Table of Contents 1975

FEATURES

Other Articles

• Some Fundamentals of Concrete

   

• Understanding the Subgrade and Preparing It

   

• Fundamentals for Footings and Foundations

   

• Keeping the House Dry

   

• Formwork for Basement Walls

   

• Concreting of Basement Walls

   

• Basement Floors and Slabs on Grade

   

• Concrete Floors Above Grade

   

• Driveways, Walks, and Patios

   

• Econocrete

   

• Old Pavement Recycled into New Subbase

   

• Colossal Creatures of Calgary

   

• New Welded Wire Fabric Numbering System Explained

   

• The Concrete Cracking Syndrome

   

• Steel-Fiber Shotcrete in Mines

   

• The End of the Backfill Problem

   

• The Concrete Manual

   

• Curling of Floor Slabs

   

• Control of Strength During Slipforming in Winter

   

• The Outlook for Concrete Paving

   

• Some Highlights from the First International Congress on Polymer Concrete

   

• Wrapping It Up with Nylon Fabric Forms

   

• Site-Precast Concrete to the Rescue

   

• Wax Seals New Bridge Decks Internally

   

• Chemical Attack on Hardened Concrete

   

• Lessons from a Tornado

   

• Concrete or Asphalt

   

• Some Economics of Precasting on Site Instead of in the Factory

   

• A New Wrinkle -- Shotcrete Containing Steel Fibers

   

• What Is Level?

   

• Picasso's "The Bather"

   

• Stop Guessing at Reshore Loads -- Measure Them

   

• A Progress Report on Concrete Reinforced with Steel Fibers

   

• Preventing D-Cracking in Pavements and Flatwork

   

• Vacuum Dewatering Procedures

  Traditional concrete slab-finishing practice entails an unavoidable wait for the concrete to stiffen sufficiently before the first floating operation can take place. This delay often entails considerable overtime.

   

• Cutting Costs by Reducing Overhead

   

• How to Avoid Problems with Lightweight Mixes

   

• How to Avoid Problems with Pumping Equipment

   

• Realistic Calculations of Costs for Wall Formwork Systems

   

PROBLEM CLINIC

• Curing sawed joints

  Do sawed joints require reapplication of curing compound after sawing?

   

• Durability of sawed joints

  Are sawed joints more susceptible to salt damage than the rest of the concrete? Sawed joints tend to be raveled. Does this do significant damage to the concrete at joints?

   

• Avoiding alkali burns from fresh mortar or concrete

  Has any experience been noted as to the cause of cement burns suffered by laborers or masons? I would also like to know how or why this occurs and the methods to be followed to minimize this rare occurrence.

   

• Concrete in pipeline between deliveries

  What do you do between deliveries of concrete on a pumping job? Do you clean the pipeline and waste the concrete in it, or do you save the old concrete in the line and the hopper to use with the next delivery? If you save it you're mixing concrete that has a lower slump or is nearly ready to set...

   

• Pumping may increase compressive strength

  How much higher design strength do we have to ask for if we decide to pump a 4000 psi concrete mix?

   

• Slow building

  What is the longest period of construction for a modern building?

   

• Lightning protection system built into structure

  If lightning protection is to be used in a structure is it normally installed during construction or added after the building is completed?

   

• Lightning damage to concrete

  Do concrete structures ever suffer seriously by being struck by lightning?

   

• Unwashed ocean beach sand can corrode reinforcement

  Is there likely to be any difficulty in using unwashed ocean beach sand in reinforced concrete?

   

• Vibrating concrete under water

  In tremie concreting under water what is done about vibrating?

   

• Concreting on a slope under water

  Can tremie concrete be placed on a slope under water?

   

• Muddy mix water

  Our jobsite is located where we will have to set up our own batch plant and the only source of water is a stream that is muddy. Can we use this water in our mixes?

   

• Don't field-weld rebars

  Will it improve the strength of crossing rebars to tack weld them on the job?

   

• Effect of Age on Cement

  Our company has a small amount of cement that is 5 years old, but it has been stored in steel drums and seems to be perfectly dry. There are only a few lumps, which were probably caused by the cement settling and thereby packing in the drums. We can easil

   

• Successful Bar Bending

  What is the secret of bending rebars in the fabricating shop so that the radius is always correct and the bars are not twisted?

   

• Protect Against Unintentional Deicer Application

  We have a sidewalk that has been through one winter. It was built in August. The top has peeled off to a depth of about 1/8 inch in spots, some about the size of a 25-centpiece and some as big as 2 feet square. Our city uses a thrower to salt the streets. I wonder if the throw from this machine has...

   

• Thermal Coefficients of Concretes

  We've been told that the coefficient of thermal expansion of concrete is about 5.5 millionths per degree F but that it can be higher or lower. There wasn't any explanation of what changes it. What's the story?

   

• Air for workability in lightweight aggregate concrete

  Why should a lightweight aggregate concrete mix for an inside wall in a heated building be air-entrained?

   

• Removing reshoring

  What is the best rule of thumb on when it is safe to remove reshoring?

   

• Bundling of rebars

  How many rebars can be bundled, and what must be done about splicing within the bundle?

   

• Pumping high density concrete

  Is it possible to place high density concrete by pumping? If so, are there any special methods or precautions needed?

   

• Number of test cylinders required

  How many test cylinders are required to be taken per day on a normal job?

