Fibre reinforced autoclaved aerated concrete

One focus in the area of improvement of quality and durability of load-bearing structures in the transport construction sector is the search for, and the invention and use of construction materials with required characteristics.

On a global scale, the development of dispersed reinforcement, as an alternative to the bar type of reinforcement, has occurred gradually. Adding fibre to construction materials was first practiced in ancient times when horsehair and straw were used to reinforce bricks. In 1911, Porter discovered that similar fibres can also be used in concrete. In the early 1900s, asbestos wool was used for the first time in reinforcement. In 1963, Romualdi and Batson published their acclaimed work on FRC (fibre-reinforced concrete). Since then glass, steel, PPR fibre and other fibrous materials have been used to reinforce concrete.

Dispersed reinforcement of aerated concrete — basis, practice

Well-known shortcomings in conventional aerated concrete and in autoclaved aerated concrete (AAC), in particular, include the low tensile strength resistance and increased brittleness, resulting in undesirable chipping and fissures during manufacturing, transporting or installing of such materials, thus degrading the performance properties of the material. Research in the domain of reinforcement of aerated concrete has a long history. In the works of F. N. Rabinovich “Dispersed reinforcement of concrete” (“Dispersno armirovannye betony”) [21] and L. V. Morgun [17; 19], it is recommended to use fibre for the dispersed reinforcement of concrete.

Assessment of the impact of dispersed reinforcement with fibrous materials on the stress-related properties of aerated concrete has revealed that embedding randomly oriented polyamide fibres in AAC results in a structure that renders it essentially equivalent to concrete with steel reinforcement in terms of bending. Already since 1980, in the “Instructions on the Manufacture of Products from Aerated Concrete”, SN 277-80 (Russian construction norms and regulations), 5th and 6th grade asbestos has been recommended as a fibrous additive. However, this additive was recommended for aerated concrete in general. In this regard, one core problem of dispersed reinforcement of autoclaved aerated concrete was overlooked, namely, the cutting of the green cake to smaller units. In 1982, an overview “New technological solutions in the production of aerated concrete” (Merkin, A. P.; Zeifman, M. I.) also presented the possibility of improving the strength profile of aerated concrete if it had dispersed reinforcement made of fibrous additives. Yet, the authors drew attention to the challenge of introducing these additives in the context of control of their dispersion in the mixture and also to the problems arising during the manufacture of aerated concrete products with cutting technology.

Today, many manufacturers offer reinforced foam concrete with basalt or polypropylene fibre and claim it to be resistant to shock, wear, frost, and water, and also claim that it features increased durability (http://penoblok-tvport.ru/ etc.). More-over, it is reported that the flexural and compressive strength of such products is 2-4 times higher than that of conventional foam concrete products of the same density, and they show minimum shrinkage and extreme resistance to crack formation.

It has led to an unprecedented situation: common and scientifically justified dispersed reinforcement of foam concrete and a complete absence of a scientific evidence base for the analogous reinforcement of AAC.

Technological preparation and trial production of fibre-reinforced autoclaved aerated concrete

The first experiments with dispersed reinforcement of aerated concrete in real production conditions were completed in March 2014. Initially, casting was done in lab molds of 300 x 300 mm. Parameters for cutting of the green cake were defined in actual manufacturing conditions. During the initial trials, the following main questions were asked:

• What kind of reinforcement to use?

• How to achieve an even distribution of the reinforcement in the green cake? Several aspects must be considered concurrently — preparation of the reinforcement, dosage, feeding to mixture.

• The main question concerns further technological operations. How to cut and autoclave the reinforced green cake?

Reinforcement

The fibre length was chosen empirically, and the best result was achieved with basalt fibre of 6.4 mm in length.

The minimum permissible thickness of interpore partitions with 1-2 mm pores and 600 kg/m³ density was 0.17-0.32 mm, whereas with 400 kg/m³ density it was 0.1-0.2 mm; hence the maximum diameter of fibres must be 3-5 times smaller than the minimum thickness of partition and must not exceed 20 microns. Chemical and high temperature resistance of individual fibres is also a crucial factor. All polymer-based fibrous materials were sorted out at this stage. Three types of fibrous reinforcement were chosen for the reinforcement of AAC: basalt fibre, glass fibre and carbon fibre. In order to determine the actual alkali resistance under real manufacturing conditions, the specialists at LLC “Glavstroj-Ust-Labinsk”, where all trials and experiments were carried out, performed two-step trials:

1. Soaking fibre in 70% alkaline solution for 24 hours and visually determining the integrity of the fibre after the trial.

