Water is necessary to make concrete.

It is also one of the easiest ways to ruin it.

Concrete consists primarily of cementitious material, water, sand, and coarse aggregate. When water contacts Portland cement, a chemical reaction called hydration begins.

Hydration allows the cement paste to harden and bind the aggregates together.

However, adding more water than the concrete needs can dramatically change its strength and durability.

The Water-Cement Ratio

One of the most important concepts in concrete technology is the water-cement ratio, commonly abbreviated w/c ratio.

It compares the weight of water in the mixture with the weight of cement.

For example:

A mixture containing 300 pounds of water and 600 pounds of cement has a water-cement ratio of:

300 ÷ 600 = 0.50

Reducing the water-cement ratio generally increases strength, provided the concrete can still be properly mixed, placed, consolidated, and cured.

Why Excess Water Weakens Concrete

Fresh concrete needs enough water to be workable.

Unfortunately, contractors sometimes add additional water because wetter concrete is easier to move, rake, screed, and finish.

The problem appears later.

Only part of the mixing water becomes chemically bound during hydration.

Excess water eventually evaporates.

When that water leaves the concrete, microscopic spaces remain behind.

The more excess water that was added, the greater the volume of capillary pores that can remain in the hardened concrete.

These pores make it easier for:

  • Water to penetrate.
  • Chlorides to reach reinforcing steel.
  • Freeze-thaw damage to occur.
  • Cracks to develop.
  • Surface deterioration to occur.

They also reduce compressive strength.

This is why simply adding water to make concrete easier to pour can be expensive in the long term.

Water and Concrete Slump

Adding water normally increases slump.

Higher-slump concrete flows more easily, which can make placement much easier.

But increasing slump by adding water is very different from increasing slump using chemical admixtures.

This distinction is extremely important.

You can have highly workable concrete without necessarily having a high water-cement ratio.

Water Reducers

Modern concrete technology allows producers to increase workability without simply dumping additional water into the truck.

This is accomplished with water-reducing admixtures.

Water reducers modify the behavior of cement particles so the concrete flows more easily using less water.

Depending on the admixture, producers can:

  • Maintain the same slump using less water.
  • Increase slump while maintaining roughly the same water content.
  • Improve strength by lowering the water-cement ratio.
  • Improve pumpability and placement.

Conventional Water Reducers

Traditional water reducers provide a moderate increase in workability.

They can be useful for slabs, foundations, walls, and many general concrete applications.

Because less water can be used while maintaining workability, the resulting concrete can potentially have:

  • Higher compressive strength
  • Lower permeability
  • Better durability
  • Less drying shrinkage

Mid-Range Water Reducers

Mid-range water reducers are commonly used in modern ready-mix concrete.

They can provide substantially improved flow without causing the mixture to behave like extremely fluid self-consolidating concrete.

For flatwork contractors, this can be especially useful.

The concrete can remain relatively easy to place and screed while maintaining a lower water-cement ratio than would otherwise be possible.

High-Range Water Reducers

High-range water reducers are often called superplasticizers.

These admixtures can produce very high-slump concrete without requiring large amounts of additional water.

They are frequently used for:

  • Heavily reinforced structures
  • Pumped concrete
  • High-strength concrete
  • Architectural concrete
  • Self-consolidating mixtures

A concrete mix can look extremely wet while still having a relatively low water-cement ratio if its workability comes from admixtures rather than added water.

That is why slump alone does not tell the entire story.

The Problem With Adding Water at the Jobsite

Adding a small amount of water at the jobsite may sometimes be permitted within the limits of the approved mix design and delivery specifications.

However, uncontrolled water addition can create serious problems.

Excess water can contribute to:

  • Lower compressive strength
  • Increased shrinkage
  • Increased cracking
  • Greater permeability
  • Scaling
  • Dusting
  • Poor freeze-thaw resistance
  • Color variations

The goal should not be to make concrete as dry as possible.

The goal is to achieve the required workability using an appropriate water-cement ratio and properly designed admixtures.

Modern concrete gives contractors much better tools for accomplishing this than simply reaching for the water hose.

Air Voids in Concrete and Their Effects on Cracking

At first glance, air inside concrete sounds like a defect.

Sometimes it is.

Other times, microscopic air bubbles are intentionally placed in concrete to make it considerably more durable.

Understanding the difference between entrapped air and entrained air is essential to understanding concrete durability.

Entrapped Air

Entrapped air consists of relatively large, irregular voids accidentally trapped during concrete placement.

Concrete does not naturally flow into every tiny space.

If it is not properly consolidated, pockets of air can remain around:

  • Reinforcing steel
  • Forms
  • Corners
  • Embedded objects
  • Large aggregate

Large air pockets reduce the effective cross-sectional area of the concrete and can create weak locations.

Visible examples include:

  • Bug holes
  • Honeycombing
  • Rock pockets
  • Voids along forms

Proper placement and consolidation help eliminate excessive entrapped air.

Entrained Air

Entrained air is different.

Air-entraining admixtures intentionally create millions of extremely small, evenly distributed bubbles throughout the concrete.

These microscopic bubbles improve durability in climates where concrete experiences freezing and thawing.

Why Concrete Needs Air in Cold Climates

Concrete contains pores that can absorb water.

When water freezes, it expands.

If water trapped inside concrete freezes without room to expand, tremendous internal pressure can develop.

Repeated freezing and thawing can eventually cause:

  • Scaling
  • Cracking
  • Spalling
  • Surface deterioration

Entrained air provides tiny relief chambers.

When water freezes and expands, some of that pressure can move into the nearby air voids instead of damaging the surrounding cement paste.

Properly air-entrained concrete can therefore survive freeze-thaw exposure dramatically better than non-air-entrained concrete.

Air Content and Concrete Strength

There is a tradeoff.

Increasing air content generally reduces compressive strength when all other factors remain equal.

The microscopic bubbles occupy space that would otherwise contain solid material.

However, that does not mean air entrainment is undesirable.

For exterior concrete exposed to freeze-thaw cycles, slightly lower compressive strength may be a worthwhile trade for dramatically improved durability.

Mix designers compensate by adjusting:

  • Cementitious content
  • Water-cement ratio
  • Aggregate proportions
  • Admixtures

The goal is to obtain both the required strength and the required air system.

Air Voids and Cracking

Air entrainment can indirectly help reduce certain forms of cracking and deterioration caused by freezing.

However, entrained air does not eliminate ordinary drying shrinkage cracks.

Concrete still shrinks as moisture leaves it.

Proper crack control requires several strategies working together:

  • Good subgrade preparation
  • Proper water content
  • Appropriate reinforcement
  • Proper joint spacing
  • Correct joint depth
  • Good curing
  • Appropriate concrete mix design

Finishing Air-Entrained Concrete

Air-entrained exterior concrete requires careful finishing.

Overworking the surface can create problems.

Aggressive steel troweling of air-entrained concrete can contribute to surface delamination because the finishing process may trap water and air beneath a dense surface layer.

For exterior sidewalks, patios, and driveways in freeze-thaw climates, a properly timed float-and-broom finish is generally more appropriate.

Not Every Air Bubble Is the Same

Air in concrete is a perfect example of why concrete construction is more complicated than it appears.

Large accidental voids can weaken concrete.

Millions of properly distributed microscopic air bubbles can protect it.

The difference comes down to the size, spacing, and distribution of those voids.