Concrete Tip of the Day #1: What Is Concrete?

Cement and Concrete — Not the Same Thing

A common misconception is that cement and concrete are interchangeable terms. In fact, they’re quite different. Cement is a dry, powdered substance that reacts chemically with water to form a hard, solid material. Concrete, however, is a composite mix consisting of cement, water, and aggregates of various sizes. The cement-water mixture forms a paste that binds the aggregates together as it hardens.

In its fresh state, concrete is plastic and mouldable, allowing it to be shaped as required. It begins to stiffen gradually but stays workable for several hours—enough time for placement and finishing. After setting, it continues to gain strength for months, and sometimes years, if moisture is maintained.

The Components of Concrete

Concrete is made up of two main parts: paste and aggregates.

• Paste includes cement, water, and sometimes air that’s intentionally entrained.

• Aggregates come in two categories:

  • Fine aggregates (sand) are particles smaller than 5mm, sourced from rivers, beaches, or manufactured by crushing rock.

  • Coarse aggregates are larger than 5mm and include gravel and crushed stone.

Occasionally, industrial by-products such as fly ash or silica fume are added to the paste to enhance performance. These materials also contribute to sustainability and durability.

How Concrete Hardens

The strength-giving process in concrete is called hydration—a chemical reaction between water and cement. Contrary to the common belief that concrete "dries out" to harden, it actually needs continuous moisture to develop strength. Curing is the process of maintaining favourable moisture and temperature conditions to allow hydration to continue over time. Proper curing leads to stronger, longer-lasting concrete.

Understanding Concrete Strength

The primary measure of concrete’s strength is its compressive strength, or how much pressure it can withstand before crushing. In flat surfaces like pavements, flexural strength—resistance to bending—is often more relevant.

Two key factors influence strength:

1. The water-to-cement ratio (lower is generally better for strength), and

2. The degree of hydration (which depends on curing conditions).

Adding too much water may make concrete easier to place, but it weakens the final product even if all other steps are done correctly.

Fresh Concrete Properties: Workability and Bleeding

Freshly mixed concrete has short-lived but critical characteristics. One of the most important is workability, which refers to how easily it can be placed and compacted without segregation or excessive bleeding.

• Slump (typically 100–120mm for general use) is used to measure consistency.

• Bleeding is when water rises to the surface of the mix. Excessive bleeding can increase the water-cement ratio at the surface, leading to weak top layers, especially if finishing is done too early.

Ordering the Right Concrete

Concrete is sold by volume, typically in cubic metres of fresh mix. Orders usually specify:

• A target strength (often based on 28-day test cylinders),

• A particular cement content or water-cement ratio,

• Slump (consistency),

• Air content.

Some projects require specialised mixes. It’s your responsibility to ensure that the right mix is ordered in time and matches the job’s specification.

Admixtures: Enhancing Concrete Performance

Modern concrete often includes admixtures—chemical additives used to modify its behaviour. These may:

• Accelerate or slow down setting time,

• Increase strength or density,

• Improve durability and workability.

Common admixtures include:

• Air-entraining agents: introduce tiny air bubbles to enhance durability and freeze-thaw resistance.

• Water reducers: allow for a workable mix with less water, improving strength.

• Superplasticisers: dramatically increase workability (slump) for up to two hours, useful for flowing concrete or mixes with low shrinkage requirements.

Stay tuned for the next episode of Concrete Tip of the Day!