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Concrete Condition Assessment

 

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Concrete Inspection, Testing, Coating Selection and Failure Analysis: 
Our highly experienced concrete specialists are available for onsite, laboratory testing and root cause determination for failures of concrete and concrete coatings. Our comprehensive technical reports are well structured and include photographic documentation, detailed test procedures, and results that will hold up in a court of law.

Our team of qualified materials and corrosion engineers, and concrete (petrographic) specialists have extensive experience evaluating concrete bridges, historic buildings, driveways, pole structures, foundations, floors, sea walls, and other asset type.  Factors such as erosion, fatigue, cycles of freezing and thawing, and adverse chemical reactions can have a deleterious effect on the performance of this material.  One of the most common problems in reinforced concrete is the corrosion of embedded metals.  The corrosion of steel in concrete is primarily caused by exposure to chloride contained in seawater, roadway de-icing chemicals, and soils.  Chloride contamination can also occur in new construction when using chloride-containing set accelerators, contaminated aggregate, or the use of non-potable mix water.  Call us for a quote to visit your project site or to speak with one of our experts.

Onsite Concrete Inspection and Testing

Onsite concrete inspection and condition assessment of reinforced concrete typically includes the following test protocol:

  • Review of background information.
  • Photographic documentation, site evaluation, environmental considerations.
  • Visual examination – to identify surface defects.
  • Hammer/chain – to detect delaminations.
  • Phenolphthalein – to determine depth of carbonation.
  • Chloride content – to identify chloride corrosion.
  • Half-cell potential mapping – to determine corrosion risk (map corrosion hot areas).
  • Linear polarization – to determine corrosion rate.
  • Continuity – to determine continuity / connectedness of rebar.
  • Stray current identification – to determine stray current corrosion risk.
  • Resistivity – to determine concrete resistivity and corrosion risk.

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If required, concrete core samples are marked and retrieved for petrographic analysis.  Cores are used for visual observation, compression testing, split tensile testing, verification of NDT and petrographic analysis. Core locations are selected on the basis of visual observations, hammer soundings and impact-echo testing. Concrete core samples can be shipped to Matergenics for a petrographic analysis.

Matergenics specializes in Petrographic Analysis, a review of the concrete matrix using microscopic techniques described in ASTM C856 to determine concrete constituents, quality, and cause of inferior performance, distress, or deterioration. Concrete is composed of sand, gravel, crushed rock, or other aggregates held together by a hardened paste of cement and water. Important properties of concrete are: durability (weather resistance, resistance to chemical deterioration, resistance to erosion), workability, water tightness, strength, elasticity, creep, extensibility, and thermal properties. Entrained air content, cement and water content and type, distribution and quality of aggregates are various factors that affect properties of concrete. Estimating future performance and structural safety of concrete elements can thus be facilitated.

        

 Petrographic analysis uses microscopic techniques described in ASTM C856 to determine the constituents of concrete, concrete  quality, and cause of inferior performance, distress, or deterioration.

There are several mechanisms that can lead to corrosion of embedded steel.  First, if the concrete cracks for any reason these cracks will often propagate towards the embedded steel resulting in direct access to the exterior environment of rain, snow, dirt, and wind borne contaminants. Second, as concrete ages it will chemically react with carbon dioxide in the atmosphere in a process known as carbonation. Carbonation results in a decrease in the alkalinity of the concrete such that passivation of the steel will no longer occur. Shallow depths of concrete cover over embedded steel will cause corrosion to occur at shorter time intervals. Also, high water cement ratios cause the concrete to be more porous leading to corrosion at shorter time intervals. And third, the presence of chlorides can lead to severe corrosion issues. As the steel corrodes the iron corrosion products have a greater volume than the metallic steel leading to internal pressures within the concrete which then causes cracking and spalling of the concrete above and adjacent to the corroded steel.

Based on the findings of the onsite inspection, condition assessment and laboratory petrographic analysis, an analytical review can be performed on key structural elements. This analysis can be used to determine deficiencies in an original design as well as to determine if the existing structure, even in a deteriorated condition, is still adequate to support the imposed loading.

Concrete Floors and Moisture – Hardened concrete is a permeable medium. The rate at which moisture can permeate through a concrete slab is dependent upon the overall quality of the concrete. Modern construction practices often use vapor retarders to produce a moisture resistant floor. They are not often present in older constructions. High moisture vapor transmission rates for concrete slabs can result in the debonding of tile and carpet; warping of wooden floors and even microbial growth. The moisture vapor emission rate can be determined in accordance with ASTM F1869. Most manufacturers of floor coverings will specify a maximum acceptable moisture transmission rate.