2. In the second step, the vessels with fibre placed in alkaline solution were put inside the autoclave for one processing cycle (10.5 hours in an environment of 100 saturated water steam under pressure of 12.5 bars and at 195^оС).

In relation to technological operations of cutting the green cake into smaller units, the main role plays the length of the reinforcement fibres. Theoretical calculations allow concluding that a fibre shorter than 3.8 mm, in case of cutting of the green cake with cords of 0.8 mm in diameter, will not damage the quality of cutting. The paper “Research of autoclaved aerated concrete, reinforced with fibres by CT scan method” by Gerd Weidemann, co-authoring with colleagues from the German “Federal Institute for Materials Research and Testing” (Bundesanstalt für Materialforschung und -prüfung, BAM), has scrupulously discussed spatial orientation of reinforcement fibres in AAC. They noticed only a small amount of vertically

oriented fibres. For experimental work, the author of the present paper chose 6.4 mm long fibre.

Preparation and distribution of the reinforcement in the green cake

Taking into account the recommendations of reinforcement manufacturers, the author of the present paper split the application of this component into two steps. After the preliminary weighing, fibre was mixed with dry components and mixed in a machine consisting of a vessel with a rotary drill and a whisk installed under a leak-proof lid. The best result (time and efficiency of mixing) in terms of the dry component was demonstrated by the cement. Use of lime, entrainment dust from a ventilated filter unit, finely dispersed residues from processing reinforced aerated concrete products proved to be less effective.

Preliminarily prepared fibre must be introduced to the AAC mixture in the initial stages of preparation to attain the best quality dispersion across the entire mixture inside the mixer. In order to prevent uneven distribution, it is reasonable to add the

fibre with previously weighed sand slime before dosing of the dry components to the mixer. Mixing at this stage can last 1-2 minutes. Further addition of components, mixing, emptying of the mixer must follow the conventional regime, without any changes.

Cutting and autoclaving of the reinforced green cake

Modern aerated concrete manufacturers mainly feature automated cutting equipment with metal cords for the cutting of the green cake into smaller units. The presence of fibre in the green cake (pressed, to be more accurate) leads to several unwanted consequences:

1. Significant increase in width and unevenness of the cut.

2. Due to lack of firm fixation, the cords cause vertical fluctuation when moving through the green cake.

3. Vertical knives form agglomerates of fibre which cause deep furrows along the surface of the green cake.

To counteract these adverse effects, we chose cutting modes with different cutting speeds, changed the angle of the cords and varied the amount of material to be cut as corrective measures.

Since dispersed reinforcement is applied mainly in the manufacture of large-sized products, treatment in autoclave takes additional time at the rising and isothermal curing stages. The cycle time can reach 11.5 to 12.5 hours. All other technological operations do not differ from the conventional ones.

4. Practical application of products made of fibre-reinforced autoclaved aerated concrete 

If aerated concrete has dispersed fibre reinforcement, the maximum tensile strength increases by 9 to 15%, while the increase in bending tensile strength, depending on technological and mixture specifics, can reach 550%. 

Besides, dispersed reinforcement of aerated concrete mixture, according to data provided by A. M. Krokhin [10], provides increased resistance to segregation.

Results of trials of dispersed reinforced elements made of AAC using non-metallic fibres are shown in Table 1.

Trials in a laboratory were done also with finished products — reinforced lintels made from autoclaved aerated concrete. By the end of the trials, lintels with dispersed fibre reinforcement showed a 30% increase in maximum load capacity (Table 2).

The key factor for the use of fibre-reinforced autoclaved aerated concrete at the LLC “Glavstroy-

Ust-Labinsk” production plant is the increase of the manufacturing output of reinforced products.

At the end of bench tests, the company could partially replace metal reinforcement in conventional lintels, and the entire production of such products was converted to production with fibres.

The number of complaints about lintel quality from end users decreased significantly. Occurrence of chippings and fissures decreased both in technological operations in the plant and during transport to the building sites.

5. Conclusion

Dispersed reinforcement of autoclaved aerated concrete opens up new development perspectives in the construction industry in general and in the production of construction materials. Factory-based development of large-sized AAC products with improved performance in terms of tensile strength will allow an increase in the proportion of autoclaved aerated concrete in objects. Technologies that have been long applied in developed countries can serve as a conceptual basis for the development of production output, meeting the demands of large buildings.