Self-consolidating concrete (SCC) suits cast-in-place bridge applications in northern-tier states such as Nebraska and South Dakota, according to research
Self-consolidating concrete (SCC) suits cast-in-place bridge applications in northern-tier states such as Nebraska and South Dakota, according to research findings presented at the 87th annual Transportation Research Board meeting earlier this year in Washington, D.C. Following are summaries of the SCC research and mix design development, plus other papers addressing ready mixed production or cast-in-place concrete practice. Summaries of precast concrete research appeared in February’s Technical Talk. More information about TRB is posted at www.trb.org.
NEBRASKA’S SCC MIX FOR C-I-P
A mix design for SCC was developed, tested and found appropriate for Nebraska cast-in-place bridge projects, according to P. Paczkowski, M. Kaszynska, G. Morcous and A.S. Nowak, University of Nebraska-Lincoln Department of Engineering, in their peer-reviewed paper, Development of SCC Procedures for On-Site Bridge Applications.
For cast-in-place applications, the [SCC] has to satisfy basic requirements, such as flowing ability, passing ability and resistance to segregation, in addition to pumpability, for delivery times up to 90 minutes, the authors write. These properties depend on the type of materials used, mix design (e.g., water/cement ratio and admixture content), placing temperature, and delivery time. Their paper confirmed the mix’s utility for bridge applications in Nebraska.
The major advantage of SCC compared to ordinary concrete is its flowability, so it can fill hard-to-reach corners and reinforcement-congested areas within the formwork, without mechanical consolidation, the researchers explain. Properly mixed, SCC is not affected by segregation, they add, while pointing out that its use has been confined mostly to precast plants, due to excellent filling properties and the elimination of noise from vibrators.
It turns out that SCC also can be used for cast-in-place applications (slabs, abutments, piers) and for repair of existing structures, especially in hard-to-reach repair projects with a restricted access for pumping concrete, the authors affirm. In the case of ordinary concrete, the major problem for on-site placement is the assurance of good quality of material and proper degree of vibration to avoid gaps and reduce the shrinkage cracks. Self-consolidating concrete offers an excellent solution [that] can improve the quality of concrete structures by eliminating some of the potential for human error.
However, SCC needs a more advanced mix design than ordinary concrete and exacting quality assurance with more testing and checking, the researchers emphasize, at least during the startup period. At present, a viscosity-modifying admixture (VMA) is used to improve the stability of the mix, [and] eliminate bleeding and segregation, they write. However, application of a high-range water reducer (HRWR) and VMA make the design of the mix much more complicated.
SCC pressure on formwork, indicated by strain gauges for different mixes and casting methods, has produced varied results. High pressure values, exceeding the hydrostatic pressure, occurred when SCC was pumped from the base, and high variation in pressure values occurred when SCC was pumped from the top at different rates, the authors assert. These studies concluded that more research is needed to gain a better understanding of SCC behavior for different site conditions. Therefore, [the] Nebraska Department of Roads has initiated a one-year research project prior to applying cast-in-place SCC in bridge construction.
The first step involved investigation of HRWRs for different delivery times and the impact of VMAs on mortar flowability. The researchers found that the addition of VMA has to be compensated for with an extra HRWR dose. Trials on mortars also showed that high dosage of VMA can significantly reduce the workability time. Slump flow retention curves were obtained for the selected SCC mixes. Two different types of retarding admixtures were checked for their ability to extend the workability time. The effect of air-entraining admixtures on air content and compressive strength also was investigated.
The second step involved testing the selected SCC recipe on four, 4-ft.-high walls, each one cast by pouring the mix at a different delivery time. This test also checked the passing ability of the mix as the minimum spacing between bars was Ê in. In another test, the mix was pumped for over 300 ft. to verify the pumpability and its impact on the fresh concrete properties.
Nebraska’s basic SCC mix design was developed for on-site bridge applications. Materials include cement mixed with Type F fly ash, sand and gravel, with the maximum aggregate size of 0.5 in. (12.5 mm), and admixtures. The reduced aggregate size helped increase resistance to segregation and eliminated problems with blocking.
The analysis of delivery time effect on SCC properties showed that for on-site applications a retarder should be used and, if needed, an additional dosage of the HRWR, the authors write. However, the amount of retarder should be carefully selected because an excessive amount does not increase workability time÷it only affects the setting time.
