The topic of physical salt attack (abbreviated as PSA) on concrete is relatively new. ACI 201 “Guide to Durable Concrete” started to mention physical salt attack in their 2016 version. PSA was a subject of extensive research on stones and rocks for a relatively long time.
So, What is Physical Salt Attack on Concrete?
Physical salt attack is a damage mechanism that under a range of environments leads to progressive scaling and flaking of concrete surface disrupting its durability performance.
PSA occurrence is due to the existence of salts in the exposure environment and at the same time, the ambient conditions (temperature and relative humidity) are favorable for salt crystallization.
This means that we should have two things for PSA to happen: salt solution and ambient conditions that cause this salt to form crystals.
Damage is observed on the evaporative surfaces of concrete in contact with salt-rich media (e.g. soil, groundwater or seawater) such as the exposed portions of basement walls, abutments, piers, slabs on grade, and tunnels.
Several salt solution can cause PSA such as sodium sulfate, sodium carbonate and sodium chloride with sodium sulfate is the most common type of salt to cause this damage.
Features of Physical Salt Attack
The main feature here is that damage does not happen below the ground but it occurs above the ground where the evaporation of water in salt happens and salt crystals form.
Another important feature of physical salt attack is that damage is mainly physical in nature. This means no chemical alteration of hydration products is involved.
Physical Sulfate Attack vs. Chemical Sulfate Attack
Sometimes specialists call physical salt attack as physical sulfate attack when the salt causing the damage is sodium sulphate.
In the classical form of external sulfate attack (chemical sulfate attack), sulfates from the surrounding environment enter through concrete to react with the hydration products. The cation type bound with sulphate ions would determine the mechanisms and complexity of chemical interactions. These chemical reactions might produce gypsum, ettringite, brucite, resulting in multiple damage manifestations such as expansion, cracking, spalling, softening and decalcification of calcium-silicate-hydrate [C-S-H].
More about chemical sulfate attack: Chemical Sulfate Attack on Concrete
On contrary, in the case of PSA, salt compounds such as sodium sulfate crystallize in the near-surface pores of concrete producing deleterious crystallization pressure to concrete.
Flatt (a famous scientist in the field) reported that the value of this pressure can reach 10–20 MPa, which is far greater than the tensile strength of normal and even high-performance concrete.
Historical confusion between Physical and Chemical Sulfate Attack
In the past, there was confusion between chemical and physical attacks caused by sulfate salts. Scientists initially thought that PSA was chemical attack when they tested samples in sulfate soils.
But later they noticed that damage occurred only above the ground and minor damage experienced under this level which was not characteristic of chemical attack.
In 1982, St John published a paper titled “An unusual case of ground water sulphate attack on concrete”. The paper was about damage on tunnel arches. He reported that apart from chemical attack on tunnel arches by Sulphur gases in coal smoke, severe exfoliation of concrete linings along the walls of three railway tunnels in New Zealand was found 0.25-1.5 m above ballast level and around construction joints.
The author described the damage cause to be somehow unusual and the characteristic feature was a coating of fine, silky, acicular crystals of sodium sulphate on the affected locations.
The above damage is actually PSA but they did not know that at that time due to lack of characterization techniques like microscopes and the appearance of this unusual case of deterioration at that time made it difficult to thoroughly understand the underlying damage mechanism.
Similar cases were reported like that but recently scientists recognized this damage mechanism. In general, PSA is expected to be a prevailing damage mechanism of concrete in various locations worldwide, where salts exist in the exposure environment (e.g. soils).
For instance, salt damp is an issue associated with soils in some parts in South Australia. Salts rise through the concrete foundations and cyclic seasons between summers (drying) and winters (wetting) cause deposition of salts and concrete damage.
Soils in some parts of the Canadian Prairie Provinces are sulphate-laden which makes structures constructed in these environments susceptible to PSA.
Similarly, cases of PSA were reported in the salty Sabkha soils in the Arabian Gulf region. It is envisaged that as a result of the attention directed towards this damage mechanism, other field cases will be reported and considerable research efforts are still to be made to fully understand the various aspects related to PSA of concrete.
Emerging Recognition of PSA
Recent research has clarified PSA as a distinct mechanism. Key findings include:
- Damage correlates with salt deposition cycles (e.g., seasonal drying/wetting).
- Advanced characterization tools (e.g., microscopy) now differentiate PSA from chemical degradation.
- Regions with saline soils or groundwater are particularly vulnerable, necessitating further research and mitigation strategies.
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