Showing papers in "Aci Materials Journal in 1998"
TL;DR: In this article, a literature review was conducted to determine the extant data characterizing the effects of strain rate on the tensile strength of concrete, and additional new data by Ross and colleagues were considered.
Abstract: A literature review was conducted to determine the extant data characterizing the effects of strain rate on the tensile strength of concrete. In particular, additional new data by Ross and colleagues were considered. These data support the dynamic increase factor (DIF) being a bilinear function of the strain rate in a log-log plot, with no increases for strain rates below 0.000001 with a slope change at a strain rate of 1/s. A DIF of approximately 7 was obtained at the highest reported experimental strain rate of 157/s. The DIF formulation recommended by the European CEB was described, together with its origins. It was found that the data differed somewhat from the CEB recommendations, mostly for strain rates beyond 1/s. Therefore, an alternate formulation was proposed based on the experimental data.
687Â citations
TL;DR: In this paper, a literature review of the effects of high strain rates on the properties of steel reinforcing bars was conducted, and a simple relationship was proposed that gives the DIF (for both yield and ultimate stress) as a function of strain rate and yield stress.
Abstract: A literature review of the effects of high strain rates on the properties of steel reinforcing bars was conducted. Static and dynamic properties were gathered for bars satisfying ASTM A615, A15, A432, A431, and A706, with yield stresses ranging from 42 ksi (290 MPa) to 103 ksi (710 MPa). Strength enhancement with strain rate was expressed in the form of a dynamic increase factor (DIF) defined as the ratio of the dynamic to static yield (or ultimate) stress. It was observed that the DIF would increase for lower reinforcing bar yield stress. A simple relationship is proposed that gives the DIF (for both yield and ultimate stress) as a function of strain rate and yield stress. This relationship is of importance for the analysis of reinforced concrete structures subjected to blast or highly dynamic loads.
382Â citations
Journal Article•
TL;DR: Palaniswamy and Shah as mentioned in this paper closed a discussion of Richart's equation when applied to specimens with very high confining pressures, and the authors responded to points raised by the discussers involving values of the lateral pressure corresponding to the transition point and the limited success of the Richart equation.
Abstract: The authors' closure of a discussion by R. Palaniswamy and S.P. Shah of a paper with the aforementioned title is presented. The original paper was published in this journal (Volume 93, Number 6, November-December 1996), as was the discussion (Volume 94, Number 6, November-December 1997). The authors respond to points raised by the discussers involving values of the lateral pressure corresponding to the transition point and the limited success of Richart's equation when applied to specimens with very high confining pressures.
255Â citations
TL;DR: In this article, an alternative nomograph and various formulas were proposed to predict an evaporation rate of surface water (primarily bleed water) from freshly placed concrete surfaces, and other factors related to evaporic and plastic shrinkage cracking were also addressed.
Abstract: Freshly placed concrete exposed to hot, windy conditions is often prone to plastic shrinkage cracking, though other conditions can also promote this phenomenon This type of cracking is normally noticed on slabs, pavements, beams, and generally other flat concrete surfaces Many factors affect plastic shrinkage cracking, in particular the evaporation of water from the surface of freshly placed concrete Other factors also influence the likelihood of plastic shrinkage cracking such as water-cement ratio, fines content, member size, admixtures, and on-site building practices Evaporation itself is a function of climatic variables such as relative humidity, air temperature, the temperature of the evaporating surface, and very importantly the wind velocity at the surface This paper primarily explains the background to the evaporation nomograph found in ACI 305R-96, "Hot Weather Concreting" (Manual of Concrete Practice, Part 2-1996), where the graph provides a means of estimating the rate of evaporation of surface moisture from concrete The paper offers an alternative nomograph and various formulas to predict an evaporation rate of surface water (primarily bleed water) from freshly placed concrete surfaces Other factors related to evaporation and plastic shrinkage cracking are also addressed
183Â citations
TL;DR: In this paper, the performance of high-strength concrete under multiaxial states of stress was studied and the axial strength of concretes with different compressive strength levels was quantitatively determined.
Abstract: The behavior of high-strength concrete subjected to multiaxial states of stress was studied. An experimental program was undertaken to quantitatively determine the failure surface for high-strength concrete. Results from this study provide the means to predict the failure condition for high-strength concrete under combined stresses. The experimental program was comprised of testing high-strength concretes at three different compressive strength levels. The three strength levels included concretes with compressive strengths of 6 ksi (42 MPa), 10 ksi (69 MPa), and 15 ksi (103 MPa). The triaxial tests were performed on 4-by-8 in (100-by-200 mm) cylindrical specimens. The confining pressures employed in the experiments ranged from 1,200 psi (8.3 MPa) to 12,000 psi (82 MPa). A series of uniaxial tension and compression tests were also performed to develop the necessary data for establishment of the failure criterion. Empirical relationships were developed for prediction of axial strength as a function of confining pressure. In general, the axial strength of high-strength concrete increases with increased confining pressure. However, in comparison with the normal-strength concrete, the effect of confining pressure on the failure strength of high-strength concrete is less pronounced.
