Nalliah Thayabharan In 2009, retail business owners in the Algo Centre mall were complaining about leaking water damage to their stores. The Bank of Nova Scotia located inside the mall, had to temporarily close its doors in order to repair the water damage. .Sudden failure of normal reinforced concrete is very rare. Provided regular inspections are made the condition of the concrete can be observed and ongoing deterioration noted.
Reinforced concrete can fail due to inadequate strength, leading to mechanical failure, or due to a reduction in its durability. Corrosion and freeze/thaw cycles may damage poorly designed or constructed reinforced concrete. Recent failures of reinforced concrete have lead to new standards such as CSA S413 and the recognition of the following factors:
• Dense impermeable concrete slows the movement of salts towards the steel reinforcing.
• A greater thickness of concrete over the steel reinforcing lengthens the time before salts reach the steel.
• Epoxy coated reinforcing steel can inhibit rust. Note, however that local rusting can occur more quickly where the epoxy has been damaged.
• Good drainage in slabs in conjunction with a waterproof membrane prevents the movement of salts and water into the concrete.
Even with these measures, some concrete deterioration will eventually appear. The most common cause of concrete deterioration in Canada is the de-icing road salt. Highly corrosive salt water reach the steel reinforcing of the parking decks through cracks and by permeating the concrete. Due to high amounts of salt the reinforcing steel rusts. When rebar corrodes, the oxidation products (rust) expand and tends to flake, cracking the concrete and unbonding the rebar from the concrete. Further rusting often breaks the bond between the steel and concrete resulting in delamination. At this point, strength is significantly reduced because the steel no longer transfers the necessary tensile strength to the concrete. Unless all of the concrete contaminated with salt is removed, deterioration will continue once the new concrete is poured. In fact, the deterioration may even speed up! Unfortunately, because of cost and the time a structure must be under repair, total removal is often not an option. At a typical parking garage, only the most deteriorated concrete is replaced every three to five years. Rusting of the reinforcing steel accelerates during the following situations:
• higher salt (soluble chloride ion) concentration in the concrete
• moisture intrusion in the concrete
• oxygen
• warm temperature, deterioration advances more quickly at warmer temperatures.
Chlorides, including sodium chloride, can promote the corrosion of embedded steel rebar if present in sufficienty high concentration. Chloride anions induce both localized corrosion (pitting corrosion) and generalized corrosion of steel reinforcements. For this reason, one should only use fresh raw water or potable water for mixing concrete, ensure that the coarse and fine aggregates do not contain chlorides, and not use admixtures that contain chlorides.
It was once common for calcium chloride to be used as an admixture to promote rapid set-up of the concrete. It was also mistakenly believed that it would prevent freezing. However, this practice has fallen into disfavor once the deleterious effects of chlorides became known. It should be avoided when ever possible.
The use of de-icing salts is probably one of the primary causes of premature failure of reinforced or prestressed concrete parking garages. The use of epoxy-coated reinforcing bars and the application of cathodic protection has mitigated this problem to some extent. Also FRP rebars are known to be less susceptible to chlorides. Properly designed concrete mixtures that have been allowed to cure properly are effectively impervious to the effects of deicers.
Deterioration of concrete begins out of sight. Regular inspection will identify the signs of deterioration; the appearance of small cracks over and parallel to the reinforcing steel locations, and rust staining on the surface of the concrete. Not all cracks in concrete are due to rusting reinforcement. Concrete also cracks due to thermal stresses and other causes).
Concrete structures constructed with techniques other than standard reinforcing are also subject to deterioration. Many post tensioned parking garages have been constructed in the last decade. Steel cables are run through small diameter openings in these structures. The cables are tightened with anchors at either end of a span. The taut cables put the concrete in compression over the length of the span, squashing it if you like. This takes advantage of concrete's high natural strength in compression and avoids stressing its low tensile strength. It has the further advantage of closing any cracks that might otherwise appear. This keeps salt and water infiltration to a minimum.
Unfortunately, the high construction standards needed for this type of construction have not always been maintained. Problems include inadequate concrete cover over anchors and punctured protective sheaths around the cables. Both may lead to rusting and sudden failure. In comparison to the number of reinforcing bars found in typical construction, there are relatively few post-tensioning cables. Thus, each one is critical and a single failure may lead to a large collapse. Because of the hidden construction of the cables and anchors it is nearly impossible to inspect for deterioration without breaking away concrete.
Reinforced concrete for parking structures that may be exposed to deicing salt may benefit from use of epoxy-coated, hot dip galvanised or stainless steel rebar, although good design and a well-chosen cement mix may provide sufficient protection for many applications. Epoxy coated rebar can easily be identified by the light green colour of its epoxy coating. Hot dip galvanized rebar may be bright or dull grey depending on length of exposure, and stainless rebar exhibits a typical white metallic sheen that is readily distinguishable from carbon steel reinforcing bar. Another, cheaper way of protecting rebars is coating them with zinc phosphate. Zinc phosphate slowly reacts with calcium cations and the hydroxyl anions present in the cement pore water and forms a stable hydroxyapatite layer.
Penetrating sealants typically must be applied some time after curing. Sealants include paint, plastic foams, films and aluminum foil, felts or fabric mats sealed with tar, and layers of bentonite clay, sometimes used to seal roadbeds.
Corrosion inhibitors, such as calcium nitrite can also be added to the water mix before pouring concrete. Generally, 1–2 wt. % of with respect to cement weight is needed to prevent corrosion of the rebars. The nitrite anion is a mild oxidizer that oxidizes the soluble and mobile ferrous ions present at the surface of the corroding steel and causes it to precipitate as an insoluble ferric hydroxide . This causes the passivation of steel at the anodic oxidation sites. Nitrite is a much more active corrosion inhibitor than nitrate, a less powerful oxidizer of the divalent iron.
Reinforced concrete flat slabs are extensively used in buildings and parking garages. At the ultimate limit state, their design is usually governed by punching shear. Punching shear failure of one column may propagate to adjacent columns, eventually leading to the total collapse of the structure. In the case where no punching shear reinforcement is provided, failure develops in a brittle manner. Punching shear failure occurs with almost no warning signs, because deflections are small and cracks at top side of the slab are usually not visible .Over the past decades, several collapses due to punching shear failures have occurred, resulting in human casualties and large damages.
In the late 70’s, the punching failure of a slab during the construction phase led to the progressive collapse of a large part of a shopping center in Switzerland. During the winter of 1981, another collapse occurred at an underground parking garage at Bluche, Switzerland, which caused the death of two children, Figure 1b. In 2004, a catastrophic collapse occurred in an underground parking garage at Gretzenbach, Switzerland. This collapse resulted in the death of 7 firemen, who were extinguishing a fire in the garage. Failure likely started from one column and propagated to a large part of the structure.
These structural collapses show that there are some shortcomings in current codes of practice. As a consequence, there is a question whether flat plates designed according to current structural concrete codes really fulfill the basic requirements for structural integrity. The failure of reinforced concrete slabs is in most cases ductile, and causes only limited redistribution of loading.
Punching failure of concret slabs without shear reinforcement is an exception, and the drop in resistance at failure is considerable and thus leads to a large redistribution of loads, which can trigger failure at adjacent columns and eventually lead to the progressive collapse of large parts of the concrete structure.
One possibility to avoid these failures is to provide alternative load paths to transfer the load of a column after it has failed in punching shear. This may be achieved by having some ductility after failure,which can be provided by means of punching shear reinforcement or integrity reinforcement.
Nalliah Thayabharan BSSO
Building Experts Canada
www.buildingexperts.ca Posted June 27, 2012 08:17 AM
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