Concrete Failure | Causes Of Concrete Structure Damage | Types Of Concrete Failure

The list of potential causes of concrete failures is a long one. A few examples include chemical reactions, shrinkage, weathering, and erosion. Many other potential causes exist, and we will explore them individually. Understanding the causes of concrete structure damage is an important element in the business of rehab and repair work.

 

UNINTENTIONAL LOADS

Unintentional loads are not common, which is why they are accidental. When an earthquake occurs and affects concrete structures, that action is considered to be an accidental loading. This type of damage is generally short in duration and few and far between in occurrences.

The visual inspection will likely find spalling or cracking when accidental loadings occur. How is this type of damage stopped? Generally speaking, the damage cannot be prevented, because the causes are unexpected and difficult to anticipate. For example, an engineer is not expecting a ship to hit a piling for a bridge, but it happens. The only defence is to build with as much caution and anticipation as possible.

 

 

CHEMICAL REACTIONS

Concrete damage can occur when chemical reactions are present. It is surprising how little it takes for a chemical attack on concrete to do serious structural damage. The following sections include examples of chemical reactions and how they affect concrete.

 

ACIDIC REACTIONS

Most people know that acid can have serious reactions with a number of materials, and concrete is no exception. When acid attacks concrete, it concentrates on its products of hydration. For example, calcium silicate hydrate can be adversely affected by exposure to acid. Sulfuric acid works to weaken the concrete and if it is able to reach the steel reinforcing members, the steel can be compromised. All of this contributes to a failing concrete structure.

Visual inspections may reveal a loss of cement paste and aggregate from the matrix. Crack- ing, spalling, and discolouration can be expected when acid deteriorates steel reinforcements, and laboratory analysis may be needed to identify the type of chemical causing the damage.

 

Code Consideration

It is a code violation to embed aluminium conduits and pipes in concrete unless the aluminium is coated or covered to prevent aluminium–concrete reaction or electrolytic action between aluminium and steel.

 

How can you create a more defensive concrete where chemical reactions are anticipated? Portland cement concrete does not fare well when exposed to acid. When faced with this type of concrete, an approved coating or treatment is about the best that you can do. Using dense concrete with a low water-cement ratio can provide acceptable protection against mild acid exposure.

 

Aggressive Water

Aggressive water is water with a low concentration of dissolved minerals. Soft water is considered aggressive water and it will leach calcium from cement paste or aggregates. This is not common in the United States. When this type of attack occurs, however, it is a slow process. The danger is greater in flowing waters because a fresh supply of aggressive water continually comes into contact with the concrete.

If you conduct a visual inspection and find rough concrete where the paste has been leached away, it could be an aggressive-water defect. Water can be tested to determine if water quality is such that it may be responsible for the damage. When testing indicates that water may create problems prior to construction, a non-Portland-cement-based coating can be applied to the exposed concrete structures.

 

There are many factors that can contribute to problems with a concrete installation. The table below outlines examples of such problems.

1.   Accidental loadings

2.   Chemical reactions

·       Acid attack

·       Aggressive-water attack

·       Alkali-carbonate rock reaction

·       Alkali-silica reaction

·       Miscellaneous chemical attack

·       Sulfate attack

4.   Construction errors

5.   Corrosion of embedded metals

6.   Design errors

·       Inadequate structural design

·       Poor design details

7.   Erosion

·       Abrasion

·       Cavitation

8.   Freezing and thawing

9.   Settlement and movement

10.       Shrinkage

·       Plastic

·       Drying

11.       Temperature changes

·       Internally generated

·       Externally generated

·       Fire

12.       Weathering

 

Alkali-Carbonate Rock Reaction

Alkali-carbonate rock reaction can result in damage to concrete, but it can also be beneficial. Our focus is on the destructive side of this action, which occurs when impure dolomitic aggregates exist. When this type of damage occurs, there is usually map or pattern cracking and the concrete can appear to be swelling.

Alkali-carbonate rock reaction differs from alkali-silica reaction because there is a lack of silica gel exudations at cracks. A petrographic examination can be used to confirm the presence of an alkali-carbonate rock reaction. To prevent this type of problem, contractors should avoid using aggregates that are or are suspected to be, reactive.

 

Code Consideration

Conduits embedded in concrete must be spaced a minimum distance that equals not less than three times the diameter of the conduit being installed.

 

Alkali-Silica Reaction

An alkali-silica reaction can occur when aggregates containing silica that is soluble in highly alkaline solutions may react to form a solid, nonexpansive, calcium-alkali-silica complex or an alkali-silica complex that can absorb considerable amounts of water and expand. This can be disruptive to concrete.

