his is the first of a two-part article on rust-jacking phenomenon. When the client asks repairing aging, normally reinforced, what should be done, the response is typically: Structural garage structures existing in “Are you buying or selling?” That simply means aggressive weather environments. The does the client hold a long-term position in the Tfirst article is from the perspective of the engineer facility, or are they simply looking to improve Rehabilitation and the second article from that of the contrac- life-safety conditions in order to extend the service tor. While the topic is the same, and the articles life for a short period? This article assumes that are complementary, they are not intended to be the prospective client is in a long-term position. renovation and restoration of a point-counterpoint. The garage study begins with a determination of existing structures the extent of deterioration. Spalling concrete on The Designer’s Perspective travel surfaces and on missing pieces from con- crete soffits is readily observable. Beneath a surface Although construction sophistication has that may visually appear to be secure could be an improved remarkably over the years, there are ongoing corrosion issue. Forty (40) years ago, a many parking structures, particularly those in the reliable process called “sounding” was utilized to northeast built forty to sixty years ago, that were estimate the extent ®of what was called “concealed constructed without much thought or under- deterioration”. Concealed deterioration suggests standing about how the aggressive nature of the that the surface may appear to be sound or solid, environment and the world we live in would yet the corrosion process beneath the surface has deteriorate these structures. reached a degree where concrete has debonded When one has been in the business of garage or layered as a result of the rust-jacking process. Copyrightrepair for a lengthy period, Sounding requires nothing more sophisticated you encounter interesting than a hammer, a steel rod, or a chain to be situations and meet inter- able to begin mapping the extent of debonding. Life before the esting people. This article is Through the years, technology has developed International Concrete intended to be a useful tool some rather exotic means for determining the for the younger engineer to extent of debonding. Many an “older” engineer Repair Institute (ICRI) be able to absorb some his- has noted that it would be great if someone would tory and lots of experiences. invent a way to assess debonding without the need Parking garages come to survey a surface on hands and knees while using By Craig E. Barnes, P.E., SECB in many shapes, sizes, and structural configu- a hammer. Technology has answered that request rations: Steel frame, pre-cast, CMU-bearing, in the form of ultrasound, x-ray, or a rotary per- metal-deckmagazine with concrete, post-tensioned decks cussion tool on an extension pole. The newer S T(filigree-type), R toppedU and Cpre-topped T pre-cast, U techniques R (still undergoingE development for bituminous surface, free-standing garages, and preciseness) are good for quantity mapping and garages beneath structures. This article will con- relatively quick, general overviews. This author centrate on normally reinforced, cast-in-place still finds that the precision of sounding is still concrete parking structures. the most reliable. How does concrete in a garage structure deterio- Sounding is best done in a relatively quiet Craig E. Barnes is the Founding rate? Well-researched over the years, deterioration environment so that the individual doing the Principal of CBI Consulting Inc. is largely due to the oxidation of the structural sounding can hear the distinction between solid Craig can be reached at reinforcement which in turn creates a phenom- concrete (a clear ring) and spalled concrete (a dull [email protected]. enon called rust-jacking, resulting in spalled thud). In a noisy environment, it is still possible to concrete. The rust-jacking phenomenon is evi- do sounding in a localized top of slab area, with denced in bulging and/or cracked concrete, often a hammer, by lightly sprinkling small pieces of with rust staining, and is a direct result of the aggregate over the surface before sounding. The pressures created as embedded reinforcement spalled concrete will vibrate, and a vibrating sand rusts within the concrete structure. Low qual- line demarking solid and debonded concrete can ity concrete, lack of air-retention, and concrete be easily observed. When marking for excavation, carbonation are all accelerants to the deteriora- chalk or paint may be marked directly on the tion process. concrete surface. A good practice is to extend A project usually starts when a facility user beyond the specific spall line by several inches (usually the Owner) contacts the engineer or to allow for oxidizing steel that may not yet have contractor stating that pieces of concrete are generated to the point of active spalling. unusually erupting from the floor surface, expos- ing reinforcing rods, or falling from overhead. A filigree concrete system consists of a relatively A quick inspection and a phone call advises the thin panel that by itself is not prospective client that water, making its way sufficient to support full span line loads but, through or into the concrete, has been pres- in composite combination with a cast-in-place ent around the reinforcement over a sufficient topping, becomes a longer span, code load sup- period of time to cause steel oxidation and the porting structural floor or roof.

