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Causes of Distress and Deterioration of Concrete

Introduction

a.  General: Once the evaluation phase has been completed for a structure, the next step is to establish the cause  or  causes  for  the  damage  that  has  been  detected. Since many of the symptoms may be caused by more than one mechanism acting upon the concrete, it is necessary to have an understanding of the basic underlying causes of damage and deterioration.   This chapter presents infor- mation on the common causes of problems in concrete. These causes are shown in Table 3-1.  Items shown in the table  are  discussed  in  the  subsequent  sections  of  this chapter with the following given for each:   (1) brief dis- cussion  of  the  basic  mechanism;  (2) description  of  the most typical symptoms, both those that would be observed during a visual examination and those that would be seen during a laboratory evaluation; and (3) recommendations for preventing further damage to new or replacement concrete.   The last section of the chapter presents a logi- cal method for relating the symptoms or observations to the various causes.

b.  Approach to evaluation:  Deterioration of concrete is an extremely complex subject.   It would be simplistic to suggest that it will be possible to identify a specific, single cause of deterioration for every symptom detected during  an  evaluation  of  a  structure.    In  most  cases,  thedamage   detected   will   be   the   result   of   more   than   one mechanism.   For example, corrosion of reinforcing steel may open cracks that allow moisture greater access to the interior of the concrete.   This moisture could lead to additional damage by freezing and thawing.   In spite of the complexity of several causes working simultaneously, given   a   basic   understanding   of   the   various   damage- causing mechanisms, it should be possible, in most cases, to determine the primary cause or causes of the damage seen  on  a  particular  structure  and  to  make  intelligent choices concerning selection of repair materials and methods.

Causes of Distress and Deterioration

Table 3-1 Causes of Distress and Deterioration of Concrete

Accidental Loadings
Chemical Reactions
Acid attack
Aggressive-water attack
Alkali-carbonate rock reaction
Alkali-silica reaction
Miscellaneous chemical attack
Sulfate attack
Construction Errors
Corrosion of Embedded Metals
Design Errors
Inadequate structural design
Poor design details
Erosion
Abrasion
Cavitation
Freezing and Thawing
Settlement and Movement
Shrinkage
Plastic
Drying
Temperature Changes
Internally generated
Externally generated
Fire
Weathering

a.  Accidental loadings.

  1. Mechanism: Accidental loadings may be charac- terized  as  short-duration,  one-time  events  such  as  the impact of a barge against a lock wall or an earthquake.These  loadings  can  generate  stresses  higher  than  thestrength of the concrete, resulting in localized or general failure. Determination  of  whether  accidental  loading caused damage to the concrete will require knowledge of the events preceding discovery of the damage. Usually, damage caused by accidental loading will be easy to diagnose.
  2. Symptoms: Visual   examination   will   usually show spalling or cracking of concrete which has been subjected to accidental loadings.Laboratory analysis is generally not necessary.
  3. Prevention: Accidental  loadings  by  their  very nature  cannot  be  prevented.    Minimizing  the  effects  of some occurrences by following proper design procedures (an example is the design for earthquakes) or by proper attention  to  detailing  (wall  armor  in   areas  of  likely impact) will reduce the impacts of accidental loadings.

b.  Chemical reactions.

This category includes sev- eral specific causes of deterioration that exhibit a wide variety of symptoms.   In general, deleterious chemical reactions  may  be  classified  as  those  that  occur  as  the result of external chemicals attacking the concrete (acid attack, aggressive water attack, miscellaneous chemical attack, and sulfate attack) or those that occur as a result of internal chemical reactions between the constituents of the concrete (alkali-silica and alkali-carbonate rock reac- tions). Each  of  these  chemical  reactions  is  described below.

(1)  Acid attack.

  1. Mechanism: Portland-cement concrete is a highly alkaline  material  and  is  not  very  resistant  to  attack  by acids.   The deterioration of concrete by acids is primarily the result of a reaction between the acid and the products of the hydration of cement.  Calcium silicate hydrate may be attacked if highly concentrated acid exists in the envir- onment of the concrete structures.   In most cases, the chemical reaction results in the formation of water-soluble calcium compounds that are  then leached away.  In the case of sulfuric acid attack, additional or accelerated deterioration  results  because  the  calcium  sulfate  formed may affect the concrete by the sulfate attack mechanism (Section 3-2b(6)).   If the  acid is able to  reach the rein- forcing steel through cracks or pores in the concrete, corrosion of the reinforcing steel will result and will cause further deterioration of the concrete (ACI 201.2R). 
  2. Symptoms: Visual examination will show disin- tegration  of  the  concrete  evidenced  by  loss  of  cement paste and aggregate from the matrix (Figure 2-13).   If reinforcing  steel  has  been  reached  by  the  acid,  rust staining,  cracking,  and  spalling  may  be  present.If  the nature of the solution in which the deteriorating concrete is located is unknown, laboratory analysis can be used to identify the specific acid involved.
  3. Prevention: A   dense   concrete   with   a   low water-cement   ratio   (w/c)   may   provide   an   acceptable degree of protection against a mild acid attack.   Portland- cement concrete, because of its composition, is unable to withstand   attack   by   highly   acidic   solutions   for   long periods of time.   Under such conditions, an appropriate surface coating or treatment may be necessary.   ACI Committee 515  has  extensive  recommendations  for  such coatings (ACI 515.1R).

A dense concrete with a low water-cement ratio (w/c) may provide an acceptable degree of protection against a mild acid attack. Portlandcement concrete, because of its composition, is unable to withstand attack by  highly acidic solutions for long periods of time.

(2)  Aggressive-water attack.

  1. Mechanism: Some waters have been reported to have extremely low concentrations of dissolved minerals. These soft or aggressive waters will leach calcium from cement paste or aggregates.   This phenomenon has been infrequently reported in the United States.   From the few cases  that  have been  reported,  there  are  indications  that this  attack  takes  place  very  slowly.    For  an  aggressive- water attack to have a serious effect on hydraulic struc- tures, the attack must occur in flowing water.   This keeps a constant supply of aggressive water in contact with the concrete and washes away aggregate particles that become loosened as a result of leaching of the paste (Holland, Husbands, Buck, and Wong 1980).
  2. Symptoms: Visual examination will show con- crete surfaces that are very rough in areas where the paste has  been  leached  (Figure 2-12).     Sand grains  may  be present on the surface of the concrete, making it resemble a coarse sandpaper.   If the aggregate is susceptible to leaching, holes where the coarse aggregate has been dis- solved  will  be  evident.    Water  samples  from  structures where aggressive-water attack is suspected may be ana- lyzed to calculate the Langlier Index, which is a measure of the aggressiveness of the water (Langlier 1936).
  3. Prevention: The  aggressive  nature  of  water  at the  site  of  a  structure  can  be  determined  before  con- struction  or  during  a  major rehabilitation.    Additionally, the water-quality evaluation at many structures can be expanded to monitor the aggressiveness of water at the structure. If there are indications that the water is aggres- sive or is becoming aggressive, areas susceptible to high flows may be coated with a nonportland-cement-based coating.