Concrete that shows map or pattern cracking and a general appearance of swelling could be a result of an alkali-silica reaction. This can be avoided by using concrete that contains less than 0.60% alkali.

 

Various Chemical Attacks

Concrete is fairly resistant to chemical attacks. For a substantial chemical attack to have degrading effects of a measurable nature, a high concentration of the chemical is required. Solid dry chemicals are rarely a risk to concrete. Chemicals that are circulated in contact with concrete do the most damage.

When concrete is subjected to aggressive solutions under positive differential pressure, the concrete is particularly vulnerable. The pressure can force aggressive solutions into the matrix. Any concentration of salt can create problems for concrete structures. Temperature plays a role in concrete destruction with some chemical attacks. Dense concrete that has a low water-cement ratio provides the greatest resistance. The application of an approved coating is another potential option for avoiding various chemical attacks.

 

Alkali-Silica Reaction

An alkali-silica reaction can occur when aggregates containing silica that is soluble in highly alkaline solutions may react to form a solid, nonexpansive, calcium-alkali-silica complex or an alkali-silica complex that can absorb considerable amounts of water and expand. This can be disruptive to concrete.

Concrete that shows map or pattern cracking and a general appearance of swelling could be a result of an alkali-silica reaction. This can be avoided by using concrete that contains less than 0.60% alkali.

 

Various Chemical Attacks

Concrete is fairly resistant to chemical attack. For a substantial chemical attack to have degrading effects of a measurable nature, a high concentration of the chemical is required. Solid dry chemicals are rarely a risk to concrete. Chemicals that are circulated in contact with concrete do the most damage.

When concrete is subjected to aggressive solutions under positive differential pressure, the concrete is particularly vulnerable. The pressure can force aggressive solutions into the matrix. Any concentration of salt can create problems for concrete structures. Temperature plays a role in concrete destruction with some chemical attacks. Dense concrete that has a low water-cement ratio provides the greatest resistance. The application of an approved coating is another potential option for avoiding various chemical attacks.

 

Code Consideration

When concrete joints are created, they must be located in a manner that will not have an adverse effect on the strength of the concrete in which they are installed.

 

Sulfate Situations

A sulfate attack on concrete can occur from naturally occurring sulfates of sodium, potassium, calcium, or magnesium. These elements can be found in soil or in groundwater. Sulfate ions in solution will attack concrete. Free calcium hydroxide reacts with sulfate to form calcium sulfate, also known as gypsum. When gypsum combines with hydrated calcium aluminate it forms calcium sulfoaluminate. Either reaction can result in an increase in volume. Additionally, a purely physical phenomenon occurs where the growth of crystals of sulfate salts disrupts the concrete. Map and pattern cracking are signs of a sulfate attack. General disintegration of concrete is also a signal of the occurrence.

Sulfate attacks can be prevented with the use of dense, high-quality concrete that has a low water-cement ratio. A Type V or Type II cement is a good choice. If pozzolan is used, a laboratory evaluation should be done to establish the expected improvement in performance.

 

Code Consideration

Unless otherwise authorized, all bending of reinforcement material for concrete must be done while the material is cold.

 

Before you can correct problems with concrete installations, you must identify the problem. See the table below for reference material in completing this task.

Relating Symptoms to Causes of Distress and Deterioration of Concrete

 

Courtesy of United States Army Corps of Engineers

POOR WORKMANSHIP

Poor workmanship accounts for a number of concrete issues. It is simple enough to follow proper procedures, but there are always times when good practices are not employed. The solution to poor workmanship is to prevent it. This is much easier said than done. All sorts of problems can occur when quality workmanship is not assured and some of the key causes for these problems are as noted below:

• Adding too much water to concrete mixtures

• Poor alignment of formwork

• Improper consolidation

• Improper curing

• Improper location and installation of reinforcing steel members

• Movement of formwork

• Premature removal of shores or restores

• Settling of the concrete

• Settling of subgrade

• Vibration of freshly placed concrete

• Adding water to the surface of fresh concrete

• Miscalculating the timing for finishing concrete

• Adding a layer of concrete to an existing surface

• Use of a tamper

• Jointing

All concrete forms for the placement of concrete need to be inspected closely. This includes confirming that the forms are properly built and that all needed reinforcement is in place (see Figure below).

