Matthew Banville, PE Gale Associates Weymouth, Massachusetts ABSTRACT When concrete structures age, various forms of deterioration occur. Spalled concrete, corrosion of reinforcing steel, and stress cracks are just a few of the defects that develop. This presentation will review the process of analyzing existing concrete structures to determine types and causes of defective conditions, and repair recom¬ mendations. The presenter will detail concrete framing types and methods of docu¬ menting and categorizing concrete deficiencies. Discussions on the cause and origin of various conditions will address cracks, joints, delaminations, spalling, water infil¬ tration, corrosion, and freeze/thaw concerns. The presenter will also discuss field and laboratory testing. SPEAKER Matthew H. Banville is a registered professional structural engineer with 10 years of experience in evaluation, design, and consulting for restoration projects involving existing buildings and structures. He is responsible for structural engineering for building projects, including building investigations and evaluations, analysis, design, coordination, specifications, and construction administration. Contact Information: Phone – 800-659-4753; E-mail – mhb@gainc.com Banville- 16 Proceedings of the RCI 23rd International Convention
Concrete-the precise combi¬ nation of cement, sand, and aggregate-produces a building material that is both ubiquitous and integral to the constructed world. Long known for its com¬ pressive strength and durability, concrete attained a greater impor¬ tance in construction with the advent of reinforced concrete in the latter half of the nineteenth century. The compressive strength of concrete, combined with its ability to act as a support matrix to bond to and protect the embedded steel, results in a com¬ posite material that exploits the best features of both materials. In flexure, the externally applied load is resisted by inter¬ nal stresses that create a force couple between tensile forces within the embedded steel and compressive forces developed within the concrete. In compres¬ sion, the reinforcing steel serves the dual purpose of strengthen¬ ing, through the inclusion of lon¬ gitudinal reinforcing; and confine¬ ment, through the inclusion of transverse bands or ties that restrain the concrete’s tendency to expand. Through proper place¬ ment within the concrete cross¬ section, building elements that exploit both the flexural and com¬ pressive properties of reinforced concrete can be constructed. As strong as reinforced con¬ crete can be, it also possesses several weaknesses that can con¬ tribute to premature failure. If installed incorrectly or without proper maintenance, the useful life of concrete is greatly de¬ creased. Weaknesses can be chemical or physical, and inter¬ nally or externally applied. This paper will describe the causes of concrete deterioration and how to evaluate and repair the damage. DETERIORATION MECHANISMS Most people can identify the basic problems observed in con¬ crete structures. A crack in a floor slab, a spall at the base of a col¬ umn, or rust stains that discolor the underside of a beam are conthe concrete normally protects the steel by sig¬ nificantly reducing the rate of corrosion; however, open cracks, reduction of concrete alkalinity, expo¬ sure to corrosive chemi¬ cals, and dissimilar met¬ als can all increase the rate of corrosion. ditions that are present in many structures built of reinforced concrete. Deter¬ mining why these deficien¬ cies are occurring can be the difficult part. By understand¬ ing the ways in which con¬ crete can deteriorate, one can deduce the cause of the sus¬ tained damage, determine the necessary remedial mea¬ sures, and reduce the poten¬ tial for future deterioration. Deterioration can be caused by an attack of the chemical makeup of concrete or from chemical reactions with embedded steel. Some of the different types of chemi¬ cal attack include: Photo 1 – Corrosion of embedded steel and common failure mecha¬ nisms. Corrosion. Corrosion occurs from an electri¬ cal reaction within the concrete matrix. Expo¬ sure to oxygen and moisture are required for corrosion to occur. The electrical reaction causes the iron in the steel to oxi¬ dize. As the steel oxidizes, it expands, inducing ten¬ sile stresses in the sur¬ rounding concrete. When the tensile strength of the concrete is exceeded, the concrete fails with a crack or spall. The alkalinity of Chlorides. Chlorides are normally introduced to concrete structures through de-icing salts or sea water. The chlorides penetrate the concrete, eventually making contact with the embedded steel. Once the chlorides com¬ bine with oxygen and moisture, corrosion of the steel occurs. With the presence of chlorides, the Proceciiinijs