About the research
A two-stage mixing process for concrete involves mixing a slurry of cementitious materials and water, then adding the slurry to coarse and fine aggregate to form concrete. Some research has indicated that this process might facilitate dispersion of cementitious materials and improve cement hydration, the characteristics of the interfacial transition zone (ITZ) between aggregate and paste, and concrete homogeneity.
The goal of the study was to find optimal mixing procedures for production of a homogeneous and workable mixture and quality concrete using a two-stage mixing operation. The specific objectives of the study are as follows: (1) To achieve optimal mixing energy and time for a homogeneous cementitious material, (2) To characterize the homogeneity and flow property of the pastes, (3) To investigate effective methods for coating aggregate particles with cement slurry, (4) To study the effect of the two-stage mixing procedure on concrete properties, (5) To obtain the improved production rates. Parameters measured for Phase I included: heat of hydration, maturity, and rheology tests were performed on the fresh paste samples, and compressive strength, degree of hydration, and scanning electron microscope (SEM) imaging tests were conducted on the cured specimens. For Phases II and III tests included slump and air content on fresh concrete and compressive and tensile strengths, rapid air void analysis, and rapid chloride permeability on hardened concrete.
About the research
Volume I: Engineering Properties and Construction Guidelines (6.7 mb pdf) Soil treated with self-cementing fly ash is increasingly being used in Iowa to stabilize fine-grained pavement subgrades, but without a complete understanding of the short- and long-term behavior. To develop a broader understanding of fly ash engineering properties, mixtures of five different soil types, ranging from ML to CH, and several different fly ash sources (including hydrated and conditioned fly ashes) were evaluated. Results show that soil compaction characteristics, compressive strength, wet/dry durability, freeze/thaw durability, hydration characteristics, rate of strength gain, and plasticity characteristics are all affected by the addition of fly ash. Specifically, Iowa self-cementing fly ashes are effective at stabilizing fine-grained Iowa soils for earthwork and paving operations; fly ash increases compacted dry density and reduces the optimum moisture content; strength gain in soil-fly ash mixtures depends on cure time and temperature, compaction energy, and compaction delay; sulfur contents can form expansive minerals in soil-fly ash mixtures, which severely reduces the long-term strength and durability; fly ash increases the California bearing ratio of fine-grained soil-fly ash effectively dries wet soils and provides an initial rapid strength gain; fly ash decreases swell potential of expansive soils; soil-fly ash mixtures cured below freezing temperatures and then soaked in water are highly susceptible to slaking and strength loss; soil stabilized with fly ash exhibits increased freeze-thaw durability; soil strength can be increased with the addition of hydrated fly ash and conditioned fly ash, but at higher rates and not as effectively as self-cementing fly ash. Based on the results of this study, three proposed specifications were developed for the use of self-cementing fly ash, hydrated fly ash, and conditioned fly ash. The specifications describe laboratory evaluation, field placement, moisture conditioning, compaction, quality control testing procedures, and basis of payment.
Volume II: Influence of Subgrade Non-Uniformity on PCC Pavement Performance (1.3 mb pdf) To provide insight into subgrade non-uniformity and its effects on pavement performance, this study investigated the influence of non-uniform subgrade support on pavement responses (stress and deflection) that affect pavement performance. Several reconstructed PCC pavement projects in Iowa were studied to document and evaluate the influence of subgrade/subbase non-uniformity on pavement performance. In situ field tests were performed at 12 sites to determine the subgrade/subbase engineering properties and develop a database of engineering parameter values for statistical and numerical analysis. Results of stiffness, moisture and density, strength, and soil classification were used to determine the spatial variability of a given property. Natural subgrade soils, fly ash-stabilized subgrade, reclaimed hydrated fly ash subbase, and granular subbase were studied. The influence of the spatial variability of subgrade/subbase on pavement performance was then evaluated by modeling the elastic properties of the pavement and subgrade using the ISLAB2000 finite element analysis program. A major conclusion from this study is that non-uniform subgrade/subbase stiffness increases localized deflections and causes principal stress concentrations in the pavement, which can lead to fatigue cracking and other types of pavement distresses. Field data show that hydrated fly ash, self-cementing fly ash-stabilized subgrade, and granular subbases exhibit lower variability than natural subgrade soils. Pavement life should be increased through the use of more uniform subgrade support. Subgrade/subbase construction in the future should consider uniformity as a key to long-term pavement performance.
About the research
Temperature and moisture fluctuations associated with winter environments can damage PCC materials. When deicing chemicals are added, the damage to PCC materials can increase significantly. Although models describing the mechanisms of deicer damage have been proposed for some time, the relative influence of each mechanism and their interactions is still not completely understood. This project investigated the effects of exposing concrete to deicing chemicals in winter environments, particularly the impact of such exposure on the material strength, scaling, and mass loss.
The research results indicate that calcium chloride causes the most damage to portland cement materials, particularly in freeze/thaw conditions among all chemicals tested. Although there is no definitive evidence for chemical interactions between calcium chloride salts and PCC material, scaling and salt precipitation observed in calcium chloride samples suggests that crystal growth and hydraulic or osmotic pressures are primarily responsible for initial damage to PCC.
About the research
This project was triggered by interest in sustainable development and new environmental regulations on waste disposal. The addition of supplementary cementitious materials (SCMs) such as fly ash, slag, and other industrial byproducts to cements can improve concrete workability, durability, and long-term strength, but a gap in knowledge about the performance of SCM concrete under a variety of conditions has limited its use by the PCC paving industry. In this project, correlations were found among the source and proportion of the SCMs, curing conditions, concrete set time, maturity, strength development, and cracking potential.
Other findings include the following:
- Concrete performance varies with the source and proportion of cementitious materials used.
- As SCM content increases, longer curing times or higher curing temperatures may be needed.
- SCM concrete can perform comparably to or better than ordinary portland cement concrete under hot weather conditions.
- Traffic opening time of pavement should be based on strength and time-temperature factor.
- Potential benefits of a more informed use of SCM concrete include improved concrete workability, lower risk of thermal cracking, improved concrete durability and long-term strength, and reduced overall concrete cost.