Concrete Surface Strains Research Articles

The Prototype Test on Sealing Capacity and Mechanical Behavior of the Underground Segmental Lining in a Low Vacuum Environment

The super-speed tube transport (SSTT) system is called the fifth traffic mode internationally. One of the critical technical problems for the design of SSTT system is the sealing reliability in vacuum tube structures. In this paper, an experimental study of the sealing capacity and mechanical behavior of the segmental lining is carried out, and the following conclusions are drawn. To meet the requirements of the SSTT system, the existing sealing of the concrete segment structure needs to be further strengthened. The joint spacing decreases with increasing vacuum and increases with vacuum pressure relief. There is an obvious correlation between the strain change and the vacuum pumping, which can be divided into four stages. The influence of the pressure difference and the local airflow movement on the concrete surface strain is dominant by turns at different stages. The concrete surface is suggested to be reinforce on account of the tensile strain caused by the local airflow movement of vacuum pumping is significant. The relative pressure caused by vacuum has little effect on the stress and deformation of the structure. The results of the study can help to understand the sealing capacity and mechanical behavior of the underground segmental lining in a low vacuum environment and be beneficial in choosing the appropriate structure type for the SSTT system.

Experimental testing of residual strain and displacement development in pretensioned BFRP reinforced concrete beams

The utilisation of fibre reinforced polymers (FRPs) as reinforcement for concrete elements has attracted attention mainly due to their high tensile strength, light weight and corrosion resistance. Lately, there has been an interest in basalt FRPs as a more economically competitive and environmentally friendly option. Basalt FRP is manufactured from a widely available volcanic rock in a process which does not require any additives, giving it an edge over the currently more popular glass-fibre reinforced plastic (GFRP) reinforcement. Pretensioning of FRP reinforcement with low longitudinal modulus of elasticity, such as GFRP and BFRP has been proposed as an effective solution to the concerns regarding the serviceability performance of flexural elements reinforced with these composite materials. In previous research it has been demonstrated that prestressing even at low levels can significantly reduce deflections and postpone cracking of BFRP reinforced concrete elements. This research presents an experimental investigation of pretensioned BFRP reinforced concrete tested under quasi-static loading and unloading cycles at 5kN load increments until failure. A comparison with an unprestressed sample is also provided to examine the effectiveness of prestressing at improving the structural performance of the beams. The samples were equipped with internal strain gauges and linear displacement transducers to monitor the development of strains in the reinforcement, deflections and concrete surface strains during testing. Close monitoring of the anchorage zone and the development of cracks was also conducted. Based on the experimental results it can be concluded that the prestressing of BFRP reinforced beams delays the development of residual strains and residual displacements upon unloading. Furthermore, the increase in the prestress level further reduced the residual strain and displacements.

Shear Strength of Reinforced Fibrous-self Compacted Concrete Box Girder using Recycled Concrete Aggregate

Protection of environmental and conservation of natural resources is a fundamental issue in today’s world. In this research, the shear behavior of reinforced concrete box girders with recycled aggregate (RA), steel fiber, and internal diaphragms were investigated. Eleven reinforced concrete box girders with typical longitudinal and transverse reinforcement were tested under two point loading until failure. The RA was prepared by crushing the collected waste of concrete from the laboratory test cylinders and cubes. The experimental variables considered include; RA percent of 50%, 75%, and 100%(replacing from NA), steel fiber with volumetric ratios (Vf) of 0.5%, 1.0%, and 2%, diaphragm numbers (two and three).The test results revealed that the shear strength of the box girders affected by the RA content, the ultimate load was decreased by (32, 25, and 19) % for the (100, 75, and 50) %RA concrete, respectively in compared with the control specimen. In contrast the steel fiber was more effective in strengthening of the RA concrete specimen, for the (Vf) of 0.5%, 1.0%, and 2.0% with non-fibrous 100% RA concrete, the strengthening were (25, 40, and 77) % respectively. Moreover, when 1.0% steel fibers added to the 100%, 75%, and 50%RA concrete respectively, the strengthening were (40 ,45 ,and48 )% compared each with its reference specimen. On the other hand, when two and three diaphragms used, the strengthening for the non-fibrous 100% RA concrete was (6% and 9%) respectively. cracking load, ultimate load, load- deflection, and concrete surface strain has been taken into consideration in this research.

