Showing papers in "Aci Materials Journal in 2004"

On the development of fly ash-based geopolymer concrete

Abstract: To reduce greenhouse gas emissions, efforts are needed to develop environmentally friendly construction materials. This paper presents the development of fly ash-based geopolymer concrete. In geopolymer concrete, a by-product material rich in silicon and aluminum, such as low-calcium (ASTM C 618 Class F) fly ash, is chemically activated by a high-alkaline solution to form a paste that binds the loose coarse and fine aggregates, and other unreacted materials in the mixture. The test results presented in this paper show the effects of various parameters on the properties of geopolymer concrete. The application of geopolymer concrete and future research needs are also identified.

Micromechanics-Based Durability Study of Polyvinyl Alcohol-Engineered Cementitious Composite

Abstract: Durability of engineered cementitious composites (ECC) reinforced with polyvinyl alcohol (PVA) fiber is investigated. ECCs have been realized as ductile strain-hardening cementitious composites with tensile strain capacity up to 5%. This material is being applied in new construction and for the repair and retrofit of structures. A micromechanics-based approach is adopted in this durability study. The micromechanics-based model relates fiber, matrix, and interface parameters to composite properties through knowledge of microdeformation mechanisms beyond the elastic stage. Composite property changes resulting from environmental loading are expected to be a manifestation of changes in properties at the fiber, matrix, and/or interface level. This concept is examined in this paper by experimentally determining the changes in the fiber and fiber-matrix interface properties with specimens exposed to accelerated testing and correlating such changes to changes in the ductility of composites exposed to the same accelerated testing conditions. The accelerated test used in this study is a hot water immersion test simulating a long-term hot and humid environment. It is found that the fiber-matrix interface chemical bond increases while the apparent fiber strength decreases when the exposure time reaches 26 weeks. Correspondingly the composite ductility also decreases. The micromechanical model provides a rational means of interpreting and correlating the data from these 2 levels of testing. Despite the deterioration, PVA-ECC is found to retain tensile ductility more than 200 times that of normal concrete or normal fiber-reinforced concrete after exposure to an equivalent of 70 years or more of hot and humid environmental conditions.

Load-Deformation Responses of Slender Reinforced Concrete Walls

Abstract: A detailed review of experimental data obtained from select slender reinforced concrete (RC) wall tests was conducted to assess the relative contributions of flexural and shear deformations to inelastic lateral displacements. An important feature of the study is to assess the accuracy and consistency of the experimental results, including any coupling between inelastic flexural and shear deformations, as well as to provide vital data to support the development and calibration of nonlinear models. Based on these studies, it was found that commonly used approaches, which rely on diagonal displacement transducers mounted within the yielding region of the wall, tend to overestimate shear distortions by as much as 30%. An approach to correct the results based on the use of vertical displacement transducers within the yielding region is evaluated and found to produce consistent results for the tests evaluated. The use of 4 to 6 pairs of vertical displacement transducer pairs was found to be effective. Evaluation of the test results also indicates coupling between inelastic flexural and shear deformations, despite nominal shear strengths of approximately twice the shear force applied during the test.

Concrete Cracking by Localized Steel Corrosion-Geometric Effects

Abstract: Steel reinforcement embedded in normal, uncontaminated concrete is usually well protected against corrosion. When the chloride-ion concentration at the reinforcing bar depth exceeds a critical threshold value, however, the protective passive layer on the steel surface breaks down and active steel corrosion begins. This article reports on a study that experimentally estimated the critical amount of steel corrosion (xCRIT) needed for concrete cover cracking of a reinforced concrete element when only a fraction of the steel bar length is corroding. The authors established that the amount of corrosion needed to crack the concrete cover (xCRIT) was 0.030 to 0.272 mm in specimens with localized corrosion, in comparison to 0.003 to 0.074 mm for more uniform corrosion reported for other investigations in comparable systems. The authors propose an empirical equation for xCRIT as a function of specimen dimensions (concrete clear cover C, reinforcing bar diameter f , and anodic length L).