12 April 2016 Concrete Removal Build Your Electric or pneumatic chipping guns and hydro-demolition (hydro) are the tools of first Career choice for removing deteriorated concrete. Site conditions will often determine which method is the preferred technique. Hydro- with Us demolition creates a significant amount of waste water that needs to be controlled. Although water-recovery systems and recy- cling can be utilized, there is still containment and waste water to be dealt with. One fea- Figure 1. ture of hydro-demolition that mechanical demolition doesn’t provide is that hydro- Removing oxidation corrosion from rein- demolition will often prepare the exposed forcing bars using wire wheels, hammers and reinforcement by removing oxidized material. chisels, sand blasting, and hydro-blasting can ® Impact hammer is a more labor-intensive be appropriate. The engineer should also be process and is a generator of airborne particles mindful of evaluating the required struc- and greater airborne noise. Noise from impact tural strength. At times, the area/quantity of tools and hydro equipment travels through a steel can be reduced and still satisfy the live concrete frame for long distances. The hydro load demand. As an example, a specification process overall creates less noise. that reads “steelCopyright with a 10% cross-section Once the sounding is done and the areas reduction is to be supplemented” might be are marked out for excavation, the floor will an unnecessary regiment if the existing steel visit www.STRUCTUREmag.orgADVERTISEMENT–For Information, Advertiser probably look like an area of randomly located could satisfy the intended design purpose with “amoebas” (Figure 1). In an ideal situation, a cross-section reduction of 25%. isolated amoebas should be circular. In the Why is it often suggested to have more cover case of concrete removal by pneumatic equip- protection for the reinforcement that is to ment, the easiest process from the contractor’s remain in the repair area than was necessary perspective, however, contains straight lines during the original construction? In a final and large rectangles. Be mindful that extend- product where a parking garage will receive ing well beyond the spall line removes sound an opaque elastomeric waterproof coating, a concrete, where it will be more difficult to typicalmagazine recommendation, the conditions that expose the encapsulated reinforcementS T that Rpromoted U the original C deterioration T willU be R E has not corroded. Leaving feathered excava- largely controlled. With a system that will tion edges at concrete spall is a poor practice, remain uncoated, wisdom suggests that more and one that is almost guaranteed to result attention should be paid to the protection of in lifting of the patch material at edges, even the reinforcement. Protecting the reinforce- when a manufacturer of an engineered patch- ment in the patch area reduces the corrosion ing product permits feathering the edges. potential for that steel, but forces the corro- Take your career in a new direction. Understanding a practical approach to con- sion potential to the unprotected steel that Engineers at Simpson Strong‑Tie crete demolition requires an understanding is adjacent to or entering the patch area. affect building construction on of excavation equipment. If full demolition This often gives rise to a phenomenon called of concrete with embedded reinforcement the “halo-effect”, where the patch appears a massive scale – helping our is required, large impact equipment greater to be working well, yet through the years customers design and build safer, than 25 pounds could be appropriate. To the adjacent concrete begins to deteriorate. stronger structures all over the globe. retain solid concrete for patching, mid-size Surrounding the perimeter of the patch area equipment to remove some of the most dis- with sacrificial anodic devices will prolong We have multiple openings for tressed concrete could be appropriate. Using the life of the original reinforcement adja- structural engineers who enjoy smaller equipment, such as rivet busters and cent to the patch and is a great technological working on innovative and electric chipping hammers, for fine-tuning advancement. There are several manufacturers challenging projects, giving would be a next step. Detailed excavation who will provide devices for this particular presentations and being on utilizing hand-held hammers and cold chisels purpose. Vector Corrosion Technologies has jobsites. Learn more and apply at is appropriate. Removal of 2500 psi versus been responsive in assisting the author’s firm strongtie.com/engineerjobs. 6000 psi concrete will yield different experi- with technical support in this respect. ences and require different techniques. Even Parent Concrete to Patch Interface though it may appear to be efficient, don’t use equipment that is larger than necessary, There are several possibilities to address the as there is an inherent danger of micro- interface between new and existing concrete. cracking of concrete that is not intended For the feather edge spall, a definite demarca- to be removed. tion is necessary. A logical approach is to make ©2016 Simpson Strong-Tie Company Inc. 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STRUCTURE magazine13 April 2016

