Suntaris

Pre-topped vs. Field-topped – Which option is better for a precast garage?

For precast parking structures, there are two types of construction systems for floor surfaces – pre-topped and field-topped. Building code requirements for supporting concentrated loads along with requirements for providing fire resistance, necessitate a 4 in. to 5 in. total flange thickness for precast double-tee members. This thickness can be attained with either a single monolithic placement at the precast plant (identified as pre-topped construction) or by using two-layer construction, with an initial 2-in. thick plant-cast flange overlayed with a 2 in. to 3 in. concrete topping (known as field-topped) after the erection of precast members onsite.

Clients often ask me, “Which of the two construction systems is better suited for my precast parking structure?”

Both construction systems have their own pros and cons. Here are a few comparison points between both construction systems based on a few key variables:

• Construction schedule: Use of pre-topped system significantly reduces the concrete topping area for onsite concrete placement and that in turn reduces the construction schedule.

• Initial construction cost: Keeping other factors constant, reduction in onsite concrete placement efforts for a pre-topped system provides savings in the form of initial construction cost.
• Construction quality: Due to controlled construction environment, construction quality for individual precast members based on the variables like concrete mix (strength, aggregate distribution, curing) and steel placement is typically of higher quality in a pre-topped system. Additionally, unlike field-topped system, bond strength of the topping and joint alignment with that of underlying precast members are not the critical factors for the service-life costs for the pre-topped system.

• Concrete cover: While pre-topped system provides more concrete cover for the flange steel, due to low surface area of steel, the role of this additional cover in itself is not as relatively significant from the durability perspective, as long as the minimum required cover of a quality concrete mix is provided in the field-topped system.

• Driving experience: Tee-to-tee joint width and vertical differential between adjoining double-tees govern the driving experience in a precast garage. Tee-to-tee joint width for pre-topped system ranges between 1”-2” whereas for field-topped construction, this width typically lies between 1/4” to 3/4”. Additionally, despite good fabrication and erection efforts, vertical differential in pre-topped system at the tee-to-tee joint is significantly higher than that in field-topped system. Wider joints and higher vertical differential across tee-to-tee joints make driving in a pre-topped garage a relatively bumpier and noisier ride.

• Drainage effectiveness: Field-topped construction provides more flexibility to ensure proper slopes for effective drainage of the floor slab surfaces. Without proper drainage, localized water ponding typically occurs. For the geographic regions affected by freeze-thaw cycles, water ponding not only acts as a maintenance problem, it also leads to localized damage of concrete while posing slip hazards.

• Joint sealant service life: Durability of tee-to-tee joint sealants, among many factors, depend upon the exposed surface area of sealant, profile of sealant, sealant continuity at or around tee-to-tee connections, and vertical differential between the edges of the joints. Assuming other factors (traffic rate, UV exposure level, structural movement, sealant material, installation quality etc.) constant; sealants in properly tooled field-topped joints last longer than that in pre-topped joints.

• Selection of shear connections: Following the guidelines of “ACI 362.1R – Design of Durable Parking Structures”, tee-to-tee connections for pre-topped construction in specific geographical zones should be of stainless steel whereas for field-topped construction, galvanized connections can also be used. Use of stainless steel tee-to-tee connections increases the initial construction cost and lack thereof increases the repair and maintenance costs for the service life of garage.

• Welding of connections: The phenomenon of overheating of embedded tee-to-tee connections during welding resulting in micro-cracking of the concrete surrounding the connections is well-documented. In the case of field-topped system, micro-cracked flange section is protected with concrete topping whereas for pre-topped system, the cracked region around the connection becomes a direct source of water permeation into the concrete, resulting in localized long-term delaminations and spalls in the later life of structure.

• Diaphragm action: Field-topped system provides a better option to place drag strut reinforcement needed to transfer the floor diaphragm loads into the shear walls.

• Future application of traffic coating: Use of traffic-bearing waterproofing membrane provides an effective barrier against water leakage and penetration of chlorides (from road salts, ice melts and environment) into concrete elements. Relative flatness and narrow joints in the field-topped construction makes it a better system for the durability of traffic-bearing waterproofing membranes.

• Planning and coordination: While both systems require planning and coordination to attain an effective and durable structure, the focus of this effort is different for each system. For pre-topped systems extensive planning is targeted early in the project to attain proper floor elevations and slopes whereas for field-topped systems extensive effort is placed in substrate surface preparation, onsite quality control and joint creation.

Bottom line:

Pre-topped systems are preferred from the perspective of construction schedule, original construction cost and construction quality of individual precast members. Field-topped systems have an upper hand in drainage effectiveness, driving experience, joint sealant service life, and traffic coating efficiency.              

Successful delivery of both construction systems require planning and coordination between owner, design engineer/consultant, precaster and contractor to not only attain a code-compliant and safe structure but also to construct a garage that is durable and meets the life-cycle maintenance costs projected by Owner.

Can all of the above-mentioned factors be compared on a single variable of present cost?

Yes, by using project specific information (location, design parameters, construction quality, garage usage), repair cost data from historic precast garages, and of course a few assumptions, a life-cycle cost analysis can be performed to compare life-cycle costs of both construction options.

If you have any questions or comments on this topic, I can be reached at spuri@suntaris.com or 404-308-2376.