[article]
| Titre : |
No magic number : coating concrete after 28 days |
| Type de document : |
texte imprimé |
| Auteurs : |
R. A. Nixon, Auteur |
| Année de publication : |
2018 |
| Article en page(s) : |
p. 24-31 |
| Note générale : |
Bibliogr. |
| Langues : |
Américain (ame) |
| Catégories : |
Matériaux cimentaires -- Revêtements:Matériaux cimentaires -- Peinture
Matériaux cimentaires -- Séchage
Rétrécissement (matériaux)
|
| Index. décimale : |
667.9 Revêtements et enduits |
| Résumé : |
Historically, the coating industry has prescribed 28 days of cure time for concrete before coating application. The 28-day threshold was based on conventionally specified strength gain values such as a minimum of 5,000 psi compressive strength at 28 days. The logic behind the adoption of the 28-day rule was twofold.
Firstly, the majority of minimum specified strength gain is empirically reached within the first 28 days. Therefore, if the compressive strength has mostly been achieved, then most of the tensile stress that forms cracks will have developed. This assumes that most of the shrinkage of the cement paste has largely occurred but ignores the all-important water-to-cementitious-materials ratio and the paste-to-aggregate ratio. These are the major contributors to shrinkage along with temperature and timing of excess moisture loss from the concrete. Coincidently, these factors are both inextricably tied to the water content in the mix design, which leads us to point number two.
Typically, it is believed that all of the excess water not necessary for cement hydration has left the concrete after 28 days. Therefore, the detrimental effects from the exit of moisture from the concrete on coating cure or coating adhesion have been eliminated. This assumes that excess moisture leaving the concrete after 28 days will not inhibit a coating's curing mechanisms. This also ignores the material principles at work in cement hydration.
The problem with the magic 28-day number is that there is just as much empirical evidence that 28 days isn't necessary, as there is for its requirement or a longer cure time prior to coating.
This article will discuss the many factors affecting the timing for concrete shrinkage (and related cracking) as well as the rate of loss of excess moisture - factors that are not reliant on 28 days of concrete cure time. In addition, case histories will be presented evidencing sucessful coating application on concrete well before 28 days of cure time versus coating problems experienced well beyong 28 days of concrete cure.
Further, we will examine mix design and material-related parameters that should be followed when scheldule needs for coating concrete prior to 28 days of cure are essential. |
| Note de contenu : |
- The basics
- Drying shrinkage and excess water in concrete substrates
- Real-world experience
- Fig. 1 : Concrete shrinkage increases with the water-to-cement for a given aggregate content
- Fig. 2 : This graph shows the typical effect of water content on drying shrinkage
- Fig. 3 : Reflective cracking which appeared approximately one month after a floor coating system was installed in a pharmaceutical plant
- Fig. 4 : No reflective cracking and no moisture-related problems have been observed since this floor coating was installed in a building at a wastewater treatment plant after two years in service
- Fig. 5 : Blisters formed in the coating on concrete walls in a wastewater treatment plant only two months after installation |
| Permalink : |
https://e-campus.itech.fr/pmb/opac_css/index.php?lvl=notice_display&id=31893 |
in JOURNAL OF PROTECTIVE COATINGS & LININGS (JPCL) > Vol. 35, N° 9 (09-10/2018) . - p. 24-31
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No magic number in JOURNAL OF PROTECTIVE COATINGS & LININGS (JPCL), Vol. 35, N° 9 (09-10/2018)