Quantification of the Effect of Train Type on Concrete Sleeper Ballast Pressure Using a Support Condition Back-Calculator

Monitoring ballast support condition and improving current sub-structure and ballast maintenance strategies is critical to ensuring safe and efficient railroad operations. Researchers at the University of Illinois at Urbana-Champaign (Illinois) have developed a ballast support condition back-calculator, a non-destructive instrumentation method and corresponding analysis tool that quantifies ballast pressure distributions under concrete sleepers without interrupting revenue service train operations. This laboratory-validated non-intrusive method uses concrete sleeper bending moment profile and rail seat loads as inputs to back-calculate the reaction distribution using a Simulated Annealing optimization algorithm that incorporates Pareto Distribution as the random variable generator. In order to further understand in-service ballast support conditions, concrete surface strain gauges were installed on concrete sleepers at a revenue service field site to measure strains that could subsequently be converted into bending moments. This site is on a shared use rail corridor with traffic ranging from high speed passenger to heavy axle load (HAL) freight trains. Rail-mounted strain gauges were used to measure strains that were used to calculate the vertical wheel-rail loads to approximate rail seat loads. This paper quantifies the ballast pressure distributions beneath concrete sleepers under different types of rolling stock and evaluates how ballast support condition changes as a function of accumulated tonnage. A wide range of loads were observed at the field site, ranging from 4 to 35 kips (18–156 kN). Corresponding ballast pressures ranged from 14 to 175 psi (97–1,207 kPa), with sleeper-ballast contact area corresponding to 60% of the bottom of the sleeper area. The accumulation of 12.24 million gross tons (MGT) (12.44 million tons) did not generate a quantifiable change in ballast pressure values nor did it generate a change in the ballast support condition. The research results presented in this paper demonstrate the potential of the back-calculator to provide a stand-alone non-invasive method to quantify ballast support conditions, sleeper health, and sleeper bearing stress. Back calculator data will aid the rail industry in optimizing tamping cycles, enhancing safety, and developing more representative concrete sleeper flexural designs based on actual support conditions.

USING CARBON FIBER STRIPS IN JOINTS STRENGTHING OF BOX SEGMENTAL SELF-COMPACTING CONCRETE BEAMS

The main objective of this research is to study the effect of carbon fibers used to strengthen the joints of the box segmental beams. For this research, four beams were produced and tested. One of these beams, monolithically, was cast as a reference beam and the three others were segmental beams. All beams were produced with Self-Compact Concrete (SCC) and box cross section. Each segmental beam consisted of three precast concrete segments were connected by post tensioning tendons. The three segmental beams have same characteristics, but different in joint types between the segments. The types of joints used were (dried, epoxied and dried strengthen by CFRP sheets). All beams were tested under static two point loads up to failure. For each test, deflections at mid-span location were recorded for each (5kN). Also, first cracking, mode of failure and ultimate loads values were recorded as well as the concrete surface strains at the specified locations for both loadings.

Effect of joint positions on flexural behaviors of box segmental specimens internally post tensioned under cyclic and static loadings

The goal of this paper is to get a better knowledge of effect of joint positions on flexural behavior of box segmental specimens under bending stresses. Three segmental specimens with internal tendons were produced and tested. Each segmental specimen was produced assembling three precast concrete segments and they were connected by using four post tensioning tendons. All segmental specimens were formed with dry joint type, but they different in joint positions. All specimens were tested under two types of loadings cyclic and static two point loads. The test of cyclic loading was carried out by exposure each specimen to three loading cycles. The static load test was carried out in same rate of loading that of cyclic load test and all girder specimens were tested up to failure. The loads versus deflections at specified points were recorded. Also, cracking, mode of failure and ultimate loads values were recorded as well as the concrete surface strains at the specified locations for both loadings.

