Why Aggregate Size Matters in Concrete Mix Design?

Concrete is vital for modern construction, serving as the primary structural element in buildings, bridges, highways, and other infrastructure.

Although cement, water, and admixtures are often put at the forefront of concrete discussions, aggregate is an essential component that impacts performance and can often be overlooked. The size of the aggregates in a concrete mix has a great impact on the strength, durability, workability, and performance of concrete. 

At Sakshi Chem Sciences, we believe that understanding the importance of making the best aggregate size selection will be the best driver of performance in construction projects.

The size and shape of the aggregates have an impact on demand for cement and water, and other conditions such as shrinkage, cracking, and ultimately durability. 

If you are designing conventional concrete or designing high-strength concrete, aggregate size is an important factor to improve your mix design.

In this blog, we discuss the reasons why aggregate size and overall construction material are vital to your mix design and the science behind it, but also practical reasons to help both enhance performance and save money.

Take Away Summary

• Aggregate size impacts cement and water requirements; smaller aggregate requires more cement paste due to the overall surface area to be covered.
• The aggregate size, where compressive strength is concerned, is generally in the range of 25-38 mm
• Larger aggregates also improve workability, reduce void space, and promote durability.
• Aggregates with good gradation fill voids well, ultimately reducing the needed cement paste and lowering the cost.  
• The size (and gradation) of aggregate affects the amount of shrinkage, cracking, permeability, and other key attributes of concrete that influence its long-term performance.  
• Proper aggregate selection is an important component of developing high-strength concrete and a model for sustainable construction practices.  

Relationship of Aggregate Size to Cement and Water Demand

Aggregate Size and Surface Area

Aggregate size has a direct influence on cement and water demand in a concrete mixture. Smaller aggregates have a larger surface area and require more cement paste to adequately coat each particle.

This not only affects the cost of material but also increases the risk of shrinkage and cracking without proper management.

The surface area of a typical concrete mixture with aggregates less than 10 mm in size can increase the total surface area to be coated, potentially increasing cement demand by 5–10%.

But, larger aggregates would reduce the total surface area, lower cement paste requirements, improve efficiency, and lower the chance of cracking.

Well-graded Aggregates: Enhancing void filling

Well-graded aggregates are intended to combine a range of particle sizes that fit effectively with each other and fill voids. This helps reduce water and cement demand while maintaining workability. Proper aggregate size and gradation will yield a dense, cohesive mixture that is easy to handle.

 For projects involving high-strength concrete, well-graded aggregates help achieve maximum compressive strength and durability.

Example Table: Effect of Aggregate Size on Cement and Water Requirement

Relationship Between Aggregate Size and Concrete Strength

Compressive Strength Considerations:

Factors affecting concrete strength and durability – Aggregates:

Concrete strength is significantly affected by the size of the aggregate. Since larger aggregates require less water for similar workability to smaller aggregates, larger aggregates frequently provide increased strength.

Research and field experience suggest that optimum compressive strength may be obtained when using aggregates with a maximum nominal size greater than 25 mm (1 inch) but less than 38 mm (1.5 inches), depending on considerations for workability and structural resistance.

While smaller aggregates can improve the surface finish of concrete, smaller aggregates often require more cement and possibly more water, which may diminish the long-term strength of concrete due to increased shrinkage and cracking.

The best performance of high-strength concrete requires a suitable selection of aggregate size, with consideration of attainable short-term strength and the ability of the concrete to withstand environmental stresses over the long term.

Shrinkage and Micro-Cracking:

Because larger aggregate sizes provide a benefit of restricting drying shrinkage and micro-cracking, larger aggregates are beneficial in reducing drying shrinkage. 

Larger aggregate sizes can also provide improved performance with respect to minimizing drying shrinkage cracking through the distribution of stress.

More specifically, aggregate sizes can be used to restrict the development of micro-cracks, and stress distribution can better resist the development of larger cracks that may develop over time.

