3.2. Preparation of Test Specimens and Fresh Properties Testing

All the basic materials, including water, were stored under controlled laboratory conditions at a temperature of 20 ± 2 °C and relative humidity higher than 60%. The target temperature of the materials before mixing of CBMs was 20 °C. Each gravel fraction’s initial water content was measured 7 days before mixing the MOC and the water required for the fraction to reach a fully saturated surface dry state was added to the aggregate grains kept in a plastic container. After mixing of aggregate grains and added water, the container was sealed with a lid. The water content of the 4–11 and 8–16 fractions, before the mixing of MOC, was 2.76% and 2.56%, respectively and surface water was considered in calculating the water that was ultimately added to the mixer.

The mixing procedures are described in the Testing protocols of the RRT⁺ programme [33]. The mixing procedure of MCP closely followed the procedure described in standard EN 196-3 [36], and the procedure of mixing MOM and MOC followed the procedure from the standard EN 480-1 [37]. For each of the three CBM mixtures, 45 test specimens were made—9 for each testing stage. Tests were carried out in 5 stages; at the ages of 30, 36, 48, 72 h, and 7 days (168 h).

After the mixing of MCP and MOM, the mixture temperature was measured and consistency was evaluated by flow table test according to the EN 1015-3 [38]. Due to the MCP’s high flowability, a modified procedure was applied, which excluded dropping (15-times) of the flow table. The procedure is described in [39] and the obtained value is referred to as the slump-flow value. Fresh density was evaluated following the EN 1015-6 [40] procedure. After that, mixtures were moulded into prism-shaped moulds with standard dimensions of 40 × 40 × 160 mm³. Three specimens were made for every single test at a particular age. For standard tests of elastic modulus, the standard prism moulds were halved to dimensions 40 × 40 × 80 mm³, where EN 196-1 [41] tolerance requirements were considered (Figure 2). Six specimens with dimensions of 40 × 40 × 80 mm³ were made for each stage of testing (3 for compressive strength test and 3 for determination of elastic modulus and σ-ε behaviour in compression).

An example of a set of moulds filled with cement paste (MCP) mixture for testing at a single age.

In the case of modified ordinary concrete (MOC), the mixture temperature was measured and consistency and fresh density were evaluated following EN 12350-2 [42] and EN 12350-6 [43] procedures, respectively. Due to slump values [42] higher than 210 mm and associated reduced sensitivity of the test method [44], the spread of the specimen after the slump measurement was recorded as well. The method is similar to that used for the MCP specimen. For each testing age, three prism specimens with dimensions of 100 × 100 × 400 mm³, three cube specimens with dimensions of 150 × 150 × 150 mm³ and three cylinders with a diameter of 100 mm and height of 200 mm (100/200 mm cylinder) were produced. All the specimens in moulds were stored at relative humidity higher than 95% and a temperature of 20 ± 2 °C. Specimens for determination of elastic modulus and Poison’s ratio were equipped by double direction strain gauges, measuring longitudinal and transverse strains simultaneously.

Specimens for measuring temperature and electrical conductivity of mixtures were cast into the custom-made extruded polystyrene container, and the upper surface was covered with a polyvinyl sheet to avoid water evaporation, as shown in Figure 3. Inside dimensions of the container were 5 × 10 × 15 cm³, to ensure complete immersion of the sensors into the mixture. For each of the three mixtures, 2 parallel test specimens were prepared.

Custom-made container (left) and moulded specimens prepared for temperature and conductivity measurements (right).

The custom-made containers were of the same dimensions as those used in [8]. In this way, the comparison of test results and determination of setting and hardening parameters of the CBMs at an early age is possible.