How Does Concrete Mixer Truck Work

Overview of Concrete Mixer Trucks

Concrete mixer trucks are vehicles designed to transport and mix concrete, keeping it fresh and ready for use at construction sites. Here’s a detailed look at their key features and functions:

Mixing Process

Inside the truck, the drum rotates constantly during transit, keeping the concrete liquid. This rotation prevents the concrete from hardening too soon and ensures it arrives ready for immediate use.

Drum Mechanism

The drum has spiral blades that mix the concrete thoroughly. When rotated in reverse, these blades act like an Archimedes screw, guiding the concrete out smoothly through a chute for precise placement.

Water and Dry Materials

Many mixer trucks have a separate water tank for on-the-go mixing. They can carry both dry ingredients and water to mix en route or be pre-loaded with mixed concrete from a central plant.

Capacity and Weight

Most concrete mixer trucks hold about 20,000 kilograms (40,000 lb) of concrete, with typical capacities of 10 cubic yards (7.6 m³). When empty, these trucks weigh between 9,100 to 14,000 kilograms (20,000 to 30,000 lb).

Axles and Braking System

To handle heavy loads and meet road weight limits, these trucks are equipped with multiple axles. Common configurations include four-, five-, and six-axle trucks, depending on load needs and regulations.

Speed and Operation

In some areas, such as the UK, concrete mixer trucks are limited to 90 km/h (56 mph). During transport, the drum spins at 2-3 revolutions per minute, maintaining the concrete mix’s consistency.

Discharge Mechanism

By reversing the drum’s rotation, the truck controls the concrete’s discharge through a chute. For sites with limited access, concrete can be discharged into a pump, hose, or conveyor belt for flexible placement.

Cleaning Process

After each job, concrete mixer trucks go through a washout process to clean the drum and other components, preventing any leftover material from affecting future batches.

Types of Mixers

Concrete mixer trucks come in various types, including traditional drum mixers, volumetric mixers that mix on-site, and self-loading mixers suited for remote areas without batching facilities.

Components of a Concrete Mixer Truck

Drum

• The drum, a large cylindrical tank on the truck, is the main vessel for mixing concrete. It rotates to blend ingredients consistently.
• Key parts of the drum include mixing blades, essential for the mixing process. There are two main types:
  • Spiral Blades: These push the concrete ingredients toward the drum’s center, ensuring a uniform mix.
  • Paddles: They lift and drop the mixture repeatedly to ensure even blending.
• High-speed blade rotation makes the drum design critical for efficient mixing.

Blades

• The drum’s internal blades have special shapes for effective mixing. Some advanced trucks offer adjustable blades, allowing customization based on different construction materials, which improves mix quality.

Engine

• The engine powers both the mixing drum and the truck’s hydraulic systems. Its performance is crucial for the truck’s efficiency in mixing and transporting concrete.

Transmission and Hydraulic System

• The transmission system transfers power from the engine to the mixing drum and other components.
• The hydraulic system, which includes a high-pressure pump, manages the movement of heavy parts like the drum. It also controls support legs and drum rotation.

Chassis and Axles

• The chassis provides structural support and stability, while the axles bear the load and enable movement.
• Pedestal components are mounted on the chassis and include the hydraulic system, bearings, and support legs. These legs stabilize the truck during loading and unloading, preventing tipping.

Pedestal Components

• The pedestal contains key parts, including the hydraulic system for vertical movement, bearings for smooth drum rotation, and support legs for stability during operation.

Bearings and Support Legs

• Bearings need regular lubrication to stay functional and prevent breakdowns in tough conditions.
• Support legs, controlled hydraulically from the cab, are essential for stabilizing the truck during mixing, loading, and unloading tasks.

How Concrete Mixer Trucks Work

Concrete mixer trucks are essential for delivering freshly mixed concrete to construction sites. These trucks are built to keep the concrete in a liquid state during transport, preventing it from hardening before reaching the job site.

Mixing Process

The drum inside the concrete mixer truck rotates during transport. This keeps the concrete mixture in liquid form, preventing it from setting too soon. The concrete stays usable until it is ready to pour at the construction site.

Drum Mechanism

The inside of the drum has blades that push the mixed concrete against the drum’s gravity. These blades ensure the cement, water, and aggregates mix thoroughly, keeping the concrete from drying during transport. This action keeps the concrete in the right state for pouring.

