Precast Structure Durability
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Precast Structure Durability

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Creating a precast structure that ...

Creating a precast structure that
will retain its durability and
aesthetic appeal over time
remains a key concern of owners and
designers. Although first costs always
are among owners’ top priorities, lifecycle
and long-term maintenance
expenses can eliminate any initial
savings. Recently, two detailed studies
of precast components sponsored by the
Precast/Prestressed Concrete Institute
(PCI) were released that can help
owners and designers balance these
needs.

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  • 1. Studies Offer ProofDURABILITY Of Precast’s Durability Survey of parking garages offers tips on design and maintenance, while chloride-permeability tests show existing standards are confusing C reating a precast structure that Northbrook, Ill. The one-year Durability in parking structures can be will retain its durability and experimental study was designed to enhanced with a lower water/cement ratio and a long-term maintenance plan. aesthetic appeal over time answer questions relating to chloride Photo: Hedrich-Blessing remains a key concern of owners and permeability, water-absorption rates, designers. Although first costs always and other durability considerations. are among owners’ top priorities, life- Among its findings was that in many cycle and long-term maintenance cases, accepted tests for determining the expenses can eliminate any initial durability of concrete needed for a savings. Recently, two detailed studies project may be overstated, costing of precast components sponsored by the owners money by making them pay for Precast/Prestressed Concrete Institute methods and materials that aren’t (PCI) were released that can help necessary. owners and designers balance these needs. Parking Maintenance “Durability of Precast Prestressed CEG’s study of parking garages ASCENT, SUMMER 1996 Concrete Structures” reports on the found that “the overall performance of long-term durability of parking precast prestressed concrete parking structures nationwide. The study was structures can be rated as very good, conducted by The Consulting with the general performance rated as Engineers Group Inc. (CEG) of San 84 percent out of 100 percent.” The Antonio. The report summarizes a primary areas of concern if problems28 survey undertaken by CEG of 62 were to arise, it notes, are derived from parking structures. The firm evaluated field-placed toppings, excessive leaking, various performance characteristics and poor drainage-system design, and inspected several sites to determine what cracking in precast spandrels. This latter methods and design criteria work best rating area was due primarily to hairline in various situations. cracks, the study says. Even so, it notes, In general, the study found that pre- “The performance of the precast cast parking garages perform extremely products was excellent,” and these well over time and those that are well- problems seldom arose. maintained and regularly inspected However, it found that maintenance perform best. It also offers a host of efforts average just 5.76 on the 10 scale. suggestions on how to achieve these This represents a major concern, as lack ends through design, construction, and of maintenance often lies at the heart of on-going inspection plans. any problems that do arise with precast The second report, “Durability garages, it notes. Aspects of Precast Concrete, Parts I and The examined structures ranged in II, Historical Review and Chloride age from two to 24 years and were Permeability Study,” was conducted by located in 17 states across the country. the engineering firm of Wiss, Janney, The dimensions were representative of Elstner Associates Inc. (WJE) of typical structures, the study says, with a
  • 2. minimum width of 105 feet, a particularly when it freezes. crickets that direct water to the drains.maximum length of 1,200 feet, and up Good drainage includes five key This, in fact, was achieved for most ofto 12 levels of parking. About 65 components, the study reports: primary the garages studied.percent of the structures were located in span floor slope, cross slope, drain Drains located as close to the columnregions where salt was applied, with 69 grid lines as possible minimize localpercent in regions where freeze-thaw ponding, while drains measuring at leastconditions were a consideration. Some ‘Poor drainage was 11 inches in diameter perform the best,41 percent of the structures also fea-tured warped double tees, which remains found to be one of the the study reports. Trench drains are used at the bottom of the roof-levela key focus for design considerations. principle culprits in the ramp. They tend to be spaced so theyConcrete strength averaged 5,878 psi don’t interrupt the floor-framing sys-and ranged from 5,000 to 7,000 psi. deterioration of tem, rather than being used continu- These