Following our April report on precast topics covered at the 85th Transportation Research Board meeting in Washington, D.C., Concrete Products surveys
TOM KUENNEN
Following our April report on precast topics covered at the 85th Transportation Research Board meeting in Washington, D.C., Concrete Products surveys presentations focused on cast-in-place methods and ready mixed concrete. More information can be obtained at www.trb.org.
Stay-in-place bridge deck forms Û easy-erect formwork that encourages use of ready-mixed for bridge decks, rather than precast Û have been under fire in recent years. Detractors say SIP forms trap moisture, which can be detrimental to concrete bridge decks, rusting unprotected rebar as well as the metal forms. Furthermore, critics say, SIP forms don’t permit inspection of the underside of a concrete deck. Accordingly, adoption of SIP formwork is limited or nonexistent in regions typified by aggressive environments.
Two TRB papers targeted SIPs, one cautioning against their use without substantial chloride mitigation techniques, the other examining how chloride problems are avoided by incorporating Fiber-Reinforced Polymers (FRPs) into the deck design. The SIP form studies and other meeting presentations of consequence to cast-in-place concrete practitioners are summarized here.
CAUTION: SPECIFYING SIP FORMS WITHOUT CHLORIDE SUPPRESSION
Specifying stay-in-place forms without chloride suppression in the deck requires great caution, say W. Spencer Guthrie, Ph.D., Stephen Frost, E.I.T., Aimee Birdsall, Ellen Linford, E.I.T., Loren Ross, Rebecca Crane, and Dennis Eggett, Ph.D., all of Brigham Young University, in Effect of Stay-in-Place Metal Forms on Performance of Concrete Bridge Decks.
The use of stay-in-place metal forms (SIPMFs) in concrete bridge deck construction has increased over recent decades, the authors report. Some state departments of transportation (DOTs) use SIPMFs frequently and are pleased with their performance. However, other DOTs only allow SIPMFs in special situations, and still other DOTs forbid their use altogether, fearing that the presence of SIPMFs may accelerate reinforcement corrosion and compromise long-term deck durability.
The authors cite 39 responses received in a national survey of state DOTs, among which 13 indicated that SIPMFs are not allowed in concrete bridge deck construction. Past research has investigated state-of-the-practice concerning deck construction using SIPMFs, determined the effect of SIPMFs on moisture content in connection with freeze-thaw deterioration, and compared overall performance of bridge decks with and without SIPMFs by visual inspection and compressive strength testing, they say. However, the issue of chloride concentration, which is a key factor in the corrosion of deck reinforcement, remains largely unaddressed in the literature.
The effect of SIPMFs on the corrosion of steel reinforcement was evaluated by analyzing chloride concentration together with half-cell potential, Schmidt rebound number, and deck distress. Half-cell potential testing was included to evaluate reinforcing-steel corrosion activity; the Schmidt hammer test was used to estimate concrete strength; and, deck distress surveys were conducted to quantify existing deck distress. In addition, an analysis of covariance (ANOCOVA) was performed to evaluate the effect of SIPMFs on each of the deck properties measured in the study.
The research program included 12 concrete bridge decks located within the I-215 corridor in the Salt Lake City vicinity. All of the bridge decks were constructed between 1984 and 1989 using epoxy-coated rebar. Five of the 13 analyses yielded significant results, including age, cover, Schmidt rebound number, half-cell potential, and chloride concentration at 2-in. depth. While differences in age and cover resulted from limitations associated with the deck selection process Û and were accounted for by ANOCOVA Û variations in Schmidt rebound number, half-cell potential, and chloride concentration were attributed to elevated moisture content in SIPMF decks.
Higher moisture content leads to enhanced curing conditions that ultimately provide greater concrete strengths, the authors explain, yet elevated moisture contents also facilitate ionic current flow necessary to sustain higher reinforcement corrosion rates and accelerate diffusion of chlorides into the concrete.
Given these research findings, engineers should carefully compare short-term advantages against potential long-term disadvantages associated with the use of SIPMFs for concrete bridge deck construction, the researchers assert. If SIPMFs are approved for use, engineers may consider applying surface treatments to the affected decks to minimize ingress of chlorides into the concrete over time and thereby retard the onset of reinforcement corrosion.