Water and Dry Materials

Before transport, dry concrete mix, which includes cement, sand, and aggregates, is loaded into the drum. Water is usually added during the trip, though some concrete mixers have a separate water tank to add water closer to the job site. This keeps the concrete at the right consistency for easy pouring.

Spiral Blade Action

The spiral blades inside the drum rotate in one direction to push the concrete deeper into the drum. This continuous movement ensures the mix stays uniform and prevents the concrete from hardening during transit. The concrete stays in a workable, liquid state until it’s ready to pour.

Discharging Concrete

When it’s time to pour the concrete, the drum rotates backward, allowing the concrete to flow out smoothly through chutes. This backward rotation gives crews full control over the placement of the concrete.

Chutes and Extensions

Concrete is discharged through chutes that can be extended to direct the flow to the desired location. These chutes are usually around 4.5 feet long, giving flexibility in where the concrete is placed.

Variations in Discharge

• Rear-discharge: In this method, a worker operates the rear of the truck to control the chutes, allowing precise concrete placement.
• Front-discharge: This setup lets the truck drive directly into the job site, with the operator controlling the concrete pour from the cab for easier operation.

Concrete Pumps

If the mixer truck can’t reach the required location, a concrete pump is used to move the concrete to areas that are hard to access, such as high-rise buildings. The pump is an efficient way to transfer concrete to tough-to-reach spots.

Time Constraints

To maintain quality, concrete must be poured within two hours of mixing to prevent it from hardening in the truck. Construction managers need to prepare the site ahead of time to avoid delays in pouring.

Capacity

Most concrete mixer trucks carry 11 to 15 cubic yards of concrete. However, weight limits on certain roads can restrict this to between 8 and 11 cubic yards.

Post-Job Cleaning

After finishing a job, the concrete mixer truck is cleaned through a process called washout. This removes any leftover cement from the drum and other parts, preventing contamination in future batches. Proper cleaning ensures the truck stays in good condition for future use.

Loading the Ingredients

To load ingredients into a concrete mixer truck, follow a precise sequence to maintain quality and consistency.

• Add Water: Start by adding about half of the water to the mixer. This helps dissolve the cement and reduces early clumping.
• Add Cement: Slowly add the cement while the drum rotates. This gradual addition ensures the cement mixes well with water and evenly coats the aggregates, preventing uneven mixtures.
• Add Aggregates and Sand: Add aggregates, like gravel, first, followed by sand, as the drum continues to spin. Add the remaining water afterward. This order reduces clumping and ensures a smooth, consistent mix.
• Adjust Drum Rotation Speeds: The drum rotates at specific speeds depending on the mixing stage. During loading, the drum operates at charging speed to blend all components. The speed is adjusted to thoroughly mix the concrete before transport. Transit-mix concrete often rotates faster at the plant than during transportation.
• Transit Mix Method: In this method, raw ingredients are loaded directly into the truck mixer for mixing during transit or on-site.
• Central Mix Method: All ingredients are pre-mixed in a stationary mixer at a central plant, ensuring better consistency and quality before being loaded into the truck.
• Preparation Steps: Clean the drum to remove any leftover material from previous batches, preventing contamination. Lubricate moving parts to keep operations smooth and avoid mechanical issues during loading.

Following these carefully controlled steps helps ensure a uniform and high-quality concrete mix, ready for delivery to construction sites.

Mixing the Concrete

The process of mixing concrete in a mixer truck starts when the dry concrete mix, made up of cement, sand, and aggregates, is added to the drum. Water can be added before transport or at the job site, depending on the truck’s design. Some trucks have a water tank, allowing the driver to add water closer to the construction site for better control over the mix’s consistency.

Drum Rotation and Blade Action

After the dry mix is loaded, the drum begins rotating, and the internal spiral blades start working. These blades mix the concrete thoroughly by pushing it deeper into the drum. The rotation ensures the materials are mixed evenly while preventing them from drying out during transportation. As the drum turns, the concrete stays workable and ready to use upon arrival at the site.

Continuous Mixing During Transport

The drum must rotate continuously during transit to avoid premature hardening of the concrete. This constant rotation keeps the concrete well-mixed and prevents it from setting before reaching the construction site. Typically, the drum rotates at 2 to 3 revolutions per minute (rpm) during transport, maintaining the mix’s consistency.

Mixing Speed and Revolutions

To achieve a fully mixed batch, the drum should rotate 70 to 100 times at the correct speed. If additional water is added at the job site, the concrete should be mixed for at least 30 more revolutions to ensure uniform consistency and prevent material separation.

