Aggregates as sprayed concrete constituent material in Mining Industry - South Africa
Aggregates for sprayed concrete shall comply with the requirements of the national standards and regulations valid in the place of use. They shall be appropriate for placement of sprayed concrete according to the specifications of the work. It is the duty of the contractor to select the most suitable grading for the process and materials available.
Aggregate sources can be natural aggregates (e.g. river sand/gravel) and recycled aggregates from excavation material or from recycled concrete. Recycled aggregates may absorb some content from the mixing water in the sprayed concrete mix.
Figure 3-6: Round aggregate (left), crushed aggregate (center) and the sphericity of crushed aggregates (right)
Compared to rounded aggregates, crushed aggregates (natural or recycled) display a negative effect on pumpability. Some crushed aggregates, e.g. recycled concrete, may require more water due to a waterabsorbing porous structure. Water absorption can range from approx. 0.1–5 %. The natural moisture content in the aggregate for dry mixes must be as constant as possible. It should be limited to 6 % [29].
The dry density of aggregate varies between 300 to more than 1700 kg/ m3. Aggregate is classified as:
- ultra-lightweight (rarely)
- 300 kg/m3 < lightweight ≤ 1000 kg/m3; (refractory, dry-mix)
- dense ≤ 1700 kg/m3 (typically used for sprayed concrete)
- extra dense > 1700 kg/m3
Aggregate quality is critical to the fresh concrete as well as the hardened product. It is essential that aggregate grading and other characteristics show only small variations. Of particular importance are the amount and characteristics of fine aggregates, i.e. the grain size distribution and grain size analysis. However, it is not relevant to talk about choice of aggregate, as normally the available material must be used and the aggregate specification has to be adapted to it.
Nevertheless, for wet-mix sprayed concrete, the following aspects should be considered:
- The maximum aggregate size should be limited to 8 or 10 mm, because of limitations of pumping equipment and to avoid excessive rebound and resulting material losses.
- Aggregate grading is also very important, particularly its lower part (fine aggregate). The content of fine material in sieve size of 0.125 mm should vary from 4 or 5 % to 8 or 9 %.
- An insufficient amount of fine material leads to segregation and risk of clogging.
- Too high content of fine material in the sand increases the water demand and can reduce pumpability.
- For fiber-reinforced sprayed concrete, the surplus of fine material is important, both for pumping and compaction. A high content of fine aggregate will provide a viscid concrete.
- Alkali aggregate reaction, often called alkali silica reaction, can lead to serious damage caused by the expansion of aggregates containing reactive silicates. The reaction is initiated by free alkali from the cement or from alkaline set accelerators. The reactivity (AKR, Alkali Kieselsäure Reaction) is strongly dependent on the place of use (EN 12620:2002-Annex G and [42] to [46]). Precautions according to EN 206-1 or national standards and regulations valid in the place of use shall be applied.
The picture below shows typical damage of the microstructure of concrete caused by alkali silica reaction.
![Figure 3 7: Typical result of alkali silica reaction, creating cracks [46]](https://assets.construction-chemicals.mbcc-group.com/en-za/typical%20result%20of%20alkali%20silica%20reaction%2c%20creating%20cracks.jpg)
Figure 3 7: Typical result of alkali silica reaction, creating cracks [46]
Adding coarse aggregate to sprayed concrete is required for the following reasons:
- The pumpability of wet-mix sprayed concrete may be improved by the addition of coarse aggregate (see also section 2.2.1).
- The addition of coarse aggregate leads to better in-place density of the sprayed concrete.
- Compared to fine aggregate, coarse aggregate possesses a reduced surface area which allows a lower water content in the mix.
- Adding coarse aggregate leads to less drying shrinkage by reducing the content of fine aggregate.
- The density / load bearing capacity of aggregates should be taken into account. Normally dense aggregates display values of ≤ 1700 kg/m3.
- The shape, roundness and sphericity of aggregates play an important role in the pumpability of concrete. It is obvious that round materials are easier to pump than crushed ones (see EN 12620:2002 for details of the shape for each type of aggregate). Sphericity ranges from well rounded to very angular.
However, if sprayed concrete has more than 40 % coarse aggregate (of sand/aggregate ratio), it causes increased rebound for both wet-mix and dry-mix spraying. Further consequences are:
- It cannot be used for thin layers and it is more difficult to use for finishing.
- It requires large-diameter hoses.
- When applied at high velocities, it forms craters on the surface of the plastic sprayed concrete.
Aggregate distribution should lie within an accepted gradation range according to current local standards or as specified. The grain size distribution curve for the aggregate should fall within the shaded area of Figure 3-8. As the gradation margins are relatively small, it may often be convenient to combine two or more aggregate fractions, e.g. 0–2 mm, 2–4 mm and 4–8 mm, by adjusting the proportion between them to achieve a grading that fits within the ideal curve limits. Too little fine material will be compensated for by using more cement or microsilica. Too much fine material is primarily compensated for by increasing the dosage of water-reducing admixtures.
The aggregates shall be well graded, and no fraction (according to ISO sieve shown in the following Table 3-5) shall constitute more than 30 % of the total. Although there is a trend to use more crushed aggregates from excavation material, the contents of crushed and non-cubical material should not exceed 10 %. An improvement in the grain size curve for natural sand by using crushed materials often results in an increased water demand and poorer pumpability and compaction. Before crushed materials are employed as part of the aggregates, comparison performance tests are recommended.
Figure 3-8: Recommended grading zone for combined aggregates (indicated in gray)
The following Table 3-5 shows recommended grading limits for sprayed concrete aggregates, with percentage by weight passing individual sieves. Further recommended aggregate gradation zones can be found in [29] and [30].
The percentage of 8 mm aggregate should preferably not exceed 10 %, since it will rebound when spraying on a hard surface (when starting the application) or penetrate previously placed concrete, producing craters which are difficult to fill.
During screening, storing and handling of the aggregates, measures should be taken to prevent the presence of particles in excess of 8 mm. Coarse particles may block the nozzle and subsequent cleaning can be very time consuming. Furthermore, this presents a safety risk for the nozzleman as he/she will have to clear blocked lines while working in the insufficiently supported areas.
Recommended grading limits for sprayed concrete aggregates (fraction 0 to 8 mm) are provided in the following Table 3-6.
Table 3-6: Recommended grading limits for sprayed concrete aggregates fraction 0 to 8 mm