Extrusion processing is a technique used to produce high-performance fiber-reinforced cement-based composites (HPFRCC), which
has shown great promise for manufacturing materials that are strong, ductile, durable, design versatile and environmentally
friendly. Despite these advantages, extrusion is still primarily limited to laboratory-scale work. One reason this technology
has not been adopted by industry is the high cost of the cellulose ether processing aids that are required for extrusion.
In this research, the possibility of partially replacing cellulose ethers with less expensive clay binders is investigated.
Extrudable and not extrudable mixes are identified and capillary rheology is used to describe the rheological parameters of
the various mixes. The results indicate that clay binders can be used as a partial replacement for cellulose ethers and that
capillary rheology can be used to describe extrudability.
The incremental core-drilling method (ICDM) is a nondestructive technique to assess in-situ stresses in concrete. In contrast
to other available methods of in-situ stress measurement in concrete, the ICDM can quantify stresses that vary through the
thickness of the concrete member under investigation, such as those due to bending or eccentric prestressing. In this method,
a core is drilled into a concrete structure in discrete increments. The displacements which occur locally around the perimeter
of the core at each increment are measured and related to the in-situ stresses by an elastic calculation process known as
the influence function method. This paper presents the analytical and numerical techniques necessary for practical use of
the ICDM, as well as results from experimental tests in which simple concrete beams were subjected to controlled loads and
insitu stresses measured via the ICDM were compared to known stress distributions. The ability of the technique to accurately
measure a variety of different stress distributions is demonstrated, and practical considerations for an ICDM investigation
The ACI recommendations for the prevention of cracking of plastic concrete attempt to eliminate such cracking by ensuring that the rate of evaporation from unprotected concrete surfaces does not exceed the estimated rate of bleed water production. The current recommendations, however do not account for the large scatter of the underlying experimental evaporation data nor the effect of altitude on evaporation rate. Ignoring the scatter of the evaporation data frequently leads to an unacceptably high probability that the evaporation rate will exceed the bleed rate. Ignoring the effect of altitude leads to similar high probabilities, but in only a comparatively small number of cases. Simple modifications of the ACI recommendations are suggested that can account for both effects. However; insufficient data on the variability of bleed rates are currently available to allow the scatter of the evaporation data to be accounted for completely.
No ACI Committee 544's repeated drop-weight impact test for concrete is often criticized for large variations within the results. This paper identifies the sources of these large variations and accordingly suggests modifications to the ACI test. The proposed modifications were evaluated and compared to the current ACI test by conducting impact resistance tests on 40 specimens from two batches of polypropylene fiber-reinforced concrete (PPFRC). The results obtained from both methods were statistically analyzed and compared. The variations in the results were investigated within the same batch and between different batches of concrete. The impact resistance of PPFRC specimens tested with the current ACI test exhibited large coefficients of variation (COV) of 58.6% and 50.2% for the first-crack and the ultimate impact resistance, respectively. The corresponding COV for PPFRC specimens tested according to the modified technique were 39.4% and 35.2%, indicating that the reliability of the results was significantly improved. It has been shown that, using the current ACI test, the minimum number of replications needed per each concrete mixture to obtain an error below 10% was 41 compared to 20 specimens for the modified test. Although such a large number of specimens is not good enough for practical and economical reasons, the reduction presents a good step on the development of a standard impact test.
The acoustic emission (AE) technique is widely used for real time damage detection in concrete. It uses stress waves emerging from nucleation and propagation of cracks recorded on the surface of the material by suitable sensors. In the present study, AE is used to monitor the deterioration progress of reinforced concrete beams subjected to four-point bending. The specimens consist of two layers of plain and fiber concrete. At different loading steps, ultrasonic pulse velocity measurements were also conducted to obtain the transient three-dimensional tomographic reconstruction of the internal structure. The AE source location is in good agreement with the velocity structure visualization and the results are confirmed by visual observation of the actual cracks developed. The results show that the AE technique and velocity tomography are useful tools to study the failure progress of concrete.
