To assume that the mix-design specifications for tilt-up projects are the same as for other concrete jobs is a grave mistake. Certain factors must be given careful consideration when designing a mix for today's increasingly complex and architecturally sophisticated tilt-up projects. Strength is one of the most important concerns on any tilt-up job. But early-age flexural strength is just as important, if not more so, than 28-day compressive strength. The variables of slump, pumpability, and finishability as well as the balance of aggregate gradation and admixtures also play a role. Clearly, one key to success is early collaboration with concrete producers and eliminating the “one-size-fits-all” mentality.
The first step in tackling the issue of today's tilt-panel mix design is to understand how concrete should be specified. Specifying concrete for tilt-up construction is a two-step process. Typically, the architect or engineer who is responsible for structural and architectural design of the building, specifies the hardened properties of the concrete (such as 28-day compressive strength, entrained air-content, finish, and color). In contrast, the contractor responsible for constructing the tilt-up building should specify the properties of the plastic concrete (such as slump), indicate how the concrete will be placed (pump, conveyor), and specify the strength (compressive and flexural) required for lifting. Both the project engineer and contractor should use performance-based specifications and avoid specifying mixture proportions or material quantities (prescriptive specifications).
For common applications, such as large warehouses, the typical hardened properties of the concrete will have a specified compressive strength (f'c) of 3000 to 5000 psi at 28 days with a maximum aggregate size of 0.75 to 1.5 inches, depending on panel thickness and bar spacing. Entrained air for freeze/thaw resistance is not normally required for tilt-up panels since the surfaces are primarily vertical and protected with a paint or sealer. However, if panels are being placed directly against earth or remain untreated, specify air-entrainment in accordance with ACI 318 Chapter Four, Durability Requirements.
According to ACI 551-05, “Tilt-Up Concrete Construction Guide,” concrete used for tilt-up panels should provide adequate strength for the in-place condition and for panel erection requirements. A minimum 28-day strength of 3500 psi is typical for in-place design requirements. However, lifting the panels between three and seven days after casting may require the 28-day strength to be specified higher in order to achieve the required lifting strength. When it comes to specifying strength of lifting, the contractor usually relies on the lifting insert manufacturer. Lifting insert manufacturers typically require a minimum compressive strength of 2500 psi at the time of the lift to ensure that the full load capacity of their products can be achieved. In addition, the insert manufacturer may require that the flexural strength, as defined by the modulus of rupture, be 400 to 500 psi in order to avoid cracking the panel during lifting. Often, the lifting strength will be more critical for mix design purposes. Supplementary cementitious materials (SCMs), such as fly ash and slag, are often used in mixes. These products can affect early-age strength gain and finishing characteristics so producers and contractors should work together to establish the right mix, especially when large proportions of SCMs are used.
The contractor typically specifies a slump of 6 to 9 inches and indicates that the concrete will be placed by pump. Given these specifications from the project engineer and contractor, the concrete producer can then provide a mix that meets both the hardened and plastic properties required.
Strength tests (both flexural and compressive) should be performed according to the appropriate ACI and ASTM requirements before erection to verify that the concrete strength meets the erection requirements. These tests involve breaking beam and cylinder specimens that are cast from the same concrete used in the panels and stored onsite in conditions similar to those experienced by the panels. Maturity methods may also be used to determine the in-place strength of the concrete. The testing agency should be notified when panels are to be cast so they can prepare these specimens and perform other onsite tests of the fresh concrete such as slump and air content.
In a recent survey of tilt-up contractors conducted by the Tilt-Up Concrete Association (TCA), nearly half cited flexural strength as the most crucial aspect of the finished project. Not only do panels have to be strong, but they have to achieve that strength in a shorter time than usual to allow the panels to be lifted safely and on schedule.
“People only think of how fast strength can be achieved in order to lift,” says Andrew S. McPherson of Seretta Construction, “but once up, the building will be there for many decades, and having a higher ultimate strength is more favorable.”
Dennis Brunner of Concrete Enterprises concurs, stating that jobs are much faster paced these days. In fact, nearly 30 percent of contractors surveyed say they've seen a change in mix designs over the past few years as projects have become more complex—and most of these contractors cite faster work turnarounds as the major cause for the change, requiring mixes that gain strength more quickly.
“Other forms of concrete construction may focus on compressive strength, but with tilt-up, we focus on flexural strength due to the speed of construction and the need to tilt as soon as possible after pouring,” said Tony Williams of Miller-Valentine Group.