The document* gives guidance on the characteristic performance of fresh concrete and its method of placement using tremie methods in bored piles and diaphragm walls.
It says that getting the mix right can only be achieved in a joint approach by the specialist contractor (to achieve the execution requirements), the designer (to meet the durability and structural needs), and the supplier (to produce an economic and practical mix).
Recent trends have favoured higher strength classes and lower water/cement ratios, resulting in greater dependence on admixtures to compensate for reduced workability and to meet the competing demands for workability in the fresh state and setting time. The application of testing methods which reflect the rheological properties of the concrete has not developed at the same rate as the mixes themselves and it is still not uncommon for the slump or flow table test to be used as the only acceptance test for the fresh concrete.
A review of problems in bored piles and diaphragm walls cast using tremie methods by both the European Federation of Foundation Contractors (EFFC) and the Deep Foundations Institute (DFI) in the United States identified a common issue that many of the problems were caused, at least partially, by the use of unsuitable concrete mixes. Inadequate concrete specifications and insufficient testing procedures were identified as a primary cause. The consequences of these problems are often significant and it has been recognised that spending more on getting the concrete right is the most cost effective approach.
A joint concrete task group was set up by EFFC and DFI in 2014 to look at this issue and its best practice guide is now available.
This first edition of the guide proposes appropriate performance criteria for the concrete together with test methods and initial recommendations on acceptance values. A second edition of the guide will be published on completion of further research and development work as this will allow definitive acceptance criteria to be presented.
The guide also addresses design considerations including concrete rheology, mix design, reinforcement detailing and concrete cover as well as best practice rules for placement. A review of methods to test the as-built elements is presented together with advice on the identification and interpretation of results.
* The EFFC/DFI Best Practice Guide to Tremie Concrete for Deep Foundations (1st Edition) is available for free download at www.effc.org
Got a story? Email news@theconstructionindex.co.uk