The accurate prediction of thermal gradients in concrete calls for models that characterize the temperature sensitivity of the hydration of cementitious materials. The most common method used for this purpose is the Arrhenius equation, which requires the selection of an activation energy Ea to define the temperature sensitivity of the reaction. For cementitious materials, Ea is typically computed using either isothermal calorimetry or compressive strength data. There is disagreement in literature as to the proper method to determine Ea. The Ea of different cementitious pastes was determined from isothermal calorimeter results using three different computational methods. The results were used to develop a systematic computational method for characterizing Ea to account for the effect of temperature on the overall rate of hydration of cementitious materials. This work lays the groundwork for more extensive studies to determine the effect of a wide variety of variables on Ea.
The use of permeability-reducing admixtures is a potential preventative of the chloride-induced corrosion of steel reinforcement, which is the main cause of the deterioration of concrete structures exposed to coastal environments. This paper presents an experimental investigation into the effectiveness of two typical commercially available permeability-reducing admixtures: one characterized by crystallization activity and the other by hydrophobic and pore-blocking effects. Concrete specimens were exposed to simulated coastal environments, and chloride concentration profiles at 28-, 365-, and 730-day exposures were determined by X-ray fluorescence spectrometry. The results suggested that the incorporation of the admixture, characterized by hydrophobic and pore-blocking effects, appeared to considerably enhance the concrete durability with respect to chloride-induced corrosion. The inclusion of the admixture characterized by crystallization activity however, seemed to have almost no detectable effect. This implies the necessity of exercising a degree of caution during specification.
Laboratory tests showed that the alkali contents of the pore solutions in concrete cores taken from the Saunders Generating Station, Cornwall, Ontario, Canada, were higher than the estimated original value. The structure was built 30 years ago, using a marginally expansive, alkali-silica-reactive, argillaceous limestone. The excess alkali in the pore solution could possibly have been derived from the clay minerals in the limestone aggregate. This hypothesis was tested by making pastes of pulverized limestone, calcium hydroxide, and water, and storing them at 38 C for 90 days. Samples were removed periodically, and the amount of alkali in the calcium hydroxide solution was determined. The results of the determinations indicate that the amount of leachable alkali would probably be sufficient to account for the enhanced alkali contents in the pore solution in concrete cores from the structure.
Bridge deck temperature changes in the first few days after placement due to the concrete heat of hydration and changes in ambient conditions have long been identified as a significant contributor to early-age cracking. The goal of this project was to develop a method of quantifying how materials and construction methods can influence the thermal stresses in bridge decks. A series of tests on concrete mixtures were then performed to quantify the concrete material thermal stress behavior in bridge decks with different placement times and coefficients of thermal expansion. Concrete with a high coefficient of thermal expansion placed in the morning led to the development of thermal stresses equal to 75% of the stress at cracking. It was also found that the thermal stresses could be reduced by up to 50% by using concrete with a lower coefficient of thermal expansion and placing at night.
In order to show that alkali-aggregate reaction is the primary cause of concrete deterioration, the concrete cracking characteristics of alkali-aggregate reaction was observed in Saunders Generation Station. The extent of concrete deterioration was evaluated and the remaining expansion potential of the concrete was determined. Experimental results indicate that a considerable amount of alkalies in the aggregate can be extracted by cation exchange in pastes of calcium hydroxide. This observation supports the hypothesis that excess alkalies found in the concrete are derived from impure limestone aggregates.
Corrosion characteristics of reinforcement in modified high-alumina cement (HAC) containing a conversion-preventing additive (CPA) was studied. An investigation was carried out using ordinary portland cement, high-alumina cement, and modified high-alumina cement. Results including compressive strength, bond strength, porosity, Cl- permeability, and extent of steel bar corrosion are presented. The effect of a deicing salt, calcium chloride, on formation of hydrogarnet and strätlingite in HAC and modified HAC mortars is also reported. This work indicates that HAC mortar, after conversion, is unable to protect the reinforcement from corrosion in an environment containing chloride ions. Conversion-inhibited high-alumina cement has the capability of protecting the reinforcement from corrosion under severe test conditions due to its effective resistance to the penetration of chloride ions.
