Concrete Behavior

7/30/2019 Concrete Behavior

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Concrete BehaviorTable of ContentsTable of Contents ................................................................................................................ 1

Structural Cracking ............................................................................................................. 2Process ............................................................................................................................ 2

Contributing Factors ....................................................................................................... 2

Results ............................................................................................................................. 3Remedies ......................................................................................................................... 3

Investigations of Structural Cracking ............................................................................. 3

Further References .......................................................................................................... 3Shrinkage Cracking ............................................................................................................. 4

Process ............................................................................................................................ 4


High water-cement ratio ............................................................................................. 4Temperature extremes ................................................................................................. 4

Lack of adequate reinforcement.................................................................................. 4

Lack of adequate curing of concrete at initial placement ........................................... 4Results ............................................................................................................................. 4Remedies ......................................................................................................................... 5

Investigations of Shrinkage Cracking ............................................................................. 5

Further References .......................................................................................................... 5Freeze-Thaw Effects ........................................................................................................... 5

Process ............................................................................................................................ 6


Freeze-thaw cycles ...................................................................................................... 6Moisture pathways ...................................................................................................... 6

High water-cement ratio ............................................................................................. 6

Lack of entrained air ................................................................................................... 6Poorly consolidated concrete ...................................................................................... 6

Results ............................................................................................................................. 6

Remedies ......................................................................................................................... 7

Investigations of Freeze-Thaw Cracking ........................................................................ 7Further References .......................................................................................................... 7

Reinforcement Corrosion .................................................................................................... 7

Process ............................................................................................................................ 7Contributing Factors ....................................................................................................... 8

Moisture Pathways ...................................................................................................... 8

High water-cement ratio ............................................................................................. 8

Presence of Chloride Ions ........................................................................................... 8Low concrete tensile strength ..................................................................................... 8

Electrical contact with dissimilar metals .................................................................... 8

Remedies ......................................................................................................................... 9Investigations of Corrosion Damage .............................................................................. 9

Structural Failure .............................................................................................................. 10

Process .......................................................................................................................... 10Contributing Factors ..................................................................................................... 10
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Foundation Settlement .............................................................................................. 10

Poor or uncertain concrete quality ............................................................................ 10

Placement of reinforcement ...................................................................................... 11Corrosion of reinforcement ....................................................................................... 11

Remedies ....................................................................................................................... 11

Investigations of Structural Failure ............................................................................... 11Other Effects ..................................................................................................................... 11Thermal Cracking ......................................................................................................... 11

Alkali-Aggregate Reactivity ......................................................................................... 11

D-Cracking .................................................................................................................... 11Glossary ............................................................................................................................ 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 dependson 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 thereinforcement, 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 fullytransferred to the reinforcement until the concrete has cracked. Before cracking, the concrete iscarrying 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 tobendingtend 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.Shearcracks 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.
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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 stressmagnitudes.

Poorly consolidated concrete: Poorconsolidation 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 thiskind 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 thereinforcement, and may hasten the

corrosionof the reinforcement (although this is the topic of a vigorous debate in the engineering
http://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#consolidationhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#consolidationhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#reinforcementhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#reinforcementhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#reinforcementhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#corrosionhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#corrosionhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#corrosionhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#reinforcementhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#consolidation


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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 becomesunsightly, or admits too much moisture to the interior of the structure, some sort of repair may bewarranted. If the structure is deficient, external reinforcement, or external post tensioning may be

undertaken. If the problem is superficial, the cracks may be repaired byepoxy injection.

Investigations of Structural CrackingStructural 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 morecomplex 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 bespecifically 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 theMiami Marine Stadiumcase study. Investigations ofstructural cracking, and in some cases, incipientstructural failure are described in the Kresge

Arena case study.

