FAQ - Frequently Asked Questions

Concrete Floor Polishing, Sealing, Grinding & Blasting FAQs

What types of grinding / blasting equipment do you use?

How slippery is polished concrete?

What are the requirements for concrete intended to be polished?

Can lightweight / air-entrained concrete be polished?

How do I calculate the quantity of concrete for the project?

Are trowel marks accepted for finished concrete floors?

What is the the maintenance cost of polished concrete?

How long will polished floors last and what is the warranty on polished concrete?

How deep do you grind concrete?

How clean is shot blasting profile and what is producton speed?

Why is the polished concrete floor NOT to expectation?

Why does concrete crack?

Why control joints are needed and how are they installed correctly?

What is the difference between a control joint and an expansion joint?

What is the difference between epoxy coating and paint application?

 

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What type of grinding/polishing equipment do you use?


Concrete Polishing & Sealing employ different machines and various tools to complete the service in the most appropriate fashion in accordance with job requirements. The workhorses of our industrial production line are robust HTC-800 HD Classic floor grinders. They are extremely effective for industrial and commercial projects of 3,000 FT2 and up.


HTC-800 HD Classic floor grinder

HTC-800 HD Classic floor grinder specifications

 


The most versatile floor preparation machine used for industrial floors and parking garages, warehouses and factories, roads and bridges, airports and supermarkets - the self-propelled Blastrac 10DPS75 is an environmentally sound choice for any surface preparation.

self propelled Blastrac 10DPS75 floor grinder

self propelled Blastrac 10DPS75 floor grinder specifications

 


For light industrial work we use the versatile HTC-500 machines which are an excellent choice for the small areas of 500 to 3,000 FT2

HTC-500 floor grinder

HTC-500 floor grinder specifications


How slippery is polished concrete?

Slip resistance is a concern with many floor systems, but particularly for concrete floors. Risk of injury is one of the biggest drawbacks to concrete. Intuitively, one might suspect that a highly polished concrete floor would become highly slippery-and thus more of a problem. It turns out that's not the case.


Contrary to a common perception, the degree of polish or shine is not directly related to slip resistance. A 400-grit finish usually is less slip-resistant than a higher 1500 or 3000-grit final finish. It helps to think of it in this way: when a floor is wet and a person walks on it, the peaks and valleys in the landscape of the surface create a tendency for the person to “hydroplane.” On a completely flat surface such as that produced by 1500 or 3000 grit, the water is pressed out from under the shoe, putting the sole directly in contact with the concrete surface— actually making the shoe stick to the surface. A true grinding and polishing system with the highest levels of shine (using 3000-grit polishing) and gloss (gloss meter readings in the range of 45 to 65) exceeds OSHA (Occupational Safety and Health Administration) and Americans with Disabilities Act (ADA) standards for coefficient of friction and slip resistance, which are the two most widely accepted standards for these safety criteria.

The process creates an attractive environment with increased light reflectivity, a desirable characteristic in today’s safety-conscious marketplace.
Standard coefficient of friction (COF) numbers for an 800-grit or semi-gloss polished-concrete surface will range from 0.79 to 0.84, a 1500-grit or gloss finish will yield a COF of 0.84 to 0.87, and a 3000-grit or high gloss finish will yield a COF of 0.87 to 0.89. These numbers all exceed the OSHA standard of 0.50 and the ADA standard of 0.60 on flat surfaces. The flatter the floor, the higher the standard of coefficient of friction.


Our polishing system is chosen and certified by NFSI as a high traction and considered to be very safe.


What are the requirements for concrete intended to be polished?

Requirements for concrete to be polished as follows:

1. Minimum concrete compressive strength of 23 MPa (3500 psi).
2. Normal weight concrete and no lightweight aggregate.
3. Preferrable non-air entrained. (Still can be successfully polished).
4. Flatness requirements: ASTM International E-1155
5. Tight hard troweled (three passes) concrete.
6. No burn marks
7. Curing options (ASTM International C-309, C-171) - no acrylic curing and sealing compound

8. Admixtures may be used

Concrete must dry out for at least 28 days after installation.


Can Lightweight / Air-Entrained concrete be polished?

Lightweight structural concrete is actually so because of the aggregate, and not the cement. Various aggregate recipes use various light weight materials to reduce the weight of concrete by as much as 40%. Lightweight concrete also provides greater fire resistance and greater R-values than normal weight concrete, in part due to air entrainment, which also allows the internal retention of more water, which enhances internal curing and hence, the strength and durability of the concrete.

Air entrainment principally is recommended to improve resistance to freezing when exposed to water and de-icing chemicals. However, there are other important benefits of entrained air in both freshly mixed and hardened concrete. Air-entrained concrete contains billions of microscopic air cells. These relieve internal pressure on the concrete by providing tiny chambers for the expansion of water when it freezes.

The amount of entrained air is usually between 5 percent and 8 percent of the volume of the concrete, but may be varied as required by special conditions. The use of air-entraining agents results in concrete that is highly resistant to severe frost action and cycles of wetting and drying or freezing and thawing and has a high degree of workability and durability.

