The Importance of Rock Washing and Chipping in Concrete Central Plant Mixing Drums

by Jon S. Belkowitz, PhD, PE

Abstract

Maintaining the efficiency and longevity of your concrete central plant's mixing drum is vital to ensuring high-quality concrete production. Over time, residual and hardened concrete can build up inside the drum, leading to decreased efficiency, higher energy consumption, and suboptimal mixing performance. Fortunately, proactive maintenance practices like rock washing and chipping can prevent these issues and safeguard your equipment.

Understanding Concrete Buildup and Adhesion

The accompanying images illustrate the progressive development of concrete buildup within mixing drums, particularly focusing on how concrete adheres to the metal fins. Both images detail the mechanical interface at the junction between the metal fin and the initial concrete layers. At this stage, the roughness and microtextural features of the metal create numerous anchoring points where concrete particles physically lock onto the fin surfaces. Over time, vibrations and thermal expansion further solidify this bond, hardening the connection and making removal increasingly difficult.

From a chemical perspective, the interplay between hydrophilic properties of cementitious materials and the moisture left on the drum’s surface aids in the adhesion process. Calcium hydroxide, a byproduct of the concrete hydration reaction, can react with residual water and carbon dioxide in the environment, forming a tightly adhered calcium carbonate layer directly on the metal. This creates a chemical bridge, enhancing both adhesion strength and resilience. Over subsequent mixing cycles, new concrete continuously bonds to the hardened buildup due to its natural ability to form cohesive layers.

The last image highlights this progressive adhesion, showing how successive layers of different types of concrete accumulate over time. This phenomenon arises because fresh concrete exploits the porous nature and microfractures of the underlying cementitious material. The unhydrated particles within the older layer partially react with the water and chemicals in the new concrete mix, effectively forming a seamless interface. While this bonding ability is advantageous in construction applications ensuring monolithic structures, it poses a significant challenge in maintaining mixing drums, as the buildup becomes increasingly stubborn and detrimental to operational efficiency.

Addressing this problem requires a dual approach targeting both mechanical and chemical interactions. Mechanical interventions aim to break the physical bond at the interface, while chemical treatments could help dissolve or weaken the calcium-based compounds adhering to the fins. Understanding these layers’ dynamics not only informs maintenance strategies but also mitigates production impacts, enhancing overall equipment performance and extending operational lifespans.

Why Regular Maintenance is Critical

Concrete buildup inside a mixing drum isn’t just a nuisance—it directly impacts your operations. Hardened layers of concrete increase the drum's workload, reducing mixing efficiency and shortening its lifespan. These buildups can also lead to uneven mixes, compromising the quality of your concrete and thus your projects. Regular maintenance, including rock washing and chipping, helps mitigate these problems.

Effects of Concrete Buildup

• Reduced Mixing Capacity: Excess concrete decreases the drum's effective volume, limiting capacity and creating inefficiencies.
• Energy Wastage: More power is required to turn a drum overloaded with hardened materials.
• Decreased Quality: Improper mixing leads to inconsistent concrete quality in the final product.
• Higher Costs: Neglecting buildup adds wear and tear to the equipment, leading to costly repairs and shortened drum life.

What is Rock Washing?

Rock washing is a preventive maintenance technique that uses high-pressure water jets to remove loose concrete and aggregate from the inside of the drum. During this process, coarse aggregates, combined with water, scour the interior surface and blades of the drum, cleaning away residual materials.

Benefits of Rock Washing

• Removes fresh concrete buildup before it hardens.
• Prevents the accumulation of thick layers of hardened material.
• Keeps the drum running efficiently with minimal downtime.

How to Rock Wash Properly

1. Add Water: Introduce 200 gallons of water or recycled wash water into the drum.
2. Run the Mixer: Operate the drum at mixing speed for a minimum of 2 minutes to allow the slurry to scrub away residual materials.
3. Final Rinse: If necessary, use a high-pressure hose to remove stubborn areas along the blades and discharge end.

When Chipping is Necessary

While routine rock washing is effective for fresh concrete, hardened layers require a more thorough approach. Chipping involves manually breaking apart hardened buildup using specialized tools, such as air-powered chipping hammers.

How to Identify When Chipping is Due

• Visual Inspection: Thick, hardened layers visible inside the drum.
• Dull Drum Sound: A muffled sound when tapping the drum indicates significant buildup.
• Performance Issues: Poor mixing quality or reduced discharge efficiency.
• Batch Complaints: If the mixing drum can no longer hold its rated capacity, chipping is overdue.

The Chipping Process

A. Conduct a Lockout/Tagout Procedure

Inform personnel and park the vehicle in a secure location.

Shut off the truck completely, remove the ignition key, and disconnect the power supply.

Secure the drum with wedges, straps, or chains to prevent accidental movement.

B. Inspect the Drum

Use proper lighting to identify hazards like sharp edges, water, or falling concrete.

Place a ventilation fan to ensure airflow and reduce dust exposure.

C. Begin Chipping

Start with the bottom section of the drum, focusing on large, loose pieces of hardened concrete.

Use an 18-pound air-powered chipping hammer for precision, avoiding damage to the drum's surface and blades.

Rotate the drum, section by section, to clear the entire interior evenly.

D. Make Repairs as Needed

Inspect the blades and drum interior thoroughly while it's cleaned.

Look for signs of wear, cracks, or damaged blades, and make repairs before finishing the process.

E. Complete the Job Safely

Discharge all chipped concrete chunks and conduct a final rinse of the interior.

Remove lockout devices, restore power, and notify personnel that the equipment is back in service.

Safety First

Maintaining a central plant mixing drum is physically demanding and involves significant safety risks. Ensure all personnel follow strict protocols and wear appropriate personal protective equipment (PPE), including hard hats, gloves, goggles, particulate respirators, and safety shoes. Additionally, keep confined space safety procedures in place to avoid accidents when inside the drum.

Establishing Maintenance Frequency

To keep your mixing drum in its best condition, follow these guidelines for frequency based on usage and production levels:

• Rock Washing: Perform after every production cycle or end of the day.
• Chipping: Conduct a minimum of once or twice annually or more frequently for high production levels or when handling sticky concrete mixes.

Long-Term Benefits of Regular Maintenance

Proactive drum maintenance pays dividends in the long run. The key benefits include:

• Improved Efficiency: Clean drums ensure uniform mixing and discharge.
• Extended Equipment Life: Regular upkeep reduces wear and tear on your equipment.
• Cost Savings: Avoid expensive downtime and repairs associated with heavy concrete buildup.
• Better Concrete Quality: Consistent and reliable mixing improves the final product for your clients.

Final Thoughts

Maintaining your central plant mixing drum through regular rock washing and periodic chipping is crucial to delivering premium concrete while keeping operational costs low. Ignoring concrete buildup can lead to inefficiencies, equipment damage, and compromised safety. With these processes integrated into your maintenance routine, you'll experience smoother operations, better concrete quality, and a longer service life for your equipment.

The best time to start prioritizing your maintenance schedule is today—the true strength of your concrete depends on the condition of your mixing equipment.