TWO CONCURRENT exhibitions at the National Building Museum, an institution that has always interested itself to both the material and the immaterial aspects of architecture, beautifully illustrate those very polarities. On the one hand, as its name implies, "Liquid Stone: New Architecture in Concrete" exemplifies the tangible, while "Samuel Mockbee and the Rural Studio: Community Architecture" focuses on something more insubstantial.

As it did in 2000 with "Wood: An American Tradition" and the more recent "Masonry Variations," the Building Museum looks at one specific building material from many sides in "Liquid Stone." Here, it divides the subject into three broad areas: "Structure," which explores the more technical side of the medium; "Surface," which highlights concrete's surprising visual and tactile extremes; and "Sculptural Forms," which promotes concrete's remarkable design versatility.

It's in this section that you'll see illustrated such projects as Spanish architect Santiago Calatrava's Auditorio de Tenerife, a concert hall in the Canary Islands whose design includes a dramatic but utterly nonfunctional winglike structure that arches over the building in a gymnastic way that belies its weight.

A final chapter, called, appropriately enough, "The Future of Concrete," includes such cutting-edge concepts as translucent concrete. Here, you'll likely be amazed by thin panels embedded with chopped fiberglass, plus examples of LiTraCon (for Light Transmitting Concrete) and something called Pixel Panels, each of which features, respectively, internal fiber optics of glass and plastic, allowing for the passage of light.

Call it showboating, but these technological advances deserve to be bragged about.

So does the show itself, which takes pains to celebrate the history and science of concrete along with its aesthetics. To "Structure," "Surface" and "Sculptural Form," I'll add a fourth S-word: Sensuality. As demonstrated in "Liquid Stone," concrete is a material with the strength and grace of a dancer, as often as not possessing a "skin" that begs to be stroked. (A word of warning: Please obey the "Do not touch" signs where applicable.)

At first glance, "Samuel Mockbee and the Rural Studio: Community Architecture" might seem to be another material showcase. It's centerpiece, after all, is a walk-in structure made largely out of bales of discarded yarn, and it documents building projects that have included old tires and cast-off automobile windshields. One home featured in photographs has walls made entirely from stacks of carpet tile scraps donated by Interface Inc., a company whose stated goal is to become completely sustainable by 2020. Part of sustainability, of course, means eliminating waste.

This is not, however, a show about recycling or ecology. What it is, is a show about a philosophy.

While that philosophy, espoused by architect and teacher Samuel Mockbee (1944-2001), includes a emphasis on the use of unorthodox building materials and the reclamation of otherwise unusable junk, it also emphasizes something more ineffable -- the concepts of giving and of participation in community.

Through the Rural Studio, an academic program Mockbee co-founded in 1993 with fellow Auburn University architecture professor Dennis K. Ruth, participating architecture students would live and work in rural Alabama's impoverished Hale County, where, as part of their training in architectural pedagogy, they would design -- and build with their own hands -- homes and community buildings for the county's residents.

The structures themselves, which are typically based on a Southern vernacular architectural idiom -- porches, tin roofs, etc. -- but which frequently extrapolate from that base in all sorts of contemporary, yet startlingly organic ways are things of crazy beauty. More beautiful, though, is the idea contained in the mission of the Rural Studio, which exists to push students past preconceptions about the poor by encouraging them to live among and collaborate with the disenfranchised beneficiaries of their talent.

While "Samuel Mockbee" includes several of Mockbee's folk-art-style paintings and sketchbooks along with student projects, the art that is at the core of this extraordinary exhibition, and that is its true subject, is the art of human kindness.

SAMUEL MOCKBEE AND THE RURAL STUDIO: COMMUNITY ARCHITECTURE -- Through Sept. 6.

LIQUID STONE: NEW ARCHITECTURE IN CONCRETE -- Through Jan. 23.

Both at the National Building Museum, 401 F St. NW (Metro: Judiciary Square). 202-272-2448. www.nbm.org. Open Monday-Saturday 10 to 5; Sundays 11 to 5. Free.

Public programs associated with the "Liquid Stone" exhibition include:

Sunday at 2 -- Exhibition tour (repeated July 16 at 1 and Aug. 15 and 25 at 2).

July 24 from 1 to 2 -- Film screening: "Concrete."

A prototype wall of LiTraCon, a translucent concrete product, above. At left, a wavelike roof over the Auditorio de Tenerife, designed by Santiago Calatrava, in the Canary Islands illustrates the sculptural possibilities of reinforced concrete. The Yancey Chapel in Sawyerville, Ala. Architecture students lived in Hale County and designed (and built) structures for the community.