Comite Euro-International Du Beton Research Articles

Early age dynamic mechanical properties of basalt fiber reinforced early strength concrete

AbstractThe dynamic compression test was carried out on basalt fiber reinforced early strength concrete (BFRESC) with ages of 4–24 h by the split Hopkinson pressure bar (SHPB) test system. The effects of curing age and basalt fiber content on dynamic mechanical properties of ESC was analyzed. The result shows that the dynamic compressive strength and specific energy absorption of BFRESC at early ages increase continuously with the increase of fiber content. The dynamic compressive strength of concrete at the early ages increases exponentially with the strain rate. With the increase of curing age, the strain rate sensitivity of dynamic compressive strength of concrete increases gradually, and the strain rate sensitivity of dynamic compressive strength of BFRESC with fiber content of 0.1% is the strongest. The specific energy absorption of concrete at early ages increases linearly with the strain rate. With the increase of curing age, the strain rate sensitivity of specific energy absorption of concrete increases gradually, and basalt fiber can enhance the strain rate sensitivity of specific energy absorption of concrete. Based on the comparison with the formula for dynamic increase factor (DIF) calculation provided by Comite Euro‐International du Beton (CEB), etc., a model of DIF based on the increase of curing age is proposed.

Influence of moisture content on the concrete response under dynamic loading

Several researches have highlighted the influence of external environmental conditions (e.g. moisture and temperature) on the concrete response at different strain rates, however, limited data are available considering such factors. This paper experimentally assesses the compressive strain rate effects of concrete at different saturation levels. The experimental assessment is conducted on sixty-six specimens tested using the Split-Hopkinson Pressure Bar. The results of the experimental program were compared against the Comite Euro-International du Beton (CEB) equations and calibration factors to account for the degree of saturation were introduced. Furthermore, the proposed calibrated equation was tested against a large database collected from the literature which showed more reliable predictions than the current CEB equation. The presented work aims to enrich the concrete database subjected to high strain rates and to facilitate reliable material predictions to be used in further numerical assessments.

Effect of strength grade on mechanical properties of concrete under dynamic load

AbstractThe uniaxial compression test and dynamic compression test of C30, C50, and C80 concrete were carried out. The effects of strength grade and strain rate () on dynamic mechanical properties of concrete were analyzed in terms of dynamic compressive strength, dynamic increase factor (DIF), dynamic peak strain, and dynamic peak toughness. And the strain rate sensitivity of mechanical indexes and DIF fitting model of concrete with different strength grades were explored. The results show that the dynamic compressive strength, DIF and dynamic peak toughness of concrete with different strength grades all have obvious strain rate effects. With the increase of strength grade of concrete, the dynamic peak strain of concrete decreases continuously, and the strain rate sensitivity of dynamic compressive strength and dynamic peak toughness of concrete increases continuously. The mechanical indexes of C80 concrete have the strongest sensitivity to the strain rate. The DIF empirical formula of Comite Euro‐International du Beton (CEB) cannot fit the DIF of C30 and C80 concrete effectively. There is a critical strain rate beyond which the strain rate effect of DIF is significantly enhanced. With the increase of strength grade of concrete, the critical strain rate of concrete decreases continuously. The piecewise linear fitting of DIF and of concrete with different strength grades shows good results.

Time-dependent Behaviour Analysis of Long-span Concrete Arch Bridge

This paper continues the previous study on clarifying the time-dependent behaviour of Beipanjiang Bridge ‒ a reinforced concrete arch bridge with concrete-filled steel tubular stiffened skeleton. The obtained prediction models and the Finite Element Models were used to simulate the long-term behaviour and stress redistribution of the concrete arch bridge. Three-dimensional beam elements simulated the stiffened skeleton and surrounding concrete. Then, a parameters study was carried out to analyse the time-dependent behaviour of the arch bridge influenced by different concrete creep and shrinkage models. The simulation results demonstrate that concrete creep and shrinkage have a significant influence on the time-dependent behaviour of the concrete arch bridge. After the bridge completion, the Comite Euro-International du Beton mean deviation of displacements obtained by 1990 CEBFIP Model Code: Design Code model and fib Model Code for Concrete Structures 2010 model are 3.4%, 31.9% larger than the results predicted by the modified fib Model Code for Concrete Structures 2010 model. The stresses between the steel and the concrete redistribute with time because of the concrete long-term effect. The steel will yield if the fib Model Code for Concrete Structures 2010 model is used in the analysis. The stresses in a different part of the surrounding concrete are non-uniformly distributed.

