Experimental investigations of the strength and deformability of steel fibre reinforced shotcrete linings are presented. The tests show that there are several advantages associated with the use of high strength steel fibres with hooks in shotcrete linings:
These fibres slip under a certain resistance in the matrix and the failure becomes very plastic-both in bending and in shear.
Advanced support systems utilizing the interaction of rock bolts and shotcrete are simplified in the design when steel fibre reinforcement is used.
The fibres prevent the breaking up of the shotcrete, and the risk of downfall is decreased.
La resistance et les deformations de beton projete à fibres d'acier ontfait l'objet de recherches experimentales. Les essais mettent en evidence plusieurs avantages que presentent l'emploi, dans les revêtements de beton projete, les fibres en acier de haute resistance terminees par des crochets:
Ces fibres glissent dans la matrice en developpant une certaine resistance et la rupture devient très plastique aussi bien en flexion qu'en cisaillement.
L'emploi de revêtements armes de fibres d'acier simplifie la conception de systèmes de soutènement bases sur l'action reciproque d'ancrages et de beton projete.
Les fibres empêchent la desagregation du beton projete et diminuent le risque d'effondrement.
Experimentelle Untersuchungen ueber die Starke und Biegsamkeit des Stahlfaserspritzbetons liegen vor. Bei Tunnelausklei- dungen aus Spritzbeton unter Verwendung von Fasern aus hochfestem Stahl mit gebogenen Enden, erweisen die Experimente mehrere Vorteile:
Diese Fasern gleiten im Material unter gewissem Widerstand und resultieren in ausgesprochen plastischen Biegeund Schubbruechen.
Der Entwurf schwieriger Konstruktionen aus Spritzbeton und Felsankern wird vereinfacht bei Verwendung von stahlfaserbewehrtem Spritzbeton.
Die Fasern verhindern das Zerbrechen des Betons und reduzieren das Einsturzrisiko.
In 1973 a joint project between the Swedish Rock Engineering Research Foundation and the Royal Swedish Fortifications Administration started. Its aim was to get better knowledge of the strengthening function of a shotcrete lining when applied to a hard rock. The investigation started with large scale tests on the failure mechanism and strength of shotcreted jointed rock (Holmgren, 1979). It was then continued with laboratory tests on the tensile adhesion of shotcrete to different rock types (Hahn, 1979). It was found that the adhesion between the rock and the shotcrete plays a determining role for the strength of the lining unless other support is arranged e.g. systematic rock bolting. In Holmgren, 1979 there are described tests on bolt supported reinforced shotcrete linings. These results served as a basis for a standardized design for bolt supported shotcrete linings in military caverns. This solution is also used in an underground storage for radioactive waste. There are many advantages with this design:
The strength is independent of the obtained adhesion strength.
The strength is possible to calculate according to principles for concrete plates on columns.
A systematic rock bolting may be required - for other reasons, too.
End anchored rock bolts with rolled threads interacting with reinforced shotcrete seems to be the most promising support in squeezing rock.
The disadvantages are:
The installation of mesh reinforcement is very costly and time consuming.
The irregularities of the rock surface make it difficult to shotcrete a reinforced lining.
The probability that the reinforcement fits the distribution of the bending moments in the lining is very small.
It is impossible to reinforce a shotcrete lining against shear failure using conventional reinforcement.
By using fibre reinforcement instead of a conventional one most of the above mentioned difficulties disappear. The most important remaining one is the cost because of fibres being an expensive material.
The punching of a single block through a shotcrete lining which is supported by rock bolts was also studied by Holmgren, 1979. The tests were performed in a rig consisting of three large granite blocks on a steel stand. The shotcrete was produced according to the wet mix method and the fibres were added at the nozzle by means of a so called Fibre Feeder which is patented by the Swedish company Besab. The fibres were hooked steel fibres Bekært Lz 35x.35 and Lz 45x.35 with a failure strength of about 1200 MPa. Standard cement and 0 - 4 mm sand were used for the concrete. Most test specimens were cured for eight days.
The test program consisted of four parts:
Large scale tests on possible designs of a fibre reinforced shotcrete lining supported by rock bolts. There were tested stiff washers of two sizes at the end of the bolt, flexible washers and also bolts without washers.
Large scale tests for the comparison of mesh reinforced shotcrete and fibre rein- forced shotcrete. There were tested meshes of cold tensioned steel and meshes of mild steel.