Oregon concrete foundation inspections.
Foundations are the backbone of your home or building. There are many reasons foundations will crack and become a long term expensive issue.
Most information available about concrete is written for contractors, for those who design concrete mixes, and for those who perform invasive testing. The information provided here is specifically for those who perform non-invasive, visual inspections of concrete, such as home inspectors or commercial property inspectors using the International Standards of Practice for Inspecting Commercial Properties.
In evaluating concrete problems, one of the important decisions inspectors must make is determining whether a problem is the result of conditions that have stabilized with a low chance of continuing future problems, or whether the conditions that caused the problem are such that there is a high probability that problems will continue or worsen.
Chemically, concrete is a complicated material, and a visual inspection will not always answer those questions. Basic knowledge of concrete mixes, installation, weathering, and the other factors that can affect how it ages, in addition to the illustrations and photo examples provided here, will give inspectors the best chance of making sound decisions and recommendations to their clients.
Constituent Material Properties
Concrete is a composite material composed of various constituent materials: a binder, usually cement, aggregates, and water.
Cement + water = cement paste
Cement paste + sand = mortar
Mortar + aggregate = concrete
Each of these constituent materials can vary in its chemical makeup and performance characteristics, depending on where it was quarried, and the manufacturing methods and conditions used to produce it.
Factors Affecting Concrete
Different factors can affect concrete and the problems that inspectors will see. How concrete hardens, strengthens and the qualities of its surface depend on a number of things, including the properties of its constituent materials.
Although Portland cement is the most commonly used binder, pozzolans may be substituted. Pozzolans are materials that, in addition to undergoing primary hydration, undergo a secondary hydration, producing a gel that fills tiny voids between cement particles, making concrete less porous and less likely to absorb moisture or chemical solutions that can damage concrete or steel reinforcement.
Because aggregates are quarried locally, their texture, weight, strength, and absorptive and reactive properties vary, and this variation can affect the concrete’s properties.
The chemical composition of water can affect concrete, but more important is the ratio of water-to-cement paste used in the mix. Cement needs only about 25% water for hydration to take place. To improve its workability, the ratio is commonly increased to around 45%. Increasing the ratio of water makes concrete more porous; as this extra water rises to the surface as bleed water and through evaporative drying, it leaves behind capillary voids. These capillary voids weaken concrete and make it more absorbent, increasing the chances of freeze damage and attack from liquid chemicals. It is not uncommon for the water-to-cement ratio to be well over 45%. If concrete starts to harden before it’s placed, water is sometimes added at the job site to maintain workability.
The constituent materials which are included in the mix, their proportions, the order in which they are combined, the length of time and method by which they are mixed, and the length of time between mixing to placing all affect the quality of concrete. With each decision and operation, there is a chance that mistakes will be made.
The environmental conditions that exist during placing, finishing and curing concrete will have an effect on how it develops. The ground and air temperatures, wind speed, cloud cover, and the absorbent qualities of the substrate will affect newly placed concrete.
The quality of workmanship can have an effect. Here are some common mistakes:
inadequate cover of reinforcement steel. Inadequate cover can lower steel’s resistance to corrosion. This is a very common problem;
incomplete consolidation. Concrete is usually consolidated with a mechanical vibrator. Concrete being placed in forms should be placed in layers, with each layer being vibrated when it is placed. Placing too much concrete at any one area at a time, or failing to vibrate concrete adequately can result in incomplete consolidation, causing a honeycomb pattern;
Honeycombing due to poor consolidation
creating cold joints. Cold joints are created when concrete is poured against concrete that has already hardened to some degree. This condition results in a weakened bond between the existing and the newly poured concrete;
A cold joint in concrete
finishing the surface too soon. Troweling while the bleed water is still on the surface forces water back into the surface layer of the concrete. This increases the water-to-cement ratio and results in a concrete surface that is overly porous and of substandard durability; and
improper curing methods or lack of curing. Improper curing can encourage cracking and reduce strength development.
The advantage to inspectors in being able to accurately determine the source of cracking is in understanding whether the condition that caused the cracking has stabilized so that it is no longer likely to cause additional cracking or encourage the propagation of existing cracks. Many cracks, like those caused by concrete shrinkage, are shallow cracks caused by forces that allow conditions to stabilize relatively quickly and do not lead to structural problems. Others, like those caused by soil subsidence or changes in soil volume, are caused by forces that can continue to affect concrete for a long time. This long-term instability can continue to cause serious structural problems over the long term.