(3)  Alkali-carbonate rock reaction.

  1. Mechanism: Certain  carbonate  rock  aggregates have been reactive in concrete.  The results of these reac- tions have been characterized as ranging from beneficial to  destructive.     The  destructive  category  is  apparently limited to reactions with impure dolomitic aggregates and are a result of either dedolomitization or rim-silicification reactions.   The mechanism of alkali-carbonate rock reac- tion is covered in detail in EM 1110-2-2000.
  2. Symptoms: Visual  examinationof  those  reac- tions that are serious enough to disrupt the concrete in a structure will generally show map or pattern cracking and a general appearance which indicates that the concrete is swelling (Figure 2-7).   A distinguishing feature which differentiates     alkali-carbonate     rock     reaction     from alkali-silica reaction is the lack of silica gel exudations at cracks (ACI 201.2R).   Petrographic examination in accor- dance  with  ASTM C 295  (CRD-C 127)  may  be  used  to confirm the presence of alkali-carbonate rock reaction.
  3. Prevention: In general, the best prevention is to avoid using aggregates that are or suspected of being reactive.      Appendix E   of   EM 1110-2-2000  prescribes procedures for testing rocks for reactivity and for mini- mizing effects when reactive aggregates must be used.

(4)  Alkali-silica reaction.

  1. Mechanism: Some  aggregates  containing  silica that  is  soluble  in  highly  alkaline  solutions  may  react  to form  a  solid  nonexpansive  calciumalkali-silica  complex or an alkali-silica complex which can imbibe considerable amounts   of   water   and   then   expand,   disrupting   the concrete Additional    details    may    be    found    in EM 1110-2-2000.
  2. Symptoms: Visual examination of those concrete structures that are affected will generally show map or pattern  cracking  and  a  general  appearance  that  indicates that the concrete is swelling (Figure 2-6).  Petrographic examination  may  be  used  to  confirm  the  presence  of alkali-silica reaction.
  3. Prevention: In general, the best prevention is to avoid using aggregates that are known or suspected to be reactive   or   to   use   a   cement  containing   less   than 0.60 percent   alkalies   (percent   Na20 +   (0.658)   percent K20).   Appendix D of EM 1110-2-2000 prescribes proce- dures for testing aggregates for reactivity and for mini- mizing the effects when reactive aggregates must be used.

(5)  Miscellaneous chemical attack.

  1. Mechanism: Concrete will resist chemical attack to  varying  degrees,  depending  upon  the  exact  nature  of the chemical.  ACI 515.1R includes an extensive listing of the resistance of concrete to various chemicals.   To pro- duce significant attack on concrete, most chemicals must be in solution that is above some minimum concentration. Concrete  is  seldom  attacked  by  solid  dry  chemicals. Also, for maximum effect, the chemical solution needs to be circulated in contact with the concrete.   Concrete sub- jected to aggressive solutions under positive differential pressure is particularly vulnerable.  The pressure gradients tend to force the aggressive solutions into the matrix.   If the low-pressure face of the concrete is exposed to evapo- ration, a concentration of salts tends to accumulate at that face,  resulting  in  increased  attack.    In  addition  to  the specific  nature  of  the  chemical  involved,  the  degree  to which concrete resists attack depends upon the tempera- ture  of the  aggressive solution,  the w/c  of the  concrete, the  type  of  cement  used  (in  some  circumstances),  the degree of consolidation of the concrete, the permeability of the concrete, the degree of wetting and drying of the chemical on the concrete, and the extent of chemically induced corrosion of the reinforcing steel (ACI 201.1R).
  2. Symptoms: Visual   examination   of   concrete which has been subjected to chemical attack will usually show surface disintegration and spalling and the opening of  joints  and  cracks.    There  may  also  be  swelling  and general disruption of the concrete mass.  Coarse aggregate particles are generally more inert than the cement paste matrix; therefore, aggregate particles may be seen as protruding from the matrix.   Laboratory analysis may be required to identify the unknown chemicals which are causing the damage.
  3. Prevention: Typically, dense concretes with low w/c  (maximum  w/c =  0.40)  provide  the  greatest  resis- tance.   The best known method of providing long-term resistance is to provide a suitable coating as outlined in ACI 515.1R.

(6)  Sulfate attack.

  1. Mechanism: Naturally   occurring   sulfates   of sodium, potassium, calcium, or magnesium are sometimes found in soil or in solution in ground water adjacent to concrete  structures.     The  sulfate  ions  in  solution  will attack the concrete.  There are apparently two chemical reactions involved in sulfate attack on concrete.  First, the sulfate reacts with free calcium hydroxide which is libera- ted during the hydration of the cement to form calcium sulfate (gypsum).   Next, the gypsum combines with hydrated calcium aluminate to form calcium sulfoaluminate (ettringite).   Both of these reactions result in an increase in volume.   The second reaction is mainly responsible for most of the disruption caused by volume increase of the concrete (ACI 201.2R).   In addition to the two chemical reactions, there may also be a purely physi- cal phenomenon in which the growth of crystals of sulfate salts disrupts the concrete.
  2. Symptoms: Visual examination will show map and pattern cracking as well as a general disintegration of the concrete (Figure 2-14).  Laboratory analysis can verify the occurrence of the reactions described.
  3. Prevention: Protection against sulfate attack can generally be obtained by the following: Use of a dense, high-quality concrete with a low water-cement ratio; Use of either a Type V or a Type II cement, depending upon the  anticipated  severity  of  the  exposure  (EM 1110-2-2000); Use of a suitable pozzolan (some pozzolans, added as part of a blended cement or separately, have improved resistance,  while  others  have  hastened  deterioration) If use  of  a  pozzolan  is  anticipated,  laboratory  testing  to verify the degree of improvement to be expected is recommended.


c. Construction errors.

Failure to follow specified procedures and good practice or outright carelessness may lead  to  a  number  of  conditions  that  may  be  grouped together as construction errors. Typically, most of these errors do not lead directly to failure or deterioration of concrete.    Instead,  they  enhance  the  adverse  impacts  of other mechanisms identified in this chapter.   Each error will be briefly described below along with preventative methods.   In general, the best preventive measure is a thorough knowledge of what these construction errors are plus an aggressive inspection program.  It should be noted that errors of the type described in this section are equally as likely to occur during repair or rehabilitation projects as they are likely to occur during new construction.

(1) Adding water to concrete: Water is usually added to concrete in one or both of the following circumstances: First, water is added to the concrete in a delivery truck to increase slump and decrease emplacement effort.   This practice  will  generally  lead  to  concrete  with  lowered strength and reduced durability.   As the w/c of the con- crete increases, the strength and durability will decrease. In the second case, water is commonly added during finishing of flatwork.  This practice leads to scaling, craz- ing, and dusting of the concrete in service.

