The 23-story concrete tower Shirley Highway motorists saw yesterday afternoon rising from the crest of a hill in Alexandria should be 26 stories high sometime today.

The structure, the core of what will be a 32-story, $56 million hotel, has been growing at a rate of an inch every eight minutes--the result of a new concrete pouring process called slip-forming, which construction officials say should enable the tower to set a speed record for construction of a high-rise building in the Washington area.

"It's hard to see how fast it's going," said 31-year-old Tom Leyden, one of a score of rubbernecks who has been watching the building from the nearby the Southern Towers apartment complex. "But it's zooming."

"It has amazed me--the speed and production of the building," said Bobby Jean Lancaster, 53, whose 10th floor apartment faces the new tower.

Today, after four weeks of constant concrete pouring from sunup Monday until sundown Saturday, workers should reach the end of the first phase of upward construction on the hotel. A more conventional construction process will add six more stories to the building by January, giving it the highest elevation of any building in the Virginia suburbs--only eight feet shy of the Washington Monument's elevation.

When completed by the end of 1984, the 500-room Radisson Mark Plaza Hotel, at Seminary Road and Beauregard Street, will be the centerpiece of a 14-acre development with a 1.2-acre lake, tennis courts, an indoor-outdoor pool, retail shops, and views of Washington on one side and 80 acres of woodland on another.

The slip-forming concrete process is something like building a tall sand castle using a steel tube. Pour a little wet sand--or concrete--into the tube, wait until it hardens, then pour a little more. Between pours, slip the tube up higher, adding to the tower's height almost as you pour.

At the Radisson, construction workers are pouring cement into 60-foot-long steel sheets, called slips, that line the tower walls. According to Tom Ulness, one of the construction managers, the method saves time--an estimated four months--and money.

A 100-yard-high crane lifts concrete by buckets and pours it into the slips that line the inside and outside of the tower.

The suddenly rising structure has drawn workers in the nearby Baileys Cross Roads area into wagers on how fast it will rise and brought to a halt many passing joggers.

"Even the traffic--they'll stop," said Leo W. Palmer, 51, who is the security chief for Southern Towers.

"We tend to get a lot of people at noon hour," said Ulness. "They line the fence. They're taking bets at how fast it's going to rise and when it's going to top out."

Not only by sunlight, but also by moonlight people have been watching the tower grow.

Palmer said he sometimes gazes upon the tower at night from rooftop of his 16-story apartment building.

At night the construction crews scurrying in front of the bright lights look like "bees in a hive," he said.

Some apartment complex residents express concern about how fast the tower is rising, recalling the 1973 collapse of one of the nearby Skyline Towers buildings, an accident that killed 14 workers and injured 34 others.

The premature removal of wooden shoring from freshly poured concrete was blamed for the accident, which allowed 20,000 tons of concrete chunks to bury the construction workers.

Ulness, construction project manager, said his crews make certain the concrete is dry before pouring more. "It's been carefully watched," he said.