And while this program is just beginning, it has substantial implications for contractors, material suppliers and pavement-owning customers. 

Unlike the Superpave system of performance-based asphalt mix designs, HPC is not a product of the Strategic Highway Research Program (SHRP). 

SHRP investigated some aspects of concrete as a material, but left much work to be done to provide a comprehensive overview of the use and performance of concrete as a material in highway applications. 

HPC has been developed over the last two decades, and was primarily introduced through private sector architectural design and construction such as high rises and parking garages. Public agencies tend to be more conservative than the private sector when it comes to changing specifications, but the public sector now is committed to incorporating this technology in the field. 

HPC is often called "durable" concrete because its strength and 
impermeability to chloride penetration makes it last much longer than 
conventional PCC. It's an engineered concrete made up of the classic elements of water, portland cement and fine and coarse aggregates, but with added components. 

In HPC, materials and admixtures are carefully selected and proportioned ("optimized") to form high early strengths, high ultimate strengths and high durability beyond conventional concrete. 

Some industrial "waste" materials of a few decades ago now are integral elements of this new engineered concrete. These admixtures, such as coal fly ash, silica fume and ground granulated blast furnace slag, add both strength and durability to the concrete, and enhance its marketability as an environmentally friendly product. 

HPC provides enhanced mechanical properties in precast concrete 
structural elements, including higher tensile and compressive strengths, and heightened modulus of elasticity (stiffness). In frost-prone regions the benefits of HPC are great. The enhanced durability of HPC helps it resist penetration of chloride-laden snow and ice melt water. This results in longer life for the reinforcing steel within, and a reduction in spalling, cracking and associated repairs. 

The proportions in which fundamental components are mixed, and the 
admixtures that are used, constitute the main difference between conventional PCC and HPC. A high-range water reducing admixture may provide a required low water/cement ratio, perhaps as low as 0.35. 
 

HPC at TRB

A precedent-setting Workshop on Durable Concrete for the 21st Century at the 76th annual meeting of the Transportation Research Board in Washington D.C. in January 1997 launched large-scale technology transfer on HPC in transportation facilities. There, a  variety of thought-provoking technical papers and serious sessions on portland cement concrete were presented. 

The workshop was designed to introduce the TRB registrants to current research and field practice in durable concrete, a "hot button" topic toward which much research has been directed. Dr. Kenneth Hover, Cornell University, established the foundation on which three subsequent speakers would build with his innovative and lively overview of concrete fundamentals. 

In an engaging style well-suited for a university professor, Hover took his listeners deep within the concrete matrix in both fresh, intermediate and cured stages, and described the variety of admixtures and how they form a functioning system within the matrix. 

"I want to go deep inside the concrete," Hover said. "We want to go all the way inside, we want to walk around inside concrete at the microscopic level, get familiar and comfortable there." And inside the 6 X 12 cylinder, he said, we would see that concrete fundamentally is a composite material, made up of coarse, intermediate and fine aggregates glued together by an "industrial-strength adhesive which we refer to as hardened portland cement paste." 

The aggregate particles are sometimes mistakenly referred to as "inert aggregates", Hover said. "The reality is they play a very heavy role in the final performance of the product. The coarse aggregates are, generally speaking, stronger in compression and tension, and are far stiffer, than the concrete as a whole. The aggregate performance from a mechanical point of view is critical to the behavior of the entire composite, and then we have to take a look at their performance from a chemical or a durability point of view." 

Regarding the "glue" of the cement paste, Hover said its performance was key to a recent court decision in Pennsylvania, in which some expert witnesses spoke in terms of "cement" and others in "concrete", confusing jurors. Finally the judge threatened to hold the trial in abeyance until clear definitions could be agreed on. 

The legal definition of cement in the Commonwealth of Pennsylvania now is "cement is to concrete as flour is to fruitcake". At 500,000-times magnification, Hover said, hydrated cement appears as fuzzy "dust bunnies" which began as smooth, hard particles, but expanded on reaction with water to connect with each other to form a strong matrix. "This [interaction] is the source of the fundamental strength and mechanical properties of concrete," he said, adding that like Velcro, it's the interlocking action of crystals that gives the connectional capability of holding the aggregate together. 

