MSD- Causes of Damage to Concrete (2023)

This report discusses the most common causes of salvage concrete damage. The discussion for each cause of failure consists of (1) a description of the cause and how it damages the concrete and (2) a discussion and/or list of appropriate methods/materials for repairing that particular type of concrete failure. The text format of this report was chosen considering the importance of first determining the cause(s) of concrete failure before attempting to select the repair method. It is expected that the complete discussion of the selected repair method will be consulted before the work is carried out.

1. Excess water in the concrete mix

Using too much water in the concrete mix is ​​the number one cause of concrete damage. Excess water decreases strength, increases curing and drying shrinkage, increases porosity, increases creep, and decreases abrasion resistance of concrete. In fact, high durability is associated with a low water/cement ratio and the use of trapped air.

Damage caused by too much mixed water can be difficult to diagnose as it is often masked by damage from other causes. For example, freeze-thaw cracking, abrasion erosion, or dry shrinkage cracking are often blamed for damage to concrete, when in reality, excess water in the mix caused the poor durability that allowed these other causes to attack the concrete. On petrographic examination, extreme cases of excessive mixing of water in hardened concrete can sometimes be detected by the presence of bleed channels or pockets of water under large aggregates. Typically, examination of batch sheets, mix records, and field inspection reports will confirm excessive use of mix water in damaged concrete. However, it should be noted that water added to on-site transit truck mixes or applied to concrete surfaces during finishing operations is generally not documented.

The only permanent repair to concrete damaged by excessive mixing water is removal and replacement. However, depending on the extent and nature of the damage, various maintenance or repair methods can be helpful in extending the life of this concrete.

2. Defective design

Construction errors can cause many types of damage to concrete. It is beyond the scope of this guide to discuss all types of damage that can result from a faulty design. A rather common design error, however, is placing embedded metal, such as electrical wires or plugs, too close to the exterior surfaces of concrete structures. Cracks form in the concrete over and around such metallic elements and allow for accelerated deterioration by freezing and thawing. The base of handrails or guardrails are placed very close to the outside corners of walls, walkways and parapets with similar results. These sockets or intrusions in concrete expand and contract at a different rate than concrete with changes in temperature. Tensile stresses created by expanding metal in concrete cause cracking and subsequent freeze-thaw damage. Barriers or long handrails can pose another problem. The tubing used for such splints also experiences thermal expansion and contraction. If proper sliding joints are not provided in the rails, expansion and contraction will result in cracks at the points where the rail anchor pads enter the concrete. This crack also allows for accelerated freezing and melt damage to the concrete.

Inadequate concrete coverage over rebar is a common cause of damage to road bridge structures. This can also be an issue with hydroelectric and irrigation structures. Inadequate coverage will initiate rebar corrosion. The iron oxide by-products of this corrosion take up more space in the concrete than the rebar and cause cracking and delamination of the concrete.

Failure to provide adequate contraction joints or expansion joints wide enough to accommodate temperature expansion in concrete slabs will result in damage. Concrete with insufficient contraction joints will crack and form a joint whenever a joint is needed but not intended. Unfortunately, these cracks are not as visually appealing as a molded or sawn joint. Crack formation relieves tensile stresses and, although unsightly, rarely requires repair. Concrete slabs with insufficient or very narrow expansion joints can cause serious damage to bridge decks, sidewalks and floors of long, steeply sloping, south-facing spillways. This concrete undergoes large daily and seasonal temperature changes due to solar radiation. The resulting concrete expansion is greatest at the top of the slabs, where concrete temperatures are highest, and lowest at the coldest bottom edges. Such expansion can result in upper concrete sections of adjacent slabs abutting against the joints between the slabs. The only possible direction of discharge movement in such slabs is upwards, leading to the formation of delamination in the concrete. These delaminations are usually located in the upper mesh of the rebar. In temperate climates, the formation of delaminations relieves stretching stresses and further damage generally ceases. However, in cold climates, water can enter delaminations where it undergoes a daily cycle of freezing and thawing.

Repairing damage caused by faulty construction is useless until building faults are mitigated. Embedded metal elements can be removed, handrails can be fitted with sliding joints, and crash barrier mounting pads can be placed in locations with sufficient concrete to withstand the pulling forces. Mitigating insufficient coverage of concrete over rebar is very difficult, but repair materials resistant to these specific types of corrosion can be selected for repair. Repairs can also be protected by sealants or concrete coatings to reduce water ingress. Boards with insufficient expansion joints can be sawn to increase the number of joints and/or widen the joints to allow for expected thermal expansion.