of the RCI 23rd International Convention Banville – 17 Photo 2 – Spalling of concrete due to chloride attack of embed¬ ded reinforcing steel. corrosion process is more aggressive, occurring even with high alkalinity and accelerating the corrosion process in concretes where the alkalinity is reduced. Carbonation. Carbon di¬ oxide in the air can react chemically with cement paste when moisture is present. The resulting chemical reaction reduces the pH of the concrete. The carbonation process penetrates the pores of the concrete, eventually penetrating to the embed¬ ded steel, increasing the potential for corrosion. Since the carbonation process penetrates the concrete, open cracks will accelerate the depth of carbonation penetration. High quality concrete is less susceptible to the carbonation process. Alkali-Silica Reaction. Because of the intimate interaction between the cement paste and the coarse aggregate, using Photo 3 – Alkali-silica deteri¬ oration of overpass substruc¬ ture. compatible materials is important. When certain types of aggregate are used, the silica in these stones can react with the hydroxides in the cement paste. The potential reac¬ tivity of suspect aggregate can be determined through ASTM C289 (Standard test method for Potential Alkali-Silica Re¬ activity of Aggregates (Chemical Method)) or ASTM C-1260 [Standard test method for Potential Alkali Reactivity of Aggre¬ gates (Mortar-Bar Meth¬ od)]. When this chemical reaction takes place, a gel develops on the aggregate surface. When moisture is introduced to the gel, the gel reacts by expanding and inducing tensile stresses in the cement paste, causing cracking. The cracks allow addition¬ al moisture to enter the concrete, accelerating the reaction. Alkali-silica re¬ action is observed on the surface of the concrete by Photo 4 – Freeze/thaw effects on air-entrained concrete. The top photo is a slab con¬ taining air entrainment; the bottom slab is concrete with¬ out air entrainment. Banville – 18 Proceedings of the RCI 23rd International Convention severe map cracking on the exposed surfaces. Freeze/Thaw. Freeze/ thaw is the process that occurs when moisture within the pores of con¬ crete freezes and expands. The expansion causes tensile forces to develop within the cement, caus¬ ing cracking and scaling of the concrete. For freeze/thaw to occur, both moisture and freezing temperatures must be present. Freeze/thaw damage is exacerbated when the concrete is exposed to cyclic freezing and thawing. Freeze/thaw is resisted in concrete through air entrained into the cement paste. En¬ trained air is the presence of large amounts of equal¬ ly spaced microscopic air bubbles within the cement paste. The freezing pore water expands into the spaces provided by the air bubbles. Chemical Attack. Due to the alkaline nature of con¬ crete, there are a number of chemicals that can directly react with the ce¬ ment paste. The most common chemicals are acid and salt solutions. These chemicals may also react with certain aggre¬ gates, resulting in aggre¬ gate deterioration. Concrete is also subject to physical attack. Physical attack can originate from a number of sources, ranging from environ¬ mental effects to construction defects. Some of the different types of physical attack are as fol¬ lows: Thermal Effects. As con¬ crete is heated and cooled, the mass can expand and contract. If this movement is not accounted for, cracking can develop when a warm mass is cooled. Excessive compressive stresses may develop when a restrained concrete mass expands due to rising temperatures, po¬ tentially producing spalls and cracks. Thermal effects can also be found in struc¬ tures that are exposed to differential temperatures. For example, a concrete bridge may develop cracks due to differential thermal movement when the top surface is heated by the sun, but the temperature in the cool, shaded underside changes only minimally. In this case, the upper layer of the concrete is trying to expand, while the lower layer is not. These expansive forces may induce cracking in the lower layers if the ten¬ sile stresses in the concrete are excessive. Shrinkage Effects. Freshly placed concrete shrinks as part of the curing process, known as drying shrinkage. This process is a result of water evaporating from the mix as it cures. Shrinkage can also occur when rapid evaporation of water in placed concrete results in differential shrinkage of the member; this is known as plastic shrinkage. Slabs placed in hot weather are most susceptible to this phenomenon where the cur¬ ing process has begun and water can readily evaporate from the slab surface. The upper layer of the concrete then shrinks at a faster rate than the lower layer, result¬ ing in the development of cracks in the upper layer. Loading