(2) Improper alignment of formwork: Improper alignment of the formwork will lead to discontinuities on the  surface  of  the  concrete.   While  these  discontinuities are  unsightly  in  all  circumstances,  their  occurrence  may be  more  critical  in  areas  that  are  subjected  to  high- velocity flow of water, where cavitation-erosion may be induced,  or  in  lock  chambers  where  the  "rubbing"  sur- faces must be straight.

(3)  Improper  consolidation: Improper  consolidation of concrete may result in a variety of defects, the most common being bugholes, honeycombing, and cold joints."Bugholes" are formed when small pockets of air or water are trapped against the forms.  A change in the mixture to make it less "sticky" or the use of small vibrators worked near the form has been used to help eliminate bugholes. Honeycombing can be reduced by inserting the vibrator more   frequently, inserting the vibrator as close as pos- sible  to  the  form  face  without  touching  the  form,  and slower   withdrawal of the vibrator.   Obviously, any or all of    these    defects    make    it    much    easier    for    any damage-causing mechanism to initiate deterioration of the concrete.    Frequently,  a  fear  of  "overconsolidation"  is used to justify a lack of effort in consolidating concrete. Overconsolidation  is  usually  defined  as  a  situation  in which the consolidation effort causes all of the coarse aggregate to settle to the bottom while the paste rises to the surface.   If this situation occurs, it is reasonable to conclude that there is a problem of a poorly proportioned concrete rather than too much consolidation.

(4) Improper curing: Curing is probably the most abused   aspect   of   the   concrete   construction   process. Unless concrete is given adequate time to cure at a proper humidity and temperature, it will not develop the charac- teristics that are expected and that are necessary to pro- vide durability.   Symptoms of improperly cured concrete can include various types of cracking and surface disinte- gration.    In  extreme  cases  where  poor  curing  leads  to failure to achieve anticipated concrete strengths, structural cracking may occur.

(5)  Improper  location  of  reinforcing  steel:  This section   refers   to   reinforcing   steel   that   is   improperly located or is not adequately secured in the proper location. Either of these faults may lead to two general types of problems.  First, the steel may not function structurally as intended, resulting in structural cracking or failure.   A particularly prevalent example is the placement of welded wire mesh in floor slabs.   In many cases, the mesh ends up  on  the  bottom  of  the  slab  which  will  subsequently crack because the steel is not in the proper location.   The second   type   of   problem   stemming   from   improperly located or tied reinforcing steel is one of durability.   The tendency  seems  to  be  for  the  steel  to  end  up  near  the surface of the concrete.   As the concrete cover over the steel is reduced, it is much easier for corrosion to begin.

(6)  Movement  of  formwork:  Movement  of  form- work during the period while the concrete is going from a fluid to a rigid material may induce cracking and separa- tion within the concrete.  A crack open to the surface will allow access of water to the interior of the concrete.   An internal void may give rise to freezing or corrosion prob- lems if the void becomes saturated.

(7)  Premature  removal  of  shores  or  reshores:  If shores or reshores are removed too soon, the concrete affected   may   become   overstressed   and   cracked.      In extreme cases there may be major failures.

(8)  Settling   of   the   concrete:  During   the   period between placing and initial setting of the concrete, the heavier components of the concrete will settle under the influence of gravity.  This situation may be aggravated by the use of highly fluid concretes.  If any restraint tends to prevent this settling, cracking or separations may result. These cracks or separations may also develop problems of corrosion or freezing if saturated.

(9)  Settling of the subgrade:  If there is any settling of the subgrade during the period after the concrete begins to become rigid but before it gains enough strength to support its own weight, cracking may also occur.

(10)  Vibration of freshly placed concrete:  Most con- struction sites are subjected to vibration from various sources, such as blasting, pile driving, and from the oper- ation of construction equipment.   Freshly placed concrete is vulnerable to weakening of its properties if subjected to forces  which  disrupt  the  concrete  matrix  during  setting. The  vibration  limits  for  concrete,  expressed  in  terms  of peak particle velocity and given in Table 3-2, were estab- lished as a result of laboratory and field test programs.

(11)  Improper finishing of flat work: The most com- mon improper finishing procedures which are detrimental to the durability of flat work are discussed below.

(a)  Adding water to the surface: This procedure was discussed  in  paragraph 3-2c(1)  above. Evidence  that water is being added to the surface is the presence of a large paint brush, along with other finishing tools. The brush is dipped in water and water is "slung" onto the surface being finished.

 Table 3-2 Vibration Limits for Freshly Placed Concrete (Hulshizer and Desci 1984) 
Age of Concrete at Time of Vibration (hr) Peak Particle Velocity of Ground Vibrations
Up to 3 102 mm/sec (4.0 in./sec)
3 to 11 38 mm/sec (1.5 in./sec)
11 to 24 51 mm/sec (2.0 in./sec)
24 to 48 102 mm/sec (4.0 in./sec)
over 48 178 mm/sec (7.0 in./sec)

(b)  Timing  of  finishing: Final  finishing  operations must  be done  after the  concrete  has taken  its  initial set and bleeding has stopped.  The waiting period depends on the  amounts  of  water,  cement,  and  admixtures  in  the mixture but primarily on the temperature of the concrete surface.   On a partially shaded slab, the part in the sun will  usually  be  ready  to  finish  before  the  part  in  the shade.

(c)  Adding cement to the surface: This practice is often done to dry up bleed water to allow finishing to proceed  and  will  result  in  a  thin  cement-rich  coating which will craze or flake off easily.

(d) Use of tamper: A tamper or "jitterbug" is unnecessarily  used  on  many  jobs.    This  tool  forces  the coarse aggregate away from the surface and can make finishing   easier.      This   practice,   however,   creates   a cement-rich mortar surface layer which can scale or craze. A jitterbug should  not be  allowed with  a well  designed mixture.   If a harsh mixture must be finished, the judi- cious use of a jitterbug could be useful.

(e)  Jointing: The most frequent cause of cracking in flatwork  is  the  incorrect  spacing  and  location  of  joints. Joint spacing is discussed in ACI 330R.

d. Corrosion of embedded metals.

(1)  Mechanisms: Steel reinforcement is deliberately and almost invariably placed within a few inches of a concrete surface.   Under most circumstances, portland- cement concrete provides good protection to the embed- ded   reinforcing   steel.      This   protection   is   generally attributed to the high alkalinity of the concrete adjacent to the steel and to the relatively high electrical resistance of the concrete.   Still, corrosion of the reinforcing steel is among the most frequent causes of damage to concrete.

(a) High alkalinity and electrical resistivity of the concrete: The high alkalinity of the concrete pore solu- tion can be reduced over a long period of time by car- bonation.   The electrical resistivity can be decreased by the presence of chemicals in the concrete.   The chemical most commonly applied to concrete is chloride salts in the form of deicers.   As the chloride ions penetrate the con- crete, the capability of the concrete to carry an electrical current is increased significantly.   If there are differences within the concrete such as moisture content, chloride content, oxygen content, or if dissimilar metals are in contact, electrical potential differences will occur and a corrosion cell may be established.  The anodes will exper- ience  corrosion  while  the  cathodes  will  be  undamaged. On  an  individual  reinforcing  bar  there  may  be  many anodes and cathodes, some adjacent, and some widely spaced.