3. Construction defects

Some of the most common types of concrete damage caused by design defects are rock nests and honeycombs, form defects, dimensional defects, and manufacturing defects.

Honeycombs and rock pockets are areas of concrete where they are left empty because the cement mortar does not fill the spaces around and between the coarse aggregate particles. These imperfections, if minor, can be repaired with cement mortar if less than 24 hours have passed since the formwork was removed. If repair takes longer than 24 hours after formwork removal, or if the pocket is extended, the area should be prepared and the failed concrete removed and replaced with dry pack and proprietary products such as pre-packaged cementitious materials; polymeric cement mortar; polymeric mortar; and replacement mortar or concrete. Some minor defects created by mold movement or failure can be repaired by surface grinding.

There are many ways to create dimensional errors in concrete construction. Wherever possible, it is usually best to accept the resulting deficiency rather than try to correct it. If the nature of the defect is unacceptable, complete removal and restoration is probably the best course of action. Occasionally, dimensional errors can be corrected by removing defective concrete and replacing it with proprietary products such as prepackaged cementitious materials; polymeric cement mortar; polymeric mortar; and replacement mortar or concrete.

Finishing errors usually involve reworking or adding water and/or cement to the surface during finishing procedures. In both cases, the resulting surface is porous and permeable and has little durability. Badly machined surfaces show surface fragmentation at the beginning of their service life. Surface chip repair involves removing weakened concrete and replacing it with epoxy bonded concrete. If deterioration is caught early, surface life can be extended using concrete sealing compounds.

4. Sulfatabbau

Sodium, magnesium and calcium sulfates are salts commonly found in alkaline soils and groundwater. These sulfates chemically react with hydrated lime and hydrated aluminate in the cement paste to form calcium sulfate and calcium sulfoaluminate. The volume of these reaction by-products is greater than the volume of the cement paste from which they are formed, resulting in concrete expansion failure. Type V Portland cement, which is low in calcium aluminate, is highly resistant to sulfate reaction and attack and should be specified in recognition that concrete must be exposed to sulfates from soil and groundwater.

Concrete that is subject to active deterioration and damage from sulfate exposure can sometimes be remedied by applying a thin layer of polymeric concrete or concrete sealant. Alternating wetting and drying cycles accelerate sulfate degradation, and some reduction in the rate of deterioration can be achieved by interrupting the cyclic wetting and drying cycle. Water-based processes to eliminate or remove sulfates are also useful if this is the source of the sulfates. Otherwise, deteriorated concrete should be monitored for removal and replacement with V-cement concrete, if necessary.

5. Alkaline Aggregate Reaction

Certain types of sand and aggregates such as opal, flint and flint or high silica volcanic rock react with the alkalis calcium, sodium and potassium hydroxide in Portland cement concrete. Some concretes containing alkali-reactive aggregates show immediate signs of destructive expansion and deterioration. Other concretes can remain intact for many years. Petrographic examination of reactive concrete shows that a gel forms around the reactive aggregate. This gel expands greatly in the presence of water or water vapor (80 to 85 percent relative humidity is all that is needed for water), causing stress cracks around the aggregate and causing the concrete to expand. In free fall, expansion in concrete is initially reflected in surface pattern cracks. A whitish type of ooze is usually evident in and around cracked concrete. In extreme cases, these cracks opened 40-50 mm. It is common for such expansion to cause significant displacements in the concrete and jamming or jamming of control gates in dikes. In large concrete structures, an alkali-aggregate reaction can only occur in certain areas of the structure. Until it is realized that various aggregate sources are commonly used to build large concrete structures, it can be confusing. Only the parts of the structure built with concrete containing alkaline reactive sand and/or aggregates show expansion due to the reaction of the alkaline aggregates.

In new construction, low alkali Portland cements and fly ash pozzolan can be used to eliminate or significantly reduce reactive aggregate degradation. In existing concrete structures, reactive aggregate degradation is virtually impossible to mediate. There are no proven methods to eliminate degradation from the alkali-aggregate reaction, although the rate of expansion can sometimes be reduced by taking measures to keep the concrete as dry as possible. It is generally futile to try to repair concrete that is actively undergoing an alkaline-aggregate reaction. Continued expansion within the concrete will simply disturb and destroy the repair material. Structures subject to active deterioration should be monitored for rate of expansion and movement, and only those repairs necessary to maintain the safe operation of the facility should be made.