155Â citations
TL;DR: In this paper, a physically motivated model for the cyclic stress-strain relationships is proposed to model concrete under general cyclic loading and the validity of the proposed model is confirmed through a comparison of the experimental results and simulated behavior of the model.
Abstract: A rational analysis of reinforced concrete (RC) structures requires satisfactory modeling of the behavior of concrete under general loading patterns. The behavioral characteristics of concrete dominantly depends upon its load history. For the study of concrete behavior, parametric study and experimental investigation into the behavior of concrete under load history of random cycles are performed. Through parametric study, the applicability of the previous concrete models is examined and a physically motivated modeling for the cyclic stress-strain relationships is proposed. The present modeling of concrete under general cyclic loading is initiated to provide substantial applicability, flexibility of mathematical expressions, and furthermore, to describe the behavior of random cycles. For the experimental study of concrete subjected to cyclic axial compressions, tests of 3 in (76 mm) by 6 in (152 mm) concrete cylinders are conducted under four different loading regimes to determine the major experimental parameters for the proposed analytical expressions. The model developed is based on the results of parametric study and experimental data obtained for the present study. The validity of the proposed general cyclic model is confirmed through a comparison of the experimental results and simulated behavior of the model.
155Â citations
TL;DR: In this paper, an experimental program was conducted to evaluate ultrasonic techniques for measuring distributed cracking in concrete structures, which refers primarily to microcracking and other high porosity regions that generally precede large cracks.
Abstract: Bridge decks deteriorate due to many causes including low level fatigue cycling, thermal loading, chemical attack, and reinforcing steel corrosion. This deterioration takes the form of distributed microscopic damage that may evolve into large defects such as cracks, delaminations, spalling, and scaling. An experimental program was conducted to evaluate ultrasonic techniques for measuring distributed cracking in concrete structures. Distributed cracking refers primarily to microcracking and other high porosity regions that generally precede large cracks. An investigation of distributed cracking yields information on weaknesses in the materials that may ultimately lead to major cracking and failure, but also can be used to evaluate distress mechanisms that do not necessarily result in large cracks. Distributed cracking in concrete was induced by freeze-thaw cycling and salt-scaling. Ultrasonic tests were used to measure changes in attenuation, pulse velocity, and peak frequency of the ultrasonic waves due to the distributed damage. The ultrasonic measurements were correlated with damage observed using optical microscopy. It was found that ultrasonic pulse velocity was not very sensitive to changes caused by distributed microcracking. The change in signal amplitude (a measure of ultrasonic attenuation) was quite sensitive to changes caused by microcracking, although the measurements showed considerable scatter. The peak frequency of the ultrasonic signal was also quite sensitive to the condition of the concrete. These results must be considered in the development field tests for evaluation of concrete structures.
143Â citations
TL;DR: In this article, the residual mechanical properties of two high-strength concretes (f sub c=72 and 95 MPa), with siliceous aggregates (mostly flint) were studied under uniaxial compression after a single thermal cycle at 105, 250, 400, and 500 deg C.
Abstract: The use of high-strength concretes (f sub c > 60 MPa) in special structures designed to work in a high-temperature environment or to withstand severe thermal accidents requires the mechanical properties of the material to be assessed with regard to high-temperature effects. In this context, the residual mechanical properties of two high-strength concretes (f sub c=72 and 95 MPa), with siliceous aggregates (mostly flint) are studied under uniaxial compression after a single thermal cycle at 105, 250, 400, and 500 deg C. The results show that while concrete toughness increases after a cycle at high temperature, strength and stiffness decrease dramatically, and the recovery of strength in time is either nil or negligible. Furthermore, the stress-strain curves exhibit a rather pronounced softening branch, which has never so far been measured in a high-temperature context and is instrumental in assessing concrete toughness in compression. The knowledge of the residual mechanical properties of a concrete is necessary whenever the thermally damaged structure is required to bear a significant share of the loads even after a severe accident.
135Â citations
TL;DR: The AASHTO Test Method T277-Rapid Determination of the Chloride Permeability of Concrete and the American Society of Testing and Materials (ASTM) C1202-Electrical Indication of concrete's Ability to Resist Chlorides Ion Penetration have specified a rapid test method to rank the chloride penetration resistance of various concretes by applying a potential of 60 V DC to a concrete specimen and measuring the charge passed through the specimen during 6 hours of testing as mentioned in this paper.
