Concrete surface is not homogenous like steel which makes the coating of concrete somewhat different from coating steel. Concrete prepared with the same raw materials may vary from one pour to the next due to variations in cement, sand and gravel, non uniform mixing, the degree of vibration, and weather conditions. In addition to being non – homogenous, concrete surfaces may contain irregularities such as bug-holes, fins, tie rod holes, and form joints; or form release agents, or air – entraining materials. Surfaces may be contaminated with efflorescence or laitance or may contain excessive moisture.
The following issues need to be specially addressed
- Residual moisture.
- Weak top layer in new concrete – laitance or efflorescence.
- Presence of small air pockets (bugholes) on or near the surface on the surface of vertical formed concrete not removed during the vibration process.
- Surface roughness & defects.
Concrete is normally allowed to cure fully before coating. This period is normally considered as 28 days. The purpose of waiting for full cure is to allow the excess moisture to evaporate and not be trapped within the coated concrete. This is true of concrete to be encapsulated. However, for one side open concrete special damp tolerant primers allow coating much before the 28 day period since excess moisture can evaporate from the negative side.
New concrete has a thin, weak surface layer with a high ratio of water to cement called ‘laitance’. This needs to be removed by blast cleaning the surface with disposable abrasive like sand. The blast cleaning has to be done from a distance and a quick hand action is required- this process is called ‘sweep blasting’. Dwelling near the surface or for a longer period will cause the concrete surface to erode quickly. Blast cleaned concrete will have the surface finish of coarse grade sandpaper.
Bugholes present just under the surface may open partially during the blast cleaning process. When coated, the trapped air will expand due to heat generated by chemical reaction of the coating. This air will pass through the liquid coating and cause a pinhole. If the coating gels fast enough, this pinholing will be reduced. Pinholing is also reduced by use of damp tolerant primer or by flooding with coating. If a pinhole free surface is required, such as in acid containment, the bugholes must be manually filled with a surfacer prior to coating. Overhanging protrusions can cause shadows and skips in the coating – these may either be removed by surface grinding or coated by a ‘four way’ pass system.
Although 100% Solids Polyurethane Coatings adhere well to dry cured concrete, the use of damp tolerant penetrating primers are invariably recommended due to the following reasons:
- The primer penetrates into the concrete and toughens the surface. This will support long term adhesion better than un-primed concrete.
- The primer reacts with residual moisture and seals the surface. The moisture is effectively isolated from the 100% Solids Polyurethane Coating. In the absence of this sealing, adhesion may be compromised. On new construction sites, dry concrete is rarely encountered.
100% Solids Polyurethane Coatings are designed for unlimited build using multiple ‘wet-on-wet- passes. The two components react rapidly. Gel time (conversion from liquid to semi solid), tack free time (cannot be deformed by thumb), handle dry and full cure time depends upon the specific formulation. Tack free time varies from 3 to 60 minutes.
Spray is by hand held spray gun. Pipeline external and internal are however sprayed using automated/ robotic equipment.
Due to their unlimited build, 100% Solids Polyurethane will produce similar thickness over the peaks as well as the valleys of the concrete surface. The coating surface will largely replicate the profile of the underlying concrete. If aesthetics are a consideration, the concrete must be ground smooth prior to coating using diamond tipped grinders. This must be specified in the contract document since this is a very slow and costly exercise.
Spray Equipment & Process
Fast setting, 100% Solids Polyurethanes comprise of two components, the Resin (Polyol/ Polyamine/ Blend) and the Activator (Isocyanate). These are supplied in separate 200 Litre drums and are sprayed using plural component airless spray equipment. The spray system comprises Hydraulic pumps driven by air motors which lightly pressurise the two components and supply them to a proportioner. This unit proportions the two liquids in the designed ratio by volume (3:1 / 2:1/ 1:1) and develops a pressure of 2000-3000 Psi.
The two liquids under pressure are pumped to a mixer manifold / static mixer where they mix intimately and flow through a spray hose into a spray gun. The mixed material atomises on exiting the spray nozzle and is sprayed onto the surface. The mutual reaction of the two components is very fast causing an instant viscosity build up prior to spray and post spray fast cure into a solid. The whole system is dynamic with supply, mixing & spraying done continuously. There is no mixing of lots in containers and spraying. It is a high volume spray operation with typical spray tip spraying 120-200 Litres/ Hour. As soon as the spray is stopped, the section between the manifold and spray gun is flushed clean with a purge liquid to prevent the coating from blocking the system.
Using the plural component system needs factory trained operators.
Regular monitoring of the surface temperature and ambient conditions (dew point) is carried out during the process of applying 100% Solids Polyurethane Coating over the primed concrete. This is to ensure that the surface temperature always exceeds the dew point by at least 3° C. If not dew can form on the surface and the coating is applied on the microscopic layer of water. Dew point depends upon the relative humidity and the ambient temperature – warmer air is able to hold more water than cold air. Therefore this situation is aggravated during cold, humid environments. In situations where the dew point criterion is not met dehumidifiers can be used to remove excess moisture from the air.
Final cured coating is tested for the following parameters to ensure that it has been correctly applied. This also confirms the quality of coating materials since the parameters would not be achieved if the material was non-conformant.
- Adhesion using ‘Pull Off Adhesion Tester’ as per ASTM D 4541 / similar standard.
- Dry film thickness by manual measurement of coating detached with dollies in the Adhesion Test. Spot variation in thickness to a larger degree than steel is encountered in coating concrete due to its inherent roughness and a ± 25% spot variation is normally acceptable.
- Hardness using ‘Durometer’ as per ASTM D 2240 / similar standard.