MC Magazine

TECHNICALLY SPEAKING
Spalling
By Evan Gurley

Is it freeze/thaw? Is it the aggregate? Is it too much water? Are we overvibrating? Several factors play into the troubleshooting of spalling.

Finishing Techniques. Concrete finishing operations should not begin until the water sheen on the surface is gone and the excess bleed water on the surface has had a chance to evaporate. If the excess water is worked into the concrete because finishing operations have begun too soon, the concrete on the surface will have too high of a water content and the surface will be weaker and less durable.

Sealants. If you are using sealants for precast products, please be aware of certain issues. Avoid film-forming sealants. These products usually contain silicone, or animal fats or waxes. Look for products that contain silanes, siloxanes or a blend of these two chemicals.Concrete needs to be able to transmit water vapor from the soil into the air. Film-forming sealants block this process. Silanes and siloxanes act like the fabric Gortex: Liquid water can’t pass by the silanes/siloxanes, but vapor can. The sealant will most likely need to be approved by the project engineer.

Consolidation. Spalling may also result from the improper consolidation of concrete during construction. Since most mix designs are made at 5,000 psi, a lower water-to-cement (w/c) ratio is ideal for fighting against freeze/thaw damage. However, improper consolidation causes severe problems. Since the w/c is lower, it takes longer to consolidate the concrete. Overconsolidation could be a key issue in this case. Overconsolidation causes water-rich cement paste to rise to the surface (a condition known as laitance). This in turn weakens the surface of the concrete, which encourages scaling (spalling in thin layers).

Laitance. Laitance is a soupy mixture of cement, fine sand and water that accumulates on the surface when using wet concrete mixes that bleed excessively. In structural concrete it leaves a light colored streak of poor concrete between lifts. In hydraulic structures it has a high permeability and is a source of water leakage. The laitance layer is particularly vulnerable to deterioration by freezing and thawing.

The use of a stiff mix of low bleeding capacity will prevent the accumulation of laitance. The use of air-entrained concrete, because of its cohesive nature and low bleeding capacity, reduces the occurrence of laitance.

Paint. A paint that does not breathe can trap moisture within the concrete and cause the surface to spall.

Air Void Spacing.Another commonly accepted hardened concrete parameter for freeze-thaw resistance is an air-void spacing factor (ASTM C 457) of 0.2 millimeters or less. A number of early research studies reported that a spacing factor of approximately 0.25 millimeters or less signified adequate freeze/thaw resistance.

Materials. Numerous factors are involved when considering aggregates in the mix design and how they can lead to spalling. To get a full description of all the different problems aggregates experience, reference ACI 201.2R-01. Some problems identified within this document include the use of ice removal agents; chemical reactions of aggregates (alkali-silica reaction); the oxidation or hydration of unstable oxides; metallic iron deposits; organic impurities; and others. Careful testing and examination can reveal the presence of the reactive aggregate impurities and limit their use in concrete mixes.

 There are many possibilities when dealing with causes of spalling. To be able to answer the question “What causes spalling?” it is recommended that you monitor/check all avenues previously suggested. It is possible that these conditions are just the tip of what could cause the spalling. A good gauge for improving concrete’s resistance to spalling would be best described below:

Resistance to concrete spalling is significantly improved with the use of a low water-cement ratio, a moderate portland cement content, adequate air entrainment, proper finishing and curing, and an air-drying period prior to exposure of the concrete to salts and freezing temperatures.

Evan Gurley is a staff engineer with NPCA.