Application of Simplex-Centroid Design Methodologies to Optimize the Proportions of Ternary Cementitious Blends in High Performance Concretes

High performance concrete (HPC) mixtures often contain multiple cementitious components. Optimizing the proportion of these individual components to achieve the desired properties is extremely tedious requiring a large number of trial batches. This process is expensive and time consuming. The use of statistical mixture design technique provides a useful approach where in multiple outcomes can be met with fewer number of test runs. This is particularly true when multiple cementitious components are used in concrete. The research in progress here uses a statistical design of experiments approach, simplex-centroid design, with three cementitious components and seven minimum design points that represent specific mixture proportions. In this study, a ternary mixture blend of portland cement, slag and Class F fly ash was used to prepare concrete mixtures. Fresh and hardened properties of concrete were evaluated, including mechanical properties such as compressive strength and split tensile strength and durability indicators such as rapid chloride-ion permeability and expansion due to alkali-silica reaction. Results from this study suggest that simplex-centroid design method is a valuable tool in minimizing the number of trial batches needed to identify the optimal concrete proportions for achieving the desired properties

Effect of selected parameters on aggregate reactivity in Accelerated Mortar bar test method: Aggregate Size & Deicers

Abstract Alkali Silica Reaction (ASR) is a chemical reaction between reactive siliceous aggregates and the alkali hydroxides present in the pore solution of hydrated cement paste in concrete. The chemical reaction produces ASR gel that is hygroscopic in nature and is volumetrically unstable in the presence of moisture. Expansion resulting from the swelling of the gel creates tensile stresses in the concrete leading to cracking and distress. While ASR distress has been known to occur in concrete for over last 70 years, there has been an increase in the frequency of this distress in recent years. This is primarily due to a combination of four factors. Firstly, marginal aggregates are increasingly being used in concrete due to shortage of good quality aggregate, particularly in urban locations where development of new source of aggregates is restricted. Secondly, the alkali content of cement has gradually increased over last two decades due to increased environmental regulations on the emissions from cement industries. Thirdly, the existing test methods employed to evaluate the reactivity of aggregates are not entirely effective with different rock types. Lastly, the development and use of new types of alkaline chemical deicing agents on concrete has created a hitherto unanticipated situation. Among the first defenses to combat ASR is an effective test method to screen reactive aggregates. Screening of aggregates for their reactive nature has been conducted by several laboratory test methods available in the concrete industry, but none of them have proven to be very reliable to assess the reactive nature of all the aggregate types accurately. Among the several tests available, the Accelerated Mortar Bar Test (AMBT) method is widely used, followed by Concrete Prism Test (CPT). The AMBT method suffers from high variability in test results and has the potential to mischaracterize a good performing aggregate as reactive. Also, recently a variant of this test method (EB-70 protocol) to evaluate impact of deicing chemicals on aggregate reactivity was introduced through Federal Aviation Administration (FAA). However, poor correlation between the results of deicer-based AMBT and the standard AMBT has required additional investigation to develop better test procedures. The CPT method is more reliable than AMBT method, however it take a long time to complete and is considered to impractical from field perspective. This thesis describes research conducted to improve existing test methods to better characterize the aggregate reactivity. The three principal objectives of this study are: (1) Determine the impact of re-sizing coarse aggregates to meet the gradation requirements of ASTM C 1260 test method in assessing the reactivity of the aggregates. (2) Develop a better test method to evaluate the reactivity of aggregate in presence of potassium acetate deicing chemical (3) Decrease the length of ASTM C 1293 test method in assessing the reactivity of the aggregates. The first objective of this study is to study the impact of processing coarse aggregate (i.e. crushing, sieving and washing) on its reactivity. One of main factors that could be affecting the reliability of the AMBT method is the aggregate gradation. When coarse aggregates are to be evaluated in this method, it needs to be crushed and processed. In processing the coarse aggregates, the distribution of the reactive siliceous phases in the aggregate can be significantly altered relative to the surface of the aggregate particles, thus affecting the reactivity of the aggregate. Aggregates depending on their source and formation have different mineralogical phases and consequently the reactive nature of silica present in those aggregates differs. The size of an aggregate plays an important role in evaluating the ASR potential of aggregates, as the presence of reactive silica and the ease of availability to surrounding alkali\u27s to cause the reaction determines the initiation of the reaction and potential of aggregate and concrete to crack with age. To investigate the effect of aggregate size in the AMBT method 4 different reactive aggregates were used representing a wide variety of mineral composition. SEM and EDX mapping techniques were employed to confirm the presence of variety of elements in the mineral structure and the variation between different aggregates. Each of the aggregates were crushed, sieved and batched according the standard ASTM C 1260 gradation. Each fraction of the ASTM C 1260 gradation was replaced with 20% reactive aggregate and 80% non reactive to meet the total mass requirements for the test. The cement to aggregate ratio was increased to 1: 1.75 for ease of workability. The mortar bars were prepared and kept in 1N NaOH soak solution at 80\u27C for 28 days and expansion readings were measured at regular intervals. The results showed that each individual aggregate size had different levels of expansion and the size factor is predominant in evaluating ASR potential of aggregates. The second objective of this study is to evaluate the effect of deicers on aggregates in the AMBT method 16 different aggregates were used that represent aggregates with different mineralogy. In this study, the standard ASTM C 1260 test method was adopted along with 3 different versions of the deicer-modified AMBT methods. These included AMBT protocols with three different soak solutions - 6.4M potassium acetate soak solution, 3M potassium acetate soak solution and combination of 3M potassium acetate + 1M sodium hydroxide soak solution. Of the 3 methods evaluated the combination solution of 3M potassium acetate + 1M sodium hydroxide proved to be most effective test method to evaluate aggregate reactivity in the presence of deicing chemicals. SEM and EDX were used to confirm the presence of ASR gel and study the deleterious behavior of ASR gel. The third objective of this research study is to investigate the possibility of decreasing the duration of CPT method by pre-saturating the aggregates with alkali solutions to increase the pace of reaction mechanism. However, in order to investigate the concept of pre-saturating the aggregate, ASTM C 1260 test method was considered due to its shorter test duration. In this study, the standard ASTM C 1260 test method was modified by pre-saturation of aggregates with 1N sodium hydroxide for 24 hrs and then the standard procedure was followed in preparing and using the aggregates. The results from this study indicate that the modification of aggregates by pre-saturation with 1N sodium hydroxide did not provide any better results at 14 days age compared to the standard ASTM C 1260 test. However for some highly reactive aggregates 3 and 7 day mortar bar expansions were higher compared to standard test. The pre-saturation of aggregates with deicer solution was also adopted, but the presence of deicing chemicals did not allow the cement hydration reaction and the mortar bars were not cured and set. In this thesis, based on the research studies conducted it is clear that aggregate size plays an important role in determining ASR potential of aggregates in the AMBT method. Thus it\u27s appropriate to use the aggregates in their natural available state and no fine crushing of aggregates should be done for employing in the test methods. Aggregates with wide range of mineralogical content behave differently on exposure to different deicer soak solutions. Thus it is beneficial to screen the aggregates with combination of alkali and deicer soak solution to assess their reactivity potential. However, further petrographical investigations on aggregates would clarify the presence of deleterious silica, and aggregates can then be grouped based on their reactive silica phase content and different ASR mitigation measure can be suggested for each group of aggregates

