TECON Seminar 2013
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A very successful event in terms of content and organisation, with excellent speakers and relevant topics. The attendance levels were however, somewhat disappointing with approx. half of the average number of people attending.
The Seminar began with an overview by Bryan Perrie from the newly-formed Concrete Institute, of current and planned specifications and test methods for concrete. He explained that revisions were currently being carried out on:
- Loading code and basis of design
- Concrete water retaining standard
- Concrete design standard
- Geotechnical design
For the design of concrete structures, the working group decision taken in 2007 involved the adoption of EN 1992-1-1 with its own set of nationally determined parameters. The draft code was planned to be finished for comment by the end of 2012, but it would seem that this is now likely to be the end of 2013.
New specifications are already in place for cement (SANS 50197); fly ash (SANS 50450); silica fume (SANS 53263); Blast Furnace Slag (SANS 55167); admixtures (SANS 50934 1-6) and water (SANS 51008). SANS 1083 covering aggregates will remain as is, whilst a decision is awaited on the specification for metallurgical slags.
With regard to testing of concrete (fresh and hardened), the SANS 3001 series will be adopted, with ‘Co’ indicating the test methods for concrete. Nomenclature would read:
3001 Co 1 Parts 1- ? Fresh Concrete
3001 Co 2 Parts 1- ? Hardened Concrete
3001 Co 3 Parts 1- ? Concrete in Structures
On the subject of specifying concrete, Perrie indicated that the traditional approach was to specify certain properties and actions for aggregates, concrete, construction process and quality control (strength). This is a very prescriptive approach with some performance requirements. The new design philosophy is to determine the environment and required longevity/durability; choose an approach to achieve durability, and then determine structural design
Marius Grassman from Concrete Testing Equipment was next to present and provided delegates with a review of equipment used for general testing in a construction environment.
Plastic cube moulds are now often used
The more common tests were covered in some detail including slump, compression strength, non-destructive tests as well as more advanced testing with very sophisticated machinery.
The focus was on the equipment used such as cones, presses, moulds, vibrating tables, compactors and curing tanks, but also covered was state-of-the-art machinery for specialised tests such as flexural, elastic modulus, deflection, deformation of first crack strength, ductility of fibre-reinforced concrete, etc.
The importance of maintenance and calibration of equipment was strongly emphasised, with details of the requirements set out by ILAC-G2 (International laboratory accreditation system) and OIML D 10 (International organization of Legal Metrology).
‘Materials Sampling and Testing for Concrete’, was presented by Johan van Wyk of the Southern African Readymix Association. Materials covered were cement, stone, sand, water, extenders and admixtures. Testing these materials was critical to producing a consistently successful concrete, and in identifying any suspect materials before committing them to production quantities.
The importance of always preparing trials mixes was stressed.
Bruce Raath, of Letaba Management Services spoke on the interpretation of test results and what they meant to the engineer and other specifiers. He began by outlining the major properties of concrete that cause disputes following interpretation of test results:
- Bond between reinforcement and concrete
- Characteristic (specified) strength
- Shrinkage limits
- Crack acceptance by type and width
- Surface flatness and hardness
- Elastic modulus
Great attention was paid in the presentation to the cube strength of the concrete as so much importance is placed on this by the specifier. It is common practice for the contractor to supply concrete against a specified cube strength, and yet, according to Raath, is it probably the most poorly interpreted property of all. In many contracts cube strength is the only specified concrete property, and much of the contract hinges on it. Ironically, cube strength is not, and was never meant to be, the strength of the concrete in the structure!
Raath went on to describe the drilling of cores in concrete and the relationship between cube test results and core test results. Cores are taken to estimate what the cube strength was at the time of concrete placement.
A case study in concrete testing was presented by Jaco van Niekerk, of the Department of Water Affairs. The project in question was the De Hoop Dam near Steelpoort, Limpopo and the testing regime covering immersion-vibrated roller-compacted concrete was featured.
|Immersion vibration of the concrete|
This impressive dam utilised over 1,1 million m3 of concrete and the importance of preparing trial mixes was clearly demonstrated again with the final concrete mix providing impermeability; workability;no segregation; a good surface finish with no honeycombing; durability andcost effectiveness.
