The Right Stone for Your Concrete Base: A Comprehensive Guide

The Right Stone for Your Concrete Base: A Comprehensive Guide

When pouring concrete, it is essential to have the right base material to ensure a durable and long-lasting surface. This base is typically made up of crushed stone, gravel, or crushed rock, each serving distinct purposes in providing a stable foundation. This guide will explore the most common types of base materials used under concrete and how to properly prepare them for your project.

Common Base Materials for Concrete

When pouring concrete, a layer of crushed stone, gravel, or crushed rock is typically used as a base to provide stability and promote proper drainage. Here are the most common types used:

Crushed Stone

Crushed stone is often made from limestone, granite, or other rocks. It provides a solid foundation and helps with drainage. The typical size range for crushed stone is 3/4 inch to 1 inch. It is a popular choice for providing a stable base under concrete.

Gravel

Gravel is similar to crushed stone but is usually rounded and less angular. It also helps with drainage and compaction. A good quality of gravel will have a consistent size, making it easy to work with and ensuring a smooth surface upon compaction.

Crushed Rock

Crushed rock can be a mix of different types of rock, often used for its stability. It is suitable for providing a strong and durable base for concrete pours. Crushed rock is often used in areas with varying soil types or where a more stable base is required.

Granular Fill

Granular fill is sometimes a mixture of sand and gravel, creating a stable base. It is particularly useful in situations where a more flexible base is required. Granular fill is compacted to ensure a level and stable surface for pouring concrete.

Compaction and Preparation

A properly compacted layer of these materials under concrete helps prevent cracking and shifting, ensuring a durable surface. Here are some tips on how to prepare and compact the base materials:

Pea Stone

Pea stone can work as long as it is compacted, although it is usually not the primary choice for a concrete base. If using pea stone, ensure it is compacted well to avoid settling and movement. An alternative that is easier to work with and more commonly used is 3/4-inch crushed gravel. Crushed gravel does not contain any rocks larger than 3/4 of an inch and can be easily raked out. As with all base materials, it is important to compact this layer as well.

It is also important to note that recycled concrete should not be used under any concrete floor installations. This ensures a clean and reliable base for the new concrete structure.

What Do You Use Under Concrete?

The best answer for what you use under concrete depends on the substrate and its properties. In many cases, it is advisable to break out a small section of the concrete and dig a trial pit to investigate the material and properties of the substrate. An even better method is to inspect the construction as-built drawings for your concrete structure. The foundation notes should be recorded in these drawings, which can help avoid intrusive investigations.

If there was a ground investigation conducted before the concrete was laid, the answer to your question can be found in the interpretive report from that study. In my area (Northwest United States), the usual product is 5/8 minus, also called road mix. This mix contains crushed rock no larger than 5/8 inches, along with a significant amount of sand. It compacts well with a plate tamper and a 4 to 5-inch layer of it makes an excellent base for pouring concrete.

Conclusion

Choosing the right stone for your concrete base is crucial to the longevity and strength of your project. Whether you opt for crushed stone, gravel, crushed rock, or granular fill, proper preparation and compaction are essential. By following best practices and consulting construction documents if available, you can ensure a solid foundation for your concrete structure.