International Concrete Abstracts Portal

Portland cement has played a significant role in the construction of major infrastructure and building structures. However, in light of the substantial CO2 emissions associated with its production, there is a growing concern about environmental issues. Accordingly, the development of eco-friendly alternatives is actively underway. Geopolymer represents a class of inorganic polymers formed through a chemical interaction between solid aluminosilicate powder with alkali hydroxide and/or alkali silicate compounds. Concrete made with geopolymers, as an alternative to portland cement, generally demonstrates comparable physical and durability characteristics to ordinary portland cement (OPC) concrete. Research on the material properties of geopolymer concrete (GPC) has made extensive progress. However, the number of large-scale tests conducted to assess its structural performance is still insufficient. Additionally, there is a shortage of comprehensive studies that compile and analyze all the structural experiments conducted thus far to evaluate GPC’s potential. Therefore, this study aimed to compile and analyze a number of bond, flexural, shear, and axial strength tests of GPC to assess its potential as a substitute for OPC and identify its distinctive characteristics compared to OPC. As a result, it is considered that GPC can be used as a substitute for OPC without any structural safety issues. However, caution is needed in terms of deflection and ductility, and additional experiments are deemed necessary in the aspect of compressive strength of large-scale members.

Recent research data was evaluated with the aim of extending the applicability of using deformed steel fiber-reinforced concrete (SFRC) to enhance the shear strength of beams and one-way slabs. Experimental results were assessed for influences on the shear strength of SFRC members that do not contain stirrups of factors, including size effect, concrete density (normalweight and lightweight) and compressive strength, fiber-volume fraction (Vf), and the longitudinal steel reinforcement ratio. Estimates of steel stresses in longitudinal bars at the time of shear failure were carried out to identify differences in members with distinct longitudinal steel ratios and bar grades, consistent with the range of flexural design parameters in ACI 318-19. Results of these analyses and a reliability investigation of design equations applicable to members without fibers were used for proposing new provisions for the shear design of SFRC beams and one-way slabs based on the ACI 318-19 shear-strength model.

Seven one-half-scale reinforced concrete coupling beams, designed using ACI 318-19, were tested with constant stiffness axial restraint. The test variables were the span-depth ratio, reinforcement configuration (conventional or diagonal), primary reinforcement ratio and bar diameter, and level of axial restraint. Six beams consisted of three nominally identical pairs, with the two beams in each pair tested at a different level of axial restraint. The two conventionally reinforced beams reached peak strength at 2.0 and 3.0% chord rotation and experienced rapid post-peak strength degradation with the opening of diagonal cracks and the formation of splitting cracks along the longitudinal reinforcement. Strength degradation in diagonally reinforced beams initiated with buckling of diagonal reinforcement, and variation in axial restraint on identical pairs of beams did not lead to a significant difference in deformation capacity. Deformation capacity was larger for beams with a larger diagonal bar diameter, which corresponded to a larger reinforcement ratio and a larger ratio of transverse reinforcement spacing to diagonal bar diameter (s/db). For the diagonally reinforced test beams, the maximum measured shear strength reached as high as 2.4 times the nominal shear strength computed using ACI 318-19 and exceeded the 0.83 √____fc ′ A cw MPa (10 √ ____fc ′ A cw psi) limit on nominal shear strength by more than a factor of 2.0 in the test with the smallest span-depth ratio. Based on strut-and-tie behavior, modifications to the ACI 318-19 equation to include axial load were examined. When the location of the compressive strut and tension tie at the beam ends was consistent with nominal moment calculations, the resulting ratio of the average maximum measured shear strength in the positive and negative loading directions to shear strength calculated using the modified equation ranged from 1.16 to 1.33. For the diagonally reinforced beams, a larger spandepth ratio, bar size, and reinforcement ratio were associated with larger rotation at yielding and larger effective flexural rigidity.