If polished, lightweight concrete would require the same finishing approach as the regular concrete, i.e. grinding, densifying, and lithium coatings.
However, some limitations to the finished result are to be seriously considered.
The water retained by the air entrained concrete and the more porous, lightweight aggregates also greatly increases the cure time for fully dry material. The air entrainment also causes the concrete to exhibit a more porous surface after grinding, which can cause irregular results in finish applications. The more porous aggregates, such as shale, will often fracture during grinding. These imperfections can cause inconsistency in the way the concrete accepts applied finishes.
Industry standard does not completely rule out the polishing the surface of lightweight concrete, nevertheless, customers might not get the type of finish they were expecting.


Are trowel marks accepted for finished concrete floors?

Some specifications require concrete contractors to produce floor surfaces that are free of trowel marks. However, the specifications don’t define the term “trowel marks.”

Both ACI 301-99, “Specifications for Structural Concrete for Buildings,” and ACI 302.1R-96, “Guide for Concrete Floor and Slab Construction,” use the term “trowel marks” (Section 5.3.4.2c in ACI 301 and Section 11.9 in ACI 302.1R). Neither document defines the term, nor is the meaning clear from the context in which the term is used. ACI 116R­00, “Cement and Concrete Terminology,” doesn’t include a definition of trowel marks.
Because differing interpretations of this term can cause problems with acceptance of the finished floor, the American Society of Concrete Contractors (ASCC) seeks to establish a consensus definition for trowel marks that is applicable during both the bidding phase and execution/acceptance of the finished floor. Establishing a common industry definition provides all parties with a fair and equitable ability to judge the acceptability of a finished floor surface.
To provide a clear and specific understanding, this position statement includes a definition and photo for two terms: “trowel pattern” and “trowel marks.”


Trowel pattern: A concrete surface feature–produced by troweling–that can be seen but can’t be felt (has no vertical profile).


Trowel marks: Concrete surface features–produced by troweling–that can be seen and felt (have a vertical profile).


ASCC concrete contractors will remove trowel marks from concrete surfaces by rubbing, grinding, or other appropriate methods. A trowel pattern is not considered to be a surface defect and will not be removed from concrete floors. If trowel patterns are unacceptable, the general contractor must make a specific requirement in the specification.


What is the maintenace cost of polished concrete?


Maintenace cost of Polished Concrete


How long will polished floors last and what is the warranty on polished concrete?

Hardness and wear resistance of any substance (including marble, concrete, limestone and terrazzo) depend on its molecular density, mineral composition and structural integrity.
During our polishing process the diamond segments mechanically close the "pores" of marble or concrete surface in a way, that the light rays reflecting from the polished floor returning back with less aberration on every progressively higher step. That is why polished surface, actually, shines. We do not change core "ingredients" of material being polished.

Therefore, the lifespan of polished surface (i.e. shiny look) directly related to 1). Material matrix structure: for marble - percent of calcite, for concrete - type of embedded aggregate and MPa, for terrazzo / agglomerates - epoxy component 2). Amount of traffic, type of traffic (dragging generator or stove over a polished floor is not a great idea) and maintenance program.
Above factors are not relevant to diamond polishing process applied.

Now, - how fast the surface will get scratched again? - It is in hands of its owner.


How deep do you grind concrete?

The thickness of the concrete floor that is removed depends on the condition of the slab and the needs and preferences of the client. With older slabs, or where a terrazzo look is desired (with aggregate exposed), it may be necessary to remove as much as 1?4 inch (6 mm). More commonly, a much thinner layer is removed (about 1/16 inch, 1.6 mm); this does not extend into the concrete aggregate, but only into the hardened cement fines or paste.
With new concrete, it is possible to obtain various appearances by adding colored aggregate (glass cullet, bits of metal, colored stone, etc.), by "seeding" the surface of the slab with additional aggregate before the concrete sets, and by adding integral pigments or other ingredients to the concrete. Various tips, such as using a low-slump mix (to keep aggregate near the surface) and avoiding walking through the wet concrete (which can push the aggregate down and result in noticeable pattern differences in the polished surface), will help to ensure an attractive finish.


How clean is shot blasting profile and what is production speed?




Why is the polished concrete floor NOT to expectation?

Before concrete gets ready for polishing it had been fabricated by a ready-mix concrete supplier and installed by third-party finishing company. It is critical to choose the right specification for would-be polished concrete and even more important to employ the reliable and diligent installer.

These are just a few examples of installation defects, improperly finished, due to lack of knowledge and negligence towards concrete surfaces which intended to be spotless and consistent.




















Why does concrete crack?

Shrinkage is a main cause of cracking in concrete slabs. As the concrete hardens and dries, it shrinks due to the evaporation of excess mixing water. The wetter or soupier the concrete mix, the greater the shrinkage will be and the greater the likelihood for cracks to develop. Using concrete with a low water-cement ratio and not adding excess water at the jobsite will help to keep shrinkage in check.
Even if your floor does crack, there are a number of remedies available for repairing the damage.
We inject large cracks with a resin, such as an epoxy or polyurethane, or fill smaller cracks with a concrete caulk or patching compound.


