Concrete, Mortars and Specialty Chemicals for Shotcrete Applications

The Batimix range of products includes a number of dry-mix ready to use concrete and mortar products for shotcreting applications for new construction, repair and rehabilitation works, specialty chemicals as well as shotcrete admixtures for the preparation of on-site concrete and mortar mixes.

Shotcreting is the projection at a high velocity onto a surface of concrete or sometimes mortar. A nozzle man applies the concrete or mortar from a pressurized hose. Shotcrete undergoes placement and compaction at the same time due to the force with which it is projected from the nozzle. The projected material encompasses the reinforcement forming structural shapes that include walls, floors, roofs, and other assemblies. Typical shotcrete applications include:

• Architectural rehabilitation and design works
• Rehabilitation of different kinds of structures such as bridges and dams.
• Repair works
• Construction of swimming pools, waterparks, and skateboard parks.
• Slope stabilization and channel/irrigation linings
• Temporary and final rock support
• Surface and underground mining
• Tunneling projects.

Although shotcrete uses all the components typically found in different types and applications of concrete, it is cast in a unique and special way. The dry mix and wet mix processes are the two application methods used, with the former still dominant in a large number of countries

In the dry process, the material consisting of cement and aggregates must be either dry or pre-dampened with a maximum of 8% humidity. It is pushed/conveyed by air through a hose. At the nozzle, it is mixed with the water and, if required, a liquid accelerator. This liquid is transformed within a split second into concrete and projected under high velocity onto the surface. The operator adjusts the water-cement ratio (W/C) to achieve the desired mortar consistency, by controlling the addition of the mixing water at the nozzle. Shotcrete applications are done in layers until the desired thickness is achieved.

In the wet mixed method, wet mixed concrete is conveyed either by a piston pump or by a rotor-stator. At the nozzle, the accelerator and air are introduced. The air breaks the concrete into small particles projecting them onto the substrate. To minimize the rebound effect, wet-mix shotcrete is always applied with the aid of a set accelerator.

A comparative analysis of the two different shotcrete methods outlined above is as follows:

Advantages

The dry-mix process offers amongst others the following advantages:

• Long conveying distances of up to 300 m horizontally and 100 m vertically.
• Long working time as the material does not set in the hose before it meets the water at the nozzle.
• High compaction and consolidation of the sprayed material due to the high velocity under which it is projected onto the surface.
• The pumps and hoses can be easily cleaned by air. No water is required and no mixed concrete has to be dealt with.
• Reasonable dispensing equipment cost.

The wet-mix process offers amongst others the following advantages:

• Accurate blending and dosage of all the components including water under operators’ control. A complete and thorough cement hydration yields enhanced performance.
• Less rebound by around 15 to 20% compared to dry-mix spraying.
• The method does not require the same level of operators’ skills as the drymix process.
• Easier use of steel fibers and new admixture technology for large volume applications and high strength concrete requirements.

Limitations

The dry-mix process has the following limitations:

• The permanent adjustment of the mixing water can lead to inconsistent concrete quality.
• Dry applied shotcrete can be relatively hard and stiff and not appropriate for some finishing operations.
• Limited use in interior applications in narrow and confined areas particularly without ventilation because of potential dust problems.
• The rebounding factor can range from 20 to 40% depending on the application, type of material, and operator’s skills.
• The lower production capacity of up to 7 m³ per hour as compared to around 20 m³ per hour with the wet mixed method.

The dry mix process offers the following limitations:

• Difficult and rather messy clean-up of the equipment and washed-out concrete.
• Limited open time in the hose as the hydration process and concrete setting start promptly after the addition of water.
• Practical limitations in the conveying distance, which should not exceed around 100 m horizontally and 50 m vertically.
• Wet mix equipment is normally more expensive, larger in size, and thus harder to maneuver on the job site.

Admixtures used for shotcrete applications

ASTM C1141 covers materials proposed for use as admixtures to be added to a Portland- cement shotcrete mixture for the purpose of altering the properties of the mixture. Admixture Types and Grades, classified as per ASTM and used in either one of the two processes are described herein:

Type I - Drymix shotcrete

Grade 1 - Accelerating admixture, conventional - (ASTM D98, C494 Type C or E) Grade 2 - Retarding admixture - (ASTM C494 Type B or D) Grade 3 - Pozzolanic admixture - (ASTM C618, C989, C1240) Grade 4 - Metallic iron admixture - Not established Grade 5 - Coloring admixtures - (ATM C979) Grade 6 - Organic polymer admixtures - (ASTM C1438) Grade 7 - Not applicable Grade 8 - Not applicable Grade 9 - Accelerating Admixture - Quick setting - (ASTM C1102)

Type II – Wet mix shotcrete

Grade 1 - Accelerating admixture, conventional - (ASTM D98, C494 Type C or E) Grade 2 - Retarding Admixture - (ASTM C494 – Type B, D, or G) Grade 3 - Pozzolanic admixture - (ASTM C618, C989, C1240) Grade 4 - Metallic iron admixture - Not established Grade 5 - Coloring admixture - (ASTM C979) Grade 6 - Organic polymer admixture - (ASTM C1438) Grade 7 - Water reducing admixture - (ASTM C494 Type A, D, E, F, or G) Grade 8 - Air entraining admixture - (ASTM C260) Grade 9 - Accelerating admixture - Quick setting - (ASTM C1102)