Blade Function and Discharge

The blades inside the drum serve two purposes: mixing the concrete and helping with its discharge. When it’s time to release the concrete, the truck rotates the drum in the opposite direction, allowing the concrete to flow out through a chute. The spiral blades, designed like Archimedes’ Screw, ensure smooth discharge. This design helps the mix flow evenly without clogging or uneven distribution.

Transportation

• Continuous Rotation: During transit, the drum of a concrete mixer truck must keep rotating to prevent the concrete from setting. This process, known as “charging,” keeps the concrete in a liquid state by constant stirring. Continuous rotation is essential to prevent hardening and to ensure the concrete is ready for unloading upon arrival.
• Drum Speed Adjustment: The drum speed can be adjusted to maintain the concrete’s consistency. Changing the rotation speed helps prevent the concrete from becoming too thick or too watery, keeping it workable throughout the journey.
• Interior Mechanism: Inside the drum, a spiral blade pushes the concrete deeper into the mixer, continuously blending it to prevent drying. A built-in turbine inside the drum also assists by forcing the concrete upward, enhancing the mixing process as the truck moves.
• Water Addition: Some concrete mixer trucks have a water tank, allowing the driver to add water to the mix closer to the construction site. This is especially helpful for long trips, as it keeps the cement from setting too soon. By adding water right before arrival, the concrete remains easy to pour and spread on-site.
• Discharge Mechanism: When it’s time to unload, the mixer truck uses an Archimedes’ screw system to release the concrete from the drum. The concrete flows through chutes, directing it precisely to areas like sidewalks or foundations with minimal handling.
• Time Constraints: Cement manufacturers recommend that concrete be unloaded within two hours of mixing. This time limit requires careful planning to ensure that, once the truck arrives, the concrete can be poured immediately. Delays can cause hardening, making the concrete unusable.
• Travel Distance and Dry Batching: The travel distance for mixer trucks generally ranges from six to fifteen miles. For longer trips, “dry batching” is sometimes used, where only dry materials go into the mixer initially, and water is added at the job site. This method helps keep the concrete workable when construction sites are far from the mixing facility.
• Site Preparation and Drum Cleaning: Construction sites must be prepared in advance so the truck can unload immediately, minimizing the risk of the concrete hardening. Drivers often clean out the drum at the site after unloading to prevent any leftover cement from hardening inside, ensuring the truck is ready for its next load.

Unloading the Concrete

Prepare the Discharge Area

Ensure the construction area is level and ready to receive the concrete. This minimizes delays and helps the unloading process go smoothly.

Control Drum Rotation

Rotate the drum at a steady speed, opposite to the mixing direction, to release the concrete evenly.

Maintain Even Distribution

Keep a consistent discharge rate to spread the concrete uniformly across the site, ensuring a well-mixed result.

Use Chutes and Hoppers

Direct the concrete flow with chutes and hoppers to reach the right spot, reducing spillage and waste.

Avoid Over-Discharge

Discharge only the necessary amount to prevent excess, avoid quality issues, and reduce cleanup.

Check Consistency

Regularly monitor the concrete’s consistency as you unload. Adjust with water if needed to keep it workable.

Clean Thoroughly

Clean the drum, chutes, and other parts of the truck after unloading to prevent hardening and prepare for future use.

Types of Concrete Mixer Trucks

Standard Transit Concrete Mixer Trucks

• Description: Standard transit mixer trucks are commonly used for ready-mix concrete.
• Capacity: They usually handle 2 to 8 cubic meters of concrete.
• Features: These trucks have a rotating drum with spiral blades that keep the concrete fluid, preventing hardening during transit.

Rear-Discharge Mixer Trucks

• Description: Concrete is discharged from the rear of the truck using an adjustable chute.
• Features: Built for heavy loads and tough environments, these trucks have strong frames, heavy-duty tires, and advanced suspension systems. They are ideal for demanding construction sites.

Front-Discharge Mixer Trucks

• Description: Less common than rear-discharge trucks, these trucks offer better control when discharging concrete.
• Features: The driver-operated chute gives more control and precision, which is useful on tight, congested construction sites where maneuverability is limited.

Volumetric Concrete Mixer Trucks

• Description: These trucks combine a concrete batching plant with a mobile mixer. Materials are stored separately and mixed on-site as needed.
• Benefits: They reduce waste by mixing only the amount needed. Customers pay for the concrete used, which can save money.
• Flexibility: These trucks allow precise mixing and the ability to switch between different mix types on-site, making them cost-effective and environmentally friendly.

