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 concrete 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 precast concrete 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 Advertiser ADVERTISEMENT–For Information, visit 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 www.STRUCTUREmag.org 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.