Moreover, with conventional concrete, there is more pore space than filled space. Hover displayed a microphotograph of ordinary cement paste. "Looking at enlarged portland cement paste, there is more 'nothing' than there is something'," Hover said. "There is more pore volume than there is solid volume present, [and] the pore volume essentially is the origin of our durability problems. If the connection is the origin of our strength benefits, the void space there is the origin of our durability difficulty, where water can enter, where sulfates, deicing salts, the chlorides and alkalis can move. Most of our deterioration mechanisms will be associated with our ability to get aggressive agents into the concrete, and they get in through that pore space. Concrete is fundamentally a very porous material." 

Hover estimated that something as "solid as concrete" actually is very porous, with cement paste 50 to 60 percent void space, and with aggregates added, about 50:50 porous/solid. "Concrete is fundamentally porous, whether we like it or not," he said. "In terms of cost-effective engineering, it's doubtful that we can make the material absolutely nonporous. What we need to do is recognize the porosity and make it more resistant to penetration so it takes longer for these aggressive substances to make their way into the concrete." 

Diluting the glue

Many problems of concrete, Hover said, could be eliminated by weighing out portland cement and adding water. The "liquid glue" would then be evaluated before adding to aggregates. "It would be obvious to anyone mixing the glue -- without having to look on any graph or chart or diagram -- that for a fixed amount of cement, the more water you add to it, the more dilute that glue is going to be," Hover said. More water will make it more runny, and more workable, but more dilute, with reduced strength and durability. But because concrete is not batched in that way, persons responsible lose sight of the fact that water dilutes the cement glue. 

Ultrafine admixtures such as fly ash, blast furnace slag and silica fume work by optimizing the packing density of concrete in the paste phase. Recent research by Don Streeter at the New York State DOT is determining optimum packing proportions of fly ash, silica fume and cement to obtain a dense paste phase, Hover said. 

Hover used foam plastic balls added to a box of water to illustrate water/cement ratios. He said that water/cement ratios of 0.54 were used during the early days of the Interstate system, which led to durability problems that were not recognized until the 1970s. Even at high-performance concrete, low ratios of 0.33, there still is a great deal of porosity in the concrete, he said. 
 

'Blessing of the Slab'

Scaling ultimately will occur when water is added to the surface of a slab in the field, in a ritual Hover called the "blessing of the slab", in which "tough spots" are sprinkled with water and reworked to improve ridability. 

"When we indiscriminately start adding water in the field, we are increasing the volume of the water, which immediately translates to increased spacing between the grains, and increased porosity," Hover said. "The potential quality of the concrete will diminished."

'Jacket weather' -- 50 to 55 deg F -- is best for concrete curing, he said. "Procedures are set up to keep the concrete warmer in winter, and cooler in summer, because we can't make an across-the-board statement that colder or hotter is better. We have to make our adjustments in both directions." 

There is no magic number, or flat rule, of days for a successful cure, Hover said.

Modified, high-performance concrete mixes with superplasticizers which don't produce bleeding are "zero-evaporation tolerance" mixes, Hover said. "There is a process, including fogging, spray cure, wet cure, slow drying and slow cooling. We have to stop thinking that curing is one thing we do at the end of the job just before we go home." 
 

Controlling environmental distress

Dr. Celik Ozyildirim, principal research scientist, Virginia Transportation Research Council, built on Hover's foundation with how concrete can be fortified against premature failure. He described the solutions in great detail, while emphasizing control of the final product through design, material selection, and proper construction practices, inspection and specify addresses include: 

• Corrosion. It can be fought by reducing permeability of concrete, or by protecting reinforcing steel from the onslaught of chlorides. Low-permeability concretes, such as latex-modified concrete (LMC), silica fume concretes, and low-slump concretes, reduce permeability. Sealers, coatings and polymer overlays will help protect steel. "One option is epoxy-coated steel, but states are showing concern as to the cost-effectiveness of this system," Ozyildirim said. Corrosion inhibitors are another means, he said, describing a 14-state study now underway studying their efficacy.