6. Deterioration by freeze-thaw cycles

Freeze-thaw deterioration is a common cause of damage to concrete constructed in colder climates. For freeze-thaw damage to occur, the following conditions must be met:

• Concrete must undergo cyclic freezing and thawing.
• Concrete pores must be almost saturated with water (greater than 90% saturation) during freezing.

Water expands in volume by about 15% when it freezes. When the pores and capillaries of concrete are nearly saturated when it freezes, the expansion exerts tensile forces that break down the matrix of the cement mortar. This deterioration occurs almost in layers from the outer surfaces inward. The rate of progression of freeze-thaw deterioration depends on the number of freeze-thaw cycles, the degree of saturation during freezing, the porosity of the concrete, and the exposure conditions. Tops of walls exposed to melting or spray snow, horizontal panels exposed to water, and vertical walls at the waterline are the most commonly damaged locations in freeze-thaw cycles.

Damage caused by the cyclic freezing and thawing of concrete does not occur until the concrete is nearly saturated. Successful mitigation of freeze-thaw deterioration therefore involves reducing or eliminating freeze-thaw cycles or reducing water absorption in concrete. It is generally not practical to protect or insulate concrete from freeze-thaw cycles, but concrete sealants can be applied to exposed concrete surfaces to prevent or reduce water absorption. Sealants are not effective in protecting flooded concrete, but they can protect concrete exposed to rain, wind spray or melt water.

Repair of freeze-thaw damaged concrete is most commonly done with replacement concrete when damage is 150 mm or deeper, or replacement concrete bonded with epoxy or polymer concrete when damage is between 40 and 150 mm deep. .

7. Abrasion erosion damage

Concrete structures that transport water containing silt, sand and rock or water at high speeds are subject to abrasion damage. Dam stills suffer damage from abrasion when currents do not sweep debris from basins. Some stills have imperfect flow patterns that cause downstream sand and rock to be washed into upstream basins. This material is trapped in basins where it causes significant damage during periods of high current. Abrasion damage results from the abrasive action of silt, sand and rock. Concrete surfaces damaged in this way usually have a polished appearance. Coarse aggregate is often exposed and somewhat polished due to the action of silt and sand on the cement mortar matrix. The extent of abrasion-erosion damage is a function of so many variables – duration of exposure, shape of concrete surfaces, rate and pattern of flow, direction of flow and aggregate load – that it is difficult to develop general theories to predict performance. concrete under these conditions. Consequently, hydraulic modeling studies are often required to define conditions and flow patterns in damaged basins and to assess necessary changes. If the conditions that caused abrasion erosion damage are not addressed, the best repair materials will be damaged and have a short lifespan.

It is well known that high quality concrete is much more resistant to abrasion damage than low quality concrete and several studies (Smoak, 1991) clearly show that the strength of concrete increases as the compressive strength of concrete increases. .

Abrasion damage is best repaired with fumed silica concrete or polymer concrete. These materials have shown the highest resistance to abrasion damage in laboratory and field tests. If the damage does not extend behind the rebar or at least 150mm into the concrete, the fumed concrete should be applied over a fresh coat of epoxy bonding.

8. Cavitation damage

Cavitation damage occurs when high velocity water currents encounter discontinuities in the flowing surface. Discontinuities in the flow path cause water to move away from the surface of the flow, creating low pressure zones and resulting water vapor bubbles. These bubbles travel downstream and collapse. When bubbles collapse against a concrete surface, a very high pressure zone is created over an infinitesimally small surface area. These severe impacts can dislodge concrete particles and create another discontinuity that can cause more extensive cavitation damage.

Cavitation damage is common to water control gates and gate structures. Very high velocity flows occur when spools are first opened or in small spool openings. These flows cause cavitation damage directly downstream of the gates or gate frames.

The cavitation resistance of many different repair materials has been tested by Reclamation Laboratories, the U.S. Army Corps of Engineers and others tested. To date, no material has been found, including stainless steel and cast iron, capable of withstanding fully developed cases of cavitation. A successful repair must first include mediating the causes of cavitation.

9. Corrosion of rebar

Rebar corrosion is usually a symptom of concrete damage rather than the cause of the damage. That is, another cause weakens the concrete and allows the steel to corrode. However, corroded rebar is so commonly found in damaged concrete that it better serves the purposes of this guide to discuss it as if it were the cause of the damage.

The alkalinity of the Portland cement used in concrete typically creates a passive, basic environment (pH around 12) around the rebar, protecting it from corrosion. If this passivity is lost or destroyed, or if the concrete cracks or delaminates enough to allow free entry of water, corrosion can occur. Iron oxides produced by corrosion of steel take up more space in concrete than the original reinforcing steel. This creates tensile stresses in the concrete and leads to further cracking and/or delamination, which accelerates the corrosion process.