Static and impact behaviour of recycled aggregate concrete under daily temperature variations

Abstract Concrete continuously suffers from impact loading and variations in daily temperature. In this study, tests on static compressive, static flexural, and impact compressive concrete specimens with different replacement percentages of recycled coarse aggregate (RCA), additional water, and cyclic temperatures were carried out to investigate the mechanical behaviour of recycled aggregate concrete (RAC). The static tests were conducted using a universal mechanical testing machine, and the impact tests were then applied using a 100-mm diameter split Hopkinson pressure bar facility. The failure mode, compressive strength, flexural strength, brittleness coefficient, strain-strain curve, and dynamic increase factor (DIF) were analysed and discussed. The results showed that (1) the influence of the replacement percentage of RCA and additional water on the static behaviour of RAC mainly depend on the effective water-to-cement ratio, RCA strength, and bond strength between the mortar and RCA, (2) the strain rate has a significant effect on the impact performance of RAC, and (3) the cyclic temperature significantly weakens the material properties of RAC. Finally, based on an empirical formula given by Comite Euro-International du Beton (CEB), a modified CEB model was proposed to accurately estimate the relationship between the strain rate and DIF of RAC.

Effect of notch position on fracture energy for foamed concrete

Foamed concrete is one of the lightweight concrete used to replace normal concrete. Foamed concrete has potential as a building construction material in Malaysia due to low density range. However, the behavior of fracture energy on foamed concrete still under investigation. Therefore, a study to determine the fracture energy of foamed concrete was conducted. In this study, foamed concrete fracture energy was obtained using the three-point bending test methods develop by RILEM and Hillerborg. A total of 12 beams with different types of notch and positions of notch were tested on the load–deflection condition. In addition, a total of 9 cube samples were cast to support the result of fracture energy by using model from Bazant and Becq-Giraudon and Comite Euro-International du Beton (CEB). Results showed the far the position of the notch from midpoint, the higher the value of fracture energy. In this study, the value of fracture energy ranges between 15 N/m and 40 N/m.

Effect of Span-to-Depth Ratio on the Fracture Energy of Foam Concrete

The use of foam concrete as an alternative material in the construction industry has become popular in structure industries due to the low density range, good strength, high serviceability, and lightness of this material. Several studies have been conducted to determine the mechanical properties and strength of foam concrete. However, research on fracture energy is still ongoing. Therefore, these studies aim to experimentally investigate the effect of span-to-depth (S/W) ratio on the fracture energy of foam concrete using beam specimens with V-notches through a three-point bending test. Beam specimens were encased and prepared at a density of 1600 kg/m3. Moreover, three different S/W ratios (4, 5, and 6) with a notch height of 20 mm were adopted. Fracture energy was determined using the Hillerborg, Bazant, and Comite Euro-International du Beton models. From the experiment, results showed that S/W ratio affected fracture energy.

Dynamic Properties of Concrete at Moderately Elevated Temperatures

Reinforced concrete is widely used to not only provide structural support but also to mitigate the effects of radiation, malevolent attacks, and dynamic accidents. Dynamic tests were performed on 102 x 203 mm (4 x 8 in.) normal weight concrete (NWC) and fiber-reinforced concrete (FRC) cylinders using a drop hammer. Dynamic splitting tension and dynamic compression tests were performed at room temperature and at 204°C (400°F). The dynamic increase factor (DIF), which is the ratio of dynamic to quasi-static strength, and the strain rate were compared to existing models. The DIF for tension and compression increased with strain rate. For compression, a DIF up to 3.2 was recorded for strain rates up to 12 s⁻¹. For tension, a DIF up to 4.1 was recorded for strain rates up to 1.8 s⁻¹. Heated NWC and FRC specimens had lower DIFs compared to room-temperature specimens for dynamic compression and tension. The experimental DIFs followed closely the Modified Comite Euro-International du Beton (CEB) model for tension.

