The issue of concrete durability has been emerged an issue of increasing concern for Australia’s building and property sector, as problems with spalling or “concrete cancer” become more acute in the country’s coastal areas.
While the obvious solution to the problem of spalling is to augment the strength of concrete at the time of its manufacture via the addition of admixtures such as crystalline powders, other expedients must be employed for completed buildings whose original developers were less preoccupied with the quality of the construction materials employed.
According to Dr. Farhad Nabavi, senior technical director at Xypex, a convenient solution may be at hand in the form of crystalline technologies that are applied to the surface of pre-existing concrete as a coating.
Nabavi notes that the chief source of the degradation of reinforced concrete is the ingress of a variety of chemicals from the external environment that can different corrosive effects.
“Diffusion or penetration of aggressive substances into concrete through the interconnected pores or capillary pores, as well as micro and macro-cracks, causes deterioration of the reinforced concrete structures,” he said.
“Depending on the type of diffusive substances, the degradation of the cement matrix or corrosion of steel reinforcement can occur. For example, sulphate attack and acid attack result in concrete matrix degradation, while chloride diffusion and carbonation will result in corrosion of steel reinforcement.”
The application of crystalline materials as external coatings can serve to prevent damage to existing concrete structures by entering and filling the very channels of ingress for corrosive chemicals – the pores and cracks that permeate the building material.
“Crystalline material, as a coating system or it is better to say surface treatment, is applied on the concrete surface and due to chemical gradient generated by the chemical concentration, the crystalline material diffuses through the concrete through interconnected pores and micro-cracks,” said Nabavi.
After the crystalline material enters the concrete, it not only fills in the channels of diffusion for aggressive chemicals, it also bonds with the concrete to become an intrinsic part of it.
“The reactions between active chemical ingredients and a broad range of cement hydration byproducts generate non-soluble crystals which blocks the pores and heals micro-cracks of up to 0.4 millimetres permanently. These crystals will be an integral part of concrete.”
According to Nabavi, the use of crystalline coatings present other advantages in addition to their ability to form integral bonds with concrete building materials.
These include these include the ability of the crystalline coating to spread through the full thickness of the concrete if there are any available interconnected pores or micro-cracks, the permanence of the coating which removes the need for repeat applications, as well as the non-toxic nature of the material, given that it eventually becomes an intrinsic part of the concrete, which means it can be used with drinking water treatment structures.
The coating systems are also highly resistant to extreme hydrostatic pressure and chemical environments with pH levels of between three and 11, and are applicable to both the positive and negative sides of water-retaining concrete structures.
Testing has already shown that the application of crystalline coatings to the concrete beams of bridges can achieve dramatic improvements in both compressive strength and water permeability.
“We selected two sections beneath the slab between the main beams of the bridge trail testing, and applied a crystalline coating system to one of the sections while the other section was left untreated as reference concrete,” said Nabavi.
“After 10 months of applying the coating system, 10 cores with 100 millimetres of diameter and 200 millimetres of length were extracted to examine the compressive strength and water permeability of the cores. During this time, the bridge has been under service, and water permeability was tested by applying water pressure of 25 metres water head for 16 hours.
“The crystalline coating increased the compressive strength by 36 per cent, while also reducing the water permeability by 99.97 per cent.”