Concrete Prismatic Specimens Research Articles

Study on the influence mechanism of polypropylene fiber on crack propagation of concrete with existing cracks under uniaxial compression

In the process of long-term service of concrete, cracks are inevitable, especially in the state of tension and compression, the crack expansion speed is accelerating, which seriously affects the service performance of concrete, and even causes major disasters. While polypropylene fiber has remarkable cracking resistance, there is a lack of theoretical analysis on crack initiation and path expansion of concrete cracks with existing cracks during the compression process. In order to analyze the inhibitory effect of fiber on the propagation of pre-existing cracks, we performed uniaxial compression tests on concrete prisms containing pre-existing cracks. Acoustic emission technology was used to monitor each test group and obtain relevant data on the cracking process of the specimens. GDEM software was used to simulate typical test conditions, and the results were compared with the test results to further explain the mechanism of crack propagation and fiber suppression. The results show that: (1) fiber can inhibit the initiation and propagation of tension wing cracks at the existing crack tip. The higher the fiber content, the shorter the symmetrical wing cracks become. (2) With the increase of fiber content (0 ∼ 4 kg/m3), the failure mode of polypropylene fiber-reinforced concrete prismatic specimens changes from tensile failure to tensile-shear failure, and finally shear failure. (3) The crack initiation stress and peak strength of prismatic specimens first increase and then decrease with the increase of fiber content. (4) The increase of fiber content delays the crack initiation time, prolongs the time to the disappearance of the peak of the acoustic emission signal, and prolongs the total time of the failure process.

Effect of the type of posttensioning anchorage on the behavior of anchorage zones

AbstractIn order to study the cracking behavior and the bearing strength of anchorage zones of posttensioning tendons using different types of anchorages, an experimental research has been performed using concrete prismatic specimens. The studied variables included the type of anchorage and the cross‐section dimensions, so the experimental program included eight prismatic specimens, four with 265 mm × 265 mm × 580 mm and four with 300 mm × 300 mm × 600 mm, reinforced with spirals and stirrups, and with two different types of anchorages. The concrete strains were measured to monitor cracking and to assess the relation between all the different variables involved on cracking and on the ultimate loads. The experimental cracking and ultimate loads were compared with models found in the literature. From the presented work, it can be concluded that the use of simple anchorage bearing plates leads to higher cracking and failure loads. This difference is more significant for cases with smaller cross‐sections.

Behavior of pre-damaged normal-strength concrete prisms repaired with basalt FRP sheets under axial compressive monotonic and repeated loading

The damaged concrete columns under earthquake can be retrofitted with fiber-reinforced polymer (FRP) and the axial mechanical properties of the strengthened columns can be greatly enhanced. In the present study, monotonic and repeated axial compressive behavior of damaged concrete prisms repaired by basalt FRP (BFRP) was investigated. Pre-damage tests were performed on plain concrete prismatic specimens under three axial compressive loading levels. Then, the damaged specimens were repaired with 1-, 2-, 4-, and 6-layer BFRP composites, followed by monotonic or repeated compression tests. The test results showed that the compressive strength and axial deformation capacities of the damaged prismatic specimens under monotonic compressive loading were improved remarkably after repairing. However, in the stress–strain curves of BFRP-repaired specimens, the initial elastic modulus, and the strength at the turning and ultimate states decreased with the increase of initial damage. Based on the test results of this study and existing literature, the strength and strain at the turning and ultimate states were estimated, and a monotonic stress–strain relationship was proposed for BFRP-repaired concrete prisms considering the adverse effect of pre-damage level. Further, a modified repeated compressive stress–strain model was proposed for BFRP-repaired damaged concrete prisms. The proposed models predicted well the test results.

Effect of admixed chloride and molarity of NaOH solution on early-age strength and reinforcement corrosion in FA/GGBS geopolymer concrete