Air content in concrete is currently the subject of intensive research, they say. Comparison of the results obtained using the ASTM 231 Pressure Method and the ASTM C457 Modified Point-Count Method shows close agreement. Therefore, the Pressure Method is recommended on-site for a fast assessment of the air content because of its simplicity. The plastic air content can cause a reduction of concrete’s compressive strength, and therefore, practical quality control procedures are needed.
Laboratory tests showed that it is possible for the mix with admixtures to maintain SCC properties up to 70 minutes, the authors conclude. If after that time, the spread decreases below acceptable limit, they write, the flowability can be recovered by an additional dosage of HRWR. On-site pilot tests showed that the SCC mix remains pumpable even in highly elevated temperatures.
SCC MIXES PROVE A-OK FOR SOUTH DAKOTA BRIDGES
Local aggregates will work in SCC in South Dakota, and SCC mixes are appropriate for bridges in the state, note Nadim I. Wehbe, Arden B. Sigl and Zachary D. Gutzmer, Department of Civil and Environmental Engineering, South Dakota State University; and, Amanda L. Boushek, CTA Group, Billings, Mont., in their paper, Laboratory Evaluation of Self-Consolidating Concrete Mixtures for Highway Structures in South Dakota.
Their research is part of a comprehensive study evaluating the feasibility and performance of SCC made with local South Dakota aggregates in an effort to develop related draft specifications, acceptance criteria, mix qualifications, and guidelines that South Dakota DOT can use for precast and cast-in-place, nonprestressed structural applications. Twelve SCC mixes were studied in the lab, varying the parameters of aggregate type, water-to-cementitious material ratio (w/c), and mixing duration. Two types of coarse aggregate were used in developing the mixes: quartzite from eastern South Dakota and limestone from western South Dakota.
Two mixing durations were utilized to simulate precast and cast-in-place applications, the authors write. Three w/c ratios Û 0.38, 0.42, and 0.46 Û were investigated. The experimental results show that South Dakota aggregates having proper shape and size can be used to produce stable SCC.
For cast-in-place applications, better control of SCC flowability can be achieved when the HRWR is added and mixed immediately before concrete discharge, the researchers report. It was observed that a longer mixing duration resulted in higher air content, they add. The strength characteristics of the hardened SCC in this research followed a trend similar to that of conventional concrete.
Because of its high workability, SCC flows into narrow spaces and form corners and around closely spaced steel reinforcement, without the need for mechanical vibration, the authors explain. While the cost of SCC is slightly higher than that of standard concrete, the use of SCC would result in enhanced finished quality, reduced labor cost, higher productivity, and increased safety as a consequence of the reduced labor force needed to place the concrete, they observe.
There are many potential applications for SCC in bridge structures, the engineers affirm. Precast and cast-in-place box culverts, precast girders, retaining structures, pile foundations, narrow and thin walls, and other structural elements with heavy reinforcement are a few examples of where using SCC may be advantageous. Past experience in South Dakota indicates that the construction of narrow-walled box culverts using conventional concrete mixtures may result in internal voids and construction defects, they say. The use of SCC may eliminate the occurrence of such defects and may result in better final products.
SCC mix proportions and properties are dependent, among other factors, on the physical properties of aggregates used in the mix. The writers conclude that for fresh SCC, as the w/c ratio increased, the SCC flowability (slump flow) typically increased; blocking potential typically decreased; T20 value decreased; L-Box H2/H1 ratio typically increased; and, the air content of the precast mixes (short-duration mixing) decreased, while the air content of the cast-in-place mixes remained practically unchanged.
For hardened SCC, as w/c ratio increased, the SCC compressive strength decreased. The compressive strength development rate of SCC is comparable to or slightly greater than that of conventional concrete; the relationship between the splitting tensile strength and the compressive strength of SCC is comparable to that of conventional concrete; and, the relationship between the modulus of rupture and the compressive strength of SCC is comparable to that of conventional concrete, the authors find. The ACI empirical equation for determining the modulus of rupture is suitable for use with SCC, they add.
In general, when properly sized and shaped, South Dakota aggregates were found to be suitable for producing SCC. Under laboratory conditions, all SCC mix designs considered in the study were found to be stable with no signs of segregation. The highest w/c ratio used (0.46) resulted in the most economical SCC mix (least amounts of cement and HRWR) and greatest flowability.
The measured 28-day compressive strength varied between 5,880 and 7,650 psi, the researchers note. The attained concrete strength is adequate for the majority of cast-in-place and precast box culvert applications in the state of South Dakota. The measured air content was either within or higher than the limits set by South Dakota DOT for conventional concrete. The air content can be easily modified by adjusting the amount of the air-entraining admixture.
