Time-Dependency of the Slump Test, or “What Slump is it Now?”
Presented By: Kenneth Hover
Affiliation: Cornell University
Description: Slump is specified, evaluated for compliance, and argued about as if it were an intrinsic property of concrete, with a desired fixed value for any given batch of fresh concrete. More correctly, slump is a time-dependent property of concrete, and its value at any moment and its rate of change depends on mixture ingredients, mixing, and temperature. Rate of slump loss may therefore be a more reliable characteristic of a given mixture in a given environment. This time-dependency is important when attempting to correlate slump with various fresh or hardened concrete behaviors, or even when using relatively uniform values of slump as an indicator of relatively uniform concrete composition. Multiple records of slump vs. time for the same mix-design, commercially batched, sampled from the truck chute, and tested over more than 10 years are used to present simple linear and non-linear ways to characterize time dependency to include “Slump Half-Life,” i.e., the time required for the slump to diminish to ½ of its current value. This is not to be confused with the “Double-Slump,” i.e., the amount of added water required to double the current slump. This concept may be more useful that the traditional rule-of-thumb that the addition of 1 gallon of water per CY will increase the slump by 1 inch. (adding 5 liters of water/m 3 will increase slump by 25 mm).
Extending the Slump Test to Improve Quality Control Towards Sustainability
Presented By: Nathan Tregger
Affiliation: GCP Applied Technologies
Description: In its first adoption, the slump test was a consistency test that was quite useful to determine when the water content varied. With the advent of chemical admixtures, changes in slump can be attributed not only to water but the influence of admixtures such as superplasticizers and viscosity modifying admixtures. In other words, with a fundamentally changing concrete over the decades, the slump test remains to address consistency of concrete from load to load. In practice, it is rare that a project or pour will test every load of concrete, never mind more than one slump test per truck load. With the advent of IIoT (Industrial Internet-of-Things), concrete trucks instrumented with sensors have been used extensively in commercial practice allowing for rheological characterization of every truck load, on the entire concrete sample. Over 150 million cubic yards have been placed through these types of systems, yielding extremely large data sets that provide valuable insight into the entire concrete production from producer to contractor. This talk will focus on the enabled capabilities of having a rheological measurement for every load of concrete, and how it has benefitted both the producer and contractor towards a sustainable and consistent concrete product.
The Customer Can Slump Test Any Concrete so Long as it is Black
Presented By: Peter Stynoski
Affiliation: U.S. Army Engineer Research and Development Center
Description: For a century, the slump test has provided a repeatable, facile, field-ready indication of workability for normal weight concretes with mixture designs that conform to ordinary ACI 211 proportioning methods. More recently, designers and producers have specified and developed mixtures with unconventional requirements for pumpability, self-consolidation around tight rebar spacing, or buildability for additive construction. The slump test alone does not provide enough information to qualify such mixtures on the jobsite. In other presentations during this session, attendees will learn how and why the slump test convolves the distinct plastic state properties of viscosity and yield strength. The relationship between measurable slump and tangible, yet subjective workability breaks down for mixtures with relatively low or high yield strength. This presentation will discuss complementary test methods to better understand the workability of high and low yield strength concretes, without rendering the slump test obsolete.
Using Slump to Predict Pumping Pressure: A Scientific Fact or a Historical Coincidence?
Presented By: Dimitri Feys
Affiliation: Missouri S&T
Description: Pumping nomograms have been developed to predict pressure during pumping of concrete. These pump design figures include equivalent pipeline length, diameter, desired flow rate and a concrete workability parameter: slump. Recent research findings on more modern concrete mixtures have shown that pumping pressure is rather influenced by concrete viscosity and the capacity to form the lubrication layer near the pipe wall, compared to yield stress or slump. New nomograms were proposed replacing slump as a workability input. The slope of the pressure-flow rate relationship of the SLIPER, a device developed to measure flow resistance during pumping, works well in those nomograms. In this presentation, the relationship between pumping pressure and viscosity, SLIPER results and slump will be highlighted. When all other factors remain constant, there is a relationship between pumping pressure and slump, but its importance is relatively small compared to concrete viscosity. So, why did a pressure prediction based on slump work in the past? This is because highly efficient chemical admixtures were not yet developed. As such, concrete producers had limited tools to alter the mix design: w/cm, aggregate content, aggregate gradation and SCMs. And for most of these parameters, viscosity, and yield stress, and thus slump, are influenced simultaneously in similar fashion. With the development of these new chemical admixtures, the industry now has the opportunity to control viscosity and yield stress independently. As such, predicting pumping pressure based on slump is rather a historical coincidence.