Ji-Hyung Lee

Microstructural Investigation of Heat-Treated Ultra-High Performance Concrete for Optimum Production

For optimum production of ultra-high performance concrete (UHPC), the material and microstructural properties of UHPC cured under various heat treatment (HT) conditions are studied. The effects of HT temperature and duration on the hydration reaction, microstructure, and mechanical properties of UHPC are investigated. Increasing HT temperature accelerates both cement hydration and pozzolanic reaction, but the latter is more significantly affected. This accelerated pozzolanic reaction in UHPC clearly enhances compressive strength. However, strength after the HT becomes stable as most of the hydration finishes during the HT period. Particularly, it was concluded that the mechanical benefit of the increased temperature and duration on the 28 day-strength is not noticeable when the HT temperature is above 60 °C (with a 48 h duration) or the HT duration is longer than 12 h (with 90 °C temperature). On the other hand, even with a minimal HT condition such as 1 day at 60 °C or 12 h at 90 °C, outstanding compressive strength of 179 MPa and flexural tensile strength of 49 MPa are achieved at 28 days. Microstructural investigation conducted herein suggests that portlandite content can be a good indicator for the mechanical performance of UHPC regardless of its HT curing conditions. These findings can contribute to reducing manufacturing energy consumption, cost, and environmental impact in the production of UHPC and be helpful for practitioners to better understand the effect of HT on UHPC and optimize its production.

Shear Friction Strength based on Limit Analysis for Ultra-High Performance Fiber Reinforced Concrete

Ultra High Performance Fiber Reinforced Concrete (UHPFRC) is distinguished from the normal concrete by outstanding compressive and tensile strength. Cracked normal concrete resists shear by aggregate interlocking while clamped by transverse reinforcement, which is called as shear friction theory. Cracked UHPFRC is expected to have a different shear transfer mechanism due to rather smooth crack face and post-cracking behavior under tensile force. Twenty-four push-off specimens with transverse reinforcement are tested for four different fiber volume ratio and three different ratio of reinforcement along the shear plane. The shear friction strength for monolithic concrete are suggested by limit analysis of plasticity and verified by test results. Plastic analysis gives a conservative, but reasonable estimate. The suggested shear friction factor and effectiveness factor of UHPFRC can be applied for interface shear transfer design of high-strength concrete and fiber reinforced concrete with post-cracking tensile strength.

Effect of Specimen Size and Initial Crack Location on Flexural Shear Behavior of UHPFRC

Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) has outstanding post-cracking behavior which is explained by hardening part due to micro cracking and softening part of crack localization. Most UHPFRC structures are governed by concrete tensile behavior represented as critical crack localization, unlike ordinary reinforced concrete structures are designed to be governed by the yielding of steel reinforced rebar. In this research, crack localization characteristics of UHPFRC were investigated on three point bending test of twenty-four single notched prisms with varied types of location of notch, height of specimens and fiber volume ratio. The tensile fracture properties of UHPFRC were found out based on two parameter fracture model from the three point bending test results of center notched prism and direct tension test results of doubly notched plate. The structural implications of size effect in UHPFRC was examined by the fracture properties in terms of fracture process zone and specimen size. Within the height of 500 mm, size effect on flexural behavior was found only in structural ductility, not in the maximum strength. With respect to the location of notch, crack growth orientation were investigated and analyzed by maximum stress criteria. The results show that in-plane shear behavior of UHPFRC is much higher than ordinary concrete and it implies that the in-plane shear behavior of UHPFRC should be considered for designing structural elements.

Shear Strength of UHPFRC Beams Without Stirrups: Fracture Mechanics Approach

Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) has outstanding capacity in post-cracking phase with residual tensile strength and a large tensile strain. The stress redistribution after dispersed crack formation enhances shear strength of UHPFRC I-beams. This paper investigates in-plane shear behavior in the web of UHPFRC I-beams without stirrups of varying shear span ratio and beam depths. And this study proposes a new parameter to relate the stress redistribution capacity at material level and structural performance as structural level in terms of characteristic length. From the test results, all specimens for shear strength were found to fail by critical crack localization in the diagonal crack zones. The tensile behavior of UHPFRC at material level is necessary to understand UHPFRC structural behavior which fails by crack localization. Thereby fracture mechanics approach may lead to more reasonable explanation. Characteristic length representing a hardening tensile behavior of UHPFRC is defined as one of representative parameters for fracture behavior. In this paper the effective length of concrete tension strut in diagonal crack zone is suggested as a characteristic length for structural element with a new brittleness factor. The shear strength equation for UHPFRC beam without stirrup is proposed using the lower bound approach with the brittleness factor based on fracture mechanics.