Do I need gravel for concrete slabs?

This article aims to answer the question “Do I need gravel for concrete slabs?“. It will also discuss the importance of adding sand to the cement mixtures. 

Do I need gravel for concrete slabs?

Yes, under a concrete slab, there is a requirement for gravel. As a result of its compactability, gravel serves as a stable base for your concrete. Water will no longer pool under the concrete as a result of this.

Some may claim that clay soils are equally as excellent as gravel when it comes to providing a foundation, however this argument ignores the fact that clay soils do not offer appropriate drainage. As a result of this, the slab begins to sink and fracture due to the accumulation of moisture and erosion.

We’ll go over all the benefits of putting gravel under your concrete slab in this post. We’ll also quickly cover the process of building a concrete slab and several situations in which pouring concrete directly onto the ground is suitable.

Why do I need gravel under concrete?

Continue reading the article to understand why you need gravel under concrete:

For a concrete slab, crushed stone or gravel offers a sturdy base. Your slab will be undermined and cracked because even the most solid, tightly compacted earth may move considerably with the weather.

You should be aware that not all gravel is created equal. Crushed gravel is available in a wide range of sizes, each suited to a particular use. When I refer to gravel in this essay, I mean crushed rock with a diameter ranging from 34” to 1” approximately.

It is possible to compress gravel. A level surface may be achieved by using this method. The amount of concrete you’ll require may be calculated with precision on a flat surface. However, drainage is the most important advantage. 

It is possible for water to infiltrate through the slab and create holes in between the ground and the slab. Your concrete slab will become unstable as a result of these voids. Compacted gravel prevents water from accumulating and allowing it to drain into the ground.

Gravel also serves as a moisture barrier, making it an excellent choice for landscaping. The path of least resistance is frequently the most direct route taken by water, therefore water that collects under a slab may eventually seep up through the slab’s microscopic capillaries and onto the slab’s surface.

The huge crevices between the pebbles act as a capillary barrier, preventing water from travelling “up.” Due to water wicking up from dirt into the concrete, slabs that are placed directly on soil typically have moist areas on top.

What is the purpose of gravel under concrete?

Concrete gravel may be used for a wide range of applications. Additionally, it aggregates to enhance the concrete’s volume, serving as both a filler and an aggregate. Because of the greater volume, the product contains less air. The size of the gravel also influences the strength of the concrete.

Although mixing is more difficult with larger gravel particles, the final concrete is more long-lasting. The quantity of water to be used also affects the amount of sand and gravel to be used. Adding more water weakens the combo.

The finished combination requires less water since aggregate is added to it. Concrete with a lower water content lasts longer and is less likely to shrink or break. Smaller gravel particles are found in “fine” concrete.

Fine aggregate is used in concrete slabs and other flat surfaces. The term “coarse aggregate” refers to larger gravel bits. For more durable structures, coarse concrete is used. However, coarse-grained concrete is more durable than fine-grained concrete.

Most home improvement stores carry pre-mixed concrete. You just need to add water to finish the recipe. If you’re doing a minor job around the house, pre-mixed concrete in a smaller bag is a great option.

Pre-mixed concrete may be used to strengthen fence posts and railings.

What type of gravel to put under concrete?

 Washed and screened gravel, ranging in size from 34” to 1”, is the most suitable choice. It’s known by a variety of names in North America, including “57” and “driveway gravel,” so we’ll stick to the particular size instead of a nickname for accuracy.

It may be difficult to crush gravel bigger than 1″ and may be incompatible with certain concrete. As a result, levelling bigger pebbles might be difficult, making it difficult to estimate the quantity of concrete required. 

Rocks larger than a few centimetres in diameter might make it difficult to mount firm foam on top of the gravel. 3/8″ to 1″ of rock is still rough enough to utilise as a base for a concrete slab foundation. 

Many people are troubled by the sharp edges of the pieces and question whether they should be used. When compressed, the jags and sharp edges work as anchors to keep the pieces together. When levelled and compacted, the irregular form of these rocks works as a “glue.”

It’s critical to use gravel that has been cleaned and screened. We don’t want our gravel to act as a “wick” for water, as previously stated. When water seeps up through the gravel and onto the slab, it is said to be wicking. It’s a risk to buy gravel that hasn’t been cleaned, since water might seep up through the pebbles and cause problems.

How much gravel should I put under concrete slab?

Under a 4-inch-thick concrete slab, you’ll need 3 inches of gravel to keep it stable. A 4″ slab requires a minimum of three inches of gravel, but more is preferable. 34″ washed and screened gravel should be used, and then compacted to level the area.

