Sprayed Concrete

Introduction

Spayed concrete is the term used for the high pressure spraying of concrete or mortar onto existing surfaces. It is extremely versatile as it can sprayed over any shape or profile and in vertical as well as overhead situations.

Shotcrete and Gunnite are both sprayed concrete, although shotcrete is generally used to describe the dry mix method whilst gunnite the we mix method.

Because it is pneumatically forced through the nozzle at high pressure when the concrete hits the surface it is both placed and compacted.

The dry mix concrete/mortar is placed into the hopper and it is only at the delivery end of the end of the nozzle that water is injected.

The dry mix method involves placing the dry ingredients into a hopper and then conveying them pneumatically through a hose to the nozzle. The nozzleman controls the addition of water at the nozzle. The water and the dry mixture is not completely mixed, but is completed as the mixture hits the receiving surface. This requires a skilled nozzleman, especially in the case of thick or heavily reinforced sections. Advantages of the dry mix process are that the water content can be adjusted instantaneously by the nozzleman, allowing more effective placement in overhead and vertical applications without using accelerators. The dry mix process is useful in repair applications when it is necessary to stop frequently, as the dry material is easily discharged from the hose.

Wet-mix shotcrete involves pumping of a previously prepared concrete, typically ready-mixed concrete, to the nozzle. Compressed air is introduced at the nozzle to impel the mixture onto the receiving surface. The wet-gun procedure generally produces less rebound, waste (when material falls to the floor), and dust compared to the dry-mix procedure. The greatest advantage of the wet-mix process is that larger volumes can be placed in less time.

Shotcrete is today an all-inclusive term that describes spraying concrete or mortar with either a dry or wet mix process. However, it may also sometimes be used to distinguish from gunite as a wet-mix. The term shotcrete was first defined by the American Railway Engineers Association (AREA) in the early 1930s. By 1951, shotcrete had become the official generic name of the sprayed concrete process.