Some of the most common causes of rebar corrosion are cracks associated with freeze-thaw deterioration, exposure to sulfate and alkali-aggregate reaction, exposure to acids, loss of alkalinity due to carbonation, lack of adequate concrete layer depth, and exposure to chlorides.

Exposure to chlorides greatly accelerates corrosion rates and can occur in a variety of ways. The application of de-icing salts (sodium chloride) to concrete to accelerate the melting of snow and ice is a common source of chlorides. Chlorides can also be found in sand, aggregates and mixing water used to make concrete mixes. Concrete structures in marine environments are subject to chloride loading from seawater or windblown spray. Finally, it used to be quite common practice to use concrete admixtures containing chlorides to accelerate the hydration of concrete placed during winter conditions.

The occurrence of corroded rebar can usually, but not always, be recognized by the presence of rust spots on external surfaces and the hollow sounds produced when affected concrete is struck with a hammer. It can also be detected by measuring the half-cell potentials of the affected concrete with specially manufactured electronic equipment. Once the presence of corroded steel is confirmed, it is important to define what actually caused the corrosion, as the cause(s) of the corrosion will generally determine which repair procedure should be used. Once the cause of the damage is defined and mitigated if necessary, it becomes important to properly prepare the corroded steel that will be exposed when the damaged concrete is removed. Steel that has been reduced to less than half of its original cross section by the corrosion process must be removed and replaced. The remaining steel should then be cleaned to remove any loose rust, scale, and corrosion by-products that may interfere with adhesion of the repair material. Corroded rebar can extend from areas of apparently deteriorated concrete to areas of apparently solid concrete. Care must be taken to remove enough concrete to encompass all of the corroded steel.

10. Exposure to acids

The most common sources of acid exposure associated with concrete structures occur near underground mines. The drainage water from these mines can contain acids with sometimes unexpectedly low pH. A pH of 7 is considered neutral. Values ​​above 7 are referred to as basic, pH values ​​below 7 as acidic. A 15 to 20 percent sulfuric acid solution has a pH of about 1. This solution will damage concrete very quickly. Acidic water with a pH of 5 to 6 also damages concrete, but only after prolonged exposure.

Acid damaged concrete is very easy to spot. The acid reacts with the Portland cement mortar matrix of the concrete, converting the cement into calcium salts that either break off or are washed away by running water. Coarse aggregate is usually undamaged but remains exposed. The appearance of acid-damaged concrete is similar to that of abrasion damage, but the coarse aggregate exposure is more pronounced and does not appear polished. Acid damage begins and is most pronounced on the exposed concrete surface, but always progressively extends into the core of the structure. The acid is more concentrated on the surface. When penetrating the concrete, it is neutralized by the reaction with Portland cement. However, the cement deep within the structure is weakened by the reaction. When acid-damaged concrete is processed, more and more concrete is removed than would be expected. Failure to remove all acid-etched and weakened concrete will result in failure of repair material adhesion. Acid washing was once permitted as a method of cleaning concrete surfaces in preparation for repairs. However, it was found that bond failure would occur unless extensive efforts were made to remove all traces of acid from the concrete. Remediation specifications no longer permit the use of acid washes to prepare concrete for repair or to clean cracks subject to resin injection repair.

As with any cause of damage to concrete, it is usually necessary to eliminate the source of damage prior to repair. The most common technique for acid damage is to dilute the acid with water. Low pH acidic solutions can be converted this way to higher pH solutions that have much less potential for damage. Alternatively, if the pH of the acidic solution is relatively high, coatings such as the Thin Polymer Concrete Coating System can be applied over repair materials to prevent the acid from damaging the surfaces again. Laboratory testing has revealed very few cost effective coatings capable of protecting repair materials from low pH solutions. Repairs to acid-damaged concrete can be made with epoxy-bonded replacement concrete, replacement concrete, polymeric concrete, and in some cases epoxy-bonded epoxy mortar. Polymeric concrete and epoxy mortars that do not contain Portland cement offer the greatest resistance to acid attack.

11. Cracks

Cracks, such as rebar corrosion, are generally not a cause of damage to concrete. Rather, the crack is a symptom of damage caused by another cause.

All Portland cement concrete will undergo some degree of shrinkage during hydration. This shrinkage produces multidirectional drying shrinkage and cure shrinkage cracking with a somewhat circular pattern. These cracks rarely penetrate very deep into the concrete and can often be overlooked.

