October 1978 Table of Contents

Problem Clinic Gap-Graded Concrete—Pros and Cons

We are bidding a project which calls for gap-graded mixes. What are the advantages and disadvantages? Read more

Problem Clinic Gap-Graded Mixes Require More Work

In spite of the lower cement content we pay about the same for gap-graded concrete as for continuously graded mixes. We use it in architectural concrete work. It costs us a lot more to handle and place it, however, so our total costs are higher. We don't Read more

Problem Clinic Gap-Grading Good for Prestressing Too

Some prestressed concrete girders will have an architectural sandblasted finish. For this reason they will be made from a gap-graded mix. What will this mix design do to the prestress loss from creep and shrinkage of the prestressed girders? Read more

Problem Clinic Type II Cement Useful for Architectural Concrete

When we are particularly concerned about getting concrete to flow well around the steel and to respond satisfactorily to vibration we would like to do whatever we can to minimize loss of slump. Is it true that Type II cement can be used for architectural Read more

Problem Clinic Uniform Color With Buff Cement

Do buff cements produce better colors than a mixture of pigment with white or gray cement? Read more

Problem Clinic Choosing Between Buff Cement and Colored Aggregate

Is it more effective to achieve a light brown or tan color with brown or tan aggregate or with a buff cement? Read more

Problem Clinic White Cement in Colored Concrete

It has been suggested that we use white cement in the mix for some architectural concrete work. Why white cement? The concrete will be colored with pigment. Read more

Problem Clinic Granite-Colored Concrete

How do you make a granite facing mix for precast panels? We plan to use a granite coarse aggregate which is predominantly white but which contains flecks of black. We've made trial mixes with white cement and with gray cement, as well as with mixtures of Read more

Problem Clinic Rustic Finish

Is there an inexpensive and easy way of giving a coarse-textured, boardmarked finish a reddish-brown hue so that the color tint stays in the hollows only--in other words to give a natural, rustic effect without providing an artificial painted or stained a Read more

Problem Clinic Black Exposed Aggregate Finish

In the first floor area of an office building I would like to place battered base shear walls with an exposed aggregate finish utilizing local gray aggregate, and would also like to integrally color the mix so as to produce a color that is at least dark o Read more

Problem Clinic Avoid Smooth Surfaces on White Concrete

What is the best way to get a smooth surface on white concrete? Read more

Problem Clinic Antique Finish for Concrete

How would you go about producing an antique finish on concrete? Read more

Problem Clinic Getting Uniformly Black Flatwork

I recently examined a one-month-old black-pigmented slab on grade that was uniformly light gray except for a few small dark gray areas. These dark areas were less than one square yard in size. I scrubbed the surface of the concrete with acid using a clean Read more

Problem Clinic Stripping Time Affects Color

Does the time when the forms are stripped have any effect on the color of the concrete? As nearly as we can tell, all the mix proportions were the same. Read more

Problem Clinic Stiffer Forms for Architectural Concrete

A set of specifications requires stiffer forms for the exterior surface of an architectural concrete wall than the interior. Is this common, and if so, why? Read more

Problem Clinic Grade B-B Plywood for Architectural Surfaces?

Is B-B plywood (oil-impregnated construction grade plywood) suitable for use in casting architectural concrete? Read more

Problem Clinic Plywood for Forms

We recently acquired a load of cheap, expendable plywood for use as form liner material. But because the plywood is intreated, it has been suggested that wood sugars could be present on the surface and act as a retarder. Is this conceivable? Read more

Problem Clinic Foamed Plastic Handy for Recesses and Blockouts

Is there a simple way to make recesses in concrete without using wood inserts that are difficult to remove? Read more

Problem Clinic Fiber-Glass-Reinforced Plastics Seldom Discolor Concrete

Because of experience with disruption from alkali-silica reaction with the aggregates in our area (unless necessary precautions are taken), I always feel a little bit nervous about contact of portland cement with anything that can set this reaction going, Read more

Problem Clinic Uniform, Neat, Sharp Surfaces and Edges

How did the contractor get such good surfaces on the Morningside House in New York City, the 1975 Construction Industries Board award winner? Read more

Problem Clinic Form Release Agents for White Concrete

What form release agents should be used with white concrete? Read more

Problem Clinic Form Stripping

We are using white pine ribs attached to 3/4-inch plywood to form a fluted wall. The ribs tend to swell and make it virtually impossible to strip the forms without severely damaging them. Is there anything you can recommend? Read more