Rough Terrain Concrete Mixer Trucks

• Description: Designed for difficult terrains, these trucks have 4×4 self-loading capabilities.
• Capacity: They can mix, load, and discharge over 100 cubic meters of concrete daily.
• Features: Equipped with electronic weight-batching systems and a slewing drum, these trucks can discharge from any side.

Reversible Drum Mixer Trucks

• Description: These trucks have drums that rotate both forward and reverse, making mixing and discharge easier.
• Use Case: They are useful for projects that need precise concrete consistency.

Self-Loading Concrete Mixer Trucks

• Description: These trucks load their own concrete, increasing efficiency and reducing the need for manual labor.
• Advantage: This feature helps automate the mixing process on-site, making it more efficient.

Trailer-Mounted Concrete Mixer Trucks

• Description: These smaller, highly maneuverable trucks are ideal for projects with limited space.
• Application: They are suited for small to medium-sized projects, particularly in urban areas with tight spaces.

Job Description of a Concrete Mixer Truck Driver

Driving to Job Sites

• Concrete mixer truck drivers deliver concrete to job sites within a radius of up to 45 miles from the plant. They ensure timely deliveries to keep construction projects on track.

Safe Driving Practices

• Safety is critical for mixer truck drivers. They strictly follow company safety rules, staying aware of traffic conditions, job site hazards, and safety at the plant.

Operating the Truck

• Operating a concrete mixer truck requires good coordination. Drivers must manage the clutch and shift through eight or nine gears, stopping and starting over 100 times a day.

Loading Ingredients

• At the plant, drivers position the truck under the hopper to load sand, gravel, cement, and water. They then start the mixer to blend the concrete.

Mixing Concrete

• Drivers monitor the mixing process to ensure the concrete is properly blended and ready for delivery upon arrival at the job site.

Unloading Concrete

• Drivers operate levers to unload concrete into designated areas or containers, following directions from the foreman or contractor.

Assembling Chutes

• For accurate unloading, drivers attach and position up to three chutes at the truck’s rear to guide the concrete flow.

Pre-Trip and Post-Trip Inspections

• Each day, drivers inspect their trucks and equipment before and after shifts, documenting any maintenance needs for reliability.

Fluid Checks

• Drivers check and refill vehicle fluids daily to keep the truck running smoothly and prevent breakdowns.

Cleaning the Truck

• After each delivery, drivers clean the mixer and truck, removing concrete to prevent hardening. They use special wash-down procedures and cleaning agents.

Minor Maintenance

• Drivers handle basic maintenance tasks, such as checking tire pressure, changing oil, and lubricating parts to ensure the vehicle remains in good working order.

Record Keeping

• Accurate records are essential. Drivers log daily mileage, complete inspection reports, and document deliveries to maintain detailed records.

Cost and Economic Aspects

Purchase Cost

• New Concrete Trucks: A new concrete mixer truck costs $100,000 and above, depending on size, mixer type, and brand.
• Used Concrete Trucks: Used trucks range from $30,000 to $150,000, based on age, truck size, and type.

Operating Costs

• Fuel Costs: Fuel expenses can be high due to the large engine size and constant drum operation.
• Maintenance Costs: Regular maintenance is required to maintain performance and avoid breakdowns, adding to operating costs.
• Labor Costs: Skilled drivers and maintenance staff are needed, which contributes to high operational expenses.

Income for Drivers

• Concrete mixer truck drivers typically earn a moderate to good income, with wages varying based on location, experience, and employer.

Return on Investment (ROI)

• Volumetric Mixers: Some users save $100,000 per year, with the equipment paying for itself within 24 months.
• Example – Baytown, Texas: Volumetric mixers helped the city reduce concrete costs by 45%, cutting the price per cubic yard from $96.40 to $54.45.

Operational and Upkeep Expenses

• Gasoline and Upkeep: In addition to the purchase cost, ongoing expenses include gasoline, maintenance, and driver wages.
• Compliance with Emission Standards: Meeting environmental regulations may require purchasing newer, more expensive models to meet emission standards.

Economic Impact

• Construction Demand: Demand for concrete mixer trucks depends on construction sector activity, which fluctuates with the economy.
• Infrastructure Investments: Government funding in infrastructure projects can increase demand for concrete mixer trucks.