Shotcrete & FRC Services

The structural design of shotcrete and FRC structures is presently not covered by AS3600 or AS5100, so designers of structures incorporating these materials are required to use alternative guidelines. The most commonly used structural guideline is the Model Code produced by the fib (fédération internationale du béton) now available as a 2020 edition. Dr Bernard is a member of the technical groups related to creep and shrinkage of FRC, and durability of FRC, within the fib TG framework for development and maintenance of the Model Code. He can help designers understand how to use the Model Code, and its limitations. Dr Bernard is also a member of Standards Australia sub-committee BD2/6 on use of fibres in hybrid FRC/RC members in AS3600 where he has used his research expertise in crack width and flexural strength estimation to contribute to upgrades in this document.

Shotcrete is a specialised material with properties that are sensitive to many parameters related to the ingredients used. Development of an effective shotcrete mixture is therefore more challenging than mix design for conventional cast concrete. Properties such as stickiness, internal cohesion, early-age strength gain are difficult to achieve at the same time as high strength, durability, and low shrinkage. With 25 years of experience across many countries developing and testing shotcrete mixtures with often strict performance demands and unfavourable ingredients, Dr Bernard has few rivals in the field of shotcrete mix design development. If you have problems achieving performance requirements, or face challenges identifying suitable ingredients to use in a mixture, contact Dr Bernard for advice in this regard.

Fibres are most commonly added to concrete to achieve post-cracking flexural-tensile strength in an otherwise brittle material. Depending on the application, fibres can replace conventional reinforcement or they can supplement the performance of conventional reinforcement, especially in relation to achieving satisfactory crack widths. There are a multitude of fibres available on the market, with some extra-ordinary performance claims in some cases. Dr. Bernard has personally tested more fibre types than just about any other person internationally, as can be attested by the many papers he has published describing the performance of present and past fibre types (see reference list below). If you need help selecting fibres for a particular application, or understanding their strengths and weaknesses, contact Dr Bernard for advice.

Problems in the application for shotcrete are quite common; understanding the cause of these problems is often challenging. This is because many factors contribute to the satisfactory or unsatisfactory performance of this material and identifying what has gone wrong is often hard. Trouble-shooting difficult mixes, and optimising performance, has been the bread-and-butter of Dr Bernard’s career in consulting relating to shotcrete over the last 20 years. If you have a problem getting shotcrete to spray properly, stick to the ground or rock, gain strength as required, or perform satisfactorily with regard to specifications, then contact Dr Bernard for advice.

Adding fibres to concrete, especially at high dosage rates, can cause problems such as low workability, poor pumping, balling of fibres, and poor surface finish. Changes to mix design may help to overcome these problems in some circumstances, but in other cases it may be necessary to moderate expectations given the materials and equipment available. Knowing what is possible and achievable at reasonable cost when fibres are added to concrete is an important part of mix development and trouble-shooting.

The in-service performance of shotcrete and FRC are sometimes marred by two common problems: excessively wide shrinkage cracks, and poor durability. Shotcrete is an inherently high-shrinkage material so it is not surprising that shrinkage cracks frequently occur in shotcrete structures. However, excessively wide shrinkage cracks could be attributable to any of a number of factors, including poor mix design, poor curing, excessive set acceleration, a low degree of restraint, etc. Understanding the cause of the problem is often unique to a particular location and may be difficult to identify. It is therefore important to employ an expert in shrinkage and creep who has many years of experience in assessing the causes of shrinkage (and other) cracks in shotcrete structures when seeking a solution to the problem. Fibres are sometimes claimed as a ‘solution’ to excessively wide crack widths, but this may not be possible depending on the circumstances. Understanding how shrinkage and creep occur in FRC structures is critical in assessing what the causes and solutions for cracks in FRC structures are.

Durability is another area of in-service performance that is sometimes unsatisfactory. The corrosion of steel fibres is quite different from the corrosion of steel reinforcing bars, so special guidelines must be followed for maximum acceptable crack widths and mix designs when using steel FRC or FRS in aggressive environments. Mechanisms such as evaporation can also concentrate aggressive agents in cracks so that apparently benign environments may end up being aggressive over time. Understanding the special durability requirements of FRC is a unique area of expertise that Dr Bernard can assist clients with that is backed up by numerous research papers published in this area (see below).

Dr Bernard has been involving in the testing of shotcrete and FRC for 30 years and has both developed test methods and standards, and published papers on their implementation, in many countries. His particular expertise is in post-crack performance assessment and determination of unusual properties such as shrinkage and creep coefficients. He was sub-committee chair of ASTM C9.42 responsible for FRC testing and remains Task Group chair responsible for the ASTM C1550 round panel test and ASTM C1812/C1812M support rollers for the ASTM C1609/C1609M beam test. He has also been involved in test development with RILEM and the fib. He is currently active in development of improved tests for drying shrinkage of set accelerated shotcrete and a guideline for the assessment of the durability of shotcrete linings.

As part of his previous work in laboratory testing of FRC and FRS, Dr Bernard was closely involved in the development of appropriate Quality Control measures for these materials, and suitable statistical assessment of results. The high variability in post-crack performance of FRC requires that special consideration be given to the sources of this variability and the effect on the performance of full-sized in situ structures. Technical references on this subject are included below.

Dr Bernard has acted as an expert witness in numerous litigation cases related to shotcrete and fibre reinforced concrete, include investigations of death and injury related to rock falls in tunnels and mines, disputes over poor early-age and in situ shotcrete performance, excessive shrinkage cracking, and failure to achieve minimum levels of environmental resistance. These litigations have ranged from a small project lasting a few weeks to projects lasting several years and involving hundreds of millions of dollars.