Abstract: The American Association of States Highway and Transportation Officials (AASHTO) Test Method T277--Rapid Determination of the Chloride Permeability of Concrete and the American Society of Testing and Materials (ASTM) C1202--Electrical Indication of Concrete's Ability to Resist Chloride Ion Penetration have specified a rapid test method to rank the chloride penetration resistance of various concretes by applying a potential of 60 V DC to a concrete specimen and measuring the charge passed through the specimen during 6 hours of testing. The method is essentially a measurement of electrical conductivity of concrete, which depends on both the pore structure and the chemistry of the pore solution. Analyses based on published results have indicated that the replacement of portland cement with supplementary cementing materials, such as silica fume, can reduce the electrical conductivity of concrete more than 90% because of the change in pore solution composition in the concrete. Chemical composition of pore solution has little to do with the transport of chloride ions in the concrete; thus, it is not correct to use passed charge to rank the chloride penetration resistance of concrete made with supplementary cementing materials.
123Â citations
TL;DR: In this article, the effects of blended fly ash on mechanical properties and durability of concrete were evaluated using two reference mixtures, one was a mixture without fly ash, and the other contained 35% ASTM Class C fly ash.
Abstract: This study focused on evaluating the effects of blended fly ash on mechanical properties and durability of concrete. In this investigation two reference mixtures were used. One was a mixture without fly ash, and the other contained 35% ASTM Class C fly ash. Additional mixtures were composed of three blends of ASTM Class C and Class F fly ash while maintaining a total fly ash content of 40% of the total cementitious materials. Mechanical properties such as compressive strength, tensile strength, flexural strength, and modulus of elasticity were determined. Durability related properties determined were drying shrinkage, abrasion resistance, salt scaling resistance, and electrical prediction of chloride ion penetration. The results showed that blending of Class C fly ash with Class F fly ash showed either comparable or better results than either the reference mixture without fly ash or the unblended Class C fly ash. Blending of fly ash, therefore, leads to comparable or better quality and reduced cost, attributed to the use of Class F versus Class C fly ash in concrete.
113Â citations
TL;DR: In this article, the effects of specialty cellulose fibers on the restrained plastic shrinkage cracking of conventional and high-performance concrete were investigated, where cellulose fiber was used at 0.06% volume fraction, which is equivalent to a fiber content of 0.9 kg/cu m (1.5 lb/cu yd).
Abstract: Specialty cellulose fibers processed for the reinforcement of concrete offer relatively high levels of elastic modulus and bond strength. The hydrophilic surfaces of cellulose fibers facilitate their dispersion and bonding in concrete. Cellulose fibers have small effective diameters that are comparable to the cement particle size, and thus promote close packing and development of a dense bulk and interface microstructure in the matrix. The relatively high surface area and the close spacing of cellulose fibers when combined with their desirable mechanical characteristics make them quite effective in the suppression and stabilization of microcracks in the concrete matrix. The investigation reported concerns the effects of specialty cellulose fibers on the restrained plastic shrinkage cracking of conventional and high-performance concrete. Cellulose fibers were used at 0.06% volume fraction, which is equivalent to a fiber content of 0.9 kg/cu m (1.5 lb/cu yd). Plastic shrinkage cracks occur when the early-age shrinkage movements (prior to final set) are restrained; this commonly occurs on the surfaces of concrete flatwork in windy, hot, and dry conditions that promote rapid evaporation. Under such conditions, a moisture gradient develops in concrete that produces internal restraint against shrinkage movements of the surface layers. In the experimental program on restrained shrinkage cracking of conventional and high-performance concrete, noting that plastic shrinkage cracking test results show an inherently high variability, statistical analysis of replicated test results confirmed that cellulose fibers are effective in reducing the plastic shrinkage cracking of conventional and high-performance concrete. Cellulose fibers had statistically comparable effects on plastic shrinkage cracking of conventional and high-performance concrete.
TL;DR: In this article, the effect of four coarse aggregate types, locally available in central Texas, on the mechanical properties of low water-cement ratio mixes, namely compressive strength, elastic modulus, and flexural strength was investigated.