Bond Behavior of Embedded Reinforcing Steel Bars for Varying Levels of Transversal Pressure

Because of columns loads, reinforced concrete (RC) continuous beams in skeleton structures are subjected to transverse compressive stress at support locations. Such lateral pressure can enhance the bond between the main top reinforcing bars and the surrounding concrete because of the confinement effect. Thus, the development length can be reasonably decreased compared to the case of no lateral pressure. However, different codes of practice, such as the Egyptian code standard ECP 203-2007 and the American Concrete Institute (ACI) code standard ACI 318-11, stipulate increasing the reinforcement location factor for upper reinforcement implemented in the calculation of the development length of such bars. On the other hand, the Comite Euro-International du Beton (CEB-FIP) model code standard CEB-FIP 2010 considers this enhancement in bond stress calculations as being due to transverse compression stress. To assess such effects, pull-out tests were performed on reinforcing bars embedded in short RC columns subjected to different levels of axial pressure. The experimental findings showed the same trend as manifested in the CEB-FIP 2010 results, where increasing the lateral pressure on the reinforcing bar resulted in decreasing the development length for both smooth and deformed steel bars. In contrast, the results of development length calculations based on both ECP 203-2007 and ACI 408R-03 were constant regardless of the level of lateral pressure on the reinforcing bar. This highlights the need for both ECP 203-2007 and ACI 408R-03 to consider the effect of transverse lateral pressure on development length calculations due to varying lateral pressure on the reinforcing bar.

Mechanical behavior of geopolymer concrete subjected to high strain rate compressive loadings

The effect of strain rate on the compressive behaviours of geopolymer concrete and mortar is reported. Split Hopkinson pressure bar was adopted for the high strain rate testings. The dynamic increase factors for compressive strength ( $${\text{DIF}}_{{f_{\text{c}} }}$$ ) and critical strain ( $${\text{DIF}}_{{\varepsilon_{\text{c}} }}$$ ) were measured and compared with Concrete Comite Euro-international du Beton (CEB) recommendations. The results show that alkaline activators have significant influence on the quasi-static compressive strength of geopolymer concrete. With high strain rate loading, the $${\text{DIF}}_{{f_{\text{c}} }}$$ of geopolymer concrete and mortar mixes increase with respect to increasing strain rates and in agreement with CEB recommendations. In addition, the coarse aggregates in geopolymer concrete mixes play important role in the increase of compressive strength. However, CEB recommendations underestimate the $${\text{DIF}}_{{\varepsilon_{\text{c}} }}$$ of critical strain for geopolymer concrete in the high strain rate loading. It is found that for the quasi-static loading and low strain rate loading, cracks propagate along interface transition zone (ITZ) and matrix of geopolymer concrete specimens whereas cracks occur at both the aggregates and ITZ under high strain rate loading.

Excessive Long-Time Deflections of Prestressed Box Girders. I: Record-Span Bridge in Palau and Other Paradigms