The strength reduction of high alumina cement (HAC) concrete due to conversion is one of the major reasons given for limiting the use of HAC in structural members. A conversion-inhibited concrete is introduced in this paper. The effect of curing and exposure conditions (e.g., temperature) on the compressive strength of HAC or modified HAC concretes was studied. Ground granulated blastfurnace slag (ggbs) and a conversion-preventing additive (CPA) containing natural zeolite or silica fume in combination wall sodium sulfate were used to inhibit the strength reduction of the HAC concretes. The results indicated that conversion-inhibited HAC concrete containing a CPA has a one-day compressive strength greater than 55 MPa when cured at 4-5 deg C. The strength of the HAC/CPA concrete is much less affected by the concrete temperature than plain HAC or HAC/ggbs concrete.
Hydration and strength development characteristics of high alumina cement (HAC) containing sodium sulfate and a variety of different zeolites were studied. The zeolites obtained from different sources included different types containing clinoptilolite, chabazite, stilbite, and natrolite. The one-day compressive strength of HAC mortars containing a commercially available zeolite and sodium sulfate was as high as 60 MPa. No strength reduction occurred in the HAC mortars water-cured at 38 C for 330 days. Hydrogarnet formation was significantly inhibited. Clinoptilolite or chabazite-based zeolites in combination with sodium sulfate were more effective in preventing the formation of hydrogarnet in HAC paste than stilbite or natrolite-based zeolites. Zeolite alone was not able to prevent the hydrogarnet formation in the HAC paste. Chabazite was the most effective zeolite in promoting strätlingite formation in the HAC paste.
This paper describes the performance of steel reinforcement in portland cement and high-volume fly ash (HVFA) concretes exposed to a chloride solution. A number of large slabs, 833 x 600 x 153 mm in size, were cast from six air-entrained concrete mixtures. Four of these mixtures were made with normal portland cement with the water-cement ratio (w/c) of the mixtures ranging from 0.32 to 0.76; the remaining two mixtures were made with the HVFA concrete with a w/(c + FA) of 0.32. The steel reinforcing bars were placed in concrete with cover thickness ranging from 13 to 76 mm. The concrete slabs were ponded with a 3.4% sodium chloride solution for a period of 6 months, and half-cell potential, linear polarization, and AC impedance techniques were applied to monitor the progress of the corrosion of steel reinforcement. The results indicate that the performance of the reinforcing steel bars in the HVFA concrete after 6 months of ponding with a 3.4% sodium chloride solution was excellent. There was no significant steel corrosion taking place on the reinforcing bars embedded in the HVFA concrete, even with 13 mm concrete cover. This performance of the HVFA concrete is equivalent to that of the control concrete with a w/ c of 0.32, and is better than the control concrete with w/c ≥ 0.43. Significant corrosion rates were observed for the reinforcing bars embedded in control portland cement concrete with w/c ≥ 0.43. As expected, the poorest performance was of the control concrete with a w/c of 0.76, where even the reinforcing bars with 51 mm cover exhibited corrosion.
The mechanisms that contribute to early-age cracking are complex. Determining the relative importance of each mechanism as well as the combined cracking potential for a given concrete material is essential for the concrete industry to construct structures with a long service life. A method for quantifying the cracking risk of a concrete mixture is presented. The method involves testing for the concrete heat of hydration, setting time, free thermal and autogenous movement, restrained stress, and mechanical property development. The concrete uniaxial stress under restrained conditions is measured using a rigid cracking frame. This test setup was used to quantify the effects of using fly ash on the concrete cracking risk using four different fly ashes with varying calcium oxide contents. All fly ashes reduced the cracking risk because of the decrease in the heat of hydration of the cementitious materials and, to a lesser extent, the increased early-age creep.
This paper presents an experimental study on the investigation of concrete properties by guided and surface wave nondestructive testing. Applications were made on two large slabs simulating homogeneity and layering in concrete, respectively. An efficient nonintrusive method was used to evaluate the concrete quality by solving the modal propagation problem of Lamb guided waves and Rayleigh surface waves. Lamb waves were used to determine the Poisson's ratio and the Young's modulus of the concrete slabs. Rayleigh waves were identified using Lamb wave fundamental-modes; thereafter, the inverse problem of Rayleigh waves was solved to evaluate the variation of shear wave velocity with depth and thus characterize the layered slab. The obtained results demonstrate the high potential of this tool that can easily be used for in-place assessment of concrete structures.
