Question: I am bidding on treating a concrete slab and one of the specified requirements is that there not be any birdbaths. The final surface will be flooded to see if this has been achieved. Do you have any suggestions?

Answer: The architect's goal of not having any birdbaths is not realistic. Birdbaths are low spots in the concrete surface that can trap water. Many owners and designers consider these to be finishing problems. However, even when reasonable standard tolerances are applied to the finish, birdbaths are possible and even likely. You should discuss this issue with the design team.

Question: What else should I discuss with the designer?

Answer: First, ask the designer if the floor is sloped to drain water. If the area isn't sloped to drain, water will collect. Even perfectly flat, perfectly level surfaces will not shed water. Since most commercial floors have some element of drainage, I am assuming we are discussing surface areas that are sloped.

Second, you should advise the designer that while the surface is sloped, birdbaths could exist. Since many designers believe there should not be any ponding in areas sloped to drains, you must check the specifications to verify that standard tolerances apply.

For example, many specifications include flatness/levelness numbers (FF/FL) but prohibit puddling. This is a problem.

Architects never want birdbaths to form, but they inevitably do.
Architects never want birdbaths to form, but they inevitably do.

ACI 117-06, “Specifications for Tolerances for Concrete Construction and Materials and Commentary,” states, “The levelness tolerances shall apply only to level slabs-on-ground or to level suspended slabs that are shored when tested.”

They do not apply to sloped areas. However, flatness (FF) numbers can be used to define the tolerance of sloped areas to being planar. The key word is tolerance. These surfaces are not perfectly flat. This means that it is acceptable to have gaps under a straight-edge. The higher the flatness number, the smaller the gaps.

To help solve this problem, ACI Committee 117 analyzed data from 600 profile lines to define the relationship between flatness number (FF) and the size of the gap beneath a 10-foot straightedge. The results are shown in Table 1.

As shown in Figure 1, if flatness tolerances are applied to an inclined surface, ponding (birdbaths) can occur.

Question: What does ACI 302, “Guide for Concrete Floor and Slab Construction,” recommend for sloped surfaces?

Answer: Before bidding the resurface project, it's important to see if the original floor was properly designed and installed. One of the primary causes for low spots or poor drainage is inadequate slope. Section 11.10 of ACI 302 states, “puddles or birdbaths on an outdoor concrete slab after a rain, or on a floor after hosing, characterize poor slab or floor surface drainage or serviceability.”

However, the minimum slope recommended (1/16 inch per foot) for interior floors is not sufficient to promote free drainage. A slope of ¼ inch per foot enables water flow.

Another primary cause listed in 302 is “failure to frequently check grades, levels, and slopes with long straightedges, and to properly build up low spots in areas detected.” This recommendation implies that concrete surfaces can be finished without any low spots. As discussed above, this is an unrealistic expectation; standard tolerances should apply.

Question: Are birdbaths only caused by finishing?

Answer: Of course not. Advise the designer that birdbaths could occur, no matter the surface treatment. Differential drying shrinkage causes slab warping to occur where edges of slabs-on-ground deform upward, creating deviations in surface elevation where birdbaths can develop.

Likewise, differential temperature contraction causes slab curling deformation. These slab distortions can start to occur relatively soon after placement, which is why flatness measurements must be recorded within 72 hours.

For elevated slabs, removing form-work and shoring can result in deflection sufficient to create birdbaths. Also, post-tensioning of balconies can result in changes in levelness and drainage.

Water will collect on any concrete surface that is not sloped to drain.
Water will collect on any concrete surface that is not sloped to drain.

Question: If we must live with birdbaths, is there an acceptable size they can be?

Answer: Some specifiers attempt to limit the depth and/or area of birdbaths. Applying standard tolerances from ACI 117 can define the expected depth of birdbaths on a floor surface with specified flatness (FF) criteria.

Table 1 – Relationship between Floor Flatness (FF) and gap under a 10-ft straightedge (from ACI 117-06).
Table 1 – Relationship between Floor Flatness (FF) and gap under a 10-ft straightedge (from ACI 117-06).

Table 1 shows the maximum gap that can be expected under a 10-foot straight-edge for various floor classifications, from conventional to superflat. As indicated, the higher the F-number, the shallower the birdbath. In fact, the number is related to the frequency and depth of the waves shown in Figure 1.

Low F-numbers have more frequent, more exaggerated waves, which result in deep birdbaths that generally encompass small areas. Higher F-numbers produce fewer, gentler waves that result in shallow birdbaths that typically envelope large areas. Therefore, trying to limit the depth and area of birdbaths using the F-number system is contradictory.

Question: How can we remedy birdbaths?

Answer: Areas of extreme low spots may be corrected somewhat by grinding adjacent high spots or, depending on the floor use, by installing a leveling course or topping. However, as discussed, birdbaths should be expected because perfect flatness is unlikely, and achieving greater flatness increases the cost.

Figure 1: If flatness tolerances are applied to an inclined surface, birthbaths, or ponding, can occur.
Figure 1: If flatness tolerances are applied to an inclined surface, birthbaths, or ponding, can occur.

ACI 117 suggests that the remedy for noncompliance with specified defined flatness tolerances should be included in the specification. However, 117 is silent on the issue of birdbaths specifically.

The American Society of Concrete Contractors (ASCC) has issued Position Statement No. 7. It states, “Birdbaths are an unavoidable consequence of a flatness tolerance. ASCC contractors will meet the flatness requirements but will not be responsible for corrective action to eliminate birdbaths.”

Scott Tarr is a partner with Concrete Engineering Specialists (CES). Before joining CES, he was a principal engineer in the Structural Evaluation Section of CTLGroup, Skokie, Ill., for 16 years. E-mailSTarr@ConcreteES.com.