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Discoloration of concrete is a source of displeasure for both the contractor as well as the owner. It may appear as gross variations in color across large concrete surfaces, light or dark spots, or early light efflorescence. Many factors contribute to this issue, which is why it can be problematic and difficult to prevent.
Factors influencing discoloration in concrete include inconsistent mixture, adding water to the mixture at a project site, changing cement color, using cement replacements, poor workmanship, etc. Discoloration due to these factors appears soon after concrete placement.
Contents:
What are the Causes of Concrete Discoloration?
Inconsistent Materials
Maintaining consistency throughout the concreting operation, especially in mix proportioning, is important to avoid unpleasant colors. Variations in different batches of concrete delivered to the project site will result in inconsistent color.
Also, production of consistent concrete mix is difficult because it consists of various materials with inherent variability. Concrete mixture constituents are cement, supplementary cementitious materials, sand, water, admixtures, and possibly fibers. Achieving consistent interaction between these materials in various concrete batches is difficult.
So, to achieve the best possible consistency in a concrete mix from different batches, all concrete constituents must comply with ASTM standards.
Adding Water at Construction Site
Contractors may add water to the concrete batches to improve workability and make finishing operations as easy as practical. Modifications in mixture on the field without adequate mixing produces concrete with a blotchy appearance.
Differences in concrete color become noticeable in side-by-side placement. For instance, when water is added to one batch, and then the concrete is poured next to an earlier non-tempered concrete.
Change in Cement Color
Changing the color of the cement used in the ready-mix plant is another cause of concrete discoloration. Individual cement color varies due to burning or grinding conditions during cement production.
The effect of unconventional cement color on discoloration can be observed when a certain cement type is replaced with another one during an ongoing placement, or the adjusted mix is placed immediately next to concrete produced with cement from a different source.
Utilization of Cement Replacements
Using recycled products, like slag and fly ash, as partial cement replacements can create color variations based on the percentage of recycled material put into the mix, water-to-cementitious materials ratio, and how concrete is cured.
Slag results in an initial slight green or bluish tint that lightens with time, whereas fly ash produces different hues ranging from buff to brown.
Addition of Calcium Chloride
Generally, calcium chloride is added to the concrete mixture to accelerate its setting time. This substance is the reason for a darkened concrete appearance, especially when its dosage approaches 2% by the weight of cement.
The coloring issue is further exacerbated when the mixture is not mixed adequately after adding calcium chloride, producing concrete with a more irregular or blotchy color.
Poor Workmanship
Contractors should consider weather conditions before and during concrete pouring because it has a major impact on the final result. For instance, wetting subgrade in a dry environment before pouring a concrete slab is a good practice.
Moreover, the placement of concrete in hot weather that increases the evaporation rate will require evaporative retardants, windbreaks, or any other suitable practices.
Poor practices such as spraying additional water on the concrete surface or carrying out final finishing while bleed water is still on the concrete surface result in increased variability in the surface appearance or delamination.
Placement of Objects on Concrete during Initial Curing
When plastic, lumber, or construction debris is left on a colored concrete during the curing process, the surface color gets blotchy and irregular, with the color being lighter where the materials had been sitting. This issue is called curing differential, which is caused by different moisture levels at the surface of concrete during the critical first few days to weeks after pouring concrete. So, uniform curing is key for producing a consistent colored concrete surface.
Plastic sheets placed on concrete to improve curing can play a major role in concrete surface discoloration. The discoloration that occurs by wrinkles or air gaps in plastic covers or curing blankets results in what is called leopard spotting.
How to Minimize Concrete Discoloration?
- Avoid the use of calcium chloride in concrete.
- Utilize consistent concrete constituents that are provided continuously from batch to batch.
- Proper concrete pouring, finishing, and curing on time will reduce discoloration.
- Do not change concrete construction practices such as a change in the concrete mix, formwork, surface treatment, or curing, which sometimes result in significant and permanent discoloration.
FAQs
1. Inconsistent Materials
2. Adding Water at Construction Site
3. Change in Cement Color
4. Utilization of Cement Replacements
5. Addition of Calcium Chloride
6. Poor Workmanship
So, to achieve the best possible consistency in a concrete mix between different batches, all concrete ingredients must comply with ASTM standards.
When plastic, lumber, or construction debris is left on a colored concrete during the curing process, the surface color gets blotchy and irregular, with the color lighter where the materials had been sitting. This problem is called curing differential, caused by different levels of moisture at the surface of concrete during the critical first few days to weeks after pouring concrete. So, uniform curing is key to producing a consistent color concrete surface.
Mixture modification in the field without adequate mixing produces concrete with a blotchy appearance. Differences in concrete color become noticeable in side-by-side placement. For instance, when water is added to one batch, the concrete is poured next to an earlier non-tempered concrete.
1. Avoid the use of calcium chloride in concrete.
2. Utilize consistent concrete constituents that are provided consistently from batch to batch.
3. Proper concrete pouring, finishing, and curing on time will reduce discoloration.
4. Do not change concrete construction practices such as a change in a concrete mix, formwork, surface treatment, or curing, which sometimes result in significant and permanent discoloration.
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