August 10, 2017
by George R. Mulholland, PE, SE, and W. Joseph Macicak, PE, SE
Concrete has long reigned as the material of choice for many structural floors, but inherent difficulties with its use continue to be a source of disputes on construction projects. Concrete cracks, shrinks, and curls, requiring careful design and skilled installers to achieve flat and level surface profiles. Slabs incorporate joints that move and require maintenance, and their deflections are not completely predictable. Often, it is the treatment of these inherent difficulties that makes the difference between a successful slab construction project and one plagued by defects and missed expectations.
When design and construction practices do not meet the owner’s expectations, disputes arise, often followed by legal action. It is therefore critical the entire construction team—design professionals, owners, construction managers, general contractors, and subcontractors—understand the limitations and potential pitfalls associated with concrete slab construction. This understanding is just as important to the success of the project as is adhering to proper design and construction practices. Active involvement of the entire construction team, including the owner, is strongly suggested. Highly effective communication between team members allows realistic expectations to be set and adjustments made, if necessary, to achieve desired outcomes.
Ideally, the owner and design professional will begin discussing expectations prior to designing the concrete slab. The owner typically wants an economical, worry-free slab requiring little maintenance. To meet these expectations, provisions to minimize control and construction joints, limit crack widths and number of cracks, and achieve a flat, level surface will likely be considered. For elevated slabs, minimizing deflections and vibrations must also be taken into account.
The designer can use these discussions as an opportunity to explain cost versus benefit tradeoffs, which allows owners to refine their expectations. For instance, control of crack size and spacing is largely determined by the quantity of distributed steel reinforcement within the slab and the shrinkage limits of the concrete mix. Additional distributed reinforcement provides better crack control, but increases costs; further, local availability of mix ingredients may limit the design professional’s selection of mix types. Discussions between the owner and design professional prior to design should address the slab’s anticipated use, life expectancy, and maintenance requirements, as well as any overall expectations.
Communication between the owner and design professional should be ongoing during the predesign, design, and construction phases of the project. Owners should have the opportunity to be part of discussions when critical construction decisions are being made. They should also consider attending project meetings, despite the fact this has not historically been common.
Typically, communication begins during the bidding phase. Potential slab construction issues should be discussed thoroughly with the contractor prior to the start of construction. The design professional should strive to provide a slab specification that is as complete as possible, based on the specific expectations of the owner, and ensure the contractor understands its extent and limitations. The specification should acknowledge the uncertainties associated with slab construction and provide proactive protocols, such as crack remediation procedures or the inclusion of allowances for leveling material to address flatness or deflection deficiencies.
Further, miscues can occur if the design professional is unfamiliar with local limitations, so these should be discussed during preconstruction meetings. For example, adjusting the design (e.g. revising the mix design based on the local availability of ingredients) before slab construction commences may be beneficial.
Successful completion of a slab project is streamlined and simplified when team members fully understand their own and each other’s responsibilities. All should be willing to closely coordinate the work. The following are responsibilities typically assumed by each team member.
The owner’s responsibilities generally include communicating expectations for floor finishes, flatness, coverings, and how the slab is to be loaded and used. Detailed descriptions—including anticipated forklift loads, rack loads, storage loads, and other live loads—should be provided. It should also detail any unusual usage conditions, which may include pushing pallets versus lifting them, unusual temperature conditions, and movement of large equipment.
The slab form and finish should be well-defined by the owner, including desired construction joint spacing and acceptable floor flatness and levelness. The owner should make clear its expectations regarding slab edge curling, acceptable quantity and width of cracks, and deflection and vibration limitations for elevated slabs. Expectations regarding maintenance should also be conveyed, as this will impact the spacing of joints within the slabs-on-grade and the finish.
The design professional’s responsibilities typically involve identifying the owner’s performance requirements and preparing contract documents, including drawings and specifications, to meet them. He or she should review and discuss the proposed design along with preliminary budgetary information with the owner, revising the design depending on the results of these discussions. The design professional should also discuss owner expectations with the contractor and its subcontractors and suppliers, and should understand and address the contractor’s concerns regarding the design and local practice limitations.
