Frequently Asked Questions

How does Auramix BCI differ from a surface-applied retarder for achieving exposed aggregate concrete finishes?

Traditional exposed aggregate concrete uses a surface retarder — a liquid or gel applied to the face of the formwork before the concrete is cast. The retarder chemical on the form face contacts the cement paste at the cast surface and retards its hydration. When the form is struck, the retarded surface paste (to a depth of 2 to 6 mm depending on the product and application rate) is easily washed off to expose the aggregate. Auramix BCI is an integral admixture incorporated into the concrete mix that achieves the same result but through a different mechanism. The Auramix BCI in the mix migrates with the bleed water to the concrete surface (for horizontal slabs) or the form face (for vertical elements and precast), concentrating in the near-surface zone and retarding the surface paste from the inside. The advantage of an integral admixture approach is that the retarder concentration in the concrete surface is determined by the admixture dosage in the mix — it is consistent across every pour from the same batch, eliminating the variability in exposure depth that can occur when form-applied retarder is applied unevenly by different operatives across a large formwork area. For large-scale precast production where consistency of finish is critical, Auramix BCI provides more reliable and reproducible results than a form-applied retarder.

What factors determine the depth of aggregate exposure with Auramix BCI and how is the exposure depth controlled?

The depth of aggregate exposure with Auramix BCI depends on three main factors: the admixture dosage (higher dosage creates deeper retardation and greater exposure depth); the timing of the washing operation after casting (washing later allows the surface retardation to penetrate slightly deeper before washing, increasing exposure depth — but waiting too long risks the surface hardening beyond the retardation depth); and the ambient temperature (higher temperature accelerates both the bulk hydration and the surface retardation, so washing must be timed earlier at high temperature). For a target exposure depth of 3 to 5 mm (revealing the coarse aggregate particles but leaving most of the fine aggregate and paste intact for a medium exposed aggregate texture), the admixture dosage and washing timing are established by a series of trial pours at the ambient temperature and concrete mix proportions to be used in production. The washing trial should be conducted at 8, 12, 16, and 24 hours after casting to identify the optimal washing window that achieves the target texture. Once the optimum window is established, it must be maintained consistently in production.

How is the wash water from Auramix BCI exposed aggregate washing managed on a construction site?

The wash water from the exposed aggregate washing operation contains cement paste, fine aggregate particles, and the residual Auramix BCI admixture. This wash water is highly alkaline (pH 11 to 13) due to the dissolved cement calcium hydroxide and must be managed as construction waste — it must not be allowed to run uncontrolled into stormwater drains, watercourses, or public sewers without treatment to reduce the pH to an acceptable level (typically pH 6 to 9). On precast factory sites, the wash water is typically collected in a settlement lagoon where the solids settle and the clarified water is recycled in the factory for further use. The biodegradable formulation of Auramix BCI means the admixture component of the wash water is not a persistent environmental concern. On in-situ construction sites, the wash water must be contained and collected — typically by providing a perimeter bund around the washing area and a pump to collect the wash water for controlled disposal. Check the project environmental management plan for specific requirements for wash water management.