The performance of eight commercial corrosion-inhibiting systems was assessed in the field over 10 years on reinforced concrete barrier walls of a highway bridge that was subjected to severe environmental conditions. These systems were composed of one or more of the following components: anticorrosion concrete admixtures, reinforcement coatings, and concrete surface coatings/sealers. The field evaluation consisted of annual surveys of corrosion potential and corrosion rate, as well as visual inspections and testing of concrete cores. After 10 years, the main reinforcement of the barrier walls, at a depth of 75 mm (3 in.), was found in relatively good condition due to an initially high-quality concrete. Special bars embedded at a depth of 13 mm (1/2 in.) in the barrier walls showed signs of advanced corrosion for all systems; however, no visible signs of corrosion were found on 25 mm (1 in.) deep bars. Nondestructive corrosion evaluation over the 25 mm (1 in.) deep ladder reinforcing bars indicated that the system containing the inorganic anticorrosion admixture provided consistently lower risks of corrosion, followed by systems containing organic anticorrosion admixtures, in comparison to the control system and other systems. The low concrete permeability and different stability of the protective layer forming on the bars may explain the observed differences in the effectiveness of these systems.
The temperature development of mass concrete elements is strongly dependent on constituent materials and mixture proportions, as well as the formwork type, geometry, and environmental conditions. This paper presents a method to account for the effects of convection, radiation, and shading on the surface temperature of mass concrete. Solar radiation, atmospheric radiation, surface-emitted radiation, and formwork radiation exchange were considered. Wind speed, ambient temperature, and surface roughness were included in the convection model. The model described was incorporated into a mass concrete temperature prediction model. The predicted temperatures were then compared with measured near-surface concrete temperatures. The ability of the model to predict the maximum temperature and maximum temperature difference were also examined. The results show that the model accurately estimates the near-surface concrete temperatures, the maximum temperature, and maximum temperature difference of the 12 concrete members instrumented.
Six proprietary concrete repair systems were evaluated for their performance and durability under field conditions. The fieldwork consisted of repairing corrosion-damaged reinforced concrete barrier walls of a highway bridge, and installing embedded instrumentation for their continuous monitoring over three years. The results indicated that the proprietary repair systems reduced the one risk of corrosion in the patches; however, the risk of corrosion in the existing concrete was not reduced. All concrete repair systems suffered from shrinkage cracking. Six (6) systèmes de réfection d'ouvrages de béton de propriété exclusive ont été évalués quant à la performance et à la durabilité en conditions in situ. Le travail sur le terrain comportait la réfection de parois de pont routier faites de béton armé qui étaient endommagées par la corrosion, et la mise en place d'appareillage encastré permettant le contrôle continu de ces parois durant une période de trois (3) ans. Les résultats ont révélé que les systèmes de réfection de propriété exclusive réduisaient le risque de la corrosion seulement dans les parties rapiécées; le risque de corrosion n'était nullement réduit dans le béton existant. Par ailleurs, tous ces systèmes de réfection du béton présentaient des fissures de retrait. RES
A mathematical model to calculate the fire resistance of circular reinforced concrete columns made with siliceous aggregate is described. Calculated column temperatures and fire resistances are compared with those measured. The results indicate that the mathematical model employed in this study is capable of predicting the fire resistance of circular reinforced concrete columns with an accuracy that is adequate for practical purposes. Using the model, the fire resistance of circular reinforced concrete columns can be evaluated for any value of the significant parameters, such as load, column section size, column length, concrete strength, and percentage of reinforcing steel, without the necessity of testing. The model can also be used for the calculation of the fire resistance of columns made with concretes other than those investigated in this study, for example, lightweight concretes, if the relevant material properties are known.
