Ask the Dot

Check back weekly for updated and additional content.  Browse our Ask the Dot questions for help with your projects and research.  If you don't see what you're looking for, let us know, we'll be happy to help you any way we can. 

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Q: Are there standards for polishing concrete, and what is the process?

There are no published standards for polished concrete, but it is generally agreed that the concrete must be polished through the sequence of disks ending with 1800-3500 grit diamonds to be considered polished concrete.  At this level the concrete will exhibit a glossy sheen and high reflectivity without the use of a topical coating. Polished concrete is not simply exposing the rock in the concrete mix then applying a sealer.  More...
 

Q: What are masonry cements composed of and what types are there?

Masonry cements are hydraulic cements designed for use in mortar for masonry construction. They are composed of one or more of the following:
 

Portland cement, Portland cement – pozzolan cement, Portland blast – furnace slag cement, hydraulic lime, and natural cement. In addition, usually contain materials such as hydrated lime, limestone, chalk, calcareous shell, talc, slag, or clay.  More...
 

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Q: What are the best methods for cleaning concrete, and how do I know which

method to use?

Concrete surfaces are not always uniform in color when forms are removed; they may have a somewhat blotchy appearance and there may be a slight film of form-release agent in certain areas. There may be mortar stains from leaky forms or there may be rust stains.  More...

 

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Q: What is concrete yield and how do I avoid concrete yield discrepancies?

Concrete yield is defined as the volume of freshly mixed concrete from a known quantity of ingredients. Ready mixed concrete is sold on the basis of the volume of fresh, unhardened concrete‐in cubic yards or cubic meters as discharged from a truck mixer. The volume of freshly mixed and unhardened concrete in a given batch is determined by dividing the total weight of the
materials by the average unit weight or density of the concrete determined in accordance with ASTM C‐138. Three unit weight tests must be made, More...

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Q: What causes concrete to bleed and what can be done to avoid problems?

After concrete is placed and before it begins to harden the aggregate and cement particles tend to sink, which causes water to appear on the surface of the concrete. The amount of bleeding is effected mostly by;  More...

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Q: What is Fly Ash, why is it useful, and what are the benefits?


Fly ash is a byproduct of coal-fired electric generator plants. The coal is pulverized and blown into a burning chamber where it ignites to heat boiler tubes. Heavier ash particles (bottom ash or slag) fall to the bottom of the burning chamber and the lighter ash particles (fly ash) remain suspended in the exhaust gases. It is the finely divided residue that results from the combustion of pulverized coal and is transported from the combustion chamber by exhaust gases. Before leaving the stack, these fly ash particles are removed by an electrostatic precipitator, or bag house.  More...

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Q: How can I accelerate concrete set time, and what is the difference between Calcium Chloride and Non-Chloride Accelerators?


Calcium Chloride is the most commonly used accelerating admixture and is used to accelerate strength development of concrete at an early age. The strength development of concrete can also be accelerated by (1) Using Type III Cement, (2) Lowering the water-cement ratio by increasing classification, or (3) Curing at higher temperatures (steam, applied heat, hot water)..  More...

Q: What is slump? How is it tested? And what are its effects?


Slump is the workability, consistency, and the plasticity of concrete.  Workability is a measure of how easy or difficult it is to place, consolidate, and finish concrete. Consistency is the ability of freshly mixed concrete to flow. And Plasticity determines concrete’s ease of molding.  More...

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Q: Why is it called Portland Cement, and how is it processed?


The invention of Portland cement is generally credited to Joseph Aspdin, an English mason. In 1824, he obtained a patent for his product, which he named Portland cement because it produced a concrete that resembled the color of the natural limestone quarried on the Isle of Portland, a peninsula in the English Channel. The name has endured and is used throughout the world, with many manufacturers adding their own trade and brand names.  More...

Q: What causes concrete to bleed and what can be done to avoid problems?  How do I know when it's time to start the finishing process?


After concrete is placed and before it begins to harden the aggregate and cement particles tend to sink, which causes water to appear on the surface of the concrete. The amount of bleeding is effected mostly by;  More...

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Q: What is the Standard Test Method for Sampling and Testing Grout?


ASTM C‐1019 covers sampling and testing of masonry grout. Lab tests are performed to establish material proportions for compliance with project specifications. Field tests are done to monitor uniformity of grout during construction for quality assurance. More...

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Q: What is the purpose of consolidating concrete and what part do mechanical vibrators play in this process?


Consolidation eliminates rock pockets and air bubbles and brings enough fine material both to the surface and against the forms to produce the desired finish.  You can use such hand tools as spades, puddling sticks, or tampers, but mechanical vibrators are best. Any compacting device must reach the bottom of the form and be small enough to pass between reinforcing bars. The process involves carefully working around all reinforcing steel with the compacting device to assure proper embedding of reinforcing steel in the concrete. Since the strength of the concrete member depends on proper reinforcement location, be careful not to displace the reinforcing steel.  More...

Q: What is a Super Flat Floor, how is it produced, and how is it measured?


By definition, Super flat =F min 100. In order to achieve this high degree of flatness/levelness, Super flat floors must be placed in long narrow strips. The forms for Super flat placements must be set with great precision. Specialized finishing techniques and continuous quality control measurements are also required. The services of a floor design/construction consultant are usually advisable. More...

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Q: Why is curling so detrimental, especially on floors with a high volume of forklift traffic?