• Alkali-Silica Reaction (ASR). A chemical reaction can occur between the reactive silica and the alkalis in concrete, and the resultant gel expands and causes distress. "We see this in our abutments and pavements," he said of Virginia. There, higher alkalis require use of pozzolanic material in concrete, removing cement in favor of fly ash, slag or silica fume.

• Freeze/thaw cycles. "Critically saturated concrete, if not air-entrained or protected, can be disintegrated. Scaling and exposure of coarse aggregate can occur. The best solution is use of air entrainment and stabilizing small bubbles in concrete," Ozyildirim said. "We like to have a proper air void system, with bubbles less than 0.2 mm from any point in the matrix, and we like to use sound aggregates, and make sure we have mature concrete." D-cracking is the result of unsound aggregates.

• Sulfate attack. The formation of ettringite -- the result of calcium aluminates reacting with sulfates -- which occupies a larger volume, causes distress in concretes. When it's a problem in a particular locale, it can be fought by using cements with lower levels of calcium aluminates, such as Type II or Type V cements.

Durable concrete in the field

Field applications of durable concrete by the Federal Highway Administration and state DOTs in Virginia, Florida and Texas were described by Mary Lou Ralls, bridge design engineer, Texas Department of Transportation. 

Virginia has seven high-performance concrete (HPC) within its boundaries, five of which involve new permeability specifications. Two have been built, and the remaining five are under contract or in the design stage. 

Unlike other enhanced construction technologies, in which higher upfront costs are overcome by long-term savings, HPC offers savings upfront and in the long run. The strength afforded by HPC offers the savings of being able to use less material to start with, complemented by HPC's long-term durability through its resistance to environmental stresses. 

Virginia has demonstrated the immediate benefits of HPC with smaller bridge sections, and fewer girders used. "Both of the two HPC bridges built [so far] in Virginia have been below the cost of 34 bridges built in the federal-aid highway system in Virginia," Ralls said. 

On Virginia's Route 40 bridge, two beams per span were saved, using AASHTO Type IV girders with 8,000 psi, vs. the conventional Type IV girders with 6,000 psi. Deck depth on the HPC bridge was increased from 8 in. to 8.5 in. to reflect the wider spacing. 

Silica fume and air entraining agent were used in the girder mix, with 0.32 water/cement ratio. The cast-in-place deck mix contained slag and a 0.4 water/cement ratio, with strengths of 8,000 to 9,000 psi. 

The second completed bridge, Route 629, had the same number of girders, but had a smaller section, going from a Type V girder to a Type IV girder, she said. The bridge has 12 100-ft. spans, with girder mixes incorporating slag, air entrainment and a 0.33 water/cement ratio. One-day compressive strengths were over 6,000 psi, and approximately 9,000 psi at 28 days. Beams for the seventh Virginia bridge will be the first to use a larger 0.6-in. diameter strand, and 10,000 psi concrete. These Type III girders will save two girders per span and will be instrumented to monitor temperature, concrete strain, deflection, transfer length and end slip. 

Florida's steel-reinforced bridges suffer not from deicing salts, but marine corrosion. Epoxy-coated rebar or penetrating sealers are not allowed by Florida specs. Florida Class IV concrete with higher compressive strengths is indicated for a moderate to extremely aggressive marine environment. 

Florida's famous cable-stay Sunshine Skyway Bridge, across Tampa Bay near Bradenton, Fla., was the first to use these new Florida concrete durability specs. Constructed in 1987, it shows no signs of concrete distress. The Dodge Island Bridge in Miami was built in 1991 using Florida Class IV concrete specs in that harsh marine environment. The Acosta Bridge in Jacksonville and U.S. 17 & 92 Bridge over the St. John's River in Orlando both were built with Class IV concrete and are performing well. "Durable concrete is the standard in Florida, which is moving forward with permeability-based and performance-based specifications," Ralls said. 
 