Chloride persuaded steel reinforcement corrosion is an important durability issue in concrete structures. In this present study, the corrosion of reinforcing steel was evaluated in 0% and 3% sodium chloride admixed fly ash (FA)/ground granulated blast furnace slag (GGBS) GPC mixes. Two different molarities of NaOH solution i.e. 8 M and 10 M were used for preparing the GPC mixes with FA/GGBS proportions of 100/0 (wt.%), and 70/30 (wt.%). Reinforced geopolymer concrete prismatic specimens were prepared for half-cell potential and linear polarization resistance (i.e. LPR) measurements after 28 days and 180 days of its manufacturing. The obtained results showed that the early age 7 days compressive strength of GPC mixtures reduced with the existence of NaCl. The higher value of compressive strength was detected in GPC prepared with FA/GGBS proportion of 70/30 as compared to that made with FA/GGBS proportion of 100/0. Further, GPC made with higher molarity (10 M) of NaOH solution showed greater strength than that made with lower molarity (8 M) of NaOH solution. The less negative half-cell potential value and lower corrosion current density (Icorr) value of steel reinforcement in GPC was observed at the later age i.e., at 180 days in all GPC mixes as compared to 28 days. The higher chance of rate of corrosion, and the higher Icorr of steel reinforcement was detected in 3.0% NaCl added geopolymer concrete. Further, more negative half-cell potential and higher Icorr of steel reinforcement were detected with a rise in molarity of NaOH solution in GPC made with FA/GGBS proportion of 70/30 at both ages (i.e. at 28 days and 180 days). However, mostly less negative half-cell potential and lower Icorr of reinforcing steel were detected with a rise in molarity of NaOH solution in GPC made with FA/GGBS proportion of 100/0 at both ages.

Transformation of concrete deforming diagrams in conditions of uniform and non-uniform stress states

The aim of the research is formulated, which is the theoretical substantiation, development and testing of the concrete state diagram, suitable for use to calculations of the strength of bended reinforced concrete elements based on the deformational model, as well as for more complete assessment of non-uniform stress-strain state of the cross-sections of elements at all stages of their loading up to the destruction directly on the values of the relative strains of concrete and the corresponding stresses.
 In this paper, an analytical method of converting the parameters of reference concrete deforming diagrams, which are determined experimentally during standard tests of concrete prismatic specimens by short-term axial compression and tension up to the destruction in «soft» mode of loading, into parameters of simulated transformed diagrams, which correspond to the «rigid» mode of loading of concrete, is developed. The way of simultaneous application of analytical expressions for the specified defoming diagrams, which describe the operation of concrete in conditions of uniform compression, for transition to the state diagram, which displays the functional relationship between the stresses and strains of the concrete compressed zone in bended concrete and reinforced concrete elements, cross-sections of which operate in conditions of non-uniform stress state, is proposed.
 The results of testing the obtained mathematical model of the concrete state diagram during the calculation by different methodics of strength of the normal cross-sections of bendable reinforced concrete elements are presented.
 It has been determined that a promising direction for further experimental and theoretical research is the study of the application of the developed methodics of graphical building and mathematical descripting of concrete deforming diagrams under conditions of uniform and non-uniform compression to the calculation of concrete and reinforced concrete elements that are operated under the action of long-term and short-term low-cycle loadings.

Application of natural fiber composite materials in the strengthening of reinforced concrete structures

This paper presents an experimental investigation of strengthened reinforced concrete structure by bonded natural fiber composite materials. Nonetheless, the composite plates are externally constituted of a matrix based on epoxy adhesive in which the aligned continuous natural fibers are embedded. In recent years, the natural fibers are of great interest as a result of policies to reduce the negative impact on the environment. They are a renewable source, helping to capture carbon dioxide during their cultivation. Numerous technical, economic, and environmental advantages such as low cost, high strength-to-weight ratio, low density, and non-corrosive properties make them very attractive to use. The objective of this research is to assess the strengthened capacity of natural fibers. This research mainly used hemp and flax natural fibers. So, these natural fibers are an alternative to synthetic reinforcing fibers like carbon fiber or glass fiber, which are widely used in civil engineering. This article briefly presents the results of the tests carried out on concrete prismatic specimens strengthened by external bonding different composite plates based on natural fibers. The tests were presented by examining the influence of reinforcement on the ultimate capacity and the element of rigidity. The results show a significant increase in the ultimate load of the strengthened concrete prismatic specimens compared to the reference specimen. The cracking and failure modes are presented. The results show that the natural fibers are a good alternative to glass and carbon fibers.