With a plate compactor or similar equipment, you’ll have to compress every 5 inches or so if you want to add additional gravel.

Using stiff foam boards under a concrete slab does not eliminate the need for gravel, since the foam’s thickness does not replace it. As a result, a 4″ thick slab will need a minimum of 3″ of gravel under 2″ of stiff foam.

How to compact gravel under concrete slab?

It is recommended to utilise a machine when compacting gravel for the foundation of a concrete slab. Gravel may be compacted in a quarter of the time using a plate compactor or jumping jack. More than 5″ of gravel will need more than one pass through the compactor. 

In order to achieve uniform compaction over the crushed rock bed, it is advised that you compress every 5″. Compactors may be rented almost everywhere that rents equipment. Big-box retailers that provide hourly rentals are likely to have one or more available. 

However, even though they are cumbersome, they may easily fit into the back of a family car or truck. If you have a large slab, it may take all day, but the time you’ll save compared to hand tamping makes it well worth the money. 

In order to obtain a level of compaction even somewhat comparable to that of machine compactors, manually compacting takes significantly longer since you must tamp every inch or so.

What is gravel?

Gravel is made from rocks, crushed stone is mined. Gravel, by definition, has a rounded shape because of its water-transported content.

The rounded edges of crushed stone provide visual and tactile contrasts with gravel. The angular and jagged edges of crushed stone may be noticed during the crushing process.

Unlike sand, the natural weathering and wear of exposure to running water has left gravel with an exceptionally smooth texture and surface. Crushed stone, on the other hand, is more often sold and used in its natural state. When gravel is crushed, it loses its rounded, smooth feel.

Depending on the size range employed, even crushed stone might be referred to as “gravel” by contractors. Stones with diameters between a few millimetres and two inches are known as “gravel,” even if they have been crushed and are no longer smooth.

Our definition of gravel will be that it cannot be used to refer to crushed stone and must stay in its original state in order to avoid confusion. The three most common types of rock and stone found in nature are igneous, sedimentary, and metamorphic.

Conclusion 

It takes a lot of effort and time to pour a slab. But if you want anything to survive for a long time, you have to put in a lot of effort. In other words, if you are going to spend the time and money to pour a slab, you may as well go all out. Add stiff foam and a vapour barrier for extra protection while you are at it.

Your compacted rock is already protected from moving and breaking by the addition of a layer of foam. Additionally, a well-built slab may be used for many years to come. There may come a time in the future when you change your mind and decide to heat the area.

Frequently asked questions (FAQS): Why put gravel under concrete?

Do I need gravel for concrete slabs?

Yes, under a concrete slab, there is a requirement for gravel. As a result of its compactability, gravel serves as a stable base for your concrete. Water will no longer pool under the concrete as a result of this.

Some may claim that clay soils are equally as excellent as gravel when it comes to providing a foundation, however this argument ignores the fact that clay soils do not offer appropriate drainage. As a result of this, the slab begins to sink and fracture due to the accumulation of moisture and erosion.

How much gravel should I put under concrete slab?

Under a 4-inch-thick concrete slab, you’ll need 3 inches of gravel to keep it stable. A 4″ slab requires a minimum of three inches of gravel, but more is preferable. 34″ washed and screened gravel should be used, and then compacted to level the area.

With a plate compactor or similar equipment, you’ll have to compress every 5 inches or so if you want to add additional gravel.

Using stiff foam boards under a concrete slab does not eliminate the need for gravel, since the foam’s thickness does not replace it. As a result, a 4″ thick slab will need a minimum of 3″ of gravel under 2″ of stiff foam.

How to compact gravel under concrete slab?

It is recommended to utilise a machine when compacting gravel for the foundation of a concrete slab. Gravel may be compacted in a quarter of the time using a plate compactor or jumping jack. More than 5″ of gravel will need more than one pass through the compactor. 

In order to achieve uniform compaction over the crushed rock bed, it is advised that you compress every 5″. Compactors may be rented almost everywhere that rents equipment. Big-box retailers that provide hourly rentals are likely to have one or more available. 

Bibliography 

DARIUS NEGAHBANI. How to mix cement to make Mortar or Concrete. Marshalls: Gardens and driveways. Retrieved from: https://www.marshalls.co.uk/gardens-and-driveways/blog/how-to-mix-cement-to-make-mortar-or-concrete#:~:text=In%20terms%20of%20the%20ratio,parts%20aggregates%20can%20be%20used

Cement to sand ratio for mortar, brickwork and plastering. CivilSir. Retrieved from: https://civilsir.com/cement-to-sand-ratio/

What was missing from this post which could have made it better?

Leave a Comment