Frequently Asked Questions

How much does the air entrainment from Conplast F297 reduce concrete compressive strength and how is this managed in mix design?

Entrained air reduces concrete compressive strength because the air voids replace solid cementitious paste in the concrete matrix. The reduction is approximately 5 per cent compressive strength for each 1 per cent increase in air content. For typical freeze-thaw protection requiring 5 per cent air content, this is a 25 per cent reduction in strength compared to a non-air-entrained concrete at the same mix proportions. However, in practice this strength reduction is partially offset by the workability improvement that entrained air provides — the improved workability allows a reduction in the water content of the mix, which partially compensates for the air-related strength loss. In mix design for freeze-thaw exposed concrete, the target strength is first determined (e.g., M35), then the mix is designed to achieve this strength at the increased air content (by reducing water-cement ratio to compensate for the strength reduction from air). The combined result is concrete that meets the structural strength requirement while providing the air void system for freeze-thaw protection.

What is the correct method to measure air content in fresh concrete on site when using Conplast F297?

Air content in fresh concrete is measured using the pressure method — the most common site measurement method. The two standard pressure-method apparatus are the Type B (cylinder) pressure meter and the Type A (bowl) pressure meter. The fresh concrete is placed in the container in standard layers, rodded or vibrated to consolidate each layer, the lid is sealed, the pressure gauge is zeroed (to subtract the air in the aggregate pores — the aggregate correction factor), and the test pressure applied. The dial reading gives the air content as a percentage of the fresh concrete volume. Measurements should be taken from the same location in the truck as the samples used for slump test and cube making — not from the initial or final discharge of the truck where mix variability is greatest. For continuous production with Conplast F297, air content should be checked on every truck initially and then at reduced frequency once production consistency is confirmed. Temperature significantly affects air content — hot conditions reduce entrained air, so dosage may need to be increased slightly in summer to maintain the target air content range.

Can Conplast F297 be used together with superplasticisers in the same concrete mix?

Yes. Conplast F297 can be used in combination with superplasticisers from the Conplast SP range or the Fosroc Auramix range, which is a common requirement for freeze-thaw resistant high-performance concrete in cold climate infrastructure. However, the compatibility of the specific admixture combination must be confirmed by trial mix, as some superplasticiser chemistries can interact with air-entraining admixtures and reduce the stability or volume of the entrained air. The general guidance is to add the superplasticiser and the air-entraining agent separately (not pre-mixed together before addition to the concrete) — add the superplasticiser first with the mix water, then add the air-entraining agent. Trial mixes with the actual cement, superplasticiser, and Conplast F297 at the intended project dosages should confirm the target air content is achievable with the combined admixture system before committing to production.