Efficiency of the confinement reinforcement in anchorage zones of posttensioning tendons

In order to study the efficiency of the confinement reinforcement in anchorage zones of posttensioning tendons, using both ordinary reinforced concrete (ORC) and high‐performance fiber reinforced concrete (HPFRC), an experimental research has been performed on the bearing strength of concrete prismatic specimens. This research also intended to assess both ultimate capacity and adequate serviceability of the local anchorage zone when reducing the concrete cross section and the confining reinforcement, both specified by the anchorage device supplier, by using an HPFRC. The experimental program included ORC prismatic specimens with 305 mm × 305 mm × 650 mm reinforced with spirals and a combination of spirals and stirrups, and HPFRC specimens with 210 mm × 210 mm × 420 mm reinforced with stirrups. The reinforcement steel strains were measured to monitor the steel yielding, to assess the relation between the strains of the inner (spiral) and outer (stirrups) levels of reinforcement and the uniformity of the confinement forces along the confined length. A comparison between the bearing strength obtained by experimental tests and by models found in the literature is also presented. From the strain measurements, it can be concluded that in the case where there are, simultaneously, two types of confinement reinforcement, the outer reinforcement is not as effective as the inner one. Therefore, when computing the bearing strength, the linear superposition of contributions of both spiral and stirrups for confinement can be nonconservative, leading to too high estimated load capacity.

Experimental study on fatigue behaviour of CFRP plates externally bonded to concrete substrate

AbstractThe behaviour of the bond between fibre‐reinforced polymers (FRP) and concrete greatly influences the behaviour of concrete structures strengthened with FRP composites. Although numerous experimental studies have investigated this bond, experimental data concerning fatigue tests on carbon FRP plates attached to concrete blocks are still lacking. Therefore, a series of double‐lap shear tests under monotonic and fatigue loadings were performed on concrete prismatic specimens reinforced with CFRP plates. First, a series of experimental investigations are summarized. Thereafter, the fatigue behaviour of CFRP plate debonding is characterized using S‐N diagrams that represent the relationship of the upper‐limit fatigue load with the monotonic load strength and the number of cycles to debonding on a semi‐logarithmic scale.

Investigation of effect of fineness dependent volume of fine aggregate on passive layer formation and corresponding corrosion of steel reinforced concrete using electrochemical approach

This paper aims to determine the effect of the fineness modulus of fine aggregate on the quality and development of passive film and its subsequent effect on the corrosion of reinforced concrete structures. The study uses both electrochemical laboratory controlled experimentation and finite element modelling approach. Corrosion is an electrochemical process that requires an electrolyte for the occurrence of a corrosion reaction. Therefore, it is necessary to calculate the effective corrosion rate with reference to the saturated area of the concrete only when there is corrosion of reinforced concrete structures. Theoretically and numerically, the saturated area depends on the capillary zone porosity, gel zone porosity and degree of saturation in the aggregate free volume of concrete, which is a heterogeneous material. It is thus important to understand the effect of aggregate fineness on the corrosion of steel rebars. Therefore, detailed investigations were conducted using a variety of fine aggregates and combinations thereof. The initiative for this research came from the observation that the interfacial transition zone (ITZ) around the steel bar in concrete does not contain coarse aggregate, but could be surrounded by mortar and not only paste. Hence, the fine aggregate volume may also influence the corrosion rate. Some fine aggregate, which is finer than coarse aggregate, could be present in the vicinity of the steel bar surface, depending on the fineness modulus, which may affect both the development of the passive layer and the corrosion rate. The previous research data in this area were found to be limited. Hence, these factors have been investigated in the present study. Both mortar and concrete prismatic specimens were cast, and the quality of passive layer development and the corrosion severity were compared between specimens to determine the effect of fine aggregate volume with respect to the respective fineness modulus values. The corrosion potential, current and rate values obtained through experimentation showed significant differences in their magnitudes with respect to the variation in the fineness modulus. A directly proportional relation was observed between the fineness modulus of fine aggregate and the corrosion of rebars. Therefore, in the light of the experimental results obtained in this study, a finite element based numerical model incorporating the effect of the fineness modulus of fine aggregate on corrosion of steel rebars in concrete has been developed and successfully verified in this study.

Prestress losses evaluation in prestressed concrete prismatic specimens

This paper presents an experimental research work to evaluate prestress losses in pretensioned prestressed concrete. An experimental program including variables such as concrete mix design, specimen cross-section size and concrete age at the prestress transfer was carried out. Several pretensioned prestressed concrete prismatic specimens were made and tested using the ECADA+ test method, based on measuring prestressing reinforcement force. In addition, specimens were instrumented to obtain the longitudinal concrete strains profiles at any time. Measurements from both techniques were taken over 1year. Measured prestress losses included elastic shortening losses and time-dependent losses due to concrete shrinkage and creep. A coefficient to account for the relationship between the prestress losses from the measured prestressing forces and the actual prestress losses from concrete compressive strains is proposed. The experimental results were compared with the predicted prestress losses using methods from several codes.

