November 1991 Table of Contents

Features How many concrete control tests are needed on an average job?

On an average job, how often should slump and air content tests be made? How many test cylinders should be made? Read more

Features High-Rise Apartment Framed in Concrete

After two of four planned Detroit high-rises were built with steel frames, owners began to hear concerns from tenants about wind-induced movement at the upper floor levels. To resolve this problem, the designer of the third high-rise decided to use a concrete frame. By switching to concrete, the owner realized savings in both construction time and overall costs, as well as the benefit of shielding the two buildings with steel frames from the wind. Substituting cassions for the costly pile foundations used on the first two buildings also contributed to overall economy. Read more

Features Choosing Support Members for Wall and Deck Forms

The performance of any form sheathing, whether made of plywood, metal, or fiberglass, is only as good as the performance of its support members. Form support is provided by various components. For wall panels, studs supported by cross members called walers provide direct support. For horizontal decking, joists give direct support. To hold support members in place, tension members are used. Because of their important function, support members must be carefully chosen to coordinate with sheathing selection. Read more

Features Aluminum Wall Forms Speed Pour Down for High-Rise

Lightweight aluminum wall forms and a "pour down" sequence contributed to the successful construction of the 44-story Nauru Tower. The tower is a mixed-use structure with the first five levels of retail and commercial space requiring special architectural treatment. Concrete for the 39 residential floors was placed using a construction sequence similar to that of tunnel forming systems; the slab plus walls and columns for the level below are poured at the same time, working from the deck forming surface. This system allowed the concrete superstructure to be built in just 17 months' time. Read more

Features Record-Setting Waffle Slab Tops Underground Convention Center Wing

Concrete contractor Landavazo Bros. Inc., Hayward, California, recently fulfilled a contract to place and pump the 110,000 cubic yards of concrete needed for the expansion of San Francisco's Moscone Convention Center. The 100-million-dollar project involved construction of a 487,500-square-foot underground wing connected to the existing convention center by a series of tunnels. Crews poured a 168,000-square-foot, post-tensioned waffle slab comprising 22,500 cubic yards of concrete. The waffle slab forms part of the street-level roof topping the underground wing. Engineers chose a waffle-slab because it can distribute heavy superimposed loads in two directions and can accommodate the large spans needed for open exhibit space. Read more

Features Conveyor System Speeds RCC Dam Construction

To build the roller-compacted-concrete (RCC) Cuchillo Negro Dam, PCL Civil Constructors Inc. used a conveyor system that delivered the RCC mix from the mixing plant to the dam site. The 1,500 feet of conveyors with a maximum belt capacity of almost 9 cubic yards per minute delivered the RCC mix to the jobsite without interruption or downtime. PCL placed an average of 158 cubic yards an hour during the 62-day pour. Read more

Features What Makes Concrete Pumpable?

In a pump line, concrete moves in the form of a cylinder or slug separated from the pump line wall by a lubricating layer of water, cement, and fine sand. To keep moving through the line, the mix must be dense, cohesive, and have sufficient mortar. When concrete is pumped, if spaces or voids between aggregates are not filled with mortar, or if the mortar is too thin and runny, pump pressures cause segregation, forcing water through the mix. When this happens, the lubricating layer is lost, causing coarse particles interlock, friction to increase, and the concrete to stop moving. To prevent plugs, the pressure at which segregation occurs must be greater than the pressure needed to pump the concrete. This can be accomplished by filling the spaces between aggregate particles with smaller aggregate. Read more

Problem Clinic Formless Footings Permitted?

Soils in our area are stable enough for us to dig and form footings using the ground only. Is it permissible to build footings this way? Read more

Problem Clinic Time Limit Between Pours to Avoid Cold Joints

Is there an allowable time limit for successive placements of concrete during a monolithic pour? In other words, once one truckload has been discharged, how long can I wait before placing the next truckload? Read more

Problem Clinic Difference Between Type I and III Cements

I know that Type III cement is high-early strength cement, but what gives it a higher early strength than type I cement? Read more

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