Abstract: Test results are reported from an experimental study in which the effect of four coarse aggregate types, locally available in central Texas, on the mechanical properties of low water-cement ratio mixes, namely compressive strength, elastic modulus, and flexural strength was investigated. Crushed river gravel, trap rock, dolomitic limestone, and calcitic limestone were used in high-performance concrete (HPC) production in varying amounts: 36%, 40%, and 44% by concrete volume. A constant water-cement ratio of 0.28 was employed for all mixes. The mineralogical characteristics of coarse aggregate, as well as the aggregate shape, surface texture, and hardness, appear to be responsible for the differences in the performance of HPCs. It was observed that HPCs with different coarse aggregates appear to lack a single equation that estimates the elastic modulus or flexural strength with sufficient accuracy as in the case of normal strength concretes. This could be attributed to the increased role of coarse aggregate in concrete mixes with low water-cement ratios, as a result of improved cement paste and transition zone.
TL;DR: In this paper, the authors investigated and compared how the frost resistance of the RAC and natural aggregate concrete (NAC) is affected by using mineral and chemical admixtures in their production.
Abstract: Recycling demolished concrete as an alternative source of coarse aggregates for the production of new concrete can help solve the growing waste disposal crisis and the problem of depleted natural aggregates. However, to make such recycling feasible, the strength and durability of the recycled aggregate concrete (RAC) must be assured. Even though much is known about the mechanical properties of recycled aggregate concrete, an important question remains about its freeze-thaw durability, and it appears that widespread utilization of such concrete will be limited until this question is addressed. This study was an attempt to investigate and compare how the frost resistance of the RAC and natural aggregate concrete (NAC) is affected by using mineral and chemical admixtures in their production. From the strength point of view, the recycled aggregate compared well with the natural aggregate in all cases, and therefore, could be considered for various potential applications. From the frost resistance point of view, the recycled aggregate had negative effects on durability performance. However, with the use of chemical admixtures, particularly air entrainment, the RAC was found to be as durable as the NAC.
TL;DR: In this paper, the authors explored the strain response and damage behavior of concrete under uniaxial compression cyclic loading and found that the fatigue failure process is related to strain response behavior.
Abstract: This paper explores the strain response and damage behavior of concrete under uniaxial compression cyclic loading. Through present experimental investigation on irreversible strain accumulation, strain range variation, and the analysis of fatigue modulus of concrete with microcracks, the fatigue failure process has been found to be related to the strain response behavior. It has also been found that irreversible strain accumulation, strain range variation, fatigue modules degradation of concrete under fatigue loading are associated with and can be used to describe fatigue damage evolution of concrete. On the basis of the experimental results and continuum damage mechanics, a fatigue model is proposed in this paper, which can be used to account for the strain response properties, fatigue damage evolution, and S-N relation of concrete under uniaxial compression cyclic loading.
TL;DR: In this paper, the results of experimental studies of the micromechanical behavior of concrete under different loading conditions were presented, where cylindrical specimens of normal-and high-strength concrete were tested under uniaxial and confined compression.
Abstract: This paper presents the results of experimental studies of the micromechanical behavior of concrete under different loading conditions. Cylindrical specimens of normal- and high-strength concrete were tested under uniaxial and confined compression. Cracks and pores in the concrete specimens were impregnated with an alloy that has a low melting point. At the stress of interest, this alloy was solidified to preserve the stress-induced microcracks as they exist under load and images from the cross sections of the concrete specimens obtained using scanning electron microscopy (SEM). Stereological analysis that interprets three-dimensional structures by means of two-dimensional sections was used on the computerized images to determine the density, orientation, and branching of the compressive stress-induced microcracks and the effect of confinement on microcrack behavior. The density and branching of the microcracks decreased as the confining stress increased. The confining stress had a pronounced influence on microcracks in the interfacial transition zone (ITZ) between the cement paste and aggregate. The amount of interfacial cracking decreased significantly as the confining stress was increased. Under uniaxial compression there were significant differences in the crack patterns observed in normal- and high-strength concretes. Under confined conditions the two types of concrete had similar microcrack patterns.
TL;DR: Tire-added latex concrete (TALC) as discussed by the authors was developed to incorporate recycled tire rubber as part of concrete and showed higher flexural and impact strengths than those of portland cement and latex-modified and rubber-added concretes.
Abstract: Tire-added latex concrete (TALC) was developed to incorporate recycled tire rubber as part of concrete. Crumb rubbers from tires were used in TALC as a substitute for fine aggregates or styrene-butadiene rubber latex, while maintaining the same water-cement ratio. Various static and dynamic strengths of TALC were measured and compared with those of conventional and latex-modified concrete. TALC showed higher flexural and impact strengths than those of portland cement and latex-modified and rubber-added concretes. Microscopic pictures taken using the scanning electron microscope seem to support that there is better bonding between crumb rubbers and portland cement paste because of latex. TALC showed potential of becoming a viable construction material to enhance brittle concrete while incorporating waste tires.
Journal Article•
TL;DR: In this paper, the authors explored the feasibility of using acoustic emission (AE) technique in rebar corrosion detection in concrete and investigated the possibility of the corrosion detection of rebar inside of concrete through an accelerated corrosion experimental method.