Steel reinforcement corrosion in chloride contaminated concrete is a costly problem that can be mitigated by cathodic protection. A variety of anode systems are currently available for the cathodic protection of reinforced concrete. One of the most effective anode systems is made of a noble metal oxide coated titanium mesh. The titanium mesh is first mechanically fastened on the concrete and subsequently encapsulated with a shotcrete or concrete overlay. This system is performing extremely well on Horizontal surfaces. However, delamination of the encapsulating shotcrete overlay has occasionally been observed on vertical surfaces such as piers, columns, and walls. The present research project was initiated to address this problem. The performance of three different cementitious overlays (two proprietary and one designed in-house) were investigated under impressed current over a period of 31 months. Samples were polarized outdoors or indoors under constant relative humidity. Numerous performance parameters were monitored on the samples that were energized at three different current densities. The worst performance was obtained with the Thorotop HCR overlay for which high circuit resistance and appreciable increase in the driving voltage were observed. The best overlay for cathodic protection was designed to have maximum adhesion using silica fume and minimum resistivity by the addition of carbon fibers.
Sulfate adsorption from internal sources in systems containing gypsum and C-S-H gel derived from either hydrating C3S or portland cement was studied with respect to delayed ettringite formation. The influence of C3A addition on sulfate desorption was also investigated. Research indicates that C-S-H gel will adsorb sulfate faster at elevated temperatures, resulting in quick depletion of the gypsum phase in C-S-H-gypsum mixes. The critical temperature for sulfate adsorption by C-S-H gel is above 65 C. Sulfate adsorbed at high temperatures is desorbed more slowly than that adsorbed at normal temperatures. The slower release of sulfate from an internal sulfate source may be a critical condition for delayed ettringite formation in high-temperature-cured portland cement paste. Two competitive reactions in terms of gypsum consumption are discussed. These involve both C-S-H gel and the hydrated calcium aluminates. Further evidence in support of a hypothesis for preferred nucleation of delayed ettringite in cracks in portland cement paste is presented. Results presented indicate that the reactants for delayed ettringite formation in cracks can come from sources away from the cracks. Diffusion of reactant ions through concrete pore solution appears to be a key element of the process responsible for initiation of delayed ettringite nucleation in cracks.
In this study, the relationship between inhomogeneity in cementitious material and stress wave parameters is investigated by measuring parameters of through transmission measurements of ultrasonic waves. Except from the inherent inhomogeneity of this type of material, the presence of damage in the form of cracks can lead to even more highlighted velocity dispersion and attenuation phenomena for specific bands of frequencies. Therefore, different contents of crack-like, film-shaped particles are included during casting of concrete to evaluate the contribution of distributed damage in the observed wave parameters. Experiments are carried out using low- and high-frequency sensors with the range of frequencies also covering those used for in-place application.
A simplified method of measuring concrete resistivity, as an index of permeability, has been developed that is similar to ASTM C1202 or the rapid chloride permeability test (RCPT). It is significantly faster and easier to perform, however. In this test, cylinders 100 x 200 mm (4 x 8 in.) were cured in 100% relative humidity and tested using the same solutions, test cells, and rubber gaskets as specified in ASTM C1202. To eliminate the problem of the temperature rise of the sample during the test, only one current reading was taken (after 5 minutes) that could be used to calculate the concrete resistivity. Testing was conducted on various different concrete mixtures after 91 days of moist curing using both the new quicker method and the standard ASTM C1202 method. An empirical correlation between the new method and the standard method demonstrates the validity and promise of the new method.
Electrochemical impedance spectroscopy and linear polarization techniques were used to study five-year-old lollipop-like concrete specimens containing sodium nitrite and dinitrobenzoic acid. An equivalent circuit model considering the physical characteristics of the rebar/concrete interface was used to simulate the impedance spectra. The RC parameters obtained from the impedance spectra simulation including the maximum phase angle shift and polarization resistance were used to characterize the rebar corrosion. The effectiveness of the corrosion-inhibitnig additives in the presence and absence of chloride ions was evaluated. The corrosion current densities estimated by impedance measurement were confirmed by those determined using linear polarization techniques. The purpose of this study was to evaluate the long-term performance potential of sodium nitrite and dinitrobenzoic acid used as the corrosion-inhibiting additives in chloride contaminated reinforced concrete.
High-quality impermeable concrete as cover of reinforcing steel is one of the best methods of preventing chlorides from initiating corrosion. AASHTO T 277 and the ASTM C 1202-91 Rapid Chloride Permeability Test were developed because of a need to rapidly measure permeability of concrete to chloride ions. Some criticisms have been made, mainly concerning the fact that conditions under which measurements are made may cause changes to the specimens. This work was designed to observe how changes in the testing procedure affect results. Factors such as temperature, AC impedance, initial DC current, charge passed, and chloride ion profiles were monitored during polarization of four different concretes. It was found that simple measurement of initial current or resistivity gave the same ranking as conventional tests for the four concretes and can replace the rapid chloride test with a considerable time saving.
