Some specifications and guides for concrete still set a maximum slump. Should you specify slump?
Historically, maximum slump was used by engineers to control water content. However, the wide variety of materials used in concrete means that slump is not a direct indication of water content. Admixtures, SCMs, and aggregates dramatically affect slump. Also, differences in site conditions, such as transit time and concrete temperature, affect the amount of water needed to reach a certain slump at the site.
Slump is primarily the concern of the contractor. The engineer typically focuses on water content and hardened properties. As long as the water content, hardened properties, and all workability parameters, such as segregation and bleeding, are well controlled, the contractor should not be needlessly limited by maximum slump. Yet many specifications and guides still limit slump in an effort to control water content.
The following are examples in American Concrete Institute (ACI) documents.
ACI 211 has recommendations in Table 6.3.1 for slump of concrete without chemical admixture. The footnote suggests the slump limits are in place to control water. Since nearly all concrete contains chemical admixture, these requirements are largely irrelevant.
ACI 301 requires that “unless otherwise specified or permitted, concrete shall have, at the point of delivery, a slump of 4 in.” The maximum slump can be increased to 8 in. when high-range water reducing admixture is used and 6 in. when mid-range water reducing admixture is used. Verifi data indicate that concrete is often placed at higher slumps without issue.
ACI 302.1R: Guide for Concrete Slab and Floor Construction
ACI 302 recommends maximum slumps for floors. However, Verifi data indicate that contractors prefer higher slumps for floors.
ACI 304.2R: Placing Concrete by Pumping Methods
ACI 304 recommends slump of 2 to 6 in. for pumping, but states that slumps greater than 6 in. can be pumped without difficulty. In most cases, pump operators would prefer higher slump to reduce pumping pressure and increase throughput. Recommending a lower slump has little to do with pumpability.
Verifi data indicate that concrete is often poured successfully at higher slumps than recommended in ACI documents. As long as the mixture proportions are within design parameters and hardened concrete properties are achieved, the slump can be adjusted to meet the contractor’s needs for efficient concrete construction.