The cause of cracking can be related to:
the properties of the constituent materials;
the design mix;
the surrounding environment in which concrete is installed;
mixing, placement, finishing and curing practices;
the type of use; and
Cracks that appear before the concrete has hardened are called plastic cracks. Plastic cracks are typically due to poor mix design, placement practices or curing methods, and may also be caused by settlement, construction movement, and excessively high rates of evaporation. Cracks that appear after concrete has hardened can have a variety of causes, and sometimes have more than one cause. The table below classifies cracks according to the length of time it takes for them to appear.
CLASSIFICATION OF CRACKS
Type of Cracking
Form of Crack
Time of Appearance
The chart below classifies cracks according to whether they appear before or after hardening.
1. Formwork Movement
Relatively small movements of formwork in the early stages of hardening will cause cracks.
2. Sub-Grade Movement
Movement of the sub-grade, such as settling or heaving, can crack concrete. This may be caused by changes in soil volume in response to changes in the soil’s moisture content, or it may be caused by subsidence. Subsidence is settling that can have a number of causes. Sub-surface mining, extraction of natural gas, the dissolution of limestone or conditions related to groundwater can all cause soil to settle. An example is when groundwater dissolves the carbonate cement holding sandstone particles together, and then carries away the particles, creating a void in the soil.
This map shows areas in the U.S. where subsidence can be a problem.
3. Plastic Shrinkage
Plastic shrinkage is caused by excessively high rates of evaporation from the surface of the concrete before it has hardened. For hydration to take place, a ratio of only about 25% water-to-cement is needed. To improve workability, water is often added to around 45%. This surplus water forces cement particles apart, suspending them in water. Once the concrete has been placed, the heavier aggregate particles settle, and the weight of the mix forces excess water to the surface. This excess water is called bleed water. Once the bleed water has evaporated from the surface, the concrete will still be wet and the surplus water will continue to dry upward through evaporation from the surface, and downward through absorption by the sub-grade, unless the concrete is installed directly on a vapor barrier, such as plastic, in which case, all drying will be upward.
Drying does not take place at an even rate throughout the concrete matrix. It happens most rapidly at the upper surface through evaporation and more slowly at the bottom through absorption. The rate at which the sub-grade absorbs water will depend on its absorbent properties, including its porosity, moisture content, and even its electro-chemical properties.
Resistance to Shrinkage
As water evaporates from the exposed surface and is absorbed by the sub-grade, capillary force pulls water from the voids between the cement particles in the main body of the matrix, and the concrete will continue to shrink. Differential drying and shrinkage rates between concrete at the surface and the underlying concrete create tensile stresses. This phenomenon is called resistance to shrinkage, and this resistance, caused by moisture surface tension in capillaries in the concrete, called capillary stress, can exceed 400 pounds per square inch (psi) in normal concrete. In high-strength concrete, capillary stress can exceed 600 psi. The tensile stress created by resistance to shrinkage is relieved by cracking. Greater differences in shrinkage rates will create greater tensile stresses, with the increased likelihood of cracking. Hot and windy conditions increase surface evaporation, so the concrete is more likely to crack than if it’s placed during cool, calm and cloudy conditions.
Resistance to shrinkage
The illustration above shows the underlying layer of concrete (B) that is slower to dry and offers resistance to shrinkage. In the image on the left, a vapor barrier is installed directly beneath the concrete so that all drying must take place from the upper surface. On the right, the lack of a vapor barrier allows drying both to the air and to the sub-grade, reducing the capillary force along AB, which reduces the resistance to shrinkage and makes concrete at the surface less likely to crack.
4. Plastic Settlement
Plastic settlement can be caused by subsidence around rebar. This is sometimes related to excessive water in the mix.
As soon as concrete is placed, it begins to consolidate, and as bleed water rises to the surface, cement particles consolidate and the concrete settles. The settling process is restrained by reinforcement steel and, under certain circumstances, cracks will develop directly above the steel. This condition is best recognized by the crack pattern, which typically reflects the even spacing of the reinforcement steel.
Repeating pattern of cracks where rebar cross-members connect longitudinal rebar
Plastic settlement cracks can be caused by:
inadequate concrete cover of the steel;
excessive water in the concrete;
form movement or blowout; or
poor consolidation practices.