Plastic shrinkage cracking occurs when fresh concrete is subjected to high rates of evaporative water loss that cause shrinkage while the concrete is still plastic. Plastic shrinkage cracks are generally somewhat deeper than drying or curing cracks and can have an optically unsightly parallel orientation.

Thermal cracking is caused by the normal expansion and contraction of concrete with changes in ambient temperature. Concrete has a coefficient of linear thermal expansion of approximately 5.5 millionths of an inch per inch per degree Fahrenheit (°F). This can cause the concrete to experience length changes of approximately 0.5 inches per 100 linear feet for a temperature change of 80°F. If the structure's design does not provide enough joints to accommodate this change in length, the concrete will simply fracture and provide the joints where needed. This type of crack usually extends completely through the element and creates a source of leakage in water retaining structures. Thermal cracking can also be caused by the use of Portland cements, which develop high heat of hydration during curing. This concrete develops exothermic heat and hardens at elevated temperatures. Subsequent shrinkage on cooling develops internal tensile stresses and resultant cracks at or above the failure points.

Insufficient foundation support is another common cause of cracks in concrete structures. The tensile strength of concrete is usually only around 1.4 to 2.0 MPa. Foundation settlement can easily lead to displacement conditions that exceed the tensile strength of concrete, leading to cracking.

Cracking is also caused by alkali-aggregate reaction, sulfate exposure, and exposure to cyclic freeze-thaw conditions, as discussed in the previous sections, and by structural overloading, as discussed in the following section.

Successful crack repair is often very difficult to achieve. It is better not to repair most types of concrete cracks than to attempt poor or improper repairs. The choice of methods to repair cracked concrete depends on the cause of the crack. First, it must be determined whether the cracks are "alive" or "dead". As cracks cyclically open and close or progressively widen, structural repair becomes very complicated and often pointless. Such a crack will easily heal in the repair material or adjacent concrete. For this reason, it is common to install crack gauges over cracks to monitor their movement before attempting to repair them. Gauges should be monitored over long periods of time to determine whether cracks are opening and closing simply due to daily or seasonal temperature changes, or whether cracks are continuously or progressively expanding due to foundation conditions or loading. Repairs should only be attempted after understanding the cause and behavior of cracks.

If the cracks are "dead" or static, epoxy injection can be used to structurally rebond the concrete. If the purpose of the repair is to seal water leaks, rather than to achieve a new structural union, the cracks should be injected with polyurethane resin. Epoxy resin injection can sometimes be used to seal low-volume water leaks and structurally bond cracked concrete pieces. Epoxies cure to form hard, brittle materials that cannot withstand injected crack movement. Polyurethane resins cure to form a flexible, low-stress, closed-cell foam that effectively seals water leaks, but typically cannot be used for structural resurfacing. (Some two-part polyurethane resin systems cure to form flexible solids that can be useful for structural rebonding.) These flexible foams can exhibit 300 to 400 percent elongation due to crack movement. It is not uncommon for damaged concrete to show cracks unrelated to the cause of the primary damage. If the depth of removal of damaged or deteriorated concrete is not less than the depth and extent of existing cracks, it can be expected that the cracks will eventually be reflected in the new repair materials. Such reflective cracking is common in repairs to bonded coatings on bridge decks, spillways, and channel linings. If reflective cracking is not tolerable, repairs should be done as separate structural members, not glued to existing old concrete.

12. Structural overloads

Concrete damage caused by structural overloading is usually very obvious and easy to spot. The event that led to an overload is usually noted and logged. The voltages generated by the overload result in characteristic cracking patterns that indicate the source and cause of the excessive voltage and the point(s) of load application. Typically, structural overloads are one-off events and, once defined, the resulting damage can be repaired with the expectation that the cause of the damage will not recur and cause damage to the repaired concrete.

It should be expected that the assistance of an experienced structural engineer will be required to perform the structural analysis necessary to fully define and assess the cause of and resulting damage from most structural overloads and to assist in determining the extent of repair required. This analysis should include determining the loads for which the structure was designed and the extent to which the overload exceeded the design capacity. A thorough examination of the damaged concrete should be performed to determine the overall effect of overloading on the structure. Displacements must be discovered and any consequential damage located. Care must be taken that some other cause of damage has not first weakened the concrete and made it unable to support the design loads. Overload damage repair is probably best done with conventional replacement concrete. The need to repair and/or replace damaged rebar must be anticipated and incorporated into repair procedures.