By Gert Schmidt
Hindle Mason Projects (HMP) has mastered the shotcrete concrete repair process.
Shotcreting is an economical and effective method for repairs that require large areas of vertical and horizontal forming.
The process entails placing cementitious materials with aggregates onto prepared surfaces pneumatically. Material consolidation and bonding occur during shotcrete applications. As such, there is no need for bonding agents. In shotcrete applications, they can have the opposite effect by negatively affecting bond strength.
As is the case with all repair types, sound shotcrete methods require proper surface preparation.
Shotcrete for vertical concrete repairs
Shotcrete concrete repair is most economical for vertical and horizontal concrete repairs. These include columns, walls, beam sides, and pier caps.
Shotcreting is also feasible for repairing overhead horizontal surfaces. These include ceilings, beam undersides, slab soffits, and overhangs.
Our processes restore structural integrity and increase cover over reinforcement when forming requirements are difficult or prohibitively expensive. This approach saves on repair time and costs.
Shotcrete concrete repair preparation
Shotcrete concrete repair preparation is critical to ensure a positive outcome.
Our teams first remove deteriorated and spalled concrete back to sound substrate.
If reinforcing steel is exposed, it is undercut to provide mechanical bond for the shotcrete.
Corrosion products on rebar are also removed using abrasive or high-pressure blasting.
This is done while ensuring that the repair boundaries are left at an out-sloping 45° angle to facilitate air and rebound escape. Meanwhile, the outer edges of the repair areas are sawcut to depths of about 20 mm. This prevents feathered edges.
HMP also replaces or supplements damaged steel reinforcement. This is done together with our clients’ engineering teams to ensure the required sizes and spacing.
Lap splices are separated by one bar diameter to allow for the placement of repair materials. This is done while ensuring that new reinforcing steel is never placed directly behind or in front of existing rebar.
The surface is then sprayed with water and allowed to dry back to a saturated surface-dry condition.
Wet/dry shotcrete concrete repairs
HMP is skilled in both wet- and dry-mix shotcrete concrete repair methods.
The shotcrete mix consists of 13 mm aggregate that conforms to SANS 1083
HMP uses ready-mixed, pre-packaged, or site-batched mixes. They can include fibres [https://me.smenet.org/sika-corporation-concrete-reinforcing-fibers-for-shotcrete/], silica fume [Silica Fume in Shotcrete], or other additives that enhance physical properties.
Fibres and proper curing procedures minimise potential drying shrinkage.
We also match the substrate concrete and repair material in terms of strength characteristics. The only exception is when the substrate concrete is very weak.
Ready-mixed shotcrete is batched and delivered to SANS 878 [2010277-AfriSam-Technical-Ref-Guide-update-2021.pdf].
Prepackaged product must meet the requirements of ASTM C1480/C1480M. Refer to C1480/C1480M Standard Specification for Packaged, Pre-Blended, Dry, Combined Materials for Use in Wet or Dry Shotcrete Application (Withdrawn 2021). South Africa does not have a dedicated SANS standard specifically for packaged, pre-blended dry combined materials for shotcrete.
Shotcrete concrete repair equipment
HMP owns both wet- and dry-mix shotcrete concrete repair equipment.
For wet-mix shotcrete, we use well-maintained concrete pumps and air compressors. This is in addition to concrete and air hoses, and shotcrete nozzles with air rings.
For dry mix shotcreting, we deploy cement guns, mixer elevators, air compressors, and water pressure booster pumps. This is in addition to shotcrete nozzles and hoses with water rings, as well as water hoses.
Initiating shotcrete concrete repair
Initiating shotcrete concrete repair entails filling the corners first and continuously moving the nozzle. This encapsulates the reinforcing steel and avoids shotcrete buildup in areas. A bond coat of neat cement is placed automatically due to the initial aggregate rebound.
We use a blow pipe to remove any overspray and rebound that accumulates in corners or on reinforcing steel.
Individual layer thicknesses are not limited and can be increased by adding additives.
Any number of layers can be placed as needed. However, it is important that immediately preceding layers are correctly prepared to ensure a successful bond.
The shotcrete is applied to just past the desired plane and allowed to stiffen.
We then trim the plastic material back to the desired plane using a sharp-edged trowel. This is undertaken when the shotcrete can be worked without disturbing the bond.
We do this while ensuring that the material is never overworked, as this can result in delamination and spalling.
Thereafter, we rod to straight lines, avoiding excessive finishing.
Proper curing procedures enhance the physical properties of the shotcrete and reduce shrinkage cracking. Refer to What is the cure time for shotcrete? – American Shotcrete Association.
Shotcrete concrete repair inspection
We then undertake a thorough shotcrete concrete repair inspection.
These evaluations are undertaken according to ACI 308R. concrete.org/portals/0/files/pdf/previews/308r_16_preview.pdf is frequently referenced in South Africa. This is especially the case for effective curing of shotcreting on large projects.
One of the many benefits of shotcreting is that quality can be closely monitored during placement. This is because the concrete is not hidden inside formwork.
We can observe the way in which the corners are being filled and reinforcing steel encapsulated. By preventing voids and unconsolidated overspray and removing rebound as it accumulates, we prevent hondcombing.
A sound shotcrete repair project will typically only fail in the substrate and have a tensile strength of more than 1 MPa.
Safe shotcrete concrete repair operations
We also pride ourselves on undertaking safe shotcrete concrete repair operations. This is especially important considering that shotcreting presents many hazards that need to be managed.
Every worksite presents unique risks that must be identified and mitigated via appropriate health and safety practices. These must also align with those of our clients. Furthermore, they must comply with relevant laws and regulations. Refer to Occupational Health and Safety Act and Regulations (Downloads) – Labour Guide South Africa. These are underlined in 37305_7-2_Labour_Layout 1. Mine Health and Safety Act 29 of 1996 and Regulations Final Booklet.pdf meets the requirements of mining clients.
Our health and safety processes are also supported by material safety data sheets [Safety Data Sheets | Free SDS Database | Chemical Safety]. These inform the healthy and safe handling of repair materials, including the type of personal protective equipment to be deployed.
Our teams are always equipped with the correct protective clothing or skin barriers, eye wear, hearing protection, and dust masks. This is in addition to forced-air respirators during abrasive blasting.
Schmidt is a Director of HMP
