Steel-reinforced concrete, one of the quintessential building materials of 20th-century architecture, acquired a bad name in the United States during the second half of the century. And for good reason.

Architects, again following footsteps boldly paced out in France by Le Corbusier, became wildly enamored with the rough qualities of "le beton brut" -- raw, exposed concrete.

The name of the resulting style, brutalism, was often understandably misinterpreted as referring to the defensive, almost misanthropic -- or you might say brutal -- character of some of the buildings we had to put up with. Tiny windows and cave-like cut-outs in fortress-like walls of low-quality material and workmanship.

"Liquid Stone: New Architecture in Concrete," an exhibition opening today at the National Building Museum, is the perfect antidote to any lingering antipathies from the brutalist era. Focusing on architectural projects of the past five years and ending with a tantalizing, if tentative, glimpse into the future, the exhibition will restore faith in concrete and architectural creativity.

Concrete is, after all, a great building material. Ancient, too. Two millennia ago Roman engineers habitually made magnificent walls, splendid piers and even stirring domes out of the stuff. Imagine what they might have done had their mixes been reinforced with bars of steel.

Reinforcement, invented in the 19th century, is what made the material modern. Those familiar steel rods, abundantly visible at construction sites though not seen in the final product, add considerably to concrete's strength. They make it possible for a material that's good at supporting great weight also to span significant distances and to assume many different shapes.

Those qualities of spanning and pliability, in addition to concrete's ready adaptability to mass production, account for modern architecture's love affair with the material.

Beginning in the 1930s, for instance, Pier Luigi Nervi, the great Italian engineer-architect, used the new thinner, stronger concretes to build dramatic stadiums and exhibition halls with roofs that seem to stretch to the limit. By contrast, expressionists in northern Europe, such as Rudolf Steiner with his fluid (indeed almost liquid) Goetheanum, built in 1928, hankered after the material for the rich, smooth and sometimes odd shapes it could produce.

But though concrete's structural strengths continued to be exploited after World War II in all sorts of buildings, the concrete-as-dramatic-shape movement was pretty much put out of commission by the worldwide popularity of the more ascetic, rational, rectilinear International Style. In the United States, this was reflected in everything from elementary schools to corporate headquarters.

It was Le Corbusier, paragon of the International Style, who in the 1950s shockingly resurrected the expressive possibilities of the material in buildings such as the pilgrimage chapel of Notre Dame du Haut in Ronchamp, France. A modest wooden scale model made in 1955 shortly after the building was completed demonstrates the power of an idea. The chapel, with its thick walls, secretive windows and soaring nun's cap roof, remains today a compelling vision of emotionally affecting architectural form. You can see its influence in Eero Saarinen's TWA Terminal of 1962, and Santiago Calatrava's Tenerife Auditorium of 2003.

And for all of the many qualities of those two larger and very different exercises, you can probably bet with a degree of certainty that the petite Le Corbusier chapel packs the more powerful architectural punch.

Yet here I am, chattering away about history when, in fact, this exhibition focuses on contemporary architecture. Exhibition curator G. Martin Moeller Jr. made sure that some of the historical high points are touched upon, and also that rudimentary technical matters receive, well, rudimentary attention. Which is plenty.

Moeller divided the show into three overlapping categories corresponding to qualities of the material -- structure, surface and sculptural form. The most interesting and important, by far, is the sculptural section, because it engages architecture at the most fundamental, communicative levels of space and form.

Having said that, I must turn around and insist that a very good or even a great work of architecture these days (or any days) does not at all depend on the degree of wildness or improbability of its sculptural form.

Calatrava's Tenerife Auditorium may be a case in point. I say may because I am a huge Calatrava fan and, after all, I'm only looking at photographs and a model here, not the real thing. But, based on the evidence at hand, I would say that while the building, with its huge, arching, double-winged crest, is undeniably spectacular, it may suffer from symbolic overload. That symbolic crest, not incidentally, is not unlike the soaring avian form that spoiled Calatrava's design in the competition for an addition to the Corcoran Gallery of Art.

I have different sorts of doubts about the Museum of the 21st Century and its attached apartment tower designed by Hariri & Hariri for a waterfront site on Manhattan's West Side. Basically, it's a proposal for an elegant tower made of concrete ribbons and sheer glass walls, improbably shaped a bit like a swollen exclamation point. Not only am I befuddled by the notion of a museum devoted to a century that's not yet four years old, but I'm concerned that, unlike the elegant model, the real thing may turn out to be an embarrassing dud, Le Corbusier on steroids.

But, hey, though making snap judgments can be entertaining, it's not really the point of an exhibition such as this, which is more about information, about circulating current practices and ideas.

It's great to see, for instance, how far concrete surfaces have advanced in the last couple of decades -- and not only in the hands of Tadao Ando, the Japanese master whose poured concrete walls have the look and feel of rare stones. Ando relies on the quality of his pours, while others here are investigating new ways to add to the concrete mix so that the results are quite unconcrete-like.

Witness, for example, the light-reflecting skin -- made with concrete panels containing shards of recycled glass -- of a proposed cultural building in San Jose, Calif., designed by an architecture firm known as WW. That brings us to the future, which turns out as always to be not quite foreseeable but, possibly, amazing.

Manufacturers such as Lafarge, which sponsored this exhibition, are producing ultra-high-performance concretes that go beyond the reinforcing rod. They're thin and very strong, and can be used (or misused) as lattice-like decorations or, remarkably, to hold buildings up. And then there is the "holy grail," in Moeller's phrase, just beyond reach: transparent concrete.

Liquid Stone continues through Jan. 23 at the National Building Museum, 401 F Street NW, open Monday through Saturday from 10 a.m. to 5 p.m. and Sunday from 11 a.m. to 5 p.m. Admission is free.

Plans by Hariri & Hariri for Manhattan's new Museum of the 21st Century include a tower of concrete ribbons and sheer glass walls. Santiago Calatrava's auditorium in the Canary Islands' Tenerife shows concrete's versatility.