Abstract: Although corrosion of reinforcing steel (rebar) in concrete is held responsible for most deterioration of structural concrete, current inspection methods lack accuracy and can provide information only after significant corrosion has occurred. Along the line in searching for an effective method for early detection of rebar corrosion in concrete, the capability of acoustic emission (AE) technique in detecting a weak stress wave make it a strong candidate. The primary advantage AE offers over other conventional nondestructive evaluation techniques is that it can directly detect the process of a flaw growth. When corrosion products are formed on a corroding rebar, they swell and apply pressure to the surrounding concrete. Microcracks will be formed and stress waves will be generated during the expansion process when the pressure is high enough to break the interface layer. The growth of the microcracks is directly related to the amount of corrosion product of a corroding rebar. Thus, by detecting the AE event rate and their amplitude, the degree of the corrosion can be interpreted. This paper explores the feasibility of using AE technique in rebar corrosion detection. It examines the correlation between the characteristics of the acoustic emission event and the behavior of the rebar corrosion in HCl solution first. Then it investigates the possibility of the corrosion detection of rebar inside of concrete through an accelerated corrosion experimental method. The theoretical prediction and experiment results have shown that AE technique do have the capability to detect rebar corrosion in an early corrosion stage. It is thus expected that the information obtained in this investigation can be used for a field application to characterize the change of a corroded rebar and to help make decisions on maintenance and repair for buildings and infrastructures.
TL;DR: In this paper, an experimental investigation of the behavior of fiber reinforced concrete under cyclic flexural loading is presented, where damage evolution is described in terms of acoustic emission activity as a function of deflection (static loading) or cycles (fatigue loading).
Abstract: An experimental investigation of the behavior of fiber reinforced concrete under cyclic flexural loading is presented. One type of polypropylene and two types of steel fibers in two different volume concentrations are studied. Load-deflection response is obtained for constant amplitude fatigue loading as well as for static loading. The damage level is recorded under static and fatigue loading using acoustic emission techniques. Data are presented in terms of complete load-deflection diagrams (for static loading) and in terms of S-N diagrams (for fatigue loading). Damage evolution is described in terms of acoustic emission activity as a function of deflection (static loading) or cycles (fatigue loading). The test results show that the addition of steel fibers increases the flexural fatigue strength considerably. Compared with plain concrete, the fatigue strength for 2 million cycles is changed from 60% to 90% of the ultimate flexure strength when the steel fiber content is one volume percent. High-fiber volume concentrations (2%) further increase absolute fatigue strength; however, fatigue performance measured relative to the static strength is decreased compared with the lower fiber volume concentration. Furthermore, the results show that the accumulated damage level at failure in the static test of unreinforced concrete is of the same order of magnitude as in the fatigue testing of the same material. However, using fiber reinforced concrete, the accumulated damage level in fatigue testing is 1-2 order of magnitude higher than the level reached in static testing of the same material. Finally, the tests show that the deflection at failure of the fiber reinforced concrete specimens under constant stress range fatigue loading can be predicted using the static load-deflection curve, provided the testing time is short enough to neglect creep effects.
TL;DR: In this article, a theoretical model based on nonlinear fracture mechanics is developed for predicting transverse cracking of a concrete ring specimen caused by drying shrinkage, which is in reasonable agreement with the experimental measurement.
Abstract: A theoretical model based on nonlinear fracture mechanics is developed for predicting transverse cracking of a concrete ring specimen caused by drying shrinkage. Using the measured material fracture parameters, fracture resistance curve of the ring specimen is determined. The maximum allowable tensile strain is then calculated based on energy balance during shrinking of the ring. Age at transverse cracking of the ring specimen caused by restrained drying shrinkage is predicted by equating the difference between the measured free shrinkage and the estimated creep to the maximum allowable tensile strain. The predicted age at cracking is in reasonable agreement with the experimental measurement.
TL;DR: In this paper, the results of an experimental and analytical study on the durability of E-glass fiber reinforced plastics (FRP) rebars were presented, where a total of 160 rebar samples were placed in corrosive chemical solutions that simulated exposure in the field.