Quality control was kept simple with density testing;compressive strength verification; andcore testing. The field testing specification of the roller-compacted concrete required compaction of RCC to be within 15 minutes after spreading or a maximum of 40 minutes after batching.
Density testing was conducted by means of a Nuclear Density Gauge and temperature testing by means of an infrared device.
Typical laboratory testing conducted on the Roller-Compacted Concrete was:
– Constant Grading Analysis and Moisture Content
– Vebe Times
– Compressive Strength
– Setting Time
– Core Testing
During the construction phase, the contractor broke the South African record for RCC placement – 131,000m3 in 28 Days.
Presentation of the next paper was shared by Prof Mark Alexander, from UCT (coastal venues) and Prof Yunus Ballim from WITS (Gauteng venue). The focus here was ‘performance testing of concrete’.
Review of durability provisions in various codes including the South African code SANS 10100-2 and the presenters concluded that these codes are universally prescriptive in nature. As such, they do not meet the needs of modern code formulations or the demands of modern concrete construction.Prescriptive requirements are virtually meaningless in the light of the multi-role binder types now available and being used.
The performance-based approach to specifying concrete outlines the functional requirements for the hardened concrete depending on the application. It measures properties of concrete that account for performance, e.g. durability and provides requirements that are clear, achievable, measurable and enforceable.
The advantages of this are that specifications clearly define the functional requirements of the hardened concrete. Producers and contractors partner together, so that the right concrete mix is designed, delivered and installed. There are established and standardized field acceptance tests to determine conformance.
‘Preventing Concrete Failures’ - the presenter, Steve Schulte's passion for building quality systems for construction projects, emanating from risk assessments that reveal concrete to be one of the highest risk items for a project, was evident. Common specifying methods, acceptance criteria for concrete, and relevance of various standard test methods were debated, in relation to the cause and effect analysis of concrete failures which point more heavily towards the human factor rather than technology.
Schulte introduced the concept of a reverse engineering process including the installation, calibration and commissioning of a data logger at a batch plant, followed by the reliable, accurate and consistent collection of data from batch plant load cells and other input signals as required. Samples of collected data were presented and the reverse engineering process explained, involving the safe and reliable long term storage of all collected data and the manipulation of collected data for the purpose of pattern recognition, and the calculations required to recompile mix ingredients from data collected by the logger.
Further development on the process will be the engineering of a mathematical algorithm to automatically interpret the nature of all collected data to include pattern recognition techniques, to demonstrate the effects of wind loading, impact loading on load cells, material theft, and calibration and batching activity amongst others.
This process will also include reverse engineering calculations comparing the reverse engineered load to specifications and acceptance criteria, to automatically determine whether or not a load of concrete will meet requirements. Loads that do not meet requirements would trigger the management software to send appropriate error messages and instant notification to relevant people before the truck leaves the batch plant in an attempt to proactively avert potential defective product.
The final presentation of the seminar dealt with factors influencing test results given by George Evans from PPC Ltd. He suggested that:
|George Evans, PPC presenting in his inimitable style|
Test results can be influenced by:
Influencers of test results are:
Training given to testers
Experience of testers
Consistency of testing
Objectivity - non-biased
No faking of results!
Each aspect was dealt with in some detail, and of particular interest was the influence of time on slump test results, and the importance of sampling techniques. Evans demonstrated how the speed with which the tester raises the slump cone from the floor, determines the final slump measurement of a concrete mix. In the example he cited, the slumps achieved with lifting the cone from slow to fast, ranged from 90mm to 150mm respectively.
Sampling was defined as ‘a small part or quantity intended to show what the whole is like’ and aspects considered included the frequency, numbers of increments, the size of the sample and the point of sampling (for example, cement being delivered should always be sampled from the bottom of the tanker).
The need for experience in testing (and therefore, adequate training) was highlighted and examples were shown where the lack of knowledge and experience compromised the testing regime with sometimes seemingly ludicrous adjustments to samples. One illustration was shown where the tester had simply ‘topped up’ his cube with cement paste to bring it to the correct size for the compression machine.
Question time was lively at all venues and feedback from delegates after the presentations was very positive in terms of the content of the programme and the quality of speakers.