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Addressing Parking Garage Corrosion with Silica Fume THOMAS C

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Addressing Parking Garage Corrosion with Silica Fume THOMAS C 8 TRANSPORTATION RESEARCH RECORD 1204 Addressing Parking Garage Corrosion with Silica Fume THOMAS C. WElL Because it reduces chloride penetrability, silica-fume so does the corrosion rate. The corrosion product will concrete is gaining popularity as a corrosion-protection expand in size by roughly four times its original volume, system for parking garages, bridge decks, and other creating tensile pressures on the concrete up to 10,000 psi. structures. This paper provides an overview of experience The concrete eventually ruptures, causing cracking and with silica-fume concrete in the United States. spalling which allows more chlorides to enter at an even Twenty-two hundred parking areas were built between the faster rate. Eventually, the concrete will deteriorate, years 1979 and 1984 in what would be considered the frost requiring expensive rehabilitation or causing structural belt of the United States. Of these, 630 contained 150 failure. spaces or more (according to the McGraw-Hill Obviously, steps must be taken to protect these Construction Group). With more than 100 of the larger structures against chloride-induced corrosion. Because it is structures being constructed on an annual basis, it is very unlikely that the use of deicing salt will cease, design important that precautions be taken to protect them against engineers must rely on corrosion protection methods to deicing salt-induced corrosion. The latest exciting product protect their structures. Corrosion protection in many other to enter the corrosion protection market is silica fume (microsilica). industries is a normal part of the design package. For Concrete in a nonaggressive environment is a very reinforced concrete structures, however, it is a fairly new strong, durable, and long-lasting building material. In feature. Even so, many types of corrosion protection aggressive environments, some precautions may be needed products are already available on the market, and most have to protect the concrete or embedded steel. With the use of proven their worth. deicing salts since the early 1960s to keep our roads clear of Surface sealants and membrane systems attempt to snow and ice, concrete transportation-type structures are physically block chlorides from entering the concrete and now under attack. Bridges and parking garages are prevent them from reaching the rebar. Coated reinforcing deteriorating at an alarming rate due to chloride-induced steel attempts to place a physical barrier between the corrosion. Bridge decks have deicing chemicals deposited concrete and steel, thus preventing the chlorides from directly on the surface where the chemicals affect not only attacking the steel. Calcium nitrite corrosion inhibitor the deck but also the supporting structural members due to promotes the stabilization of the steel’s natural passivating leakage. Although deicing chemicals are used only layer, thereby controlling the corrosion rate. And finally, sporadically in parking garages, cars carry salt-infested snow latex modifiers and silica fume reduce the permeability of into the garages. Much of this salt remains after the cars the concrete, considerably slowing the ingress of chlorides. leave. Each of these systems has a different operational Unlike bridges, however, which are washed by spring mechanism, but all help extend the service life of the rains, parking garages are rarely washed down in the spring. structure. The salts deposited in the winter remain all year. Indeed, Silica fume is a by-product of silicon, ferrosilicon, or when comparing concrete chloride contents of bridge decks other silicon alloy production in a submerged arc electric and parking garage decks in the same location, and all other furnace. It contains a silica (SiO2) content of 85 percent or parameters being equal, the garage decks usually show a greater and has as an extremely fine particle size, which has higher chloride content at all slab depths. This chloride caused some researchers to call it a “super pozzolan.” The content is due to the presence of larger amounts of chlorides ultra-fine particle size of roughly 0.1 micrometers allows the on the slab surface during warmer weather. As ambient silica fume to fill the voids in the cement paste and between temperatures rise, chlorides are able to diffuse through the the cement paste and aggregate while the pozzolanic feature concrete pores at a greater rate of speed. causes reaction with the excess calcium hydroxide The highly alkaline environment of concrete creates a (Ca(OH)2). This process results in a far less permeable protective passivating layer on steel that inhibits the electro- microstructure matrix that appears homogeneous and that chemical reaction of corrosion under normal conditions. has no gaps and no large crystals of Ca(OH)2 (1). Chlorides will move through the concrete pores as well as Fairly long-term studies (2) have shown that silica fume through the transition zone between the paste and aggregate reduces the apparent