Each of the above grades is further classified according to the following classes:

• Class A - Liquid
• Class B - Non-liquid

The Batimix range of admixtures for shotcrete includes the following:

Shotcrete accelerating admixtures

Conventional shotcrete accelerating admixtures help the mix gel by reacting with the water. Dosage can be adjusted to optimize setting characteristics for different environments. While minimizing particle rebound, they are not designed to significantly accelerate concrete setting times to allow workers to finish concrete placing.

Quick setting shotcrete accelerating concrete admixtures minimize particle rebound and are intended for applications requiring quick setting within minutes such as may be required in tunneling and mining operations.

Shotcrete retarding admixtures

Set retarding admixtures can be added to wet shotcrete mixes to allow for longer transport distances. They will lengthen setting times so as to allow for traffic, longer transport distances and hot climates. These are also designed to enhance the flow characteristics of shotcrete mixes particularly when using fibers and for reducing the pumping pressure of shotcrete mixes.

Pozzolanic admixtures

Silica fume, fly ash, and slag are often added in different proportions in wet and dry mixes to the shotcrete mix to enhance strength and durability and reduce the permeability of the hardened concrete.

Metallic iron admixtures

Metallic Iron admixtures are carefully graded ground metallic particles free from rust, oil, foreign materials and non-ferrous metal particles. These are classified as follows

• Metallic Iron, a Type I Grade 4 Class B admixture.
• Metallic Iron, a Type II Grade 4 Class B admixture.

Synthetic structural fibers

The performance of a fiber-reinforced concrete or shotcrete depends strongly upon the susceptibility of the fibers to physical damage during the mixing or shotcreting process, their chemical compatibility with the normally alkaline environment of the cement paste, and their resistance to service conditions encountered within un-cracked concrete or as a consequence of cracking. ASTM C1116 classifies fiber-reinforced concrete or shotcrete by the material type of the fiber incorporated.

• [Visit: https://holderchem.net/products/details/batimix-fibers-type-i]Fibers Type I or steel fibers. They contain stainless steel, alloy steel, or carbon steel fibers.
• [Visit: https://holderchem.net/products/details/batimix-fibers-type-ii]Fibers Type II or glass fibers. They contain alkali-resistant glass fibers.
• Fibers Type III or synthetic fiber. They contain virgin polypropylene or other synthetic fibers.

Coloring admixtures

Coloring shotcrete admixtures are mixes based on various pigments. A limited range of standard colors is available. Different colors can also be provided upon request. Users should be aware that shades might vary.

Organic polymers

Bonding admixtures based on organic polymers or based on latex are used to create higher tensile strength and greater durability of polymer-modified shotcrete. Polymer modifiers are classified as per ASTM C1438 into two product types:

• Type I – For use in areas not exposed to moisture.
• Type II – For general use.

Flow enhancement admixtures

Flow enhancement admixtures are high range water reducers that significantly enhance gravity flow and ease the pumping pressure of the shotcrete mix. While essential when using structural fibers in a mix, they greatly contribute in increasing the slump of a shotcrete mix without the addition of water.

Air-entraining admixtures

Air entraining agents are often added to shotcrete mix designs for flowability and most especially freeze / thaw protection. ASTM C1141 defines the shotcrete air-entraining admixture requirements, which like concrete admixtures must conform to ASTM C requirements.

Dry-mix line of premium products

ASTM C1480 provides the standard specification for packaged, pre-blended, dry combined mix of materials for use in wet or dry shotcrete applications. The products supplied are characterized by type and grade and delivered to customers with the ingredients uniformly mixed. They are designed to significantly enhance the performance of shotcrete applications and are specifically formulated to generate less dust in dry mix processes and reduce the rebound factor whether used in dry or wet applications.

Shotcrete Types are based on aggregate use:

• Type FA shotcrete shall contain aggregate in accordance with the requirements of Specification C1436 Grading #1. Aggregates failing to comply with Grading #1 can be used if preconstruction testing proves that they give satisfactory results.
• Type CA shotcrete shall contain coarse and fine aggregates in accordance with the requirements of specification C1436 Grading #2. Aggregates failing to comply with Grading #2 can be used only if preconstruction testing proves that they give satisfactory results or if acceptable service records exist.

Shotcrete Grades are based on the desired physical properties of the material:

• Grade GR shotcrete is a general use product.
• Grade SR shotcrete has sulfate resistance.
• Grade LP shotcrete has permeability values.
• Grade FR Class I shotcrete incorporating fibers is intended to minimize plastic shrinkage cracking.
• Grade FR Class II shotcrete incorporating fibers is intended to provide a minimum level of post-crack flexural load capacity for hardened shotcrete.

It is to be noted that a single material may meet more than one grade. The purchaser may specify other properties.