   

• Fire endurance of prestressed beams

  What is to prevent the prestressing steel in a beam from losing its prestress during a fire ant causing an early, quick collapse?

   

• Expansive cement and shrinkage-compensating cement

  What is the difference between shrinkage-compensating cement and expansive cement?

   

• Curing within an enclosure using steam

  We plan to place concrete during the coming winter and are wondering about steam curing. is there danger of cracking the concrete if we build an enclosure around the beams and introduce live steam?

   

• Causes of changes in air content

  On a recent job we had excessive fluctuations in air content. The mix contained 490 pounds of cement per cubic yard and was made with 1-inch maximum size aggregate. The air was required to be between 3.5 and 6.5 percent, but sometimes it was considerably higher and sometimes a little lower. What...

   

• Don't remix an old mix with a new one

  Could a mix of concrete that is too old (2 hours or older) be mixed with a fresh load of the same mix design?

   

• Rubber void formers shorten when inflated

  If we use pneumatic tubes to form voids how much extra length should we allow?

   

• Frost protection from vacuum treatment

  Does vacuum treatment remove enough water from a floor so that it is adequately protected against early frost damage without supplying heat from salamanders or providing other winter protection?

   

• Concrete for stairways

  We had trouble placing a stairway with some leftover concrete. What is a good mix for stairways?

   

• Reducing popouts from alkali-aggregate reaction

  Popouts from alkali-silica reaction are common in our area, but worse on some jobs than others. Besides using low-alkali cement what can we do to minimize the problem?

   

• Admixtures and Additions

  What is the difference between an admixture and an addition?

   

• Driving dowels into stiff concrete

  Is it all right to stick or drive dowels into concrete that has been placed 60 to 90 minutes earlier when the temperature is about 70 degrees F?

   

• Age of mix being placed doesn't in itself predetermine cold joint

  Can a mix that is more than two hours old be placed without producing a cold joint?

   

• One-way and two-way slabs

  What is meant by a one-way and a two-way slab?

   

• Special trowels for white concrete

  How can we avoid trowel burns on hard-troweled concrete made with white cement?

   

• Preventing Curling of Floor Slabs

  What is a reliable method of preventing curling in floor slabs?

   

• Finding the rebars

  Does anyone make any kind of gadget for locating reinforcing steel in hardened concrete?

   

• Building a segmented prestressed bridge

  Where can I get information about bridge construction by post-tensioning a series of segmented boxes?

   

• Cement for nuclear reactors

  Must Type II cement be used for all prestressed concrete reactor vessels?

   

• Predicting fire resistance

  Is there any way to estimate fire endurance of floors or to design for it without running an actual test?

   

• Response to earthquakes

  What kind of motion is it that an earthquake imposes on a structure?

   

• Repairing ferro-cement hull damage

  Is it satisfactory to use standard ferro-cement methods to repair a hole in a ferro-cement boat?

   

• Cost of ferro-cement vessels

  Is it substantially cheaper to build a yacht out of ferro-cement than conventional materials?

   

• Choice of lightweight aggregate

  What is the best kind of lightweight aggregate to use for a loadbearing wall?

   

• Freeze-thaw resistance of epoxy adhesives

  If we use an epoxy adhesive will it resist freezing and thawing as well as the concrete?

   

• Slopes achieved with conveyors

  How steep a slope can we figure on using with a concrete conveyor?

   

• Depth of folded plate roof

  How deep a fold is needed for a folded plate roof that will span 120 feet?

   

• Proper time to remove inflatable void former

  How soon can rubber void formers be removed?

   

• Standard mix designs

  Where can we obtain a set of standard mix designs for a given strength? Our testing laboratories are frequently asked for mix designs on such short notice that it does not allow time to calculate them.

   

• Wet mix probably can't be salvaged by remixing

  Would it be all right to mix a load of concrete that is too wet (7-inch slump or wetter) with a dry load of the same mix design to obtain concrete of allowable slump (under 6 inches)?

   

• Construction loads

  Is it structurally safe to store supplies on a shored floor?

   

• Rock and soil anchors

  Where can I get information on the proper construction of prestressed soil anchors?

   

• White floors light up the room

  We'd like to put in a white concrete floor to improve the lighting in an assembly plant. How fast will the beneficial effect on lighting disappear as the floor gets older and duller?

   

• Soniscope uses

  Is the soniscope used to evaluate the strength of concrete or is it used to find out whether it has been weakened by cracking?

   

• Bracing for precast panels

  We are having trouble figuring the requirements for bracing for some factory precast walls because we don't know how much to allow for wind loads or the possibility of impacts. Can you give us a formula or guide?

   

• Is aluminum always aluminum?

  If aluminum pipe is so bad for pumping because it generates so much gas in the concrete, and if aluminum powder is purposely used to produce gas in some grout, why is it permissible to use aluminum forms? Don't they cause trouble by making gas bubbles on the surface?

   

• Changes in concrete at high temperatures

  What happens to concrete during exposure to various temperatures up to 1,000 degrees F?

   

• Self desiccation

  What is meant by the term "self-desiccation"?

   

• Reuse of form liners

  What is a reasonable number of reuses for plastic and elastomeric form liners?

   

• Meaning of "laitance"

  In the article on sealing joints of tilt-up panels (April 1974, page 171) the word "laitance" is used. This word has frequently appeared in our concrete specifications over the years but we have never been able to find it in our dictionary. What is a good definition?