Q: As a materials engineer, I am trying to discover the cause of discoloration. The contractor involved is very experienced in tilt-up construction and reports that this discoloration has only occurred three or four times over recent years. The concrete supplier operates a very good quality-control program. The concrete used for the panels does not incorporate superplasticizer, and no admixtures were added onsite. The concrete was also well-mixed and placed at a slump of 4 to 5 inches.The discoloration shown is evident only on the underside of the tilt-up panels (the side cast against the slab). The dark patches are characterized by a surface full of pinholes and a weak matrix, which is easily abraded by rubbing with a finger. The dark patches are more common around the perimeter of the tilt-up panels. They also seem to occur at high sports on the panels (which correspond to low spots in the slab), but there are some exceptions. The panels were cast directly on the slab, which was coated with a bond breaker. The contractor indicates that the concrete was thoroughly vibrated with internal vibrators, including the use of a poker vibrator along the edge forms. I would like to know if any readers have experienced this phenomenon, especially if they have learned the cause.

A. Excessive release agent was applied to the slab, and it ponded in the low spots. Too much release agent can act as a retarder. The solution is to make certain the release agent is applied according to the manufacturer's recommendations and that excessive amounts of release agent are not allowed to accumulate.

James M. Shilstone Jr.
The Shilstone Companies Inc.,
Dallas, Tex.

The cause is too much bond breaker. The reason the dark patches occur around the [panel] perimeter and at low spots on the slab is because bond breaker was applied so heavily that it puddled. The solution is to instruct workers to use less bond breaker and to spread it out thinly and evenly with a household mop or rags. If the pinholes and a weak matrix persist, the bond breaker has too high a fatty-acid content; get a higher-quality bond breaker.

David G. Markle
Tulsa Dynaspan Inc.,
Broken Arrow, Okla.

The problem is not in the concrete mix. If it were, the discoloration would be consistent throughout the thickness of the panel. I believe the problem is an excessive amount of bond breaker, which can tend to act as a retarder. As the panel forms were sprayed, the excess oozed down to the slab. As the slab was sprayed, the excess shed from high points of the slab to low points. Solution: Pour a flatter slab (to eliminate low points), and spray it with two light coats of bond breaker prior to rebar placement. With bond breaker, more is definitely not better.

Ted N. Mefelli
La Russo Concrete Co. Inc.,
Lakewood, Colo.

It appears that the panels were either coated with an inappropriate bond breaker, or an overabundance of bond breaker was used and [excess] was not sufficiently wiped off prior to casting the concrete. Bond breaker, if not used properly, can act as a set retarder, causing powdering of the concrete surface in areas where there may have been a buildup. The solution is to use a drying type of product, spray it on lightly (more is not better), and wipe off all puddles or overabundance of bond breaker prior to concrete placement.

William Pompili
Pompili Precast Concrete,
Garfield Heights, Ohio

If the contractor applied one heavy coat of the bond breaker and it settled in the low spots on the slab, it may easily have had a retarding and mottling effect on the panels. Bond breaker should be applied to the slab in two light coats in opposite directions. Application of a single heavy coat, especially if done in cool and damp weather, may have resulted in ponding and uncured bond breaker left on the slab when the panels were poured. To test bond breaker [application], splash a little water on the slab. If a bond breaker has been properly applied, the water tends to bead up like it does on a freshly waxed car.

William J. Zens
Allied Building Products,
Seattle, Wash.

Discoloration problems have been around for some time, but they have resurfaced at a more frequent rate here in Las Vegas due to more tilt-up [buildings] being placed. The Tilt-Up Concrete Association's newsletter Tilt Tips (No. 1) addresses this problem. It is called the "osmotic effect." TCA says that surface defects found on the downside of tilt-up panels, such as spalling, delamination, or dusting, are often the result of incomplete hydration of the cement due to insufficient water. Water that would normally be available for hydration is drawn out of the panel, down into the casting or floor slab by osmosis. The result: a weakened downside surface. The newsletter also suggests an effective solution to preventing water penetration. Create a water-impermeable barrier between the casting slab and the tilt-up panel by using a membrane-forming combination curing compound and bond breaker. Apply the product according to the manufacturer's instructions as the curing agent for the casting slab and as the bond breaker prior to casting of the tilt-up panel. I am in the process of placing some test panels using various mix designs, admixtures, and bond breakers, including a test panel half with and half without a membrane-forming curing compound.

Wayne Stroud
WMK Materials,
Las Vegas, Nev.