Further References
http://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#epoxy%20injectionhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#epoxy%20injectionhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#epoxy%20injectionhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#petrographic%20analysishttp://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/PAGES/C856.htm?L+mystore+hmpq5371http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/PAGES/C856.htm?L+mystore+hmpq5371http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/PAGES/C856.htm?L+mystore+hmpq5371http://c/DATA/PAPERS/NCPTT/module%20III/miami_marine.ppshttp://c/DATA/PAPERS/NCPTT/module%20III/miami_marine.ppshttp://c/DATA/PAPERS/NCPTT/module%20III/miami_marine.ppshttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#struct.%20failurehttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#struct.%20failurehttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#struct.%20failurehttp://c/DATA/PAPERS/NCPTT/module%20III/miami_marine.ppshttp://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/PAGES/C856.htm?L+mystore+hmpq5371http://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#petrographic%20analysishttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#epoxy%20injection


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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 forordinary 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 thedevelopment 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, awater/cement ratioof about .25 ischemically 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 ofthe concrete.

Temperature extremes

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

Lack of adequate reinforcement

Reinforcementof the concrete cannot prevent shrinkage cracking, but can control the severity

and extent of the development of cracks.

Lack of adequatecuringof 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 tensilestrength and is much more susceptible to the development of cracks.

Results

The result of shrinkage cracking, like structural cracking in properly designed concrete is simplya 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
http://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#water/cement%20ratiohttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#water/cement%20ratiohttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#water/cement%20ratiohttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#reinforcementhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#reinforcementhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#curinghttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#curinghttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#curinghttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#curinghttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#reinforcementhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#water/cement%20ratio


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reinforcement, and may hasten thecorrosionof 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 ordamaged areas.

Severe shrinkage cracking of a concrete patch: the patching concrete mix was probably

overwateredand/or insufficiently cured.

Remedies

Usually, this is a condition that does not require remedies. Where the cracking becomesunsightly, 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 beundertaken. If the problem is superficial, the cracks may be repaired byepoxy injection.

Investigations of Shrinkage CrackingInvestigations 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 shrinkagecracking. Investigations should also be undertaken to discern shrinkage cracking from structural

cracking.

Further References
http://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#corrosionhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#corrosionhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#corrosionhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#epoxy%20injectionhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#epoxy%20injectionhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#epoxy%20injectionhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#epoxy%20injectionhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#corrosion


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Freeze-Thaw Effects


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Process

Concrete is a porous material and will absorb water, either into pores, which always exist withinthe 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 theconcrete, they may cause a number of effects: spalling of the concrete, development of further

cracks, 'popouts' of the surface of the concrete

Contributing FactorsFreeze-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 withthe 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 waterfreezes, damage to the concrete may result.

Lack of entrained air

Entrained airis 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 theproperair entrainingadmixture (AEA), at the correct concentration, is properly mixed into high
http://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#entrained%20airhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#entrained%20airhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#air%20entraining%20admixhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#air%20entraining%20admixhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#air%20entraining%20admixhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#air%20entraining%20admixhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#entrained%20air


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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 theexpansion 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 concretePoorly 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 ofdelamination

are identified by sounding with a hammer or a chain drag. When concrete spalls, the corners, orthe 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 aconcrete surface usually have a further underlying cause, such as overworking during finishing,or impropercuringprocedures.

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 tomoisture. 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. Theapplication 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 toASTM 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 analysisbyASTM C856can also be very useful: the presence or absence of

entrained aircan be detected by examination of the concrete.
http://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#delaminationhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#delaminationhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#delaminationhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#curinghttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#curinghttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#curinghttp://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/PAGES/C666.htm?L+mystore+hmpq5371http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/PAGES/C666.htm?L+mystore+hmpq5371http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/PAGES/C666.htm?L+mystore+hmpq5371http://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#petrographic%20analysishttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#petrographic%20analysishttp://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/PAGES/C856.htm?L+mystore+hmpq5371http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/PAGES/C856.htm?L+mystore+hmpq5371http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/PAGES/C856.htm?L+mystore+hmpq5371http://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#entrained%20airhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#entrained%20airhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#entrained%20airhttp://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/PAGES/C856.htm?L+mystore+hmpq5371http://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#petrographic%20analysishttp://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/PAGES/C666.htm?L+mystore+hmpq5371http://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#curinghttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#delamination


