The aim was to trial a low-carbon material to assess its performance – slump, flow, and placement, its strength gain profile and its ultimate strength gain performance, as well as measure how much CO2 would be saved against the practicality of using a new kind of material within a large-scale road construction project wherever possible. The long-term performance of Cemfree would also be monitored for future reference. As Cemfree contains no ordinary Portland cement (OPC), up to 80% of CO2 emissions could typically be saved on such a project.

The intention was to trial up to 100m3 of Cemfree concrete in varying applications where the risk of concrete requiring replacement through structural or physical failure was deemed low e.g. in ancillary concrete for kerbing, drainage, low strength infills, lightly used pavements, and minor structures with little risk of structural failure or cost of replacement. After stakeholder discussion, the trial was limited to kerbing concrete and a pipe headwall.

The batching procedure was modified slightly as a different admixture would be used as no retardation is needed with Cemfree. Trials were done to check that this process would work with the concrete batched in one location, transported for 30-90 mins (traffic dependant) to a second location for pouring into blocks of 1-1.5m3. This was repeated to give the plant operators and technical staff a feel for the mixes and how the concrete reacted to placing and finishing. Cemfree’s long open life meant that more concrete could be delivered at once for kerbing, reducing vehicle movements, impact on traffic and air quality.

The first trial took place on the 27th January 2020. This was a trial for the headwall at the drainage outlet. The concrete specified for this pour was a C40/50 at S3 consistency due to the ground conditions’ specification and in view of the actual retained height. It was agreed that a nominal C30 concrete would satisfy structural requirements. On arrival, the concrete was tested for slump (160 mm) and air content (5.1%) and was poured using a skip – there were no adverse comments from the operators carrying out the work. They reported that the material behaved in the same way as ‘normal’ concrete during placing, but that it was slightly more difficult to get a good finish on the floated surface of the concrete.

Slump-testing over time showed a steady decline in slump from an initial 160mm to 20mm at 3 hours. It was noted that the concrete, even at a slump of 20mm, still appeared to be thoroughly workable.

As the weather was cold but above freezing, there were concerns that the concrete would not be strong enough to strip at 24 hours. Cube strengths of around 2.5 N/mm2 (cured at 20oC) confirmed this and the concrete was left until the following day. The concrete was stripped between 48 hours-50 hours old, with no damage to face or edges of the concrete.

The second trial was for the kerbing concrete, this took place on the 31st January 2020. The first mix delivered was ‘wetter’ than the kerbing crew were used to but became more solid at rest. The mix design was adjusted for the second load which showed some ‘balling’. When broken open, the balls proved to be well mixed material rather than the dry balls of aggregate/cementitious binder usually found in very wet mixes from dry batch plants. It has been conjectured that these are formed by the wet/nearly dry material being turned over in the truck mixer drum and not being broken up by the mixer blades inside. The mix was used both to bed kerbs and back the kerbs. The kerb-layer’s comments were that it was possible to use the material, though it was slightly harder to move around with shovels but using slightly different methods of work could be used successfully.