Journal of Structural Engineering

Forum papers are thought-provoking opinion pieces or essays founded in fact, sometimes containing speculation, on a civil engineering topic of general interest and relevance to the readership of the journal. The views expressed in this Forum article do not necessarily reflect the views of ASCE or the Editorial Board of the journal.

Using recycled concrete aggregates (RCAs) as coarse aggregate in concrete has the potential to supplement current natural aggregate reserves, divert construction and demolition debris from landfills, and promote the adoption of sustainable civil infrastructure. As many of the design equations used to calculate structural concrete properties are based on empirical data for natural aggregate concrete, using these equations for RCA concrete may not be applicable. This study examined the bond of reinforcement in concrete produced using RCA as coarse aggregate. One natural aggregate (NA) and three RCA sources were evaluated and used as coarse aggregate in 14 separate concrete mixtures with four compressive strength levels. Various concrete mechanical properties, including compressive strength, splitting tensile strength, modulus of rupture, and fracture energy, were tested, and correlations between these properties and reinforcement bond were studied. The reinforcement bond was measured using 48 beam-end specimens incorporating several bonded lengths. The results showed that bond strength of RCA concrete was reduced by up to 21% in comparison to NA concrete, and that there was a strong correlation between bond strength and coarse aggregate crushing strength. A regression model was developed to relate bond strength to coarse aggregate strength, concrete compressive strength, and the bonded length. Using this model, experimentally predicted development lengths were calculated to be up to 9% greater for RCA concrete members in comparison to NA concrete members. Overall, this study was directed at providing guidance on the evaluation of multiple RCA sources and their respective impact on the bond of reinforcement in structural concrete.

Efforts are being made to achieve more efficient operation of buildings, with the goal of reducing the construction industry’s contribution to energy consumption and greenhouse gas emissions. That contribution also includes the energy embodied in structures; that is, the energy consumed in the processes of extracting, manufacturing, transporting, and installing construction materials (including recycled materials) and elements. In particular, in spite of the use of additives such as fly ash, reinforced concrete (RC) structures, which are large consumers of cement, are responsible for a sizable proportion of worldwide carbon emissions. These emissions can be reduced significantly through the more efficient use of both concrete and steel that can be achieved by optimization. Modern optimization tools are now available that make it possible to perform large volumes of calculations efficiently that are applicable to a wide variety of structural engineering problems. This study presents an optimization approach developed with a view to allowing decision makers to balance sustainability and economic objectives. To illustrate this approach, an RC frame under gravity and lateral loads is considered in this paper. It was found that, depending upon the parameter values used in the calculations, the design optimized with respect to the ${\mathrm{CO}}_2$ footprint yields a ${\mathrm{CO}}_2$ footprint that is lower (by 5% to 10%) than the design optimized with respect to cost. The reduction can be smaller for low-rise structures and other structures with predominantly tension-controlled members. However, for structures whose members predominantly experience large compressive forces, such as high-rise buildings, the reduction may be more significant. This also may be true of certain prestressed and poststressed concrete members. Additional research aimed at ascertaining the extent to which this is the case is warranted.

Residential energy use is a significant contributor to greenhouse gas emissions and climate change. Reducing energy efficiency in conventional wood-framed houses are thermal bridges: direct paths that allow heat to flow through the studs instead of the insulation. One suggestion for reducing thermal bridging is the staggered stud (SS) wall. SS walls use $2\times 6$ bottom and top plates with $2\times 4$ studs alternating between sides of the wall. This allows sheathing to be applied to both sides while eliminating thermal bridges. A literature review has revealed a lack of laboratory test data for SS walls used as shear walls, raising concerns about their safety. The objective of this study was to evaluate the seismic performance of typical SS walls and compare their behavior to similar conventional walls. Monotonic and cyclic laboratory tests were conducted with and without gypsum wallboard. The staggered stud specimens performed similarly to conventional walls. Some minor differences were identified, but the data raised no immediate concerns about the use of SS walls as an energy efficient option in areas of seismic hazard.