The segmental prestressed concrete box girder of Koror-Babeldaob (KB) Bridge in Palau, which had a record span of 241 m (791 ft), presents a striking paradigm of serviceability loss because of excessive multidecade deflections. The data required for analysis have recently been released and are here exploited to show how the analysis and design could be improved. Erected segmentally in 1977, this girder developed a midspan deflection of 1.61 m (5.3 ft) compared with the design camber after 18 years, and it collapsed in 1996 as a consequence of remedial prestressing, after a 3-month delay. Compared with three-dimensional analysis, the traditional beam-type analysis of box girder deflections is found to have errors up to 20%, although greater errors are likely for bridges with higher box-width-to-span ratios than the KB Bridge. However, even three-dimensional finite-element analysis with step-by-step time integration cannot explain the observed deflections when the current American Concrete Institute, Japan Society of Civil Engineers, Comite Euro-International du Beton (or Comite Euro- International du Beton—Federation internationale de la precontrainte), and Gardner and Lockman prediction models for creep and shrinkage are used. These models give 18-year deflection estimates that are 50-77% lower than measured and yield unrealistic shapes of the deflection history. They also predict the 18-year prestress loss to be 46-56% lower than the measured mean prestress loss, which was 50%. Model B3, which is the only theoretically based model, underestimates the 18-year deflection by 42% and gives a prestress loss of 40% when the default parameter values are used. However, in Model B3, several input parameters are adjustable and if they are adjusted according to the long-time laboratory tests of Brooks, a close fit of all the measurements is obtained. For early deflections and their extrapolation, it is important that Model B3 can capture realistically the differences in the rates of shrinkage and drying creep caused by the differences in the thickness of the walls of the cross section. The differences in temperature and possible cracking of the top slab also need to be taken into account. Other paradigms on which data have recently been released are four bridges in Japan and one in the Czech Republic. Their excessive deflections can also be explained. The detailed method of analysis and the lessons learned are presented in Part II. DOI: 10.1061/(ASCE)ST.1943-541X .0000487. © 2012 American Society of Civil Engineers. CE Database subject headings: Creep; Shrinkage; Box girders; Serviceability; Deflection; Span bridges. Author keywords: Prestressed box girder; Bridges; Segmental erection; Shear lag; Design standards; Concrete; Relaxation.

Analytical Model for CFRP-Strengthened Prestressed Concrete Girders Subject to Cyclic Loading

An analytical model is presented to simulate the behavior of prestressed concrete girders strengthened with various carbon fiber-reinforced polymer systems and subjected to static and cyclic loading. The initial concrete strains owing to prestressing and girder self weight load at the moment of the application of the strengthening system and the concrete cyclic creep as a result of the cyclic loading are considered in the model. Experimental results are used to validate the analytical model. Additionally, deflection and concrete strain increases on account of the cyclic loading are compared to the values provided by Comite Euro-International du Beton and Federation Internationale de la Precontrainte (CEB-FIP) and Associao Brasileira de Normas Tecnicas (NBR) 6118 codes. Deflection and concrete strain obtained from the analytical model were above those observed in the tested girders, especially after 100,000 cycles, owing to the logarithmic function used to express the fatigue behavior of concrete. In gener...

Numerical Analysis of Concrete Material Properties at High Strain Rate Under Direct Tension

The tensile strength of concrete material increases with the strain rate. Dynamic tensile strength of concrete material is usually obtained by conducting laboratory tests such as direct tensile test, flexural test, spall test or splitting test (Brazilian test). Some codes of practice such as Comite Euro-International du Beton (CEB) give empirical relations of concrete material dynamic increase factor (DIF) based on testing data. However, the reliability of the dynamic testing and the derived DIF are under debating. It is commonly agreed now that the DIF obtained from dynamic impact test is affected by lateral inertia confinement effect. Therefore, those derived from testing data do not truly reflect the dynamic material properties. The influence of the lateral inertia confinement, however, is not quantified. Moreover, concrete is a heterogeneous material with different components, but is conventionally assumed to be homogeneous, i.e. cement mortar only, in most previous experimental or numerical studies. In the present study, a mesoscale concrete material model consisting of cement mortar, aggregates and interfacial transition zone (ITZ) is developed to simulate direct tensile tests and to study the influences of the lateral inertia confinement and heterogeneity on tensile strength increment of concrete materials with respect to strain rates. The commercial software AUTODYN is used to perform the numerical simulations. The influence of lateral inertia confinement on tensile DIF of concrete material is examined.

Influence of high strain rates on stress–strain relationship, strength and elastic modulus of concrete

Abstract Earlier papers showed a method for strength evaluation at high strain rate from the results of static tests, and a method for evaluating modulus of elasticity at static loading from tests based on stress-wave propagation. This paper presents new results on the influence of high strain rates on the stress–strain relationship, strength and modulus of elasticity using the thermofluctuation theory, principle of accumulation and development damages as well as nonlinear behavior of concrete under loading. Obtained equations explain and unify the influence of high rates on the mechanical properties of concrete for different kinds of uniaxial stress. Comparative study showed again substantial agreement with existing experimental results and general equations by the Comite Euro-International du Beton (CEB) Model Code. The developed approach diminishes the labor expenses for tests of concrete under high rates of loading.