Performance concrete is about to go mainline, as in paving. A group of researchers have proven that by tailoring the concrete mixture proportions, they can create a concrete mix that becomes consolidated without internal or external vibration while it maintains its slab shape immediately following the paving process.

Their research could radically change the mainline paving practice.

Currently, the contractor orders low-slump concrete. Upon delivery, the high-tech paver consolidates and shapes the fresh concrete into a slab while finishing the surface. The critical aspect of the paving process is how a machine consolidates the stiff, fresh concrete into a homogenous volume. The paver relies on equally spaced internal vibrators to consolidate the concrete.

If the operator doesn't set the vibration frequency properly, the fresh concrete can be over-vibrated, causing aggregate segregation and poor air entrainment. When this happens, zones of paving defects, called vibrator trails, are left in the hardened surface. Over time, highway engineers have documented that vibrator trails become areas of heavy repairs.

By eliminating vibrator trails, researchers hope to increase the pavement's service life and lower annual maintenance expenses. The goal is to develop a fresh concrete mix design that doesn't require internal vibration to consolidate and keeps sufficient shape stability. The fresh pavement must retain its edges while standing free without any support after the slipform paver has passed. And this concrete mix must be stable enough to allow immediate surface texturing.

Altering SCC

The search for this wonder mix has focused on adapting self-consolidating concrete (SCC) from a spreading material to a quick self-standing substance. The approach reduces an SCC mix's flowability immediately after placement while increasing its green strength.

The researchers have been able to modify the rheological properties of a conventional SCC mixture with a variety of chemical admixtures and fine materials.

Their mix design avoided larger aggregates, as used in regular pavement concrete. So, they focused on adding Class F fly ash and three types of clay as mineral additives. They discovered that by adding a very small amount of certain clays, they reduced the mix flow ratio to near zero. They attributed this result to the combined effect of the water absorption of the fine materials (no additional water has been added) and the particle size and shape of these materials.

The researchers also discovered that adding viscosity-modifying admixtures to the concrete mix can achieve significant improvement in green strength.

They tested the mix designs in a model paver, specially developed for this research by Iowa State University to simulate the slipform paving process without applying internal or external vibration. The test showed it was possible to use the modified SCC.

It is not clear if this paving technique could potentially be used on a full scale. Currently, it is only intended to use the paver for laboratory investigations because it does not seem viable to eliminate the surface vibration that is also applied to the concrete by the traditional concrete pavers.