Cracking in Concrete Structures – Cracking and spalling of concrete in parking garage structures is almost always the result of corrosion of the steel reinforcement. Various factors such as depth of concrete cover over the rebar, depth of carbonation, moisture permeability of the concrete (related to the water/cement ratio), and the presence of waterproof coatings on the concrete surfaces can affect the occurrence of the corrosion process. The volume expansion that happens when iron corrosion products are formed can exert enough localized pressure to crack and eventually spall the concrete cover. The presence of melt water and the use of chloride deicing salts will exacerbate the problem. The potential for corrosion of uncoated reinforcing steel can be determined by performing half-cell potential mapping of uncoated concrete in accordance with ASTM C876.

Electrochemical Methods such as cathodic protection are an important tool to enhance the durability of concrete structures.  Cathodic protection systems provide a small electrical current to the embedded steel to prevent or control active corrosion.  There are many types of cathodic protection systems with various advantages and limitations.  Our staff engineers are NACE certified in cathodic protection, corrosion, coatings, and materials selection and design and can assist with these aspects as well.  Click here to learn more about cathodic protection.

Types of Concrete Deterioration

Types of deterioration include: corrosion of reinforced steel, freeze-thaw damage; sulphate attack; alkali-aggregate reactivity; fire damage; surface scaling; popouts; effects of admixtures; and several other aspects.

  • Corrosion of embedded steel-Embedded reinforcing steel (rebar) is widely used to give concrete greater strength and even flexibility. If the steel corrodes then the expansion of the corrosion products will cause cracking and spalling of the concrete. Normal concrete is very alkaline and will therefore cause passivation of any embedded steel. If theconcrete loses its high pH it will no longer offer protection to the steel.
  • Spalling –Spalling is the dislodgment of large or small pieces from a structure. The dislodged pieces often have a conical or bowl shape. Spalling can result from impact, fire damage, or corrosion of embedded steel.
  • Shrinkage cracking-During the curing of new concrete the moisture within the concrete will diffuse to the surface where it will evaporate. The concrete at the exterior surfaces will dry and shrink faster than the concrete deeper within the structure resulting in tensile stresses at the surface which if great enough cause the formation of drying shrinkage cracks. Geometry can also affect formation of shrinkage cracking with long spans between joints being most prone to cracking.
  • Carbonation-In older concrete structures carbonation occurs at the exterior surfaces. The process of carbonation will cause the carbonated surface layer to shrink and crack. Cracks from carbonation tend to be shallower than cracks from initial drying shrinkage.
  • Freeze thaw damage (Frost damage)-Concrete is an inherently porous material although the permeability can vary between different structures. The original water cement ratio is largely responsible for the degree of permeability. When water within the concrete pores is exposed to temperatures below 32 degrees Fahrenheit this water can freeze and the expansion in volume caused by the formation of ice will exert an internal hydraulic pressure on the cement matrix which can initiate internal cracking. Over time and with multiple exposures to freezing temperatures the internal cracks will grow and the cement matrix will begin to crumble. Water saturated concrete is most susceptible to freeze thaw damage. The intentional air entrainment of the cement matrix is typically done to prevent freeze thaw damage.
  • Pop outs-Pop outs are small localized pits in a concrete surface usually caused by the fracturing of a susceptible aggregate just below the surface. The susceptible aggregates are often sedimentary rocks which are inherently weak and prone to moisture absorption. The mechanism of freeze thaw causes the formation of pop outs.
  • Salt Damage-If dissolved salts are present in the water to which the concrete is exposed, these salts may be absorbed into the concrete where they crystallize and precipitate out of solution. The volumetric expansion caused by crystal formation may result in internal cracking eventually leading to disintegration and surface scaling of the concrete.
  • Sulfate attack-Ground water and seawater usually contain sulfates and higher concentrations of sulfates can often be found in industrial wastewaters and mine waters. Sulfates can chemically attack the cement matrix of the concrete leaving behind a soft powdery surface. On other occasions sulfate attack can result in the formation of compounds that cause expansion and spalling of the concrete.
  • Erosion-Erosion is the progressive deterioration of the concrete surface resulting from high velocity water flow, cavitation damage in flowing water systems, and mechanical scouring and abrasion by ice, sand and gravel, or other solid materials.
  • Structural Cracking-Structural cracking results from external loads imposed upon the structure. Direct impact or earthquakes can cause cracking. These cracks tend to be wide and deep. Differential settlement of a structure can cause cracking. Flexural loading can result in fatigue cracking.
  • Efflorescence-Water movement through hardened concrete can result in internal leaching. Calcium ions are dissolved and transported to the surface. At the surface reaction of the leachates with carbon dioxide in the air and drying of the deposits can result in whitish colored encrustations. The process is known as efflorescence. Efflorescence often occurs at cracks in the concrete. Internal crystallization can weaken the concrete.
  • Vegetation in cracks-Cracks in concrete which fill with dirt and moisture can sometimes promote the growth of vegetation within the cracks. As the plant roots propagate into the cracks they can promote further crack propagation and weaken the structure.
  • Alkali-aggregate reaction-The original cement used in the concrete mix can often contain alkali ions such as sodium and potassium. Alternatively these ions can be introduced from the environment or even be present in the aggregates or from an admixture. Certain types of amorphous silica aggregates, or aggregates containing amorphous silica, are susceptible to chemical reactions with the alkali ions. The resulting reaction products will swell in the presence of moisture leading to internal cracking which will eventually be visible at the surface as map cracking.