Abstract: Because of their unique properties, fiber reinforced plastics (FRPs) are becoming increasingly popular among researchers and engineers in the construction industry. FRP rebars in particular present an attractive alternative to steel rebars in reinforced concrete. Among the features of this type of rebar are the high strength-to-weight ratio and potential resistance to aggressive environmental factors. This paper presents the results of an experimental and analytical study on the durability of E-glass FRP rebars. A total of 160 rebar samples were placed in corrosive chemical solutions that simulated exposure in the field. Tests were performed at temperatures of 25 deg C and 60 deg C. Test variables included one type of fiber (E-glass), two matrix materials (polyester and vinylester), seven chemical solutions, and ultraviolet radiation. Rebar specimens were constructed from E-glass fibers embedded in polyester or vinylester resin matrix. Rebar sizes were 10 mm (3/8 in) and 19.5 mm (3/4 in) in diameter. Changes in weight and physical appearance were recorded over a one-year period. In addition, 10 beams each reinforced with two 10-mm (3/8-in) E-glass/polyester or E-glass/vinylester FRP rebars were subjected to deicing salt solutions. They were tested in flexure to failure after one-year and 2-year periods, and the load versus mid-span deflection relationships were recorded. Test results of rebars and beams indicate that significant loss of strength can result from the exposure of E-glass FRP rebars.
Journal Article•
TL;DR: In this article, the authors compared the sulfate resistance of a cement-bentonite suspension with additive lime and pozzolanic zeolite, and found that the cement-zeolite suspension showed an increase in compressive strength.
Abstract: The sulfate resistance of cement-bentonite suspension with additive lime is compared with that of a suspension where bentonite was totally replaced by pozzolanic zeolite. Cement-bentonite suspensions are frequently used for the construction of underground sealing walls. The disadvantage of these suspensions is due to a relatively low resistance against the aggressive media. With the purpose of obtaining sulfate-resistant suspension, a cement-zeolite suspension with the viscosity (Marsh cone) of 35 sec, the maximum settlement of 3% volume/24 hr, and minimal compressive strength (28 days curing in water) of 2.0 MPa (290 psi) was prepared. In the 10% sodium sulfate solution, a total destruction of the cement-bentonite suspension test specimens took place within 30 days. No degradation was found in the cement-zeolite suspension attacked by the sulfate solution for 365 days. Moreover, the cement-zeolite suspension showed an increase in compressive strength. It is supposed that the reason for this lies in the high reactivity of zeolite toward lime and intensive consumption of calcium hydroxide by pozzolanic reaction of zeolite. The SO3 bonding and resulting calcium sulfate dihydrate development is markedly decreased in the cement-zeolite suspension compared with cement-bentonite suspension.
TL;DR: In this paper, the effect of silica fume on bond and anchorage characteristics of reinforcing bars in high performance concrete was investigated and the validity of the upper limit of 70 MPa (10,000 psi) imposed by the American Concrete Institute (ACI) Building Code 318-95 on the concrete compressive strength for determination of development length was evaluated.
Abstract: Little research is reported in the literature on the effect of pozzolans such as silica fume on structural behavior of reinforced concrete, namely on bond and anchorage characteristics of reinforcing bars in concrete. The objectives of the study were to investigate the effect of silica fume on bond and anchorage characteristics of reinforcing bars in high performance concrete, to study the validity of the upper limit of 70 MPa (10,000 psi) imposed by the American Concrete Institute (ACI) Building Code 318-95 on the concrete compressive strength for determination of development length, and to evaluate the reliability of the empirical equation of Orangun, Jirsa, and Breen in estimating the bond strength of deformed bars embedded in high strength concrete. Sixteen beam specimens were tested. Each beam was designed to include two bars in tension, spliced at the center of the span. The splice length was selected so that bars would fail in bond, splitting the concrete cover in the splice region, before reaching the yield point. The beams were loaded in positive bending with the splice in a constant moment region. The variables used were the percentage replacement by weight of cement by silica fume, casting position, and the superplasticizer dosage. Test results indicated that replacement of 5-20% of the cement by an equal weight of silica fume resulted in an average 10% reduction in bond strength regardless of casting position or the superplasticizer dosage used.
TL;DR: In this paper, the performance of concrete slabs coated with acrylic-based surface coating against chloride and atmospheric carbon dioxide attacks was investigated and the experimental results showed practically no chloride penetration into the concrete beneath the acrylic based surface coating even after long-term exposure.
Abstract: The majority of concrete structures, particularly those in coastal environments, often suffer from both chloride and atmospheric carbonation attacks. Application of polymer-based concrete surface coating is one of the solutions available for the long-term protection of the reinforcement steel from corrosion. This paper deals with the performance of concrete slabs coated with acrylic-based coating against chloride and atmospheric carbon dioxide attacks. For this purpose, fully and partially surface-coated slabs were exposed to repeated cyclic wetting by sodium chloride solution followed by drying. After completion of the cyclic exposure regime, the slabs were exposed to laboratory drying environment and then were subjected to experimental investigations. The experimental results showed practically no chloride penetration into the concrete beneath the acrylic-based surface coating even after long-term exposure. On the other hand, for the uncoated concrete, the amount of chlorides and their depth of penetration into the concrete were found to be dependent on the water-cement (w/c) ratio of the concrete. The amount of chlorides penetrating laterally, and the distance they penetrate from the uncoated concrete into the adjoining concrete below the acrylic-based surface coating is also a function of the w/c of concrete. Further, both x-ray diffraction studies and phenolphthalein spray tests showed the acrylic-based coating to be very effective in controlling the carbonation of the concrete below it, especially when the coating was applied by spraying. The coating maintained good adhesion with the substrate concrete even after the long-term repetitive wetting and drying cycles.
