Specifying the Concrete Slab to be Polished

It is typical for concrete, a suitable material for many types of flooring, to be unsuitable for polishing. Typically, specifiers and owners will wait until after the concrete contract is signed or even after it is laid to choose floor coverings or other treatments. This approach has worked well for most types of tiles or carpeting, but it is unsuitable for polished concrete.

Acknowledge the difference by separating it into two specification sections.

To ensure that a concrete slab is polished correctly, it is essential to alert the other design professionals that it must be specified differently. This, in turn, alerts the construction team that it must also be placed differently.

The most effective way to communicate this difference is to separate the concrete slab specifications into two sections:

03 30 00 - Cast-in-place concrete for concrete not designated as polished concrete, and

03 3X XX - Cast-in-place concrete for concrete defined as polished concrete.

It is important to cross-reference the concrete specification sections in 03 35 43 - Polished Concrete Finishing. When specifying a concrete slab, it is crucial to determine how to define properties, performance, or attributes that may not be measurable and what distinguishes a concrete slab to be polished.

Specifying the concrete slab when attributes are not measurable

Although polished concrete floor slabs have been in use for 15 years, there is still a need for adequate methods to determine all performance criteria. Therefore, some specifications focus on quality assurance (QA) and quality control (QC) measures.

Pre-qualification requirements

Contractors should qualify for the job by setting minimum requirements for the number of projects or years of placing and finishing concrete to be polished.

While it is imperative to specify the correct methods to ensure the concrete is polished correctly, it is also essential to choose the right products.

Supervisory personnel experience

The company's experience is necessary, and the supervisor must have a good track record. One should specify the experience level required at the project site.

Consider specifying the number of projects with similar size and scope. Although Section 03 30 00 requires an American Concrete Institute (ACI)-certified concrete flatwork finisher, more is needed. ACI certification only ensures expertise in finishing concrete surfaces but doesn't cover polishing concrete slabs. Therefore, it is advisable to require an ACI-certified finisher and someone experienced in polishing concrete slabs for concrete slabs that need to be polished.

Quality control plan

Requiring a quality control plan is an excellent way to ensure that the contractor fully understands the scope of the work and the impact of nuances in placing, finishing, and curing on the quality of the polished floor. For example, hand-finishing around penetrations, columns, or near the slab edges produces a different concrete surface density than a power trowel machine. This difference will become evident during the polishing process.

Pre-installation conference

Conducting a pre-installation conference before starting the concrete installation project is crucial. The conference should include experienced individuals such as the concrete contractor, ready-mix producer, polishing contractor, testing agency, construction manager, and representatives of the architect, engineer, slab consultant, and owner. The purpose of the meeting is to discuss the impact of concrete installation on the final polish and the proper design, detailing, and installation techniques required. This helps avoid misunderstandings and review project conditions significantly affecting the work. Developing a checklist to guide the meeting and take minutes for distribution among the attendees is recommended.

Mockups

Making a mockup using the same materials, methods, and personnel for a project is recommended to ensure its quality. A mockup of 100 square feet, including contraction joints and concrete from two trucks, would suffice. Larger mockups may only include some parts of the final slab and are less effective. It is essential to leave half of the mockup exposed and unpolished so that the work of both the concrete and polishing contractors can be evaluated.

It's important to polish the mockup and the slab to simulate the expected delay between floor placement and polishing. If the finished slab is hand-finished, the mockup should include this process. Additionally, a section of the mockup should be intentionally damaged and then repaired to assess the impact of the repair process on the polished surface.

Identifying the differences: Part 3–Execution

To ensure proper execution, it is essential to specify the differences in Part 1 – General, Part 2 – Products, and Part 3 – Execution. It is worth noting that the QA and QC provisions have already been discussed. To achieve good execution and obtain proper concrete for polishing, the following recommended specification requirements should be used in Part 3.

Preparation

Following ASTM E 1643 and the manufacturer's written instructions, install and maintain a sheet vapor retarder. Lap joints should be 150 mm (6 in.) and sealed with the tape recommended by the manufacturer.

The rationale is that moisture can negatively impact the polished surface appearance. If a vapor retarder is specified, the slab must be reinforced with steel or welded wire with a cross-sectional area of steel less than 10 percent of the concrete cross-sectional area. It should also be continuous below saw cuts to minimize dominant joint activation. Concrete slabs on vapor retarders may slip, which can cause some joints not to open, allowing concrete shrinkage shortening to accumulate at one wide dominant joint.