Strut Confinement of Simply Supports Deep Beam Using Strut Reinforcement

This study investigates the effect of confining the Strut region of the deep beam by using Struts Reinforcement; which consists of four main bars enclosed by stirrups. Six specimens were tested for investigating the behavior of deep beams including; ultimate load, mid-span deflection, crack pattern, first shear and first flexure cracks, concrete surface strain and mode of failure. The specimens were tested under two symmetrical points load with and of 1 and compressive strength of 38 MPa. The main parameters were: first one the diameter of the main bars of Strut Reinforcement (8, 10, 12 mm) with constant spacing of stirrups equal to 80 while the other parameter was varied spacing of stirrups of strut reinforcement (120, 100, and 80 mm) with constant main bars diameter of 8 mm. The test results showed that the Strut confinement generally increased the ultimate load from 750 kN to 1250 kN and the ductility of the beam, confined shear cracks and strain surface across the strut and shear area and turned failures mode from shear failure to flexure. The increase in the diameter of the main bars enhanced the behavior of the beam more than the stirrups number.

Structural Behavior of Reinforced Concrete One-Way Slabs Cast with Self-Compacting Concrete Containing Recycled Concrete as Coarse Aggregate

This work deals with the effect of using Recycled Concrete Aggregate (RCA) as a partial replacement of coarse aggregate in Self-Compacting Concrete (SCC), on the structural behavior (flexure and shear) of reinforced concrete one-way slabs. To the authors’ knowledge, this study is one of limited studies concerning the behavior of recycled aggregate concrete one-way slabs subjected to line loading with significant replacement of conventional aggregates by recycled concrete aggregate (up to 75 %). Three replacement ratios were considered 25 %, 50 % and, 75 %. The mixes (with natural stone coarse aggregate, NCA) have an averaged compressive strength of (Fcu = 42 MPa) at the age of 28 days with a tolerance of (± 1.5 MPa). While, the mixes (with RCA) have an averaged compressive strength of (38.5, 36.5 and 34 MPa) for the three replacement ratios respectively, at the age of 28 days with a tolerance of (± 2 MPa). All the slabs were cast with length of (1600 mm), width of (600 mm), while the thickness was variable. For this purpose, sixteen reinforced concrete one-way slabs were cast and divided into five groups (G1 to G5). Different parameters that affect the behavior of one-way slabs were studied and include type of failure, replacement ratios of NCA by RCA, amount of main reinforcement, thickness and locations of line loadings along the span. Hardened concrete specimens results show that the compressive strength Fcu, tensile strength Ft, modulus of rupture Fr and modulus of elasticity E were decreased as the RCA replacement increased. The experimental results of slabs show that the ultimate capacity of slabs decreased as the RCA replacement increased, the deflection and strain increase as the RCA replacement increases and the crack width increases as the RCA replacement increases. From the results of ultimate capacity, cracking load and moment, deflections, crack width and pattern and concrete surface strains, it can be concluded that the recycled concrete aggregate can be used as a partial replacement of natural coarse aggregate to produce self-compacting concrete mixes. Also, the behavior of one way slabs cast with SCC containing RCA is acceptable.

Flexural Behaviours of Box Segmental Beams with Internal Tendons Subjected to Repeated and Static Loads

The main objective of this study is to gain a better understanding of the behaviours of different types of joints when box segmental beams are subjected to bending. Five beams with internal tendons were thus produced and tested: one beam was cast monolithically as a reference beam and four were formed as segmental beams. All beams were produced using Self Compacting-Concrete (SCC) in box sections. Each segmental beam was formed by assembling three precast concrete segments with four post tensioning tendons. The segmental beams had the same general characteristics, but with different types of joints between the segments. Four types of joints were adopted in this study;-dried, epoxied, dried strengthened with carbon fibres and multi-steel shear keyed joints. All beams were tested under two types of loadings, repeated and static. A test of repeated loading was carried out by exposing each specimen to three loading cycles. Each cycle was implemented by subjecting each specimen to up to 40% of the ultimate load then releasing the loads to return the specimen to the non-loading case. The test of static loading was carried out at the same rate of loading as the repeated load test, and all girder specimens were tested up to failure. The loads versus deflections at specified points were recorded for each 5 kN. Cracking, mode of failure and ultimate load values were recorded, along with the concrete surface strains at specified locations for both loadings. The tests results for repeated loading showed that joint type had a clear effect on behaviours in the segmental beams, which had deflection readings ranging from 16 to 61% higher than that of the monolithic (reference) beam, as well as higher tensile strain than seen in the reference beam, by 8 and 67%. Similar effects of joint type were observed under static test. At the first crack loading, the segmental beams had deflections of between 8 and 94% and tensile strains of about 5 to 18% more than the reference beam. The failure modes of the tested beams were all different. Failure of the dry joint occurred in the segment interface, while the failure of each epoxied joint and joint strengthened by carbon fibres developed in the concrete adjacent to the segment interface. The steel shear keys clearly transfer the shear resistance to the concrete parts adjacent of the joints.