NEW SIP DESIGN USES FIBER-REINFORCED POLYMERS
A cost-effective, structural Fiber Reinforced Polymer (FRP), stay-in-place (SIP) formwork bridge deck system can be effectively integrated with a modular three-dimensional FRP reinforcement cage, say Thomas Ringelstetter, Lawrence Bank, Michael Oliva, Jeffrey Russell, University of Wisconsin-Madison; and, Fabio Matta and Antonio Nanni, University of Missouri-Rolla, in Development of a Cost-Effective Structural FRP Stay-In-Place Formwork System for Accelerated and Durable Bridge Deck Construction.
The use of permanent stay-in-place (SIP) formwork systems in highway bridge construction is standard practice for many departments of transportation throughout the U.S., especially in regions where deicing agents are not typically used, the authors contend. Conventional bridge-deck forming requires labor to install and remove plywood formwork, which translates into additional time on the project and potentially increased project cost. Since SIP forms are not removed after the concrete has hardened, labor costs, and possibly project duration, are decreased.
According to the researchers, essentially two SIP formwork systems have been implemented in the U.S.: preformed steel deck panels and partial depth precast/prestressed concrete panels. In states with aggressive environments, metallic forms can corrode; and, the use of either SIP system does not allow for inspection of the deck’s underside, they observe. Use of an FRP reinforcing system reduces the need for visual inspection of the underside of the deck, since reinforcement corrosion is not an issue. A nonmetallic FRP SIP form that is not susceptible to electrochemical corrosion, therefore, provides a more acceptable system for use in highway bridge decks, even in aggressive environments.
Recent research conducted at the University of Wisconsin demonstrates the evolution of SIP reinforcing systems to include FRP SIP formwork. The evolution of the system occurred through the laboratory testing, design and construction of two Wisconsin state-owned bridge structures, the authors report. Each structure utilized different FRP reinforcing and formwork systems. These projects pointed out the need for a competitive SIP formwork system to be used in conjunction with a FRP reinforcement system.
An outcome of the research was a cost-effective, structural FRP stay-in-place formwork system integrated with reinforcement for concrete highway bridge decks, developed and tested at the University of Wisconsin-Madison. Its evolution progressed from a structural SIP form separated from the top layer of reinforcing grating, to a double-layered grating with no SIP form, to the current solution Û a double-layered grating fully integrated with an FRP structural SIP form. All the systems have been shown to support highway bridge design loads.
The ongoing search for more efficient use of FRP materials and more constructible solutions can lead to further optimization, the researchers affirm. Due to cost of materials and potential for rapid construction, the gridform system is currently the most promising option. Laboratory testing of full-scale panel and beam specimens has shown that the gridform system easily meets strength and deflection code specifications: the strength is over five times that required; and, stiffness surpasses by four times the recommended value. The FRP gridform’s benefits over conventional steel reinforcement are primarily corrosion resistance and the potential for accelerated construction due to the modular nature of the system. The gridform solution will be implemented in Greene County, Mo., in the superstructure replacement of a slab-on-girder bridge.
ELIMINATING 28-DAY WAIT IN FLOWABLE FILL SPECIMEN CURING
If a procedure explored by University of Florida-Gainesville researchers holds up, the 28-day wait for flowable fill samples to cure to strength for mix design approval may be sidestepped. The effect of moisture on the long-term strength of flowable fill was evaluated by Webert Lovencin, Fazil Najafi, Ph.D, and Hammad Chaudhry, University of Florida-Gainesville, in Assessment of Flowable Fill Strength in Pavement Construction.
Flowable fill is a self-compacted, cementitious material used primarily as a backfill in lieu of compacted fill, the authors explain. It generally consists of sand, portland cement, fly ash/slag, and water. Flowable fill does not settle, requires no vibration nor other means of compaction, can be excavated, and is safer than other forms of fill.
To predict whether or not a flowable fill mix is excavatable, establishing a correlation using the early-age strength and long-term strength is a viable approach, the authors assert. Flowable fill mixtures exhibiting strength less than 0.689 Mpa (100 psi) would be classified excavatable. Mixtures resulting in strengths higher than 0.689 Mpa would be very difficult to excavate and, thus, would be termed nonexcavatable for practical purposes.
Because flowable fill is generally used for backfill, literature review shows few studies evaluating the effect of moisture on the long-term strength of flowable fill underneath pavement. Addressing that deficiency, the researchers presented a lab study evaluating the effects of moisture on long-term strengths of flowable fill and assessed an accelerated method for predicting its 28-day strength. Their study was performed using excavatable and nonexcavatable flowable fill mixtures. Samples were categorized as Îdrained,Ì i.e., designed to allow seepage of bleed water from the mix after placement; and, Îundrained,Ì i.e., designed to prevent seepage of bleed water from the mix after placement. Flowable fill strength was measured using a proctor penetrometer and limerock bearing ratio (LBR).
