Shotcrete Additives: Building Construction

Improving Shotcrete Efficiency for Building Construction Projects

Building construction projects are rarely simple. Many require temporary excavations to install structural excavations, while those which include underground rooms, floors, or parking, require permanent excavations that must be reinforced. Some construction areas may require concrete of varying thicknesses to meet technical specifications or for repair or restoration work. In projects that include substantial curves or elaborate formwork, conventional poured concrete is not a feasible means of construction. Instead, contractors would be better served with shotcrete treated with the appropriate admixtures that help it meet the required specifications.

Shotcrete’s Building Construction Applications

Shotcrete and concrete’s fundamental components are the same, and like concrete, shotcrete is typically dosed with one or more admixtures to ensure that it retains essential properties after being pumped pneumatically and has the additional properties it needs to match building construction specifications. Special considerations and requirements could include high compressive strength, low permeability, or high abrasion resistance. Additionally, engineers and contractors must consider job-site specific conditions such as long concrete haul distances or very long pumping distances.

As a result, contractors and construction project managers should consider specific shotcrete admixtures for strength, workability, and job site requirements. One or more chemical, mineral, or fiber admixtures should be considered if shotcrete will be used for skeletal, foundational, or load-bearing structures, as well as non-structural elements, such as exteriors requiring complex aesthetic elements or those elements that will be exposed to moisture.

Admixtures for Strength

Contractors may consider using several chemical admixtures, including water-reducing admixtures and accelerators. Shotcrete’s applications typically require good workability, which stems from a higher water-to-cement ratio. However, too much water can reduce the shotcrete’s strength and durability. Water reducers and high-range water reducers (also known as superplasticizers) can help shotcrete retain and gain strength by lowering the water-to-cement ratio while maintaining good workability.

Accelerators speed up the setting rate of shotcrete and can be used to accelerate the construction schedule by achieving higher strength earlier on. However, it’s important to consider using accelerators relative to job-site conditions, as using an accelerator when temperatures are high may enhance strength but greatly decrease workability or even cause flash-set.

Certain mineral admixtures, such as fly ash, silica fume, rice husk ash, and metakaolin can also enhance shotcrete strength, particularly over the long-term. Reinforcing shotcrete with fiber may also increase strength while reducing shrinkage.

Admixtures for Durability

Water reducers and superplasticizers may also be used to enhance shotcrete durability. To protect shotcrete from the effects of natural freezing and thawing cycles, air-entrainers are used. These chemical admixtures create millions of microscopic air bubbles that absorb pressure from moisture in concrete when it freezes. Moreover, damp-proofing admixtures not only reduce permeability but can also act as accelerators.

Fiber admixtures, as well as several mineral admixtures, are also helpful when it comes to shotcrete durability. Pozzolanic admixtures, such as silica fume, fly ash, blast furnace slag, and rice husk ash, can help form dense shotcrete ideal for water-retaining structures. These, along with chemical admixtures, can decrease shrinkage induced-cracking, delaminating, and scaling. Corrosion inhibitors can also reduce spalling. Efflorescence can be greatly mitigated by the use of water reducers.

Admixtures for Workability

Using normal water reducers can help enhance workability. Normal water reducers reduce water by up to 10 percent. However, mid-range water-reducing admixtures also reduce water by up to 15 percent, and superplasticizers reduce it by up to 40 percent. Set retarding admixtures can also help improve workability, especially when ambient temperatures are high. Further, fly ash and silica fume are ideal mineral admixtures to improve shotcrete workability. These admixtures are necessary for pumping shotcrete over significant distances or heights, as well as finishing.

Admixtures for Job Site Requirements

Contractors and construction project managers must also select the appropriate admixtures for job site conditions. Soil moisture and terrain, weather, and pumping distance may all necessitate the selection of different shotcrete admixtures to meet job requirements. Accelerators, retarders, and pumping aids are commonly balanced to ensure workability is maintained without overly delaying the construction schedule.