A very large number of concrete mixes have been characterised during the course of the research and several important guidelines for sprayed concrete mix design and use have been established. The relationship between the use of cement replacement materials and durability, well established in conventionally placed concrete, was found to be applicable to sprayed concrete (Mott Macdonald & Sprayed Concrete Ltd).
The shortlisted research mixes produced good durability properties, extremely good flexural toughness characteristics and tensile strengths improved by the presence of steel fibre reinforcement (Taywood & Imperial College).
Use of the specified mixes during the field trials in Spain produced favourable results, illustrating the Pan-European credibility of the mix designs (Dragados).
The development of suitable mathematical models has required detailed testing of both early age and mature concrete necessitating the development of multi-axial loading equipment specifically for the project. The resultant time-dependent models have been based on an extensive test matrix, involving many uniaxial-multi axial, compressive-tensile and monotonous creep tests for sprayed concrete ranging from 2 hours to 270 days.
The early age testing has demonstrated that accelerated plain and fibre reinforced sprayed concrete mixes exhibit considerable strength and stiffness from a very early age (2 hours) with the elastic regime becoming predominant within 12 hours (IMMG).
The problems associated with sampling and testing at early age, prompted the development of an instrument capable of providing reliable insitu measurements of bearing strength which has been found to produce a good alternative to defining strength parameters. (IMMG)
The orthotropic nature of the structural response of mature sprayed concrete, created by the spraying process, has been highlighted. The beneficial effects of fibres, clearly utilised in empirical designs for rock tunnel linings, are less apparent in engineered soft ground linings subject to predominantly hoop compression (Imperial College).
Mathematical models describing the constitutive relationships for sprayed concrete have been proposed. The available concrete model developed at Imperial College on the basis of extensive testing of conventionally cast concrete appears to adequately describe the mature behaviour of the sprayed material, with only minor modification (IMMG & Imperial College).
A series of large scale tests were designed and implemented to generate test data that could be used to calibrate and update if necessary the mathematical models. The tests involved spraying circular fibre reinforced concrete linings onto a simulated clay surround. Loading was applied from the outset to simulate ground interaction. The linings were taken up to failure after reaching predefined maturity at approximately 14 days. The results have clearly highlighted the inherent variability in the sprayed concrete process and the need to take it into account in the design process (Taywood). Continued interpretation of the results will take place outside the project timescale (Mott MacDonald).
The research will develop non ferrous fibre reinforced sprayed concrete mixes using pfa and micro silica for the use in underground structures and will produce design and construction guidelines to allow these developments to be fulley exploited.
The major research tasks are:
i) develop and validate mathematical models to allow realistic simulation of the new materials and enable economic design;
ii) produce design and construction guidelines to exploit the research findings;
iii) identify the optimum fibre characteristics, and pozzolan quantities for optimum mix production from a construction and user viewpoint.
iv) refine the selected mixes based on a strength and durability criteria.
Successful completion of the research tasks would lead to:
i) wider actual and potential application of sprayed concrete;
ii) more economical design utilising sprayed concrete with very significant savings if durable materials are developed;
iii) reduction of dust pollution during construction and a consequent reduction on respiratory hazards and construction accidents;
iv) reduced wastage due to redound with a realistic reduction target from current 30% wastage due to 5%;
v) increased speed of construction in many underground operations in specific cases possibly 10 flod e.g. single sided wall in top down construction.
Funding SchemeCSC - Cost-sharing contracts
SW7 2BU London
ME17 2LA Maidstone
UB1 2QX Southall