TL;DR: In this paper, a study was made of the behavior of concrete reinforced with randomly distributed short fibers under uniaxial tension, and it was concluded that strain hardening and multiple cracking responses can be achieved for concrete incorporating short fibers when the parameters are carefully selected.
Abstract: A study was made of the behavior of concrete reinforced with randomly distributed short fibers under uniaxial tension. Five kinds of fibers were used to prepare the fiber reinforced concrete specimens for the uniaxial tension test. These included two kinds of steel fibers, two kinds of polyvinyl alcohol fibers, and one kind of polypropylene fiber. Fiber volume fraction varied from 2% to 6% in most cases. The matrices were normal strength concrete and/or admixture modified concrete. Plain concrete specimens were also prepared as a reference for the purpose of comparison. The adaptive control test method was used in the experimental study to obtain complete stress-deformation responses for both plain concrete and fiber reinforced concrete. Both strain softening and strain hardening behavior was observed in the experiment, and it seemed that a transition point did exist for the composites. It was concluded that strain hardening and multiple cracking responses can be achieved for concrete incorporating short fibers when the parameters are carefully selected.
TL;DR: In this paper, the authors describe how several of these chemical admixtures affect mortar-bar expansion and show that the results are in agreement with a theoretical model presented in a previous paper that explains the volume change behavior of reaction product gels and attributes the swelling of the reaction product gel to double-layer repulsion forces.
Abstract: Many aggregates are susceptible to the alkali-silica reaction. As a result of this reaction, mortar bars and concrete elements containing portland cement expand. In order to limit this expansion, chemical admixtures that interfere with the alkali-silica reaction can be introduced into the mixing water. The research discussed below describes how several of these chemical admixtures affect mortar-bar expansion. Mortar bars containing any of several chemical admixtures in the mixing water at initial molar concentrations of 1 or 2 were subjected to ASTM C 1260 tests. After the expansion test, samples were prepared from each of the mortar bars and examined in a scanning electron microscope with EDX capabilities. The following chloride salts and hydroxides were used: NaOH, KOH, LiOH, NaCl, KCl, LiCl, CaCl2, MgCl2, and AlCl3. For a given initial molar concentration, the expansion test results indicated that the chloride salts with monovalent cations were the most damaging, followed by those with divalent and trivalent cations. These results are in agreement with a theoretical model presented in a previous paper that explains the volume change behavior of the reaction product gels. This model attributes the swelling of the reaction product gel to double-layer repulsion forces.
TL;DR: In this paper, laboratory-made roller compacted concretes with various combinations of cement (Type I and Type V for sulfate-resistant concrete), lignite dry bottom ash, and crushed limestone coarse aggregate were tested to ascertain the suitability of this type of concrete for pavement applications.
Abstract: Laboratory-made roller compacted concretes with various combinations of cement (Type I and Type V for sulfate-resistant concrete), lignite dry bottom ash, and crushed limestone coarse aggregate were tested to ascertain the suitability of this type of concrete for pavement applications. The fresh properties and strength and deformation of the hardened roller compacted concrete containing bottom ash have been discussed in the companion article (Part I). This paper describes the data pertaining to long-term durability of bottom ash roller compacted concretes. The analysis of the test results leads to the conclusions that durable concrete can be produced with the high-calcium dry bottom ash used in this investigation. Resistance to sulfate attack, rapid freezing and thawing, and wear improved with increases in cement and/or coarse aggregate contents. Length change caused by external sulfate attack varied from 0.0203% to 0.0388%, whereas no mass loss or reduction in strength were found in any of the test samples. Abrasion testing under wet conditions was consistently worse than under dry conditions. After 300 rapid freezing and thawing cycles, the mixture proportions of this investigation displayed a maximum mass loss of 2.3% and a minimum durability factor of 91.2%.
TL;DR: In this paper, the influence of two pozzolanic materials (high-reactivity metakaolin (HRM) and silica fume) on the toughness characteristics of high-performance fiber-reinforced concrete was examined.
