The constituent materials used in the production of concrete were unpacked at the recent seminar ConSem 2014, organised by the Concrete Society of Southern Africa. A total of 250 delegates attended the event, which was held in Durban, Port Elizabeth, Cape Town and Johannesburg, where industry experts in concrete technology and application describe the latest developments in cement, extenders, aggregate, admixtures, specifications and concrete research.
The scene was set by Bryan Perrie, from The Concrete Institute, who presented an overview of the manufacture of cement, the various types that can be produced in terms of SANS 50197 and SANS 50413, and the availability of the various generic types of cement by supplier by region. Particular focus was made on the industry measures being taken in reducing the CO2 emissions in the production of both cement and concrete.
His presentation went a long way to assist delegates in understanding why the current range of cements exists, the benefits of the various cement types and why it is so important to specify cements by their standard nomenclature rather than brand names.
Prof Mark Alexander from the University of Cape Town presented an overview of the influence of aggregates on hardened concrete and reminded delegates that aggregates make concrete 'fit-for-purpose' by imparting to concrete:
- Volumetric stability
- Thermal expansion compatibility with reinforcing steel
- Stiffness, strength and toughness
- Ease of use
He continued by describing the effects of aggregates on concrete in terms of the ITZ (Interfacial Transition Zone), strength, deformation properties, elastic modulus, shrinkage, creep, thermal properties and durability. The section on Alkali-Silica Reaction was particularly interesting and relevant in the South African context.
Bryan Perrie presented his second paper of the seminar which focused on cementitious extenders and their application. Aspects covered included types of extenders, their availability, methods of use, and application examples. The latest specifications for these supplementary cementitious materials were described as well as their use and the specific features and benefits they offered when added to concrete.
Once referred to as 'snake oil', chemical admixtures were described by Graeme Smith (and by Hennie van Heerden in Port Elizabeth and Cape Town) of Sephaku as complex, formulated chemical products that enhance the performance and final properties of concrete. They have become the "fifth ingredient of concrete".
The large number of admixtures available in South Africa were described and ranged from plasticisers to anti-washout, underwater concrete admixtures. He estimated that over 80% of all Ready Mixed and Precast Concrete currently produced contained some form of admixture and that approximately 80% of that was in the form of plasticisers and super-plasticisers.
Smith (and van Heerden) covered in some detail the dispersant role of plasticisers explaining that the normal pplasticiser is adsorbed onto cement particles, repelling cement particles by electrostatic or steric repulsion and thereby stopping cement from flocculating in clumps and trapping water. This increases the fluidity of the mix.
The presentation continued with a detailed overview of self-compacting concrete (SCC) which resulted from the development of PolyCarboxylate Ether (PCE) Superplasticisers. It was said that any SCC mix design must consistently achieve:
- Good flow
- No segregation
- Good passing ability through reinforcement /no blocking
- Appropriate viscosity / speed of flow
In addition, SCC mix design requires an in depth knowledge of concrete mix design and materials as well as consistently good constituents.
In conclusion, mention was made of the wide range of other admixtures that were available to change and/or enhance the properties and performance of concrete, including those for air entrainment, waterproofing, shrinkage compensation, acceleration, retardation and spray applications.
In each centre hosting the event, a local project featuring the modern approach to concrete mix design, application and use of materials was presented, and these were:
Mahatma Gandhi Road Sewer Pump Station: Kendall Slater, Hatch Goba
Dedisa Peaking Power Plant, Coega: Campbell Moloi, Ansaldo Energia
Portside Building: Cyril Attwell, Murray and Roberts
Electrical Cabins for Wind Farms: Justin Kretzmar, ISG Group
The afternoon session began with a fascinating look into the world of concrete research presented by Professor Elsabé Kearsley from the University of Pretoria. Delegates were exposed to the future of concrete with recycled aggregate; 'green cement'; ultra-thin, high-strength concrete; ultra-thin, continuously reinforced concrete pavement; more appropriate test methods and specifications and centrifuge modelling.
She concluded that the use of cementitious materials in the construction industry can be significantly increased by using modern technology to optimize material properties and mechanical behaviour. By using materials with suitable properties; optimizing material properties; using appropriate specifications; using suitable test methods; measuring relevant properties and curing the concrete properly.
The final session of the day was given to George Evans of PPC who, in his own inimitable style brought the various topics of all the presentations together into one and showing how good (but not perfect) concrete could be produced, consistently.
Good concrete, he concluded, is the result of good planning involving everyone in the chain. There is no such thing as perfect concrete he suggested - many well-designed concrete mixes are perfect for a given application, but some are better than others.
Time will tell how durable the structure is, and therefore how good the concrete really is.......