Potential of fiber-reinforced concrete as a high-performance technical mixture

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Reinforcing the concrete pavement with structural fibers improves its durability and helps protect against potential flaws and cracks. The MnDOT has used fiber reinforced concrete (FRC) on some concrete bridge decks and pavements. However, winter weather conditions, freeze-thaw cycles and road salt further accelerate the deterioration of concrete. In a recent study, researchers evaluated FRC in the context of engineering performance mix design methods, giving the MnDOT confidence in the parameters that FRC must meet to help withstand Minnesota’s inclement weather.

MnDOT is a leader in the use of the Performance Engineering Mix (PEM) design method for pavements, which provides a variety of tests to simulate actual conditions during pour and throughout the life of concrete life. Traditional concrete testing methods have been used for decades, but they are generally empirical, do not represent field conditions, and are not specific to FRC. PEM design, on the other hand, uses new measurement technologies to identify the parameters a compound should have to maximize strength and durability.

“This was our first study evaluating fiber-containing concrete as a performing engineering mix, and we now understand the ranges of parameters that our fiber mixes need to have,” said Robert Golish, concrete engineer, MnDOT Office of Materials and Road Research.

To ensure that fiber-containing concrete mixes have the qualities that can help withstand Minnesota’s often harsh weather conditions, the MnDOT wanted to apply PEM design methods to verify the FRC property values ​​that would be specified to produce durable pavements that reduce maintenance costs and improve road quality.

What was our goal?

The objective of the study was to identify target PEM specifications for FRC blends to provide a durable and durable pavement.

What have we done?

To begin, the researchers conducted a literature review on the PEM design procedure and key engineering parameters, fiber types, and FRC properties. In the lab, they designed and tested 57 concrete mixes with PEM design methods. Blends varied in fiber and aggregate type, workability and air voids.

Box testing is a simple and inexpensive method of testing in which fresh concrete is poured into a formwork, subjected to vibration, and observed as the formwork is removed.

With input from MnDOT, the researchers selected two synthetic fibers commonly used in pavements and three classes of coarse aggregate based on standard MnDOT construction specifications. Three target plastic air contents (4%, 6%, and 8%) were tested, and the blends were kept within acceptable workability ranges.

Researchers performed concrete tests on each mix to understand the effect of structural fibers. Significant tests of the PEM design method for fresh concrete included:

  • Super Air Meter (SAM) test: Measures the air content with a modified manometer at different pressures and evaluates the resistance of concrete to freeze-thaw distress. Ordinary concrete should generally achieve a SAM number of 0.2, which corresponds to a durability factor of 70%.
  • Box test: Evaluates concrete’s response to vibration and its ability to hold its shape without spalling during paving operations. The recommended representative numerical rating for ordinary concrete, based on visual inspection, is 1 to 2, which corresponds to 10% to 30% surface voids (small cavities in the surface of the concrete).
  • Vibrating Kelly bullet test (V-Kelly): Measures fluidity under vibration to simulate field conditions such as the rate of penetration of a Kelly ball attached to a vibrator into a concrete trough. A V-Kelly index between 0.8 and 1.2 is appropriate for ordinary concrete.

Other tests measured properties of hardened concrete, including compressive strength, toughness and flexural stiffness, as well as resistivity to water penetration and freeze-thaw cycles.

“MnDOT engineers can have confidence in the use of fiber reinforced concrete in the PEM platform because they now have target ranges for more durable concrete,” said Manik Barman, associate professor in the Department of Civil Engineering. from the University of Minnesota Duluth.

Using the test results, the researchers developed preliminary specifications for recommended parameter ranges for FRC blends.

What have we learned?

By applying the PEM design procedure to FRC, the researchers demonstrated the appropriate ranges of fresh concrete properties:

  • SAM number-FRC achieves a SAM number comparable to regular concrete (0.2), although it may require more water-reducing admixture to be as usable and have the same air content. The researchers recommend that the SAM number for FRC blends should be as high as 0.3.
  • Box test rating—Adding fibers to the concrete resulted in higher ratings. The researchers recommend, again, adding a water-reducing additive to keep surface air voids below 30%.
  • V-Kelly Index—Increased fiber dosages create a stiffer mix with reduced workability (a lower index). To achieve the desired range, steps must be taken to improve handling. However, a lower V-Kelly rating may be appropriate for FRC as the strength and durability criteria have been met. But more workability study is needed before making this recommendation.

Tests for hardened concrete showed that the fiber had little effect on the concrete’s compressive strength, while flexural toughness, or crack resistance, and residual strength improved significantly.

And after?

This work assists the MnDOT in incorporating the fresh concrete property specifications of concrete containing fibers into the MnDOT standard specification for construction and provides assurance that FRC mixtures developed under the PEM design procedure will be durable and long lasting for use in roadways and bridge decks. .

Further investigation of blends and field testing, particularly to find an appropriate range for the V-Kelly Index for FRC, may be warranted.

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