The Impact Of Workability RetentionContractors want concrete to be as workable as possible to facilitate placement and consolidation without compromising performance in the hardened state.
JOSEPH DACZKO, BASF
Contractors want concrete to be as workable as possible to facilitate placement and consolidation without compromising performance in the hardened state. Concrete producers strive to consistently deliver expected performance attributes. Yet, hydration of portland cement leads to a change in fresh concrete properties immediately after batching, most notably with respect to mix slump or workability. In air-entrained concrete mixtures, furthermore, modified slump can lead to changes in air content, typically a reduction, and potential rejection of the load. Invariably, rejected loads result in loss of revenue and reduced profits for concrete producers.
In 2009, the estimated annual concrete production volume in North America will be approximately 290 million yards. Accordingly, if 0.1 percent (290,000 yards) of the projected 2009 total concrete volume were to be rejected for failure to meet specified slump and air content values, a revenue loss exceeding $20 million would result, assuming an average selling price of $70.00/yd. and no reuse of the rejected concrete. Thus, the significant impact on the concrete industry of slump management over time, i.e., slump retention, is readily apparent.
Slump loss may lead to retempering of the concrete or implementation of other measures to counteract the expected loss in workability. Such practices can have a significant impact on consistency and the quality of in-place concrete. Following is a discussion of workability retention and the various methods traditionally used to manage it. Additionally, an innovative, new admixture is introduced that provides workability retention without the adverse effects of current practices.
MANAGING WORKABILITY LOSS
Concrete technologists necessarily have adopted various methods for managing the inevitable loss of workability over time. Current practices include:
Retempering with water at the job site Û This practice increases total water content, lowering strength and potentially resulting in a water-cementitious materials ratio (w/cm) that exceeds the target value.
Redosing high-range water reducer (HRWR) at the job site Û Plant-added HRWRs, introduced in the mid-1980s, considerably improved slump retention compared to first-generation HRWRs. Today's polycarboxylate-based HRWRs further refine slump-retention properties. And, while HRWRs can be redosed on site when additional slump is needed for placement, trained personnel typically must be present to determine and record the appropriate amount. Moreover, accurate measuring devices are required to ensure proper dosages.
Increasing slump at the time of batching to compensate for slump loss Û This practice requires knowledge of slump loss over time, as well as continuous monitoring and adjustments to ensure consistency.
Addition of a hydration-controlling or retarding admixture to slow down hydration and stiffening Û Hydration-controlling and retarding admixtures have a successful history of improving slump retention. Although these products often provide the desired performance, in some instances, extended setting time and lower early-age compressive strength development may occur, especially when the retarder is not used to offset hot-weather effects.
Mechanical methods of retempering Û Now on the market is new truck-mounted equipment that automatically adds water to the concrete mix during transit, thereby removing the human variable from the retempering process. The end result, nevertheless, is variable w/cm, compressive strengths, and entrained air contents.
Frequently, these practices are modified to account for specific requirements or to compensate for such factors as prevailing ambient conditions and material changes. If a retarder is used to extend a concrete mixture's slump life, for example, overall effectiveness ultimately will depend on retarder type and dosage, taking into account the following additional factors:
- Prevailing ambient and concrete temperatures
- Travel time to the jobsite
- Reasonable slump retention that can be achieved
- Rate of hardening requirements
- Early-age compressive strength requirements
A caveat must be noted: the techniques listed above may not be applicable to concrete mixtures such as high-strength concrete or self-consolidating concrete (SCC). In particular, the practice of adding water at the job site to SCC is not recommended, due to its sensitivity to water fluctuations and stability requirements.
CURRENT PRACTICE VS. NEW TECHNOLOGY
When used in combination with normal, mid- and high-range water-reducing admixtures, a recently developed admixture technology provides flexible amounts of workability retention. The product is added separately to provide workability retention without impacting initial workability and negatively affecting setting time or early compressive strength. Two evaluations were conducted to assess the impact of current techniques to manage slump loss as compared to that of the new workability-retaining admixture.
Tests were conducted in the laboratory on nominal 8-in. (200-mm)-slump, air-entrained concrete mixtures over a period of 90 minutes. All testing was performed at 70_F (23_C) on concrete comprising Type I portland cement; Class C fly ash; a naturally mined, glacial-deposit fine aggregate; a #57 limestone coarse aggregate; and, a HRWR. The evaluation targeted three mixtures: 1) a reference sample retempered repeatedly over time, 2) a reference batch redosed over time with additional HRWR, and 3) concrete incorporating a novel chemistry developed specifically to promote workability retention without retardation.