Effects. When a reinforced concrete member is overloaded or the design load is applied with an inad¬ equate amount of curing time, damage to the con¬ crete may occur due to these loads. The result of excessive or early loading is often represented by cracks developing in the tensile zone of the member or with shear cracking developing where the shear capacity of the beam was exceeded. Honeycombing. Honey¬ combs occur when the cement paste does not fill the voids between the aggre¬ gate. Common causes of honeycombing include inad¬ equate consolidation or vibration during placement, improper placement of the concrete, congested or im¬ properly spaced reinforcing steel, excessive mixing of the concrete, and low work¬ ability of the concrete. Segregation. The separa¬ tion of the concrete compo¬ nents is called segregation. It refers to the process where the aggregate sepa¬ rates by size and weight. The larger aggregate collect at the bottom of the section and the cement paste col¬ lects at the surface. Segre¬ gation can be attributed to improper placement proce¬ dures (dropping concrete from an excessive height), a high concrete slump, and over -vibration of the placed concrete. Cold Joints. When concrete pours are interrupted inten¬ tionally or unintentionally, full concrete strength and continuity may not occur between the mating con¬ crete surfaces. Intentional examples of this condition include structures where multiple lifts of concrete are required (e.g., tall walls); unintentional examples in¬ clude delivery delays during the placement operation. Proceedings of the RCI 23rd International Convention Banville – 19 For proper bond between the two pours, the mating surfaces of the joint must be properly prepared. When not properly prepared, these joints provide sources of moisture intrusion into the concrete section, possibly permitting direct attack of the reinforcing steel. Placement of Reinforcing. The placement of reinforcing steel within a concrete assembly can affect the con¬ crete’s ability to properly bond to and protect the reinforcing steel. Examples of this condition include placing reinforcing without adequate concrete cover for the service environment, placing reinforcing without proper spacing, and not accounting for lap splices. Inadequate cover can accel¬ erate the rate of a chloride attack or the effect of car¬ bonation. Cracks may devel¬ op in the concrete if the reinforcing is not located at the proper location, either through improper design or construction practices. Construction Tolerances. All construction requires tolerances to achieve ac¬ ceptable structures within a reasonable amount of time and effort. Extraordinary tolerancing of a structure requires precise planning by the designers and careful execution of the work by the contractor. In instances where tolerances are not adhered to, the as-built con¬ dition may affect the strength and durability of the structure. For example, improper slope in a floor slab may permit the collec¬ tion of surface runoff that is prevented from reaching an area drain. In this instance, not only is the concrete sur¬ face exposed to a direct and possibly cyclic moisture source, the unintentional collection of water may af¬ fect the response of the sup¬ porting structure and in¬ duce flexural cracking. In another instance, tied rebar assemblies may not main¬ tain an adequate amount of concrete cover if excessive form deflection occurs dur¬ ing concrete placement. These deficiencies will re¬ duce the ability of the con¬ crete to protect the embed¬ ded reinforcing, possibly accelerating the deteriora¬ tion of both the reinforcing and the concrete. FIELD CONDITION SURVEY It is important to research a building’s history before perform¬ ing the field evaluation. The plans, specifications, as-builts and any other pertinent construc¬ tion documentation will provide a description of how the building is constructed, the strengths of the materials used, and the intended purpose of individual building components. This information, combined with information on previous repairs and additions, can assist with assessing the inuse loading conditions for com¬ parison to the design intent for altered structures experiencing distress. Repair and maintenance history information, obtained either through records or through interviews with the building owner and maintenance staff, is also useful in researching reoc¬ curring problems and for under¬ standing new problems that could be a result of improper repairs and maintenance. If some or all of the informa¬ tion described above is not avail¬ able, it can be attained through a number of different sources. In¬ terviews with the building owner and maintenance staff can be helpful if there are long-term per¬ sonnel on the maintenance staff. Historic