Additionally, repair guidelines and allowances within the project documents must state the requirements for correcting floor cracking, flatness, curling, and levelness. Limitations on the number of cracks, crack widths, floor flatness and levelness, deflection, and curling should be provided within the project documents with proposed repair techniques.
The contractor is responsible for carefully reviewing the contract documents to understand the project requirements. Prior to construction commencement, contractors should discuss the design and owner’s expectations with its subcontractors, suppliers, and the design professional, and understand any limitations or special conditions that may affect the work. Through discussions with the subcontractors and suppliers, it should communicate any local practices that may impact the project. Finally, once all expectations and design requirements are understood, the contractor should perform the work per the construction documents and industry standards.
MasterFormat specifically addresses meetings related to the construction of the structure, not its design. MasterFormat Section 01 31 00–Project Management and Coordination and Section 03 30 00–Cast-in-place Concrete both address construction meetings involving cast-in-place concrete construction. The referenced documents discuss up to six different meeting types and list the required attendees. The owner and design professional are required at each of these meetings except the pre-installation conference. Since the pre-installation meeting is the last ‘best chance’ prior to construction to ensure all team members have adequate understanding of the expectations for the slab and any limitations that need to be discussed, owner and design professional attendance is strongly recommended. Attendance allows the owner and design professional to hear first-hand all construction team concerns and assist in making consensus compromises to the design and construction procedures.
When disputes occur
Despite the availability of numerous highly skilled and experienced concrete slab designers and contractors, there is still frequently defective construction, missed expectations, and legal disputes, often arising from paying inadequate attention to the aforementioned limitations and difficulties prior to construction. These authors have come across several projects illustrating how disputes—and sometimes legal actions—arise when owner expectations are not properly understood or repair actions are not clearly defined. The following are a few examples of aspects that should have been addressed before construction.
A large warehouse incorporated a lightly reinforced slab-on-grade by a design professional from another part of the country. Prior to design, the owner expressed its desire for minimal cracking within the slab. Although several discussions regarding the likelihood of cracking and the proposed slab joint spacing took place between the design professional and the local contractor, they disagreed over whether the slab, as designed, could be constructed to meet the owner’s expectations for crack control. Ultimately, the design professional elected to dismiss the contractor’s concerns, and left the original design intact with regard to slab reinforcement, shrinkage limit on the concrete mix, and joint spacing (which exceeded industry standard recommendations). Additionally, the project documents contained no specific slab repair guidelines addressing the number and size of cracks, slab curling limits, or control joint width limits.
When the finished slab exhibited excessive cracking, curling, and control joint widths unacceptable to the owner, investigation and legal action resulted. During subsequent floor slab investigations, reinforcement placement was found to be inconsistent, some slab areas appeared to have been improperly cured, and the subbase was not consistently level. As a result of the inadequacies of the original design and poorly executed construction, the finished slab could not support the intended loads, and its anticipated life cycle was diminished. These defects may have been avoided had the construction team members understood the importance of achieving mutual understanding and consensus prior to construction.
Even if the design professionals and contractors disagreed on specific portions of the design or construction methodology, the establishment of repair guidelines with acceptable limits for slab cracking, crack size, curling, and joint widths could have been used to provide a protocol for resolving issues without resorting to legal proceedings. These repair guidelines acknowledge uncertainties exist, help the owner manage expectations, and emphasize the importance of maintaining proper construction techniques.
Another large warehouse incorporated a 152-mm (6-in.) thick concrete slab-on-grade, lightly reinforced with welded-wire fabric and originally designed for light-to medium-duty usage. After approximately 10 years of light-duty use, the slab exhibited little to no signs of distress. At that time, the property was leased to a new tenant, who planned to change the usage within the warehouse to heavy-duty, but did not convey the intended use to the owner.