Results show that during the early curing period, undrained samples exhibit lower strength than the drained samples, the authors conclude. However, the strength of undrained samples increased with longer periods and to some extent exceeded the drained samples at 28 days.
A regression equation was developed to estimate the 28-day LBR values of flowable fill mixtures using oven-cured samples, a method that will allow field engineers to approve or disapprove a mix design relatively soon, instead of waiting 28 days for a sample to cure. It should be noted that the number of points used in the regression analysis is quite small, the researchers cautioned. Based on limited data, a trend is seen between 28-day LBR and oven-cured LBR samples. Additional samples should be obtained and are recommended to draw a more rigorous evaluation of the correlations between oven-cured and 28-day LBR samples.
ÎTAGUCHIÌ METHOD ELUCIDATES CONCRETE BONDING
The Taguchi method of describing bonding between concrete mortar and aggregate enhances our understanding of what goes on at that interface, write Juanyu Liu, Anal Mukhopadhyay, Ph.D., and Dan Zollinger, Ph.D., P.E., all with Texas A&M University, in Contribution of Aggregates to the Bonding Performance of Concrete.
Evidence collected from extensive field studies has indicated that the bond of the aggregate-mortar interface at an early age is one of the most significant factors affecting the development of delamination and eventual spalling, which is a distress type that plagues both jointed and continuously reinforced concrete (CRC) pavements, the authors note. Coarse aggregates generally occupy about 70 to 80 percent of the volume of concrete and form its skeleton. Therefore, a better understanding of the contribution of aggregates to the bonding performance of concrete is central to successful construction practice using aggregates to prevent delamination and spalling distresses.
In this study, a fractional factorial design Û the Taguchi method Û was applied to investigate the bearing on the aggregate-mortar interfacial bond of key construction design factors, as well as determine what kinds of aggregate provide best performance. In the ÎTaguchiÌ design, four factors Û aggregate type, water/cementitious ratio, replacement of ultra-fine fly ash, and curing method Û were considered, the researchers point out, including three levels for each factor based on the orthogonal array.
A fracture mechanics parameter, fracture toughness (KIf), was used to represent the aggregate-mortar bond strength. The associated comprehensive investigation of aggregate characteristics through physical, geometric, and chemical categories Û and the overall evaluation of aggregate properties based on a rating system using utility theory Û provided not only better understanding of the contribution of aggregate to the bonding performance of concrete, but also valuable recommendations on selecting right aggregate type in concrete [pavement].
Within the scope of the study, aggregate type was identified as the factor most strongly affecting the integrity of the aggregate-mortar bond, the authors state. The investigation indicated that the aggregate-mortar interfacial bond for a given cement paste was found not to be a simple function of any one aggregate property, but rather, a function of all three properties Û physical, geometric and chemical Û in combination. Yet, differing relative importance was detected among aggregate properties, and the impact of components within a property varied as well, which affected the overall contribution of aggregates to the interfacial bond.
Therefore, with appropriate combination of properties, any coarse aggregate type can produce positive effects on the bonding performance between aggregate and mortar, the authors conclude. Based on utility theory, this study also proposed that for a given aggregate type, the bonding performance of concrete can be improved by selecting optimum levels of other design factors and aggregate blending.
PERMEAMETER SUITS PERVIOUS CONCRETE
A simple device can gauge pervious-pavement performance note L. K. Crouch, Ph.D., P.E., Tennessee Technological University Department of Civil Engineering, Cookeville, Tenn.; Nathan Smith, P.E., L.I. Smith and Associates, Inc., Paris, Tenn.; Adam Walker, E.I., U.S. Army Corps of Engineers-Nashville District; Tim Dunn, Center for Energy Systems Research, Tennessee Technological University; and, Alan Sparkman, Tennessee Concrete Association, Nashville, in Determining Pervious PCC Permeability with a Simple Triaxial Flexible-Wall Constant Head Permeameter.
Pervious concrete is a mixture of coarse aggregate, water, portland cement, and possibly admixtures, the authors elaborate. Unlike traditional portland cement concrete, pervious concrete contains little or no fine aggregate, and has been called Îno-finesÌ concrete for many years. This lack of fine aggregate gives the pavement its open void structure and produces a permeable concrete. [Editor’s note: Pervious concrete is getting a close look for drainable pavements in urban areas and parking lots, where rainwater may percolate through to soils prior to discharge to waterways or aquifers. The Tennessee Concrete Association has been a leader on this front.]
