Background
Here is some background information on our research topic that should be understood before proceeding into the methodology of our project
What is Concrete?
Aggregate
Water
Cement
Concrete refers to aggregates of stone, sand or gravel binded together with a mixture of cement and water, essential for construction of infrastructure such as bridges and buildings
Challenges with Sedimentary Rock
While replacing granite with sedimentary rock can potentially provide numerous benefits, sedimentary rock does pose some challenges with regards to concrete design.
ALKALI - SILICA REACTION, or ASR, is a reaction that occurs between the silica in sedimentary rock, and the alkalis in Portland Cement. This produces a alkali - silicate gel, that expands greatly in volume, causing cracks in the concrete. An expansion of equal to or less than 0.1% due to ASR will deem the aggregate suitable for use as concrete.
LOWER STRENGTH OF SEDIMENTARY ROCK has also been reported (Rock Mechanics: Properties of Rock Materials, n.d.) As the type of aggregate greatly affects the properties of concrete (Shetty, 2005), this can result in the resulting concrete having less strength than granite concrete, making it unsuitable to be used for construction work. For concrete to be deemed suitable for construction work, its compressive strength should be at least 50N/mm2 and tensile strength should be be at least 2.5N/mm2.
Solution Design
Our solution design will be divided into 2 parts to investigate each of the challenges brought about by sedimentary rock.
ALKALI - SILICA REACTION. There have been numerous methods to attempt to mitigate ASR reaction, and these methods typically involve eliminating or minimizing any of the following factors: reactive silica in the aggregate, alkaline pore cement solution, and moisture (Farny, & Kerkhoff, n.d.) Our project focuses on 2 methods, namely the use of fly ash and chemical additives.
For fly ash, it is a supplementary cementitious material (SCM) that is effective in mitigating ASR due to several factors. It limits the permeability of concrete thus minimizing moisture, and by replacing cement in the concrete reduces the amount of free alkali ions. For our testing we will replace 25% of cement by mass with fly ash. This is the optimal mass because while adding more fly ash may increase effectiveness of ASR mitigation, replacing more than 25 - 30% cement with fly ash can result in lowered strength of the concrete.
Lithium salts have been well-documented to mitigate ASR as well. Its exact inhibiting mechanism is not exactly known, but a possible explanation is the preference in forming non-expansive lithium silicate instead of the alkali-silicate, thus forming less alkali silicate.
LOWER STRENGTH OF SEDIMENTARY ROCK. It is very challenging to come up with a solution design if sedimentary rock does indeed possess physical properties that are unsuitable for concrete production, such as compressive strength below the acceptable limit, because as mentioned aggregates do affect the properties of the concrete greatly.
Hence instead of coming up with a solution design, our project will conduct testing on both the sedimentary rock aggregate and concrete samples to verify if using sedimentary rock in concrete production does result in concrete of lower compressive and tensile strength. This is because the reported physical properties by Farny and Kerkhoff may differ from the properties of local sedimentary rock due to different geographical location and hence different composition.
From background research, the objectives of our experimentation are to:
1. Test physical properties of concrete with sedimentary rock as aggregate, and compare it with that of granite
2. Test for effectiveness of methods to mitigate ASR, namely addition of fly ash and lithium salts