key areas were spotlighted. ously. parking structures.’Proper Drainage A Must Sealants Make A Difference “Poor drainage was found to be one location, drain size, and joint sealing. Sealants play a critical role inof the principle culprits in the deteriora- Excessive slopes can lead to warping maintenance programs, the studytion of [the studied] parking problems, minimized headroom, or found. “The structures that performedstructures,” the study reports. In the few safety issues, the study says. The goal the best were sealed at all circumstancescases where it occurred, the study says, should be to create a minimum slope where precast members come together,”poor drainage allows water to collect on that still drains water efficiently. The it says. This includes joints connectingthe deck’s surface, creating ponds that survey found that in most cases, the flange to flange, tee to beam andallow road salt to concentrate and attack desired slope ran between 1.5 percent spandrel, tee and beam to wall panels,the concrete. This in turn lets chloride and two percent for the primary long- and all those around columns. If theseions penetrate the concrete. Ponding span slope and 0.5 percent to one were properly sealed, the study says,also can create a safety problem, percent for the beam lines that create across the board “the deck had very few ASCENT, SUMMER 1996 29
  • 3. if any leaks.” Urethane was the most flange cracks if a field-placed topping is be ones that minimize field-placed typical sealant used, the study adds. to be applied.” If the system is pre- concretes which typically have high w/c “Properly installed urethane appears to topped, however, even small cracks ratios,” the report summarizes. “The have an effective life of seven to 10 should be sealed to ensure water and pretopped double tee system would years, with touch-up maintenance when water-borne chlorides do not attack the produce the most durable concrete at and where required.” underlying system. the riding surface as well as throughout Joints should be prepared properly Any cracks in pretopped systems, the structure. If field-placed topping is prior to sealing them, the study stresses. should they arise, as well as larger cracks employed, then a w/c ratio of no more In topped systems the topping should in field-topped decks, typically are than 0.45 is recommended.” In fact, be tooled (not sawn) to a depth of 1/2 sealed by a surface application of a clear, PCI recommends that field-placed inch and all scales or latence removed low-viscosity epoxy, the report adds. topping should offer a w/c ratio as low by grinding. Flange joints in pretopped Working cracks are best treated with a as possible, preferably 0.40 or lower. systems should be ground or stoned to sealant such as urethane. It also says that Steel embeds often are left exposed to remove sharp edges and loose material. cracks larger than 0.0010 inch should the elements in a precast structure, the be examined by an experienced engi- study observes, and these usually consist Flange Cracking neer. Silane sealants have also proven to of precast-member bearing plates and Strains caused by shrinkage create be effective for small cracks (less than connection plates between precast most of the cracking in double tee 0.0005 inch). members in welded connections. These flanges that occurs, the study reports, Although no type appeared in the can be subjected to water from leaks or emphasizing that such cracking is not surveyed garages with regularity, harsh climates and should be protected. typical. These cracks most often develop spandrel cracking was reported to be the In regions where road salt isn’t used, most prevalent type, the report says. only a shop paint rust preventative coat Such cracking typically is observed as a is needed to ensure smooth operation, it ‘Flange cracking, unless diagonal torsion crack, particularly in L- reports. In regions where salt is used, beam spandrels or cracks radiating from the embeds should be galvanized or severe, seldom leads to the bearing or connections. To alleviate protected with zinc or epoxy paint. It this, connections should be used that also points out that regular maintenance poor performance of the allow spandrels to relieve temperature of the structure will provide additional strains, the report suggests. “Bolted protection by ensuring caulks and double tee.’ connections have proven to perform sealers are working properly. quite well,” it notes. In addition, “Fiber- during the production and curing reinforced bearing pads at least one- Shear Walls Predominate cycles, and they build up in the flange half-inch thick also were observed to The predominant lateral-load system regions between the stems. During the perform better than plain bearing pads.” used by the studied garages consisted of curing process, the stem concrete is held a shear wall system. Some 89 percent of rigidly by the steel forms. This restrains Admixtures Can Help the structures examined used this ap-ASCENT, SUMMER 1996 the flange’s ability to