(b) Corrosion-enhanced  reduction  in  load-carrying capacity of concrete.   As the corrosion progresses, two things occur:   First, the cross-sectional area of the rein- forcement is reduced, which in turn reduces the load- carrying capacity of the steel.  Second, the products of the corrosion,  iron  oxide  (rust),  expand  since  they  occupy about  eight  times  the  volume  of  the  original  material. This increase in volume leads to cracking and ultimately spalling of the concrete.   For mild steel reinforcing, the damage to the concrete will become evident long before the capacity of the steel is reduced enough to affect its load-carrying capacity.   However, for prestressing steel, slight   reductions   in   section   can   lead   to   catastrophic failure.

(c)  Other   mechanisms   for   corrosion   of   embedded metals.   In addition to the development of an electrolytic cell, corrosion may be developed under several other situations.  The first of these is corrosion produced by the presence  of  a  stray  electrical  current.    In  this  case,  the current  necessary  for  the  corrosion  reaction  is  provided from an outside source.   A second additional source of corrosion is that produced by chemicals that may be able to act directly on the reinforcing steel.   Since this section has dealt only with the corrosion of steel embedded in concrete, for information on the behavior of other metals in concrete, see ACI 201.2R and ACI 222R.

(2) Symptoms: Visual  examination  will  typically reveal rust staining of the concrete.   This staining will be followed by cracking.   Cracks produced by corrosion generally run in straight, parallel lines at uniform intervals corresponding to the spacing of the reinforcement.   As deterioration continues, spalling of the concrete over the reinforcing  steel  will  occur  with  the  reinforcing  bar becoming visible (Figure 2-27).   One area where labora- tory analysis may be beneficial is the determination of the chloride contents in the concrete.   This procedure may be used to determine the amount of concrete to be removed during a rehabilitation project.

(3) Prevention: ACI 201.2R describes the considera- tions for protecting reinforcing steel in concrete:   use of concrete with low permeability; use of properly propor- tioned concrete having a low w/c; use of as low a con- crete  slump  as  practical;  use  of  good  workmanship  in placing the concrete; curing the concrete properly; provid- ing adequate concrete cover over the reinforcing steel; providing good  drainage to prevent  water from standing on  the  concrete;  limiting  chlorides  in  the  concrete  mix- ture; and paying careful attention to protruding items such as bolts or other anchors.

e.  Design  errors:

Design  errors  may  be  divided into two general types:   those resulting from inadequate structural design and those resulting from lack of attention to relatively minor design details.   Each of the two types of design errors is discussed below.

(1)  Inadequate structural design:

  1. Mechanism: The failure mechanism is simple-- the concrete is exposed to greater stress than it is capable of carrying or it sustains greater strain than its strain capacity.
  2. Symptoms: Visual examinations of failures resulting  from  inadequate  structural  design  will  usually show one of two symptoms.  First, errors in design result- ing in excessively high compressive stresses will result in spalling.    Similarly,  high  torsion  or  shear  stresses  may also result in spalling or cracking.   Second, high tensile stresses will result in cracking.   To identify inadequate design as a cause of damage, the locations of the damage should be compared to the types of stresses that should be present in the concrete.  For example, if spalls are present on the underside of a simple-supported beam, high com- pressive  stresses  are  not  present  and  inadequate  design may be eliminated as a cause.   However, if the type and location of the damage and the probable stress are in agreement, a detailed stress analysis will be required to determine whether inadequate design is the cause.   Labo- ratory analysis is generally not applicable in the case of suspected inadequate design.   However, for rehabilitation projects, thorough petrographic analysis and strength test- ing  of  concrete  from  elements  to  be  reused  will  be necessary.
  3. Prevention: Inadequate  design  is  best  prevented by thorough and careful review of all design calculations. Any rehabilitation method that makes use of existing concrete structural members must be carefully reviewed.

(2)  Poor design details:

While a structure may be adequately designed to meet loadings and other overall requirements, poor detailing may result in localized con- centrations of high stresses in otherwise satisfactory con- crete.    These  high  stresses  may  result  in  cracking  that allows water or chemicals access to the concrete.  In other cases,  poor  design  detailing  may  simply  allow  water  to pond on a structure, resulting in saturated concrete.   In general, poor detailing does not lead directly to concrete failure; rather, it contributes to the action of one of the other causes of concrete deterioration described in this chapter.  Several specific types of poor detailing and their possible effects on a structure are described in the fol- lowing paragraphs.   In general, all of these problems can be prevented by a thorough and careful review of plans and specifications for the project.   In the case of existing structures, problems resulting from poor detailing should be handled by correcting the detailing and not by simply responding to the symptoms.

  1. Abrupt  changes  in  section: Abrupt  changes  in section may cause stress concentrations that may result in cracking.   Typical examples would include the use of relatively thin sections such as bridge decks rigidly tied into massive abutments or patches and replacement con- crete that are not uniform in plan dimensions.
  2. Insufficient reinforcement at reentrant corners and openings: Reentrant  corners  and  openings  also  tend  to cause stress concentrations that may cause cracking.   In this case, the best prevention is to provide additional reinforcement in areas where stress concentrations are expected to occur.
  3. Inadequate  provision  for  deflection: Deflections in excess of those anticipated may result in loading of members or sections beyond the capacities for which they were designed.   Typically, these loadings will be induced in walls or partitions, resulting in cracking.
  4. Inadequate provision for drainage: Poor attention to the details of draining a structure may result in the ponding of water.   This ponding may result in leakage or saturation of concrete.   Leakage may result in damage to the  interior  of  the  structure  or  in  staining  and  encrus- tations on the structure.  Saturation may result in severely damaged concrete if the structure is in an area that is subjected to freezing and thawing.
  5. Insufficient travel in expansion joints: Inade- quately  designed expansion joints  may result in spalling of concrete adjacent to the joints.   The full range of pos- sible temperature differentials that a concrete may be expected to experience should be taken into account in the specification for expansion joints.   There is no single expansion joint that will work for all cases of temperature differential.
  6. Incompatibility  of  materials: The  use  of  mate- rials with different properties (modulus of elasticity or coefficient of thermal expansion) adjacent to one another may  result  in  cracking  or  spalling  as  the  structure  is loaded or as it is subjected to daily or annual temperature variations.
  7. Neglect of creep  effect: Neglect of creep  may have similar effects as noted earlier for inadequate provi- sion for deflections (paragraph 3-2e(2)(c)).   Additionally, neglect  of  creep  in  prestressed  concrete  members  may lead  to  excessive  prestress  loss  that  in  turn  results  in cracking as loads are applied.
  8. Rigid  joints  between  precast  units: Designs utilizing precast elements must provide for movement between adjacent precast elements or between the precast elements and the supporting frame.  Failure to provide for this movement can result in cracking or spalling.
  9. Unanticipated shear stresses in piers, columns, or abutments: If, through lack of maintenance, expansion bearing assembles are allowed to become frozen, horizon- tal loading may be transferred to the concrete elements supporting  the  bearings.   The result  will  be  cracking  in the  concrete,  usually  compounded  by  other  problems which  will  be  caused  by  the  entry  of  water  into  the concrete.
  10. Inadequate joint spacing in slabs: This is one of the most frequent causes of cracking of slabs-on-grade. Guidance on joint spacing and depth of contraction joints may be found in ACI 332R.

f. Abrasion:

Abrasion  damage  caused  by  water- borne debris and the techniques used to repair the damage on several Corps' structures are described by McDonald (1980).   Also, causes of abrasion-erosion damage and procedures for repair and prevention of damage are described in ACI 210R.