Evaluation of Electrically Conductive Concrete Containing Carbon Products for Deicing

Abstract: Using electrically conductive concrete for deicing is an emerging material technology. Due to its electrical resistance, a thin layer of conductive concrete can generate enough heat to prevent ice formation on concrete pavement when energized by a power source. Under research sponsored by the Nebraska Department of Roads, a concrete mixture containing steel fibers and shavings was developed specifically for concrete bridge deck deicing. The mixture has a compressive strength of 31 MPa and provides average thermal power density of 590 W/m-sq. with a heating rate of 0.14 deg C/min in a winter environment. During development of the conductive concrete, several drawbacks about using steel shavings in the mixture were noticed. As a follow-up effort, carbon and graphite products were used to replace steel shavings in the conductive concrete design. The electrical conductivity and the associated heating rate were improved with the carbon products. A conductive concrete deck has been implemented for deicing on a highway bridge at Roca, located approximately 15 miles south of Lincoln, Nebraska. The Roca Spur Bridge has a 117-ft-long and 28-ft-wide conductive concrete inlay, which has been instrumented with temperature and current sensors for heating performance monitoring during winter storms. Experimental data and operating costs are presented herein.

Fiber Synergy in Fiber-Reinforced Self-Consolidating Concrete

Abstract: Self-consolidating concrete (SCC) was developed to respond to the need for concrete that can improve durability while eliminating the need for compaction and vibration work. SCC can compact itself into complicated formwork and congested structural elements under its own weight without the need for mechanical vibration. Thus, SCC decreases construction time, labor, and equipment needed on construction sites; makes the construction of heavily congested structural elements and hard-to-reach areas easier; reduces noise and injuries related to vibration work of concrete; and helps achieve higher-quality finish surfaces. Fiber reinforcement can extend the technical benefits of SCC by also providing crack bridging ability, higher toughness, and long-term durability. This article reports on a study of the potential synergistic effects in SCC incorporating different steel and synthetic polymer macro- and microfibers in various hybrid (single, binary, and ternary) combinations. The study included a total of 31 SCC mixtures, with combinations of fibers in varying proportions from 0.25 to 1.0%. The fiber length varied from 30 to 50 mm for the macrofibers, while microfibers were approximately 12 mm long. Each mixture was evaluated using the slump flow and L-Box flow tests, compressive strength tests, flexural toughness, first-crack strength in bending, and post-first-crack behavior. Results of this study show that fibers can have rheological and mechanical synergistic effects. The authors conclude that optimized fiber combinations can better increase toughness and flexural strength while maintaining adequate flow properties for fiber-reinforced SCC.

Development of Statistical Models for Mixture Design of High-Volume Fly Ash Self-Consolidating Concrete

Abstract: Self-consolidating concrete (SCC) in the fresh state is known for its excellent deformability, high resistance to segregation, and use, without applying vibration, in congested reinforced concrete structures characterized by difficult casting conditions. Such concrete can be obtained by incorporating either mineral admixtures such as fly ash (FA) or viscosity-modifying admixtures (VMA). The use of VMA has proven very effective in stabilizing rheology of SCC, and recent research has focused on development of new, cheaper VMAs compared with currently available, costly commercial ones. Research to produce an economical SCC with desired properties has been conducted in recent years into the use of FA. In this paper, 21 statistically balanced concrete mixtures were investigated to minimize the use of high-range water-reducing admixtures (HRWRA) and to optimize the use of fly ash in SCC. The minimum use of HRWRA and optimum use of FA were desired in this study. Four independent variables were used for design of SCC mixtures. The fresh concrete properties were determined from slump flow, V-funnel flow, filling capacity, bleeding, air content, and segregation tests. The mechanical properties and durability characteristics of SCC such as compressive strength, freezing/thawing resistance, rapid chloride permeability, surface scaling resistance, and drying shrinkage were determined to evaluate the performance of SCC. Four statistical models to predict the slump flow, 1- and 28-day compressive strength, and the rapid chloride permeability of SCC were developed and their performances were validated.

Low-Cycle Fatigue Failure of Reinforcing Steel Bars

Abstract: This article reports on a comprehensive experimental study that was carried out to examine the low-cycle fatigue behavior of ordinary reinforcing bars used in reinforced concrete (RC) construction. The objective of the study was to gain a better understanding of low-cycle fatigue failure of the longitudinal steel reinforcement in potential plastic hinge zones of RC members subjected to seismic loads and to develop a fatigue life relationship to characterize the response. In the study, a pair of aluminum strips was securely fastened between the specimen and the custom-built gripping blocks that were used to act as the force transfer media. The results of both monotonic tension tests and low-cycle fatigue tests using constant amplitude cyclic strain histories on four different bar sizes are reported in this article. Preliminary findings indicate that fatigue life is influenced by the diameter of the bar and the geometry of the rolled on deformations. The authors develop fatigue life relationships based on total strain amplitude and energy, for use in damage and failure modeling. The authors point to the need for including low-cycle fatigue behavior of the bars, in addition to monotonic properties, for applications that subject the bar to reversed cyclic loads.

