Ultra Fast-Track Concrete Paving Belgian Experience

For three years the Roads and Traffic Administration (AWV) of Flemish Brabant in Belgium has been using the technique of ultrafast-track (UFT) concrete paving for the rehabilitation of several road sections. The aim is to achieve a compressive strength of 40 N/mm² on cores after 30 to 36 h of curing. The materials used are commonly available in concrete mixing plants, without the addition of fly ash or silica fume. Cement type and content and the water-cement (w/c) ratio are set in a preliminary laboratory study. On most of the work sites the results have been excellent and the goals have been achieved. However, a number of lessons have been learnt from field practice. In the preparation stage of the project workability plays a key role in the final design of the concrete mixture, particularly in the dosage of the superplasticizer. The base layer must resist heavy traffic loading within a day, since the paving concrete is to be delivered on that layer. Because of the limited tolerance in timing, all necessary precautions – such as checking the existing road for the presence of obstacles in the construction zone – have to be taken in advance, and measures must be taken if the bearing capacity of the subgrade is inadequate. In the execution stage a steady supply of concrete mixture has to be ensured, as the time window between mixing at the plant and laying on site is very narrow. Depending on temperature conditions, the composition of the concrete mixture may have to be altered to guarantee sufficient workability. Adjacent pavements have to be demolished carefully, to avoid damaging the newly laid section. And because of the high cement contents used, sawing and curing are even more crucial than for normal concrete pavements. The technique is suitable for application mainly on road sections with a length between 50 and 200 m (max. 1200 m²). Further laboratory research is planned in 2005-2006 with a view to extending experience to different mixtures and ambient temperatures. Besides the aspects of concrete technology, good organization of the work and quality management are the key factors for the success of a UFT project.