Concrete technology at UCL

Interest in the plastic analysis of structures has led to two broad categories of work: the residual strength of reinforced concrete elements damaged by reinforcement corrosion and membrane action in concrete slabs.

Residual strength of damaged elements and membrane action in slabs

Residual strength work involves the assessment of the confining effect of links, albeit low-level in non-earthquake resisting columns, and the effect of bond loss in flexural elements. The work on membrane action is concerned with identifying the in-plane stiffness of slab areas surrounding a failure mechanism.

A particular application being considered is a groundbearing reinforced concrete slab supporting a localised load, for example, from the column of a storage rack or a wheel. Current design practice makes use of either charts, which are heavily dependent on empirical data, or formulae based on pure flexure yield line analysis. The formulae, both in their prediction of fan radius and load capacity, do not account for the large scatter in experimental results. The capacity of the slab is enhanced by the membrane action produced by the restraining effect of areas of the slab surrounding the mechanism. Work at UCL shows that membrane action consideration leads to a rational basis for the determination of the fan radius. Figures 1 and 2 show our test rig and a failed slab during an experimental programme putting a new theoretical approach to the test.

Loads near to a slab edge, or a construction joint, will not be significantly enhanced by this membrane effect. However, ground-bearing slabs are being cast in increasingly large pours and hence large areas exist where loads can now be distant from an edge. This means that economies arising from the consideration of membrane action are well worth an improved design approach.

Reinforcement corrosion and infrastructure maintenance and repair

Another long-standing area of interest in durability focuses on the global, multi-billion pound problem of reinforcement corrosion in transport infrastructure, in particular bridges, due to the presence of water laden with chloride ions from de-icing salt or seawater. Fundamental research has included developing a deeper understanding of the effect on corrosion of chloride ion binding, cathode to anode area ratio, cracking in the concrete cover and patch repairs. Other investigations, both theoretical and practical, have been carried out on preventative maintenance and repair techniques, including pore liners, waterproofing membranes, cathodic protection and electrochemical chloride removal. Results from these projects are being used to develop maintenance strategies for managing stocks of bridges suffering from chloride attack and cost-effective methods of preventing reinforcement corrosion in new construction.

A major drawback of many of the measures for enhancing durability is that, although they promise to reduce future maintenance costs, they increase initial costs and therefore may not be affordable when application is required (see Figure 3). Work on life-cycle costing is being extended to take account of this and other factors that are preventing the more widespread use of this technique.

Concerns about the impact of construction on sustainable development have led to a review of existing practices and recommendations for design, repair and management of structures (see Figure 4). Work is focused on developing straightforward models to assess the sustainability of alternative strategies.

Self-compacting concrete

Studies on the exciting area of self-compacting concrete (SCC) have been ongoing for ten years and a research group at UCL has focused on the choice of component materials, mix proportioning and test methods. In particular, the use of tests on the mortar fraction as part of the mix design process continues to be of interest, with recent work concentrating on the effect of viscosity-modifying agents and the extent to which secondary materials can be successfully incorporated in the concrete.

Knowledge of the rheology of the both the mortar fraction and the concrete itself is essential for a full understanding of SCC. Previous work at UCL in this area, which included the development of an improved version of the two-point workability test for fresh concrete originally developed at Sheffield University (see Figure 5), has been extended to SCC. The recent comparative rheometer tests, sponsored by the American Concrete Institute, in which UCL participated, gave some useful correlations to enable comparison of results obtained by different workers using different instruments.

Major recent work has included participation in the EU Growth Project Testing-SCC, which developed standardised tests for SCC. The author was directly involved in the initial selection of materials and the development of mixes with the three key properties of filling ability, passing ability and resistance to segregation. He also coordinated the UK section of the 'round-robin' test programme with the preferred test methods, in which five laboratories participated (see Figure 6). Results and recommendations have been taken forward to a pan-European group producing guidelines for SCC and will be used in the production of Standards for test methods. UCL also made a significant contribution to the recently published joint Concrete Society/BRE Technical Report 62, Self-compacting Concrete: A review(1).

Cement in waste management

Cement has a spectrum of applications in waste management. At one extreme, industrial by-products such as pulverised-fuel ash (pfa) and ground-granulated blast furnace slag (ggbs) are valuable and accepted additions to cement and concrete products, known to increase durability; at the other, cement is used to contain radioactive wastes. There is the potential for a variety of complex interactions between cementing materials and other components of these systems, which can have important effects on the final product properties, such as durability and contaminent immobilisation. A group is working on both the use of industrial by-products and the treatment of wastes and contaminated land by cement-based stabilisation/solidification. This has involved developing technologies and test methods in the laboratory, preparing regulatory guidance documents, implementing and evaluating technology performance at field scale, and doing computer modelling. Recent work has studied the effects of interactions between waste components and cement on important properties by the use of neural network analysis to identify general trends in existing data from a variety of sources (see Figure 7).

Current interests also include development of operating windows for common UK waste types (e.g. filtercakes from treatment of liquid wastes, air pollution control residues and contaminated soil) to facilitate implementation of the waste pre-treatment requirement under the EC Landfill Directive, development of novel processes for containment of soluble contaminants (e.g., chlorides in solidified air pollution control residues from municipal waste incineration (see Figure 8)) and understanding of contaminant immobilisation and transport mechanisms in cement-based wasteforms and other media.

Concluding remarks

The department has very recently substantially increased its academic staff numbers, and is developing research interests in a number of exciting and challenging new areas. Cement and concrete technology will continue to be a major interest. Further details of the department and all research areas can be found at www. civeng. ucl. ac. uk/