Performance of supplementary cementitious materials in concrete resistivity and corrosion monitoring evaluations

Abstract: A testing regime was developed to optimize strength and durability characteristics of a wide range of high-performance concrete mixtures The aim of the selected designs was to offer multiple solutions for creating a highly durable and effective structural material that would be implemented on Pennsylvania bridge decks, with a life expectancy of 75-100 years A prime method for optimizing the mixtures was to implement supplemental cementitious materials, at their most advantageous levels Fly ash, slag cement and microsilica all proved effective in creating more durable concrete design mixtures These materials have also shown success in substantially lowering chloride ingress, thus extending the initiation phase of corrosion An additional benefit studied in this program is the ability of these materials to extend the propagation phase of corrosion due to the high resistivity they impart to the concrete Ternary mixtures from these materials were particularly effective, showing much higher resistivity values than the materials used separately

Precast concrete products using industrial by-products

Abstract: This work aimed to help establish the use of high volumes of fly ash, bottom ash, and used foundry sand in manufacture of precast molded concrete products such as wet-cast concrete bricks and paving stones. ASTM Class F fly ash was used as a partial replacement for 0 (reference), 25, and 35% of portland cement. Bottom ash combined with used foundry sand replaced 0, 50, and 70% of natural sand. Tests for compressive strength, freeze-thaw resistance, drying shrinkage, and abrasion resistance were conducted on the wet-cast concrete masonry units manufactured at a commercial manufacturing plant. It was concluded that all wet-cast bricks could be used for both exterior and interior walls in regions where freezing and thawing is not a concern, and for interior walls in regions where freezing and thawing is a concern. No wet-cast paving-stone mixtures, including the reference mixture, met all ASTM requirements for paving stones.

Anchorage Length of Near-Surface Mounted Fiber-Reinforced Polymer Rods for Concrete Strengthening-Analytical Modeling

Abstract: Among strengthening techniques based on fiber-reinforced polymer (FRP) composites, near-surface mounted (NSM) FRP rods are one of the most recent and promising acquisitions. Because bond performance is a critical aspect of the technology, experimental and theoretical investigations on this issue were performed, with this paper focusing on the latter. Analytical modeling is carried out in 2 phases. First, the experimental local bond stress-slip curves are modeled analytically and the numerical solution of the differential equation of bond allows computing the bond failure load as a function of the bonded length and the anchorage length of NSM FRP rods required in design. In the 2nd phase, an approximate bi-dimensional analysis in the elastic range and a limit analysis assuming plastic behavior of concrete are conducted to compute upper and lower bounds to the local bond strength. In both phases, analytical predictions are compared with experimental results, showing good agreement.

Bi-Surface Shear Test for Evaluating Bond between Existing and New Concrete

Abstract: Even though bond tests have been developed for several specific applications, there is no consensus among practitioners for evaluating the bond strength under a shear state of stress that is commonly encountered in structures. This study is aimed at presenting a new shear test method for evaluating the bond between repair materials and concrete substrate. This method uses a 150 mm cube where the repair material constitutes 1/3 of the specimen volume. Three point loads are applied such that they produce a state of shear stress on the repaired interface. Test variables include specimen size, maximum aggregate size of repair materials, type of repair material, interface roughness, and age at loading. This test method is easy to carry out on both lab and field samples. Bond strength was improved with surface roughness, silica fume content, and age at testing. Larger specimens yielded lower bond strength. Bi-surface shear is a reliable test for determining bond between existing concrete and repair materials.

Mechanical Performance of Sprayed Engineered Cementitious Composite Using Wet-Mix Shotcreting Process for Repair Applications

Abstract: , V. 101, No. 1, January-February 2004.MS No. 02-473 received December 11, 2002, and reviewed under Institute publicationpolicies. Copyright © 2004, American Concrete Institute. All rights reserved, includingthe making of copies unless permission is obtained from the copyright proprietors.Pertinent discussion including authors’ closure, if any, will be published in the November-December 2004

Effects of air entrainment on rheology

Abstract: This article reports on a study in which the effects of air entrainment on rheological parameters were investigated using cement paste. The addition of air-entraining agent increased the air content up to a saturation level, above which no further increase in air content was observed. The results showed that with increasing air content, the yield stress increased and the plastic viscosity decreased. The authors note that the increase in yield stress was an unexpected result because increasing air is well known to cause an increase in slump, and yield stress and slump are known to be negatively correlated (as yield stress increases, slump decreases). The authors propose two explanations for the effects of entrained air bubbles on rheological parameters: the attraction of cement particles and bubbles to form bubble bridges, and a fluid response of air bubbles due to their deformability. The authors contend that bubble bridges dominate in the yield stress and the fluid response will dominate when the sample is flowing.