shrink between “Certain admixtures are fairly univer- proach, while 10 percent used moment the stems as it cures, occasionally sally employed by precast concrete frames and one percent used only stair producing a crack in the flange adjacent producers to greatly enhance the dura- and elevator shafts. However, examina- to the inside corner of the juncture bility of precast concrete products,” the tion of the systems showed there is no between the stem and the flange. study reports. The two most common need to isolate stair and elevator shafts “These cracks can be minimized by are air-entraining agents used in away from the structure, the study30 proper curing techniques that retain freeze/thaw regions and super plasti- found. “If appropriate connections are moisture,” the report says. cizers that reduce the amount of water Cracking also can occur during the needed to produce a workable concrete stripping process, it notes, when one mix, thereby lowering the water/cement end of the stem is elevated higher, (w/c) ratio. putting more stress on the other end. It The report refers to the tests and also can result from excessive wracking experiments run in the WJE study, of the tee as it is transported to the adding that its own survey bears out the jobsite. Although none of these occur findings that reducing the w/c ratio to often, they are the key areas to focus on within the range of 0.35 to 0.40 greatly to prevent any cracking that may arise. reduces the ability of chloride ions to Even then, the report team’s penetrate the concrete. “Concrete with inspections showed that “flange crack- this w/c ratio has proven to be nearly ing, unless severe, seldom leads to poor impervious, and the additions of other performance of the double tee.” Small admixtures such as silica fume provide This diagram of a typical ductile bolted cracks (those less than 0.004 inch in little substantial benefit.” It also notes connection shows the voids in the sleeve width) “typically are not through cracks that 57 percent of respondents said they surrounding the bolt permit lateral movement and seldom affect the performance of employed w/c ratios of this range in without impairing performance. In not one case in the study where such a connection the member,” the report says. “There- their projects already. was inspected was cracking at the connection fore, nothing typically is done to such “The most durable structures would observed.
  • 4. employed, such shafts can be includedin the lateral-load system.” The most effective connection systemappeared to be one using boltedconnections between the spandrel andthe columns, with those connectionsrelieving thermal stresses. Thepredominant connection type betweenthe shear walls themselves was a weldedconnection in a recessed pocket.Maximum Warping Analyzed Double tees at times are warpeddeliberately in a parking garage project This side view of a stress pattern in a warped double tee from CEG’s durability study shows thewhen drainage plans provide different patterns of compression. The maximum tensile stresses occur in the top of the flange near theslopes for the bearings at each end of unwarped stem, the survey found.the double tee floor member. Althoughthis presents no problems in general andcan be accomplished routinely, thereport says, large warps may result insome cracking in the double tee flange.To measure the extent of this atdifferent amounts of warping, thesurvey team ran a finite-elementanalysis. The result showed thatmaximum tensile stresses occur in thetop of the flange near the unwarpedstem. This follows classical bending-stress distribution patterns, except thetop surface of the flange exhibits a stressdistribution similar to a concentratedload-stress distribution. The results ofthe finite-element analysis were This diagram shows the stress pattern that develops in a warped double tee that cracked. Thesupported by an actual test of durable study found that the crack will propagate along the flange rather than through it, supportingtees with various degrees of warping. the theory that most through-flange cracking is created by handling of the warped tee rather than the actual warping process. The study says that double tees can ASCENT, SUMMER 1996 ‘The overall Changes Suggested For AASHTO Standards In its findings, the WJE study suggests several key changes to the existing performance of precast AASHTO standards for curing and ponding tests. “The current AASHTO moist- and heat- or steam-curing specifications [in prestressed concrete Section 8.11] need to be revised,” it says. “There are numerous technical parking structures can reasons for requiring moist-cured field concrete that will see chloride exposure 31 in its lifetime to have moist curing beyond the age when the concrete reaches be rated as very good.’ 70 percent of the design strength.” The present provision allows these field concretes to receive as little as one to three days of moist curing with existing low w/c ratios and cements.be warped safely to the amount of 3/4 It also suggests revisions to the T 259 Ponding Test. These include:of an inch between stems and 1.5 inchesfor a full tee width. In general, it adds, • A “more realistic” curing period, halving the current 14-day period.