5. Autogenous Shrinkage
Plastic shrinkage is shrinkage caused by the loss of water to the atmosphere. Autogenous shrinkage is shrinkage that takes place with no loss of water to the atmosphere. Autogenous shrinkage is caused by internal drying, with water being absorbed by the constituent materials in the concrete. As the long-term chemical hydration process continues – and it can continue for many years -- water in the pores within the cement paste is absorbed, and the pores are filled, to some degree, by materials produced during hydration. This process leads to decreased permeability and increased strength and durability of the cement paste. Absorption of water from the pores also causes shrinkage.
Since there is no loss of water to one exposed surface, autogenous shrinkage is more uniform than plastic shrinkage. However, tensile stresses still develop, and embedded steel can cause anomalies in an area of concrete with relatively uniform stress. These anomalies can cause variations in stress within the concrete that are relieved by cracking. Autogenous shrinkage cracking will be shallow and is not a structural issue. The cracks may look similar to those formed during plastic shrinkage and are often propagations of cracks created during plastic shrinkage.
6. Premature Freezing
Concrete is porous; it can absorb water. When absorbed water freezes, it expands, and if no method is used to accommodate the expansion, it can lead to cracking, or flakes of concrete may break loose and separate from the surface. This is especially true if absorbed moisture freezes before the concrete has aged enough to gain significant strength. Concrete should be kept warm until it has had a chance to harden adequately. The appearance of concrete damaged by premature freezing will vary with environmental conditions, but it often appears as widespread spalling or delamination of the surface layer, since cracking is often parallel to the surface.
7. Scaling and Crazing
Scaling is the shedding of flakes of hardened concrete at the surface. It can be caused by a number of conditions:
Exposure to freezing and thawing can cause scaling, which can be made worse by the application of de-icing salts.
Concrete that has been improperly cured or that has inadequate air entrainment will be less resistant to scaling caused by freezing.
Finishing operations started while bleed water is still on the surface can weaken the surface layer and cause dusting or scaling. When concrete is placed during hot and dry conditions, the bleed water may appear to be gone, but the surface may still be actively bleeding. The bleed water may be evaporating as it reaches the surface. During such conditions, finishing operations may be started under the mistaken impression that the surface is done bleeding.
Over-working the surface during finishing will reduce the air content of the surface concrete, leaving it weaker and more vulnerable to scaling due to freezing conditions.
Fertilizers, such as ammonium sulfate and ammonium nitrate, will chemically attack the concrete surface.
Poor drainage causes water to pool, and water containing de-icing salts can also lead to pooling on the surface for extended periods of time. This can happen where snowplows pile snow on sidewalks and driveways.
Smaller patches of damage have expanded to form a large area of damage.
In addition to properly timing finishing operations, proper curing will help prevent dusting and scaling. In hot and dry environments, the sub-grade should be dampened before the concrete is placed, and the surface should be kept damp to keep it from drying too quickly. In cool and damp environments, a water-repelling sealer should be applied to keep the surface from absorbing too much water. Concrete is most fragile during the first year after placing, so de-icing chemicals should be avoided during that time, and the concrete should be protected from absorbing moisture just before freezing weather develops.
During inspections, the most common places to find this type of damage are driveways and garage floors. Even if ice-melt is not used on walkways by a home’s occupant, the undercarriage of vehicles can accumulate frozen slush from roadways that contains chloride solutions. This slush will melt from cars parked in driveways and garages and be absorbed into the concrete. Poor finishing practices, such as over-working the concrete or working bleed water back into the surface, will leave the surface weak and more likely to flake. Concrete less than a year old that may be exposed to chlorides should have a sealer applied that is designed specifically for concrete to help prevent freeze damage. Sealers may have to be re-applied periodically, depending on the type of chemicals used on the roadways in a given area, as well as the climate zone. It is sometimes possible to remove a weak or damaged surface layer of concrete and apply a thin, bonded re-surfacing product based on Portland cement, latex-modified concrete or polymer-modified mortar. Inspectors can recommend that their clients investigate products or methods that have been used successfully in the area where the home is located.
In the photo above, the flake has detached from the surface of the footing, which was caused by the expansion of a pocket of minerals in the concrete. Negative grade around the home caused the soil to become saturated. Moisture eventually diffused through the porous concrete, eventually reaching the expansive minerals. The clue that this was caused by moisture is the white mineral salts, called efflorescence.
Pattern cracking, also called map cracking and craze cracking, appears as a network of random cracking on the concrete’s surface. The cracking is usually shallow (less than 1/8-inch deep) and not a structural issue. It’s seldom a durability problem but more of a cosmetic one.