Optimizing SCM Proportions to Meet Multiple Performance Characters of Ternary Concrete Mixtures Using Simplex-Centroid Design and Analysis Techniques

High performance concrete mixtures often contain multiple cementitious components. Among these, cement is the most expensive in addition to having a higher carbon footprint. Life cycle assessment of cement production reveals that the cement content is the most important factor in determining a concrete mixture\u27s embodied energy and carbon footprint. Compressive strength, an important property of concrete, is directly related to the quantity of cement used in the mixture. However, higher quantities of cement lead to durability issues. The increased concerns about the durability of concrete over the past decade have increased focus on improving the long-term performance of concrete structures. The goal of reducing the quantity of cement has led the use of supplementary cementitious materials (SCM) such as slag, fly ash, silica fume and others as a replacement. The traditional method for optimizing high performance concrete mixtures involves systematically varying the individual proportions of the components in small increments and studying the resultant effect. In this method, the basis for selecting SCM dosage is arbitrary and often focuses on a specific set of requirements such as strength or durability. Optimizing the component proportions in the traditional way to achieve the desired properties is time-consuming, requiring a large number of trial batches, making this process expensive and inefficient. The use of statistical mixture design techniques has the potential to reduce the number of test runs needed, especially when multiple cementitious components are used and multiple requirements have to be simultaneously satisfied. The research reported here investigates the use of a statistical design of experiments approach, specifically the simplex-centroid mixture design, using three cementitious components and a minimum of seven design points representing specific mixture proportions. In this study, a ternary blend of portland cement, slag and Class F fly ash was used. The total cementitious content of the concrete was kept constant although the individual proportions were varied. Fresh and hardened properties of concrete were evaluated, including mechanical properties such as compressive strength and split tensile strength and durability indicators such as rapid chloride ion permeability and expansion due to alkali-silica reaction. With the use of statistical design software (JMP), strength and durability prediction equations were developed and subsequently validated using an additional five concrete mixtures. These prediction equations investigated here generated a response surface for a given property as a function of the proportions of the three cementitious components using the seven concrete mixtures. Multiple response surfaces were superimposed on the simplex design region, and optimum cementitious mixtures were identified. The ternary blends were also used to evaluate mortars for alkali-silica reaction potential in mortar bars, and fundamental studies on cementitious paste systems involved pore solution extraction analysis and electrical resistivity. The results obtained from this study showed that the properties of concrete such as compressive strength and rapid chloride ion permeability had a good correlation between the actual and predicted values whereas properties such as split tensile strength did not a show good correlation. The deleterious effects of alkali-silica reaction in mortar and concrete were evaluated using a threshold expansion value. These evaluations indicated that the mixtures below the threshold expansion contour in the simplex region did not show any alkali-silica reaction distress. The results from the cementitious paste studies showed that the electrical resistivity of the cementitious paste systems increased with decreasing ionic concentrations in the pore solution due to the replacements of cement with SCMs. In addition, the pore solution analysis showed that because of the pozzolanic reaction of SCMs, the alkali ions become trapped in the secondary C-S-H gel and the pore solution alkalinity is reduced with age. At elevated temperatures due to the instability of the calcium sulfo-aluminate phases, the sulfate ions (SO4-2) dissolved back into the pore solution. Using the simplex centroid design technique the pore solution results can be used to generate response surface for ionic concentrations of cementitious paste systems. Results from this research suggest that the simplex-centroid design could be a valuable tool for minimizing the number of trial batches needed to identify the optimal concrete proportions for achieving the desired properties. As an outcome of this research, guidelines were developed for using the simplex-centroid method for concrete mixture design applications. The optimum mixtures obtained for various concrete applications within the simplex region yielded optimum cement dosages, in turn reducing the cost of concrete and its carbon footprint. Future work in this area should include using different SCMs to optimize desired properties of concrete. In addition, this concept can be extended to include the w/c ratio, another important property of concrete. Various statistical mixture designs techniques can also be explored to improve the predictability power. Ultimately, the research in this area should lead to more cost-effective concrete with a smaller carbon footprint that can be adopted for use in the field