Workability and Mechanical Properties of Heavyweight Magnetite Concrete

Yang, Keun-Hyeok,Mun, Jae-Sung,Lee, Ho American Concrete Institute 2014 ACI materials journal Vol.111 No.3

<P>To provide basic data for developing mixing details and design models for structural heavyweight magnetite concrete, 18 concrete mixtures were prepared under different replacement levels with natural sand and granite coarse particles for magnetite fide and coarse aggregates, respectively. The density of concrete ranged between 3182 and 3400 kg/m(3) (197.28 and 210.8 lb/ft(3)) for replacement by natural sand, and 2446 and 3118 kg/m3 (151.65 and 193.31 lb/ft(3)) for replacement by granite coarse particles. Test results revealed that the replacement by natural sand was preferable to that by granite coarse particles for enhancing the initial slump, tensile resistance capacity, shear strength and bond behavior with a reinforcing bar of heavyweight magnetite concrete, and for minimizing the decrease in concrete density due to replacement by normalweight aggregates. ACI 349 equations for moduli of elasticity and rupture were generally conservative for heavyweight magnetite concrete. The CEB-FIP equations were not conservative for the moduli of elasticity and rupture, strain at the peak stress, and splitting tensile strength of heavyweight magnetite concrete, whereas they were conservative in predicting the bond stress-slip response of heavyweight magnetite concrete. Overall, it can be concluded that the density of concrete should be considered as a critical factor, together with its compressive strength, in determining the various mechanical properties of heavyweight concrete:</P>

Shearhead Reinforcement for Concrete Slab to Concrete-Filled Tube Column Connections

Kim, Jin-Won,Lee, Cheol-Ho,Kang, Thomas H.-K. American Concrete Institute 2014 ACI structural journal Vol.111 No.3

<p>This paper presents full-scale gravity-load test results on 10 concrete-filled tube (CFT) column-reinforced concrete (RC) flat plate connections with shearheads, The CFT construction has many structural and constructional advantages over conventional steel and RC column construction. Use of RC flat plate systems in the basement and residential floors of tall buildings is often demanded to reduce story height and enable rapid construction. Combining CFT columns and flat plate flooring is expected to result in synergetic effects. Issues related to connecting CFT columns to flat plates, however, have not yet been fully addressed. Several promising connecting schemes using steel shearheads were proposed and tested in this study. Test results show that the proposed connection can exhibit punching shear strength higher than its RC flat plate counterparts. An empirical model that can reasonably predict the punching shear strength of the proposed connection was also proposed.</p>

Three-Dimensional Grid Strut-and-Tie Model Approach in Structural Concrete Design

Yun, Young Mook,Kim, Byunghun,Ramirez, Julio A. American Concrete Institute 2018 ACI structural journal Vol. No.

<P>The strut-and-tie model approach has now been incorporated in current U.S. design codes and guidelines for the design of disturbed regions in structural concrete elements. However, more work is needed to extend the approach to design of three-dimensional (3-D) structural concrete. It is also important to consider its verification with experimental evidence. The application to 3-D design situations of this approach brings more uncertainties with its proper application and limitations. To reduce uncertainty and assist designers in the application of the strut-and-tie model to 3-D situations, the authors present in this paper a 3-D grid strut-and-tie model approach consisting of three key steps: 1) grid elements to construct a 3-D strut-and-tie model; 2) triaxial failure model of concrete to determine effective strengths of concrete struts and nodal zones; and 3) iterative technique to evaluate the axial stiffness of struts and ties. In this paper, the authors also incorporate in the strut-and-tie model a new concept of maximum cross-sectional areas of struts and ties to examine the strut-and-tie model's geometrical compatibility. The approach is illustrated with the redesign of a deep pile cap with tension piles available in the literature. In a subsequent paper, the authors will evaluate the approach with test results of 157 specimens tested to failure. The tests include 78 reinforced concrete pile caps, 19 slab-column joints, and 60 beams subjected to torsion.</P>

Analysis of Compressive Strength Development and Carbonation Depth of High-Volume Fly Ash Cement Pastes

Wang, Xiao-Yong,Park, Ki-Bong American Concrete Institute 2016 ACI materials journal Vol.113 No.2

<P>High-volume fly ash (HVFA) concrete, which typically has 50 to 60% fly ash as the total cementitious material content, is widely used to achieve sustainable development in the concrete industry. Strength development and carbonation are critical research topics for using HVFA concrete. This paper presents a numerical procedure to evaluate the strength development and carbonation depth of HVFA concrete. This numerical procedure consists of a hydration model and a carbonation reaction model. The hydration model analyzes the fly ash dilution effect and the pozzolanic reaction. The amount of carbonatable materials, such as calcium hydroxide (CH) and calcium silicate hydrate (CSH), are calculated using reaction degrees of cement and fly ash. The compressive strength development of cement-fly ash blends are evaluated from CSH contents. The calculation results from the hydration model, such as the amount of carbonatable materials and the porosity, are used as input parameters for the carbonation reaction model. By considering the effects of material properties and environmental conditions, the carbonation reaction model analyzes the diffusivity of carbon dioxide and the carbonation depth of HVFA concrete with different curing conditions, different fly ash contents, and different water-binder (w/b) ratios.</P>

Multiscale Modeling of Concrete in Anchorage Region of Post-Tensioned NSM CFRP

Kim, Yail J.,Ji, Yongcheng,Chang, Wei-Tze,Kang, Jae-Yoon,Park, Jong-Sup,Jung, Woo-Tai American Concrete Institute 2016 ACI structural journal Vol.113 No.6