The area enclosed by pattern cracking may be anywhere from 1/2-inch to 4 inches across.
Pattern cracking can be caused by the following:
poor curing practices. Environmental conditions such as low humidity, high outside temperatures, direct sunlight, and wind can create high rates of evaporation from the surface layer of concrete. Resistance to shrinkage from the underlying concrete causes stress that is relieved by craze cracking;
excessive water in the mix;
over-vibration of the concrete, causing coarse aggregate to settle and cement paste to concentrate at the surface;
over-working the surface with a steel trowel during finishing;
performing finishing operations while bleed water is still on the surface; and
sprinkling cement dust on the surface to soak up bleed water.
8. Drying Shrinkage
Drying shrinkage is shrinkage that takes place after the concrete has hardened and some degree of bonding has developed between the cement paste and the aggregate. As concrete continues to dry, it will continue to shrink. Drying shrinkage includes both shrinkage that takes place while losing moisture to the air and autogenous shrinking, as noted previously. The cracks will look similar to those formed during plastic shrinkage. Cracking during drying shrinkage may be the propagation of cracks that initially developed during plastic shrinkage.
Drying shrinkage cracks typically extend across the face of a wall or across pavement, which is called transverse cracking.
Wall shrinkage cracks are often diagonal and do not extend to the corners.
They are typically shallow and linear, although such cracks are not always continuous.
Notice the lack of control joints in this shrinkage crack.
A typical interruption in a shrinkage crack
Transverse shrinkage cracks appear where the tensile strength of concrete is lowest, such as where concrete is thinner at a control joint, or across an area of concrete with cracks on either side. Shrinkage cracks propagate from the tips, so concrete adjacent to the termination of a shrinkage crack is under more tensile stress than concrete in other parts of a slab.
A transverse shrinkage crack in pavement extends between cracks located in control joints.
In the photo above, areas A and B were the points at which the highest stress developed due to adjacent cracking in the control joints, so that is where the walkway panel split first, following the path of greatest stress. As the slab continued to shrink, the two cracks connected with each other to form a single transverse crack.
Another type of configuration where stresses are concentrated is where concrete forms a re-entrant corner.
A re-entrant corner crack
A re-entrant corner with properly placed control joints
This close-up of the corner shows cracking taking place in the control joints, as it should.
Re-entrant corners are high-stress areas prone to transverse cracking from plastic shrinkage. A re-entrant corner is where any inside corner forms an angle of less than 180 degrees into the body of the slab. As the concrete dries and shrinks, the wedge shape of the re-entrant corner encourages concrete to crack off the point of the angle into the slab.
Re-entrant cracking at the apex of a radiused re-entrant corner
The photo above shows concrete cracking off the apex of a radiused re-entrant corner, even though there is a control joint located nearby.
Red areas show approximate locations of stress concentrations
This image shows how steep stress gradients can be, or how concentrated stresses can be according to shape. If the stress gradient were shallower, the stress at the apex would be less concentrated and would have been relieved by cracking at the control joint.
9. Thermal Cracking
Variations in the temperature of concrete cause it to expand and contract. Significant differences in temperature between the outer and inner portions can cause concrete to crack. Especially when concrete is fairly new and has not gained much strength and is in an expanded condition from the heat generated during hydration, as it cools, cracks can develop. Cracking can be stimulated by temperature gradients that may be the result of differences in thickness, such as when the exterior cools faster than the interior, although this is more typical in massive structures. It may be caused by environmental conditions, such as cold weather. The forces at work are similar to resistance to shrinkage.
Although we may think of concrete as being brittle, it will actually bend instead of breaking under certain conditions. Concrete creep is the deformation of a concrete structure under sustained load. Long-term pressure or stress on concrete can make it change shape. This deformation typically happens in the direction in which the force is applied. Inspectors are most likely to see creep in retaining walls, or in foundation walls that are not reinforced by a floor structure. Creep does not necessarily cause concrete to fail. The photos below show creep and cracking caused by long-term pressure from tree roots.
Concrete in high-humidity environments, such as damp basements and crawlspaces, will experience accelerated rates of deformation compared to similar concrete in low-humidity conditions.
11. Design Load and Overload
Cracking can be the result of a design that is inadequate for a particular type of load. Cracking may be caused by the failure of a foundation to resist the weight of water-saturated soil, as in the illustrations below.
