air voids from plastic concrete during placement of the concrete.  In modern concrete work, consolidation is accomplished by vibrating, either using an external vibrator, or by vibrating the forms.  In historic concrete, consolidation of the concrete may have been accomplished by rodding or tamping.  Consolidation is further described in Module II. 

Poorly consolidated concrete displays rock pockets or honeycombing, where the cement paste has not fully penetrated the coarse aggregate.  

 

corrosion

 

A general term for the degradation of metals by oxidizing into other compounds.   Corrosion of ferrous metals, such as steel, is particularly damaging to reinforced concrete because the products of corrosion occupy a greater volume than the base metal. 

 

corrosion cell

 

A corrosion cell consists of three components, an anode, a cathode, and an electrolyte.  Electrons migrate from the anode, through the electrolyte, or solution, to the cathode. 

 

creep

 

Long-term deformations of concrete under continuous compressive loading. 

 

curing

 

Concrete, especially concrete slab surfaces, must be cured after placement, by keeping the entire thickness of concrete moist.  This is generally accomplished by placing an impervious membrane on the surface of the concrete, causing retention of the water from the concrete.  Curing is necessary to ensure complete hydration of the cement. 

 

delamination

 

de-bonding of the cover over a layer of reinforcement in a slab due to the effect of corrosion of the reinforcement. 

 

depassivation

 

activation of a corrosion cell in steel reinforcement by lowering of the pH of the porewater of the concrete, usually either through carbonation or intrusion of chloride ions. 

 

differential settlement

 

unequal settlement of different portions of the foundation of a structure.  If the entire structure settles at the same rate, no new stresses are introduced.  Differential settlement of a concrete structure invariably causes structural cracking or structural failure. 

 

diffusion

 

dispersion of a material through a medium, such as dispersion of chloride through the pores in concrete. 

 

electrolyte

 

a solution containing a significant concentration of ions. 

 

encapsulation

 

protection of an object by removing it from contact with the environment    

 

entrained air

 

air intentionally introduced into plastic concrete in the form of small bubbles

 

epoxy injection

 

a crack repair procedure in which an epoxy is injected into the concrete member under pressure, so that it fills the cracks before it hardens. 

 

external prestressing

 

A repair procedure in which precompression is applied to concrete by means of tensioned strands applied to the outside of the structure. 

 

fiber reinforced polymer

 

A composite material consisting of glass or carbon fibers in a matrix.  These materials have recently come into use for concrete reinforcement and repair. 

 

linear polarization resistance

 

A test to determine the approximate rate of corrosion of concrete reinforcement.  The electrical potential of the concrete porewater  is varied from the open-circuit potential of the reinforcement in small increments.  Precise measurements of the change in current for changes in voltage allow the 'polarization resistance' of the corrosion cell to be determined.  From this information, knowing the exposed surface area of the reinforcement, the rate of corrosion can be inferred.  The accuracy of this test suffers from the uncertainty of the resistance of the medium, and the lack of certainty of the area of reinforcement exposed in a test.  Nevertheless, carefully applied, the test can give more complete information about reinforcement corrosion than the customary half-cell potential measurement.  This test is also explained in the text.

 

macro-cell

 

A corrosion cell in reinforcing steel in which the anode and the cathode are physically separated along the length of the reinforcement. 

 

micro-cell

 

A corrosion cell in reinforcing steel in which the anode and the cathode are in the same physical location within the reinforcement. 

 

needle scaling

 

Removal of deteriorated or delaminated concrete by means of a vibrating needle.  This link provides a view of MacDonald's NG10, a compressed air driven needle scaler.

 

 

open-circuit potential

 

The electrical potential of the reinforcement measured without any current flowing.  This measurement is made by the method of half-cell potential measurement, using a copper/copper sulfate reference electrode. 

 

oxidation

 

Loss of electrons at the anode of a corrosion cell.  The loss of electrons from a ferrous metal promotes combining with atmospheric oxygen, and formation of products of corrosion.  (See text)

 

petrographic analysis

 

Microscopic analysis of a thin section specimen according to ASTM C856

 

post tensioning

 

Prestressing applied to concrete after the concrete is in place in the structure.  This is distinguished from pretensioning, which is a manufacturing procedure for new prestressed concrete structures.   See external prestressing

 

prestressing

 

Applying an initial compressive stress to concrete using high-strength steel wires or strands. 

 

reduction

 

Gain of electrons at the cathode of a corrosion cell.  (See text)

 

re-entrant corners

 

Interior corners, such as the corners of a window opening in a concrete wall. 

 

reinforcement

 

Steel rods, bars, or wires added to concrete to improve its resistance to tensile stresses. 

 

shear

 

An internal force acting in a direction perpendicular to a beam or slab

 

spall  

 

Superficial popping out of a relatively small (up to 100 square inches) of concrete.  The depth of a spall is usually to the depth of the reinforcement. 

 


A small spall in a concrete roof slab

 

structural failure

 

A deficiency in load-carrying capacity, or excessive deflections in a structure.  Structural failures can be long-term or sudden. 

 

water/cement ratio

 

The ratio by weight of water to cement in a concrete mix.  Approximately 25% water/cement is required to hydrate the cement.  Additional water results in pores in the concrete, and may result in lower strength and lower durability.