PAVEMENT UNDERSEALING (HIGHWAY GROUTING)
Pavement undersealing is the process of filling the small voids beneath the concrete pavement. The voids were created by heavy traffic, particularly trucks, flexing the concrete slab. Water enters the base through cracks and joints in the concrete. Undersealing is the process of mixing and pumping a thin grout into the voids beneath the slab. Undersealing should be performed before the voids become so thick that the pavement fails. This grout is composed of 1 part Portland cement to 3 parts fly ash or natural pozzolan to 1.5-3 parts water by volume. Undersealing is followed by joint sealing to prevent surface water from entering the base.
Other mineral fillers such as fine sand or limestone dust have also been tried but fly ash with its particle size, gradation and shape better penetrates thin voids. The grout is more easily pumped. It is important to test the fly ash in the grout as some fly ashes are unsuitable due to quality variations or due to their inherent chemical properties. Changes in coal source at the power plant may change the chemical and physical properties of the fly ash.
The highway grout has some unique properties. The quantity of water exceeds that required to hydrate the components. The physical shape and gradation of the fly ash gives the grout high permeability. This allows the grout to shed excess water with the application of low pumping pressure. The grout becomes more viscous. The grout sets in 1-6 hours and this initial set can be accelerated or retarded with the appropriate admixture. The 7 day compressive strength is at least 600 psi. ASTM and state DOT specifications must be followed.