Lateral pressure resulting in a vertical crack
The photos below show a garage built into a hillside. Cracking along the back wall of the garage is the result of the concrete’s inability to resist lateral forces from the soil. The force may have been created by machinery during the original backfill operations, or it may have resulted over time from the weight of water-saturated soil. The correction was to raise a section of the end wall to help resist rotation, and to install a steel beam as a strongback. Inspectors should not recommend specific corrections but should recommend that a structural engineer be consulted to design corrective measures.
A steel I-beam acts as a strongback. (The top flange of the beam is facing the camera).
12. Design and Sub-Grade
Concrete foundations must be designed in such a way that they resist forces created by problems with the sub-grade, including settling and heaving.
There are two types of settling: compaction and consolidation.
Compaction is the displacement of air-filled voids between soil particles. Inadequately compacted soil will settle as the weight of the structure presses the particles together. This process continues until the force required to further compact the soil exceeds the force placed upon the soil by the structural load – in other words, until the state of compaction reaches equilibrium with the load, and the condition becomes stable. The time required to reach stabilization varies with soil type and its characteristics.
Vertical displacement occurred as the lower end of the walkway settled.
The crack shown in the photos above was caused by inadequate compaction. If shrinkage were the cause, the crack would have appeared in the control joint. Another clue is the vertical displacement that appeared as the lower end of the walkway settled.
Hillside lots are sometimes created by cutting into the hillside and using the soil that’s removed from the uphill side as fill on the low side. Poor compaction can cause cracking of the foundation and the interior and exterior wall coverin
Settling caused by inadequate compaction during cut and fill
Poor soil consolidation is another cause of settling. Consolidation is the displacement of water-filled voids between soil particles. Settling from consolidation may take longer to reach equilibrium than settling caused by compaction, since water has greater resistance to moving through soil than air.
Settling may be uniform, or it may occur at different rates at different parts of a foundation. Although sandy and coarse-grain soils settle faster than fine-grain soils such as clay, the soil beneath a home often varies in composition. Settling rates can vary not only with the type and characteristics of the soil, but also with the loads carried by and the configuration of the foundation footings. The illustration below shows how poor compaction or consolidation, or soil with poor load-bearing capacity, can cause settling that damages foundations.
In the photo above, a column transferred the roof load to the soil, which settled under the weight. Portions of the wall that bear no roof load settled less. Note that the cracking is slightly wider at the top.
Heaving may be caused by clay-based soils that expand in volume when they absorb water. These soils, called expansive soils, can damage foundations and concrete floor slabs through uplift. Soils capable of damaging foundations may contain as little as 5% of the active mineral, and may exert as much as 5,500 pounds per square foot (psf) of pressure against the concrete. Special methods must be used when building foundations on expansive soil in order to avoid future damage as the soil expands. One method is to rest the foundation on piers that extend down below the zone of water-content fluctuation, where the soil is stable, or to bedrock.
In the photo above, holes extending down to the bedrock at the corners and mid-spans of the walls will be filled with reinforced concrete to form piers. The top 3 feet of rebar are visibly protruding from the holes. Cardboard void forms placed on the ground between the piers and inside the forms are strong enough to bear the weight of the concrete when it is poured, creating voids beneath the walls. If the soil heaves, the void forms will be crushed. The amount of heaving will have to be greater than the thickness of the void forms (typically, 6 inches) for the soil to come into contact with the bottom of the foundation walls. Inspectors may be able to verify that a drilled-pier foundation has been installed but they will not be able to confirm the depth to which the piers were drilled, so they should observe carefully for signs of foundation movement.
The doubled bottom plate accommodates heaving.
Another method used to accommodate heaving is the use of doubled bottom plates. When inspecting a home in an area known to have expansive soil, inspectors should identify basement walls framed without this method as defective. In a newer home inspectors should recommend that correction be made. If walls have been in place for a significant period of time with no damage occurring, this condition will probably not be a problem unless something happens that introduces water to the soil beneath the basement floor. This might be a leak from plumbing pipes, valves or fixtures, or leaking of laundry equipment. Inspectors should state this in their reports.
A doubled bottom plate connected to a wall with a single bottom plate
The photo above shows a wall with doubled bottom plates connected to a wall with single bottom plates. Heaving will damage the connection between the two walls, and the walls with the single bottom plates may transfer the force of the heaving to the home’s structure above. This should be identified as a defect in the inspection report. If the walls have been in place for many years, the chances are good that unless changes occur that introduce moisture to the soil beneath the slab, such as plumbing leaks, no problem will result, and this should be mentioned in the report. In newer homes, this condition may cause problems, and that should be stated in the report.