Properties of Early-Age Portland Cement Mortar Monitored with Shear Wave Reflection Method

Abstract: The setting and hardening process of concrete is considered the most critical time period during the life of a concrete structure. Thus, the nondestructive, in-place testing of early-age concrete properties is an important tool for the progress of many construction projects. This article reports on a study that examined the ability of a shear wave reflection method to monitor the hydration kinetics of Portland cement mortar. The wave reflection method measures the reflection loss of shear waves at an interface between a steel plate and the mortar. The study included mortars with water-cement ratios (w/c) of 0.35, 0.5, and 0.6, which were tested at isothermal curing conditions of 25 degrees C. The authors characterized the hydration behavior of the mortars by setting time, heat of hydration, compressive strength, dynamic shear modulus, and degree of hydration. The results show that compressive strength has a bilinear relationship to the reflection loss. Additional results suggest that the wave reflection measurements are qualitatively governed by the dynamic shear modulus of the mortar. Finally, the reflection loss data of all three tested mortars were found to be uniquely related to the gel-space ratio of the cement paste phase of the mortars.

Drying shrinkage of concrete reinforced with fibers and welded-wire fabric

Abstract: This research compares the performance of different fiber types, fiber blends, and welded-wire fabric (WWF) in their ability to prevent and control drying-shrinkage cracking. Restrained shrinkage ring tests were conducted on fiber-reinforced concrete mixtures to determine the age of the first visible crack and maximum crack width. Results for various steel and polypropylene fibers in diverse fiber volumes and shapes are compared with plain concrete and concrete reinforced with WWF. The WWF is not effective in prolonging the age of a first crack but does reduce the maximum crack width. Effectiveness of steel and polypropylene fibers in reducing crack width and increasing the age of first cracking depends on the type of fiber. Commonly used parameters such as volume of fibers, number of fibers, and aspect ratio did not uniquely explain the effectiveness of the fibers. A novel model that equally accounts for influences of all constituents of a composite on its properties is used and extended to describe the shrinkage cracking behavior of the tested materials.

High-Performance Fiber-Reinforced Concrete Mixture Proportions with High Fiber Volume Fractions

Abstract: An experimental study of high-performance fiber-reinforced concrete (FRC) was performed, showing that high-performance FRC with fiber volume fractions up to 3.75% of 30 mm-long hooked steel fibers can be achieved using conventional mixing and appropriate matrix compositions. Based on this investigation, the optimum fiber content was dependent on mixture design constituents, fiber types, and mixing procedure. Test results showed high-performance steel FRC (SFRC) with 3.75% fiber volume fraction is attainable and can be successfully applied in the field. Fibers were mixable with the mortar and concrete matrices. The mixture was workable and flowable and fiber distribution was uniform. Six mortar mixture proportions, 1 lightweight concrete mixture, and 3 fiber types were tested. Mortar mixtures included cement Types II and V, mortar sand, condensed silica fume, and high-range water-reducing admixtures. Fiber types were hooked steel and straight steel fibers, and polypropylene macrofibers. The mixtures were successfully produced in a ready-mix plant. Flexural strengths obtained varied from 12-24 MPa. Compressive strengths varied from 74-100 MPa and splitting tensile strengths were from 12.6-17 MPa. Flexural toughness of mixtures achieved in this study was 2-3 times higher than those of conventional FRC.

Applications of statistical models in proportioning medium-strength self-consolidating concrete