This paper describes the performance of steel reinforcement in concrete slabs that were ponded with a 3.4% sodium chloride solution for a period of 6 months. The concrete slabs were cast using portland cement concrete with and without silica fume and blast-furnace slag. The concrete cover to steel reinforcing bars rangedfrom 13 to 76 mm. The concrete resistance to chloride ion penetration was determined according to ASTAI C 1202. The corrosion resistance of the steel reinforcing bars was determined using half-cell potential, linear polarization, and AC impedance techniques. The test results showed that both the silica fume and slag concretes exhibited very low penetrability to chloride ions, with the value of charge passed being less than 1000 coulombs. All control concretes had the value of charge passed greater than WOO coulombs, regardless of the water-cement ratio (\v/c). There was no significant corrosion of the reinforcing steel in the silica fume, slag, and control portland cement concrete with water-cementitious materials ratio (w/cm) of 0.32 after 6 months of ponding with a 3.4% sodium chloride solution. Significant corrosion rates were observed for the reinforcing steel embedded in control portland cement concrete with w/c 0.43. As expected, the poorest performance was of the control concrete with a \v/c ofo.76, where the corrosion of reinforcing steel was detected, even with a 51 mm concrete cover. There was a good correlation among the test results obtained using the half-cell potential, linear polarization, and AC impedance techniques.
Although there are several procedures predicting concrete compressive strength, reliable methodologies involve either extensive testing or voluminous databases. This paper presents a simple and efficient procedure to evaluate the activation energy and the rate constant of concrete. These two parameters can be used for a rapid prediction of the mechanical properties of concrete and particularly the evolution of compressive strength. They also allow separation of effects due to physical phenomena such as humidity loss. The procedure uses an experimentally-determined parameter called "hardening time" as an indicator of equivalent maturity when comparing two hardening profiles. Test results from specimens of six concrete types validate the approach.
The depths of cracks in desiccating plastic concrete are estimated by considering the effects of the suction (negative pore pressure) associated with desiccation and applying five failure models derived from fracture, theories combined with theories drawn from geotechnical engineering under the assumption that plastic concrete is a frictional particulate material. The estimated crack depths vary with the depth of desiccation, the suction profile, and a small number of material parameters that depend on the model adopted and are comparatively easy to estimate accurately. Four of the models predict excessively large crack depths. The fifth, however, predicts shallower crack depths that increase with the age of the concrete and are consistent with those of analogous desiccation cracks in coal mine tailings. It thus offers a relatively robust method of estimating the depth of desiccation cracks. Confirmation of this with data for plastic concrete is clearly desirable but not possible at present.
The injection of cementitious grout into deteriorated civil structures is a common application for strengthening or repair purposes. The nondestructive estimation of the repair effect, which is always desirable, is not an easy task because geometry constituent materials' property mismatch, and temperature-dependent hydration rate often impose difficulties to the characterization. In many cases, after the grout injection and the elimination of voids, the pulse velocity decreases unexpectedly, complicating the task of repair evaluation. In this work, an experimental study of stress wave propagation in concrete impregnated with injection cement is presented. It is revealed that, despite the common impression, wave velocity should not be expected to rise after repair in any case because the properties of grout are initially lower than the surrounding concrete. This effect can be emphasized by the low hardening rate that is imposed to the grout material in the case of low environmental temperature. The transmitted frequencies are also decreased shortly after injection and, therefore, the decrease of wave parameters such as velocity or amplitude should not necessarily be taken as a sign of unsuccessful repair or even deterioration.
Material characterization tests of an Ultra High Performance Fiber Reinforced Concrete (UHPFRC) were performed at various ages. A linear relationship was obtained between the mechanical properties and the degree of hydration. In parallel, the influence of curing conditions on the physico-mechanical properties and the time dependent behavior of this UHPFRC was investigated. A temperature increase accelerated the hydration process at early age and therefore improved the material’s compressive strength and the carrying capacity in four point bending tests, but at a long term, a higher temperature had inverse effects on the mechanical properties. Moreover, at a 20 °C temperature cure, the UHPFRC exhibited autogenous shrinkage at long term comparable to normal concrete. An increase of curing temperature increased the autogenous shrinkage. This effect may be due to the hydration and the self-desiccation processes which are accelerated at high temperatures.