records, such as pho¬ tographs, are helpful in identify¬ ing changes to the building. Tax and building inspector records can also provide a history of the permitted alterations. Reviewing the construction drawings and/or examining the structure will help determine the type of concrete construction. Some of the con¬ crete configurations encountered include plain concrete, often found in footings, dams, and resi¬ dential construction; cast-inplace reinforced concrete; prestressed- precast concrete; and post-tensioned concrete. During the field survey, the dimensions listed on the con¬ struction plans should be spotchecked for consistency and veri¬ fication of the plans. Should plans not be available, the existing con¬ ditions will need to be measured and the necessary plans, grid, ele¬ vations and sections will need to be developed by hand. The level of detail of the field sketches will be contingent upon the level of detail required for the survey. On copies of drawings or field sketches, the existing conditions should be doc¬ umented and categorized. These conditions include: Cracks. The type and width of the crack should be recorded. If a crack is believed to be active, a monitor may be installed to record any move¬ ments. Joints. The configuration and condition of all joints should be recorded along with any noted deficiencies. Delamination. Areas of delamination should be iden¬ tified by type (partial or full) and their depth recorded. Spalling. Location, depth, and conditions of spall should be recorded. Paste Erosion. Paste erosion may be due to a chemical reaction with the paste or Banville – 20 Proceedings of tbe RCI 23rd International Convention through erosion. Environ¬ mental conditions that may have impacted the area should be noted. Water Infiltration. Signs of water infiltration should be documented, along with whe¬ ther the leak was active at the time of the survey. Infiltration associated with rust staining or efflorescence should be identified accordingly. Exposed Steel. The extent and condition of exposed steel should be documented. Corrosion. Noted corrosion may include surface staining due to corrosion of the embedded steel and surface¬ mounted components. Structural Distress. Possible indications of structural dis¬ tress include excessive deflec¬ tion, shear cracking, tension zone cracking, radial cracking at columns, etc. Freeze/Thaw. Areas of freeze/thaw damage should be identified and the depth of the damage recorded. Alkali-Silica. Areas of alkali¬ silica damage should be iden¬ tified. Alkali-silica damage should be sampled for confir¬ mation of the condition through laboratory testing. Organics. Organic matter growing on concrete surfaces is often indicative of excess moisture. Both the moisture and organic growth can dete¬ riorate the concrete. Organic growth may also obscure damage to the concrete. The area should be carefully reviewed for signs of concrete distress. Any previous repairs should be documented, including if the repair coincides with an observed defect. General conditions of the facility should also be document¬ ed. The location, condition, and configuration of any surface treat¬ ments, equipment, fixtures, and utilities should also be document¬ ed. TESTING Field Testing – Non-Destructive Numerous testing options are available to assist in completing pachometer. While both systems result in identifying the size and location of embedded reinforcing, the ground-penetrating radar also provides a three-dimensional rep¬ resentation of the concrete, iden¬ tifying the differing layers of rein¬ forcing. the field condition survey. The most common method of non-destructive field testing is through a process called sounding. Sounding involves striking the concrete surface and observing the sound produced. Solid concrete will produce a ringing sound while concrete that is spalled, delaminated, or con¬ tains voids will produce a flat or hollow sound. Sounding can be accomplished using a variety of tools. Sounding of small areas and vertical or overhead structural ele¬ ments is best achieved by using a hammer or steel rod. A steel chain can also be dragged over the surface under evaluation. This method is best suited for slab surfaces where large areas can be tested in a rea¬ sonable amount of time. Non-destructive evalua- Photo 5 – Sounding of concrete by chain dragging. Photo 6 – Sounding of concrete by striking with a hammer. tions can also be accom¬ plished using ultrasonic methods. Two common approaches include a pulse velocity meter and an impact echo system. The pulse velocity meter can detect defects such as the depth of cracks and loss of bond. The impact echo system can detect the thickness of a thin con¬ crete section, locate a crack with¬ in the concrete, and locate voids or