Cracks caused by heaving may be wider at the top, as shown in the illustration above, but this condition can also be caused by settling toward one corner.
Vertical displacement creates trip hazards and you may see concrete ground down to alleviate this problem, as in the photo above. Inspectors who observe these types of trip hazards should recommend correction. In areas with expansive soils, inspectors need to be diligent in identifying negative and neutral grade around the foundation and recommending mitigation. Typical mitigation involves correcting the grade and/or installing a plastic membrane beneath the topsoil around the home’s perimeter to act as a barrier to runoff seeping down next to the foundation.
A swimming pool located close to a home’s foundation can saturate the soil. Water diffusing through the air-blown mortar (gunite) wall of a swimming pool introduces moisture into the soil. In most cases, the pool’s walls are not designed to resist the swelling forces created by expanding soil. If the pool is installed near the home’s foundation, the resulting pressure can damage the pool’s structure and the home’s structure.
Soils may also heave when water-saturated soil freezes.
Tree roots may also cause concrete to heave. As shown in the photo below, it was less expensive to grind off the protruding portion than to demolish and replace the walkway.
Expansive soil expands when it absorbs water, but it also shrinks as it loses water. This process can leave voids that can reduce the soil’s ability to support a structural load. When foundations are supported by piers that depend on friction to support their structural loads, shrinking soil can cause a loss of friction, reducing the foundation’s ability to support the structure and allowing foundation settlement.
Concrete fatigue is long-term failure due to repeated loading (cycles). Loads may be applied in tension, compression, torsion (twisting), bending, or a combination of these actions. In fatigue, each individual load is less than a single static or unchanging load that would exceed the strength of the concrete. Fatigue has to do with the development and propagation or growth of cracks and micro-cracks, and the failure of the bond between the cement paste and aggregate. Crack development and bond failure worsen slowly over time and are influenced by a number of variables. Fatigue is not something most inspectors will be able to identify and is less common in residential construction than in commercial and especially industrial structures. An in-depth discussion of fatigue requires engineering expertise, and recognizing or diagnosing it lies beyond InterNACHI’s Standards of Practice.
14. AAR, ASR and DEF
Ideally, aggregates used in concrete would be chemically inert. But because aggregates are quarried locally, there is sometimes little choice in the types of aggregate that are available for use in concrete. Some types of aggregate react chemically with alkali hydroxides in cement to produce a gel. When this gel is exposed to moisture, it expands, and this expansion can crack concrete. There are two types of alkali-aggregate reactions (AAR): alkali-silica reaction (ASR), and alkali-carbonate reaction (ACR).
ASR is of more concern because aggregates containing reactive silica materials are more common. Inspectors will not be able to conclusively identify damage from ASR visually. It can be identified through petrographic analysis, which is examination by a petrographer using a special microscope. Petrography is the branch of geology that concerns the study of rocks.
Exudation is a liquid or viscous, gel-like material discharged through a pore, crack or opening in the surface of concrete. The presence of exudation is a strong indication that cracking has been caused by ASR.
This condition may be caused by ASR, but only laboratory testing can confirm this.
Inspectors do not need to identify the source of the condition – only its severity.
The recommendation here would be to consult with a concrete contractor
to gain an understanding of options and cost for replacement.
Although this cracking looks similar to pattern cracking, the fact that it graduates in intensity,
starting at the control joint, indicates that absorption from moisture accumulating in the control joint is the cause. Pattern cracking would be fairly uniform across the surface.
Delayed ettringite formation (DEF) is a condition that describes a chemical reaction caused by curing at excessively high temperatures, resulting in gaps and cracks that form around the aggregate, weakening the concrete. This is another condition that inspectors will not be able to identify visually.
15. Steel Corrosion
Corrosion of embedded steel is the most common cause of concrete problems. As steel corrodes, the corrosion product expands, and this expansion can crack concrete and cause sections to break loose in flakes. Steel is typically protected by a thin film that develops as part of a chemical reaction with cement. This coating, called passivity, lowers the rate of corrosion to a point at which it becomes insignificant. Several conditions can damage the passive coating and dramatically increase the rate at which reinforcement steel corrodes.
