Time flies for Tarmac PQ-X Cement
21st February 2007
Special rapid-hardening PQ-X cement from CMS Pozament was specified by MJS Construction as part of extensive reinstatement works on the taxiway at East Midlands Airport. The time-contingent works meant that MJS had just 12 hours to break out a 50m2, 350mm-thick slab of concrete and reinstate it with new PQ-X concrete, ready to withstand the weight of an airliner.
The 14-day refurbishment programme, which took place in (month) (year), involved replacing more than 850m2 of concrete taxiway, carried out during strict 12-hour daytime ‘possession’ periods. Tarmac’s PQ-X is a rapid-hardening cement with high early strength properties. It can be made with standard aggregates using normal ready-mix, or on-site batching, to produce an air-entrained concrete that complies with stringent BAA specifications. PQ-X has a compressive strength of over 20N/mm2, increasing to 65N/mm2 after 28 days.
The tight time schedules meant that MJS had to work with military precision so that the PQ-X concrete has sufficient time to set, ready for use by large aircraft the same evening.
Mike Smith, managing director of MJS Construction, has a wealth of experience of similar projects, which enabled him to identify CMS Pozament’s PQ-X cement as possessing the best performance properties for the job in hand, and for convenient on-site concrete production. MJS has used Tarmac’s PQ-X concrete on many projects, and so was aware of its exceptional rapid setting properties, as well as greater freeze/thaw resistance, compared with conventional concrete. This was a vital asset when completing winter pouring jobs like the one at East Midlands airport.
Project outline
Site works at the East Midlands Airport typically started at 8.00am, when the MJS team took possession of the site. Works vehicles were moved in convoy along the taxiway, guided by the airport’s facilities management supervisor.
Two excavators equipped with breaker tools would begin excavating a 12m x 4m slab of concrete, and by 10:00 the bay would be fully cleared and all concrete waste removed off site. A mini excavator would then begin cutting a 500mm trench in the sub-base of the slab to hold four cable ducts. Before 11.00am the walls of the bay were lined with fibreboard and the cable ducts are installed and connected to ducting in the neighbouring slab.
By midday the PQ-X concrete for the ducting trench is being mixed, and by 1.00pm the concrete is placed in the trench, screeded and left for a short time for initial set. Crews then prepare the bay, and by 2:00pm two layers of reinforcement mesh and a plastic membrane are installed and the bay is ready to receive concrete.
Within the hour, the first of three Tarmac ready-mix trucks arrive, loaded with enough aggregate, sand and water to batch 5m3 of concrete. Because of its quick setting properties, the PQ-X is added to the mix on-site, straight from IBC bags, followed by final water adjustments to produce the specific compacting factor. Tarmac CMS Pozament supplied the PQ-X at a precise weight of 455 kg for each cubic metre of concrete.
At 3.15pm the mix is emptied and the first pour levelled within 20 minutes using poker vibrators and a roller striker, followed almost seamlessly by pouring from the other two ready-mix trucks. After each pour is laid, a good working finish is achieved using a float, followed by a brush, with the new slab complete at 5.00pm.
Over the next three hours the fast-curing PQ-X concrete will reach a compressive strength of over 20N/mm2 (increasing to 65N/mm2 after 28 days) enabling the taxiway to be opened to aircraft on schedule at the end of the possession period.
