2.4. Preparation of the Concrete Samples

Cylindrical concrete specimens (Figure 3A) were cast from local sand (0/4 mm), white CEM-I type Portland cement (Aalborg White^®, Aalborg, Denmark), tap water and EC aggregates (4/8 mm, Liapor GmbH, Hallerndorf, Germany). Concrete mix proportion was adopted from [13] and is given in Table 2. This mix proportion was used to produce concrete samples with both coated and uncoated EC particles. To ensure that the same amount of a healing agent is present in all samples, bacteria-immobilized EC particles were weighted before the coating procedure. By doing so, the total weight of the active healing agent remained the same for all concrete samples, whereas each coating added a particular additional weight to the bacteria-immobilized EC. For each coating listed in Table 1, six concrete samples were cast. Three samples were used for viability studies and three for monitoring the crack closure.

An illustrative representation of the sample preparation (A) and crack width measurement procedure (B).

Concrete mix proportions.

Each concrete sample was mixed in a separate plastic cup, sealed and left to cure for 7 days (Figure 3A). After the curing period, specimens were split using a vise, as described in [24]. A radial tension crack separated the sample into two parts (Figure 3A). To keep the separated parts together, the split samples were tightened using waterproof power duct tape. To avoid a complete closure of cracks, 0.1 mm crack spacers were inserted before tightening the cracked specimens (Figure 3A). Finally, the cracked specimens were immersed in tap water (Figure 3B). The pH of the water was periodically measured during the incubation period.