|
How
slippery the polished concrete is?
What
are the requirements for concrete intended to be polished?
Are
trowel marks accepted for finished concrete floors?
How
long polished floor will last and what is a warranty on polished
concrete?
Why
concrete cracks?
Why
control joints are needed and how to install them correctly?
What
is the difference between epoxy coating and paint application?
====================================================
How
slippery the polished concrete is?
Polished
concrete is a safe walking surface.
SCOF (Static Coefficient of Friction) test indicates finer grit
levels and higher gloss actually provide a very safe floor for foot
traffic, with SCOF increasing (i.e. becoming more slip-resistant)
with higher gloss levels.
The National
Floor Safety Institute (NFSI) is an independent tester of floors
including polished concrete. Its method, NFSI 101-A, Standard
for Evaluating High-traction Flooring Materials, Coatings, and Finishes,
tests treated surfaces twice:
- Once, soon after a floor is polished and densified; and
- 30 days later to evaluate the effects of normal facility traffic
of the original reading. Surfaces must register SCOF readings complying
with the Americans with Disabilities Act and Occupational
Safety and Health Administration (OSHA) recommendations at both
times under both wet and dry conditions.
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. Non-air entrained
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
be dry out for at least 28 days after installation.
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 specifier must
make a specific requirement in the specification.
How
long polished floor will last and what is a 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.
Why
concrete cracks?
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 to install them 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 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.
|