Abstract: This paper reviews statistical models obtained from a factorial design carried out to determine the influence of 4 key parameters on filling and passing ability, segregation, and compressive strength. These parameters are important for successful development of medium-strength self-consolidating concrete (MS-SCC). The parameters considered were the contents of cement and pulverized-fuel ash (PFA), water-powder ratio (W/P), and dosage of HRWRA. The responses of the derived statistical models are slump flow, fluidity loss, rheological parameters, Orimet time, V-funnel time, L-box, JRing combined with Orimet, JRing combined with cone, fresh segregation, and compressive strength at 7, 28, and 90 days. The models are valid for mixtures made with 0.38-0.72 W/P, 60-216 kg/m3 of cement content, 183-317 kg/m3 of PFA, and 0-1% of high-range water-reducing admixture (HRWRA), by mass of powder. The utility of such models to optimize concrete mixtures to achieve good balance between filling ability, passing ability, segregation, compressive strength, and cost is discussed. Examples highlighting usefulness of the models are presented using isoresponse surfaces to demonstrate single and coupled effects of mixture parameters on slump flow, loss of fluidity, flow resistance, segregation, JRing combined with Orimet, and compressive strength at 7 and 28 days. Cost analysis is carried out to show tradeoffs between cost of materials and specified consistency levels and compressive strength at 7 and 28 days that can be used to identify economic mixtures. The paper establishes usefulness of the mathematical models as tools to facilitate the test protocol required to optimize MS-SCC.

Effect of Aggregate Type on Mechanical Behavior of Dam Concrete

Abstract: Due to the practical difficulties in performing a full size specimen test, dam concrete is usually sieved to remove the aggregate larger than 40 mm (1.6 in) when tested, which is called sieved concrete. This paper reports on a study that examined the influence of surface roughness of aggregate via two types of aggregates on the mechanical and fracture properties for dam and sieved concretes. The authors used two types of aggregates by full size specimens of 450 x 450 x 900 mm (18 x 18 x 35 in.) for tension and 450 x 450 x 450 mm (18 x 18 x 18 in.) for compression and by standard size specimens 150 x 150 x 550 mm (6 x 6 x 22 in.) for tension and 150 x 150 x 150 mm (6 x 6 x 6 in.) for compression, respectively. The complete stress-strain responses both in tension and compression were acquired through a systematic experimental program. The authors propose models to estimate the basic mechanical properties and the fracture parameters of the concretes with specimen ages; they propose theoretical models for normalized axial stress-strain curve in compression and for normalized axial stress versus normalized maximum crack width curve in uniaxial tension. Results include: the strengths, modulus of elasticity, and fracture energy increase with specimen age; the specimen size and maximum aggregate diameter significantly affect the fracture energy, peak strain, and crack width of concretes in uniaxial tension; and the performance of concrete with crushed coarse aggregate is higher than that of concrete with natural coarse aggregate.

Creep and shrinkage of high-performance lightweight concrete

Abstract: Two high-performance lightweight concrete (HPLC) mixtures with average compressive strengths of 68.5 and 75.4 MPa (9950 and 10,950 psi) were developed. Their air-dry unit weights were 1875 and 1905 kg/cubic m, respectively. 26 creep specimens were loaded at 16 or 24 hrs to 40 or 60% of their initial strength. This preliminary investigation showed that expanded slate HPLC experienced less creep, but slightly more shrinkage than normalweight HPC of similar paste content, mixture proportions, and strength. The 620-day creep coefficients of the 68.5 and 75.4 MPa (9950 and 10,950 psi) HPLC were 1.66 and 1.29, respectively. Creep and shrinkage were compared with estimates from 12 models.

Variations of lateral and pore water pressure of self-consolidating concrete at early age

Abstract: This study investigates the kinetics of variations in lateral pressure and pore water pressure that can be exerted by self-consolidating concrete (SCC) during the plastic stage of cement hydration and until the early stage of hardening. Nine mixtures of constant slump flow values of 650 +/- 15 mm were evaluated by varying the sand-to-total aggregate ratio R from 1.0 to 0.30. The mixture proportioned with an R value of 0.50 was tested with 3 different coarse aggregate gradations of 10-5, 14-5, and 20-5 mm. Each concrete was tested to determine setting time, adiabatic heat rise and temperature development, and pressure variations with time. Test results show that kinetics of lateral pressure drop during the plastic stage are significantly affected by the degree of internal friction that can be related to coarse aggregate concentration. For mixtures with R values greater than 0.50, the concrete behaved as an exclusively viscous material with a limited degree of internal friction, which resulted in greater development of lateral pressure. For mixtures with R values lower than 0.50, the concrete appeared to exhibit greater resistance to shear stress given the higher degree of aggregate friction, which led to considerably lower lateral pressure development. The drop in lateral pressure toward zero occurs after the end of the dormant period as the rate of cement hydration is accelerated. Beyond the dormant period, progressive formation of hydration products leads to the creation of a structural network, and the pore water pressure begins to drop abruptly towards negative values. The time for cessation lateral pressure is shown to be influenced by the R value. For R values greater than or equal to 0.40, the concrete can continue to exert some lateral pressure until the onset of final setting. For mixtures made with R values of 0.36 and 0.30, the cessation of lateral pressure occurred before initial setting as the rate of pressure drop was more accentuated than that resulting from SCC with higher R values. The increase in maximum aggregate size from 10 to 14 mm resulted in a sharper rate of pressure drop, which is associated with an increase in the degree of internal friction. A lower rate of pressure drop, however, can be obtained with a further increase in aggregate size to 20 mm, given the relatively decreased packing density of the 20-5 mm coarse aggregate as compared to that of the 14-5 mm.

