After heavy rain hit Adelaide, a concrete wall at the Darlington Upgrade crumbled after rain got behind the wall, resulting in traffic delays for up to 6 weeks. They had used spray-on concrete “shotcrete” on the wall and was unsure why so little rain would do so much damage. The damage was located to one side of the wall; the geotechnical engineer checked the entire length of the road to ensure it did not have the same issues.

What is shotcrete?

Shotcrete is concrete delivered by a hose and pneumatically projected at high speeds onto any type or shape of surface. It is popular to be used on vertical surfaces like tunnel walls, pools and road side walls.

There’s two application methods – Gunite is the term referred to dry concrete applied using pressurised air, where water is applied at the nozzle. It allows a professional to add precisely the correct amount of water to obtain the correct viscosity of the concrete, so that it dries to the right amount of strength.

Wet application is ideal for covering large areas quickly. A traditional concrete pump pumps the wet shotcrete mix through large hoses and compressed air is introduced at the nozzle. Neither application methods are superior to the other, and depends on the project regarding which is best to use.

The advantages of Shotcrete

Spray on concrete is one of the primary growth factors for the market in the construction industry as it has become increasingly popular among builders due to its application being economical and flexible. It provides higher mechanical strength to buildings and construction compared to normal concrete due to the structure being less porous and increased bond strength with most substrates and rapid or instant capabilities particularly on complex forms and shapes.

It is very cost effective and reduces construction time. It has been said that Shotcrete is a sustainable option mainly due to its cost efficiency. One of the biggest savings is labour-saving benefits of at least 50% due to the use of minimal formwork. The material is gunned in from the open side against a prepared substrate, eliminating the need for forming, saving money on forming materials and labour in the time-consuming process of making the forms and conveying the concrete.

Shotcrete methods are capable of virtually producing the same in-place product as traditional cast-in-place concrete construction. However, Shotcrete is more time-efficient and a method used to save time by up to 50% more than normal concrete methods.

Disadvantages of Shotcrete

Even though overall, Shotcrete is very advantageous in the construction industry, there are a few disadvantages of Shotcrete. One of the biggest disadvantages of Shotcrete is that its success is dependent on the performance or professionalism of the operator/builder as human error is the biggest factor as to why Shotcrete would fail. A professional need to know how much water (if using dry mix method) or air (if using wet mix method) is needed at the nozzle to ensure the concrete is the right viscosity, so that when it dries it is the right strength and bonds correctly. Therefore, ideally it requires someone who is skilled and has many experienced hours with greater geotechnical knowledge to operate the Shotcrete application.

Shotcrete is usually reinforced by metal rods, mesh or fibres. The preparation of the surface before Shotcrete is applied is important. The surface should be free of lose materials, clean and smooth. The finished Shotcrete layer should be kept continuously wet for at least a week after completion.

Two problems that can occur are gravel in the concrete aggregate is too large, which interferes with the spraying of the concrete; and ready mixed concrete is too wet so it doesn’t stick well to vertical and overhead surfaces.

Shotcrete is a very cost effective and time-saving method of concrete application that has been becoming increasingly popular in the construction industry. However, the Shotcrete application should only be done by an experienced and knowledgeable professional due to the biggest factor towards failure is human error. 

In regards to the Darlington Upgrade, the state government blamed recent heavy rain, saying the wet weather caused significant movement in the clay, which resulted in spray-on concrete disintegrating, however engineers are saying the collapse raises serious questions about the structural integrity of the project if it is unable to withstand rain.