  1. Mechanism: Abrasion-erosion damage is caused by the action of debris rolling and grinding against a concrete surface.   In hydraulic structures, the areas most likely  to  be  damaged  are  spillway  aprons,  stilling  basin slabs, and lock culverts and laterals.   The sources of the debris include construction trash left in a structure, riprap brought  back  into  a  basin  by  eddy  currents  because  of poor  hydraulic  design  or  asymmetrical  discharge,  and riprap or other debris thrown into a basin by the public. Also barges and towboats impacting or scraping on lock wells   and   guide   wells   can   cause   abrasions   erosion damage.
  2. Symptoms: Concrete surfaces abraded by water- borne debris are generally smooth (Figure 2-20) and may contain localized depressions.   Most of the debris remain- ing   in   the   structure   will   be   spherical   and   smooth. Mechanical abrasion is usually characterized by long shallow grooves in the concrete surface and spalling along monolith joints.  Armor plates is often torn away or bent.
  3. Prevention: The following measures should be followed to prevent or minimize abrasion-erosion damage to concrete hydraulic structures (Liu 1980 and McDonald 1980).

(a) Design: It appears that given appropriate flow conditions in the presence of debris, all of the construc- tion materials currently being used in hydraulic structures are to some degree susceptible to erosion.    While improvements in materials should reduce the rate of con- crete damage caused by erosion, this improvement alone will not solve the problem.   Until the adverse hydraulic conditions that can cause abrasion-erosion damage are minimized or eliminated, it will be extremely difficult for any of the construction materials currently being used to avoid damage by erosion.   Prior to construction or repair of major structures, hydraulic model studies of the struc- ture may be required to identify potential causes of ero- sion damage and to evaluate the effectiveness of various modifications in eliminating those undesirable hydraulic conditions.       Many   older   structures   have   spillways designed with a vertical end-sill.   This design is usually efficient in trapping the erosion-causing debris within the spillway.   In some structures, a 45-deg fillet installed on the upstream side of the end sill has resulted in a self- cleaning stilling basin.   Recessing monolith joints in lock walls and guide walls will minimize stilling basin spalling caused     by     barge     impact     and     abrasion     (See paragraph 8-1e(2)(e)).

(b)  Operation: In existing structures, balanced flows should  be  maintained  into  basins  by  using  all  gates  to avoid discharge conditions where eddy action is prevalent. Substantial discharges that can provide a good hydraulic jump without creating eddy action should be released periodically in an attempt to flush debris from the stilling basin.   Guidance as to discharge and tailwater relations required for flushing should be developed through model and   prototype   tests.      Periodic   inspections   should   be required to determine the presence of debris in the stilling basin and the extent of erosion.   If the debris cannot be removed  by  flushing  operations,  the  basin  should  be cleaned by other means.

(c)  Materials: It  is  imperative  that  materials  be tested and evaluated, in accordance with ASTM C 1138 (CRD-C 63), prior to use in the repair of abrasion-erosion damaged hydraulic structures. Abrasion-resistant concrete should include the maximum amount of the hardest coarse aggregate  that  is  available  and  the  lowest  practical  w/c. In  some  cases  where  hard  aggregate  was  not  available, high-range water-reducing admixtures (HRWRA) and condensed silica fume have been used to develop igh compressive  strength  concrete  97 MPa  (14,000 psi)  to overcome  problems  of  unsatisfactory  aggregate (Holland 1983).). Apparently,  at  these  high  compressive  strengths the  hardened  cement  paste  assumes  a  greater  role  in resisting abrasion-erosion damage, and as such, the aggregate quality becomes correspondingly less important. The abrasion-erosion resistance of vacuum-treated con- crete,  polymer  concrete,  polymer-impregnated  concrete, and polymer portland-cement concrete is significantly superior to that of comparable conventional concrete that can also be attributed to a stronger cement matrix.   The increased costs associated with materials, production, and placing of these and any other special concretes in com- parison with conventional concrete should be considered during the evaluation process.  While the addition of steel fibers would be expected to increase the impact resistance of concrete, fiber-reinforced concrete is consistently less resistant to abrasion-erosion than conventional concrete. Therefore,  fiber-reinforced  concrete  should  not  be  used for repair  of stilling  basins or  other hydraulic  structures where  abrasion-erosion  is  of  major  concern. Several types of surface coatings have exhibited good abrasion- erosion resistance during laboratory tests.   These include polyurethanes, epoxy-resin mortar, furan-resin mortar, acrylic  mortar,  and  iron  aggregate  toppings.    However, some difficulties have been reported in field applications of surface coatings, primarily the result of improper sur- face preparation and thermal incompatibility between coatings and concrete.

g.   Cavitation:

Cavitation-erosion is the result of relatively complex flow characteristics of water over concrete surfaces (ACI 210R).

(1)  Mechanism: There is little evidence to show that water  flowing  over  concrete  surfaces  at  velocities  less than 12.2 m/sec (40 ft/sec) causes any cavitation damage to the concrete.   However, when the flow is fast enough (greater than 12.2 m/sec) and where there is surface irreg- ularity in the concrete, cavitation damage may occur. Whenever there is surface irregularity, the flowing water will separate from the concrete surface.   In the area of separation from the concrete, vapor bubbles will develop because of the lowered vapor pressure in the region.   As these  bubbles  are  carried  downstream,  they  will  soon reach areas of normal pressure.   These bubbles will col- lapse with an almost instantaneous reduction in volume. This collapse, or implosion, creates a shock wave which, upon  reaching  a  concrete  surface,  induces  very  high stresses over a small area.  The repeated collapse of vapor bubbles on or near the concrete surface will cause pitting. Concrete spillways and outlet works of many high dams have been severely damaged by cavitation.

(2)  Symptoms: Concrete that has been damaged will be severely pitted and extremely rough (Figure 2-21).  As the damage progresses, the roughness of the damaged area may induce additional cavitation.