Loss of Alkalinity
In order for concrete to sustain passivity, it must maintain a level of alkalinity between 9.5 and 13. If the alkalinity drops below the required level, the rate at which embedded steel corrodes will increase significantly. The pH of concrete can be lowered by either of two processes:
1. carbonation, in which acidic gases, such as carbon dioxide (CO2), can be absorbed into the concrete; and
2. leaching by water.
Since most concrete is porous, CO2 will be slowly absorbed by the surface and find its way deeper into the concrete. Also, water will slowly diffuse through it. If steel has inadequate concrete cover, usually 1-1/2 inches minimum, the pH of the concrete encasing the steel may be lowered to the point at which the passive layer is damaged or destroyed and the rate of the steel’s corrosion may increase dramatically.
Another way in which CO2 or water may reach the concrete surrounding steel is through cracks caused by excessive structural loading, shrinkage, plastic settlement, or other reasons. Inadequate cover is especially common in the risers of concrete stairs, as shown in the photos below.
Inadequate cover due to control joints is also common in flatwork. Where control joints are installed perpendicular to embedded rebar, the areas at which the control joint crosses each bar may develop cracking and corrosion, and eventually spalling or flaking. This common condition is called D-cracking and is shown in the photo below.
Depassivation by Chlorides
Chloride solutions commonly used to keep pavements ice-free can destroy the passive layer that protects steel. They may reach it by liquid diffusion through porous concrete or through cracks.
Contamination by Atmospheric Pollution
Reinforcement steel may be contaminated by chlorides before it is embedded in concrete, during either transportation or storage. In addition to de-icing solutions, salt air in coastal environments can contaminate exposed steel.
Galvanic corrosion occurs when two dissimilar metals are in contact with each other in the presence of moisture. This can happen when aluminum conduit or copper wire is in contact with steel.
Plastic Settlement Cracking
Plastic settling can cause cracking above steel reinforcement that can allow corrosive agents, such as water and chloride solutions, to reach and corrode steel. Cracks resulting from plastic settlement do not always cause corrosion. If reinforcement steel has adequate coverage, corrosive agents will not reach it. A closer look (below) shows discoloration that has developed along the resulting settlement crack.
If discoloration above a settling crack were caused by corroding rebar, it would be rust-colored. The discoloration in the photo above is most likely caused by reactive aggregate. Discoloration will spread and cracking will worsen slowly.
16. Freeze-Thaw Cycling
Again, as moisture is absorbed by concrete, it may freeze and expand, creating damage that often appears as spalling or surface delamination. Air entrainment is used to solve this problem. Air entrainment creates tiny voids spaced closely together in the concrete. As water turns to ice, it expands into these voids, reducing internal pressure.
Inspectors should look for spalling damage from a lack of air entrainment at projects where the concrete was mixed on-site, or if the mix was designed by someone with an inadequate knowledge of concrete.
There are two types of sulfate attack: internal and external. External attack is more common.
External Sulfate Attack
External sulfate attack occurs when water containing dissolved sulfates is absorbed into porous concrete. A chemical reaction takes place that can vary in severity and may appear as:
bond failure between the aggregate and the cement paste; and/or
a chemical alteration of the cement paste’s composition.
The purpose of control joints is to help control the location of shrinkage cracks. Control joints weaken concrete by making it thinner. Since concrete will crack where stresses are highest and the concrete is weakest, control joints should be installed at high-stress areas. Whether or not the joints are placed correctly depends on how well the person placing them understands where stresses are likely to develop.
The inspector’s main concerns with control joints are over-spaced joints, and especially a lack of joints. The spacing of control joints is the responsibility of the designer and can vary with:
slab or wall design;
slab or wall thickness;
type, amount and location of steel reinforcement;
shrinkage potential of the concrete;
floor or wall slab restraints (embedded steel or block-outs for windows);
environmental factors (temperature, wind, humidity, etc.); and
the method and quality of curing.
The ideal shape for concrete partitioned off by control joints is square, or rectangular not to exceed 1.5x1.
In general, the spacing should not exceed 30 times the thickness, not to exceed 15 feet. Joint depth should be one-quarter the thickness of the slab but not less than 1 inch. Some types of concrete are designed to maximize the spaces between control joints, but inspectors will not be able to identify these mixes visually, and if shrinkage cracking and control joint spacing both seem excessive, the cracking is probably related to spacing.
Curing is the process of controlling the temperature and rate at which moisture leaves the concrete while it’s at an early age. Controlling temperature and moisture is necessary for concrete to reach its design strength. Keeping concrete within an acceptable temperature window prevents freeze damage or damage from overheating.
