In line with Qatar’s construction development policy, the Qatar Petroleum District development has been designed to minimise its impact on the environment. From the conception of the project, the eco-friendly credentials of the development were paramount. With this in mind, the use of post-tensioning (PT) in the floor slabs of the buildings was identified as a factor which could significantly contribute to this goal.
CCL Qatar, part of CCL, a leading global specialist in this field, became involved with the project at early design stage and provided a preliminary post-tensioning scheme which convinced consultant KEO, to specify CCL post-tensioned slabs in the design, thereby providing the flat slabs and clear spans that were required. Throughout the project CCL has continued to work closely with KEO to ensure the achievement of the optimum design solution. Further support has been given by CCL Stressing Systems, the manufacturing arm of CCL based in Leeds, UK, which has contributed to the smooth running of the post-tensioning operation.
In total, there is an area of approximately 550,000m2 of post-tensioning in the QP District project. As of now, approximately 95% of the PT design work has been approved whilst the post-tensioning of the floors for towers one, two and three has been completed.
The use of post-tensioning had an immediate impact with regard to the sustainability of the project. Providing reduced structural thickness for similar spans and loads, post-tensioned concrete enabled the depth of the floor slabs to be minimised. This has led to a reduction in the amount of concrete used when compared to traditional reinforced concrete construction.
The thinner slabs achieved by CCL post-tensioning have maximised floor to ceiling heights and yielded larger living areas, at the same time as creating a lighter structure, requiring smaller foundations.
Post-tensioned slabs have also resulted in the use of less reinforcement than in traditionally constructed elements. In conventional reinforced concrete, reinforcement is placed throughout the member in areas of tension to take the tensile load arising from flexure or bending. With post-tensioning, a pre-compression is applied to the slab so that when the load is introduced and flexure occurs, the net tension is reduced, lessening the amount of reinforcement needed.
The reduction in the amounts of concrete and reinforcement being used on the project has not only impacted on material costs, but has also reduced embodied energy which in turn has contributed to a lowering of carbon emissions during the construction process. It is estimated that CO2 has been reduced by up to 25% in some areas. Lower quantities of concrete and reinforcement in the structure have also meant a reduction in the transportation of materials, decreasing environmental impacts still further.
Waste management of the post-tensioning materials on the QP development is closely regulated. The amount of waste generated by the post-tensioning operation has been minimal, but is nevertheless limited to a maximum of 4% of strand and galvanised ducting, all of which is 95% recyclable.
Following a change of management on the project, a number of late alterations were requested which necessitated the opening of some of the slabs and the cutting of tendons. These changes were carried out successfully, providing testament to the flexibility of post-tensioning and its ability to accommodate subsequent modifications. This work was undertaken whilst the construction process was under way, yet incurred only minimal additional costs for the client considering the number of changes required and the time involved.
Quality and safety are key factors on the project, and are monitored closely by all parties. Already classified as a class 1 sub-contractor by main contractor, Bouygues-Midmac-Al Jaber Joint Venture (BMJ), CCL is expected to deliver high standards of safety and quality management. The company has not disappointed, achieving an overall safety indicator of almost 97%, and receiving further recognition of its high standards of site safety in the form of commendations for a number of CCL employees, for their contribution to maintaining safe working conditions.
CCL was also recognised for its professionalism and organisational competence by Qatar Project Management (QPM), who carried out the quality control audit.
