Taking a crack at it
Soon, everybody will know you cant actually see wise cracks, not for awhile anyway
Soon, everybody will know you cant actually see wise cracks, not for awhile anyway. The proof is in a more advanced, intelligent material called high-performance concrete, which so far has proven its worth particularly when it comes to resistance. And as more and more are finding out, cracks, the instigators of bridge breakdown, are staying out of sight.
The industrys big break came in 1987, when Congress initiated the five-year Strategic Highway Research Program (SHRP) to look at various products to improve the constructibility and reduce the maintenance of the nations highways and bridges. Surfacing from the research was high-performance concrete (HPC), which has been designed to be more durable and, if necessary, stronger than conventional concrete. HPC is commonly used in the construction of bridge decks and girders, but its starting to be used in pavement applications as well. A dominant feature of HPC is its strength. Reaching the 8,000 psi level is relatively easy, and some strengths have gone as high as 10,000 psi.
The principle ingredients have remained the same, but two things are different from conventional concrete: the way the mix is designed and the special additives used.
A contractor can essentially tailor high-performance concrete to meet the needs of the project.
"If you want a certain stiffness or concrete strength, a state can write that right into their specs," Sue Lane, team leader for the high performance bridge materials team in the office of infrastructure research and development for the Federal Highway Administration (FHWA), told ROADS & BRIDGES.
In its definition of HPC in bridges, the FHWA has identified four performance grades for durability (scaling, abrasion resistance, freeze/thaw durability, permeability) and four for mechanical properties (compressive strength, stiffness, creep, shrinkage). The bridge designer specifies which performance grades are required for a project and a set of standard tests are then used to confirm that the concrete meets those requirements.
Three key mineral admixtures used exclusively for HPC are silica fume, fly ash and slag. The ingredients are very small particles that fill air voids between the grains of cement, thus producing a denser mix. The denser the concrete, the more durable it is. Another special component is super plasticizer. HPC demands a great quantity of cement, and with the use of a mineral admixture the water-to-cement ratio is greatly reduced. So, in order to increase the workability there has to be an increase in the amount of super plasticizer.
Thanks to SHRP, FHWA has been able to put HPC into words. It wasnt until 1993, however, when an important study was put into action.
Taking results from an HPC study conducted by North Carolina State University, the University of Michigan and the University of Arkansas, the FHWA forged its first cooperative agreement with a state for an HPC bridge. Teaming with the Texas DOT for construction of the Louetta Road overpass near Houston, the FHWA asked the state to design a bridge with normal concrete and design a bridge with HPC to see what benefits could be derived from the new technology. Emerging from the project was a 6/10-in. diam. pre-stressing strand that was restricted by the FHWA at the time. Research showed the strand and the spacing of the strand was feasible and resulted in no cracking. New quality control features also were developed.
Finding a good use for it
Texas wasnt the lone state discovering the benefits of HPC bridges. Under ISTEA legislation, FHWA funded 15 bridge projects in 12 states. In all, approximately 35 states have tried the mix. Confidence and enthusiasm has quickly spread.
Because the concrete is able to stand up to the cracking better than its traditional counterpart, there is a reduction of permeability to chloride penetration. And with less corrosion, there is the increase in life span. The current estimate is that HPC bridges could have a useful service life of 75 to 100 years.
Another popular advantage surfacing from the increase in HPC work is the elimination of girders, which helps offset the increased cost in materials used in the concrete mix.
Additionally, if a certain girder spacing is desired the span length can be increased, thus making the need for intermediate piers obsolete. By eliminating piers, roadways underneath can be widened in the future. If clearance is a problem, for a given span length one can actually decrease the size of the girder by using HPC.
Quality control and quality assurance also is heightened. With HPC, the contractor is expected to make trial mixes in order to get a feel of how to work with the desired mix.
"They are essentially going back to the fundamentals of how concrete should be produced," Basile Rabbat, manager, transportation structures and structural codes for the Portland Cement Association, told ROADS & BRIDGES. "Concrete is the function of the quality and workmanship that you put in completing it. If you have good quality control during the design of the mixes, if the proportions are properly controlled and are consistent, then when you deliver your concrete, you place it, consolidate it and cure it you are going to get quality concrete."
Where does HPC stand?
Several sites have served as a proving ground for HPC bridges. ROADS & BRIDGES outlines three such convincers.
Route 104 and Route 3A bridges in Bristol, N.H.
The construction of two HPC bridges has driven New Hampshire to start a third.
The Route 104 bridge is 65 ft long and 57 ft wide, while the Route 3A structure stretches 60 ft and carries a width of 40 ft. Both handle two lanes of traffic.
The mix for the deck contained Type II cement and 71/2% silica fume. The water/cement ratio maximum was .38 and the air content was 6-9%. There was a 28-day cylinder strength of 7,200 psi, and the New Hampshire DOT was shooting for a chloride ion permeability of less than 1,000 coulombs. The curing procedure was a four-day wet cure. The only difference in the Route 3A bridge was a seven-day wet cure.
High-performance concrete girders were used on both bridges. Route 104 has four AASHTO Type III girders, while 3A has New England Bulb Tee 1000 girders. Strength at release was 5,000 psi and the final 28-day strength was 8,000 psi. The length between girders for 104 was 121/2 ft center-to-center. It was 111/2 ft for 3A.
Construction on a third bridge in Rollinsford is expected to begin soon. The 110-ft-long, 341/2-ft-wide span will carry Rollins Road over Main Street and a railroad.
