Autogenous Shrinkage Of High-strength Concrete Research Articles

Effects of cooking oil on the shrinkage-reducing of high-strength concrete

Autogenous shrinkage is a serious problem for high-strength concrete, which may lead to early-age cracking. Shrinkage-reducing agents are typically used to mitigate the autogenous shrinkage of concrete. However, some negative effects, such as unstable workability and the reduction of compressive strength, will be caused. In this study, emulsified or plain cooking oil is used as a shrinkage-reducing agent in low water-binder ratio concrete. Deformational behaviours of concrete with cooking oil and normal commercially available shrinkage-reducing agent are investigated. Experimental results show that the utilization of cooking oil could significantly reduce the autogenous shrinkage of high-strength concrete due to the reduction of surface tension. Additionally, emulsified cooking oil is better than normal shrinkage-reducing agents and plain cooking oil in many aspects of the concrete. Micro-structure of the concrete with different shrinkage-reducing agents is also observed. Emulsified cooking oil is considered a promising admixture for the mitigation of autogenous shrinkage of high-strength concrete.

Early-Age Autogenous Shrinkage and Cracking Risk of 5D Hooked-End Steel Fiber–Reinforced High-Strength Concrete under Uniaxial Restrained Condition

The incorporation of steel fiber exhibits great benefits to reduce cracking risk and autogenous shrinkage (AS) of high-strength concrete (HSC). 5D hooked-end steel fiber (5DSF) is a kind of novel hooked-end steel fiber. Previous investigations mainly focused on the mechanical properties and shrinkage deformation of 5DSF reinforced HSC (5DSFRC). However, the early-age cracking risk of 5DSFRC under uniaxial restrained conditions considering temperature evolution, AS, and restrained stress simultaneously has not been thoroughly investigated. A temperature stress test machine, which is capable of measuring these factors simultaneously, was used to investigate the effect of 5DSF volume fraction (0%, 0.12%, 0.24%, and 0.36%) on the early-age cracking risk of HSC in the present study. Experimental results and analysis showed that the cracking age, cracking stress, ratio of cracking stress to axial tensile strength, and reserve strength of HSC increased with an increase in 5DSF volume fraction. The AS of HSC decreased with an increase in 5DSF volume fraction, and an AS formula was proposed to calculate the early-age AS of HSC reinforced with steel fiber considering the effect of the volume fraction and shape of the fiber. The cracking risk of HSC decreased with an increase in 5DSF volume fraction. The early-age cracking risk and AS of HSC reinforced with 5DSF were lower than that of HSC reinforced with 3D hooked-end steel fiber with a similar steel fiber volume fraction.

Effect of n-C-S-H-PCE and slag powder on the autogenous shrinkage of high-strength steam-free pipe pile concrete

This study investigates the effect of the calcium silicate hydrate–Polycarboxylate ether nanocomposite (n-C-S-H-PCE) on the development of autogenous shrinkage and other relevant properties, including strength, hydration degree, elastic modulus, and capillary pressure. To measure the autogenous shrinkage development of concrete, a strain-temperature-humidity integrated acquisition system was used to continuously monitor the interior strain, temperature, and humidity evolutions of the concrete. The results show that the n-C-S-H-PCE has a pronounced enhancement effect on the mechanical properties up to 7 d. The early strength enhancement of the n-C-S-H-PCE can reach 64% at 1 d. Due to the fast hydration, the autogenous shrinkage is higher in the concrete containing n-C-S-H-PCE. And the slag powder also increases the autogenous shrinkage. Moreover, a model suitable for predicting the autogenous shrinkage of high-strength concrete, such as PHC pipe piles, is established. The prediction values are in good agreement with the experimental values. We finally conclude that the n-C-S-H-PCE increases the hydration degree, which results in a lower internal RH and a higher capillary pressure at the early hydration period compared with the reference concrete. This study provides a reference for the application of n-C-S-H-PCE and slag powder in concrete and the further prediction of the autogenous shrinkage for concrete containing those ingredients.

Mitigation of autogenous shrinkage of high-strength concrete using rice husk ashes with distinct pore structures

The effect of particle size (50% passing size (D50) of 20 μm, 14 μm and 7 μm) and content (8%, 10% and 12% by mass) of rice husk ash (RHA) on the autogenous shrinkage evolution of high-strength concrete was investigated. RHA samples were selected based on differences in their pore structures, which were investigated by means of nitrogen adsorption isotherms, scanning electron microscopy and absorbency tests. Cement–pozzolan hydration development was studied using isothermal calorimetry. The concretes were subjected to mercury intrusion porosimetry and compressive strength tests. Considering a statistical 22 factorial analysis with a central point, the results showed that D50 and the interaction between D50 and RHA content influenced the autogenous shrinkage at 28 and 100 days. The water absorbed internally by the preserved RHA pore structure (D50 ≈ 20 μm) in the high cement replacement (12% RHA) mixes reduced the final autogenous shrinkage of concrete without a decrease in compressive strength. In addition, the RHA with a more preserved pore structure accelerated the hydration kinetics by heterogeneous nucleation. However, this behaviour did not influence the early autogenous shrinkage results.

Compressive Creep and Shrinkage of High-Strength Concrete Based on Limestone Coarse Aggregate Applied to High-Rise Buildings.

