Editor's Note: Scott Tarr, a principal engineer in the Structural Evaluation section of CTLGroup, submitted Problem Clinic this month. His principal experience is in the area of distressed concrete slabs on ground (interior and exterior) and flooring failures. He specializes in comparing the as-built to specified load-carrying capability of concrete slabs and developing repair specifications to restore the serviceability to that designed.

An International Concrete Repair Institute and American Concrete Institute member, Tarr serves on several committees, including 301 Structural Concrete Specifications, 302 Construction of Concrete Floors, 360 Design of Slabs on Ground, and 330 Concrete Parking Lots.

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Aggregate shadowing occurs when coarse aggregate particles end up close to the surface. The condition may occur with concrete mixes with low paste content.

Aggregate shadowing occurs when coarse aggregate particles end up close to the surface. The condition may occur with concrete mixes with low paste content.

Question: We've been asked to bid on a polishing project on a recently placed slab. The engineer had been concerned about controlling concrete drying shrinkage. He developed a concrete mix design optimization plan that resolved many of the problems that occur on industrial slabs on ground. The slab has performed very well, but this surface has many dark spots on the hard-troweled surface. What are these spots?

Answer: The problem you are observing is known as aggregate shadowing. This is being observed more often as concrete mixtures are being optimized for minimal drying shrinkage.

Aggregate shadowing can cause an initial concern over aesthetics. However, most often, the dark spots fade relatively quickly and blend to an acceptable appearance. With proper surface closure and adequate curing, the issue does not normally lead to subsequent deterioration.

Creating low-shrinkage concrete mixtures decreases the potential for random cracking and warping which have plagued the industry for years. However, these new mixes maximize the coarse aggregate topsize and total coarse aggregate content while minimizing the total paste content (cement and water). While this generally results in lower shrinkage, the decreased paste content sometimes results in less surface paste created during floating operations. Consequently, some coarse aggregate particles are ending up slightly closer to the surface with less paste cover. These aggregates may create dark spots or aggregate shadowing.

Question: If cement paste isn't transparent, how does aggregate “reflect” through?

Answer: The reflection can be created in several ways. First, the thinner paste above near-surface coarse aggregate receives greater compactive effort from the troweling process due to the underlying support from the aggregate particle. Greater compaction results in greater density and lower water-cement ratio, which can result in a darker paste, often called burnishing.

Aggregate particles typically become saturated during batching and placement, which can lower the water-cement ratio of the nearby paste, causing darkening. But subsequent to hardening, the saturated aggregate can maintain the nearby paste at a higher moisture condition, especially subsequent to moist curing. This can darken its appearance. While it's critical to properly cure the thin areas of paste, the author has not seen surface deterioration due to aggregate shadowing as long as the surface is closed and cured. The dark spots fade with drying.

Examine the surface the day after the first placement, as the shadows are seldom noticed during finishing.
Examine the surface the day after the first placement, as the shadows are seldom noticed during finishing.

Question: What if the surface is not completely closed above the aggregates?

Answer: If the surface is not completely closed over the aggregate particles, the paste tends to spall away, exposing the aggregate similar to mortar flaking. Sometimes the aggregate particle becomes loose in the pocket and needs repair. If surface openings are abundant, this can be problematic.

Question: Is the large topsize aggregate causing the problem?

Answer: No. More often, it is the mid-sized aggregate that ends up at the surface.