By John S. McClenahen Chief executives of privately held fast-growing manufacturers and service providers believe the U.S. economy is growing and that prospects are brighter during the next 12 months, indicates a new PricewaterhouseCoopers survey. Some 54% of 383 CEOs interviewed between July and September says the U.S. economy is now growing, up from 33% in the second quarter of this year. Also, an impressive 73% now say they are optimistic about the economy's prospects during the next year, up from 59% in the second quarter. What worries the CEOs? By a wide margin, it's weak demand. Some 66% express concern. Shrinking margins are the second-place worry, but it's a concern cited by only 34% of the CEOs. The CEOs are now more inclined to add to their workforces than they were during the second quarter of this year. Some 70% are planning net additions to their payrolls during the next year; only 4% plan further cuts; and 26% expect no change from current levels.