The winter of 2009-10 in the eastern United States was the coldest and wettest in decades. As a result, experts from Lafarge were asked to inspect many brick veneer projects whose appearance concerned owners and designers.
The cement company's representatives studied each component of the walls affected in 10 different projects, and came to some conclusions regarding the effects of the extreme weather conditions. These led to recommendations for contractors, and a study of component interaction.
Was it the mortar?
Mortar joints on the inspected projects displayed various shades of the specified mortar color, whether a colored or grey mortar was used. Masonry cement and cement lime mortars experienced the same types of color variations.
Before they are delivered, masonry cement and cement lime mortars are tested by their manufacturers for color consistency. Whether the cement is used in concrete or for masonry, color consistency is greatly important to the manufacturer. The color target is based on the “lab” scale, using a colorimeter to measure the intensity of colors (illustrated in Figures 1 and 2).
None of the investigated projects included had a complaint about pigment variation of the dry powder, or of the freshly prepared mortar.
Samples of the wall were cut out for testing. The areas of the wall face that appeared light in color cured the same light shade through the joint's depth, as shown in Figure 2. Mortar from the same batch will cure out to a lighter shade on saturated units than mortar cured between dry units.
Joint tooling is also a variable. Joints that are tooled when the mortar is “thumbprint hard” are tooled at the right time, although this is a subjective measurement. Portland Cement Association (PCA) has published a Trowel Tips document entitled “Mortar Color” that details how this color variation occurs.
Over the course of a wet, cold winter, brick with moisture content ranging from dry to fully saturated are laid together in a wall. When the dry brick are “thumbprint hard,” the entire wall is jointed. Many of the wet brick, from the same cube as the dry ones, are tooled at the same time, but these joints predictably dry lighter.
Joints that are tooled when wet will lose more cement paste from their surface, since it is diluted by tooling when wet. Cleaning can also be a factor. Even an approved cleaning agent can wash away a significant amount of diluted cement paste from the joint. Instead of the cement paste, the sand/aggregate color appears more prominently.
Was it the brick?
Brick color is approved by the designer and owner at the sample panel. In some of the Lafarge projects the brick was nearly monochromatic, while others had some range in the approved sample. In one project using red brick with a slight range in the approved sample, one wall was considered visually unacceptable. Joints were hidden with tape to eliminate the effect of joint color variations.
Where the wall appeared to show noticeable variation before the tape was applied, the brick were of a very consistent color and pattern when the joints were hidden. This highlighted the variations in joint color. In other cases, the brick were nearly witout variation in color.
Manufacturers and distributors are not required to protect brick from the elements in their yards or before delivery. If it is raining or snowing, brick will arrive at jobsites wet.
At one time, there was a specification in ASTM C 90 for Type I, moisture controlled CMU. In theory, block was to be delivered to projects with 0% moisture content. In practice, this standard proved impossible to adhere to because, on a jobsite, it is impossible to control the elements. Even if the block starts out dry, condensation is retained by the packaging designed to keep out rain and snow. The standard was amended, and Type I block are no more.