4 Important Benefits of Gravel Base for Concrete Driveway
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Do You Need Gravel Base For Concrete Driveway?
A strong and stable soil support system consisting of subgrade, sub-base and base is necessary for a concrete driveway to have the required load bearing capacity and durability.
A compacted gravel base for concrete driveway provides strength and facilitates drainage.
The material most suitable for base should have the following characteristics:
- Be Inert & Free of Foreign Matter
- Have High Strength & Durability
- Should lack Cleavage Planes to Avoid Splitting
- Be Tough & Abrasion Resistant
- Have Random Angular Profile & Texture to Facilitate Interlocking
- Be Low in Moisture Content
Angular Gravel ticks all the boxes.
What Happens If You Don’t Use a Gravel Base for Concrete Driveway?
Concrete slabs such as floors, decks or driveways are seldom poured over existing grade. Concrete has high compressive strength and durability. However, a concrete slab requires a strong and stable base too!
If you don’t use gravel base for concrete driveway, you are likely to face one or more of the following problems:
1. Prevents Structural Cracks
Concrete slabs have high compressive strength but relatively poor tensile strength. When a concrete slab rests on a stable, level surface it will not be subject to bending. The tensile stresses are kept low.
But if the surface below the concrete is not level, the slab will tend to bend and twist resulting in tensile stress. As a result, structural cracks can develop, grow and propagate.
TIP: Reinforce the concrete driveway with wire mesh (if 4” thick) or rebar (if 6” or thicker).
2. Prevents Sinking & Heaving
The grade below a concrete driveway varies from place to place. Some grades may have a high clay content while others may have a high sand content. In most locations you can expect a degree of ground movement. Change in soil moisture content, mild seismic activity or growth of tree roots could cause the movement.
A gravel base forms a buffer between the grade and the concrete slab. The transmission of ground movement is dampened somewhat.
Without the buffer of the gravel base for concrete driveway there is a higher probability of the driveway sinking or heaving.
It is good practice to use a geotextile fabric between the subgrade and the gravel base. Check out my earlier post 3 Great Benefits of Geotextile Fabric Under Concrete Driveway.
RECOMMENDED PRODUCT
The Best Geotextile Driveway Fabric for Your Driveway & Patio Base
Geotextile driveway fabric is a supportive layer that is laid between the compacted subgrade and the sub-base aggregates to prevent the migration and loss of the aggregates in to the subgrade. It helps stabilize the ground, permit drainage and prevent weed growth.
Here is the Geotextile Driveway Fabric I recommend.
Happy Buy Driveway Geotextile Fabric
A woven polypropylene fabric that is durable and corrosion-resistant, and can withstand a grab tensile strength of 600 pounds.
3. Prevents Concrete Deterioration
By using a gravel base for concrete driveway a drainage path for rain water is created. The risk of water pooling below the concrete driveway water is reduced.
Water pooling below the concrete slab is not good as the water will rise into the slab due to capillary action. Ingress of water in a concrete slab is detrimental to its structural strength and durability.
Water permeation corrodes both the concrete and the reinforcement. Both strength and durability will be compromised.
TIP: Install a Vapor Retarder Film between the base and the concrete slab. Check out my earlier post Vapor Barrier Under Concrete Driveway: Does It Increase Life?
RECOMMENDED PRODUCT
The Best Vapor Barrier for Under Concrete Slab such as Driveways, Garage Floors & Patios
A concrete vapor barrier is a material that prevents moisture from entering a concrete slab from the grade below. This helps in controlling the concrete curing conditions in the initial stages. Subsequently it prevents the wicking of moisture into the concrete slab from the damp earth.
Here is the Vapor Barrier I recommend.
Farm Plastic Supply-Concrete Vapor Barrier (10 mil)
Heavy duty durable plastic sheeting engineered from low-density polyolefin. Primary application is as a vapor barrier underneath concrete.
4. Enhances Load Bearing Capacity
Without a strong and stable gravel base for concrete driveway, the driveway will not have the required load bearing capacity.
Most residential concrete driveways are 4” thick and reinforced with welded wire mesh. This is considered as having sufficient load bearing capacity for standard passenger cars.
The caveat is that the concrete slab must rest on a strong and stable gravel base.
What Type of Gravel to Put Under Concrete?
The gravel base for concrete driveway must be built up in layers. For maximum load bearing capacity and best drainage the layers should be such that the largest gravel stones are in the bottommost layer. The size of gravel should progressively reduce in the layer above it.
The sub-base and base should use:
# 57 – The gravel stones are around ¾ “ long. This grade is ideal for use in sub-base of concrete driveways.
# 67 – The gravel stones are around ¾ “ and smaller. This grade is great for driveway base and fill.
# 8 – The gravel stones are around ¼ “ to ½ “ in size and are often referred to as pea gravel. This grade is suitable for use in the topmost layer of the sub-base or the bottommost layer of the base.
# 10 – The gravel stones are ⅛ “ or smaller and are also known as screenings. It can be used as the topmost layer of the base, as an alternative to or along with coarse sand.
An important factor to keep in mind is that the gravel profile should be sharp and angular rather than smooth and rounded. This ensures good interlocking between each piece and the result is higher strength and better drainage.
How Much Gravel Do You Put Under a Concrete Slab?
The best sub-base thickness is 4” – 6” made up of 4 layers of angular gravel that progressively decrease in size.
The best base thickness is 2” made up of 1 layer of pea gravel and 1 layer of coarse sand.
Should You Compact Gravel Before Pouring Concrete?
Yes. Each layer of gravel must be compacted before the next layer is put on.
Compacting causes the angular gravel to inter-lock and form a stable and strong base. The individual pieces will not have any lateral movement.
Parts of the gravel may disintegrate during the compacting process, but this is not a bad thing. The smaller pieces get lodged in the voids and make the inter-locking even more rigid!
Thank you very much for reading this post. I do hope you found it informative and helpful.
