Stages and shotcreting cost in the repair of reinforced concrete structures by shotcreting

Reinforced concrete structures, as a result of operation, wear out which necessitates repair. There are many methods of structural repair of reinforced concrete structures, but it is shotcreting that will be the most profitable in some instances. Shotcreting is recommended during the reconstruction of buildings, repair of bridges, berthing facilities, chimneys, industrial workshops structures, pipelines, settling tanks, reservoirs, basins, canals, and other facilities, especially in cases when access to the work site is limited. Shotcreting is particularly cost-effective and efficient for large-scale repair work where high productivity is required.

Stages of work in shotcreting

During the repair of reinforced concrete structures, the order of work involved in shotcreting includes several stages, with their number varying based on the type of structure and the objectives of the contractor.

1. Surface pretreatment
Surface pretreatment is performed in 2 steps: removal of the existing surface layer and surface pretreatment.
First, you need to remove the existing old or destroyed concrete manually or use a hammering or hydrodynamic method. The remains of the old layer must be cleaned. After that, oil and other inclusions must be removed from the surface, otherwise it will be impossible to achieve sufficient adhesion quality between the substrate and the repair surface layer. The most common way to do this is to clean the surface mechanically, for example using hydrodynamic or sandblasting methods.

2. Reinforcing the structure or replacing / modifying the old rebar
This step is only required when the facility is being structurally repaired. If it is just about restoring the protective layer in your case, there will be no need for doing this step.

3. Surface shotcreting
Shotcrete is applied to the pretreated and cleaned surface (and in some cases to the refurbished reinforcing frame). The repair layer will not be perfectly flat, so if necessary, it can be trimmed or grouted to enhance the appearance of the object. When it comes to industrial repairs, however, appearance doesn’t always matter.

4. Refuse collection
Dust and dirt cannot be avoided during shotcreting because rebound is generated in the process, as a small portion of the concrete ricochets off the surface. For this reason, thorough cleaning of the area where the work was performed is necessary at this stage.

5. Finishing works

The structural repair is already finished at this stage. Finishing works are carried out if the object requires it. Such works can include waterproofing, painting, and other options for applying finishing coating.

Factors that affect the cost of shotcrete and duration of repair works: the selection of shotcrete equipment and mixtures

It is absolutely natural to want to make the process of structural repair as simple, fast and cost-effective as possible. When it comes to shotcreting, achieving this goal is quite feasible, but to do so you will need to be particularly careful in selecting the most suitable equipment and mixture.
Selection of shotcrete equipment.

The following factors should be considered when selecting equipment:

• type and characteristics of the object to be repaired, its condition and requirements for the structure itself;
• the experience of the specialists performing the repair;
  conditions at the construction site: the availability of necessary infrastructure and additional equipment;
• the distance from the location of the equipment to the location of the repair layer; it can be as much as 200 meters;
• access to the surface to be shotcreted (from the ground, platforms, and so on);
• specifics of the project: the thickness of the required repair layer, grade of concrete and other
• requirements for it (for example, chemical or friction resistance)
• requirements for the resulting surface;
• required productivity of equipment;
• process organization: from delivery of equipment and working mixtures to their storage and refuse collection;
• whether the equipment is to be used only on one site or on several sites.

 

It is important to keep in mind that all the above factors will determine shotcrete cost per square metre.

Selection of shotcrete mix

The second most important and influential factor is the selection of a shotcrete mix. You might wonder, “How much does shotcrete cost?” To answer this question, let’s consider some selection factors. The mixture has to be selected based on the requirements of the object with respect to strength, chemical resistance, water resistance, frost resistance, and so on. It will also be necessary to decide whether in your case you should buy the factory mixture, or it will be better to prepare it on site. Your choice will depend on whether you have the necessary conditions for its preparation on the spot, what fraction and mobility of the mixture is required, and what your allocated budget is. The final decision will be based on the project documentation, the type of works, the shotcrete machine you plan to use, and the cost-effectiveness factor.

If you decide to prepare the mixture on site, you must first understand that to produce a high-quality shotcrete mixture yourself, you will have to be sufficiently experienced and qualified in this area. The wrong mixture can lead to a large amount of waste (rebound), resulting in poor-quality applied concrete or it may not be suitable for shotcrete at all.
It is appropriate to prepare your own shotcrete mix when you can use local inexpensive aggregate of natural moisture (which is mainly sand). In this case it may be worthwhile and economically and temporarily justified.

When doing this, it is necessary to:
decide whether you are going to use dry or wet mix shotcreting, it is the choice that depends on the requirements of the project (the characteristics of the finished concrete – strength, rapid strength, and so on), working conditions (open space or enclosed space (the need and the possibility of organization of protection from dust), temperature, humidity, ease of work, and so on), availability of communications (electricity, water, air) and existing equipment)
obtain the formulation of the mixture, depending on the design conditions (characteristics of ready-mixed concrete) and the available equipment
procuring and delivering aggregate, cement and additives
organize the control, warehousing, and proper storage of materials (in regard to cleanliness, moisture protection, and so on)
organize the work of the mixing area
ensure control of formulation and technology.
If you consider the skills and costs required, as well as the risks, it is often cheaper and more appropriate to buy a ready-made mixture

It is frequently cost-effectiveness that makes shotcreting the most suitable and most efficient way of performing structural repairs of reinforced concrete structures. But in order to maximize savings, you need to take into account a large number of complex factors that sometimes are not obvious for a non-professional person and related to the specifics of the project, selection of equipment and mixture. Tornado Company is ready to share its practical experience and knowledge with you: we have been repairing reinforced concrete structures by means of shotcreting for more than 30 years. Our specialists will help you avoid potential mistakes, select a cost-effective shotcrete machine, and ensure that each stage is performed with the highest quality to achieve maximum profitability, compliance with the design requirements and personnel convenience.

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