March 1990 Table of Contents

Features Petrography Reveals Parking Ramp Problems

A recent popout problem in a Minnesota parking ramp shows how valuable petrographic analysis can be. The ramp is a multi-level, precast concrete, double-tee bar structure that was constructed during a fall and winter. Shortly after the snow melted, popouts were observed throughout the ramp. Six 4-inch-diameter concrete cores were sent to the laboratory for petrographic examination. To everyone's surprise, three other problems were discovered in addition to the shale in the cores. First, the entrained air content of all six cores was very low, averaging 1.6 percent. Second, the cores showed the classic overfinishing profile- in two cores the aggregate had subsided up to 1 inch from the top surface. Last, two cores showed dark gray bands of cement paste completely lacking sand-sized aggregate. Read more

Features Computerize Carefully

Computerizing is a lot more time-consuming and costly than most contractors imagine. Transferring data takes a lot of time, often months, and inevitably involves transcription errors. Managing a fail-safe transition is costly. It always takes longer to install, transcribe, and debug a system than anyone projects. Conversion often costs as much as two or three times early estimates. It's risky to depend on a new computer system too soon. You need a formal contingency plan in case data or the system is lost. Professional advice and even insurance is available to cover the contingency. Remember to store duplicate data off the premises to further protect yourself. Read more

Features Accelerating Admixtures for Cold Weather Concreting

Cold weather concrete has superior properties to concrete placed in hot weather. At low temperatures, though, concrete sets and gains strength more slowly because the cement doesn't hydrate as fast. Setting time is increased about one-third for each 10 degree decrease in concrete temperature down to 40 degrees Fahrenheit. Accelerating admixtures can help to offset these effects of low temperatures on setting and strength gain. They should meet the requirements of ASTM C 494, Standard Specifications for Chemical Admixtures for Concrete. Read more

Features A Unique Way to Repair Flat Plates

Innovative repair methods used on a South Florida high-rise corrected problems caused by poor construction practices. The building's floor structure consists of a two-way flat plate that is continuous with 8-foot cantilever balconies. The 8.5-inch-thick balconies surrounding the building perimeter were found to be structurally deficient. The top reinforcement was placed too low in the balcony slabs. Reducing the distance between the top steel and the bottom of the concrete slab reduced the slab's ability to resist cantilever forces. Read more

Features A Contractors' Guide to Air-Entraining and Chemical Admixtures

Here's brief summary of how chemical admixtures affect properties of both plastic and hardened concrete: Read more

Features Here's a Concrete Curtain Wall That Really Is a Curtain

Shoppers who pass the Jessica McClintock storefront in a Costa Mesa, California, mall see billowing curtains made of concrete. Actually, it's not concrete but a portland cement mortar sculpted by stonework contractor Gerardo Sorrentino of San Francisco. Photos show Sorrentino tying metal lath to form the curtain, as well as the intricate detail in the completed scratch coat. Read more

Problem Clinic Using Two Courses of Concrete to Place Welded Wire Fabric

We're going to be placing a 6-inch-thick concrete slab on grade that requires welded wire fabric for crack control. Plans call for it to be in the center of the slab. We think it ought to be closer to the top. Also, we want to place the concrete in two la Read more

Problem Clinic Air-Entrained Concrete Can Be Troweled

Specifications for a job I'm doing call for a troweled surface for air-entrained concrete flatwork. I've heard that air-entrained concrete shouldn't be troweled. How can I convince the architect that troweling is a mistake? Read more

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