Properties of the fresh mix

Test methods for the evaluation of the fresh FRC are primarily for the following characteristics  

1 Workability of a mix in which the fibres are dispersed by the mixing method itself. 

2 Workability of the matrix in spray applications, in which the fibres and matrix are sprayed from individual nozzles. 

Fibres generally tend to stiffen a concrete mix, and make it seem harsh when static, though it may still respond well to vibration. Under vibration, the stiffening effect of the fibres tends to disappear, and so a properly designed FRC mix can be handled in much the same way as plain concrete in terms of mobility and ability to flow [1]. Therefore, workability tests based on static conditions, such as the slump test, are not very useful, and can be quite misleading, since the concrete is in fact workable when vibrated. Thus, in order to assess the workability of fresh FRC mixes, it is recommended that tests in which dynamic effects are involved be used [1,2]. 

Over the years, a great many workability tests have been devised. Koehler and Fowler [3] have described 61 different test methods, though only some of these have ever been adopted as standards, and even fewer are used for FRC. The most common test methods in current use include  

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It may be useful to monitor the unreacted components of the adhesive system as well as the properties of the freshly mixed adhesive, partly as a check on the adhesive supplier and partly to establish the working characteristics of the adhesive. Techniques for... 

Mineral additions mnst be distinguished from chemical admixtures, which are added to the binder only in small amounts (usually below 1%), and which influence the properties of the fresh mix or the hardened paste, but do not alter snbstantially the product of the hydration reaction. 

The general term workability is used to express these overall properties of the fresh mix and is applied to all kinds of concrete. Special concretes that do not require any additional operations for placing and compacting, so-called self-compacting concretes (SCC) and self-levelling concretes (SLC)... 

The asbestos fibre is unique in its properties. It permits control of the properties of the fresh mix, to make it processable by the efficient dewatering techniques, and, at the same time, it provides an excellent reinforcing efficiency in the hardened composite, due to its high strength, modulus of elasticity and bond. In addition, this fibre is stable in the alkaline cement environment, thus providing a composite of excellent durability. This combination of properties is difficult to match with any one type of fibre. There are fibres which can match the mechanical quality of asbestos, but they cannot, on the other hand, provide the processing characteristics. 

The addition of silica fume to dry-mix shotcrete in proportions of 10-15% by mass of Portland cement substantially improves the adhesive and cohesive properties of the freshly applied shotcrete. Silica fume creates a very dense and sticky mix with an almost complete lack of bleeding. These characteristics of the material are attributed to the extreme densification achieved by packing of the ultrafine silica fume particles between the cement particles in the plastic (fresh) shotcrete. Practical ramifications of these characteristics include the observations which follow [107, 117-119]. 

The properties of an aggregate influence both the characteristics of the fresh mix and the strength and durability of the hardened concrete. 

Testing program. The properties of the trial mixes in both the fresh and hardened state were determined. Fresh state properties measured were slump and flow parameters and workability when used in conjunction with AR fibre mesh reinforcement. Hardened state properties tested included compressive and flexural strength after 7 days of submersion curing at 26°C. Fresh state properties were measured directly after the mixing was completed. 

Diluents. These are generally incorporated to reduce the viseosity of the freshly mixed adhesive to offset the effect of the filler. This may be required to improve handling and spreading characteristics or to allow filler additions which tend to reduce cost. Other properties of the fresh and hardened adhesive can be affected by the use of diluents, for example pot life, flexibility and glass transition temperature. If the diluent is non-reactive, such as solvents which remain in the cured system, the net result is a deterioration of chemical and mechanical properties such as increased shrinkage and reduced adhesion. Reactive diluents containing epoxy compounds are capable of combining chemically with the resin/hardener system. 

The selection of an appropriate model is made according to its ability to represent experimental results and also its simplicity. The properties of cement paste are not the only factors determining the behaviour of cement-based composites, concretes, mortars, fibre concretes, etc. Other components of the fresh mix, their volume fractions and properties are also important. That is why, for practical considerations, it is not necessary to use very complicated models for the cement paste. 

Polymer composition has a major influence on the final properties of the hardened composites and also on the workability of the fresh mix. PCC is used when the following properties need to be improved ... 

Other types of production processes are also dependent on the special nature of the fresh mix, which is controlled by the special properties of the asbestos fibres. The Magnani process is based on direct deposition of slurry on a porous former, followed by dewatering and consolidation by vacuuming and pressure. The product can be produced by the build-up of several layers. 

The initial plastic state of the fresh concrete subsequent to the mixing process, where properties such as the air content, density and workability are normally measured by relevant standard tests, and utilized as a means of control of production. The magnitude of these properties is affected by the addition of water-reducing admixtures, either intentionally or as a side effect, which could result not only in a change in the characteristics in the plastic state, but could also be reflected in changed properties in the hardened state. 

The workability of the fresh mortars and the properties of the hardened mortars are determined by the mix composition (binder, water, traditional aggregate, polymeric granulate, additives) in the same way like in normal lightweight mortar technology. The dense structure of the grains is advantageous as it makes it unnecessary to take into account the water suction of the aggregate. 

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