Water-Tightness Testing Concrete Basins per ACI 350.1

The Problem

A water-tightness test on a concrete basin sounds simple: fill it, wait, measure the drop. The catch is that the water surface drops for reasons that have nothing to do with leaks. A big open basin evaporates measurably every day, rain adds water back, and new concrete drinks for its first several days full. Run the test naively and you'll fail a tight basin in August or pass a leaky one in a drizzle. ACI 350.1 structures the test so the number you compute is actually leakage [1].

The Approach

The correction comes first. The elegant version is the evaporation pan: float a calibrated pan of water in the basin, and it sees the same evaporation and rainfall as the basin surface, so the difference isolates leakage:

Δh = drop_basin − Δh_pan

Or measure the terms separately: Δh = drop_basin − evaporation + rainfall. The corrected drop converts to volume and normalizes to the standard 24-hour basis:

%/24hr = V_lost / V_test × (24 / t_hr) × 100

checked against the allowable percentage of volume per 24 hours from the spec. The tool also inverts the whole chain into the pre-test planning number: the allowable drop in inches over your test duration, so the crew knows before the test starts what reading means pass.

Calculator showing pan-corrected drop, volume lost, percent per 24 hours, and the allowable drop for the test duration
Figure 1: The pan-corrected loss rate against the ACI 350.1 allowable, with the pre-test allowable drop in inches.

Two protocol points do most of the work of making the number defensible. First, stabilization: ACI 350.1 requires the basin sit full for a stabilization period (commonly 3 days) before the measured test starts, or an early “failure” is just absorption. Second, measurement precision governs duration: with a good hook gauge you can resolve about 1/16 in., so if the allowable drop over 24 hours is smaller than that, extend the test — the standard allows it for exactly this reason, and the tool flags allowable drops under a quarter inch.

Running the test so the numbers mean something
  • Measure from fixed reference points at multiple locations, at the same time of day — thermal expansion of the water is real over a big surface.
  • Valves and bulkheads count as leaks: a weeping plug valve fails the basin on paper. Isolate and verify them before blaming the concrete.
  • Most specs distinguish acceptable exterior dampness from unacceptable flowing water regardless of the volumetric result — walk the walls, don't just read the gauge.

Single file, vanilla JS, localStorage persistence. The contract spec governs the criterion, stabilization period, and retest provisions; AWWA D110/D115 have their own tightness provisions for circular prestressed tanks [2][3].

The tool: Basin Hydrostatic Test Calculator.

References

  1. [1] ACI 350.1 — Specification for Tightness Testing of Environmental Engineering Concrete Containment Structures. https://www.concrete.org/
  2. [2] AWWA D110 / D115 — tightness testing provisions for circular prestressed tanks, where applicable.
  3. [3] Contract specifications — governing criterion, stabilization period, and retest provisions.