And, according to the authors, for cast-in-place SCC applications where the concrete is expected to remain in the mixer truck for an extended duration before it’s placed, HRWR should be added and mixed on the job site immediately before the load is discharged. This will reduce the potential for HRWR evaporation and ensure adequate slump spread.
FIBERS IMPART FLEXURAL TOUGHNESS IN WHITETOPPING
Higher-than-conventional doses of fibrillated polypropylene fibers enhance a thin whitetopping’s flexural toughness, but not its compressive strength, say Maria Carolina Rodezno and Kamil E. Kaloush, Ph.D, P.E., Arizona State University, in their paper, Effect of Different Dosages of Polypropylene Fibers in Thin Whitetopping Concrete Pavements.
The most frequently used synthetic fibers in concrete pavements are fibrillated polypropylene, the authors state. They are normally used in [thin whitetoppings] at a rate of 3 lb./yd. Fibers distributed in the concrete mix provide multidirectional reinforcement that absorbs energy and increases impact and freeze-thaw resistance, thus reducing cracking. The fiber-reinforced concrete plays a major role in increasing the ductility of [thin-whitetopped] concrete pavement structures. It allows an ultra-thin and thin sections (normally 2 to 5 in.) supported by the asphalt concrete layer to act as a structural load bearing system.
Fiber reinforcement dramatically increases the tensile strength and ductility of brittle materials, the researchers note. Through the process of bridging cracks and pull-out, the fibers result in significant toughening of brittle matrices. Cracks are intersected by random fibers that debond, pullout, and thus provide energy absorption and toughening. This effect normally is measured by evaluating the post-peak region of the load-deformation responses.
By controlling the cracks at a microscopic level, the macroscopic load-carrying response is enhanced, they say. Such effects have been observed for composites with steel and glass, as well as fibrillated polypropylene fibers. Fibers disallow the localization processes, and a homogeneous state of microcracking is formed, which dissipates energy over the entire volume.
To more closely study fiber performance, a collaborative effort between the Arizona DOT, Arizona State University (ASU), and the local concrete industry evaluated the effect and potential performance of polypropylene, fiber-reinforced, thin whitetopping pavement sections in Arizona. Accordingly, a lab-based experimental program was undertaken to evaluate the engineering properties of four different sections, comprising three polypropylene fiber-reinforced mixes using dosages of 3, 5, and 8 lb./yd., plus a fiber-free control.
Various samples were collected during construction and tested for compression, flexural, and toughness using cylindrical, prismatic and round panel specimens. While the compression results did not show any added benefit with the addition of polypropylene fibers, analysis of flexural and round panel tests demonstrated the advantage of the fibers.
Specifically, the toughness results from both tests showed an increase in these values when the percentage of fibers was increased, the authors write. However, the results of toughness cannot be taken into account using common concrete pavement design methodologies, since the traditional input value is the modulus of rupture.
Therefore, an analysis approach was performed that took into account the additional benefit of polypropylene fibers by adding a residual strength to the modulus of rupture. It was concluded that the effect of the addition of fibers was best captured using the round panel test, the researchers observe. The percentage increase in toughness values from the different fiber dosages was larger in the round panel test than the flexural test. It was also concluded that the use of 5 lb. per cubic yard fiber dosage has the best value-added benefit to the mix.
Almost two years after their construction, a field survey of the test sections showed that they are performing well with no signs of cracking or any other distress, the authors report.
USE OF RAP CAN BENEFIT FLORIDA PCC PAVEMENTS
Florida DOT always has been a national leader in the research and application of reclaimed asphalt pavement (RAP) in asphaltic concrete mixes, and new research in RAP in portland cement concrete pavements shows RAP can improve those pavements, according to Nabil Hossiney, Guangming Wang and Mang Tia, Department of Civil & Coastal Engineering, University of Florida-Gainesville; and, Michael J. Bergin, Florida DOT state structural materials engineer, in their paper, Evaluation of Concrete Containing RAP for use in Concrete Pavement.
The modulus of elasticity of concrete is known to have a major effect on the performance of concrete pavements, they write. [and is] an important input parameter to the AASHTO design equations for concrete pavements. Concrete pavements using concrete with a lower modulus of elasticity would have a lower stress due to the same applied load and, thus, could have a lower chance of cracking.
For example, the engineers note, in an investigation of the performance of I-75 concrete pavements in Sarasota and Manatee counties, the percent of cracked slabs reportedly increased with an increase in the concrete’s modulus of elasticity. In another research study undertaking, the optimal mixture for concrete pavement was found to be not necessarily a concrete with a high flexural strength, but one with a proper combination of low modulus of elasticity, low coefficient of thermal expansion, and adequate flexural strength properties.
