H. Celik Ozyildirim, Virginia Transportation Research Council
Over the years, concrete has performed well in bridge decks. However, with increasing use of deicing salts and changes in concrete constituent materials, many decks are exhibiting distress that requires costly repairs. The distress may be the result of corrosion of reinforcement, freeze-thaw deterioration, alkali-aggregate reactivity, or sulfate attack. In each case, water and solutions penetrating into the concrete initiate the deterioration. Therefore, when exposed to these environments, concretes must have a high resistance to the penetration of water and harmful solutions if the concrete is to achieve longevity. This can be achieved with a low permeability concrete. In addition, for resistance to damage from freezing and thawing, a proper air-void system is needed.
Most specifications require a minimum compressive strength, a maximum w a t e r-cementitious material ratio (w/cm), and a minimum cementitious material content. It is well established that lowering the w/cm reduces the permeability of concrete. However, at a low w/cm, the workability of concrete becomes a concern. Presently, fly ash, slag, or silica fume are used to reduce the permeability of bridge deck concretes at a conventional w/cm of 0.40 to 0.45.
The concept that strength, cementitious material content, and w/cm requirements will ensure durable concretes is misleading. Controlling w/cm in field concretes has been difficult. Recent cements are finer and have larger amounts of fast-reacting compounds. The hydration reaction occurs faster and the hydration products are not as uniform. Even though satisfactory strengths are achieved, low permeability is not always attained.
The Virginia Department of Transportation (VDOT) is attempting to obtain low permeability concrete by testing concretes for their resistance to chloride penetration. The chloride permeability test is described in AASHTO T277 or ASTM C 1202, “Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration.” In this test, the charge (in coulombs) passed through a saturated concrete specimen 2-in (50-mm) thick and 4-in (100-mm) in diameter in a
six-hour period is determined. Low values indicate high resistance to penetration by chloride solutions. It is well known that permeability decreases with concrete age, and the rate of reduction depends on the type of cementitious material. For example, fly ash concretes do not show the expected low permeability at 28 days since it takes longer for fly ash to react and exhibit its effectiveness. Therefore, an accelerated curing method is included in the VDOT special provisions. The specimens are cured one week at 73°F ( 23°C) and three weeks at 100°F ( 38°C). Results are similar to those obtained after six months of curing at 73°F (23°C). The specified maximum coulomb value is 2,500 for bridge deck concretes. This limit can be achieved with a conventional w/cm of 0.40 to 0.45 as long as sufficient amounts of Class F flyash, slag or silica fume are added into the mixture and proper construction practices are followed.
Construction practices including consolidation and curing are also addressed in the VDOT special provisions. For bridge decks, moist curing is required for a minimum of seven days and until 70 percent of the minimum 28-day design compressive strength is attained. Protection by fogging is required to prevent rapid drying of the concrete surface until application of wet burlap and plastic sheeting. After moist curing, a curing compound is applied. Curing of one of the decks constructed in accordance with the low permeability special provisions is shown in the photograph. For the 1995-97 construction seasons, five decks were successfully built using these provisions. More are under construction or planned. Proper consolidation is emphasized. Internal vibrators and screeds with vibrating elements are specified with lower limits in the vibration frequencies.
The low-permeability provisions will become a part of an end-result specification (ERS) being developed. In the ERS, limits on air content, slump, and temperature are specified as screening tests. Acceptance will be based on concrete properties such as strength and permeability as determined by the rapid permeability test, and construction practices such as concrete cover, deck thickness, and surface smoothness. Studies are continuing on a test to address the volumetric changes attributable to moisture. The new specifications addressing durability directly are expected to result in long-lasting and cost-effective bridge decks.
Further Information
Further information about VDOT’s approach to specifying durable bridge decks is available from the author at 804-293-1977 or [email protected].