Problem Clinic Wood Swells

If we embed decorative wood blocks in a concrete wall, will we have any difficulty with weathering of the blocks or the concrete? Should we treat the concrete or the wood to keep moisture out? Read more

Problem Clinic Textures and Patterns

How are intricate textures and patterns reproduced on architectural concrete surfaces? Read more

Problem Clinic Bonding the Form Liner

Should architectural form liners be bonded tightly to the inside of the form? Read more

Problem Clinic Best to Use New Material as Absorbent Liner

What causes the patchy and variable appearance of the concrete surface we're getting from absorbent form liners? Read more

Problem Clinic Reuse of Form Liners

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

Problem Clinic Obscuring Joints Between Form Liners

How can I keep joints between form liners from showing in architectural concrete? Read more

Problem Clinic Making a Dimpled Surface

What kind of form liner is used to produce a dimpled surface? Read more

Problem Clinic Retarder Useful in Making Rope Finish

How can we produce a rope finish on tilt-up panels? Read more

Problem Clinic Form Liners for Rope Finish

Isn't there some way to simulate a rope finish with form liners and thus avoid the tedious work of inlaying individual lengths of rope, as well as the complicated stripping, salvaging and cleanup? Read more

Problem Clinic Rubber Matting Form Liners

Where can I obtain rubber matting form liners for concrete? Read more

Problem Clinic Release Agents for Rubber Form Liners

What kind of form release agents are needed on form liners made of rubber matting? We are using rubber carpeting for the texture and pattern needed. Read more

Problem Clinic Steel Interference in Architectural Concrete

We are having trouble obtaining a uniform surface on architectural concrete beams where there is not enough room for the vibrator to work. Should we go to a wetter mix? Read more

Problem Clinic Tying Rebars in Architectural Concrete.

Specifications call for 2 inches of concrete cover over reinforcing steel, and this seems like something that should be particularly observed to avoid either staining or spalling of architectural concrete. But what happens where the tie wire on the rebars Read more

Problem Clinic Chairs for Architectural Concrete

What is the best kind of chair for us to use in the architectural concrete ceiling of a breezeway--plastic or stainless steel? Read more

Problem Clinic Minimizing Cracks in Architectural Concrete

Can steel be used to minimize cracking in architectural concrete? Read more

Problem Clinic Protection of Steel When Sandblasting

Is there any special requirement for steel protection when sandblasting a concrete surface? Read more

Problem Clinic Obscure Joints By Using Rustication Strips

How do you treat joints in walls and beams for best appearance? Read more

Problem Clinic Materials Suitable for Rustication Strips

Is wood the only material used for rustication strips or are other, less absorptive materials used also? Read more

Problem Clinic Attaching Rustication Strips

How are rustication strips attached to the forms? Read more

Problem Clinic Keeping Tape in Place at Formwork Joints

How can we keep tape from being displaced at the joints during placing and vibrating? It is discouraging to have an otherwise good job messed up by wandering and wrinkled tape lines. Read more

Problem Clinic Objectionable Cracking in Rustications

We have placed some beams on an architectural concrete job with an attractive rustication pattern. However, cracks forming in the bottoms of the beams have detracted seriously from their appearance. What can be done to minimize their size when we do anoth Read more

Problem Clinic Handle Architectural Concrete With Care

Can regular transporting and handling equipment be used for placing architectural concrete? Read more

Problem Clinic Eliminating Lift Lines

Some buff colored wall surfaces are marred by horizontal, slightly wavy lines. The concrete above and below appears to be uniform, so there couldn't have been any differences in the mixes. The lines appear to mark the tops of successive lifts. How can the Read more

Problem Clinic Eliminating Air Bubbles

We have been experiencing increasing problems with air bubbles (bugholes)and pockets on the faces of concrete walls formed against standard plywood forms. These voids have caused the cost of sacking and patching to soar. Do you have any thoughts or inform Read more

Problem Clinic Avoiding Air Entrapment

The article titled "Can We Get Rid of Bugholes?" (Concrete Construction, June 1972, page 266) discusses how to get rid of most or all of the entrapped air when casting walls. Is there anything that can be done to cut down on the amount of air entrapped in Read more

Problem Clinic Surface Retarders for Exposing Aggregate

Is it practical to treat formwork so that surface concrete will not harden? Read more

Problem Clinic Information About Aggregate Transfer Method

For a special job it is desired to produce colored murals by using colored aggregate accurately positioned and embedded in the surface. The aggregate transfer method has been recommended. Where can I obtain more information about this method? Read more