A design professional was engaged to detail several large-capacity in-slab scales for the existing slab-on-grade, but apparently did not realize the new scales were designed for loads far exceeding the capacity of the adjacent existing slab. The contractor, which cut and removed portions of the existing slab to install the large scales, did not recognize the lightly reinforced slab was incompatible with a large-capacity scale.
Following installation of the scales, the existing slabs experienced severe joint raveling and spalling due to excessive loading by the new tenant’s large forklift trucks. During a subsequent dispute between the owner and the new tenant, analysis showed a slab more than twice the thickness of the existing one was needed to properly support the new forklift loads. Had the proposed new usage of the facility been thoroughly discussed by the owner, the new tenant, and the design professional prior to design, it is likely the existing slab’s capacity limitations would have been obvious, and other building lease options for the tenant—or slab replacement—could have been considered.
A freezer structure including a reinforced concrete slab placed on rigid insulation was constructed within a warehouse. The slab was scheduled to be placed after completion of the surrounding freezer structure, which incorporated insulated prefabricated walls and roof. The concrete floor was cast during the summer, prior to the refrigeration system being operational. Within the enclosed, insulated space, temperatures became elevated during the concrete curing process, creating improper curing conditions that caused widespread, excessive cracking.
Had the construction team engaged in pre-installation discussions to ensure the installation procedure was understood by all, the unusual curing environment would likely have been identified. Adjustments—such as use of high-temperature concreting practices—could have been implemented to prevent rapid drying and development of plastic shrinkage cracking. The distressed slab was removed and a new slab installed after the refrigeration system was operational and could provide moderated temperatures within the freezer during concrete placement and curing.
This article’s authors have encountered many disputes that arose from unanticipated elevated slab deflections. These problems could likely have been avoided had the construction team communicated more effectively before construction, addressing owner expectations and the limitations of predicting slab deflections. These disputes often involve the construction team’s failure to address the potential differences between the requirements of MasterFormat Division 03–Concrete and Division 09–Finishes for floor flatness and levelness. In some cases, the project documents include provisions to address this potential difference, such as drawing general notes similar to the following:
Slabs have been designed in accordance with ACI deflection limitations. Provide leveling material as necessary prior to finish installation to account for allowable deflection.
While such information is helpful and appears to be all-encompassing, experience shows it may not provide adequate detail to prevent disputes over the scope of remedial floor slab surface treatments. The accuracy of floor slab deflection predictions can vary depending on the type of cast-in-place slab used, environmental conditions at the time of concrete placement, and numerous other variables within the construction process. The entire construction team should engage in discussion of elevated floor deflection issues prior to construction. Acknowledgement by all parties of the difficulties in accurately predicting slab deflections provides a basis for development of reasonable remediation provisions.
Communication amongst the construction team is critical to the success of a concrete slab project. As an owner, it is important to retain experienced design professionals and contractors that have previously performed the intended scope of work and understand the local material andconstruction limitations. Although means and methods are ultimately the responsibility of the contractor and typically the main topic of discussion during pre-installation conferences, input from the owner and design professionals is crucial to avoid common pitfalls and ensure the design intent remains intact.
George R. Mulholland, PE, SE, is a senior consulting engineer at Raths, Raths & Johnson, a national engineering, architecture, and forensic consulting practice. He has almost 30 years of experience specializing in the evaluation, investigation, and repair of distressed façades, buildings, and structures. Mulholland’s portfolio has encompassed cast-in-place, precast, and post-tensioned concrete, as well as masonry, steel, and timber structures, with a specialization in parking garages. He can be reached via e-mail at firstname.lastname@example.org.
Joseph Macicak, PE, SE, is a consulting engineer at Raths, Raths & Johnson, and has specialized in the investigation and analysis of structures, materials, and construction components for 16 years. He has applied his skills to field investigation, repair design, management and administration of repair programs, and forensic support services for dispute resolution and development of expert technical opinions. Macicak’s background includes evaluation of stucco, wood framing, and concrete distress, along with the collection and assessment of destructive and nondestructive test data. He can be reached via e-mail at email@example.com.
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