“The stresses caused by a moderate • Use of a 15-percent chloride concentration solution for the ponding solution.warp are generally not critical. The The existing three-percent NaCl solution represents sea water, not currentcracks resulting from a service warp are deicing salt solutions used in northern regions that are more concentrated andgenerally longitudinal and do not harmful.penetrate through the flange.” • Increased ponding periods from 90 days to 365. Since ponding tests require Despite its focus on key areas of little supervision, this should add little impact to its cost, the study points out.concern that arise with precastconstruction, the report emphasizes that • Doubled chloride samples, from two to four, as two data points are “totallythis approach should not be construed insufficient to determine diffusion coefficients.”to mean that precast garages offer many • Use of core rather than drilled samples, which can be contaminated anddifficulties. In fact, it stresses that just erroneous and provide inaccurate drilling depths.the opposite was the case. “It should be
  • 5. noted,” the report says in its summary, tested concrete sample was cured in a expresses the amount of electrical charge “that all structures surveyed or inspected water tank, under wetted burlap or that passes through the concrete in a set were in serviceable condition even when through heat-curing. Concretes incor- period of time and is measured in over 25 years old and in severe porating admixtures of silica fume were coulombs. The ratings are supposed to corrosion exposure and subjected to cured under wetted burlap only. allow designers and owners to specify poor maintenance.” Even with its focus Three w/c ratios were produced, at the level of permeability they desire by 0.46, 0.37 and 0.32. These represented using the mixture’s coulomb rating as a the two ends of the range typically delineator. However, the study found ‘The typical good found in precast products (0.37 to that ASTM’s existing coulomb ratings 0.32), plus one considerably higher in for concrete cured in each type of performance of precast w/c, which is typical of 0.45 AASHTO- system do not correspond well to the parking structures of all grade concrete. Samples using silica- actual permeability that can be seen in fume additions of 5.0 percent and 7.5 practice. kinds in all ranges of percent by weight of cement also were “Using coulomb ratings as an indica- tested under wetted burlap, while tor of durability will be misleading and exposure was apparent.’ standard precast concrete type mixes will add cost to a project,” says Donald made with each of the three w/c ratios W. Pfeifer, project manager at WJE. on finding where problems arise and were tested under each of the three This can be critical, because specifying how they can be eliminated, “The types of curing. This resulted in 15 concrete to meet the set coulomb typical good performance of precast different test samples. The results for ratings can be more expensive than parking structures of all kinds in all each sample are shown in Table 1. using other concretes that are less costly. ranges of exposure was apparent.” All 15 mixtures were tested according “The concrete w/c ratio is the to standards set by the American Society dominant factor in reducing chloride Chloride Ion Tests of Testing Materials (ASTM) to check permeability,” the study says. The The ability to achieve high durability for permeable voids. The three heat- report found that the concrete with a through various curing methods was cured mixtures of conventional concrete 0.37 w/c ratio had reduced its chloride enhanced by the findings of the study were found to have the lowest volume ingress by about 80 percent over the performed by WJE. It examined various of permeable voids. Adding silica fume concrete with a 0.46 w/c ratio. Further, durability properties when cured in increased the number of voids by 100 the concrete with a 0.32 w/c ratio had different ways and when created with percent for the five-percent mixture and reduced penetration of 95 percent. It different w/c ratios. The study was 50 percent for the 7.5-percent version. pointed out that this evidence conducted by WJE engineers for PCI The study also tested the coulomb confirmed a three-year corrosion study and featured several formats. Each rating of each sample. The rating performed by the Federal HighwayASCENT, SUMMER 199632 Chloride ingress, calculated diffusion coefficient and surface chloride and calculated times to corrosion for the 15 mixtures Table 1, produced by WJE for its chloride-permeability study, summarizes the key findings of the tests. Among other things, it indicates that lower water/cement ratio of any concrete sample correlated better with resisting chloride penetration than the AASHTO T277 coulomb ratings.
