Concrete mix design — why the water-cement ratio decides almost everything

Concrete is four ingredients and one decision. The ingredients are cement, water, aggregate and whatever is added to modify them. The decision is how much water, and almost everything that matters follows from it.

The water-cement ratio

Cement needs water to hydrate — roughly a quarter of its own weight, chemically. Concrete at that ratio would be unplaceable, so more water is added to make it flow.

That surplus water does not vanish. It ends up as capillary pores running through the hardened concrete, and the more of it there was, the more connected that pore network is.

Why durability usually wins

There are two separate ways to arrive at a mix.

Structurally, from the load: this element needs a characteristic strength of so many N/mm².

By exposure, from the environment: this element sits in saline ground, so it needs at most this water-cement ratio and at least this cement content.

In a Gulf plant the second nearly always produces the more demanding mix.

Cement, and what sulphate attacks

Type	What it is	Chosen for	The trade-off
Type I / OPC	Ordinary Portland cement	General construction where the ground is not aggressive	No sulphate resistance; high heat of hydration in thick pours
Type II	Moderate sulphate resistance, moderate heat	Mildly aggressive ground; large pours where heat matters	Slightly slower early strength than Type I
Type III	Rapid hardening — finer ground, same chemistry	Early formwork striking, precast, cold weather	More heat, faster, and less forgiving in hot weather
Type V	Sulphate resisting — tricalcium aluminate deliberately limited	Sulphate-bearing ground, sabkha, saline groundwater	Slower strength gain; does NOT help against chloride
Fly ash (PFA)	Pozzolan from coal combustion, replacing part of the cement	Lower heat, denser and less permeable concrete over time, better chloride resistance	Slower early strength; needs longer curing to deliver the benefit
GGBS	Ground granulated blast furnace slag	Much lower heat, strong sulphate and chloride resistance, high replacement levels possible	Slow early strength; pale colour; curing discipline is essential
Silica fume	Very fine pozzolan from silicon manufacture	Very high strength and very low permeability	Sticky mix, hard to place and finish, and it demands excellent curing

The single decision most often driven by the ground investigation is which of these to use. Sulphate in the ground attacks a cement high in tricalcium aluminate, so a sulphate-resisting cement is one with that compound deliberately limited — which is also why it gains strength more slowly.

Portland cement is several compounds, and one of them matters more than the rest when the ground is aggressive: tricalcium aluminate.

Sulphate in groundwater reacts with it in the hardened paste to form products that occupy more space than what they replaced. The expansion happens inside the concrete and breaks it up from within.

Sulphate-resisting cement — Type V — is simply cement with that compound limited. Less of it to attack, less damage. It gains strength more slowly, which is the price.

Pozzolans: better and slower

Fly ash, ground granulated slag and silica fume are pozzolans. They are not cement, and on their own they do nothing. They react with the calcium hydroxide that cement hydration produces, forming more of the same binding compounds.

Three consequences follow, and they are a package:

1. Lower heat. The reaction is slower and spread out. In a thick pour, that is the difference between a sound element and one that cracks from thermal gradient.
2. Denser, less permeable concrete — eventually. The additional reaction fills pore space that would otherwise stay open. This is the durability benefit, and it is substantial.
3. Lower early strength. Slower reaction, slower strength gain.

Aggregate

Aggregate is most of the volume, and it does more than fill space.

Grading decides how much paste is needed. A well-graded aggregate, with a spread of sizes, packs efficiently and needs less paste — and less paste means less water and less shrinkage. A uniform, single-sized aggregate leaves voids the paste has to fill.

Maximum size is limited by the section and by the reinforcement. Aggregate must pass between bars and through cover, or it arches across and leaves a void.

Cleanliness. Dust and clay on the surface stop paste bonding to the stone. Salt on aggregate from a coastal source brings chloride in with the mix, which is far worse than chloride arriving slowly from outside.

Shrinkage

Concrete shrinks as it dries, and if it is restrained from shrinking it cracks. Nothing eliminates that; the mix can only reduce it.

The drivers are paste content and water content — so a well-graded aggregate and a low water-cement ratio both help, and a rich, wet mix is the worst case. This is why an over-sanded mix, which needs more paste, shrinks more than one that is properly graded.

Carbonation, and why cover is a durability dimension

Fresh concrete is strongly alkaline, around pH 13, and that alkalinity maintains a passive oxide film on the reinforcement that stops it corroding.

Carbon dioxide from the air reacts with the concrete and lowers that pH. Where the carbonation front reaches the steel, the film is lost — the steel is depassivated — and corrosion can begin.

The concrete itself is barely affected. This is a reinforcement problem, and the defences are the two things that decide how long the front takes to arrive: low permeability, and enough cover.

The mix submission

Before any concrete is placed, the mix is submitted and approved: the constituents, the water-cement ratio, the cement content, the admixtures, and trial results demonstrating it achieves the required strength and workability.

It is a hold point for good reason. Once the pour has happened, nothing about the mix can be inspected — only its consequences, years later.

What to take away

• The water-cement ratio decides strength and, more importantly, permeability. Adding water on site raises it and the damage cannot be undone.
• Use a plasticiser for workability, never a hose.
• Durability usually sets the mix, not strength — so a passing cube proves less than it appears to.
• Sulphate attacks the concrete and is resisted by limiting tricalcium aluminate (Type V). Chloride attacks the steel and is resisted by low permeability and cover. Sabkha brings both.
• Pozzolans give lower heat and better long-term durability, at the cost of early strength — and only if curing is maintained.
• Well-graded aggregate needs less paste, which means less water and less shrinkage.
• Carbonation depassivates the steel by lowering pH. Cover and permeability are the defence.
• The mix submission is a hold point, because afterwards only the consequences are visible.