Excessively hot concrete may develop thermal cracking if temperatures are not uniform throughout, since concrete will expand and contract at different rates according to temperature. Also, overly hot concrete will lose moisture quickly, which can cause excessive shrinking and cracking from resistance to shrinkage. In some cases, water necessary for hydration will evaporate. The following describe some curing methods.
ponding or immersion: Once the surface has hardened, it is flooded. Water evaporates from the surface of the flooded area instead of from the surface of the concrete. This minimizes shrinkage and cracking.
spraying or fogging: This is effective but water is lost to evaporation.
moisture-retaining fabrics: These work well. Burlap is often used.
impervious paper: This slows the rate of evaporation.
plastic sheets: These slow the rate of evaporation.
For home inspectors, the concern with concrete curing compounds is that they can interfere with the bond between concrete and stone or ceramic tile floor coverings when mortar-based adhesives are used. There are two basic types of curing compounds: permanent and temporary.
Permanent Curing Compounds
Permanent, moisture-impermeable curing compounds fill the pores in concrete, creating a smooth surface. Mortar forms a better mechanical bond to rougher surfaces. Complete removal of the permanent curing compound is the only way to ensure a successful installation when a Portland cement-based setting material is used. A permanent curing compound can be removed by mechanically scarifying the concrete’s surface to a depth below that which the curing compound has penetrated. Scarifying exposes the concrete’s pores. Mechanical scarification involves blasting the surface with materials such as shot, beads, high-pressure water, or abrasives.
Temporary Curing Compounds
Temporary curing compounds dissipate gradually, either through chemical reactions or by oxidation from exposure to the ultraviolet radiation in sunlight. This means that concrete treated with temporary curing compounds and dependent on UV exposure for dissipation must be left exposed to sunlight for an adequate amount of time.
Temporary curing compounds can also leave a residue that interferes with the bond between mortar and concrete. Mechanical abrasion can be used to get rid of this residue. Such mechanical abrasion methods include bush hammering, planers and grinders. Abrasion doesn’t penetrate the surface as deeply as scarification, so it may not be adequate. Depending on the curing product, scarification may also have to be used on temporary curing compounds. Acids used to remove curing compounds may interfere with the bond between mortar and concrete.
Curing/sealing compounds are applied for the same purpose as curing compounds: to reduce excessive water loss from the surface. But curing/sealing compounds have UV inhibitors and are designed to remain in place to help prevent the compounds from yellowing. A curing/sealing compound might be used if the concrete floor is to be left exposed. Some of these types of compounds have UV inhibitors that are only good for the short term, and concrete protected by these will eventually start to turn yellow. If an inspector sees yellowing on a concrete surface exposed to sunlight but not on a less-exposed surface, the use of an inappropriate curing/sealing compound may be the cause of the problem.
Surface sealers are applied to cured concrete and are designed to provide a protective finish. There are a number of choices in surface sealers, depending on whether the floor will need to resist chemical attack or abrasion, and whether aesthetics are important. Sealers applied over stains will magnify the stains. When a sealer is being re-applied over an existing sealer, problems can arise from chemical incompatibility. There are two types: film-forming and penetrating.
Film-forming sealers are typically acrylic resins, and the surface must be clean and dry for proper application. For penetrating sealers, all potential contaminants must be removed first, including:
form-release agents; and
Problems with Sealers
Inspectors may find a number of problems with solvent-based sealers, which often have a combination of sources:
Bubbles in the sealer are an indication that it was applied when the concrete was too hot, causing the solvent to evaporate on contact and form bubbles.
Peeling or flaking from the surface indicates poor adhesion that may be caused by:
surface contaminants, such as dust or dirt; or
too much water present on the surface of the concrete when the sealer was applied.
A white or frosted appearance can be caused by:
efflorescence on the surface of the concrete beneath the film;
high evaporation rates during application, typically caused by excessively high air or concrete temperatures, or wind;
moisture trapped between the concrete and sealer;
adhesion failure between the concrete surface and the sealer, or between multiple coats of sealer.
The formation of globs of sealer can be caused by excessively high air or concrete temperatures during sealer application.
Sticky or tacky sealer may be caused by:
grease, oil, solvents or other substances that attack the sealer;
plasticizing agents in rubber-backed mats or weatherstripping (like that found on garage doors) can leach out, softening the sealer.