One of the most common reasons concrete slabs crack, sink, or fail early is poor base preparation. No matter how strong the concrete mix is, if it is poured on the wrong base, problems are almost guaranteed.

 

Why the Base Is Critical for Concrete?

Concrete is strong in compression, but it depends entirely on the support beneath it.
A proper base:

      a. Distributes load evenly.

      b. Prevents settlement and sinking.

      c. Controls moisture movement.

      d. Reduces cracking.

      e. Increases slab life.

Concrete poured on an improper base may look fine initially—but failures usually appear within months.

 

Common Bases Used for Pouring Concrete!

Below are the most common bases used in practice, from weakest to strongest.

1. Natural Soil. (Only If Properly Prepared)

Concrete should not be poured directly on loose soil.

When soil can be used:

     a. Soil is firm and well-compacted.

     b. Organic matter is removed.

     c. Soil is not expansive (like black cotton soil).

Required preparation:

     a. Remove topsoil and vegetation.

     b. Compact the soil thoroughly.

     c. Add a separation layer (sand or granular material).

   Never pour concrete on loose, wet, or organic soil.

 

2. Sand Base. (Limited Use)

Sand is sometimes used as a leveling layer, not as the main structural base.

Suitable for:

      a. Small walkways.

      b. Pavers (not structural slabs).

      c. Light-duty applications.

Limitations:

      a. Poor load-bearing capacity.

      b. Easily shifts if not confined.

      c. Not suitable for driveways or slabs.

Sand alone is not recommended for slabs.

 

3. Gravel or Crushed Stone Base. (Most Recommended)

This is the best and most commonly recommended base for concrete slabs.

Why gravel works well:

     a. Excellent load distribution.

     b. Good drainage.

     c. Reduces moisture-related cracking.

     d. Easy to compact.

Typical specification:

     a. Crushed stone or aggregate (20–40 mm).

     b. Thickness: 75–150 mm (depending on load).

     c. Proper mechanical compaction.

This base is ideal for:

     a. Driveways

     b. House floors

     c. Patios

     d. Footpaths

 

4. PCC (Plain Cement Concrete) Base.

PCC is a lean concrete layer placed below the main slab.

Advantages:

     a. Very stable base.

     b. Uniform support.

     c. Acts as a clean working surface.

     d. Reduces contamination from soil.

Commonly used for:

     a. Structural slabs.

     b. Industrial floors.

     c. Foundations.

     d. Heavy-load areas.

This is a premium base option, but more expensive.

 

5. Existing Concrete Slab. (Overlay Base)

Concrete can be poured over old concrete, but only under conditions.

Existing slab must be:

    a. Structurally sound.

    b. Free from major cracks.

    c. Properly cleaned and roughened.

Additional requirements:

    a. Bonding agent or cement slurry.

    b. Minimum overlay thickness.

    c. Proper curing.

Overlaying is a repair or upgrade method, not a structural fix.

 

Bases You Should NEVER Pour Concrete On -

     a. Grass or vegetation (without removal and proper base preparation).
     b. Loose soil.
     c. Mud or waterlogged ground.
     d. Organic topsoil.
     e. Uncompacted fill.

 

These conditions lead to:

      a. Settlement

      b. Cracking

      c. Delamination

      d. Short slab life

Note: Concrete can be poured where grass previously existed, but only after complete removal of grass and preparation of a stable base.

 

Choosing the Right Base (Quick Guide)

 

Moisture Barrier: An Important Addition!

In many cases, a polythene sheet (vapour barrier) is placed above the base to:

      a. Prevent moisture rising.

      b. Reduce shrinkage cracks.

      c. Improve durability.

This is especially useful for indoor slabs.

 

Common Mistakes People Make!

      a. Pouring concrete directly on grass.

      b. Skipping compaction.

      c. Using sand as the main base.

      d. Ignoring drainage.

      e. Making the base too thin.

These mistakes save time initially but cost much more later.

 

Concrete should always be poured on a stable, compacted, and well-prepared base.
In most practical situations, a compacted gravel base provides the best balance of performance and cost. For heavy or structural applications, PCC is the safest choice.

Remember:
Strong concrete starts from the ground up.