 

CORROSION

Corrosion of steel reinforcing members is a common cause of damage to concrete. Rust staining will often be present during a visual inspection if corrosion is at work. Cracks in concrete can tell a story. If they are running in straight lines, like parallel lines at uniform intervals that correspond with the spacing of steel reinforcement materials, corrosion is probably at the root of the problem. In time, spalling will occur. Eventually, the reinforcing material will become exposed to a visual inspection.

 

Code Consideration

Unless otherwise authorized, it is a code violation to weld crossing bars that will reinforce concrete.

 

Techniques for stopping, or controlling, corrosion include the use of concrete with low permeability. In addition, good workmanship is needed. Some tips to follow include:

• Use as low a concrete slump as practical.

• Cure the concrete properly.

• Provide adequate concrete cover over reinforcing material.

• Provide suitable drainage.

• Limit chlorides in the concrete mixture.

• Pay special attention to any protrusions, such as bolts and anchors.

 

DESIGNER ERRORS

Designer errors are divided into two categories: those that are a result of inadequate structural design and those that are a result of a lack of attention to relatively minor design details. In the case of structural design errors, the result can be anticipated and will generally result in a structural failure.

Identifying structural design mistakes concentrates on two types of symptoms, spalling and cracking. Spalling indicates excessively high compressive stress. Cracking and spalling can also indicate high torsion or shear stresses. High-tensile stresses will cause cracks. Petro- graphic analysis and strength testing of concrete is required if any of the concrete elements are to be reused after such failures. The best prevention requires careful attention to detail. Design calculations should be checked thoroughly. Flaws in design details account for most of these types of problems. See the following list for examples of design factors to consider:

• Poor design details

• Abrupt changes in section

• Insufficient reinforcement at reentrant corners and openings

• Inadequate provision for deflection

• Inadequate provision for drainage

• Insufficient travel in expansion joints

• Incompatibility of materials

• Neglect of creep effect

• Rigid joints between precast units

• Unanticipated shear stresses in piers, columns, or abutments

• Inadequate joint spacing in slabs

 

ABRASION

Abrasion damage can occur from waterborne debris, which typically rolls and grinds against concrete when it is in the water and in contact with concrete structures. Spillway aprons, stilling basin slabs, and lock culverts and laterals are the most likely types of structures to be affected by abrasion. The reason for this is often a result of poor hydraulic design. Another cause for abrasion can be a boat hull hitting a concrete structure.

 

Code Consideration

When groups of reinforcing bars are bundled together as a reinforcement device for concrete, the bundle must not contain more than four bars.

 

When abrasion occurs, concrete structures tend to wind up with a smooth surface. Long, shallow grooves in a concrete surface and spalling along monolith joints indicate abrasion. The three major factors in avoiding abrasion damage are design, operation, and materials.

For prevention strategies, review the list of tips below:

• Use hydraulic model studies to test designs.

• A 45 degree fillet installed on the upstream side of the end sill results 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.

• Balanced flows should be maintained into basins by using all gates to avoid discharge conditions where eddy action is prevalent.

• Periodic inspections are needed to locate the presence of debris.

• Basins should be cleaned periodically.

• All materials used must be tested and evaluated.

• Install abrasion-resistant concrete.

• Fiber-reinforced concrete should not be used for repairing stilling basins or other hydraulic structures subject to abrasion.

• Coatings that produce good results against abrasion include polyurethanes, epoxy-resin mortar, furan-resin mortar, acrylic mortar, and iron aggregate toppings.

 

CAVITATION

Cavitation erosion is a result of complex flow characteristics of water over concrete sur- faces. For damage to occur, the rate of water flow normally needs to exceed 40 feet per second. Fast water and irregular surface areas of concrete can result in cavitation. Surface irregularity and water speed create bubbles. The bubbles are carried downstream and have a lowered vapour pressure. Once the bubbles reach a stretch of water that has normal pressure, the bubbles collapse. The collapse is an implosion that creates a shock wave. Once the shock wave reaches a concrete surface, the wave causes very high stress over a small area. When this process is repeated, pitting can occur. This type of cavitation has affected concrete spillways and outlet works of many high dams. Prevention has to do with design, materials, and construction practices. The following list highlights some of the key considerations:

• Include aeration in a hydraulic design.

• Use concrete designed with a low water-cement ratio.

• Use hard, dense aggregate particles.

• Steel-fiber concrete and polymer concrete can aid in the fight against cavitation.

• Neoprene and polyurethane coatings can assist in the fight against cavitation; however, coatings are rarely used as they might prevent the best adhesion to concrete. Any rip or tear in the coating can cause a complete stripping of the coating over time.

• Maintain approved construction practices.