Development of a parametric model for the prediction of concrete railway crosstie service bending moments

Concrete crosstie usage in North America continues to increase for rail transit and heavy axle load freight railroad applications. As such, it is important to design optimized crossties to save both capital and maintenance funds. Recently, a method for quantifying concrete crosstie bending moments using concrete surface strain gauges has been developed, deployed, and validated. Data from this method are used in this paper for (1) building a model to quantify sources of variability for field bending moments and the relative influence of each source, (2) generating an accurate model to predict bending moments at the two field locations surveyed, and (3) comparing the relative effects of predictor variables on rail transit and heavy axle load freight rail modes to determine their influence on service bending moments. Results show that it is possible to develop a reliable model to predict bending moments, and that several factors have a strong influence on these predictions, namely vertical load, temperature gradient, and axle location within a railcar truck. The most significant factor is the crosstie support condition, especially with respect to center moments. While the aforementioned model’s primary utility is for the two sites and railroad systems surveyed, the model provides a valuable tool for determining which variables are the most critical for inclusion in the future mechanistic design of concrete crossties.

Effect of Temperature on Concrete Carbonation Performance

In this study, the effect of temperature on macroperformance and microcharacteristic of carbonized concrete was investigated. The carbonation depth, compressive strength, and surface strain of concrete under different simulated environments for 28 d were measured. XRD and ESEM-EDS analysis were conducted to present the phase composition, types of hydration products, and microstructure characteristics of samples before and after carbonation. The results showed that the effects of temperature on carbonation depth, strain, and compressive strength were significant. There was a linear relation between temperature and carbonation depth as well as compressive strength of concrete. The effects of environment factors on concrete surface strain after carbonation manifested as the strain value and the slope of linear segment of strain curve. Significant differences of phase composition and hydration products were observed before and after the carbonation, which mainly manifested as attenuation and disappearance of diffraction peaks of hydration products. Temperature affects the crystal form of the carbonation products.

Reinforced SCC one-way slabs: time-dependent flexural cracking control

Fibre-reinforced self-compacting concrete (FRSCC) is a high-performance building material that combines the positive aspects of the fresh properties of self-compacting concrete (SCC) with the improved characteristics of hardened concrete as a result of fibre addition. To produce SCC, either the constituent materials or the corresponding mix proportions may notably differ from conventional concrete (CC). Therefore, it is vital to investigate whether all the assumed hypotheses about CC are also valid for SCC structures. This paper reports a study in which eight SCC and FRSCC slabs of similar cross-section were monitored under service loads for up to 240 d. The recorded time-dependent cracking and deflections are presented. The steel strains within the high-moment regions, the concrete surface strains at the tensile steel level, deflection at the mid-span, crack widths and crack spacing were recorded throughout the testing period. The experimental results show that the instantaneous crack widths and spacings for the normal SCC slab series were close to and much less than, respectively, the corresponding variables for the normal CC slab series.

