concrete joint reinforcement
Concrete Joint Reinforcing is used in concrete pavement at contraction and expansion joints. There are two primary styles of joint reinforcing which are meant for separate applications: load transfer and tied joints:
Load transfer dowels are commonly placed along transverse road contraction joints and are meant to transfer traffic loads from slab to slab rather than downward onto the slab base, which can cause settlement over time and expedite overall failure of the concrete pavement. Load transfer dowels are typically smooth to allow for horizontal movement between the 2 slabs while resisting differential vertical movement between them. These dowels help multiple sections of the concrete slab move together as opposed to single sections moving independently. When single sections move independently under traffic loads, the ‘aggregate interlock’ (that was once expected to be sufficient to resist differential movement from bay to bay) will fail over time putting localized areas of pressure on the underlying base. Load transfer dowels are now becoming required in most state new concrete paving specs because they, when designed and installed correctly, can extend a pavement life up to 50 years.
Many newer concrete paving machines can place dowel and tie bars into the wet concrete in between screeding operations.
Joint Tie bars are typically deformed rebar placed at longitudinal road construction joints. Tie bars are meant to assist in keeping two adjacent slabs tied together and to resist horizontal movement. They may provide some minimal load transfer characteristics but are not intended to do so and should not be utilized as such.
‘Dowel bar retrofit’ projects are also being done quite commonly and have been known to extend the life of existing concrete pavements up to 15 additional years. Dowel bar retrofitting consists of sawcutting slots at transverse joints, placing dowel bars at middle of slab elevation, and infilling the slots with concrete patching material. The surface is then diamond ground to remove imperfections.
There are several different material types of dowels, ranging from black steel to epoxy coated steel and stainless steel.
Joint dowels must resist corrosion long term from moisture and deicing chemicals in order to maintain their purpose and effectiveness. Epoxy coated dowels are the most commonly used dowel for corrosion resistance. Dowels can range from 3/8” in diameter to 1-1/2” inch or more by 12-24” long. The most common size range used today is ¾”- 1” x 12” – 20”. The standard spacing for transverse joint dowels is typically 12” but can be reduced or increased depending on the slab thickness, dowel bar diameter, and thus the shear stress being applied to the dowel bar. The embedment length on either side is commonly a minimum of 8 bar diameters but can also vary depending on the slab thickness and dowel bar diameter specified.
Dowels are commonly placed into proper position by ‘baskets’ (pre-designed steel support cages) which are laid out at the proposed joint locations prior to concrete placement.
For expansion or construction joints, dowels may be drilled and epoxied into place after one portion of the pavement is placed, and just prior to the adjacent portions placement. WHen high productivity dowel placement is required, there are specialized drilling machines that are commonly utilized for this work.