Figures 1 through 3 demonstrate differences in concrete performance attributed to the various methods. Although repeated additions of admixtures such as HRWRs [Figure 2] typically do not occur in practice, this mix was incorporated to further demonstrate the effect of relative slump changes and their impact on air contents.
Although ASTM C 94 permits only a one-time addition of water, Figure 1 shows the impact of repeated retempering on slump and air content of a reference concrete mixture that is losing slump over time. The air and slump can be seen to track together. Also evident is variability in one-day compressive strengths as water is added and air content goes up and down. Air content fluctuation as well as inconsistent additions of water often are primary causes of variability in compressive strength results, which then lead to higher standard deviations and, potentially, greater overdesign requirements.
Figure 2 demonstrates the impact of multiple additions of HRWR over time. Similar to the retempering scenario, slump and air content change with each redosing operation. Significantly, the rate of slump loss is remarkably consistent after each successive HRWR addition, while Figure 1 shows a slowing rate of slump loss as more and more water is added to that mixture. The HRWR-dosed mixture's one-day compressive strength was affected less than that of the repeatedly retempered mixture, because the w/cm was maintained.
Also demonstrated in Figure 2 is the value of plant-added HRWRs that provide slump retention, including new formulations mentioned previously. Plant-added HRWRs allow for the most consistent performance and greater quality assurance.
The scenarios represented in Figures 1 and 2 would require labor and technical expertise to ensure that slump is appropriate at the point of placement. The ability to maintain workability over time, however, would eliminate such labor and equipment requirements while providing target concrete consistencies.
Illustrating the impact of the new workability-retaining admixture, Figure 3 indicates that slump and air contents are more consistent over time, compared to the same variables under retempering and redosing. Compressive strength is also maintained.
A second evaluation was designed to assess impact on extending workability of the new workability-retaining admixture as compared to that of retarding or hydration-controlling agents. To that end, three identical base mixtures Û a reference, a mixture with the workability-retaining admixture, and a mixture with a hydration-controlling admixture Û were run at 70_F (21_C). Standard Type I/II cement and Class C fly ash were included as cementitious materials in the three non-air-entrained mixes of equal w/cm. All mixtures incorporated a lignin-based water reducer at equivalent dosage and a HRWR dosed to achieve an initial 7- to 8-in. (175?200-mm) slump.
As shown in Figure 4, the hydration-controlling admixture provided a substantial improvement in workability retention, although it more than doubled the initial setting time. By contrast, the workability-retaining admixture exhibited minimal impact on initial setting time, as it achieved effective slump retention. The performance of the workability-retaining admixture thus indicates that slump retention can be accomplished without retardation.
PERFORMANCE ADMIXTURES - A NEW ERA
Workability-retention characteristics of a mixture have a significant impact on batch-to-batch consistency of concrete production. Whether controlled by human intervention or specialized equipment, traditional methods of managing workability loss generally have proven inexact and costly. A more effective means of providing workability retention stands to offer multiple benefits:
- Workability retention on demand
- Greater batch?to-batch consistency of delivered concrete
- More consistent compressive strengths due to minimal job-site addition of water
- Greater consistency in entrained-air content
- Minimized redosing of water-reducing admixture on the job site
- Fewer rejected loads and better customer satisfaction due to consistent quality of concrete (slump, air and strength)
- Faster truck turnaround time
- Expanded concrete delivery range
A new, innovative workability-retaining admixture technology has been developed to provide reliable slump retention without retardation, thereby enabling more consistent concrete production. It permits a wide dosage range that allows flexible amounts of workability retention, which makes slump retention possible for normal, mid- and high-range water reduction applications. Hence, the latest generation of workability-retaining admixtures opens a new era of performance technology. Û www.basf-admixtures.com/RheoTEC
This report was provided by Joseph Daczko, product manager for BASF Construction Chemicals LLC, Cleveland, Ohio. His 18-year career dedicated to the development and application of concrete construction materials includes management of many BASF investigations, among them key projects addressing concrete exposed to acidic environments; fresh mix rheology and workability; self-consolidating concrete; and, rapid developing, high-strength concrete. He holds patents covering two areas of high early-strength concrete compositions.
Daczko has published numerous papers on rheology and self-consolidating concrete. His technical presentations have been featured at such seminars as the Second and Third International Symposia on Self-Compacting Concrete, ASTM and ACI symposia, as well as a variety of State Department of Transportation Workshops.