March 1980 Table of Contents

New Developments in Steel Fibrous Shotcrete

Since its introduction in 1971, steel fibrous shotcrete has found many interesting and practical uses. Read more

The shape of things to come A Guide to Specifying Formwork for Concrete

Since formwork generally is a temporary installation (and not part of the building or other finished product with which the specifier is concerned), it may be that little attention is given to the formwork specification even though it has a vital bearing on cost and quality of the concrete. Read more

How to Produce High-Strength Concrete

Though a longtime major aim of the concrete industries has been to provide the least expensive material possible, these industries have also been concerned with constantly improving their product. Read more

Monitoring Structural Movements with New Telltale

A new telltale is now available for readily monitoring movement of cracks or joints in structures to 1 millimeter (0.040 inch). Read more

A New Role for the Wheel

Invention of the wheel has been called one of the great steps in the development of technology. And inventors today are still finding new uses for it. One is in the concrete imprinting business. Read more

Concrete Machine Tool Structures

In recent years increasing interest has been shown in the use of concrete as an alternative to mild steel or cast iron for the structural components of machine tools. Read more

Problem Clinic Heavy Clay Subgrade

Should flatwork be placed on heavy clay? Read more

Problem Clinic Effect of Vapor Barrier on Cracking

Does polyethylene vapor barrier under slabs decrease or promote surface cracking? Read more

Problem Clinic Polyethylene Keeps Water From Going Into Subbase

Does polyethylene under a slab reduce the loss of mix water? Read more

Problem Clinic Vapor Barrier May Contribute To Curling

Will a vapor barrier accentuate the curling-cracking of a slab on grade? What corrective measures can be taken for a slab that has already curled? Read more

Problem Clinic Rebar Spacing and Sizes Of Cracks

Regarding reinforcement in slabs, please clarify whether you find a rebar spacing of 16 inches as good in reducing crack widths as a spacing of 6 or 8 inches, assuming there is the same total area of reinforcement in any case. Read more

Problem Clinic Chair Height and Rebar Spacing

Suppose that 1 1/4-inch rebar chairs were specified and that some chairs used were only I inch and others 11/2 inches high. How critical would this difference in elevation be with respect to the spacing between rebars? Read more

Problem Clinic Kind of Joints to Control Cracking and Curling

Do you feel that sawed control joints cut at the proper time will reduce cracking and curling better than other control joints? Read more

Problem Clinic Tooled Joints Not Good Under Wheeled Traffic

Which is better, a joint made by hand when concrete is still plastic or a joint cut by a saw after a day or two? Why? Read more

Problem Clinic Perimeter Joints Prevent Cracking

Why is an expansion joint used between the perimeter of a basement floor and the wall? Read more

Problem Clinic Isolation Joint Material Usually Thick

Regarding isolation joints, why does the industry still use thick expansion joint material? Are there other factors than just isolation? What about expansion in heated buildings? Read more

Problem Clinic Steel Crossing Construction Joints

In construction joints that are actually control joints isn't it possible to place steel so that it will stop any movement of the concrete? Read more

Problem Clinic No Joints in Structural Slabs and Their Toppings

In an area 90 by 110 feet, completely enclosed against the weather, how important are control joints in a 2 1/2-inch topping with welded wire fabric on a 3 inch metal deck? The area will be heated in winter to keep a fairly constant temperature of 70 degrees F and cooled to the same temperature in the summer. Read more

Problem Clinic Fineness of Cement and Concrete Quality

How does the fineness of cement affect the quality of concrete? If the fineness is high, what procedures can be followed to maintain quality in the product? Read more

Problem Clinic Fly Ash in Slab Mixes

Could you comment on fly ash in concrete for slabs on grade? Will fly ash reduce shrinkage? Read more

Problem Clinic Water-Cement Ratio and Vacuum Dewatering

Can the water-cement ratio be increased in the original mix if vacuum dewatering is to be employed? Read more

Problem Clinic Calcium Chloride Effects

What does calcium chloride do to setting time and concrete strength? Read more

Problem Clinic Calcium Chloride, Strength and Workability

Does the use of I or 2 percent calcium chloride in concrete weaken the concrete measurably? Why does concrete containing calcium chloride seem to work better? Read more