The relationship between velocities of ultrasonic stress waves transmitted along direct and indirect paths was investigated. Tests were conducted on plain concrete slabs of dimensions 1000 x 1500 mm, with a thickness of 250 mm. Direct ultrasonic wave transmission tests were conducted between top and bottom surfaces of the slabs and indirect tests were conducted along the slab surface. A test procedure, described in BS 1881 to determine indirect wave velocities, was refined by defining the number and spacing of transducers. Comparisons were made between direct and indirect wave velocity measurements using statistical analysis. The statistical analysis revealed that direct and indirect wave velocities could be used interchangeably in evaluating the properties of the concrete. The minimum number of tests points required for a reliable estimate of indirect wave velocity was studied and recommendations are provided
In this paper, the tensile properties of concrete at very early ages and their measurements are reviewed, and the need for further study is clearly highlighted, A newly developed apparatus and procedures for uniaxial direct tensile testing of concrete specimens at ages of 1.5 hours or more after mixing, together with the experimental results obtained, are reported. An improved knowledge of various important early-age properties is presented. The tensile strength, Young's modulus, and fracture energy of early-age concrete are all found to increase very slowly during the first 3 hours or so, but significantly increase thereafter. Strong correlations are shown to exist between these three parameters, especially between tensile strength and fracture energy. The high values of fracture energy obtained strongly suggest that early-age cracking of concrete involves a significant zone of plastic straining or microcracking in the vicinity of the crack tip. Early-age concrete is also shown to be more ductile than mature concrete.
Advanced modeling of unsaturated water and contaminant transport in concrete requires knowledge of the unsaturated conductivity (permeability) function and the water retention curve. An assumption of simple Fickian diffusion is not sufficient due to the nonlinear water and contaminant fluxes. The Van Genuchten-Mualem conductivity model, widely used in describing hydraulic properties of soils, is validated by predicting experimental moisture content profiles obtained by nuclear magnetic resonance during simple absorption experiments with concrete cylinders. A new tortuosity parameter is suggested. Analytical methods are extended based on the Van Genuchten-Mualem model, to provide a means of estimating the saturated permeability from the much simpler sorptivity experiment. The predicted permeability is similar to the long-term experimental permeability. This suggests that only the very long-term saturated permeability is suitable for describing unsaturated moisture flux.
Internal curing is promoted as a way to mitigate autogenous shrinkage in high-performance concrete having a low water-binder ratio (w/b). Different methods of internal curing have been proposed. In this study, the effect of substituting 20% of normalweight sand by an equal mass of lightweight sand on the development of shrinkage was investigated on a 0.35 w/b high-performance concrete. Shrinkage was monitored using vibrating wire gauges cast at the center of 100 × 100 × 400 mm (4 × 4 × 16 in.) concrete samples. Two samples were sealed with self-adhesive aluminum foil to represent a closed curing system without any exchange of humidity between the concrete and its environment. After demolding at the age of 23 to 25 hours, two other samples were cured under water for 6 days. Thereafter, these two samples were removed from water and maintained at 23 °C (73 °F) and a 50% relative humidity (RH) environment. Experimental results clearly demonstrate the efficiency of a 20% substitution of normalweight by a lightweight sand to reduce autogenous and drying shrinkage. The incorporation of 20% lightweight sand did not significantly affect the 28-day compressive strength. The cementitious matrix presented low chloride ion permeability. Internal curing through the use of partial replacement of normalweight sand by lightweight sand definitely represents an efficient method to diminish autogenous and drying shrinkage in low w/b concretes where external water curing does not allow in-depth curing of concrete.