This paper presents lateral-loading test results of lightweight wood-frame shear walls with ply-bamboo sheathing panels. Twenty shear walls of five different types of sizes, nailing, and loading schemes were tested under monotonic and cyclic loading programs in accordance with international standards. Two different nails were selected for their good constructability and used for constructing the shear wall models, as follows: (1) 51-mm long nail-gun-driven 6 d spiral nails, and (2) 51-mm long staple nails. Test results show that the application of ply-bamboo sheathing panels can satisfy the shearing capacity requirements for shear walls designed as structural elements. The ply-bamboo panel sheathed lightweight wood-frame shear walls have comparable load and displacement characteristics in addition to capacities as wood panel sheathed shear walls in the literature. The results obtained from the research reported in this paper can promote the application of wood and bamboo material-based seismic-resistant structures, which have the potential to be environmentally friendly.

The arch, as a structural element, has been used since ancient times, originally in stone or timber, more recently in reinforced concrete or steel. However, the more modern materials involving steel and concrete are highly energy and carbon intensive. Hardwood timber, although renewable, takes between 30 and 50 years to grow when used structurally. But a highly sustainable tropical material, bamboo, a woody grass, requires only 4–5 years of growth before its considerable compressive and tensile strength can be used for developing structural arches for load-bearing applications. This paper investigates the structural performance of bamboo-concrete (Bamcrete) composite parabolic tied arches of span 4.5 m with a rise-to-span ratio of 0.2 under a uniformly distributed loading. An innovative technique of integrating two slender bamboos of the Dendrocalamus strictus genera together to form a rigid joint, termed a Haritha-IITD Bamcrete (HIB) joint, is described. The paper also discusses the test set-up for loading and monitoring the performance of the Bamcrete arch. Furthermore, a comparison is made between the experimental results and computer modeling using STAAD software, where good correlation is obtained within the linear response range. The techniques involved in making a Bamcrete arch are so successful as to suggest strong potential exists for widespread use as a sustainable and renewable structural load-bearing element.

The production of portland cement—the key ingredient in concrete—generates a significant amount of carbon dioxide. However, due to its incredible versatility, availability, and relatively low cost, concrete is the most consumed synthetic material on the planet. One method of reducing concrete’s contribution to greenhouse-gas emissions is the use of fly ash to replace a significant amount of the cement. This paper compares two experimental studies that were conducted to investigate the shear strength of full-scale beams constructed with both high-volume fly ash concrete (HVFAC)—concrete with at least 50% of the cement replaced with fly ash—and conventional concrete (CC). The primary difference between the two studies involved the amount of cementitious material, with one mix having a relatively high-total cementitious content [$502\mathrm{kg}/{\mathrm{m}}^3$ ($850\mathrm{lb}/{\mathrm{yd}}^3$)] and the other mix having a relatively low-total cementitious content [$337\mathrm{kg}/{\mathrm{m}}^3$ ($570\mathrm{lb}/{\mathrm{yd}}^3$)]. Both HVFAC mixes used a 70% mass replacement of portland cement with Class C fly ash. Each of these experimental programs consisted of 16 beams—eight constructed from HVFAC and eight constructed from CC—with three different longitudinal reinforcement ratios. The beams were tested under a simply supported four-point-loading condition. The experimental shear strengths of the beams were compared with both the shear provisions of selected standards (United States, Australia, Canada, Europe, and Japan) and a shear database of CC specimens. This comparison indicates that the HVFAC beams possess superior shear strength compared with the CC beams.

Binary and ternary binders are recommended for the production of concrete mixes used in infrastructure because these concretes are environmentally friendly. Moreover, concrete produced by using glass powder (GP) as a binder shows very low permeability to chloride ions. An extensive research project on the use of GP as cementitious material is underway at the University of Sherbrooke, Quebec, Canada. The northeastern Canadian province of Quebec has a policy on waste management to promote the recovery and management of materials from the municipal, industrial, commercial, and institutional sectors. Therefore, new alternatives for using recycled glass are needed. GP contains approximately 70% silicon dioxide. Thus, the replacement of 20% of cement with the incorporation of GP into the formulation of concrete provides economic and environmental benefits. Moreover, it has been shown that concrete with 20% GP has a very low permeability to chloride ions, which makes it a suitable solution for RC elements subject to corroding conditions, such as deicing products or a salty atmosphere. This paper presents results from a study on the structural behaviour of RC columns incorporating GP. Three series of six columns were tested under axial compression to show that RC columns made with GP show satisfactory structural behavior. The specimens had varying amounts of transverse reinforcement. For each series, three columns containing 20% GP as a cement replacement and three columns containing regular cement were tested. The results showed that for a water-binder ratio ($\mathrm{w}/\mathrm{b}$) of 0.40, the replacement of 20% of cement by GP delayed cracking of the concrete cover and slightly improved the load-carrying capacity and the postpeak response for columns tested at 28 days. For $\mathrm{w}/\mathrm{b}=0.55$, the results (i.e., load at first crack, peak load, and first observation of concrete spalling) for columns with GP tested at 91 days were still slightly lower than those without GP. However, the difference was smaller than for columns with the same $\mathrm{w}/\mathrm{b}$ that were tested at 28 days. Overall, the results showed that the structural behavior of RC columns made using concrete with GP is similar to columns made with normal concrete. According to these results, it is possible to use concrete with 20% GP for the construction of sustainable building structures.