Factorial design of cement slurries containing limestone powder for self-consolidating slurry-infiltrated fiber concrete

Abstract: This study investigated slurries with high penetrability for production of self-consolidating slurry-infiltrated fiber concrete (SIFCON). Factorial experimental design was adopted to assess the combined effects of 5 independent variables on mini-slump test, plate cohesion meter, induced bleeding test, J-fiber penetration test, and compressive strength at 7 and 28 days. The independent variables investigated were the proportions of limestone powder (LSP) and sand, the dosages of high-range water-reducing admixture [superplasticizer (SP)] and viscosity agent (VA), and water-binder ratio (w/b). A 2-level fractional factorial statistical method was used to model the influence of key parameters on properties affecting the behavior of fresh cement slurry and compressive strength. The models are valid for mixtures with 10-50% LSP as replacement of cement, 0.02-0.06% VA by mass of cement, 0.6-1.2% SP, and 50 to 150% sand (percent mass of binder) and 0.42-0.48 w/b. The influences of LSP, SP, VA, sand, and w/b were characterized and analyzed using polynomial regression, which identifies the primary factors and their interactions on the measured properties. Mathematical polynomials were developed for mini-slump, plate cohesion meter, J-fiber penetration test, induced bleeding, and compressive strength as functions of LSP, SP, VA, sand, and w/b. The estimated results of mini-slump, induced bleeding test, and compressive strength from the derived models are compared with results obtained from earlier proposed models that were developed for cement paste. The proposed response models of the self-consolidating SIFCON offer useful information regarding the mixture optimization to secure a high penetration of slurry with low compressive strength.

Interaction Formula for Reinforced Concrete Columns in Fire Conditions

Abstract: This paper presents a simple and rational interaction formula -- the Rankine method -- for predicting the fire resistance of reinforced concrete (RC) columns under fire conditions. Conventionally, the Rankine method has been applied to RC columns and frames subjected to increasing loads but maintained at ambient temperature. Based on a theoretical approach, the Rankine method is extended to RC columns subjected to fire conditions by taking the temperature effects on steel reinforcement and concrete material into consideration. The accuracy of the method is verified through four sets of experimental test results comprising 76 columns. The method yields reasonably accurate predictions for the fire resistance of RC columns. In addition, the paper includes two worked examples to illustrate the flexible use of such a formula for either predicting the load-bearing capacity or the fire exposure time of an RC column subjected to the standard ISO 834 fire curve.

Innovative Bridge Deck System Using High-Performance Fiber-Reinforced Cement Composites

Abstract: This paper details the basis for an innovative concept related to building better and more durable concrete bridge decks. The decks are assumed to be supported by beams of any structural material. The typical deck is made with high-performance, fiber-reinforced cement composites (HPFRCC) and reinforced with only 1 layer of transverse bottom reinforcing bars for positive moment resistance. Thus, reinforcement is as far away as possible from the top surface of the deck and protected from intrusion of corrosive agents. The HPFRCC is designed to: 1) balance part of the negative moment; 2) allow the development of an effective plastic hinge mechanism, with hinges forming at supports and in spans, under ultimate loading; and 3) keep crack widths extremely small under service conditions. A dramatically higher durability is expected. A brief summary of test results and analytical modeling is also provided.

Is microcracking really a precursor to delayed ettringite formation and consequent expansion

Abstract: Several previous studies have shown the importance of the pre-existence of microcracks to delayed ettringite formation (DEF) and consequent expansion. Other research results, however, showed clear differences of expansion behavior depending on the causes of microcracks. This article reports on a study of the expansion of 40 and 36 heat-cured mortar and concrete mixtures, respectively. The ultimate value of expansion due to DEF is always higher for slow-cooled samples than this same value for rapid-cooled samples. The thermal shock of rapid-cooled samples creates microcracks. However, the authors conclude from their results that microcracking caused by thermal shock does not promote expansion. On the contrary, this type of microcracking decreases the ultimate value of expansion, though it can accelerate it because microcracks facilitate the availability of water.