(3)  Prevention:

(a) Hydraulic design: Even the strongest materials cannot withstand the forces of cavitation indefinitely. Therefore, proper hydraulic design and the use of aeration to reduce or eliminate the parameters that trigger cavita- tion  are  extremely  important  (ACI 210R).    Since  these topics are beyond the scope of this manual, hydraulic engineers   and   appropriate   hydraulic   design   manuals should be consulted.

(b) Conventional materials: While proper material selection can increase the cavitation resistance of concrete, the only totally effective solution is to reduce or eliminate the causes of cavitation.  However, it is recognized that in the case of existing structures in need of repair, the reduc- tion  or  elimination  of  cavitation  may  be  difficult  and costly.   The next best solution is to replace the damaged concrete with more cavitation-resistant materials.   Cavita- tion resistance of concrete can be increased by use of a properly designed low  w/c, high-strength concrete.   The use  of  no  larger  than  38-mm  (1-1/2-in.) nominal  maxi- mum size aggregate is beneficial.   Furthermore, methods which have reduced the unit water content of the mixture, such as use of a water-reducing admixture, are also bene- ficial.   Vital to increased cavitation resistance are the use of hard, dense aggregate particles and a good aggregate- to-mortar bond.  Typically, cement-based materials exhibit significantly lower resistance to cavitation compared to polymer-based materials.

(c) Other cavitation-resistant materials: Cavitation- damaged areas have been successfully repaired with steel- fiber  concrete  and  polymer  concrete (Houghton,  Borge, and Paxton 1978).   Some coatings, such as neoprene and polyurethane, have reduced cavitation damage to concrete, but since near-perfect adhesion to the concrete is critical, the use of the coatings is not common.   Once a tear or a chip in the coating occurs, the entire coating is likely to be peeled off.

(d)  Construction  practices: Construction  practices are of paramount importance when concrete surfaces are exposed to high-velocity flow, particularly if aeration devices are not incorporated in the design.   Such surfaces must be as smooth as can be obtained under practical conditions.   Accordingly, good construction practices as given in EM 1110-2-2000 should be followed whether the construction is new or is a repair.   Formed and unformed surfaces should be carefully checked during each con- struction operation to confirm that they are within speci- fied tolerances.   More restrictive tolerances on surfaces should be avoided since they become highly expensive to construct and often impractical to achieve, despite the use of modern equipment and good construction practices. Where possible, transverse joints in concrete conduits or chutes should be minimized.  These joints are generally in a location where the greatest problem exists in maintain- ing a continuously smooth hydraulic surface.   One con- struction technique which has proven satisfactory in placement of reasonably smooth hydraulic surfaces is the traveling slipform screed.   This technique can be applied to tunnel inverts and to spillway chute slabs.  Hurd (1989) provides information on the slipform screed.   Since sur- face hardness improves cavitation resistance, proper cur- ing of these surfaces is essential.

h.   Freezing and thawing:

(1) Mechanism: As the temperature of a critically saturated concrete is lowered during cold weather, the freezable water held in the capillary pores of the cement paste  and  aggregates  expands  upon  freezing.    If  subse- quent thawing is followed by refreezing, the concrete is further expanded, so that repeated cycles of freezing and thawing have a cumulative effect.   By their very nature, concrete hydraulic structures are particularly vulnerable to freezing and thawing simply because there is ample opportunity for portions of these structures to become critically saturated. Concrete is especially vulnerable in areas of fluctuating water levels or under spraying condi- tions.   Exposure in such areas as the tops of walls, piers, parapets, and slabs enhances the vulnerability of concrete to the harmful effects of repeated cycles of freezing and thawing.   The use of deicing chemicals on concrete sur- faces may also accelerate damage caused by freezing and thawing and may lead to pitting and scaling.  ACI 201.2R describes the action as physical.   It involves the develop- ment of osmotic and hydraulic pressures during freezing, principally in the paste, similar to ordinary frost action.

(2)  Symptoms: Visual examination of concrete dam- aged by freezing and thawing may reveal symptoms rang- ing   from   surface   scaling   to   extensive   disintegration (Figure 2-10).     Laboratory  examination  of  cores  taken from structures that show surficial effects of freezing and thawing will often show a series of cracks parallel to the surface of the structure.

(3)  Prevention: The  following  preventive  measures are recommended by ACI 201.2R for concrete that will be exposed to freezing and thawing while saturated:

  • Designing the structure to minimize the exposure to  moisture.    For  example,  providing  positive  drainage rather than flat surfaces whenever possible.
  • Using a concrete with a low w/c.
  • Using adequate entrained air to provide a satisfac- tory air-void system in the concrete, i.e., a bubble spacing factor of 0.20 mm (0.008 in.) or less, which will provide protection for the anticipated service conditions and aggregate size.   EM 1110-2-2000 provides information on the recommended amount of entrained air.
  • Selecting  suitable  materials,  particularly  aggre- gates that perform well in properly proportioned concrete.
  • Providing adequate curing to ensure that the com- pressive  strength  of  the  concrete  is  at  least  24 MPa (3,500 psi) before the concrete is allowed to freeze in a saturated state.

i.    Settlement and movement

(1)  Mechanisms:

(a) Differential movement. Situations in which the various elements of a structure are moving with respect to one another are caused by differential movements.   Since concrete structures are typically very rigid, they can toler- ate very little differential movement.   As the differential movement  increases,  concrete  members  can  be  expected to be subjected to an overstressed condition.   Ultimately, the members will crack or spall.

(b)  Subsidence. Situations in which an entire struc- ture is moving or a single element of a structure, such as a monolith, is moving with respect to the remainder of the structure are caused by subsidence.   In these cases, the concerns are not overcracking or spalling but rather stabil- ity against overturning or sliding.   Whether portions of a single structural element are moving with respect to one another or whether entire elements are moving, the under- lying cause is more than likely to be a failure of the foun- dation   material.      This   failure   may   be   attributed   to long-term consolidations, new loading conditions, or to a wide variety of other mechanisms.   In situations in which structural movement is diagnosed as a cause of concrete deterioration, a thorough geotechnical investigation should be conducted.

(2) Symptoms: Visual examination of structures undergoing settlement or movement will usually reveal cracking or spalling or faulty alignment of structural members.   Very often, movement will be apparent in nonstructural  members  such  as  block  or  brick  masonry walls.  Another good indication of structural movement is an increase in the amount of water leaking into the struc- ture.   Since differential settlement of the foundation of a structure is usually a long-term phenomenon, review of instrumentation   data   will   be   helpful   in   determining whether apparent movement is real.  Review by structural and geotechnical engineering specialists will be required.

(3)  Prevention: Prevention of settlements and move- ments or corrective measures are beyond the scope of this manual.   Appropriate structural and geotechnical engi- neering manuals should be consulted for guidance.

j.   Shrinkage

Shrinkage is caused  by the loss of moisture from concrete.   It may be divided into two gen- eral categories:   that which occurs before setting (plastic shrinkage) and that which occurs after setting (drying shrinkage).   Each of these types of shrinkage is discussed in this section.

