Engineers probe for ways to build a better bridge

Arnold (Reggie) Cutting, in hard hat and steel-toed boots runs his hand along the underbelly of a new bridge going up across the briny Sheepscot River here. ''If you look close,'' says the Maine state engineer, peering down the length of the 2,700-foot structure, ''you can see the sheen on the concrete. This is very dense stuff.''

Mr. Cutting, who lavishs as much affection on bridges as most people do on their pets, is referring to a new type of concrete used on bridges across the country.

The tougher, less-porous material - concrete mixed with plastic polymers - is one of a growing number of subtle changes in bridge construction that will help tomorrow's structures hold up better than today's.

The nation's stock of bridges is in daunting disrepair. The federal government estimates that almost half are either deficient or obsolete. The recent collapse of the Mianus River bridge in Greenwich, Conn., killing three people, was a grim reminder of the work to be done.

Patching up the nation's infrastructure will take some creative scientific and financial footwork. But pressure is also mounting on engineers to come up with a generation of safer, longer-lasting bridges. Are the ones being forged in today's idea shops any better than yesterday's? Some answers may be gleaned from the nearly completed bridge gently arching out from this steeple-dotted town:

* The $8.5 million structure is made of prefabricated concrete sections. Prefab bridges - with sections cast in the factory and then hoisted into place - have become popular in recent years, partly as a way to speed up construction of new bridges and patch old ones.

* Concrete for today's bridges is often laced with plastic polymers or rubbery latex materials, making it less prone to water seepage. And the steel reinforcing bars in the concrete are coated with epoxies, also to prevent corrosion. Adhesives are also sandwiched between the concrete segments themselves to prevent water from getting inside the structure.

* Progress has also been made in making tougher steels, still the predominant building block in bridges. And more exotic materials are being tested. Some bridges have already been made of plastic-fiberglass composites, which are light and corrosion-free. But, say bridge experts, cost and concern about brittleness will probably confine them to military use.

The Sheepscot bridge (the first in New England to incorporate these new construction techniques, although similar structures have been used in the South for years) has some added touches to protect against saltwater and Maine winters. The concrete support pillars plunging into the frothy tidal bay are sheathed in granite. Granite curbs will line the roadway on top of the bridge. Both the sheathing and the curbs will help prevent exposure to water seepage, as will the veneer of latex-laced concrete that will eventually top the road surface itself.

In some coastal areas, engineers pump low-voltage electrical currents (undetectable to those passing over) into the steel reinforcing rods of concrete bridge decks. The current neutralizes the corrosion reaction. But the technique, some experts say, is relatively expensive and requires a lot of maintenance.

Yet new problems are popping up as fast as new designs. Engineers are now unraveling the impact on bridges of some of the latest road salts used in winter to prevent ice formation. In the longer run, warns Dr. Michael Ackroyd of Rensselaer Polytechnic Institute, acid rain and other pollutants could turn out to be a significant new source of bridge corrosion.

The moral in all of this, sums up Mr. Cutting, is that bridge building is getting better but remains an inexact science. The old Wiscasset bridge - a rickety mix of wood and steel - withstood cars and climate for 50 years. Mr. Cutting says the new bridge may last twice that long, despite having a 50-year design life. ''We have made tremendous strides,'' he says. ''But,'' he adds quickly, ''we will never get to the point of building the perfect bridge.''