Curling raises the slabs edges and corners.  This usually causes the slab to react in one of two ways.  The slab either rocks back and forth or breaks in the center, neither of which is good.  More...

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Q: What is a screed, and what is its roll in placing concrete?
 

​To remove concrete in excess of that which is required to fill the formevenly or bring the surface to grade, performed with a straight-edged pieceof wood or metal by means of a forward sawing movement or by a poweroperatedtool appropriate for this purpose. The name applied to the toolused to fill the form evenly.  More...

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Q: ACI 318, How are Strength Evaluations done on existing structures?
 

Strength considerations related to axial load, flexure, and combined axial load and flexure are well understood. There are reliable theories relating strength and short term displacement to load in terms of dimensional and material data for the structure.

 

To determine the strength of the structure by analysis, calculations should be based on data gathered on the actual dimensions of the structure, properties of the materials in place, and all pertinent details.  More...

 

Q: What is the Difference Between an Isolation Joint and an Expansion Joint?
 

Isolation and expansion joints accommodate anticipated differential horizontal and vertical movements that occur between a pavement and a structure. Their purpose is to allow movement without damaging adjacent structures. More...

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Q: What is Plastic Shrinkage Cracking? Why Does it Occur? And How Can I Minimize its Effects?
 

Plastic Shrinkage Cracks appear in the surface of fresh concrete soon after it is placed and while it is still plastic. These cracks appear mostly on horizontal surfaces. They are usually parallel to each other on the order of 1 to 3 feet apart, relatively shallow, and generally do not intersect the
perimeter of the slab. More...

 

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Q: What is the difference between "maximum size" and "nominal maximum size" when discussing aggregates?
 

The “maximum size” is the smallest sleve opening which all of the aggregates will pass.  The “nominal maximum size” is the smallest sleve opening in which of most of the aggregate will pass. More...

 

 

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Q: What is the corrosion of steel and why it is a concern? Why does steel in concrete corrode and how can it be prevented?
 

ASTM terminology defines corrosion as “the chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of material and its properties”.  For steel embedded in concrete, corrosion results in the formation of rust which has two to four times the volume of original steel and none of its good mechanical properties.  Corrosion also produces pits or holes in the surfaces of reinforcing steel, reducing strength capacity as a result of the reduced cross-sectional area.  More...

 

 

 

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Q: What Does ACI (American Concrete Institute) Call the Most Abused Tool in the Industry?
 

A jitterbug.  A jitterbug or grate tamper is sometimes used on slabs or grade, or exterior paving to push down the coarse aggregate particles below the surface.  More...
 

 

 

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Q: What is the Difference Between a “Small-Rock-Pump” Mix and a “Small-Rock” Mix?
 

Basically the small-rock-pump mixes have more sand and less #8 coarse aggregate compared to the standard small-rock mixes.  This just allows for the concrete to go through a small hose easier.  Because of the extra surface area generated by the increased sand, extra cement and fly ash is required.  More...
 

 

 

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Q: What tends to crack more, monolithic slabs or traditional slabs with stem walls?
 

There are two basic reasons why concrete cracks, either by an external applied force or a change in volume.  More...
 

 

 

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Q: A contractor had been trying to place a driveway for days.  However, the pour kept being delayed due to rain. Finally, a sunny day occurred and the concrete was placed.  Unfortunately something went wrong.  Several soft spots occurred throughout the driveway.  Some of the soft spots were so soft that a screwdriver could easily be pushed through.  What went wrong?
 

The first clue is the rainy weather for days before the pour.  The sub-grade became wet and mucky.  As the finishers screeded the concrete, their boots pulled up muck from the sub-grade and contaminated the concrete as the finishers walked around.  More...

 

 

 

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Q: What is “false set” and “flash set”? Why is it critical that ready-mix drivers be aware of both?
 

Early stiffening includes both false set and flash set, and is the early development of rigity in the work-ability or plasticity of cement paste, mortar, or concrete.  More...

 

 

 

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Q: What are fine and coarse aggregate and what role do they play in ready-mixed concrete?
 

Hydraulic cement concrete is a cement and water paste in which aggregate particles are embedded.  Aggregate is granular material such as sand (fine aggregate), graven/crushed stone (coarse aggregate) and blast furnace slag that usually occupied about 75% of the concrete’s volume.  Besides reducing volume changes due to drying shrinkage of the cement paste, aggregates are a somewhat inexpensive filler that reduces the cost of the concrete.  More...


 

 

 

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Q: How Does Cracking Happen and How Can It Be Minimized?
 

Cracks in concrete are unsightly and when it does happen the first reaction is, something is wrong with the concrete. But, why does concrete to crack? There are many reason why but, concrete cracks due to internal stress created from the hydration of the concrete. More... 

 

 

 

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Q: At Typical Dosage Rates, Fibers Do Not Increase the Compressive Strength or the Flexural Strength of the Concrete. So What Benefits Do Fibers Provide?
 

The addition of fibers brings benefits to both plastic and hardened concrete.When the concrete is still in its plastic state, fibers provided an internal support system for the concrete. It has been likened to a “bird’s nest” supporting the coarse aggregate throughout the concrete. The fibers also improve the cohesiveness of the concrete, reducing the tendency of the coarse aggregate to segregate. If you look at a cross-section of fiber-reinforced concrete, you can observe uniform distribution of the coarse aggregate, especially near the top surface. More...


 

 

 

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