Experimental Investigation of FRP-Concrete Debonding under Cyclic Actions

The effectiveness of externally bonded fiber-reinforced polymer (FRP) laminates for strengthening existing reinforced concrete (RC) structures is strictly related to the bond of the FRP reinforcement to the concrete substrate. Although numerous experimental studies have investigated this bond, experimental data concerning cyclic tests on both FRP sheets and plates applied on concrete specimens are still lacking. Thus, a series of single shear tests (SSTs) under both monotonic and cyclic actions, without inversion of sign, were performed on concrete prismatic specimens reinforced with carbon FRP (CFRP) sheets or plates. To evaluate and compare the influence of cyclic external actions on the bonding behavior of sheets and plate reinforcements, the results provided by monotonic and cyclic tests are reported and discussed in this paper. In particular, force-displacement relationships, axial strains, and shear stresses recorded along the FRP reinforcement are reported; the influence of the load path on the FRP debonding behavior is also examined.

Freeze-thaw resistance of concrete with porous aggregate

This paper deals with the influence of periodic freezing on lightweight concrete characteristics. Sets of lightweight concrete prismatic specimens are cyclically frozen in range from 20 °C to −20 °C and non-destructively tested after every 25 cycles. Freeze-thaw resistance is determined from measurement of the frost-attacked and non-frost-attacked (referential) specimens. The referential specimens are air/water-cured. Non-destructive methods, especially ultrasonic impulse method and resonance method are used for determination of specimen’s degradation. Experiments are finished with destructive test in order to determine the static modulus of elasticity.

A técnica de dopagem no tratamento da zona de interface: ligações entre concreto novo e velho

Para que as estruturas de concreto alcancem uma vida útil adequada é necessário avaliar a durabilidade e considerar a manutenção das mesmas. Isso tem estimulado o desenvolvimento de tecnologias destinadas a solucionar a problemática de peças deterioradas ou danificadas. Este trabalho analisa o comportamento da zona de interface entre concreto novo e velho após a escarificação do concreto velho (tratamento físico) e o emprego da técnica de dopagem (tratamento químico). A técnica de dopagem consiste na impregnação de pó por via seca ou pasta de alto desempenho, seguida do lançamento de concreto novo, ou no lançamento direto de um concreto novo com características de alto desempenho. O desempenho dessa ligação foi avaliado por meio de ensaios de tração na flexão, até a ruptura, em corpos de prova prismáticos de concreto simples (fck = 20 MPa) e em seguida remoldados. A partir dos resultados, verificou-se que a ruptura dos corpos de prova recuperados ocorreu fora da região de interface e que as resistências à flexão desse concreto foram mantidas tais quais as originais, demonstrando a eficácia do procedimento na ligação entre concreto novo e velho.

The effects of steel fibre reinforced concrete on system ductility

Steel fibre-reinforced concrete is being used extensively today in both field applications and experimental studies on concrete strength and ductility. The state of passive confinement generated by the fibre delays cracking and enhances ductility. The present paper reports on both experimental and analytical studies. In the former, a series of 16 steel-fibre reinforced concrete prismatic specimens were subjected to axial loads and the respective axial load-unit strain diagrams were subsequently plotted to determine the effect of steel fibres on reinforced concrete column ductility. Secondly, an analytical study was run to determine the additional ductility accruing to a frame system when steel fibres are included in the concrete. Analytical models were generated for 16 two-storey, single-span reinforced concrete frames. The columns in these frames were designed to the same characteristics as the specimens used in the experimental tests. Non-linear static (pushover) analyses were performed for each frame to obtain load-displacement curves and determine the effect of steel fibres on reinforced concrete column ductility.

Etude experimentale de la fatigue du beton en flexion 3 points

The results of fatigue tests under three point bending carried out on concrete prismatic specimens showed a significant scatter in the material life time. This scatter did not allow any prevision of the fatigue life time. The concrete damage through the fatigue history seems to be dependent on the development of deformations that grow at constant rate during a large part of the test.