Concrete Mixing On-Site: Mixtures, Ratios & Tools

Today, you'll be playing the role of a chemist.

On-site mixing of materials offers commercial subcontractors a unique set of opportunities and challenges. It's not an overstatement to say your mixing process can significantly impact the quality, efficiency, and cost-effectiveness of the finished project.

This intricate process, essential for ensuring the right consistency and performance of materials, allows subcontractors to exercise control over several critical variables and determine the best way to mix concrete for each unique application. In this guide to concrete mixing on-site, we'll delve into the three main areas where subcontractors have options:

1. Using admixtures to enhance material properties
2. Material ratio
3. Selecting mixing tools based on project scale and requirements

Each of these areas offers a path to a better construction outcome. Like a scientific formula, they also require careful consideration.

Use of Admixtures 

Ready-mix concrete or site-mix concrete? The decision might remind you of the time you chose Bisquick over a mix that required eggs, oil, and water – or vice versa. 

Concrete doesn’t have a set recipe, per se. The mixture can include one or more of the following: 

• Manufactured sand
• Crushed stone
• Gravel
• Sand
• Recycled concrete

If your purchase requires on-site you’ll need to account for extra delivery and storage. The storage site will need to be available until immediately before you use the concrete. After all, one of the top tips for mixing concrete is: keep the materials safe until it’s time to pour.

Is Mixing Your Own Concrete Cheaper? 

While the price point on any job is a concern, the cost of mixing your own concrete is more a matter of preference and what the project calls for. Several factors influence the decision between site-mixed and ready-mix concrete, including:

Ready-mix concrete is trucked in from a plant, while site-mixed concrete comes from bagged ingredients you blend together.  

(Pro Tip: There’s a prepackaged product we call a “booster” to get the mix to set quicker in cold construction conditions. You can add these at the job site, whereas most admixtures are sold in a container.)

Let’s Talk Aggregate

Before purchasing an aggregate, determine what exactly you’re buying. 

Purchasing a product that doesn’t contain pea stone means you’ll be adding some afterward to extend the mixture’s ROI and improve its performance. Keep in mind that unwashed, dry pea stone and gravel have fine particles that can mess with the ability of site-mixed concrete to set properly. Stone or gravel can absorb any water you add, upsetting the mix balance. 

Meanwhile, a lot of sand contains iron, which may introduce unwanted moisture into the mix. Sand is also affected by temperature. If you leave it baking in the sun before bringing it into a shaded parking garage, it may take 2+ hours to fully cool down. In the event this happens, add cold water to adjust its readiness.

No matter your material, get that mix out of the bucket as soon as possible. Leaving it in the portable mixer will allow it to set more quickly. You’ll end up building frustration, not concrete structures.

Ratio of Materials

Concrete's strength and durability depend on its mix ratio, a guideline of how much of each material (cement, water, sand, gravel) the mix should contain. 

What is the best mixing ratio for concrete? That depends on the grade of concrete you need. One of the most common mix ratios in construction is 1:2:4, meaning:

• One part cement
• Two parts sand
• Four parts gravel

From there, you need just enough water to facilitate mixing without making the product too soupy. The ideal amount depends on not just your concrete mix ratio, but also the job site’s temperature, humidity, and amount of sunlight.

A 4:2:1 mix isn't suitable for all projects – in commercial construction, you may need to put a unique spin on the recipe.  For projects that require extreme compressive strength (like a heavy-duty shop floor), a mix of 5:2:1.5 may be necessary.

Several factors carry weight in the mixing process and your decision of whether to adjust:

Mixing Tool Selection

The best tool for mixing concrete isn’t always the obvious choice. Assuming you’re still up in the air about where to mix the product, let’s talk about those choices.

Ready Mix vs. Portable Mixer

This decision largely depends on the amount of concrete you need to mix and the project’s scale. 

• Mobile mix – mixing is all done in the truck. It’s all metered out
• Ready mix – higher volumes
• Portable mix – that’s done on-site. “Just add water”

Advantages of mobile mix include:

• Quality control: Automated systems ensure precise measurements of materials, leading to consistent and high-quality concrete
• Volume: Can produce small and large batches efficiently
• Reduced waste: Tight control over ratios ensures optimal use of resources (without using the whole bag)

For smaller projects or in situations where accessibility is poor, portable concrete mixers are popular.

Portable mixes typically come in a bag. Advantages of portable mixers include:

• Agility: Provides fresh concrete exactly where and when you need it
• Cost-effectiveness: Doesn’t require a big up-front investment, making it suitable for smaller projects or when demand varies
• Adaptability: Can easily handle sudden changes in project requirements, allowing for different mixes as needed

Choosing a Mixing Paddle

Now it’s time to identify the best mixing paddle for concrete. The right paddle can significantly impact the quality and consistency of the mix, making it a crucial choice for any construction project.

This choice depends on the mix’s requirements (e.g., viscosity and volume), the type of mixer or drill you’ll use, and the ideal mixing motion.

You can use the same paddle to mix different product types, but they won’t be as effective. Potential consequences of choosing the wrong paddle include: 

• Wasted time (in both mixing and the inevitable repairs)
• Poor mixes – may have clumps or air bubbles
• Cost of repairs
• Destroyed buckets

Familiarize yourself with these six styles: 

1. Straight Paddle

A simple, straight design without any twists or complex shapes, this paddle is effective for mixing materials in a straightforward, circular motion. It’s best for small- or medium-volume mixes, like mortar or light concrete. It plays nicely with standard power drills and handheld mixers. 

2. Spiral or Ribbon Paddle

This mixing paddle features a helical shape that moves material from the bottom of the container upward, ensuring a thorough mix without trapping air. This product is best for mixing in medium volumes, as it works well to minimize clumps.

3. Collomix-Type Paddle

The Collomix is a specific brand known for its high-quality paddles. These paddles are strictly for professional use, with distinct shapes available for different material types. These paddles are created with the viscosity and volume of the mix in mind. 

4. Birdcage or Basket Paddle

As you might assume, this paddle’s shape resembles a birdcage or basket. This paddle can mix large volumes efficiently by pulling material into the center and then upward.

5. Whisk-Type Paddle

Similar to a kitchen whisk, this type of paddle is much larger and more robust. Try it for quick, high-shear mixing – in other words, gentle, yet thorough.

6. Dual Paddle Mixer

This is a system of two paddles that rotate in opposite directions to mix material more effectively and quickly. This setup makes it ideal for heavy-duty mixing, offering a faster and more homogenous result. 

Earning Mixing Style Points

To prevent those disastrous outcomes, you should know the three main mixing motions for concrete. The type of material, weight, and moisture content all contribute to choosing the right motion. The decision should prioritize the biggest issues first (minimizing mess if the mixture is dry, breaking up clumps, etc.).

More Tips for On-Site Concrete Protection & Repair

The practice of on-site mixing presents a dynamic blend of opportunities and challenges for commercial subcontractors. 

There are several critical areas where subcontractors can wield their expertise in on-site mixing – namely, strategic admixture use, tweaking material ratios, and picking the right mixing process. By becoming a master chemist of sorts, you’ll empower your ability to influence project quality, efficiency, and economic viability.

To get more tips like these for the entire concrete protection and repair process, visit our blog library: