• Computers and Concrete
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• 제5권4호
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• pp.279-293
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• 2008

In the present paper a 3D thermo-hygro-mechanical model for concrete is used to study explosive spalling of concrete cover at high temperature. For a given boundary conditions the distribution of moisture, pore pressure, temperature, stresses and strains are calculated by employing a three-dimensional transient finite element analysis. The used thermo-hygro-mechanical model accounts for the interaction between hygral and thermal properties of concrete. Moreover, these properties are coupled with the mechanical properties of concrete, i.e., it is assumed that the mechanical properties (damage) have an effect on distribution of moisture (pore pressure) and temperature. Stresses in concrete are calculated by employing temperature dependent microplane model. To study explosive spalling of concrete cover, a 3D finite element analysis of a concrete slab, which was locally exposed to high temperature, is performed. It is shown that relatively high pore pressure in concrete can cause explosive spalling. The numerical results indicate that the governing parameter that controls spalling is permeability of concrete. It is also shown that possible buckling of a concrete layer in the spalling zone increases the risk for explosive spalling.

• 콘크리트학회논문집
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• 제15권6호
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• pp.915-923
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• 2003

This paper presents the test results of total 35 direct tensile specimens to investigate the effect of high-strength concrete on the tension stiffening effect in axially loaded reinforced concrete tensile members. Three kinds of concrete strength 25, 60, and 80 MPa were included as a major experimental parameter together with six concrete cover thickness ratios. The results showed that as higher strength concrete was employed, not only more extensive split cracking along the reinforcement was formed, but also the transverse crack space became smaller. Thereby, the effective tensile stiffness of the high-strength concrete specimens at the stabilized cracking stage was much smaller than those of normal-strength concrete specimens. This observation is contrary to the current design provisions, and the significance in reduction of tension stiffening effect by employment of high-strength concrete is much higher than that would be expected. Based on the present results, a modification factor is proposed for accounting the effect of the cover thickness and the concrete strength.

• Corrosion Science and Technology
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• 제11권5호
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• pp.157-164
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• 2012

In first experiment series, this paper is devoted for examining progress of reinforcement corrosion due to carbonation in concrete and to quantify uncarbonation depth to protect reinforcement from corroding. The tolerance of cover depth should be considered in order to prevent carbonation-induced corrosion. From the relationship between the weight loss of reinforcement and corrosion current density for a given time, therefore, the tolerance of cover depth to prevent carbonation-induced corrosion is computed. It is observed that corrosion occurs when the distance between carbonation front and reinforcement surface (uncarbonated depth) is smaller than 5 mm.As a secondary purpose of this study, it is investigated to examine the interaction between carbonation and chloride penetration and their effects on concrete. This was examined experimentally under various boundary conditions. For concrete under the double condition, the risk of deterioration due to carbonation was not severe. However, it was found that the carbonation of concrete could significantly accelerate chloride penetration. As a result, chloride penetration in combination with carbonation is a serious cause of deterioration of concrete.

• Smart Structures and Systems
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• 제17권4호
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• pp.577-591
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• 2016

In this study, the transfer length of 2400 MPa, seven-wire high-strength steel strands with a 15.2 mm diameter in pretensioned prestressed concrete (PSC) beams utilizing high strength concrete over 58 MPa at prestress release was evaluated experimentally. 32 specimens, which have the variables of concrete compressive strength, concrete cover depth, and the number of PS strands, were fabricated and corresponding transfer lengths were measured. The strands were released gradually by slowly reducing the pressure in the hydraulic stressing rams. The measured results of transfer length showed that the transfer length decreased as the concrete compressive strength and concrete cover depth increased. The number of strands had a very small effect, and the effect varied with both the concrete cover depth and concrete strength. The results were compared to current design codes and transfer lengths predicted by other researchers. The comparison results showed that the current transfer length prediction models in design codes may be conservatively used for 2400 MPa high-strength strands in high-strength concrete beams exceeding 58 MPa at prestress release.

• Structural Engineering and Mechanics
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• 제30권6호
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• pp.713-732
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• 2008

Serviceability and durability of the concrete members can be seriously affected by the corrosion of steel rebar. Carbonation front and or chloride ingress can destroy the passive film on rebar and may set the corrosion (oxidation process). Depending on the level of oxidation (expansive corrosion products/rust) damage to the cover concrete takes place in the form of expansion, cracking and spalling or delamination. This makes the concrete unable to develop forces through bond and also become unprotected against further degradation from corrosion; and thus marks the end of service life for corrosion-affected structures. This paper presents an analytical model that predicts the weight loss of steel rebar and the corresponding time from onset of corrosion for the known corrosion rate and thus can be used for the determination of time to cover cracking in corrosion affected RC member. This model uses fully the thick-walled cylinder approach. The gradual crack propagation in radial directions (from inside) is considered when the circumferential tensile stresses at the inner surface of intact concrete have reached the tensile strength of concrete. The analysis is done separately with and without considering the stiffness of reinforcing steel and rust combine along with the assumption of zero residual strength of cracked concrete. The model accounts for the time required for corrosion products to fill a porous zone before they start inducing expansive pressure on the concrete surrounding the steel rebar. The capability of the model to produce the experimental trends is demonstrated by comparing the model's predictions with the results of experimental data published in the literature. The effect of considering the corroded reinforcing steel bar stiffness is demonstrated. A sensitivity analysis has also been carried out to show the influence of the various parameters. It has been found that material properties and their inter-relations significantly influence weight loss of rebar. Time to cover cracking from onset of corrosion for the same weight loss is influenced by corrosion rate and state of oxidation of corrosion product formed. Time to cover cracking from onset of corrosion is useful in making certain decisions pertaining to inspection, repair, rehabilitation, replacement and demolition of RC member/structure in corrosive environment.