Problem Clinic Effect of Aggregate Size on Strength

How much stronger is a floor made with 1/2-inch aggregate than one with 3/4-inch aggregate if all other ingredients remain the same? Read more

Problem Clinic Effect of Sand on Cracking

Do some sands tend to cause concrete to crack more than others? We normally use a manufactured sand and a natural sand. The manufactured sand appears to cause a lot more cracking, especially plastic shrinkage cracking. Read more

Problem Clinic Measuring and Controlling Air Content

How do you control and measure the amount of air in a slab? Read more

Problem Clinic Floating and Finishing by Power or by Hand

Can you complete the finishing of concrete with power floating and troweling or must you hand float and finish? Read more

Problem Clinic Power Float Versus Power Trowel With Float Shoes

Is there an advantage to using a disc-type powerboat instead of the conventional troweling machine with float shoes during the floating of a floor? Read more

Problem Clinic Retaining Moisture Before Machine Troweling

Under circumstances where plastic shrinkage cracking might be anticipated, would spraying curing compound just prior to machine troweling eliminate the need for fogging? Read more

Problem Clinic Rubbery or Spongy Surface

How do you overcome a rubber-like movement when troweling? Read more

Problem Clinic Manuals on Finishing

Where can we get a reliable manual on concrete finishing? Read more

Problem Clinic Protection Against Rain

What should you do if rain hits after the slab has been placed? Read more

Problem Clinic Delays From Cold Weather or High Humidity

What can be done to speed up the setting time when working in weather that is cold, highly humid or a combination of both? Read more

Problem Clinic Setting In Cold Weather

How can we make concrete set up in cold weather? Read more

Problem Clinic Cold-Weather Curing

How does cold weather affect concrete curing time and strength? Should cement content be increased in cold weather? Read more

Problem Clinic Relative Efficiency of Curing Methods

Why is waterproof curing paper considered the best curing method and curing compounds considered the poorest, assuming, of course, that proper application procedures have been exercised in the application of both? Read more

Problem Clinic Curing and the Solar Still Effect

Is use of plastic sheeting a good curing method for concrete or does this produce a solar still effect—drawing the moisture out of freshly placed concrete too quickly? Read more

Problem Clinic Best Curing Temperatures

At what temperature does concrete cure best? Read more

Problem Clinic Carbonation of Slabs Covered With Curing Compounds

How can carbonation hardening by air occur when a slab is sealed by a curing compound? Read more

Problem Clinic Cure-and-Seal Compounds Should Represent Good Quality

How good are combination cure-and-seal compounds? Read more

Problem Clinic Epoxy Cure-Seal-Harden Compound

What is your opinion of an epoxy curing compound which is said to seal, harden and dustproof? What are its advantages? Read more

Problem Clinic Effect of Drying on Hydration

Once you remove the forms how long will it take for concrete to dry too much for hydration to continue? Read more

Problem Clinic Preventing Differences in Drying Rates

How do you overcome having the surface of the slab dry slower than the base of the concrete? Read more

Problem Clinic Damage From Early Freezing

If concrete is frozen, does strength come back? What does freezing do to the finish? Read more

Problem Clinic Chert Popouts

What can a finisher do to control or eliminate chert popouts? Read more

Problem Clinic Spalling of Parking Lot Pavements

What causes spalling in concrete parking lots? Read more

Problem Clinic Spalling of Concrete at High Temperatures

What papers or publications do you know of regarding the exposure of concrete surfaces to high temperatures (1500°F)? What can you recommend from personal experience to keep the surface of concrete from spalling when shock-exposed to high temperature? Read more

Problem Clinic Applying Mineral or Metallic Aggregates for Hard Surfaces

Please discuss the method used to apply floor hardeners on a monolithic slab-on-grade floor. Read more

Problem Clinic Minimizing Curling of Slabs

When using a manual straightedge how high a slump can be allowed while still maintaining control of shrinkage and minimizing curling? What else can be done to prevent or minimize curling? Read more

Problem Clinic Preventing Blistering

In a floor made with a mix containing 4 to 6 percent air with a slump of 4 to 5 inches, we have had the experience a number of times of small pimples developing. Read more

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