Fracture properties of slag-based alkali-activated seawater coral aggregate concrete

The development of cracks inside concrete is harmful to the structural durability of seawater coral aggregate concrete (CAC) subjected to an aggressive marine environment. Thus, three-point bending (TPB) tests were performed on the notched beams to determine the fracture properties of cement-based CAC and slag-based alkali-activated CAC (AACAC) at different initial crack-depth ratios (a(0)/H = 0.2, 0.3, 0.4, and 0.5) and alkaline contents (Na2O-to-binder ratios of 3%, 4%, and 6% by mass). The initial fracture toughness (K-IC(ini)), critical effective crack length (a(c)), and unstable fracture toughness (K-IC(ini))( )of the CAC and AACAC were calculated and analyzed based on the double-K fracture criterion. Then, the fracture energy (G(F)) and characteristic length (l(ch)) were introduced to analyze the energy consumption and brittleness of concrete. It was found that increasing the a(0)/H ratio reduced the initial cracking load (P-ini), peak load (P-max), a(c), G(F), and l(ch), but increased the P-ini/P(max )ratio, K-IC(ini), K-IC(un), and K-IC(ini)/K-IC(un) ratio, implying that a higher a(0)/H ratio had a detrimental effect on the crack propagation and increased the brittleness of the AACAC. Additionally, a higher alkaline content, i.e., a greater compressive strength of concrete, upgraded the values of P-ini, P-max, K-IC(ini), K-IC(un), and G(F) due to better mechanical interaction at paste-aggregate interfaces. However, the P-ini/P-max ratio, a(c), and l(ch) were steadily lowered with increasing concrete strength, justified by an increase in brittleness. Moreover, the notched AACAC beams contained higher a(c) (approximately 12.2% increase) and lower K-IC(ini)/K-IC(un)(approximately 4.6% decrease) than those of the notched CAC beams, demonstrating that the existence of alkali-activated materials (AAMs) was beneficial to crack propagation due to improved interfacial transition zone (ITZ) between the paste matrix and aggregates.

Automated summary

Increasing crack depth ratio decreases load-bearing capacity and fracture toughness of concrete while increasing brittleness, whereas higher alkaline content improves mechanical properties while decreasing brittleness.