SSTENGIN16.indd 1 2/4/16 11:22 AM where the success of the patch is ultimately judged. The reader may have heard state- ments to the effect that the process of curing concrete is never completed. The author read in one article that concrete used by the Romans is still in the process of hydrating. Taking advantage of particles within the parent concrete that have been relatively dormant and can participate in the hydration process can be done by achieving a saturated- surface-dry (SSD) surface. From the author’s perspective, this means more than simply applying a light water spray to the surface Figure 2. Grinder with vacuum attachment. Figure 3. Remotely operated excavation hammer. immediately before patching; it means provid- a saw/grind cut, which is often restricted to ing a 12-hour saturation by first applying the The contractor and inspector should not be a ¾-inch depth. Three-quarters of an inch water spray and then covering with burlap/ the decision makers® as to: (a) how aggressive is a reasonable cut depth, or estimation of burlene/etc. to prevent drying. Without let- the concrete removal is, (b) how much of the the clear cover, so as to maintain intact the ting the surface dry, and immediately before bar is to be exposed, (c) is there excess steel existing steel beyond. Confirm this dimen- applying the patch material, apply a coating which may be removed, or (d) where and how sion before production cutting. Increasing of neat cement (cement mixed with water to much shoring should be placed. These are the depth beyond ¾ inch using impact power the consistency of heavy paint) to the SSD questions for the Engineer of Record. tools will result in a rough surface, which concrete surface.Copyright Take the time to broom the Working the edges of a patch placement to is desirable to promote bond between the neat cement into the parent concrete and a consolidate the concrete against the parent existing concrete and new patch material. perfect surface will be readied for bonding substrate is a very important step. Finishing Grinding/cutting will appear to polish the the new to the old concrete. Do not let the the concrete surface to prepare it for either cut surface of the sound concrete. For bond- neat coating dry, otherwise a surface of dry, vehicle traffic or for subsequent coating is an ing purposes, this is a poor surface. If using dehydrated cement will result and the bond important step as well. Be familiar with the grit blasting to clean the reinforcing steel, will be negatively impacted. use of the garage to determine whether an one can utilize that same process to provide The placement of concrete is one that aggressive or a lighter broom finish is required. a light raised profile to the cut surface. A requires attention by both consultant/inspec- If the garage is to be coated, a magnesium natural hair-line shrinkage crack is likely to tor and contractor. Apply the neat cement in float surface is perfectly acceptable. form at the patch-to-parent concrete sur- advance of the concrete placement so that it Facilitating the curing of the patch mate- face, resulting from concrete shrinkage while is notmagazine permitted to dry out. A color change rial takes place either with the application curing. It is almost impossibleS to eliminateT Rin the neatU from a darkC gray sheenT to a lightU of moistR curing E blankets or chemical spray- this crack, but control of water content and gray mat color is an indication that drying on compounds. If using a chemical and the good construction practices are very helpful has taken place. Drying does not mean that garage is to be coated, don’t use a chemical in minimizing it. Again, understanding if the the cement in the neat has cured. In drying that will not be removed with the track blast- final product will receive an opaque coating out, it becomes a bond-break surface that ing process. will assist in these decisions. The meeting may not regenerate during the hydration Soffit Repair line of new patch material to old concrete is process. In the early stages of the drying one of the more vulnerable locations when process, if the concrete is delayed in delivery, Repairs on vertical surfaces and overhead considering crack control. It is the location a light spray from a hose is acceptable. The repairs require different techniques altogether. where essentially dissimilar materials come light spray should not be of a concentration First, one needs to assess what has been the together and results in a natural shrinkage that allows water pockets to accumulate. contributor to the overhead spalling and steel line. If the final product does not receive a With longer delay, applying additional neat deterioration. Has the deterioration resulted topical waterproof coating, one can be assured and brooming it vigorously will break up the primarily from the passage of water from that some level of deterioration will initiate surface that has begun to dry, and allow it above, or are other issues involved? One will at this location. If the patch material and to be reestablished. One can probably get probably find the largest extended deteriora- the parent concrete begin to debond at this away with doing this once. Beyond that, the tion results