Date of Award
Master of Science (MS)
Rangaraju, Dr. Prasad
Putman , Dr. Bradley
Klotz , Dr. Leidy
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's 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'C 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's 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.
Math, Sujay, "Effect of selected parameters on aggregate reactivity in Accelerated Mortar bar test method: Aggregate Size & Deicers" (2011). All Theses. 1113.