Replacement of Cement by Ash

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This research work is based on the use of Rice Husk Ash (RHA) as filler in Asphalt concrete pavement. Asphalt mix design was carried out using Marshall Stability method to test the performance of the material in terms of its known engineering properties. Several trial mixes with bitumen contents of 4.5%, 5.5%, 6.5% and 7.5% were produced in order to obtain the optimum bitumen content. This investigation focuses on the partial replacement of cement with Rice Husk Ash in the obtained optimum bitumen content in the following order 0% (control), 5%, 7.5%, 10%, 12.5%, 15%, 17.5%, 20%, 22.5%, and 25%. A total of 42 mix specimens were produced for this experiment, 12 of these mix specimens were compacted with each percentage of bitumen content, to determine the optimum bitumen content, and 30 specimens were produced to determine the optimum Rice Husk Ash content in terms of the asphalt concrete strength. From the Marshall Stability-flow test and density-void analysis, results obtained show that the performance of mix containing 0% of RHA (control), have Stability, flow, Compacted density of mix (CDM), Void in Mix (VIM), Void in Mineral Aggregate (VMA), and Void filled with Bitumen (VFB) as 6.7KN, 3.0mm, 1.49g/cm³, 39.4%, 47.27% and 16.63% respectively at an optimum bitumen content of 5.5%. The sample prepared with 10% RHA as filler have Stability, flow, CDM, VIM, VMA, and VFB of 7.63%, 2.19mm, 1.78g/cm³, 28.23%, 36.77%, and 23.23% respectively at an optimum bitumen content of 5.5% which satisfied the provision in the Standard Specification requirement of Marshall Criteria by Asphalt Institute (1979). Thus for maximum strength, 10% RHA is recommended as partial replacement of cement as filler in Asphalt Concrete mix.


    1.      Background

A pavement could be defined as a hard surface constructed over the natural soil for the purpose of providing a stable, safe and smooth transportation medium for the vehicles (Merriam, 2013).
Hot mix asphalt (HMA) is a generic term that includes many different types of mixtures of aggregate and asphalt cement (binder) produced at elevated temperatures (generally between 300-350ºF) in an asphalt plant. Typically, HMA mixtures are divided into three mixture categories: dense-graded; open-graded; and gap-graded as a function of the aggregate gradation used in the mix (Griffiths and Thom, 2011).
1.1.2    Supplementary cementitious materials (SCMs)
Supplementary cementitious materials are often incorporated in Asphalt concrete mix to reduce cement contents, improve workability, increase strength and enhance durability.
The use of SCMs dates back to the ancient Greeks who incorporated volcanic ash with hydraulic lime to create a cementing mortar. The Greeks passed this knowledge on to the Romans, who constructed such engineering marvels as the Roman aqueducts and the coliseum, which still stands today. Early SCMs consisted of natural, readily available materials such as volcanic ash or diatomaceous earth. More recently, strict air-pollution controls and regulations have produced an abundance of industrial by-products that can be used as supplementary cementitious materials such as flyash, silica fume and blast furnace slag. The use of such by-products in concrete construction not only prevents these products from being land-filled but also enhances the properties of concrete in the fresh and hydrated states.  SCMs can be divided into two categories based on their type of reaction: hydraulic or pozzolanic. Hydraulic materials react directly with water to form cementitious compounds, while pozzolanic materials chemically react with calcium hydroxide (CH), a soluble reaction product, in the presence of moisture to form compounds possessing cementing properties. The word “pozzolan” was actually derived from a large deposit of Mt. Vesuvius volcanic ash located near the town of Pozzuoli, Italy. Pozzolanic SCMs can be used either as an addition to the cement or as a replacement for a portion of the cement. Most often an SCM will be used to replace a portion of the cement content for economical or property-enhancement reasons. Here is a brief overview of one of the more common pozzolans used in the manufactured concrete products industry (Neuwald, 2010)
1.1.3    Rice husk      
Rice husk is an agricultural residue which accounts for 20% of the 649.7 million tons of rice produced annually worldwide. The produced partially burnt husk from the milling plants when used as a fuel also contributes to pollution, and efforts are being made to overcome this environmental issue by utilizing this material as a supplementary cementitious material.
The chemical composition of rice husk is found to vary from one sample to another due to the differences in the type of paddy, crop year, climate and geographical conditions. Rice husk is one of the most widely available agricultural wastes in many rice producing countries around the world. Globally, approximately 600 million tons of rice paddy is produced each year. On average 20% of the rice paddy is husk, giving an annual total production of 120 million tonnes. In majority of rice producing countries much of the husk produced from processing of rice is either burnt or dumped as waste. Burning of RH in ambient atmosphere leaves a residue, called rice husk ash. For every 1000kg of paddy milled, about 220kg (22 %) of husk is produced, and when this husk is burnt in the boilers, about 55kg (25 %) of RHA is generated. The non-crystalline silica and high specific surface area of the RHA are responsible for its high pozzolanic reactivity (Miyagawa and Gaweesh, 2001).
A pozzolanic reaction occurs when a siliceous or aluminous material get in touch with calcium hydroxide in the presence of humidity to form compounds exhibiting cementitious properties (Papadakis et al., 2009). Data from reaction results between RHA and CH indicates that the amount of CH by 30% RHA in cement paste begins to decrease after 3 days, and by 91 days it reaches nearly zero, while in the control paste, it is considerably enlarged with hydration time (Yu et al., 1999).
1.2       Aim and Objectives of the research
1.2.1    Aim
The aim of this research work is the partial replacement of cement with rice husk ash (RHA) using Marshall Stability Method.
1.2.2    Objectives

  1. To carry out preliminary tests on rice husk ash and all other asphalt concrete constituents, to determine its physical and chemical composition.
  2. Preparation of trail mix by varying aggregates, ordinary Portland cement, rice husk ash, with predetermined percentages of bitumen content.
  3. To determine the engineering properties of the specimen mix, using Marshall Stability method.
  4.  Determination of optimum rice husk ash conten for Asphalt concrete.

Scope of Research
This study is limited to the evaluation of compressive strength of asphalt concrete having its filler been supplemented with rice husk ash. This will be achieved by carrying out preliminary studies on the constituents of asphalt concrete, and the use of Marshall Stability test in determining the mechanical properties of the asphalt concrete mix.

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