Case Study Example:

In a highway bridge project in India using high-strength concrete with a maximum nominal aggregate size of 32 mm (1.25 inches), concrete mixes were shown to provide 15 % less drying shrinkage cracking as compared to high-strength concrete mixtures that used 12 mm (0.5 inches) aggregate size.

The use of larger aggregates also helped to improve long-term durability and minimize repair costs

Impact of Aggregate Size on Workability and Consolidation

Enhancing Flow and Placement

The size of the aggregate influences the workability and consolidation phase of concrete, which is a critical part of successfully providing a concrete pour. Larger size aggregates decrease the surface area needed to be covered by paste, allowing it to flow better and require less effort to place.

Aggregates that are smooth and rounded will help flow even further by using the ball-bearing properties of the aggregate.

On the other hand, smaller-sized aggregates are better for surface finish, but they may clump together and force the person handling the pour to pay more frequent attention to getting a uniform pour.

Selecting the correct aggregate size will help ease placement while maintaining structural integrity and surface finish when the pour size is large, like for high-strength concrete.

Reinforcement Spacing in Aggregate Size Selection

Concrete with dense reinforcement needs to consider the selection of aggregate size more closely. The American Concrete Institute (ACI) states that the maximum size aggregate should never exceed three-quarters of the minimum clear distance between reinforcement bars.

Using an aggregate that is too large may impede the flow, create segregation, and affect consolidation of the concrete around the reinforcement. A smaller aggregate will serve to place the concrete in tighter spacing to be able to fully encapsulate the rebar.

Risks of Segregation or Separation of Coarse Aggregate

Segregation, or the separation of the coarse aggregate from the paste, takes place during the handling or pouring process in terms of the quality and integrity of the concrete itself.

The size and grading of coarse aggregates can lessen the separation or segregation of paste with the coarse aggregate.

Ways to minimize segregation:

• Use well-graded aggregates to optimize the space between the aggregate particles.
• Maintain proper mortar-to-aggregate ratios.
• Use limits to drop heights when placing.
• Modify vibration times to achieve proper compaction without excessive compaction.
• Consider environmental conditions such as temperature and humidity during the placement phase.

Selecting proper aggregates can limit the pace between aggregates and assist in reducing honeycombing from the past, which greatly improves texture, strength, and durability.

Durability Considerations Related to Aggregate Size

Permeability and Shrinkage

The size and shape of aggregate can influence aggregate permeability, which increases durability. Aggregate size can reduce voids, limiting water penetration and improving resistance against moisture.

The smaller size and shape of aggregates can increase the volume of paste and thereby increase shrinkage, which decreases durability.

A good balance of optimal aggregate shape and size can improve resistance to freezing and thawing, provide better abrasion resistance, and increase its overall longevity.

Example Table: Permeability and Aggregate Size

Freeze-Thaw Resistance

Concrete subjected to varying temperatures experiences freeze-thaw cycles, which can compromise the integrity of the concrete. Large aggregates can influence the overall air void distribution and pressure relief during these freeze-thaw cycles.

Proper grading and air-entraining agents, and adjusted aggregate size can be greatly beneficial in improving freeze-thaw resistance and extending the lifespan of the concrete.

Financial Implications of Aggregate Selection

Material savings

• Choosing the appropriate aggregate size can have significant impacts on material costs.
• Less cement is needed due to a lower surface area requirement.
• Less labor because better workability can provide savings.
• Less rework due to the certainty of placement and consolidation.
• A better use of recycled aggregates may satisfy the need for better pricing while being environmentally sustainable.

Example: using recycled coarse aggregates can provide a cost reduction in material as much as 20 percent and an economical and sustainable mix design.

Processing and Transportation

Bigger aggregates may require specialized processing and handling equipment, leading to higher processing costs as a result. Smaller aggregates may warrant similar costs, although the intensive crushing and washing may typically cause the operational costs to increase.

Incorporated savings may be realized depending on the appropriate aggregate size selected; understanding materials, processing, and transportation will enhance the overall efficiency of your project.