Where HPC concretes in structures and pavements progress from here was discussed by Thomas J. Pasko, Jr., director, Office of Advanced Research at FHWA. Cement varies tremendously from one manufacturer to another, and so does fly ash. Available aggregates are of four basic rock types which grade from one to the other in an infinite variation, he said. And man-made products exist, such as manufactured sand, lightweight slags and recycled materials. 

This means that concrete mixes begin with a variety of materials. A variety of tests exist, but they don't relate one material to the other. The challenge to mix designers of the future is to "optimize" the mix through analysis to make sure the variable components all work together to create a durable product. It's not easy to put the finger on what causes D-cracking or surface distress, he said. 

"It's not one broad, collective problem; it's 3,000 separate problems and we have difficulty relating them," Pasko said. Education is a major problem. Making sure staff knows how to operate sophisticated equipment is a major problem, he said. "The workers can't improve if they don't know what they're doing," Pasko said. Pasko mentioned some areas which need research: 

• Self-curing concretes, which may crust over or with oil on the surface 
• Methods of triggering set, perhaps with microwaves, particularly with pavements 
• Durability without air entrainment 
• Anything involving composites, for example, synthetic fibers, and 
• Reducing thermal curl by making concrete more conductive.

Technicians across the country are making exciting new inquiries into the longevity of PCC pavements. 

Some even have set a design life span of 50 years for PCC pavements. The goal of FHWA's High Performance Rigid Pavements (HPRP) program is to construct selected highway projects which will demonstrate promising PCC pavement design technology. 

In 1992, a team of state, industry and federal engineers toured European PCC pavements and returned with a wealth of information. This led to a 1993 1-mile test section being constructed on I-75 (Chrysler Freeway) in Detroit. Its design and construction procedures were similar to those used in Germany and Austria. 

Its performance will be monitored through 1998. The pavement consists of a 10-in. PCC pavement over a 6-in. lean PCC base, both over a 16-in. non-frost-susceptible layer. The driving course is made up of 5500 psi concrete containing a crushed basalt brought 340 miles south from Sault Ste. Marie, Ont. In Europe, it is rather common to see high-quality rock shipped hundreds of miles to a job site, if specified. 

National goals of HPRP research include: 

• Increasing the service life of PCC pavements 
• Decreasing construction time by up to half 
• Lowering life-cycle costs 
• Lowering maintenance costs by up to half 
• Constructing ultra-smooth ride quality pavements 
• Incorporation of recycled or waste products without compromising quality 
• Utilization of innovative construction equipment or procedures, and 
• Utilization of innovative quality initiatives. 

The American Concrete Pavement Association is partnering with FHWA and state agencies to answer fundamental questions about PCC pavements in an age of increased longevity. PCC pavements are historically long-lived, but expensive to repair. Now the challenge is to reduce the need to repair, or restore PCC pavements to as-new condition. 

"The industry has addressed the concerns that concrete pavement constructions take too long, cost too much and are too difficult to repair," said outgoing ACPA president Marlin J. Knutson, P.E. "Industry is focusing a great deal of attention, time and resources on improving quality and restoring our existing infrastructure." Two recent breakthroughs have been in-place concrete recycling and ultrathin whitetopping. "Whitetopping is a durable, economical option for resurfacing," Knutson said. 

Knutson elaborated on a number of design improvements that will lead to high-performance concrete pavements that would last five decades. They include: 

• Use of thickened edges, or wider pavements, to reduce edge deflections 
• Optimal thickness, or trapezoidal design with increased thickness for multilane facilities, especially for truck lanes 
• Composite pavement designs that employ lower-quality aggregates and recycled concrete and asphalt in the majority of the slab 
• Use of high-quality, skid- and abrasion-resistant aggregates and high-performance mixes for the surface 
• "Fair" mechanistic design procedures, including complete life cycle cost analyses and road user delay costs for all pavement types 
• Study of the bonding between existing PCC pavements and ultrathin concrete overlays 
• Development of a design catalog for low and high volume interstate pavements, and 
• Use of bonded overlays with dowel bar retrofit at joints and cracks for previous designs of conventional pavements that are faulted, but still exhibit good structural and load-bearing abilities.