Concrete undergoes shrinkage regardless of the influence of external forces. The deformation of concrete is crucial for the structural stability of high-rise and large-scale buildings. In this study, the shrinkage and compressive creep of 70–90 MPa high-strength concrete used in high-rise buildings were evaluated based on the curing conditions (sealed/unsealed), and the existing prediction models were examined. It was observed that the curing condition does not significantly affect the mechanical properties of high-strength concrete, but the use of limestone coarse aggregate increases the elastic modulus when compared to granite coarse aggregate. The autogenous shrinkage of high-strength concrete is greater than that of normal-strength concrete owing to self-desiccation, resulting in a large variation from the value predicted by the model. The drying shrinkage was observed to be similar to that predicted by the model. Compressive creep was affected by the curing conditions, compressive strength, loading level, and loading age. The compressive creep of high-strength concrete varied significantly from the prediction results of ACI 209; ACI 209 was modified based on the measured values. The shrinkage and compressive creep characteristics of high-strength concrete must be reflected to predict the deformation of an actual structure exposed to various conditions.

Effects of hydration heat velocity on autogenous shrinkage of high-strength concrete

The purpose of this study was to quantitatively investigate the effects of a micro encapsulated retarder (MER) on the hydration heat velocity (HHV) and autogenous shrinkage velocity (ASV) of high-strength concrete (HSC). A series of laboratory experiments was designed and conducted with the main variables being the amount of MER (0, 0·4 and 0·8% of total cement weight), specimen size (300 × 300 × 300 mm3 and 100 × 100 × 400 mm3) and two different curing conditions (semi-adiabatic and in air at 20°C). As a result, the MER was demonstrated to be effective for retarding the cement hydration process without reducing the compressive strength of the hardened concrete. Analysis of the results showed that the hydration temperature and the HHV of the HSC decreased as MER amount increased. Increasing the amount of MER also decreased the ASV and ultimate autogenous shrinkage for HSC. Finally, it was found that the HHV is strongly correlated with the ASV, and an approximate formula relating the two parameters is proposed. The findings in this study will be useful for optimum concrete design by controlling early-age shrinkage in HSC.

Creep of AFRP Confined High-Strength Concrete Columns under Low-Level Stress

This paper investigates the creep of fiber-wrapped high-strength concrete (HSC) columns (FWHSCCs) under low level axial stress by testing and modeling. Firstly, an experimental work on the creep of four FWHSCCs is presented. Secondly, by introducing modified coefficients about compressive strength, water-cement ratio and silica fume content of concrete, a modified model suitable for predicting the creep of HSC, based on Bažant's B3 model for normal concrete creep prediction, is proposed. Thirdly, considering the state of triaxial stresses of the concrete core, the autogenous shrinkage of HSC, and the interaction stress history, an analytical model based on the proposed model for HSC creep and the power law for fiber reinforced polymer (FRP) creep is developed for predicting the creep of FWHSCC. The prediction results agree well with the test data. This investigation also shows the creep of FWHSCCs is lower than that of FRP wrapped normal strength concrete column.

시험체 크기 및 수화지연 효과에 따른 초기재령 수화발열 및 자기수축 특성 분석

시멘트페이스트, 모르타르 및 콘크리트의 자기수축의 크기 및 발현율은 초기재령 내부온도의 이력과 크기에 의해 영향을 받는다고 보고된 바 있으나, 초기재령 수화열과 자기수축에 대한 구체적인 관계분석은 아직 미흡한 실정이다. 이 연구에서는 초기재령 수화온도 및 자기수축의 이력특성에 대한 분석방법을 제시한 선행연구를 기초로, 시험체 크기 및 수화지연 효과에 따른 고강도콘크리트의 초기재령 수화열과 자기수축의 거동특성 및 상관관계를 분석하였다. 또한 자기수축에 의한 균열평가시 유효자기수축의 기준이 되는 응결시간과 수화온도 및 자기수축의 이력과의 관계도 검토하였다. 그 결과, 시험체가 매스화될수록 초기재령 수화발열 상승구간의 수화온도 상승량 및 수화발열 상승속도, 자기수축 증가구간의 자기수축 증가량 및 자기수축 속도는 증가하는 것으로 나타났으며, 지연제를 사용할 경우 그 값은 감소하는 것으로 나타났다. 초기재령 콘크리트의 수화발열 상승속도가 증가하면 자기수축 속도는 증가하며, 이와 더불어 재령 91일 자기수축도 증가하는 것으로 나타나, 종국 자기수축은 초기재령 수화발열상승 속도에 의해 영향을 받을 수 있을 것으로 사료된다. 한편, 콘크리트 균열평가시 유효자기수축의 기준이 되는 응결시간과 변곡점 및 수화온도 상승시점은 시멘트의 수화반응 과정에 있어 서로 밀접한 관계가 있음을 확인할 수 있었다.

Autogenous shrinkage of high-strength concrete containing silica fume under drying at early ages

The present study investigated experimentally autogenous shrinkage of high-strength concrete containing silica fume under drying at early ages. The influence of drying on hydration of cementitious materials in the high-strength concrete with water–binder ratios of 0.25, 0.35 and 0.45 was evaluated based on bound water content (BWC), which was exposed to drying at the ages of 0.5, 1.0 and 3.0 days, respectively. By establishing the relationship between the BWC and autogenous shrinkage strain under sealed conditions, autogenous shrinkage strain under drying conditions and drying shrinkage strain were separated from total shrinkage strain, and, then, the contribution of autogenous shrinkage in total shrinkage was discussed. The results showed that the percentage of autogenous shrinkage was macroscopically 50–20% based on the present method, while that was 70–30% based on the conventional superposition principle (SP). The latter resulted in overestimating autogenous shrinkage strain under drying conditions.

고강도 플라이 애쉬 콘크리트의 자기수축 예측 모델

초록·키워드 목차 오류제보하기 등록된 정보가 없습니다. ABSTRACT1. 서론2. 실험 연구3. 실험 결과 및 분석4. 자기수축 모델의 제안5. 결론참고 문헌요약