Fibers in concrete bridges controls cracks
High-performance, fiber-reinforced concrete controls cracks and provides satisfactory bond strengths in bridge deck construction, say Levon C. Hoomes, E.I.T., Celik Ozyildirim, Ph.D., P.E., principal research scientist, and Michael C. Brown, Ph.D., P.E., associate director, Virginia Center for Transportation Innovation and Research, in their 2014 TRB paper, “Evaluation of High Performance Fiber-Reinforced Concrete for Bridge Deck Connections, Closure Pours and Joints.”
“Connections, closure pours and joints in bridges are often a source of distress due to cracks and openings in them,” say Hoomes, Ozyildirim and Brown. “Wide separation facilitates the penetration of harmful solutions which can lead to costly repairs.”
Cracks are caused by volumetric changes due to moisture and temperature and the application of service loads after the concrete has hardened, they write. Poor bonding between the existing concrete and new concrete can lead to separation or opening, they say. Wide cracks or openings within the material or at the interface and leaking joints allow the ingress of water and chemicals, causing damage to the bridge deck sections, as well as the bridge substructure through corrosion of reinforcing steel, alkali-silica reactions, sulfate attack, and freeze-thaw damage.
“The addition of a small amount of discontinuous fibers to a conventional concrete matrix minimizes cracking, but the size of these cracks still permits the intrusion of harmful solutions,” say Hoomes, Ozyildirim and Brown. “High volumes of suitable fibers used in high performance fiber reinforced concrete produce multiple very tight cracks (< 0.1-mm wide), which do not allow for the ingress of water and other harmful solutions.
The authors evaluated plastic and hardened mixture properties, with emphasis on deflection hardening, flexural toughness, and bond strength. Different systems were tested: engineered cementitious composite (ECC); hybrid fiber-reinforced concrete (HyFRC) systems, including both steel and synthetic discontinuous fibers; HyFRC including only synthetic fibers; and ultra-high performance concrete with steel fibers.
ECC has low permeability, does not contain coarse aggregate, and is generally classified as a mortar mix, they write. ECC contains cement, fly ash, sand, and polyvinyl alcohol (PVA) microfibers (2 percent by volume) in order to achieve high ductility.
A hybrid fiber-reinforced concrete (HyFRC) system with both steel and synthetic discontinuous fibers can achieve strain hardening. “In contrast to ECC, coarse aggregates are typically used in HyFRC mixtures,” say Hoomes, Ozyildirim and Brown. “The presence of coarse aggregate reduces paste requirements, which is expected to decrease the amount of shrinkage of the material and be less costly.
A hybrid fiber-reinforced concrete system with different synthetic fibers, but excluding steel fibers, has the advantage of easier handling and high corrosion resistance, they say. A variety of combinations of fibers is possible; one system investigated contained only PVA macro- and micro-fibers, while a second system had 50 mm polypropylene fibers in addition to PVA fibers. These mixes were explored in an attempt to eliminate the use of steel fibers yet maintain the flexural and crack control characteristics seen in both the HyFRC and ECC mixes.
[gtblockquote type=”right” quote_text_size=”22″ quote_text_style=”normal” quote_text_color=”#9F0226″] This mixture has high compressive and bond strengths and is very durable.[/gtblockquote]Like ECC, UHPC with steel fibers is a mortar mix. “UHPC with steel fibers has been used for field-cast connections for precast deck panels,” they say. “UHPC evaluated was a prepackaged proprietary material; water and high-range water reducing agents are added and mixed thoroughly before the addition of 14-mm (0.6-inch) long, brass-coated steel fibers. “This mixture has high compressive and bond strengths and is very durable, with high resistance to chloride ion penetration, freeze-thaw cycles, and chemical attack.
The authors conclude the following:
• ECC and HyFRC mixes achieve deflection hardening with high fiber content of steel or synthetic fibers. ECC achieved the highest deflection hardening capacity of all systems tested.
• Although UHPC attained the highest stress capacity, the material did not deflection-harden; results are partly attributed to the settling of the fibers to the bottom. High shrinkage values do not necessarily indicate wide cracks. ECC exhibited the highest shrinkage values, yet showed deflection hardening and uniformly distributed, tight cracks instead of few wide cracks.
• Workability of mixtures with steel fibers was higher than those with synthetic fibers.
• All mixtures achieved adequate bond strength for use in closure pours.