Reinforced concrete (RC) structure design codes stipulate various design limits to prevent the brittle failure of members as well as ensure serviceability. In the structural design of RC walls, the maximum shear strength is limited to prevent sudden shear failure due to concrete crushing before the yielding of shear reinforcement due to over-reinforcement. Despite the increase in wall shear strength provided by a compression strut, the maximum shear strength limit for walls in the ACI 318-19 code is the same as the maximum torsional strength. Consequently, the shear strength of large-sized walls with high-strength concrete is limited to an excessively low level. The ACI 318-19, Eurocode 2, CSA-19, and JSCE-17 standards provide similar equations for estimating wall strength, but their maximum shear strength limits for walls are all different. In this study, experimental tests were conducted on nine RC wall specimens to evaluate the maximum shear strength. The main variables of the specimens were the shear reinforcement ratio, compressive strength of concrete, and the failure mode. The experimental results showed that the maximum load was reached after the yielding of shear reinforcement even when the shear reinforcement ratio was 1.5 times higher than the maximum shear reinforcement ratio specified in the ACI 318-19 code. In addition, the measured shear crack width of all specimens at the service load level was less than 0.42 mm (0.017 in.). The shear strength limits for walls in the current codes were compared using 109 experimental results failing in shear before flexural yielding or shear friction failure, assembled from the literature. The comparison indicated that the ACI 318-19 code limit underestimates the maximum shear strength of walls, and it particularly underestimates the maximum shear strength of walls with high-strength concrete or barbell-shaped cross-sections. Additionally, this study proposes an equation for estimating the maximum shear strength limit of walls based on the truss model. The proposed equation predicted the maximum shear strength of RC walls with reasonable accuracy.

Noncorrosive fiber-reinforced polymer (FRP) reinforcement presents an attractive alternative to conventional steel reinforcement, which is prone to corrosion, especially in harsh environments exposed to deicing salt or seawater. However, FRP rebars’ lower axial stiffness leads to greater crack widths when FRP reinforcing bars elongate, resulting in significantly lower flexural stiffness for FRP-reinforcing bar-reinforced concrete members. The deeper cracks and larger crack widths also reduce the depth of the compression zone. Consequently, both the aggregate interlock and the compression zone for shear resistance are significantly reduced. Additionally, due to their limited tensile ductility, FRP reinforcing bars can rupture before the concrete crushes, potentially resulting in sudden and catastrophic member failure. Therefore, ACI Committee 440 states that through a compression-controlled design, FRP-reinforcing bar-reinforced concrete members can be intentionally designed to fail by allowing the concrete to crush before the FRP reinforcing bars rupture. However, this design approach does not yield an equivalent ductile behavior when compared to steel-reinforcing bar-reinforced concrete members, resulting in a lower strength reduction, ϕ, value of 0.65. In this regard, using FRP-reinforcing bar-reinforced ultra-high-performance concrete (UHPC) members offers a novel solution, providing high strength, stiffness, ductility, and corrosion-resistant characteristics. UHPC has a very low water-to-cementitious materials ratio (0.18 to 0.25), which results in dense particle packing. This very dense microstructure and low water ratio not only improves compressive strength but also delays liquid ingress. UHPC can be tailored to achieve exceptional compressive ductility, with a maximum usable compressive strain greater than 0.015. Unlike conventional designs where ductility is provided by steel reinforcing bars, UHPC can be used to achieve the required ductility for a flexural member, allowing FRP reinforcing bars to be designed to stay elastic. The high member ductility also justifies the use of a higher strength reduction factor, ϕ, of 0.9. This research, validated through large-scale experiments, explores this design concept by leveraging UHPC’s high compressive ductility, cracking resistance, and shear strength, along with a high quantity of noncorrosive FRP reinforcing bars. The increased amount of longitudinal reinforcement helps maintain the flexural stiffness (controlling deflection under service loads), bond strength, and shear strength of the members. Furthermore, the damage-resistant capability of UHPC and the elasticity of FRP reinforcing bars provide a structural member with a restoring force, leading to reduced residual deflection and enhanced resilience.