(1)  Plastic shrinkage:

(a)  Mechanism: During the period between placing and setting, most concrete will exhibit bleeding to some degree.   Bleeding is the appearance of moisture on the surface of the concrete; it is caused by the settling of the heavier  components  of  the  mixture.    Usually,  the  bleed water evaporates slowly from the concrete surface.   If environmental conditions are such that evaporation is occurring faster than water is being supplied to the sur- face by bleeding, high tensile stresses can develop. These stresses  can  lead  to  the  development  of  cracks  on  the concrete surface.

(b)  Symptoms: Cracking caused by plastic shrinkage will be seen within a few hours of concrete placement. Typically, the cracks are isolated rather than patterned. These cracks are generally wide and shallow.

(c)  Prevention: Determination of whether the weather conditions on the day of the placement are conducive to plastic shrinkage cracking is necessary.   If the predicted evaporation rate is high according to ACI 305R, appropri- ate  actions  such  as  erecting  windbreaks,  erecting  shade over  the  placement,  cooling  the  concrete,  and  misting should be taken after placement.   Additionally, it will be beneficial to minimize the loss of moisture from the con- crete surface  between  placing  and  finishing.     Finally, curing should be started as soon as is practical.   If crack- ing caused by plastic shrinkage does occur and if it is detected early enough, revibration and refinishing of the cracked area will resolve the immediate problem of the cracks.     Other  measures  as described  above  will  be required to prevent additional occurrences.

(2)  Drying shrinkage:

(a) Mechanism: Drying shrinkage is the long-term change in volume of concrete caused by the loss of mois- ture.     If  this  shrinkage  could  take  place without  any restraint, there would be no damage to the concrete. However, the concrete in a structure is always subject to some  degree  of  restraint  by either  the  foundation,  by another part of the structure, or by the difference in shrinkage between the concrete at the surface and that in the interior of a member.   This restraint may also be attributed to purely physical conditions such as the place- ment of a footing on a rough foundation or to chemical bonding of new concrete to earlier placements or to both. The combination of shrinkage and restraints cause tensile stresses that can ultimately lead to cracking.

(b)  Symptoms:  Visual  examination  will  typically show cracks that are characterized by their fineness and absence of any indication of movement.  They are usually shallow, a few inches in depth.  The crack pattern is typi- cally orthogonal or blocky. This type of surface cracking should not be confused with thermally induced deep cracking which occurs when dimensional change is restrained in newly placed concrete by rigid foundations or by old lifts of concrete..

(c)  Prevention: In general, the approach is either to reduce the tendency of the concrete, to shrink or to reduce the restraint, or both.   The following will help to reduce the tendency to shrink:   use of less water in the concrete; use of larger aggregate to minimize paste content; placing the concrete at as low a temperature as practical; dam- pening the subgrade and the forms; dampening aggregates if they are dry and absorptive; and providing an adequate amount of reinforcement to distribute and reduce the size of cracks that do occur.   Restraint can be reduced by providing adequate contraction joints.

k. Temperature  changes

Changes  in  temperature cause a corresponding change in the volume of concrete. As was true for moisture-induced volume change (drying shrinkage), temperature-induced volume changes must be combined with restraint before damage can occur.   Basi- cally, there are three temperature change phenomena that may cause damage to concrete.   First, there are the tem- perature changes that are generated internally by the heat of hydration of cement in large placements.  Second, there are the temperature changes generated by variations in climatic conditions.   Finally, there is a special case of externally generated temperature change--fire damage. Internally  and  externally  generated  temperature  changes are discussed in subsequent paragraphs.   Because of the infrequent  nature  of  its  occurrence  in  civil  works  struc- tures, fire damage is not included in this manual

(1)  Internally generated temperature differences.

(a)  Mechanism: The hydration of portland cement is an exothermic chemical reaction.   In large volume place- ments, significant amounts of heat may be generated and the  temperature  of  the  concrete  may  be  raised  by  more than  38 °C  (100 °F)  over  the  concrete  temperature  at placement.   Usually, this temperature rise is not uniform throughout the mass of the concrete, and steep tempera- ture gradients may develop.   These temperature gradients give  rise  to  a  situation  known  as  internal  restraint--the outer portions of the concrete may be losing heat while the inner portions are gaining (heat).   If the differential is great, cracking may occur.   Simultaneously with the development of this internal restraint condition, as the concrete mass begins to cool, a reduction in volume takes place.  If the reduction in volume is prevented by external conditions (such as by chemical bonding, by mechanical interlock, or by piles or dowels extending into the con- crete), the concrete is externally restrained.   If the strains induced by the external restraint are great enough, crack- ing may occur.   There is increasing evidence, particularly for rehabilitation work, that relatively minor temperature differences in thin, highly restrained overlays can lead to cracking.  Such cracking has been seen repeatedly in lock wall   resurfacing   (Figure   2-5)   and   in   stilling   basin overlays.   Measured temperature differentials have typi- cally been much below those normally associated with thermally induced cracking.

(b) Symptoms: Cracking resulting from internal restraint will be relatively shallow and isolated.  Cracking resulting   from   external   restraint   will   usually  extend through the full section.  Thermally induced cracking may be expected to be regularly spaced and perpendicular to the larger dimensions of the concrete.

(c)  Prevention:  An  in-depth  discussion  of  tempera- ture and cracking predictions for massive placements can be found in ACI 207.1R and ACI 207.2R.  In general, the following may be beneficial:  using as low a cement con- tent as possible; using a low-heat cement or combination of cement and pozzolans; placing the concrete at the minimum practical temperature; selecting aggregates with low moduli of elasticity and low coefficients of thermal expansion; cooling internally or insulating the placement as appropriate to minimizing temperature differentials; and minimizing the effects of stress concentrators that may instigate cracking.

(2)  Externally generated temperature differences

(a)  Mechanism: The basic failure mechanism in this case is the same as that for internally generated tempera- ture differences--the tensile strength of the concrete is exceeded.   In this case the temperature change leading to the concrete volume change is caused by external factors, usually changing climatic conditions.  This cause of deter- ioration  is  best  described  by  the  following  examples: First, a pavement slab cast in the summer.  As the air and ground temperatures drop in the fall and winter, the slab may  undergo  a  temperature  drop  of  27 °C  (80 °F),  or more. Typical parameters for such a temperature drop (coefficient  of  thermal  expansion  of  10.8 × 10-6/°C  (6 × 10-6/°F) indicate a 30-m (98-ft) slab would experience a shortening of more than 13 mm (1/2 in.). If the slab were restrained, such movement would certainly lead to crack- ing.  Second, a foundation or retaining wall that is cast in the summer.   In this case, as the weather cools, the con- crete  may  cool  at  different  rates--exposed  concrete  will cool  faster  than  that  insulated  by  soil  or  other  backfill. The  restraint  provided  by  this  differential  cooling  may lead  to  cracking  if  adequate  contraction  joints  have  not been  provided. Third,  concrete  that  experiences  sig- nificant expansion during the warmer portions of the year. Spalling  may  occur  if  there  are  no  adequate  expansion joints.   In severe cases, pavement slabs may be lifted out of alignment, resulting in so-called blowups.   Fourth, concretes  that   have   been  repaired   or   overlayed   with materials that do not have the same coefficient of thermal expansion as the underlying material.  Annual heating and cooling may lead to cracking or debonding of the two materials.

