There is no mortar in the panel
A thin brick panel is cast face down. The brick sits in the pockets of the liner, the concrete goes over the back, and when the panel strips, the concrete that flowed into the gaps between the brick is the joint. Precasters call that concrete the matrix. On the finished building it is the mortar line, and it is the second color on every square foot of the wall.
An architect who picks a brick from a sample board has picked half the wall. The other half is the matrix, and the matrix is a decision the panel drawings and the sample approval have to make on purpose.
What changes the read of the brick
Matrix color. The face mix can be gray cement, white cement, buff, or pigmented. Against the same brick, a light joint separates every unit and the coursing reads crisp from the street. A joint close to the brick color knits the wall into one tone and the coursing softens. Neither is right. They are different walls, and the sample has to show which one the job is buying.
Joint width. The liner sets it. Most Advanced liners form a 3/8 in. joint, and the PCI test assembly is built on a 3/8 in. raked joint. A wider joint puts more matrix in the wall and more of the matrix color into the read.
Joint profile. Raked, concave, tooled and flush joints throw different shadows. A raked joint in a light matrix draws a line under every course. A flush joint nearly disappears. The profile comes from the liner, so it is chosen when the liner is chosen.
Joint depth. RP 152-25 asks the designer to limit the depth of the joint so the back profile of the brick is never exposed to weather and the brick keeps full bond to the concrete. The PCI assembly for freeze thaw and pull-out testing uses a joint no deeper than 1/4 in. from the brick face.
Finishing. Sandblasting opens the matrix and lightens it. Acid etching does the same in a different way. A hot water wash to remove the wax from the brick face changes the sheen of the brick. RP 152-25 lists the cleaning, washing and rinsing the plant does as reasons the finished brick can differ from the maker’s sample board, and it asks for a precast sample so the designer sees the real thing.
Staining. The matrix can also mark the brick if the plant lets it. RP 152-25 says not to let dyed joints or additives in the face mix or the backing become soluble through improper curing, cleaning agents, acids or early washing, because they can deposit on the brick face. That is a production control, but it is also a reason a pigmented matrix needs a plant that has done it before.
What to approve
RP 152-25 puts final brick approval after a precast sample, made with the selected brick and every production process the plant will use, because the sample shows the joint the concrete mixture creates, the liner profile and the effects of finishing on color and texture. Under MNL 117 and MNL 122 that means a 12 by 12 in. initial sample, a range sample around 4 by 4 ft, and one or more full size production pieces viewed at the plant.
Before production, the plant casts one more sample with the delivered brick and the approved mix, so the run that ships is compared with the run that was approved.
What to put on the drawings
- The brick, by maker, name, product code, color, size and texture, the way RP 152-25 says to order it.
- The matrix, by cement color and any pigment, and the finish the panel gets.
- The joint, by width and profile, which is the liner.
- The sample sequence, and who signs it.
Advanced supplies the brick and makes the liner, so the joint width and profile on the drawing are ones Advanced can build. Your rep can pull a cast sample with the matrix the job is considering before the mockup is scheduled.
Common questions
- What is the matrix in a thin brick precast panel?
- The concrete cast against the back of the brick. It fills the joint the liner forms between every brick, so it is the mortar line the eye reads on the building. Its color, texture and finish are part of the wall's color, not a background to it.
- Does the same brick look different with a different matrix?
- Yes. A light gray or buff matrix lifts a dark brick and makes the coursing read clearly. A dark or pigmented matrix pulls the wall toward one tone. The brick has not changed. The joint has.
- Why does PCI want a cast sample and not a brick board?
- Because the brick board shows the brick alone. RP 152-25 says the cast sample shows the mortar joint the concrete mixture makes, the joint profile the liner makes, and what cleaning, washing and finishing do to the color and texture. Final approval belongs on that sample.
- How deep should the joint be?
- Shallow enough that the back of the brick is never exposed to weather and the brick keeps its bond to the concrete. RP 152-25 asks designers to limit joint depth for both reasons. The PCI test assembly uses a raked joint no less than 3/8 in. wide and no more than 1/4 in. deep.