April 1997 Table of Contents

Features Vibrator Trails in Slipformed Pavements

Vibrator trails are streaks of high-paste-content, low-air-content concrete found in the vibrator paths of some slipformed pavements. They are caused by excessive vibration of concrete and are prone to longitudinal cracking. In 1995, a field research project was initiated in Iowa to evaluate the effect of slipform-paver vibrations on the air content of hardened concrete. The study, which involved three separate interstate paving projects, examined three variables: vibrator frequency, paver speed, and vibrator position within the concrete slab. Read more

Features Troubleshooting Crusted Concrete

Problems occur when the surface of concrete flatwork crusts over and looks like it's ready to finish while the underlying concrete is still soft, like jelly. If finishers get on the crusted surface with a power trowel, the soft underlying concrete may bulge beneath the trowel, creating a wavy or cracked surface. But if they wait for the underlying concrete to stiffen, the top surface is usually too hard to float and trowel. Read more

Features The Drying Shrinkage Dilemma

Many designers of concrete structural elements know that concrete shrinks when it dries, but they often fail to consider this in their designs. Article describes the possible consequences of drying shrinkage, including failures at filled joints, slab curling, and excessive cracking. Read more

Features Long-Term Strength Tests of High-Strength Concrete

Limited data are available on the relationship between the strengths of test cylinders and cores taken from in-place high-strength concrete. Because high-strength concrete structures may be heavily reinforced, it can be difficult to obtain cores with the same aspect ratio (core length divided by core diameter) as cylinders. The relationship between aspect ratio and compressive strength for normal-strength concrete is well-established. But does this relationship apply to high-strength concrete? The results from five-year tests of high-strength concrete help to answer this question. Read more

Features Are They Pour Lines or Cold Joints?

Both pour lines and cold joints are common in concrete construction, but cold joints have more serious consequences. Using case histories, the authors discuss several testing methods that can be used to evaluate whether a visible pour line found on an exposed concrete surface is just a noncritical surface imperfection or a sign of a significant structural defect. Read more

Problem Clinic Cause of Surface Deterioration

We placed a 6-inch-thick, 16x40-foot air-entrained concrete slab during cold weather. The 12-cubic-yard load of 3000-psi concrete was made with hot water and contained an accelerator that permits placement at temperatures down to 20 F. Concrete arrived at Read more

Problem Clinic Which Curing Method Works Best?

Do you know of any data describing which curing methods produce the best results? Read more

Problem Clinic Cracks in Driveways: What's Acceptable?

We do everything possible to prevent cracking of the concrete driveways and patios that we place. We promptly cut joints to the correct depth and space them no more than 12 feet apart. We also use medium-slump concrete and start curing it as soon as we ca Read more

Problem Clinic Sources of Colorful Aggregates for Concrete

Where can we get some pure white and pure black pebbles for a special concrete mix? Our exposed aggregate renovation job calls for matching some existing concrete, and it looks like black and white were both used. Read more

Problem Clinic Slump of Concrete for Residential Walls

The contractor who plans to build the foundation walls for our house makes a practice of using concrete with a slump of 6 ± 1 inch. Read more

Problem Clinic Prepare Wall for Patching

Experience shows that repairs have been made at disturbingly frequent intervals on a bridge deck that bears heavy traffic. Read more

Problem Clinic Admixtures From Recycled Aggregates

We are considering purchasing concrete from a plant that uses recycled aggregate. This job will continue for some time, so we are concerned about whether the plant will be able to deliver the same quality of concrete throughout the job. Read more

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