Quantification of Loading Environment and Flexural Demand of Prestressed Concrete Crossties under Shared Corridor Operating Conditions

Optimization of the design of railway infrastructure and its components requires a comprehensive understanding of the loading demands that are expected. Currently, many design guidelines for track components use historical wheel loads and calculate bending moments based on broad assumptions. However, tools are available to accurately quantify and characterize the variability each load has on a particular component. This is particularly important in shared use rail corridors where higher speed passenger services operate on the same infrastructure as heavy axle load (HAL) freight trains. Each traffic type generates unique demand variabilities, and these should be incorporated into a holistic approach to optimized track design. Therefore, researchers at the University of Illinois at Urbana-Champaign (UIUC) are conducting field research aimed at the characterization of field conditions on Amtrak’s Northeast Corridor through the use of wheel impact load detector (WILD) data and concrete crosstie surface strain gauges. Results from this experimentation show high variability of loads resulting from varied types of train operations and significant differences in impact load ratios. Finally, laboratory measured flexural capacity for the concrete crossties showed a conservative design with a potential margin of improvement in reduction of residual capacity (i.e., factor of safety).

Simple model for time-dependent bond transfer in pretensioned concrete using draw-in data

Time evolution of prestress loss and bond transfer length holds vital information concerning long-term performance of pretensioned prestressed concrete. In this paper, it is proposed to utilize long-term measurements of strand draw-in to extract this time-varying information, which could be more effectively obtained than concrete or strand surface strain measurements. Theoretical investigations combined with numerical studies of experimental data are carried out for this purpose. This study builds on Guyon’s boundary value problem (BVP) model which quantifies draw-in for instantaneous elastic response. A one-dimensional linear viscoelastic standard solid model is employed to model the creep of concrete, which is a simple but effective model of the time-dependent response of concrete. Guyon’s BVP model is generalized to include time dependence and then combined with an existing initial value problem (IVP) model for post-tensioned concrete leading to a new mixed model for time-dependent prestress loss and bond transfer in pretensioned concrete.Analysis is undertaken in this study to (i) quantify long-term prestress loss and bond-transfer behaviors by applying the proposed model and directly utilizing measured draw-in time history data, (ii) validate the proposed quantitative analysis by using scaled pretensioned concrete beams with different types of aggregates and strengths of concrete - among other factors, and (iii) offer discussion of future work. For example, introducing concrete drying shrinkage into the proposed simple model seems promising and is necessary to provide an improved representation of the time-dependent behavior of prestressed concrete.

Quantification of concrete railway sleeper bending moments using surface strain gauges

As the use of concrete sleepers increases for heavy-haul freight railroad and rail transit applications in North America, it is becoming more critical to quantify their flexural performance under revenue service traffic in an effort to improve sleeper design and maintenance practices. The objective of improving sleeper design and performance is achieving longer service lives, lower life cycle costs, and fewer in-service failures. Presently, center cracking is one of the most common factors limiting the service life of concrete sleepers in North America, and rail seat cracking has also been documented as a performance concern. Improving the understanding of sleeper flexure can help reduce the occurrences of cracked sleepers by ensuring designs are adequate for the field conditions that are encountered. Additionally, previous laboratory research conducted at the University of Illinois at Urbana-Champaign (UIUC) found that sleeper flexure magnitude is highly dependent on support conditions. To date, few methods have been proposed to accurately quantify the in-service field bending moments of concrete sleepers and their variability due to support conditions and other factors. A method using concrete surface strain gauges has been developed, deployed, and validated by UIUC for quantification of sleeper bending moments. This method has been successfully deployed in the laboratory and in seven field locations, providing flexural demand data that can be used for the design of concrete sleepers. This paper will present the aforementioned instrumentation methodology and results from one field installation in which surface strain gauges were installed on ten concrete sleepers on a high-tonnage, heavy-haul freight railroad with the objective of quantifying sleeper-to-sleeper bending moment variability.

Experimental investigation of cracking and deformations of concrete ties reinforced with multiple bars

Interpretation of test results may be inadequate even for simple test layouts. On the one hand, conventional tests, that investigate a concrete prism reinforced with a single bar in the centre, do not allow to account for the effect of variation of the diameter-to-reinforcement ratio (∅/p). This important limitation should be related to the use of simple specimens in constitutive modelling. On the other hand, a tie-test typically provides measurements of average deformations of the internal reinforcing bar and the concrete surface. The experimental evidence, however, often contradicts the general assumption of the equivalence of the mean strains of reinforcement and concrete. The strain difference is closely related to the width of the concrete cover. This paper investigates the effect of distribution of bar reinforcement on deformation and cracking behaviour of tensile elements. Special testing equipment has been developed to investigate ties reinforced with multiple bars. The test program consists of 23 ties. The number and diameter of the bars vary from 4 to 16 and from 5mm to 14mm, respectively. Two different covers (30mm and 50mm) are considered as well. The deformation analysis is based on measurements of the average reinforcement and concrete surface strains. The development of cracks was investigated using digital image correlation (DIC). The Model Code 2010 and Eurocode 2 predictions are compared with the experimental data. While the Design Codes predict that the maximum crack spacing is dependent on the ∅/p ratio, the test results indicate that the crack distances are actually much less dependent on the reinforcement characteristics.