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Further References

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

Reinforcement CorrosionProcess

Corrosionof embedded steel in concrete, includingreinforcement, is a complex electrochemicalprocess that can result in very severe damage to a concrete structure. In order to corrode, the

reinforcement must have access to moisture, oxygen, andelectrolyte. Because concrete is a

porous material, permeable to air and water, these three elements are nearly always available inconcrete. However, thealkalinityof 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 fullcorrosion cellconsists of two components, a cathode, where free electrons combine with

oxygen and water to form hydroxide (OH) ions and an anode, where iron ionizes by the loss of

electrons, and combines with the hydroxide ions to form products of corrosion, commonlyknown 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 anddissolved ions, orelectrolytes. So, the reactions in thecorrosionprocess are

anode (oxidation) Fe Fe++

+ 2e

XFe(OH)2 + H2O iron oxide products of corrosion (rust)cathode (reduction) O2 + 2H2O + 4e 2(OH)

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 theconcrete, or by changing the environment. Examples of these conditions are:

Exposure to chloride ions (de-icing salts, chloride used as an accelerator, seawater)Carbonationof the concreteExposure to the atmosphere by development of large cracks.

When acorrosion celldevelops 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 generaldiffusionof water andelectrolytes through the concrete

to the level of the reinforcement

Macro-cell: anodes and cathodes are remote from each other within portions of thereinforcement 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 atsome 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 thereinforcement, localized spalling of the concrete cover over the area of corroding reinforcement,

ordelamination of large areas of the concrete cover.
http://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#corrosionhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#corrosionhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#reinforcementhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#reinforcementhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#reinforcementhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#electrolytehttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#electrolytehttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#electrolytehttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#alkalinityhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#alkalinityhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#alkalinityhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#corrosion%20cellhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#corrosion%20cellhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#corrosion%20cellhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#corrosion%20cellhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#electrolytehttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#electrolytehttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#electrolytehttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#corrosionhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#corrosionhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#corrosionhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#oxidationhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#oxidationhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#oxidationhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#reductionhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#reductionhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#reductionhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#carbonationhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#carbonationhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#corrosion%20cellhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#corrosion%20cellhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#corrosion%20cellhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#micro-cellhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#micro-cellhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#diffusionhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#diffusionhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#diffusionhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#electrolytehttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#electrolytehttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#electrolytehttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#macro-cellhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#macro-cellhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#delaminationhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#delaminationhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#delaminationhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#macro-cellhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#electrolytehttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#diffusionhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#micro-cellhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#corrosion%20cellhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#carbonationhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#reductionhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#oxidationhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#corrosionhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#electrolytehttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#corrosion%20cellhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#alkalinityhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#electrolytehttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#reinforcementhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#corrosion


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Contributing FactorsMoisture Pathways

If the surface of the concrete is subject to long-term wetting, the water will eventually reach thelevel of the reinforcement, either throughdiffusionthrough 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 fromfailure of the roofing membrane, poor detailing of drainage facilities, or lack of maintenance ofdrainage facilities.

High water-cement ratio

Concrete placed with a high water-cement ratio, as seen underFreeze-thaw cycles, is moreporous due to the presence of excess water in the plastic concrete. The porosity increases the rte

ofdiffusionof water and electrolytes through the concrete and makes the concrete more

susceptible to cracking.

Presence of Chloride IonsThis 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'sthrough 1960's, and chloride ions may be present in the original construction of a thin-shell

concrete roof.

Carbonation of Concrete

Carbonationrefers to the chemical process where free calcium hydroxide in the porewater of theconcrete combines with atmospheric carbon dioxide to form calcium carbonate. The chemical

reaction is

Ca(OH)2 + CO2 CaCO3 + H2OAs 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 carbondioxide 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 concretedevelops 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

ironlead

brass

copper
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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 anddamage 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.

RemediesThe 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 berepaired 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 thereinforcement byabrasive blast cleaning, high pressure water, orneedle 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 furtherprotected byencapsulationby 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 DamageDelamination 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 theopen-circuit potentialis takenbetween 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, thefollowing potentials give the following indications of corrosion activity.
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-50 to -200 mV passive

-200 to -350 mV uncertain

-350 mV or less corrosion is very likelyThis siteshows the use of a commercial system for taking half-cell potentials. (Requires AdobeAcrobat Reader)

The test gives no indication of the rate of corrosion, because it only measures open-circuitpotentials. However, it has been made easy to perform, and is a commonly used, and commonlyavailable means of obtaining some indication of corrosion activity. The rate of corrosion can be

determined using a procedure known aslinear polarization resistance. In this procedure the

resistance of the reinforcement is determined by measuring current at a number of voltagesslightly 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.Thissite shows one such device.