    

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Above, photos of concrete cracking are shown in various forms. The types of deterioration observed should be documented by photographs with drawn maps and diagrams to show the locations of each                                                                                                                             type of damage, the extent of each type of damage and the orientations of cracking.

 

Corrosion Risk Assessment Structures can be affected by many factors including erosion, fatigue, cycles of freezing and thawing, and adverse chemical reactions.  One of the most common problems in reinforced concrete is the corrosion of embedded metals.  The corrosion of steel in concrete is primarily caused by exposure to chloride contained in seawater, roadway de-icing chemicals, and soils.  Chloride contamination can also occur in new construction when using chloride-containing set accelerators, contaminated aggregate, or the use of non-potable mix water.

Electrochemical methods such as cathodic protection are an important tool to enhance the durability of structures.  Cathodic protection systems provide a small electrical current to the embedded steel to prevent or control active corrosion.  There are many types of cathodic protection systems with various advantages and limitations.

Design of Suitable Corrosion Protection Measures to Mitigate Corrosion of steel Reinforcment

Cathodic protection and corrosion inhibitors will be considered to mitigate the corrosion of reinforced steel.  Cathodic protection (CP) is a method wherein a sufficient amount of electric direct current (DC) is continuously supplied to a submerged or buried metallic structure to mitigate, slow down or temporarily stop the natural corrosion processes from occurring. The DC current corrodes a sacrificial anode when it is connected to a structure to be protected. There are two methods for supplying DC to cathodically protect a structure. They are the following:

  • Galvanic anode cathodic protection system.
  • Impressed current cathodic protection system.

The galvanic anode cathodic protection system generates DC as a result of the natural electrical potential difference (electrochemical reaction) between the metal to be protected (cathode) and another metal to be sacrificed (anode). The sacrificing metals such as magnesium (Mg), zinc (Zn) or aluminum (Al) all have a lower more negative electrical potential. The current output of this system is affected by factors such as:

  • Driving voltage difference between the anode and the cathode.
  • Resistivity of the electrolyte (environment).
  • pH factor.
  • Natural or man made environmental chemistry and/or contaminates.

The impressed current cathodic protection system comprises four main components which together constitute an electrical circuit.  They are as follows:

  • A controllable DC power source – usually a transformer rectifier.
  • An applied anode – a material placed onto or into the concrete or surrounding electrolyte to enable current flow.
  • An electrolyte – normally the pore water present within the concrete, or in the case of remote anodes, also the water, soil or mud in which the anodes are placed.
  • A return electrical path – normally the electrically continuous reinforcement steel to be protected.

The CP transformer rectifier can be powered by external power sources, such as alternating current (AC).  The CP rectifier converts the input power source into DC.  DC is discharged from impressed current anodes made of metals such as steel, high silicon cast iron, graphite, platinum and titanium mixed metal oxide.  The potential current output of an impressed current CP system is limited by factors such as available AC power, rectifier size, anode material, anode size and anode backfill material.  The current output of an impressed current cathodic protection system is far greater than the current output of a galvanic anode cathodic protection system.  However, higher maintenance during service is required and short circuiting of anode and rebar should be taken into consideration in design and implementation of this system.

It is important to determine the condition of the steel reinforcement and current requirements for the design of a reliable cathodic protection system.

Client Involvement: No evaluation can be complete without input from the project  owner or maintenance personnel. These individuals live with the project on a daily basis and typically have historical data related to building/project use, prior maintenance history, change in use of the facility, etc. In addition, a successful repair strategy can only be developed if the expectations of the client are clearly understood. For example, if the structure will be replaced in one year, it is not beneficial to the client to evaluate the structure and develop repairs for a 20-year anticipated life.

We Are Here to Help

We respond to all inquiries promptly by sending a technical proposal detailing scope of work and estimated costs.  Please call Dr. Zee at 412-952-9441 or Ed Larkin at 412-788-1263 for additional information, and how we may be able to assist you in the lab or at the project site.  Alternatively, you can send your request to info@matergenics.com.  Call us today for a quote!

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