Placement

Depositing concrete in a continuous operation is recommended to ensure uniform distribution of coarse aggregates and a polished surface. Dumping the concrete into piles and then raking it out can result in coarse aggregates 'stacking,' causing non-uniform distribution and showing through the polished surface. Therefore, it is essential to deposit the concrete continuously to avoid the segregation of coarse aggregates.

To ensure a smooth surface for polishing, use a vibratory straightedge, such as a hand-held vibratory bull float, a vibrating screed, a truss screed, or a laser screed. Hand-screeding won't provide enough surface paste, so it's recommended to screed slab surfaces with a vibratory straightedge and strike off to correct elevations. This will ensure an adequate amount of paste at the surface for polishing.

When using a vibratory screed, make sure to keep it moving constantly across the slab while the vibrator is running. Leaving the screed in one place can cause too much vibration in that area and push the coarse aggregate deeper into the slab. This can lead to uneven exposure of the coarse aggregate after polishing. Non-uniform vibration can occur if the vibratory screed is left running without advancing or if the vibrating head of a self-propelled laser-guided screed is lowered onto the surface before the machine moves.

It is essential to avoid using a highway straightedge or bump cutter when finishing the concrete surface. This method can remove too much paste from high spots, leading to an uneven polished surface (Figure 1). To achieve flatness in polished concrete, it is recommended to use pan floats.

Finishing

It is crucial not to trowel over adjacent hardened concrete surfaces. Before each pass, inspect the trowel machines and remove any accumulated mortar. Scratching of the concrete surface can occur if troweling is done over adjacent hardened concrete from previous placements. Debris falling off an uncleaned trowel machine is one of the main causes of these scratches. These scratches can often be visible on the polished surface and can cause damage to the overall appearance of the concrete surface.

To achieve a polished surface, it is vital to use a machine trowel to smooth the slab's edges and get as close as possible to the walls and columns. This method should be preferred over hand-finishing as it results in a denser and harder surface, which is more suitable for polishing. The difference in surface densities between machine-troweling and hand-troweling is apparent in a polished surface (Figure 2).

The troweled surface should not have any trowel marks, burn marks, or mottling. These features can affect the final polished appearance and reduce its uniformity. Sometimes, specifiers require three trowel passes to control these effects. However, the number of trowel passes depends on various factors, such as the concrete mix design, weather conditions, and finishing equipment. Different finishing equipment, such as a walk-behind or riding power trowel, and the blade type—steel or plastic can affect the outcome. Therefore, specifying the number of passes may not consistently achieve the desired result.

The floor flatness (FF) must be achieved within 72 hours of testing, as per ASTM E 1155 Standard Test Method for Determining FF Floor Flatness and FL Floor Levelness Numbers. The required FF value depends on the Class of Grind specified for the polished surface. It is important to note that the appearance of the polished surface is directly affected by the floor's flatness. Therefore, measuring FF within 72 hours of testing and before the polishing contractor starts working on the surface is recommended. If the FF value decreases by more than 10 percent, the polishing contractor may request a change order to cover the extra work required.

In cases where a decrease in FF is anticipated, it is advisable to specify an FF value one step higher than what is specified for the Class of Grind. For example, if the concrete slab is set to have a FF of 50, but a Class B grind (FF 40) is chosen, this allows for a decrease in FF over time.

For large placements of 20,000-sf and above, an FF of 50 or above is recommended when laser screed, check rod, highway straightedge/bump cutter, and other floor flatness tools and techniques are available. However, for retail or light commercial areas, grocery stores with drains and penetrations, or school slabs that are broken into small placements, an FF of 35 or lower should be specified. This ensures that the polished surface meets the required level of flatness and appearance.

Joints

To control out-of-joint cracking and minimize curling, the maximum contraction joint spacing should not exceed 24 times the slab thickness or 15 ft, whichever is smaller. It's important to note that curling can expose more coarse aggregate at the corners during polishing. The spacing of the joints can be adjusted based on various factors, such as the concrete properties, environmental conditions, and materials used. The engineer should determine the maximum contraction joint spacing to minimize cracking and curling. Rather than providing the maximum contraction joint spacing (Figure 5), the preferred specification offers the contraction joint layout to incorporate the joints as desired in the polished concrete.