Why control joints are needed and how are they installed correctly?

Contraction/control joints are placed in concrete slabs to control random cracking. A fresh concrete mixture is a plastic (fluid) mass that can be molded into virtually any shape, but as the material hardens there is a reduction in volume (shrinkage). When shrinkage is restrained by contact with supporting soils, granular fill, adjoining structures, or reinforcement within the concrete, tensile stresses develop within the concrete section. While concrete is very strong in compression the tensile strength is only 8% to 12% of the compressive strength. In effect, tensile stresses act against the weakest property of the concrete material. The result is cracking of the concrete.

There are two basic strategies to control cracking for good overall structural behavior. One method is to provide steel reinforcement in the slab which holds random cracks tightly. When cracks are held tightly or remain small, the aggregate particles on the faces of a crack interlock thus providing load transfer across the crack. It is important to recognize that using steel reinforcement in a concrete slab actually increases the potential for the occurrence of random hairline cracks in the exposed surface of the concrete.

The most widely used method to control random cracking in concrete slabs is to place contraction/control joints in the concrete surface at predetermined locations to create weakened planes where the concrete can crack in a straight line. This produces an aesthetically pleasing appearance since the crack takes place below the finished concrete surface. The concrete has still cracked which is normal behavior, but the absence of random cracks at the concrete surface gives the appearance of an un-cracked section.
Concrete slabs-on-ground have consistently performed very well when the following considerations are addressed. The soils or granular fill supporting the slab in service must be either undisturbed soil or well compacted. In addition, contraction joints should be placed to produce panels that are as square as possible and never exceeding a length to width ratio of 1 ½ to 1 (Figure 1). Joints are commonly spaced at distances equal to 24 to 30 times the slab thickness. Joint spacing that is greater than 15 ft. require the use of load transfer devices (dowels or diamond plates).


Figure 1a: Joint Spacing in Meters


Figure 1b: Joint Spacing in Feet

Contraction joints may be tooled into the concrete surface at the time of placement. Joints may be tooled into the surface (first pass) prior to the onset of bleeding or immediately with the first pass of the floating operation. The longer the first pass for jointing is delayed the more difficult it will be to shape clean straight line joints. Tooled joints should be re-established with each successive pass of finishing operations.

Joints may also be sawed into the hardened concrete surface. It is important to understand that the longer sawing is delayed the higher the potential for cracks to establish themselves before sawing is complete. This means that any cracks that occur before the concrete is sawed will render the sawed joint ineffective. Timing is very important. Joints should be sawed as soon as the concrete will withstand the energy of sawing without raveling or dislodging aggregate particles. For most concrete mixtures, this means sawing should be completed within the first 6 to 18 hours and never delay more than 24 hours. Early-entry saws are available which may allow cutting to begin within a few hours after placement.
Contraction/control joints must be established to a depth of ¼ the slab thickness (Figure 2). Proper joint spacing and depth are essential to effective control of random cracking.


Figure 2: Minimum Depth of Contraction Joints




What is the difference between a control joint and an expansion joint?

A control joint is an intentionally weakened break in a concrete surface to allow for contraction stress. These joints are most often seen in concrete slabs. Often these joints are cut after the concrete is poured, using a concrete saw. The concrete is generally scored only, not cut all the way through. This allows the concrete to naturally crack at a joint and not elsewhere in the slab. Control joints are often constructed to transfer lateral loads across the joint.
Expansion joints, on the other hand, are true structural joints separating different sections of the building typically used to accommodate thermal and moisture expansion in concrete. An expansion joint is a joint, left completely free of mortar and filled with elastomeric sealant to keep it watertight.


What is the difference between epoxy coating and paint application?

An epoxy floor is a coating applied over bare (or prepared) concrete that provides an attractive, durable and long lasting finish.
Epoxy is not paint, but a two-part material consisting of a 100% solid resin epoxy and hardener. Epoxy is blended just prior to application, and it quickly bonds using a thermosetting reaction.
Unlike paint, this bonding provides a highly durable material which lasts 4-5 times longer than conventional paint.


How do I calculate the quantity of concrete for the project?

Use this handy tool to calculate concrete yards for your project. It will also tell you how many bags of premixed concrete are needed. When pouring concrete, you should always order at least 10% more than the exact amount the project calls for. Just take the cubic yardage figure and multiply it by 1.10. It's better to have too much versus running short. It's a real pain if you know what you run out and have to order more. You will probably get charged extra delivery fees and fuel charges. So save yourself some money by making sure you have enough.



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Also, keep in mind that most ready mix companies will deliver a minimum of 1 yard and will take orders in 1/4 yard increments above the 1 yard. As an example, if you have a 20x20 slab that's 4 inches deep, the calculator will show 4.94 cubic yards needed. Then add 10% and you get 5.4 yards. Using the closest 1/4 yard increment, you want to order 5 1/2 yards. This is assuming that the subgrade is consistently 4 inches thick or as close as possible.