Their study evaluated the feasibility of using concrete containing RAP in concrete pavement applications. Test specimens containing different percentages of RAP were produced in the laboratory and evaluated for properties relevant to concrete pavement performance.
Using the measured properties of these concretes containing RAP, finite element analysis was then performed to determine how the concretes containing different amounts of RAP would perform if it were used in a typical concrete pavement in Florida. Results of the lab testing program indicate that compressive strength, splitting tensile strength, flexural strength, and elastic modulus of the concrete decrease as the percentage of RAP increases.
The coefficient of thermal expansion appears to be not affected by the RAP content, while the drying shrinkage appears to decrease with increasing RAP content.
But, when analysis was performed to determine the maximum stresses in a typical concrete pavement in Florida under critical temperature and load conditions, the maximum stress in the pavement was found to decrease as the RAP content of the concrete increased, due to the decrease in its elastic modulus. Though the flexural strength of the concrete decreases as the RAP content increases, the authors write, an increase in RAP content still results in a decrease in the maximum stress to flexural strength ratio for the concrete. This indicates that using a concrete containing RAP can result in improvement in the performance of concrete pavements.
POZZOLANIC ADMIXTURE OFFERS ALTERNATIVE TO SILICA FUME
In the quest for high-performance concrete, a new pozzolanic admixture Û vitreous calcium aluminosilicate Û offers performance similar to silica fume, but at higher doses, write Satiar A. Shirazi and Akhter B. Hossain, University of South Alabama; and, Jarrod Person and Narayanan Neithalath, Clarkson University, in their paper, Properties of Concrete Containing Vitreous Calcium Aluminosilicate Pozzolan.
In recent years, high-performance concrete (HPC) has become widely used in transportation structures where strength and durability are important considerations, the authors note. In HPC, a part of portland cement is replaced by pozzolanic materials. These pozzolans improve the strength and durability of concrete, and the mechanisms by which these are accomplished are well known.
Among the various pozzolans used to enhance the performance of concrete, vitreous calcium aluminosilicate (VCAS) is a relatively new one, they say. VCAS is a white pozzolanic material produced from glass fiber manufacturing waste. It is processed by grinding waste glass fibers to a fine powder form that effectively demonstrates pozzolanic behavior.
VCAS pozzolan is white in color and very consistent in chemical composition, because the waste fibers from glass industries are vitreous, clean, and low in iron and alkalis, the authors explain. Since VCAS is a new pozzolanic admixture, relatively little information is available on various physical properties of concrete made with VCAS that would allow the development of comprehensive mixture proportioning procedures.
To fill that void, the researchers conducted a study on VCAS pozzolan’s effects on several important properties of fresh and hardened concrete, providing a relative comparison of the performance of specimens containing VCAS with that of specimens containing silica fume. They saw that the addition of VCAS pozzolan increased mixtures’ slump (with improved consistency), while the addition of silica fume decreased it.
The VCAS pozzolan was found not to have significant influence on the mixtures’ plastic shrinkage cracking potential, while silica fume increased it. Thus, VCAS-containing concrete mixes show lower plastic shrinkage cracking than silica fume mixtures. This beneficial property can help alleviate one of the major problems in the construction of transportation structures, such as bridge decks and pavements, they say.
Addition of VCAS and SF increases the free shrinkage of concrete, the authors find. Therefore, proper precautions (i.e., use of external or internal curing methods, use of shrinkage reducing admixture) should be taken if these highly reactive pozzolans are to be used in concretes for bridge decks, pavements and other concrete structures that have a tendency to develop shrinkage cracking.
The short- and long-term compressive strength development of both VCAS and silica fume mixes were found to be higher than that of the control mixture, they write. Therefore, VCAS offers tremendous potential for use in transportation structures where both short- and long-term strengths are important.
Rapid chloride permeability (RCP) values for mixtures with VCAS and SF specimens were significantly lower than those of the control concrete; thus, incorporation of VCAS or silica fume as a cement replacement in concrete enhances resistance to chloride penetration. At equal replacements of cement with either VCAS or silica fume, reduction in RCP values was higher for the silica fume-modified mixtures. After 45 days of moist curing, no difference was evident between RCP values of concretes with 9 percent silica fume and 15 percent VCAS. Incorporation of either VCAS or SF also reduced the concrete mixtures’ sorptivity and moisture diffusion coefficient.