Form contraction joints using an early-entry power saw with a dry-cut blade to prevent out-of-joint cracking in concrete. It's essential to attach a vacuum to the saw to remove saw-cut residue, as this residue can cause staining on the concrete at the joint (Figure 6) and may even lead through the polished surface.

Curing

To cure concrete, three methods can be used: (1) Moisture Curing, (2) Moisture-Retaining-Cover Curing, or (3) Resin Curing Compound. Specifying a standard format for every project can be challenging due to the interaction between the curing method and the potential use of dyes, stains, and hardeners for the polished slab. Therefore, the curing method should be decided at the pre-installation conference. The final selection of the curing method will depend on the preference of the polishing contractor and the products to be utilized for the polished concrete.

Protection

Protecting the concrete slab according to the slab protection provisions outlined in section 03 35 43– Polished Concrete is essential. This is because there are no provisions in 03 30 00 for slab protection. The Concrete Polishing Association of America (CPAA) recommends the abovementioned approach. As this task involves the activities of other trades, it is the construction manager's responsibility to ensure that all trades know the importance of slab protection. To assist with this, Figure 7 illustrates a job site sign that can be used to communicate the importance of protecting the slab surface. Alternatively, the slab can be scheduled to be placed late in the schedule, which can reduce protection issues. However, this may slightly increase the cost and schedule. It is important to note that protecting the slab during construction can be challenging.

Identifying the differences: Part 3 and Part 2–Products

Using the appropriate techniques and materials to achieve polished concrete surfaces is crucial. In addition to specifying the correct methods, one must select the right products and use the proper terminology to attain the desired results.

Vapor retarder

ASTM E 1745 requires a vapor retarder with the manufacturer's recommended adhesive or tape to prevent moisture damage to polished concrete.

Concrete ingredients

When polishing cast-in-place concrete, obtaining all the necessary materials from a single manufacturer is essential. This includes concrete material of every color, size, type, and variety provided by a manufacturer with the resources to ensure consistent quality in the cast-in-place concrete. This requirement is because the variations of the materials can affect the polished surface's uniformity. This source limitation is similar to the one specified for architectural cast-in-place concrete.

Concrete mix design

The following two options should be considered when using fly ash and slag cement on concrete floors:

1. Limit the percentage of fly ash and slag cement to less than 20 percent of the total cementitious materials by weight.

2. Prohibit the use of fly ash and slag cement altogether.

The first option has potential benefits, such as reducing diamond wear and tooling expense, as these materials are highly abrasive. It also helps to ensure enough alkalis are present for chemical reactions with densifiers that aid in polishing, especially if the floor is colored with reactive stains. However, it is critical to determine if this option is feasible during the mockup. Sometimes, using fly ash in slag cement is necessary for sustainability purposes.

On the other hand, the second option could be preferred by some concrete and polishing contractors who choose to use only Portland cement mixes. This is because the delayed setting and strength-gain characteristics associated with fly ash and slag cement can create challenges in producing a uniformly finished slab surface, particularly in cooler weather. Some specifications require fly ash or slag cement in the summer to slow setting and prohibit their use in the winter due to the slower setting. However, this practice should not be allowed for polished concrete as the color variation in these two mixes will be evident.

Concrete properties

• The compressive strength requirement is 4000 psi to ensure efficient polishing.
• To prevent blisters and delamination, do not exceed 3% air content in trowel-finished floors.

Engineering the concrete slab

Concrete flatness and levelness are specified using F-numbers, measured within 72 hours after finishing operations are completed, per ASTM E 1155. This is important since polishing contractors may perform their work as many as 90 days or more after placing the concrete.

To ensure the concrete slab retains the required levelness and flatness for the polishing contractor, the engineer must design it accordingly. For slabs-on-ground, the F-numbers are affected by curling as the concrete dries and shrinks. Therefore, the engineer must provide a design that minimizes F-number change over time. Alternatively, the specifier could allow for a decrease in FF with time by specifying a higher level for the concrete floor that will be measured within 72 hours.

A team success

The pre-installation conference is crucial in building a successful team and ensuring everyone is fully invested in the polished concrete process. All members, including the specifier, engineer, construction manager, concrete contractor, and polishing contractor, should participate in this conference to ensure the project's success.

Polished concrete differs from traditional concrete floors, requiring specific treatment in specifications and construction. Experienced polished concrete designers and contractors often go beyond the recommended specifications to achieve the best results for a particular project. It is highly recommended to include such skilled contractors in the team.

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