(b)  Symptoms: Visual examination will show regu- larly spaced cracking in the case of restrained contraction. Similarly, spalling at expansion joints will be seen in the case of restrained expansion. Problems resulting from expansion-contraction caused by thermal differences will be   seen   as   pattern   cracking,   individual   cracking,   or spalling.

(c)  Prevention: The best prevention is obviously to make provision for the use of contraction and expansion joints.  Providing reinforcing steel (temperature steel) will help to distribute cracks and minimize the size of those that  do  occur.    Careful  review  of  the  properties  of  all repair materials will help to eliminate problems caused by temperature changes

l.  Weathering:

Weathering is frequently referred to as a cause of concrete deterioration.   ACI 116R defines weathering as "Changes in color, texture, strength, chemi- cal   composition,   or   other  properties   of   a  natural   or artificial material due to the action of the weather."  How- ever, since all of these effects may be more correctly attributed   to   other   causes   of   concrete   deterioration described  in  this  chapter,  weathering  itself  is  not  con- sidered to be a specific cause of deterioration.

Relating Symptoms to Causes of Distress and Deterioration

Given a detailed report of the condition of the concrete in a structure and a basic understanding of the various mech- anisms that can cause concrete deterioration, the problem becomes one of relating the observations or symptoms to the  underlying  causes.     When  many  of  the  different causes  of  deterioration  produce  the  same  symptoms,  the task of relating symptoms to causes is more difficult than it first appears.   One procedure to consider is based upon that described by Johnson (1965).   This procedure is obviously  idealized  and  makes  no  attempt  to  deal  with more than one cause that may be active at any one time. Although there will usually be a combination of causes responsible for the damage detected on a structure, this procedure should provide a starting point for an analysis.

a. Evaluate structure design to determine adequacy:

First consider what types of stress could have caused the observed symptoms.   For example, tension will cause cracking, while compression will cause spalling.   Torsion or shear will usually result in both cracking and spalling. If  the  basic  symptom  is  disintegration,  then  overstress may be eliminated as a cause.   Second, attempt to relate the probable types of stress causing the damage noted to the locations of the damage.   For example, if cracking resulting  from  excessive  tensile  stress  is  suspected,  it would not be consistent to find that type of damage in an area  that  is  under  compression.    Next,  if  the  damage seems appropriate for the location, attempt to relate the specific orientation of the damage to the stress pattern. Tension  cracks  should  be  roughly  perpendicular  to  the line of externally induced stress.   Shear usually causes failure by diagonal tension, in which the cracks will run diagonally in the concrete section.   Visualizing the basic stress patterns in the structure will aid in this phase of the evaluation.  If no inconsistency is encountered during this evaluation,  then  overstress  may  be  the  cause  of  the observed damage.  A thorough stress analysis is warranted to confirm this finding.   If an inconsistency has been detected, such as cracking in a compression zone, the next step in the procedure should be followed.

b. Relate the symptoms to potential causes:

Table 3 Relating Symptoms to Causes of Distress and Deterioration of Concrete
Causes

Construction Faults

Cracking Disintegration

Distortion Movement

Erosion

Joint Failures

Seepage Spalling
Accidental







Loadings
X






Chemical







Reactions
X
X





Construction





X

Errors X
X



X
X
X
Corrosion
X





X
Design Errors
X



X
X
X
Erosion

X

X



Freezing and







Thawing
X
X




X
Settlement







And Movement
X

X

X

X
Shrinkage X
X

X




Temperature




X


Changes
X





X

For this step, Table 3-3 will be of benefit.   Depending upon the symptom, it may be possible to eliminate several possible causes.   For example, if the symptom is disintegration or erosion, several potential causes may be eliminated by this procedure.

 

c. Eliminate  the  readily  identifiable  causes:

From the list of possible causes remaining after symptoms have been related to potential causes, it may be possible to eliminate two causes very quickly since they are relatively easy  to  identify.     The  first  of  these  is  corrosion  of embedded metals.   It will be easy to verify whether the cracking and spalling noted are a result of corrosion.  The second cause that is readily identified is accidental load- ing,  since  personnel  at  the  structure  should  be  able  to relate the observed symptoms to a specific incident. Analyze  the  available  clues. If  no  solution  has been reached at this stage, all of the evidences generated by field and laboratory investigations should be carefully reviewed.     Attention should  be  paid  to  the  following points:

(1)  If the basic symptom is that of disintegration of the concrete surface, then essentially three possible causes remain:   chemical attack, erosion, and freezing and thaw- ing.   Attempts  should  be  made  to  relate  the  nature  and type of the damage to the location in the structure and to the environment of the concrete in determining which of the three possibilities is the most likely to be the cause of the damage.

(2)  If there is evidence of swelling of the concrete, then there  are two  possibilities:   chemical reactions  and temperature changes.  Destructive chemical reactions such as alkali-silica or alkali-carbonate attack that cause swel- ling will have been identified during the laboratory inves- tigation.    Temperature-induced  swelling  should  be  ruled out unless there is additional evidence such as spalling at joints.

(3) If the evidence is spalling and corrosion and accidental loadings have been eliminated earlier, the major causes of spalling remaining are construction errors, poor detailing, freezing and thawing, and externally generated temperature changes.  Examination of the structure should have  provided  evidence as  to  the  location  and  general nature of the spalling that will allow identification of the exact cause.

(4)  If  the  evidence  is  cracking,  then  construction errors, shrinkage, temperature changes, settlement and movement, chemical reactions, and poor design details remain as possible causes of distress and deterioration of concrete.   Each of these possibilities will have to be reviewed in light of the available laboratory and field observations to establish which is responsible.

(5)  If  the  evidence  is  seepage  and  it  has  not  been related to a detrimental internal chemical reaction by this time, then it is probably the result of design errors or construction errors, such as improper location or installa- tion of a waterstop. e. Determine  why  the  deterioration  has  occurred. Once the basic cause or causes of the damage have been established, there remains one final requirement:   to understand how the causal agent acted upon the concrete. For example, if the symptoms were cracking and spalling and the cause was corrosion of the reinforcing steel, what facilitated the corrosion?   Was there chloride in the con- crete?   Was  there inadequate  cover over  the reinforcing steel?   Another example to consider is concrete damage caused by freezing and thawing.   Did the damage occur because the concrete did not contain an adequate air-void system,  or  did  the  damage  occur  because  the  concrete used was not expected to be saturated but, for whatever reason, was saturated?  Only when the cause and its mode of action are completely understood should the next step of selecting a repair material be attempted.


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