<P>This paper presents a multiscale modeling approach to understand the behavior of anchorage concrete in prestressed concrete bridge girders strengthened with post-tensioned near-surfacemounted (NSM) carbon fiber-reinforced polymer (CFRP) strips. The macroscale, mesoscale, and microscale responses of the concrete are represented by continuum finite element, discrete-object physics, and analytical representative element models, respectively. Domain decomposition and integration are implemented in accordance with an information-passing mechanism. The post-tensioned NSM CFRP results in macroscale strain softening near the anchorage and it also causes a regional net strain increase responsible for macroscale and microscale damage. The existence of a rapid crack-propagation phase is ascertained in the anchorage concrete from a mesoscale standpoint, which influences stress transfer from the anchor bolt to the contiguous concrete. The microscale damage of the concrete rapidly propagates when mechanical distress is applied due to post-tensioning the NSM CFRP and stabilizes with a decrease in stress transfer from the cementitious binder to aggregate inclusion in the representative element. The concept of effective stress is proposed to link the macroscale and microscale responses of the anchorage concrete.</P>

Non-Iterative Moment Capacity Equation for Reinforced Concrete Beams with External Post-Tensioning

Lee, Swoo-Heon,Shin, Kyung-Jae,Kang, Thomas H.-K. American Concrete Institute 2014 ACI structural journal Vol.111 No.5

<p>The stress of unbonded steel in post-tensioned concrete beams at the ultimate stage is needed to determine their moment capacity, but it is difficult to accurately predict the stress of external posttensioning steel. This is because the unbonded steel stress is governed by total steel elongation, which depends on the beam deflection and other interrelated variables. The bulk of the previously developed analytical methods are applicable only for internal tendons encased in concrete. Given the lack of analytical models for externally post-tensioned beams, in this study, a simplified non-iterative equation for the stress at ultimate in external tendons or other steel materials is developed. During this process, flexural responses of yield and ultimate moments and their corresponding deflections are also predicted. A major effort is made to simplify such a process and propose a generalized non-iterative moment capacity equation for reinforced concrete beams with external post-tensioning.</p>

Development Length of Headed Bar Based on Nonuniform Bond Stress Distribution

Hwang, Hyeon-Jong,Park, Hong-Gun,Yi, Wei-Jian American Concrete Institute 2019 ACI structural journal Vol. No.

<P>The anchorage capacity of headed bars shows large variations according to design conditions, such as diameter of reinforcing bars, location of anchorage, and the use of fiber-reinforced concrete. In the present study, a design method was studied considering such various design conditions. In the proposed model, the anchorage strength of a headed bar was defined as the sum of the contributions of the straight bar length and the head bearing. Particularly, a nonuniform bond stress distribution model was used for the straight bar length. The proposed method was applied to 361 existing test specimens with various conditions of headed bar anchorage, including compression-compression tension (CCT) node, lap splice, and beam-column joint. The predicted results were compared to the existing test results and the predictions of current design codes including ACI 318 and Model Code 2010. The results showed that the proposed model predicted the test results with reasonable precision.</P>

Flexural Testing of Circular Concrete-Filled Tubes without Axial Forces

Nghiem, Andrew,Kang, Thomas H.-K.,Lee, Minsun,Ramseyer, Chris,Lee, Cheol-Ho AMERICAN CONCRETE INSTITUTE 2018 ACI materials journal Vol.115 No.2

Flexural Testing of Reinforced Concrete Beams with Recycled Concrete Aggregates

Kang, Thomas H.-K.,Kim, Woosuk,Kwak, Yoon-Keun,Hong, Sung-Gul American Concrete Institute 2014 ACI structural journal Vol.111 No.3

<p>Owing in part to the fact that appropriate structural design guides for recycled concrete materials have not been established, only approximately 14% of all waste concrete is made into aggregate and recycled. This is an inefficient method of recycling the concrete materials, under-utilizing an important and valuable asset. To promote the use of recycled concrete aggregates (RCAs), flexural tests of 28 reinforced high-strength and normal-strength concrete beams were performed. The main objectives of this research were the structural investigation of flexural performance and the evaluation of the potential application of RCA for concrete structures. In comparing the RCA specimens to the natural aggregate specimens, the overall crack patterns were similar to each other. The flexural behavior was also not affected significantly by the use of RCA up to 30% RCA replacement ratio. The current test data and data from other researchers' studies were examined in evaluating the flexural strength of reinforced concrete beams with RCA.</p>

Shear Strength of Concrete Composite Beams with Shear Reinforcements

Kim, Chul-Goo,Park, Hong-Gun,Hong, Geon-Ho,Kang, Su-Min,Lee, Hyerin American Concrete Institute 2017 ACI structural journal Vol. No.

<P>Currently, the hybrid construction of precast concrete (PC) and cast-in-place concrete (CIPC) with different concrete strengths is frequently used for precast concrete structures. However, in ACI 318-14, the shear strength of PC-CIPC composite beams is defined as the sum of the respective shear strengths or the lower concrete strength of the two concretes, without clear evidence. In the present study, the shear strengths of simply supported composite beams with shear reinforcement were investigated. The test variables included the cross-sectional area ratio of the precast concrete and cast-in-place concrete, the spacing of the stirrups, and the shear span-depth ratio. The test results showed that the use of high-strength concrete (HSC) in the compression increased the contribution of shear reinforcement by increasing the depth of the tension zone. Thus, the use of HSC in the compression zone significantly increased the shear strength of the composite beams. The shear design equation of ACI 318 safely predicted the shear strengths of the specimens by using the sum of the individual strengths of the two concretes or by using the average concrete strength.</P>