Frequently Asked Questions

What makes Auracast HP different from the standard Auracast range and when is the HP grade needed rather than a standard PCE?

The Auracast range from Fosroc encompasses a family of PCE admixtures with varying molecular architectures — different backbone lengths, different side chain densities and lengths — that provide different combinations of water reduction, workability retention, and compatibility with different cement and SCM combinations. Standard PCE admixtures in the Auracast range provide excellent performance for most modern concrete applications: water reduction of 20 to 30 per cent, workability retention of 60 to 90 minutes at 30 degrees Celsius, and compatibility with most cement types used in India including OPC, PPC (fly ash), and PSC (GGBS). Auracast HP is the high-performance premium variant, formulated with a higher-molecular-weight PCE backbone and longer, denser polyethylene oxide side chains that provide: higher maximum water reduction (30 to 40 per cent versus 20 to 30 per cent for standard PCE), allowing concrete at W/CM below 0.22 to 0.25 — water-cement ratios that would be unworkable without PCE even at normal aggregate gradings; and enhanced steric hindrance from the longer side chains that provides stronger and more persistent workability retention in hot conditions. Auracast HP is needed rather than a standard PCE in: ultra-high-strength concrete above M80 where W/CM below 0.25 is required and standard PCE does not provide sufficient water reduction; high-performance concrete where the 90-minute workability retention of standard PCE is still inadequate for the required transit-plus-placement time; and concrete with very high silica fume content (above 10 per cent replacement) where the very high specific surface area of the silica fume demands more PCE for the equivalent workability.

What trial mix procedure should be used to calibrate the Auracast HP dosage for an ultra-high-strength M100 concrete design?

Calibrating an Auracast HP dosage for an ultra-high-strength M100 concrete design requires a structured trial mix programme because the relationships between admixture dosage, water reduction, workability, and strength are non-linear at the very low water-binder ratios used in M100 concrete — small changes in dosage or water content have large effects on workability and strength. The recommended trial mix procedure is as follows. First, establish the target concrete properties: compressive strength at 28 and 90 days (M100 typically requires 100 MPa mean strength at 28 days, allowing for the statistical safety margin above the characteristic strength), target slump flow (for SCC) or target slump (for vibrated concrete), maximum aggregate size, and maximum W/CM. Second, using the binder combination planned (OPC plus silica fume is typical for M100), establish the baseline mix proportions at the target W/CM (typically 0.20 to 0.22 for M100 with silica fume) without any admixture and confirm the mix is too stiff for placement — confirming that the admixture is needed for workability. Third, add Auracast HP in incremental dosage steps (0.3, 0.5, 0.8, 1.0, 1.2 per cent of cementitious material) while keeping the W/CM fixed, measuring slump or slump flow at each dosage step and recording the temperature and time from mixing. Fourth, identify the dosage that provides the target slump flow at the required time after mixing. Fifth, produce a full trial batch at the calibrated dosage, carrying out fresh property tests (slump flow, T500, J-ring if SCC), casting cubes for strength testing at 7, 28, and 90 days, and carrying out permeability testing (RCPT) at 28 and 56 days. The trial mix cubes should be tested before full production to confirm the strength target is met.

Can Auracast HP be used with recycled concrete aggregate in high-performance concrete production?

Using recycled concrete aggregate (RCA) in high-performance concrete (HPC) with Auracast HP introduces additional complexity compared to HPC with natural aggregate, primarily because recycled concrete aggregate has significantly different water absorption characteristics from natural crushed stone or river gravel. RCA typically has a water absorption of 4 to 8 per cent (by dry weight) compared to 0.5 to 1.5 per cent for natural aggregate — the high absorption of the RCA means that some of the mix water is absorbed into the porous RCA particles in the first 15 to 60 minutes after mixing, effectively reducing the free water available for cement hydration and workability and causing accelerated slump loss beyond what would be expected from the temperature and admixture dosage alone. This effect is compounded when Auracast HP is used: the high water-reduction capability of the PCE means that the total mix water is already very low, and any additional absorption by RCA reduces the available water to potentially deficient levels, causing rapid stiffening. For HPC with RCA and Auracast HP, the RCA should be used in a pre-wetted, saturated surface-dry (SSD) condition to prevent water absorption during mixing, or the admixture dosage should be increased and the mix design adjusted to account for the estimated RCA water absorption. The maximum percentage of RCA replacement for natural coarse aggregate in HPC applications is typically limited to 30 to 50 per cent to maintain the target strength and durability properties — 100 per cent RCA replacement is generally not compatible with ultra-high-strength concrete design.