Blast response of full-size concrete walls with chemically reactive enamel (CRE)-coated steel reinforcement

In this study, two full-size concrete walls were tested and analyzed to demonstrate the effectiveness of a chemically reactive enamel (CRE) coating in improving their mechanical behavior under blast loading: one with CRE-coated rebar and the other with uncoated rebar. Each wall was subjected in sequence to four explosive loads with equivalent 2, 4, 6-trinitrotoluene (TNT) charge weights of 1.82, 4.54, 13.6, and 20.4 kg. A finite element model of each wall under a close-in blast load was developed and validated with pressure and strain measurements, and used to predict rebar stresses and concrete surface strain distributions of the wall. The test results and visual inspections consistently indicated that, compared with the barrier wall with uncoated reinforcement, the wall with CRE-coated rebar has fewer concrete cracks on the front and back faces, more effective stress transfers from concrete to steel rebar, and stronger connections with its concrete base. The concrete surface strain distributions predicted by the model under various loading conditions are in good agreement with the crack patterns observed during the tests.

Long-term flexural cracking control of reinforced self-compacting concrete one way slabs with and without fibres

In this study experimental result of a total of eight SCC and FRSCC slabs with the same cross-section were monitored for up to 240 days to measure the time-dependent development of cracking and deformations under service loads are presented. For this purpose, four SCC mixes are considered in the test program. This study aimed to compare SCC and FRSCC experimental results with conventional concrete experimental results. The steel strains within the high moment regions, the concrete surface strains at the tensile steel level, deflection at the mid-span, crack widths and crack spacing were recorded throughout the testing period. Experimental results show that hybrid fibre reinforced SCC slabs demonstrated minimum instantaneous and time-dependent crack widths and steel fibre reinforced SCC slabs presented minimum final deflection.

Short term bond shear stress and cracking control of reinforced self-compacting concrete one way slabs under flexural loading

Fibre-reinforced self-compacting concrete (FRSCC) is a high-performance building material that combines positive aspects of fresh properties of self-compacting concrete (SCC) with improved characteristics of hardened concrete as a result of fibre addition. To produce SCC, either the constituent materials or the corresponding mix proportions may notably differ from the conventional concrete (CC). These modifications besides enhance the concrete fresh properties affect the hardened properties of the concrete. Therefore, it is vital to investigate whether all the assumed hypotheses about CC are also valid for SCC structures. In the present paper, the experimental results of short-term flexural load tests on eight reinforced SCC and FRSCC specimens slabs are presented. For this purpose, four SCC mixes - two plain SCC, two steel, two polypropylene, and two hybrid FRSCC slab specimens - are considered in the test program. The tests are conducted to study the development of SCC and FRSCC flexural cracking under increasing short-term loads from first cracking through to flexural failure. The achieved experimental results give the SCC and FRSCC slabs bond shear stresses for short-term crack width calculation. Therefore, the adopted bond shear stress for each mix slab is presented in this study. Crack width, crack patterns, deflections at mid-span, steel strains and concrete surface strains at the steel levels were recorded at each load increment in the post-cracking range.

Concrete surface strain measurement using Moiré fringes

Evaluation of the stress state of concrete in prestressed concrete (PC) structures is extremely important for construction management and O&M (operation and maintenance). We developed a strain visualization sticker using the principle of Moiré fringes, which makes it possible to read the approximate strain value of a concrete surface directly with the naked eye and also enables remote, noncontact measurement of strain with high accuracy by a digital camera, etc. This paper presents an outline of the developed strain visualization sticker and the results of verification of its performance.