• 콘크리트학회논문집
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• 제23권6호
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• pp.791-797
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• 2011

이 논문은 인장증강효과에 대한 피복두께의 영향을 알아보기 위하여 실시한 12개의 축하중을 받는 직접인장 실험체의 실험 결과를 정리 분석한 것이다. 피복두께와 철근 직경의 비를 주 변수로 선정하여 6개의 서로 다른 피복 두께를 갖는 실험체를 제작하여 실험을 실시하였다. 실험 결과에 따르면 피복두께가 얇을수록 쪼갬균열의 영향이 크게 나타났으며, 인장증강효과와 균열간격이 감소하였다. 그리고 균열안정화 단계에서의 인장증강효과도 피복두께가 얇아질수록 감소하는 것을 확인하였다. 현행 설계기준의 인장증강효과 모델들은 피복두께의 변화에 따른 인장증강 거동의 차이를 반영할 수 없으며, 특히 피복두께가 얇을수록 인장증강효과가 감소하는 현상을 고려하지 못하고 있다. 따라서 이 연구에서 수행한 실험 및 분석 결과를 근거로 하여 인장증강효과에 피복두께의 영향을 반영할 수 있는 인장증강 계수 수정식을 제안하였다.

• Structural Engineering and Mechanics
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• 제61권6호
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• pp.711-719
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• 2017

This study investigates the effects of reinforcing bar diameter and cover depth on the shrinkage behavior of restrained ultra-high-performance fiber-reinforced concrete (UHPFRC) slabs. For this, twelve large-sized UHPFRC slabs with three different rebar diameters ($d_b=9.5$, 15.9, and 22.2 mm) and four different cover depths (h=5, 10, 20, and 30 mm) were fabricated. In addition, a large-sized UHPFRC slab without steel rebar was fabricated for evaluating degree of restraint. Test results revealed that the uses of steel rebar with a large diameter, leading to a larger reinforcement ratio, and a low cover depth are unfavorable regarding the restrained shrinkage performance of UHPFRC slabs, since a larger rebar diameter and a lower cover depth result in a higher degree of restraint. The shrinkage strain near the exposed surface was high because of water evaporation. However, below a depth of 18 mm, the shrinkage strain was seldom influenced by the cover depth; this was because of the very dense microstructure of UHPFRC. Finally, owing to their superior tensile strength, all UHPFRC slabs with steel rebars tested in this study showed no shrinkage cracks until 30 days.

• Structural Engineering and Mechanics
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• 제37권6호
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• pp.649-669
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• 2011

In this paper an analytical model is presented that addresses the compressive response of short-fiber reinforced concrete members (FRC) with hooked steel fibers. This model is applicable to a wide range of concrete strengths and accounts for the interaction between the cover spalling and the concrete core confinement induced by transverse steel stirrups and also for buckling of longitudinal reinforcing bars. The load-shortening curves generated here analytically fit existing experimental data well.

• 콘크리트학회논문집
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• 제14권6호
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• pp.851-863
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• 2002

철근의 부식에 의해 발생하는 부식생성물은 부피팽창으로 인하여 주위의 콘크리트에 압력을 가한다. 이 팽창압은 철근 주의의 콘크리트에 인장응력이 생기게 하며 결국 콘크리트의 피복에 균열을 발생시킨다. 콘크리트 피복의 균열발생은 콘크리트 구조물의 사용수명을 감소시킬 뿐만 아니라 안전성에도 영향을 미친다. 본 연구의 목적은 콘크리트 피복에 균열을 일으키는 임계부식량을 조사하는 것이다. 이를 위하여 포괄적인 실험 및 이론적 연구를 수행하였다. 주요 실험변수로는 콘크리트의 강도와 피복두께이고 부식량의 증가에 따른 콘크리트 피복 표면의 인장변형률을 측정하였다. 철근 팽창에 의한 팽창압의 계산에 공극 및 균열 속으로 흡수되는 부식생성물을 고려하였다. 본 연구를 통하여 균열을 일으키는 임계부식량은 콘크리트 압축강도가 커짐에 따라 증가한다는 사실을 확인하였다. 부식층 내의 팽창압과 변형 사이의 관계를 유도하였으며 부식생성물층의 강성을 결정하였다. 팽창압과 변형 사이의 관계를 설명하기 위하여 압력을 유발하지 않는 변형량의 개념을 도입하였다. 본 연구에서 제안된 이론은 실험결과와 잘 일치하며 콘크리트 구조물의 내구성 설계에 기초가 될 수 있을 것이다.

• 한국화재소방학회논문지
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• 제25권1호
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• pp.78-84
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• 2011

최근 콘크리트 내화 연구는 대부분 고강도 콘크리트의 내화성능 확보에 많은 초점이 맞추어져 있다. 그러나 국내의 콘크리트 수요를 살펴보면 40MPa 이하의 일반강도 콘크리트의 수요가 전체 콘크리트 수요량의 대부분을 차지한다. 따라서 고강도 콘크리트의 내화성능에 대한 연구뿐만 아니라 일반콘크리트의 내화성능에 관한 연구도 필요하다. 본 연구에서는 40MPa 콘크리트 기둥을 대상으로 콘크리트 피복 두께와 단면크기를 변수로하여 내화성능을 평가하였으며, 연구 결과 단면크기가 커질수록, 피복두께가 두꺼워질수록 내화성능은 향상되는 것으로 나타났다.