Self-healing concrete has emerged as one of the prospective materials to be used in future constructions, substituting conventional concrete with the view of extending the service life of the structures. As a proof of concept, over the last several years, many studies have been executed on the effectiveness of the addition of self-healing agents on crack sealing and healing in mortar, while studies on the concrete level are still rather limited. In most cases, mix designs were not optimized regarding the properties of the fresh concrete mixture, properties of the hardened concrete and self-healing efficiency, meaning that the healing agent was just added on top of the normal mix (no adaptations of the concrete mix design for the introduction of healing agents). A comprehensive review has been conducted on the concrete mix design and the impact of healing agents (e.g., crystalline admixtures, bacteria, polymers and minerals, of which some are encapsulated in microcapsules or macrocapsules) on the properties of fresh and hardened concrete. Eventually, the remaining research gaps in knowledge are identified.

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The use of macrocapsules in self-healing applications offers a potential benefit by carrying a larger amount of healing agent in comparison with microcapsules. However, the application of macrocapsules is still limited to paste and mortar levels on lab-scale. This is due to a concern that most capsules might be broken when mixed with concrete components. In this study, cementitious tubular capsules were used and they were considered as a partial replacement of coarse aggregates (2 vol% gravel). The capsules have a dimension of 54 mm and 9 mm in length and outer diameter, respectively. A water-repellent agent (WRA) was entrapped in the capsules as a proposed agent to seal the crack. Initial results revealed high survivability of capsules during concrete mixing: 100% survival ratio when tested in a drum mixer and 70–95% when tested in a planetary mixer. The mechanical and self-sealing properties of concrete containing embedded capsules were evaluated. With the addition of capsules, around 8% reduction of compressive strength was noticed, but no further effect on splitting tensile strength was detected as compared with concrete without capsules. Ultrasonic pulse velocity (UPV) tests confirmed that the presence of capsules also did not significantly affect the compactness of the hardened concrete. Furthermore, the embedded capsules were able to break when a crack was introduced and it was found that 90% sealing efficiency was achieved by capsule-based concrete as a result of the successful release of sealing agent into the crack.

Particle packing models (PPMs) have been developed and evolved since 1929 and allow to formulate the proportioning of concrete mix constituents with the minimal voids ratio. In the past, this tool has led to important innovations in concrete technology, as the development of ultra-high performance concrete. Also with regard to the design of ecological concrete, PPM is a promising technique. In our ongoing research on the design of ecological concrete incorporating recycled concrete aggregates (RCAs), the model of Dewar will be applied. This theory is however mainly developed and validated for concrete mixtures with natural aggregates. In order to validate the theory for RCAs and understand the effect of variability on the aggregate's main characteristics (e.g. voids ratio and mean size) on the voids ratio diagram, a sensitivity analysis has been performed and is reported in the paper. It includes a sensitivity in the prediction of Un (= voids ratio of mixes of fine and coarse aggregates) and n (= fine fraction content) to the various parameters in the model of Dewar (U1 = voids ratio of the fine fraction, U0 = voids ratio of the coarse fraction, r = size ratio, m = spacing factor, kint and kp = empirical factors for determining the notional width factor). The results are graphically presented for one specific example and show that variation of U0 affects mainly the Un and n values of changing point B while variation on U1 affects mainly the Un value of point E and the n value of point B and point C. With regard to the size ratio, the effect of a variation on r depends on the size ratio range considered. Variation on m has a larger impact on n values than Un values at the changing points and variation on the kint results in a variation of Un and n values, mainly for point B and C. The effect of variations of kp on Un and n is limited.