  • 6. Complete Studies Available “Durability of Precast Prestressed Concrete Structures” was conducted by Thomas J. D’Arcy, Walter J. Korkosz and Larbi Sennour of the Consulting Engineers Group Inc. of San Antonio. Copies are available from the company at 2455 N.E. Loop 410, Suite 125, San Antonio TX 78217; 210/637-0977. The Chloride Permeability study was conducted by Matthew Sherman, David McDonald and Donald Pfeifer of Wiss, Janney, Elstner Associates Inc. of Northbrook, Ill. Copies are available from PCI. (See the in-depth story in the PCI JOURNAL July/August ’96.) Both reports can be obtained from PCI, the sponsoring associa- tion, at 175 W. Jackson Blvd., Suite 1859, Chicago IL 60604; 312/786-This diagram from WJE’s chloride-permeability study shows that four different concretes 0300; e-mail, info@pci.org.offering a wide range of coulomb ratings performed quite similarly during actual tests.Administration in 1987. “The correla- The five-percent burlap-cured silica dramatically decreased the absorptiontion between long-term chloride fume mixture with a w/c ratio of 0.46 and volume of permeable voids in had an estimated time-to-corrosion rate conventional concrete at all w/c levels. essentially equal to that of 0.37 w/c “The reasons for and the implications of ‘Using coulomb ratings conventional heat-cured concrete, while these findings merit further research.” the 7.5-percent burlap-cured mixture as an indicator of with a w/c ratio of 0.37 corresponded to No Strength Loss the heat-cured conventional concrete of The study also reports that “Proper durability will be 0.32 w/c ratio. heat-curing created no significant This shows two different approaches misleading and will strength loss at 28 days at all three w/c ASCENT, SUMMER 1996 to achieving the same durability, as levels.” This could be seen in the fact add cost to a project.’ measured by a w/c ratio. However, the that 28-day strength of the material that use of silica fume changes the overall received no supplemental moist-curing characteristics, the study stresses. “The after heat-curing averaged 90 percent ofpermeability and the coulomb test addition of silica fume to concrete the companion 28-day, water tank-recommended in the ASTM C1202 test caused a significant increase in the water cured samples for all three w/c levels.appears to be highly variable,” the study absorption and volume of permeable In addition, the study says, “The 33says, “and requires individual correla- voids in moist-cured concrete at all w/c surface chloride concentrations for thetion between these two types of tests for levels,” it says. Similarly, heat-curing heat-cured concretes are significantlyevery concrete mixture.”Heat-Cured Performs Well Heat-cured concrete had the lowest Changes Suggested For ASTM Standards The WJE study team suggests five key changes to the ASTM C1202water absorption and volume of coulomb-testing standards:permeable voids at the tested date of 42 • The age of the concrete should be specified when laboratory-producedcuring days, the report says. The 365- mixtures are being evaluated and correlated to long-term ponding tests.day, long-term ponding tests also • All laboratory-produced specimens for coulomb and ponding testing shouldshowed that heat-cured slabs generally be moist-cured for no more than seven days. Moist curing to ages or 28 orhad the lowest near-surface chloride 90 days should be prohibited as unrealistic.concentrations compared to the average • Long-term ponding tests should be specified to mean at least 365 days.of the other types. • Salt-water solutions in long-term ponding tests should be increased to 15 The silica-fume mixtures produced percent salt from the current three percent.28- and 180-day compressive strengths • The existing tables showing correlations between coulomb ratings andequal to the strengths of conventional permeability should be eliminated totally.concretes with equal w/c ratio, eitherburlap- or tank-cured, the study reports.
  • 7. way to improve current concrete is to tighten specifications to require the use of lower w/c concretes. Concrete should be specified to have a low chloride- diffusion coefficient as determined by long-term ponding tests.” Use of the ASTM standards that provide only coulomb values should not be used to specify concrete mixtures because they do not apply uniformly across all variables that can be used to reach the same durability, including different ‘The best way to improve current concrete is to tighten specifications to require the use of lower w/c concretes.’ aggregates, admixtures, and curing procedures. “The tests must be used only when proper correlations between coulomb values and long-term ponding test results have been established for the individual concretes under test, as already required in the 1991 ASTM C 1202.” The study suggests that the precast industry should carry out long-term ponding tests, of at least one year in duration, to prove the performance ofASCENT, SUMMER 1996 the material to all purchasers. “Taking a long-term view of concrete performance can increase the durability of all structures, lessening the need for repair and maintaining concrete as a state-of- the-art building material of choice for34 the transportation and construction industry.” These two studies provide owners and designers with more insight into how to create their precast concrete structures—and how to avoid costly KEY mistakes. Everyone benefits when those involved in design, construction, and maintenance are aware of the material’s design properties and how long-term care combine to keep costs to a These three diagrams fromWJE’s chloride-permeability study compare permeability levels of minimum. ■ various curing methods for concretes with water/cement ratios of 0.46 (top), 0.37 (middle) and 0.32 (bottom). They show that no matter the curing method used, durability improved as the w/c ratio was lowered. — Craig A. Shutt lower than that assumed in other lower than the surface concentrations published corrosion research for long- for the overall average of the burlap- term life estimates.” The heat-cured and water tank-cured concretes with concrete surface concentrations were and without silica fume, at all w/c levels. between 18 percent and 36 percent Overall, the study states, “The best