defects such as honeycombing. Should the approximate size and location of the embedded reinforcing steel be desired, non¬ destructive testing methods include ground-penetrating radar and magnetic testing using a Field Testing – Destructive Destructive testing methods include exploratory openings, cor¬ ings, and pull-out testing. Explor¬ atory openings can reveal condi¬ tions such as depth of cracks, delamination, reinforcing size, and pattern and coating informa¬ tion. Cutting an opening in the area of a previous repair will re¬ veal information about the prepa¬ ration, application, and perfor¬ mance of the repair. Corings will determine condi¬ tions similar to exploratory open- Proceedin^s of the RCI 23rd International Convention Banville – 21 ings but at a limited scale. Corings can provide depth of cracks, depth of delamination, and reinforcing size. A core can also be sent to a laboratory for petrographic analysis. Pull-out testing can determine the bond strength between a coat¬ ing and the concrete substrate, or between two cementitious materi¬ als. The application of this test when used for determining coat¬ ing bond is covered by ASTM D4541 – Standards Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers. The test method calls for bonding a plug to the surface coating. The area around the plug is then cut away to isolate the bond area. The testing apparatus is set over the plug and attached to the plug. A force is applied through the test¬ ing apparatus until the plug is pulled from the substrate. Review of the plug will reveal the type of failure (i.e., failure in the topping, along the bond line, or in the sub¬ strate). An approximation of the bond strength can be determined through a reading on the appara¬ tus; however, this value is a qual¬ itative answer since different ap¬ paratus will yield different results. LABORATORY TESTING Three common laboratory tests that provide concrete property information include the chloride ion content test, depth of carbon¬ ation test, and concrete petrogra¬ phy. Chloride ion content is deter¬ mined by an analyzer that mea¬ sures the amount of chloride ion in a prepared sample. The sample is prepared from pulverized sam¬ ples of concrete taken directly from the field or prepared in the laboratory from a solid sample. The application of this test is cov¬ ered by ASTM C-1218 – Standards Test Method for Water-Soluble Chloride in Mortar and Concrete. Depth of carbonation is deter¬ mined by applying phenolph¬ thalein to the sample. The phe¬ nolphthalein reacts with the alka¬ line cement paste to turn the paste a pink color. Due to its lower pH, the carbonized concrete does not change color, allowing the thickness of the carbonized layer to be measured. Petrographic analysis involves cutting a concrete sample into thin layers and observing them under a microscope. The aggre¬ gates and cement paste are exam¬ ined for conditions such as con¬ crete proportions, type of aggre¬ gate, air content, presence of dele¬ terious chemicals, alkali-silica reaction, freeze/thaw action, and depth of carbonation. METHODS OF CONCRETE REPAIR Concrete Removal Prior to repairing the deficien¬ cies observed during the field con¬ dition survey, the damaged con¬ crete must be removed. Concrete removal is achieved through a variety of methods, depending upon the repair to be made. When dealing with large sur¬ face areas, scabble, scarifying, or hydrodemolition will remove con¬ crete. A scabble removes the dete¬ riorated concrete by pulverizing the concrete’s surface with cylin¬ ders driven by compressed air. Scarifying removes concrete by scraping the surface of the con¬ crete. Hydrodemolition erodes the concrete by using high-pressure water jets. With hydrodemolition, the amount of concrete removal is determined by the pressure of the waterjets in combination with the speed of the tractor pulling the jets. When a more controlled or deeper area of concrete requires removal, hand-operated pneu¬ matic hammers or truck-mounted hydraulic hammers can remove concrete by driving a chisel into the surface, causing spalling of the concrete. SURFACE REPAIR OF CONCRETE Shallow and deep surface repairs of concrete are prepared in a similar manner. The deterio¬ rated concrete is removed from the area requiring repair. The area requiring repair should be overcut into a regular shape, such as a square or rectangle. The perime¬ ter of the repair should be cut with a slight back bevel so that the repair acts like a key to retain the material. For shallow repairs, the depth to solid concrete is less than the depth of the reinforcing steel. The concrete is removed to a uniform depth. The surface of all exposed rebar will require cleaning, most often by sand or shot blasting. For deeper repairs, where more than half of the rebar