Spurred by what the researchers note was the Tennessee Concrete Association’s recognition of a need to improve the workability of pervious PCC, a technique was developed for determining constant head permeability Û an essential first step, as maintaining adequate permeability is essential for pervious PCC performance. Thus, a simple triaxial flexible-wall constant head permeameter was constructed to measure permeability of pervious PCC in the range of 0.001 to 10 cm/sec. Laboratory samples comprising six compactive efforts were produced using three gradations of crushed limestone and two gradations of gravel with a consistent pervious PCC mixture design. Cores from four field demonstrations were also obtained. Effective air void content and constant head permeability of both field and laboratory pervious PCC mixtures were determined and compared.
The authors note several observations:
- Effective void content of similar pervious PCC decreases with increasing compactive effort.
- Constant head permeability of pervious PCC appears to be a function of three factors, given a constant paste amount and character: effective air void content, effective void size, and drain-down.
- Average constant-head permeability values of laboratory compacted specimens showed good agreement with two of three published values at low voids and fair agreement at high voids values.
- Average constant head permeability of laboratory-compacted and field-cored specimens agreed reasonably well (within 50 percent) for similar effective void contents.
EXPLORING SCC PROPERTIES
Although use of self consolidating concrete is becoming increasingly widespread in the U.S., as well as Europe and Japan, SCC properties aren’t fully understood, say Nakin Suksawang, Hani Nassif and Husam Najm, Rutgers, The State University of New Jersey, in Evaluation of Mechanical Properties for Self-Consolidating, Normal and High-Performance Concrete (HPC). The high viscosity of self-consolidating concrete (SCC) enables it to flow freely with minimal segregation, the authors explain. Accordingly, SCC flows under its own weight into corners of formwork and through closely spaced reinforcement with little or no vibration or compaction. Lower energy cost, less stress on formwork, reduced labor cost, and elimination of potential human error in concrete consolidation are benefits of this dynamic. Additionally, the concrete becomes more homogeneous due to equal dispersions of the cementitious paste and aggregates; as a result, both SCC mechanical properties and durability are improved over normal/conventional concrete.
Fresh concrete can easily attain high flowability by simply increasing the water-to-binder (w/b) ratio, the researchers assert. However, increasing the w/b ratio alone could lead to concrete segregation and less durability, they add. Thus, in order to successfully develop SCC, mineral and chemical admixtures, e.g., pozzolans, limestone filler, superplasticizer, and viscosity-modifying admixture (VMA), need to be added to the mix design to prevent segregation and enhance SCC durability. In addition, the absolute volume of coarse aggregates needs to be limited to reduce interparticle friction, allowing the SCC to flow under its self-weight without segregation.
Several departments of transportation are currently accepting SCC mix design in some of their projects, the authors contend. Moreover, the Federal Highway Administration (FHWA) is also promoting SCC, they report. Yet, because SCC is a relatively new material, its mechanical properties and durability are not fully understood.
By way of a remedy, the researchers evaluated and compared SCC mechanical properties with those of normal or conventional concrete, as well as with high-performance concrete (HPC). The effect of supplementary cementitious materials on SCC’s mechanical properties was also investigated, and the experimental results were compared to predictive equations. The authors conclude:
- No significant changes in compressive strength between SCC and conventional concrete are evident.
- SCC modulus of elasticity is slightly lower than that of conventional concrete, but its tensile splitting strength is higher. A 5 percent reduction in the elastic modulus was observed, as well as a 10 percent increase in tensile splitting strength.
- The addition of pozzolans to SCC increases the rate of change of the modulus of elasticity, i.e., the elastic modulus of SCC containing supplementary cementitious materials increases at a higher rate over time than that of regular SCC.
- SCC has higher drying shrinkage than conventional concrete and HPC. SCC drying shrinkage was noted as approximately 30 percent higher than that of conventional concrete and about 40 percent higher than that of HPC. The drying shrinkage of SCC can be reduced by adding pozzolans; fly ash, which produces a 10 percent reduction over normal SCC, is the best overall pozzolan for controlling drying shrinkage.
- The performance of SCC under rapid chloride permeability testing (ASTM C1202) is greatly enhanced with the addition of fly ash and silica fume, especially at 56 and 90 days. A 70 percent reduction in the amount of charge passed was obtained with the addition of silica fume and fly ash.
- Compared to other the models, the ACI 363 equation provides the better prediction for SCC tensile splitting test.