Half a century ago, in the early days of Concrete Construction, it was big news to hear about a project where 5000 square feet of floor was placed and finished in one day. Completing a 3000-cubic-yard mat pour in less than a day was unheard of. Advances in pumping technology that would exponentially increase placing potential started in the 1960s, but finishing and quality control techniques still had strides to make.
In the 1980s Concrete Construction reported on two developments that raised the bar for quality and productivity in slab construction. The first article, “A Time Bomb in Floor Tolerances?” by Allen Face, appeared in November 1982. An April 1987 article, “A Screeding Machine That’s More Than a Strike-Off,” was subtitled “Ride-On Vibratory Screed Spreads Concrete by Auger and Sets Grade by Laser.”
F-number system
Allen Face, president of the Edward W. Face Co., had begun to develop a new method for measuring floor flatness in 1979. Though he didn’t specifically use the term “F numbers” in his 1982 article, he introduced the concept and described an instrument that could be used during construction to monitor and automatically graph floor profiles.
Face explained that two industry trends were driving the need for flatter concrete floors. First, warehouses were using new small-profile, high-lifting material handling equipment that would not function properly on a floor that was not flat. Second, modular portable partition systems now widely used in high-rise office buildings required tightly maintained clearances between ceiling and floor.
Face’s F-number system quickly replaced the standard 10-foot straightedge method. In 1987 ASTM issued ASTM E 1155 “Standard Test Method for Determining Floor Flatness and Levelness Using the F-Number System.” ACI-117 “Standard Specification for Tolerances for Concrete Construction and Materials” adopted the system in 1990.
Concurrently, developments on the equipment front would advance quality and productivity.
The laser screed
In April 1957 Concrete Construction reported that vibrating screeds were gaining popularity. Thirty years later, a case study of the new screeding machine with laser receptors described how by striking off 240 square feet of floor in less than two minutes the machine enabled a crew of nine to screed a 14,000-square-foot floor in five hours. The screed, invented by a concrete finishing contractor, was now available for purchase.
True to journalistic form, the coverage presented the features, operation, and benefits of the screed without fanfare. A historical viewpoint allows more license to evaluate the impact of a technological breakthrough. In a January 2000 retrospective titled “The Equipment Revolution,” a segment on the laser-controlled vibratory screed stated that the machine “changed how concrete is placed and how other equipment is used.”
David and Paul Somero, the concrete contractors who invented the laser screed, had observed the use of lasers to control bulldozers and graders. The Someros couldn’t sell the idea to manufacturers, so they teamed up with a mechanical engineer and developed it themselves. They created a prototype in 1985 and sold their first machine in late 1986.
“Prior to buying and using the Someros’ machine we would average 5000 to 10,000 square feet of placed concrete a day,” Ken Endres of Middleton Construction, Middleton, Wis., told Concrete Construction in January 2000. “Now, with the screed, we average 20,000 to 25,000 square feet per day with the same manhours.”
Also in that article, Allen Face observed that the laser screed had had a “profound impact” on floor flatness and levelness.
In 1999 the laser screed was enhanced with a 3-D attachment that converted the machine from single-plane to multiple-plane screeding. The “Achilles’ heel,” according to a September 2002 article, was that workers installing control joints and applying curing agents would find it difficult to keep up with the highly productive machine. No doubt there are innovations on the horizon that will soon solve those problems, just as in the past.
For example, productivity increases resulting from concrete placing advancements created a demand for finishing equipment that could keep pace. The ride-on power trowel had been patented in 1973, but it was the demands of the 1980s that propelled the riding trowel into widespread use. According to the January 2000 equipment article: “On a 40,000- to 50,000-square-foot slab, four to five riders can do the work of 25 to 30 walk-behind trowels—or 125 to 150 finishers working on their hands and knees. Today’s mega-slab pours wouldn’t be feasible without rider trowels.”
Mega mats
Technological innovations ushered in the era of record-setting placements of thousands of cubic yards of concrete. As reported in Concrete Construction:
1981 The 485,000-square-foot George R. Moscone Convention Center in San Francisco logged in with “one of the largest foundation mats in the history of modern building construction,” requiring more than 12,000 cubic yards of cast-in-place concrete reinforced by 8000 tons of steel.
1988 On a Seattle foundation job, nine concrete pumps set West Coast records for placing concrete. After the first hour of pumping, 1350 cubic yards of concrete were in the hole. The 30,000-square-foot area required 10,700 cubic yards of concrete for an average depth of almost 10 feet. The continuous pour was completed in 13.5 hours.
1991 A 71,000-square-foot foundation mat for a multiuse building complex in Newport Beach, Calif., had to be placed over a quagmire 21 feet below sea level. The 7400 cubic yards of concrete was placed overnight in a monolithic pour using 130 ready-mix trucks and five truck-mounted pumps with 170-foot boom extensions.
2006 The concrete foundation of the Trump International Hotel and Tower in Chicago is placed in a continuous 7000-cubic-yard pour, representing North America’s largest single pour of self-consolidating concrete.