Optimizing early performance is critical for concrete paving projects. Understanding when to perform joint sawing, avoiding uncontrolled cracking, and achieving good surface quality and durability are all essential for success. The key is being able to evaluate and optimize the set times and early strength with unfamiliar materials and varied placement temperatures.
Traditional laboratory concrete batches present time and labor challenges and are difficult to use to simulate job temperature variations. A new alternative is to run tests on only the paste or mortar portion of a concrete mix and to track the temperature as the mix hydrates. This allows using the heat gain trends to look at the relative effects on strength gain and setting for different candidate materials and proportions and to investigate the effects of job temperature variations.
The temperature history of the first few hours of hydration of a paste or mortar mixture (the thermal profile) serves as a record of the relative C3A and C3S hydration rates and the interaction of CaSO4 (gypsum). We start by determining the Main peak heat rise (M) from the low point during the dormant period (caused by the interaction of the gypsum) to the main peak during C3S hydration. A consistent “fraction” of M can be used as a relative setting indication (50% is convenient), to evaluate the effects of materials and proportioning variables on setting of mixes while varying a single constituent (such as an admixture or SCM). For mortar mixtures, fractions of around 21% and 42% of M are usually reasonable approximations of initial and final times of set in actual concrete, corresponding to set times determined using ASTM C403 (Test Method for Time of Setting of Concrete Mixtures by Penetration Resistance).
Using these techniques, changing temperatures can be recorded and thermal profiles generated. The data is most useful when the thermal profiles of different test mixtures are compared on the same time scale. For example (see below graph), profiles for mixtures comparing five different water-reducing admixtures with identical water-cementitious materials ratios and temperatures, combined with one-day strength tests of the paste samples, reveal that admixture A/F 1 resulted in the least retardation of the set while still having good early strength.
This same procedure can be used for variations in cement replacement. Admixtures can be tried and incompatibilities can be spotted. Performance targets can be established for both setting and early strengths. With only a few days of testing, variations can be tried until an acceptable mix is found, which can then be tested over a temperature range that might be expected at the jobsite. Once all of this is accomplished, then it would be time to move to trial concrete batches using more time-consuming and definitive tests to refine the proportions.
Concrete producers may find that this technique is a useful quality control tool that can help to identify material variability, to explore likely effects of potential mixture variables, and/or to gage normal seasonal performance changes. Thermal profile testing can be especially useful in evaluating mixture modifications that will help assure consistent setting times on pavement projects where concrete temperatures can vary widely.
Check out a more detailed presentation on this technique.
Tim Cost is senior technical service engineer with Holcim (US).