Because of vertical clearance concerns, span lengths were stretched. Span length between the girders is 7 1/2 ft center-to-center. Piers from the existing bridge also were eliminated.
The deck mix is identical to the one used for 104 and 3A. The two-lane structure will require five New England Bulb Tee 1400 girders. Initial strength required is 5,700 psi and final strength is 8,000 psi after 28 days.
An innovative feature of the bridge is the use of fiber-reinforced polymer grid reinforcement in the deck instead of steel.
"Its a material that wont corrode," Peter Stamnas, senior project engineer for New Hampshire DOT, told ROADS & BRIDGES. "We want to increase the lifetime expectancy of the bridge deck itself."
Convington Bridge, Wash.
The state famous for apples also could be considered the core of the HPC industry. Using the material is standard practice in Washington, and one of the best examples is the three-span Covington Bridge. Two of the spans reach 80 ft in length, while the third measures in at 135 ft. All three are approximately 40 ft wide and handle two lanes of traffic. The bridge was originally designed with conventional concrete and had seven lines of girders, but the use of HPC reduced the line to five. The spanning length between each girder is 8 ft.
"There is a slight increase in cost because of HPC, but since we are eliminating some lines of girders there is an overall savings," Bijan Khalegi, concrete specialist for the Washington DOT (WashDOT), told ROADS & BRIDGES.
For the Convington, Khalegi said there was a 10% increase in cost per length of girder by using HPC as the choice material. However, since two lines of girders were eliminated there was an 18% savings in the final price tag.
Fly ash, silica fume and Type III cement were used in the mix for the girders. The release strength was 7,500 psi and the final strength was 10,000 psi for 56 days.
WashDOT stuck with an HPC mix its been using for years on the deck. It contained 660 lb of cement, 75 lb of fly ash, 1,100 lb of fine aggregate and 1,700 lb of coarse aggregate. Maximum water was 290 lb and the maximum water/ cement ratio was .39. The air entrainment was 6% and a water reducer Type A was used. The deck slab had a 14-day curing stage and reached a 28-day strength of 4,000 psi.
The structure was built in 1997, and has passed every inspection so far.
"We havent had any problems yet," said Khalegi. "It has performed very well in terms of durability and wearing."
Charenton Canal Bridge, La.
The first cajun HPC bridge was completed in Louisiana late last year.
According to HPC Bridge Views, a newsletter sponsored by the FHWA, the Charenton Canal Bridge project involved the replacement of a 55-year-old reinforced concrete bridge with a 365-ft-long continuous prestressed concrete structure using Type III AASHTO girders. Each 73-ft span consists of five girders that are spaced at 10-ft centers and support an 8-in.-thick cast-in-place concrete deck. HPC was used in all components of the bridge.
Specified compressive strength of the girders and piles was 10,000 psi no later than 56 days. A rapid chloride permeability of 2,000 coulombs or less at 56 days also was specified for concrete used in all members.
HPC allowed the bridge to be designed with one less line of girders. The contractor used fly ash in the precast members and slag in the cast-in-place members.
Looking over the competition
SHRP left with ISTEA, but many in the industry believe something better has come along. The Innovative Bridge Research and Construction program was introduced with the Transportation Equity Act for the 21st Century. It not only deals with HPC, but any kind of innovative material for bridges. So now instead of having a program set aside strictly for HPC, other materials are competing for bridge funding. High-performance steel, fiber-reinforced polymer composites and stainless steel rebar are just a few which could come into play.
To qualify, a state submits an application for a proposed project using any kind of new technology. The projects are then evaluated and only a certain number receive funding. The selection process starts in April, and the chosen ones are revealed in the fall.
The six-year program funded approximately $10 million in projects during year one and $15 million in year two, according to Terry Halkyard, structural engineer for the FHWA.
Whats riding on pavement use?
According to Steve Forster, manager of FHWAs research program on cement and concrete, there are a few factors keeping the use of HPC in pavements from taking off.
The cost effectiveness of the material over conventional concrete is still in question. A contractor cant take full advantage of the strength of HPC, and hes not eliminating any members (i.e. girders) to cancel out the increase in mixture costs. Also, HPC pavements usually require some type of curing compound to achieve the curing necessary to protect from moisture loss.
"Its more of a matter of optimizing your properties for your situation rather than maximizing them," Forster told ROADS & BRIDGES.
One situation where bridge-type HPC might come into play is in repair and rehab situations where opening traffic is an overriding concern.
"Thats when you need a more exotic mix with high early strength gain, which will allow you to reopen traffic in a matter of hours as opposed to days," he said. "But usually in those situations youre talking about limited length of pavements."
Forster added that the use of pre-cast slabs also may help accelerate the process. Another HPC pavement application peaking interest is ultra-thin whitetopping, which is the rehab of asphalt pavement with a concrete overlay.
The FHWA and U.S. Army Corps of Engineers recently created a device for measuring the workability of HPC concrete. With stiffer pavements, sometimes the ability to place and consolidate the concrete becomes an issue. The power and mass of slipform pavers today can handle the change, but what about the properties of the mix? Is the actual placement of the mix going to drive entrained air out? Is it going to overvibrate and segregate the aggregate so the coarse aggregate is farther down in the pavement? The new method, different from a slump test, is expected to monitor these and other factors.
"If you have a means to measure workability while youre doing the mix design then that should help avoid some of those potential problems," Forster said.