Concrete-filled tubes (CFTs) provide an opportunity for more sustainable structural systems through the use of concretes with high volumes of supplementary cementitious materials (SCMs) to replace cement. However, there is concern that the SCM concrete would not be able to sustain the early (construction or dead) loads due to the longer cure time, necessitating stress transfer to the steel tube, possibly leading to tube buckling. Further, these loads are permanent and the long-term response of CFTs with SCM concretes is also unknown. This research evaluates the response of CFTs with conventional and high-volume SCM concretes to immediate compressive, sustained, and extreme loadings. Two large-scale (508 mm diameter) CFT column-to-footing connection specimens, one with a high volume of SCM used to replace portland cement, were tested under long-term compressive and cyclic loadings. Shrinkage and creep were evaluated for both the large-scale CFT and companion cylinders. The total creep strain in the SCM and conventional concretes was found to be much larger in the unsealed cylinders than the CFT specimens. Creep coefficients were found to be similar for the sealed cylinders and the CFT specimens. Drying shrinkage was found to be negligible in the CFTs and the SCM concrete was found to have slightly larger shrinkage than conventional concrete in both sealed and unsealed cylinders. The CFT specimens were subjected to cyclic lateral loading to evaluate their response to extreme loads and the effects of creep loading and the SCM concrete on performance were negligible.

This paper investigates the flexural and shear behavior of reinforced concrete beams that use recycled concrete aggregates (RCA) as replacement for coarse natural aggregates (e.g., crushed stone, gravel). The experimental results from 12 twin pairs of normal strength concrete beam specimens are presented and compared with predictions from existing code methods and analytical models for conventional concrete. Each pair of beams is saw-cut from a single, longer member to investigate the inherent variability in the results, specifically focusing on locally available recycled materials with minimal processing and construction methods that are consistent with current U.S. practice. It is found that the use of RCA does not cause an observable change in the progression of nonlinear behavior and failure. The effect of RCA on the flexural and shear strength of the beams is also small; however, there is a considerable reduction in the initial stiffness and an increase in the ultimate flexural deflections as the amount of RCA is increased. In general, the predicted results are reasonably close to the measured trends, indicating that existing analytical models and code-based procedures for conventional concrete can also be applied to RCA concrete beams.

Alternative concretes are needed to reduce the significant environmental impact of ordinary portland cement concrete construction. This paper reports findings from a study that evaluated the structural performance of environmentally friendly concretes made with self-cementitious fly ashes as the sole binders and pulverized reused glass as the aggregate. Such an evaluation is essential to moving these new concretes out of the laboratory and into general structural applications. Six reinforced beams (two each of three designs) were tested using three fly ash binders and a conventional concrete control mix (total of 24 beams). The beams were configured to evaluate the performance of the concretes under three different failure mechanisms: (1) tension-controlled failure, in which the concrete crushes after the longitudinal steel yields; (2) compression-controlled failure, in which the concrete crushes before yielding of the longitudinal steel; and (3) shear-controlled failure, in which the concrete cracks in the absence of shear reinforcement. Results from these beam tests were compared with conventional concrete control specimens and standard capacity predictions. The fly ash/glass tension-controlled specimens behaved similar to the conventional concrete specimens. In the compression-controlled and shear-controlled beams, the conventional concrete outperformed the fly ash/glass concrete. With respect to the applicability of standard capacity equations to fly ash/glass aggregate concretes, observed beam capacities either closely approached or exceeded predicted capacities; although, it was determined that some empirical factors should be adjusted to account for noted differences in observed stress-strain responses.