 

Code Consideration

Bundle reinforcing bars must be enclosed within either stirrups or ties.

 

FREEZING AND THAWING

Freezing and thawing during the curing of concrete is a serious concern. Each time the concrete freezes, it expands. Hydraulic structures are especially vulnerable to this type of damage. Fluctuating water levels and under-spraying conditions increase the risk. Using deicing chemicals can accelerate damage to concrete with resultant pitting and scaling. Core samples will probably be needed to assess the damage.

Prevention is the best cure. Provide adequate drainage, where possible, and work with low water–cement ratio concrete. Use adequate entrained air to provide suitable air-void systems in the concrete. Select aggregates best suited for the application, and make sure that concrete cures properly.

 

SETTLEMENT AND MOVEMENT

Settlement and movement can be the result of differential movement or subsidence. Concrete is rigid and cannot stand much differential movement. When it occurs, stress cracks and spall are likely to occur. Subsidence causes entire structures or single elements of entire structures to move. If subsidence is occurring, the concern is not cracking or spalling; the big risk is stability against overturning or sliding.

A failure via subsidence is generally related to a faulty foundation. Long-term consolidations, new loading conditions, and related faults are contributors to subsidence. Geotechnical investigations are often needed when subsidence is evident.

 

Cracking, spalling, misaligned members, and water leakage are all evidence of structure movement. Specialists are normally needed for these types of investigations.

 

Code Consideration

Spiral reinforcement for cast-in-place concrete must not be less than 3/8 in. in diameter.

 

SHRINKAGE

Shrinkage is caused when concrete has a deficient moisture content. It can occur while the concrete is set or after it is set. When this condition happens during setting, it is called plastic shrinkage; drying shrinkage happens after the concrete has been set.

Plastic shrinkage is associated with bleeding, which is the appearance of moisture on the surface of the concrete. This is usually caused by the settling of heavier components in a mixture. Bleed water typically evaporates slowly from the surface of concrete. When evaporation occurs faster than water is supplied to the surface by bleeding, high-tensile stresses can develop. This stress can lead to cracks on the concrete surface.

Cracks caused by plastic shrinkage usually occur within a few hours of concrete placement. These cracks are normally isolated and tend to be wide and shallow. Pattern cracks are not generally caused by plastic shrinkage.

Code Consideration

Spacing requirements for shrinkage and temperature reinforcement must be spaced no farther apart than five times a slab’s thickness and no farther apart than 18 in.

 

Weather conditions contribute to plastic shrinkage. If the conditions are expected to be conducive to plastic shrinkage, protect the pour site with windbreaks, tarps, and similar arrangements to prevent excessive evaporation. In the event that early cracks are discovered, revibration and refinishing can solve the immediate problem.

 

Drying shrinkage is a long-term change in the volume of concrete caused by the loss of moisture. A combination of this shrinkage and restraints will cause tensile stresses and lead to cracking. The cracks will be fine and the absence of any indication of movement will exist. The cracks are typically shallow and only a few inches apart. Look for a blocky pattern to the cracks. They can 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.

To reduce drying shrinkage, try the following precautions:

• Use less water in concrete.

• Use larger aggregate to minimize paste content.

• Use a low temperature to cure concrete.

• Dampen the subgrade and the concrete forms.

• Dampen aggregate if it is dry and absorbent.

• Provide adequate reinforcement.

• Provide adequate contraction joints.

 

FLUCTUATIONS IN TEMPERATURE

Fluctuations in temperature can affect shrinkage. The heat of hydration of cement in large placements can present problems. Climatic conditions involving heat also affect concrete; for example, fire damage, while rare, can also contribute to problems associated with excessive heat.

Code Consideration

The code allows one to assume that the ends of columns built integrally with a structure will remain fixed. This assumption is used while computing gravity load moments on columns

Hydration of concrete can raise the temperature of freshly placed concrete by up to 100 degrees. Rarely is the temperature increase consistent throughout the concrete, which can generate problems such as shallow and isolated cracks. How can this be avoided? See the tips below:

• Use low-heat cement.

• Pour concrete at the lowest reasonable temperature.

• Select aggregates with low moduli of elasticity and low coefficients of thermal expansion.

External temperature changes can result in cracking that will appear as regularly spaced cracks. There may be spalling at expansion joints. Using contraction and expansion joints can help prevent this damage.

There are many potential causes for concrete failure. Extended education, experience, and scientific testing is often required to clearly identify these causes. There is always more to learn. Keeping an open mind and immersing yourself in the components of concrete is the best route to success.

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