diameter is exposed, the concrete should be completely removed from around the bar so that a minimum clear¬ ance of 1 inch is provided all around the exposed bar. In most instances, a bonding agent is applied to the steel and concrete surface. The product literature or manufacturer’s representative of the product that will be used should be consulted when the suitability or need of a bonding agent is in question. Most bond¬ ing agents have specific applica- Photo 7 – Properly prepared surface repair. Banville – 22 Proceedings of the RCI 23rd International Convention tion requirements with clear direction as to the minimum and maximum exposure time before the patch material is applied. Failure to observe the manufac¬ turer’s preparation, application, and timing requirements can result in a bond break at the perimeter of the patch. There are many available repair mortars (Portland cement mortar, Portland cement concrete, shrinkage-compensating con¬ crete, polymer-modified concrete (increases bond and flexural strength), and shotcrete, etc.). Many of the available mortars contain a combination of the properties of the above-mentioned materials. The physical properties of a particular product must be carefully reviewed to ensure the necessary features are included. Since particular formulations vary by product manufacturer, the product literature or man¬ ufacturer’s representative should be consulted. Repair mortar application can be accomplished in a variety of manners. Concrete can be cast¬ in-place, often used when the fulldepth repair is overhead. The repair can be hand applied or troweled into place; this method is often used for shallow and over¬ head repairs. The formed and pumped method works well for vertical and overhead repairs. This is when a form is applied over the repair and the repair material is pumped into the form. Each of these methods has pros and cons that must be evaluated for each particular repair. CRACK REPAIR OF CONCRETE The type of crack repair to be undertaken is contingent upon the objective of the repair. A crack may be repaired to restore or increase strength or stiff¬ ness, to seal against mois¬ ture intrusion and pre¬ vent corrosion, to improve serviceability, and/or to improve aesthetics. For joints that do not require structural repair, the main purpose of most repairs will be to seal the crack against water intru¬ sion and improve aesthet¬ ics. Repair methods that achieve this result Photo 8 – Epoxy injection of parking include routing and seal¬ garage slab. ing (a slot is cut into the top of the joint and filled with sealant), applying overlays, or gravity filling the crack with a flexible material. When structural repair of the crack is required, possible repair methods include epoxy injection, gravity filling, grouting, additional reinforcing, and stitching. Both the additional reinforcing and stitching methods involve installing reinforcing. When stitching, a series of pins is drilled and epoxied over the joint. The stitches are applied in differing lengths and angles to prevent cre¬ ating a stress plane in the con¬ crete. Additional reinforcing can be applied either externally (through the addition of carbon or glass-fiber reinforcing) or inter¬ nally (by installing pins that span the crack plane). Epoxy crack injection is a multi-step process. The crack is first sealed with an epoxy and injection ports are installed to permit the injection process. The epoxy is then installed in a spe¬ cially-designed pumping appara¬ tus that mixes the epoxy compo¬ nents in the proper formulation. The epoxy is then injected into the lowest or first port in the crack. The next port is monitored for evi¬ dence of epoxy extruding out the tube. Once the adjacent port indi¬ cates the presence of epoxy, the first port is sealed and the injec¬ tion proceeds to the adjacent port. Injection proceeds again until epoxy is extruded out of the next adjacent port. This procedure is repeated until all ports have been filled. In the case of a vertical crack, the injection process begins at the lowest port and fin¬ ishes at the highest port. CONCLUSION Through proper evaluation, design, and installation, concrete repairs can be made that perform as well as the surrounding mater¬ ial. A comprehensive evaluation will identify the areas that require repair, as well as assist in identi¬ fying possible sources of the dam¬ age. By understanding the extent and source of the damage incurred, a suitable repair using the most appropriate materials can be designed. Only through proper preparation and execution will a repair be successful. Many of the repair materials used have specific requirements that must be carefully followed to produce a quality repair. Proceedings oj the RCI 2 3rd International Convention Banville – 23
