Concrete is one of the most widely used building materials in the world. It is strong, durable, and relatively inexpensive. However, concrete has one Achilles heel – it needs air to reach its full strength potential. Without air entrainment, concrete can deteriorate rapidly and fail prematurely. In this article, we will explore what happens when concrete lacks adequate air, the vital role air plays in concrete, and how to properly air entrain concrete mixtures.
What is Air Entrainment?
Air entrainment refers to the intentional creation of tiny air bubbles in concrete. This is accomplished by adding an air entraining agent, usually a soap-like surfactant, during mixing. The surfactant reduces the surface tension of the concrete mixture, allowing millions of microscopic air voids to form and disperse evenly throughout the concrete as it hardens.
These air voids serve several important purposes:
- They increase concrete’s resistance to cycles of freezing and thawing.
- They improve the workability and cohesion of fresh concrete.
- They provide relief spaces for water vapor to expand into.
- They reduce segregation and bleeding.
- They increase durability.
Well-aerated concrete contains billions of non-interconnected air cells that account for 3% to 8% of the total concrete volume. This protects the concrete at a microscopic level.
What Happens Without Adequate Air Entrainment
Insufficient air entrainment leaves concrete vulnerable to damage in several ways:
Freeze-thaw deterioration
One of the most dangerous threats to concrete is repeated cycles of freezing and thawing. Water inside the concrete expands by about 9% when it freezes. This produces internal hydraulic pressure that can cause cracks, scaling, and crumbling if the concrete lacks air voids to absorb these expansive forces.
Without enough entrained air, water has no room to expand as it freezes. This results in severe internal damage to the concrete’s pore structure each winter. Over time, pieces of concrete will peel off and expose the reinforcement bars underneath, compromising strength and integrity.
Plastic shrinkage cracking
All concrete shrinks slightly as it changes from a plastic state to a solid state during curing. Concrete properly air entrained has bubbles that act as flexible ball bearings between the shrinking masses of cement paste. This allows the concrete to shrink without developing stress cracks.
Non-air-entrained concrete has no such relief zones. If plastic shrinkage occurs too quickly, it will crack either on the surface or internally. Surface cracks are unsightly and allow in water, salts, and debris. Internal cracks compromise strength.
Reduced strength
Air entrainment improves concrete’s resistance to fracture at early ages by providing discontinuous voids that halt microcrack propagation. This increases overall tensile strength.
Without air entrainment, concrete is more brittle and prone to cracking from shrinkage. Cracks act as stress risers within the concrete, reducing compressive strength.
Increased permeability
Air entrainment purposely increases the volume of non-interconnected voids within concrete. This leaves less space available for connected capillary pores that allow water penetration.
Non-air-entrained concrete will have higher permeability, increasing the risk of:
- Freeze-thaw damage
- Reinforcing steel corrosion
- Staining
- Efflorescence
- Scaling
Increased segregation
Freshly mixed concrete has a tendency to segregate, or separate its ingredients. Heavier aggregates can sink towards the bottom, while water and cement paste rise towards the top.
Air entrainment acts like a stabilizer, improving the consistency and cohesion of fresh concrete. This reduces segregation and bleeding, ensuring the concrete cures evenly.
In non-air-entrained concrete, uneven segregation while curing leads to areas of poor quality concrete within a structure.
Examples of Concrete Damage Without Air Entrainment
There are many real-world examples of concrete deterioration caused by inadequate air entrainment:
Tilghman Island Bridge
This bridge in Maryland was built in 1938 without air entrainment due to lack of knowledge at the time. The bridge deck slabs experienced severe scaling and deterioration due to seawater exposure and repeated freezing. Large chunks of concrete fell from the underside of the bridge, requiring expensive repairs.
Toronto Parking Garages
Hundreds of cracked concrete ceiling slabs have been reported in the City of Toronto’s parking garages. Experts concluded the damage was primarily caused by lack of air entrainment combined with de-icing salts and freeze-thaw exposure.
Milwaukee County Courthouse
This iconic building from the 1930s was plagued by decaying parapet walls, dangerous falling debris, and disintegrating balustrades due to missing air entrainment in the original concrete. An $80 million renovation project was required.
Lock and Dam Structures
Many U.S. lock and dam structures were found to have concrete damage caused by freeze-thaw cycles. This led the Army Corps of Engineers to mandate a minimum air content of 6% for all new lock and dam concrete projects.
Effects on Fresh Concrete Properties
Fresh Concrete Property | Effect of Air Entrainment |
---|---|
Workability | Improves – due to lubricating effect of air bubbles |
Cohesion | Increases – provides stability |
Bleeding | Reduces – air voids provide space for water |
Segregation | Reduces – increases uniformity |
Finishability | Improves – results in a creamier texture |
Effects on Hardened Concrete Properties
Hardened Concrete Property | Effect of Air Entrainment |
---|---|
Compressive Strength | Slight reduction – around 5-10% decrease |
Tensile Strength | Increase at early ages – reduces microcracks |
Flexural Strength | Slight reduction – around 5-10% decrease |
Permeability | Reduces – lowers risk of salt/water damage |
Freeze-Thaw Resistance | Greatly improves – prevents hydraulic pressure damage |
Abrasion Resistance | Slightly decreases – due to increased void space |
Proper Air Entrainment Levels
Air content requirements vary based on the agency standard and exposure conditions:
Agency Standard | Required Air Content |
---|---|
ACI 301 | 4.5% to 7.5% |
Army Corps of Engineers | 5% to 8% |
ASTM C94 | 4.5% to 8% |
Severe Exposure | 6% or higher |
Air content above 8% can lead to significant reductions in strength. Most experts recommend maintaining air content between 5% and 7% for a balance of freeze-thaw resistance and strength.
How to Properly Air Entrain Concrete
To properly air entrain concrete, follow these practices:
Use the Right Air Entraining Admixture
Air entraining admixtures (AEAs) contain soap-like surfactants that stabilize air bubbles in concrete. The admixture must be compatible with the cementitious materials and mix proportions.
Add the Admixture Correctly
For best results, AEAs should be added on site and batched with the initial mixing water. Pretesting is recommended to determine the optimal AEA dosage rate for the mix.
Allow Adequate Mixing Time
After adding the air entraining admixture, allow an additional 2-3 minutes mixing time for the air bubbles to fully generate and disperse.
Avoid Overwatering
Adding excess water can cause air bubbles to coalesce and escape. Follow ACI guidelines to maintain an appropriate water-cement ratio.
Frequently Test Air Content
Use an air meter to test air content throughout the pour at regular intervals. Adjust the admixture dosage as needed to maintain air content within specifications.
Protect Fresh Concrete
Safeguard against conditions causing rapid surface evaporation that could lead to loss of entrained air in the critical surface layer.
Conclusion
Air entrainment is a vital component of durable, long-lasting concrete. When concrete lacks adequate air voids, the consequences can be severe and expensive. Freezing and thawing cycles will rapidly degrade non-air-entrained concrete through internal hydraulic pressure, surface scaling, cracking, and reinforcing steel corrosion.
Following proper air entraining procedures, measuring frequently, and staying within recommended air content ranges is critical to concrete performance and longevity. Specifiers and builders should never treat air entrainment as optional, but as an indispensable element for concrete success.