Abstract: For steel fiber-reinforced concrete with practical fiber volume fractions, the major post-peak energy dissipation mechanism is the pull-out of fibers across a crack. With undeformed, smooth fibers, post-peak energy dissipation of "toughness" is mainly a function of fiber-matrix adhesional bond, whereas for the highly stressed deformed fibers, properties of the bulk matrix also become important. High-performance matrices tend to be brittle, and addition of pozzolanic admixtures, particularly silica fume, further increases the brittleness. An increased matrix brittleness can cause crushing and splitting of the matrix and in turn, curtail the ability of fibers to transfer stresses during pull-out, thus reducing the overall toughness. This paper examines the influence of two pozzolanic materials--high-reactivity metakaolin (HRM) and silica fume--on the toughness characteristics of high-performance fiber-reinforced concrete. It is concluded that HRM is particularly effective in improving the post-peak energy absorption capacity of concrete with fibers, and unlike silica fume, no particular post-peak brittleness is seen to occur.
TL;DR: In this paper, the performance of fusion-bonded epoxy-coated reinforcing steel (FBECR) was evaluated in chloride doped simulated concrete pore water solutions and field structures, showing that a different corrosion protection failure mode exists in the field than in short-term laboratory studies.
Abstract: Corrosion protection performance of fusion bonded epoxy-coated reinforcing steel (FBECR) was evaluated in chloride doped simulated concrete pore water solutions and field structures. Results demonstrate that a different corrosion protection failure mode exists in the field than in short-term laboratory studies. In laboratory studies, the chloride is at the bar surface area immediately and the degree of corrosion protection is a function of the quality of the coating. In field structures, the epoxy coating debonds from the steel in moist-wet concrete. The rate of epoxy debondment and the chloride increase at the bar depth determine if the chloride corrosion threshold limit is reached before or after the epoxy debonds. If the coating is debonded when the chlorides reach the corrosion threshold limit, corrosion takes place under the coating in an acidic environment and the corrosion protection of FBECR is nil. In Virginia, FBECR may provide protection for approximately 5% of the bridge decks and thus FBECR is not a cost-effective corrosion protection system for Virginia bridges.
TL;DR: In this paper, the use of a fiber architecture-based design methodology facilitates the optimization of the performance of ductile or pseudo-ductile fiber reinforced plastic (FRP) reinforcement for concrete structures.
Abstract: This paper addresses the need for a ductile or pseudo-ductile fiber reinforced plastic (FRP) reinforcement for concrete structures. The criteria to be met by the FRP, which are based on the properties of the steel reinforcing bar it is to replace, are threefold: high initial modulus, a definite yield point, and a high level of ultimate strain. It is shown that the use of a fiber architecture-based design methodology facilitates the optimization of the performance of FRP through material and geometric hybrid. Consequently, the advantages of FRP, such as high strength, low weight, and chemical inertness or noncorrosiveness, can be fully exploited. Using the material hybrid and geometric hybrid, it is demonstrated that the pseudoductility characteristic can be generated in FRP reinforcing bar. Critical material and geometric parameters such as elastic modulus, fiber volume fraction, twisting, crimp, and helical effect in the specimen components were investigated and parametric studies are reported. Ductile hybrid FRP bars were successfully fabricated at 3-mm and 5-mm nominal diameters using an in-line braiding and pultrusion process. Tensile specimens from these bars were tested and found to have consistent pseudoductile behavior and good agreement with the analytical predictions.
TL;DR: In this article, the authors present a summary of a dozen publications on modeling the strength development of portland cement during the past 40 years, and present a condensed summation of the development and capabilities of the exponential model.
Abstract: This paper is a summary of a dozen publications the writer produced on modeling the strength development of portland cement during the past 40 years. It appears that this mathematical model performs better and within wider limits than other cement models for strengths. Since the earlier papers were published in various places, mostly abroad, it seems worthwhile to present a condensed summation of the development and capabilities of the exponential model. The most recent, most advanced form of the model is presented in the first part of the paper. There are several main features of this form. First, it consists of two hardening components: C3S and everything else, mostly C2S; with the two components developing strength independently at different rates. Second, C3A acts as a catalyst on the strength development of the two hardening components. Third, the model needs only two experimental parameters with which the model is applicable to all portland cement types, a wide range of curing temperatures, and age between one day through one year or more. Fourth, the process of strength development is divided into three stages: the zeroth stage, which is the period immediately after mixing; the first stage when the hydration is controlled by the rates of chemical reactions; and the second stage where the hydration is controlled by a diffusion mechanism. The steps leading to the present form of the model are illustrated in the second half of the paper, mostly for educational purposes but also because they provide insight into the roles of the cement compounds in strength development. The support of the model by comparison with experimental data is demonstrated by several examples.