from cracks in the concrete that location, having a slight undercut of three to surface is lost and grit blast removal of the allow aggressive water to pass through. When five degrees on the ground joint will provide a dry neat is recommended. a garage ages and when it has been exposed to mechanical wedge, which will help to control Generally, spalls are a rather shallow patch, the carbon-monoxide rich environment of a the patch from lifting. The rough profile of which makes vibrating of the concrete for parking garage, carbonation of the concrete a grit blasted joint (adequate) or a mechani- consolidation purposes difficult. The use of can take place. This process dilutes the natural cally profiled surface (best) to create a friction pencil vibrators can be very helpful, par- passivity protection that concrete provides to interface can be very beneficial. A minimum ticularly where there are significant areas of the steel, thereby creating increased corrosion ¼-inch amplitude profile on the horizontal reinforcement congestion. A relatively new potential. Those practicing in New England, surface of the patch area has been found by tool called the vibrating , that can be repairing garages along the coast, are faced the author to work well. operated by one person, has been found to with an unusual situation in garage repair. Bonding the patch material to the parent be very beneficial for concrete consolidation Carbonation, chloride-laden air, and seasonal concrete is the greater challenge and may be as well as surface profile/finish. condensation cycles (Spring/Fall) initiate steel

STRUCTURE magazine14 April 2016 corrosion and concrete spalls on concrete garage surfaces of the underside of decks that have no other explanation. Overhead Repair Overhead work can be as demanding as deck work. Almost exclusively today, over- head patching is done with a combination of hand-patch materials, hand-patch and partial form work, or overhead pumping with engineered materials. With the excep- tion of rare instances where traditional sand and cement dry-pack is utilized, pre- State-of-the-Art Products packaged engineering products produced STRUCTURAL TECHNOLOGIES provides a wide range of custom by a variety of manufacturers are used today. Those products with a cement base designed systems which restore and enhance the load-carrying® are ones with which the author has had capacity of and other structure types, including the most success. With the hand-patch masonry, timber and steel. Our products can be used stand-alone or process, the construction crew is usually unable to proceed the placement with in combination to solve complex structural challenges. repeated misting of the parent surface and neat application. Here is where one faces Copyright the engineers’ conundrum. If everything V-Wrap™ noted in this discussion is necessary to Carbon Fiber System visit www.STRUCTUREmag.orgADVERTISEMENT–For Information, Advertiser achieve a good patch with long term viabil- ity, how can the engineer accept something less? The realities are quite simple. The DUCON® “old fashioned” process of hand patch Micro-Reinforced Concrete Systems with cement-sand-and-water, followed by asking the facility operator to keep traffic off the surface above the patch for at least fourteen days to control vibration that VSL would de-bond the patch, is not a dying magazineExternal Post-Tensioning Systems process…it is “dead”. Here isS a suggestion: T R U C T U R E knowing that overhead patches are ques- tionable, should something go wrong and a Tstrata™ potentially hazardous situation result from Enlargement Systems the overhead patch material dislodging, the author’s firm utilizes a combination of the existing and non-corrosive Engineered Solutions pins and wires within the patch material Our team integrates with engineers and owners to produce to provide secondary securement of the new material. high value, low impact solutions for repair and retro t of existing Soffit forming and patch material structures. We provide comprehensive technical support services placement by pumping has advanced sub- including feasibility, preliminary product design, speci cation stantially over the last 10 to 15 years. The need to over-pour through a pour pocket support, and construction budgets. Contact us today for assistance or trough to create a concrete head has with your project needs. been largely done away with, with flow- able materials [shrinkage compensated] and pressure pumps. Creating an SSD surface is much more difficult overhead. Young engineers should not despair. As self-consolidating concrete (SCC) has www.structuraltechnologies.com allowed the engineer to design a w/c = 0.4 concrete, water reducing admixtures +1-410-859-6539 have allowed that design to be placed with To learn more about Structural Group companies visit www.structuralgroup.com a 9-inch slump, a magic “goo” results that DUCON® trade names and patents are owned by DUCON GmbH and are distributed exclusively in North America can provide confidence for long-term bond by STRUCTURAL TECHNOLOGIES for strengthening and force protection applications. VSL is the registered trademark of VSL International Ltd. of patch material to parent concrete. Just wait and see!▪

STRUCTURE magazine15 April 2016