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

Structural FailureProcessStructural 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 thedevelopment 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 FactorsFoundation Settlement

Settlement, especiallydifferential settlementof the foundation produces unanticipated stresses in

the superstructure, due to loss of support at the locations of the settlement. Thin-shell concrete
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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 thebendingmoments

present in the shell. Differential vertical movements of individual supports also produce largebending 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 tocreep,or long term compressive deformations. As a low-rise arch, vault or dome creeps, the structuresags, 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 tensionoccurs. 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 throughdelamination of the concrete cover on the reinforcement,and may also remove a significant portion of the cross sectional area of the steel tension

reinforcement.

RemediesThe 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 ofexternal prestressing, or repair with bondedfiber reinforcedpolymersheets 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 FailureInvestigations 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 failureinclude 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 bytaking 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 KresgeArena case study.

Other Effects

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

Thermal Cracking

In most cases, shrinkage movements in concrete are larger than the normal thermal expansionand 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
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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 ReactivityCertain types of sulfate-containingaggregates, when wetted, react with the alkaline elements in

concrete, causing large volume changes around the aggregate. This process produces large andwidespread 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-CrackingWhereas most freeze-thaw cracking of normal weight concrete occurs in the cement matrix,

when freeze-thaw expansion and damage occurs in porousaggregate, 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 theconcrete 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

Glossaryabrasive blast cleaning

Cleaning a hard-surfaced material using grit carried by compressed air, as insandblasting. 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
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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. (seecathode)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 morefully in Module II

carbonation

The combination of free calcium hydroxide (calcite) in concrete with carbon dioxide toform 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

depassivatethe reinforcement and promotecorrosion.

The chemical reaction of carbonation is

Ca(OH)2 + CO2 CaCO3 + H2Ocathode

In a battery or electrochemical cell, the cathode is positively charged, and electronsmigrate 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 inducinggalvanic corrosionin theanode.

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 furtherdescribed in Module II.
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Poorly consolidated concrete displays rock pockets or honeycombing, where the cementpaste 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, ananode, acathode, and anelectrolyte.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 theentire thickness of concrete moist. This is generally accomplished by placing an

impervious membrane on the surface of the concrete, causing retention of the water fromthe 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
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activation of a corrosion cell in steel reinforcement by lowering of the pH of the porewater

of the concrete, usually either throughcarbonationor 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 ofa concrete structure invariably causes structural cracking or structural failure.diffusion

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

electrolyte

a solution containing a significant concentration of ions.encapsulation

protection of an object by removing it from contact with the environmententrained 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 haverecently 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 theopen-circuit potentialof

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 aboutreinforcement corrosion than the customaryhalf-cell potentialmeasurement. This test is

also explained in thetext.

macro-cell
http://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#carbonationhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#carbonationhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#carbonationhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#open-circuit%20potentialhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#open-circuit%20potentialhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#open-circuit%20potentialhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#half-cellhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#half-cellhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#half-cellhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#lprhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#lprhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#lprhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#lprhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#half-cellhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#open-circuit%20potentialhttp://www.engr.psu.edu/ae/ThinShells/module%20III/concrete_behavior_text.htm#carbonation


19/20

A corrosion cell in reinforcing steel in which theanodeand thecathodeare physically

separated along the length of the reinforcement.

micro-cell

A corrosion cell in reinforcing steel in which theanodeand thecathodeare 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 drivenneedle scaler.open-circuit potential

The electrical potential of the reinforcement measured without any current flowing. This

measurement is made by the method ofhalf-cell potentialmeasurement, using acopper/copper sulfate reference electrode.

oxidation

Loss of electrons at theanodeof acorrosion 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 toASTM C856.

post tensioning

Prestressingapplied 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. Seeexternal prestressingprestressing

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

reduction

Gain of electrons at thecathodeof acorrosion 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
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Concrete Behavior