Concrete is getting stronger and stronger
by David Beadle, Cement & Concrete Ass.
P
CONCRETE more than five times as
strong as the strongest normally used
so strong that its manufacture em-
ploys methods borrowed from ad- vanced metallurgy is one result of research being carried out at the Wex- ham Springs research station of Bri- tain's Cement and Concrete Associa- tion.
The extra-high-strength concrete was developed as a means of studying the properties of cement-based mate- rials stronger than those obtainable by conventional methods.
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The strength of concrete conven- tionally expressed in terms of the com- pressive strength needed to crush a standard cured specimen depends primarily on its cement content, and more particularly on the ratio of water to cement in the cement-paste matrix: the great majority of structural con- crete has a water/cement ratio of 0.55 to 0.45, giving strengths of 4,000 to 6,000 pounds force per square inch (28 to 42 Newton (N) per square milli- metre) and strengths of 8,000 pounds force per square inch (55 N. per square millimetre) are about the maximum in normal use.
Metallurgical technique used
Because the water/cement ratio of the matrix is the main factor in
strength, research into extra-high strengths has concentrated mainly on hardened cement pastes without ag- gregate not, strictly speaking, con- crete at all.
Using conventional vibration to compact the mix, strengths of appro- ximately 22,000 pounds force per square inch (150 N. per square milli- metre) have been reached with cement-paste having a water/cement ratio of about 0.20, but to obtain lower ratios and higher strengths the association's research team turned to compacted-powder techniques, long familiar in metallurgy and more re- cently in industrial ceramics: the cement content is increased by literal- ly "ramming" the cement grains to- gether under high pressure.
Model testing facilities at Britain's Cement and Concrete Association's research station have been particularly closely associated with the design of segmental precast concrete bridges and elevated structures. In these, large precast cross-sectional "slices" are post-tensioned together to form the final structure, a method of value where sites are limited. Three "generations" of segmental design were tested for the Mancunian Way in Manchester, in the north-west of England, seen here.
The cement can be pressed dry, producing a rigid low-strength "bis- cuit" which is then hardened by im- mersion in water under a vacuum, or a small amount of water can be added and the specimen pressed as a moist powder. A typical example of the re- sults is a strength of 44,300 pounds force per square inch (305 N. per square millimetre) obtained by die- pressing the specimen to 45,000 pounds force per square inch (310 N. per square millimetre).
Although the research into extra high strength materials will un- doubtedly find practical applications in the future, it is a good example of the basic "pure" research carried out by the association. Of more direct in- terest to the engineer is the work of the design research department, con- cerned with the understanding of the structural behaviour of all types of concrete structure, the effects of site procedures and design practice on the behaviour of the finished structure and the development of appropriate design procedures for use in practice.
Value of model testing
One of the most valuable tools used by the department is model test- ing. By examining the behaviour of a model in the laboratory, the behaviour of the full-sized structure can be pre- dicted during the design stage; indeed, model testing is often the only practic- able means of verifying design assump- tions.
Much of the model testing work carried out at Wexham Springs has been with large-scale models construct- ed of micro-concrete a scaled-down mix designed to simulate the proper- ties of the full-scale materials. Micro- concrete models can be reinforced and prestressed in a way analogous to the prototype. By testing to failure, design assumptions can be verified, or the design modified as required.
A great deal of model testing is undertaken on behalf of consulting engineers as part of the design process for structures. The model testing facili- ties have been particularly closely as- sociated with the design of segmental
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Far East BUILDER, July 1970