A large-scale regression analysis was carried out using experimental data gathered from various sources to evaluate the ratio of splitting tensile strength to cylinder compressive strength as a function of compressive strength of concrete. The reliability of the proposed equation based on experimental data for compressive strength ranging between 4 and 120 MPa (580 to 17,400 psi) was assessed by means of the integral absolute error (IAE). It is also shown that, by only knowing compressive strength and the ratio of tensile to compressive strength, the failure envelope for very high-strength concrete can be established using Johnston's strength criterion without performing triaxial compression tests. A numerical example demonstrates the application of Johnston's strength criterion.
The transient and ultimate shrinkage behavior of unsealed hardened cement paste of water-cement ratio (w/c) = 0.3 was investigated for maximum exposure temperatures up to 670 C (1238 F) and a heating rate of 1 C/min (1.8 F/min). The shrinkage phase, the duration of which was dependent on the maximum exposure temperature, was modeled mathematically using a combined logarithmic and exponential expression. The amount of shrinkage experienced and the temperature at which the trend of increasing ultimate shrinkage is reversed was found to depend on the w/c ratio.
Temperatures of significance are identified in the range 20 to 700 C (68 to 1292 F) for changes in strength and elastic modulus (both static and dynamic) of unsealed hardened cement paste measured at the various temperatures as well as after cooling. These changes are correlated with chemical and microstructural changes reported in the literature. The beneficial effects of thermal drying and loading, within limits, upon these mechanical properties are observed. It is concluded that these properties are dependent primarily on the maximum temperature of exposure as opposed to the temperature at testing.
Although intensive research related to ultra high-performance Concrete (UHPC) and its composition has been carried out over the past two decades, attaining compressive strengths of over 150 MPa (22 ksi) without special treatment, such as heat curing, pressure, and/or extensive vibration, has been nearly out of reach. This paper describes the development of a UHPC with a compressive strength exceeding 200 MPa (30 ksi), which was obtained using materials commercially available in the U.S. market and without the use of any heat treatment, pressure, or special mixer. The influence of different variables such as type of cement (C), silica fume (SF), sand, and high-range water reducer (HRWR) on compressive strength is evaluated The test results show that the spread value, measured through a slump cone test on a flow table, is a good and quick indicator to optimize the mixture packing density and thus its compressive strength.
Data on the behavior of high-strength concrete at high temperatures is of concern in predicting the safety of buildings and construction in response to certain accidents or particular service conditions. Investigations were carried out on the behavior of three concretes (high-strength concrete with and without polypropylene fibers and lightweight aggregate concrete). The three groups of specimens were subjected to identical testing conditions. After a heating-and-cooling cycle at 200 °C, mechanical tests were carried out. Thermal gradient and concrete thermal stability during heating, compressive strength, modulus of elasticity, and splitting tensile strength were analyzed. Comparisons and conclusions were drawn about thermal stability at high temperatures and the residual mechanical properties of the tested concretes.
The initial curing of concrete specimens for quality assurance is addressed in different ways in testing standards, which often specify requirements that are difficult to meet in practice unless very costly initial curing chambers are available. The failure to meet these requirements in many areas of the world does not appear to result in adverse consequences. This study analyzed six initial curing temperature schemes, all with cycles similar to natural conditions to avoid the simplifications inherent in constant temperature curing. Three strengths of concrete and two initial curing times (24 and 72 hours) were used in this study. The findings showed that initial curing time had no effect on 28-day strength. The 28-day strength also proved to be resilient to maximum and minimum initial curing temperatures outside the limits stated in the standards considered in this study.
In 1976, production of separately ground, pelletized blast-furnace slag started near Hamilton, ON, Canada, and a research program began in 1977 to study the effects of this slag cement on sulfate resistance of concrete. For this purpose, concrete cylinders were cast from eight batches using normal, moderate sulfate-resistant and highly sulfate-resistant portland cement types as well as mixtures of high-C3A portland cement plus slag at a water-cementitious materials ratio (w/cm) of 0.45 or 0.50. In the present study, samples were cut from concrete cylinders after 38 years of exposure to sodium sulfate solutions and thin sections were prepared for analysis using scanning electron microscopy. Microstructural details were investigated from the exposed surface to the center of each cylinder, and different phases were determined. Ettringite, thaumasite, gypsum, and layers of calcium carbonate were found to have formed in the samples. A 65% slag substitution of high-C3A portland cement was very effective in improving the performance of concrete exposed to sodium sulfate.