Volume and Weight Ratios in a Mix Design

The proper ratio of aggregates is always front and center for the appropriate performance of concrete. To achieve these volume-to-weight ratios, taking into consideration gravities and actual moisture contents will help to provide the needed consistency in the concrete mix.

A proper dosage of coarse and fine aggregates can lend itself to optimum packing density, strength, and workability.

Important items to consider:

• Adjust the mix design for moisture in aggregate.
• Ensure proper verification of the specific gravities of aggregates to ensure proper proportioning.
• Test batch mixes.
• Maintain control over batching to account for spending to minimize costly errors.
• Evaluate uniformity overall with larger projects.
• Effects of Aggregate Size on Shrinkage and Cracking.

Shrinkage and cracking are concerns of concrete. Larger aggregates reduce paste volume and drying shrinkage through actions of micro-reinforcement with the matrix. Smaller aggregates increase paste volume, which increases the likelihood of shrinkage and cracking.

 Properly optimized aggregate size is essential for durable and crack-resistant concrete.

Optimizing Aggregate Size for Performance and Economy

Optimizing aggregate size contributes to improved performance, longevity, and cost-effectiveness:

• Increased strength and load capacity.
• Increased workability during more complex pours.
• Less cement and other materials are used.
• Providing consistent quality across the project.
• Supporting sustainability in construction.

The selection of aggregate size must be made carefully to meet strength needs while minimizing shrinkage and cracking with high-strength concrete types.

Best Practices for Choosing Aggregate Size during Mix Design

Specifying the aggregate size is one of the most crucial decisions in concrete mix design because it affects workability, strength, durability, and even cost.

Assessing structural performance in relation to environmental conditions, as well as practicalities for the site, are important considerations in making the appropriate decision. Some of the following guidance illustrates some best practices to consider.

• Know your project-specific requirements: Every project is different. For instance, a foundation for a high-rise building will require a different aggregate than a pavement slab or decorative concrete finish. Keep in mind that you will want to start with the structural requirements, load-bearing considerations, and environmental considerations (e.g., freeze-thaw cycles; chemical exposure).
• Be cognizant of the maximum aggregate size: Reduced aggregate is often confined to the maximum size by the formwork limitation or the spacing of rebar. A general rule of thumb to use is that an aggregate size should not be greater than 1/5 of the narrowest dimension of the formwork or 3/4 of the clear distance of the spacing of the reinforcement. An aggregate that is too large can result in honeycomb or inadequate consolidation. 
• Utilize multiple sizes of aggregates: Using one size can result in voids or excessive demand for paste. By working with a combination of coarse and fine aggregates, you achieve improved packing density and improved flow of the concrete. This results in less demand for cement and water, increased strength, and liquid retention while reducing shrinkage potential. 
• Be mindful of shape and texture: Round aggregates, like natural river gravel, are smoother and promote better workability, which will improve placement and consolidation. Crushed stone, being more angular, promotes better mechanical bond but will degrade workability if not balanced in the mix. 
• Be aware of uniformity and consistency: Consistency in aggregate grading, based on the gradation criteria, promotes uniformity and enables expected mixture behavior. Uniform aggregates result in even development of strength, diminish segregation, and improve long-term durability of the structural element.

By paying attention to these details, you can design a mix that not only meets strength requirements but is also workable, economical, and durable in the long run.

Summary

Sakshi Chem Sciences knows that the aggregate size in concrete mix design is important. It affects the cement and water requirements as well as workability, strength, durability, shrinkage, and cost.

Selecting the right size aggregate is an important aspect of conventional and high-strength concrete projects. If the aggregate is selected and sized properly, you can provide economical, durable, and long-lasting high-performance concrete.

Keep in mind, the right aggregate is not just a part of the mixture, but the primary part of your construction project.

FAQ

Sagar Telrandhe

Sagar Telrandhe is a Construction Engineer with a B.Tech in Construction Engineering & Management. Passionate about infrastructure development, project planning, and sustainable construction, he specializes in modern construction techniques, project execution, and quality management, contributing to efficient and innovative building.