Problem Clinic Exposing Very Large Aggregate

Rocks of more than ordinary size (about 6-inch-diameter) are to be exposed in a panel. What is the best method to use? Read more

Problem Clinic Age for Sandblasting

How old should concrete be at the time of sandblasting? Read more

Problem Clinic Depth of Sandblast

What is the probable indentation obtained by sandblasting the surface of 3000- to 3500-psi architectural concrete? Read more

Problem Clinic Waterblasting Strong Concrete

Is there a limit to the strength and age at which concrete can be successfully roughened by waterblasting? Read more

Problem Clinic Don't Work Bushhammer to the Very Edge

What can be done to prevent ragged corners when bushhammering? No matter how carefully we work it seems impossible to keep a reasonably uniform vertical line. Read more

Problem Clinic Fractured Concrete

We would like to produce a coarse, or broken texture on the columns of a bank building. A sandblasted surface wouldn't be coarse or rough enough, and we don't think casting against a special form liner would give the right effect. What do you suggest? Read more

Problem Clinic When to Remove Form Ties From Architectural Concrete

How much time can be allowed before removing form ties from architectural concrete? Read more

Problem Clinic Twisted Wire Form Ties Not Suitable

How do you protect twisted wire form ties from rusting, and what is the best way to obscure them so they don't detract from the architectural concrete surface? Read more

Problem Clinic Obscuring Tie Holes

A client would like to have broad expanses of concrete in which no tie holes can be seen. Is it possible to obscure tie holes? Read more

Problem Clinic Prevent Staining From Ties

Is it always necessary to plug tie holes or treat the tie ends? We would like to find some system or method that avoids these operations. Read more

Problem Clinic Materials for Plugging Tie Holes

What materials can be used for plugging tie holes? Read more

Problem Clinic Sealer for Sandblasted Architectural Surfaces

It has been suggested that we apply a silicone sealer after sandblasting architectural concrete. Does sandblasting make the surface so porous that a sealer is needed? Read more

Problem Clinic Concrete for Permanence—With a Little Help

Is it necessary to seal architectural concrete surfaces? We prefer the look of the untreated concrete. Read more

Problem Clinic Surface Conditions Suitable for Applying Latex Paints

How soon can we paint concrete walls with latex paint? Is any preliminary treatment necessary and is there a preferred way to apply these paints? Read more

Problem Clinic Boardmarked Finish

We recently completed a contract involving a considerable area of boardmarked concrete. The quality of the concrete is excellent but the color varies considerably from board to board. What's the probable cause and how can it be prevented? Read more

Problem Clinic Sack Rubbed Finish

When is sack rubbing of exposed concrete necessary and what is the recommended method? Read more

Problem Clinic Stains From Mill Scale

Will mill scale on steel forms affect the concrete finish? Read more

Problem Clinic Some Form Release Agents May Stain Concrete

We know that lumber can sometimes stain concrete. But what about form release agents? Our lumber does not appear to be responsible for some recent trouble we've had. Read more

Problem Clinic Aggregates May Discolor White Concrete

The most common aggregate available for an upcoming job is a rather dark brown. Will this work against us in making white concrete? We could substitute another aggregate in whole or in part. Does the coarse aggregate affect the color more than the fine ag Read more

Problem Clinic Avoiding Staple and Tie Marks on Concrete

How can we prevent repetition of the effect we got on a recent job where every staple and tie showed up as a mark on the concrete surface, even though we were very careful to keep them flush on the formwork? Read more

Problem Clinic Sand Streaking

The surfaces of some cast-in-place walls appear streaky. The streaks look like lines of sand particles. How can this be prevented? Read more

Problem Clinic Make Form Corners Tight

By following our usual forming practices we are painfully aware that the corners we are getting are not sharp enough on a decorative concrete job. Read more

Problem Clinic Whither Rubber Form Liners?

From time to time I see references to rubber form liners in CONCRETE CONSTRUCTION, but I can't locate a supplier. Can you help? Read more

Problem Clinic Wavy Honeycomb Lines

We have undulating lines of honeycomb on some walls. Is this because of improper vibration? Read more

Problem Clinic Dusting of Vertical Surface

We have just had a serious occurrence of dusting of a vertical concrete surface, even though the wall was very carefully and thoroughly cured. Could the form oil we used be at fault? How can the dusting surface be rectified? Read more

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