diffusion coefficient of concrete by at and eventually reach the embedded steel. Penetration of the least an order of magnitude. Other long-term salt water passivating layer takes place, and corrosion of the steel ponding studies at H. R. Grace & Co.’s Construction Prod- begins. As the chloride content around the rebar increases, ucts Division, Cambridge, Massachusetts, are beginning to corroborate these results. W. R. Grace & Company, Construction Products Division, Cambridge, Mass. Weil 9 calcium hydroxide to produce more calcium silicate hydrate TABLE 1 CHLORIDE PERMEABILITY BASED ON CHARGE paste and filling the pores between the aggregate and cement PASSED grains. More paste with fewer voids creates a better bond Charge Passed Chloride Typical between the aggregate, producing higher strengths. To (coulombs) Permeability Description continue the earlier example (4), the concrete mix 4,000 High High water-cement ratio containing a cement factor of 550 lb of type I cement with a (0.6); conventional PCC water-cement ratio of 0.45 will produce a 28-day 2,000—4,000 Moderate Moderate water-cement ratio (0.4—0.5); compressive strength of 5,500 psi. Adding 7.5 percent silica conventional PCC fume by weight of cement and a high-range water reducer to 1,000—2,000 Low Low water-cement ratio insure workability produces a 28-day strength of 9,500 psi. (0.4); conventional PCC When 15 percent silica fume is added, the 28-day strength 100—1,000 Very Low Latex-modified concrete, reaches 10,300 psi (Table 2). The engineer should take internally sealed concrete advantage of this increased compressive strength and design 100 Negligible Polymer-impregnated not only for reduced concrete chloride permeability but for concrete; polymer concrete high concrete strengths as well. Actual concrete permeability is a feature that cannot, Due to reduced concrete chloride permeability, silica- unfortunately, be measured quickly for upcoming projects. fume concrete is gaining popularity as a major corrosion- In response to this problem, the Federal Highway protection system for parking garages, bridge decks, and Administration has produced a rapid test method for other structures. To date, the two major silica-fume determining the apparent chloride permeability of various admixture suppliers have participated in the construction of concretes (3). The 4-in.-diameter by 2-in.-thick specimens 39 parking garage projects (both completed and under are subjected to a 60-V potential for 6 hr to measure the construction). The first silica-fume parking garage was charge passed in coulombs. At least a dozen parameters completed in 1984, while 27 were either completed in 1986 affect the final coulomb reading, so five chloride or under construction in 1987. The average-size project 3 permeability categories were created (Table 1). Coulomb contained 3,500 yd of concrete, with the smallest 3 3 readings from different samples that fall in the same containing 150 yd . The largest, 45,000 yd is under category are considered to be equivalent as far as chloride construction. Dosage rates ranged from 3.75 percent silica permeability is concerned. Design engineers who have fume by weight of cement to 16 percent with an average of specified silica fume thus far required concrete coulomb 7.5 percent. Types of projects include less permeable over- readings to be in the 100—1,000 coulomb category (very lays, toppings on prestressed double tees, rehabilitation low). ranging from large-scale patching to completed deck Some engineers believe inaccuracies exist in the FHWA replacement, and standard cast-in-place new construction. test and are not specifying coulomb levels, but rather Even with all its advantages, silica-fume concrete does “percent silica fume by weight of cement.” Usually a require extra attention during finishing and curing. Due to specified silica fume quantity is based on prespecification the cohesiveness of silica-fume concrete, it is recommended testing at a local laboratory to see what amount will attain a that slumps at least an inch higher than normally placed be coulomb level of 1,000 or less. As an example, tests (4) run used. Another area of caution is the reduced amount of on a 550-lb cement factor mix with a water-cement ratio of bleed water after a slab is placed. Above a dosage rate of 5 0.45 yielded a coulomb reading of 3,600. When 7.5 percent percent silica fume by weight of cement, the bleed rate silica fume by weight of cement was added, a coulomb drops dramatically. As the silica-fume content increases, the reading of 850 was produced; while a 15 percent silica-fume bleed rate correspondingly decreases. ACI 302, Guide for addition gave 200 coulombs. It is not uncommon to see Concrete Floor and Slab Construction, or ACI 3()~, silica-fume specifications of 7.5 percent by weight of Standard Practice for Curing Concrete, should be followed cement. to reduce the possibility of plastic shrinkage cracks. Besides reducing concrete chloride permeability, silica- Recommended procedures include fogging during finishing fume concrete has many other benefits: increased abrasion and wet burlap or the use of curing compounds during curing.
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