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At almost 1km long the Rose Fitzgerald Kennedy Bridge The LT 104 Barrier on the Rose Fitzgerald Kennedy Bridge is a H4b class barrier and the first of its kind to be constructed in Ireland.
This CE certified barrier is 1,100mm high and includes 12 strands of reinforcement which formed a particular challenge for the placement operatives.
The reinforcement was placed through rollers with checks at 5m, 10m and 15m intervals at the commencement of each pour. Additional checks were required at 100m intervals thereafter to ensure the cover and alignment of the steel reinforcement.
Through the use of the Roadstone’s research and development laboratory in Dublin a wide range of trials were conducted to assess both the plastic and hardened properties of the concrete mixes including workability, workability retention and compressive strength.
Upon the completion of laboratory trials, operational trials were conducted to further assess the plastic and hardened properties. In addition Roadstone worked with other parties involved in the construction of the bridge to assess other parameters such as the pumpability of the different mixes.
The barrier offers high safety levels with no displacement or damage with car impact and minimum displacement, only 300mm, with high sided vehicles. This allows for effective separation of both east and west bound carriageways and ensures that the bridge stay cables are protected from impact.
Operational trials assessing the performance of the different mixes with different weather conditions such as cold and warm weather and wet and dry weather proved essential in understanding the performance of the mixes and planning accordingly for such conditions prior to each individual concrete pour.
A C35/45 air entrained mix to a 50 year design life was required to meet durability requirements with XD3 being the most onerous exposure class. Compressive strengths of greater than C32/40 are uncommon for air entrained mix types. This high strength classification resulted in the cement content being higher than is typically required for barrier mixes.
Control of the workability of the mix proved crucial. Trials were conducted to assess the mix performance. That trials demonstrated that if the mix arrived onsite too wet the placed concrete was prone too slumping resulting in levels being off.
If the mix was too dry the concrete was prone to setting too quickly, with separating and cracking due the high cement content and the use of a straight CEM II mix. The trials resulted in the slipform contractor requiring a mix that slumped onsite at 25mm ±5mm. Such tolerances are extremely tight. As a result of there was a particular focus placed on quality and service.
Every load was checked or tested at the supply plant with additional checks on raw materials also conducted. There was also close communication between the slipform contractor and the batching plant to ensure that each load arrived onsite at just the right time.