COMPARATIVE ANALYSIS OF THE STRENGTH OF CONCRETE MADE FROM VARIOUS AGGREGATE

ABSTRACT

The comparative analysis of the strength of concrete made from various aggregate importance of studying the behaviour and properties of concrete can be highlighted by considering the fact that concrete is the most used man-made material in the world.  The very first step in making concrete is its mix design and deciding the type and amount of constitutes used in the production of concrete which should fulfil the requirements of the final product. Mix design models are commonly used for the purpose of proportioning concrete ingredients while anticipating the properties of the final product.

The current document deals with the commonly used principals in mix design models namely particle packing theory and excess water/paste layer theories. The conducted studies includes an investigation on accuracy of particle packing models (Toufar, 4C, CPM) and also tries to address the issue with measurement of specific surface area of particles as an essential input to water/paste layer theories.

It has been observed that the particle packing models can predict the packing density with acceptable margin. However, it should be mentioned that the particle packing models by themselves are not mix design models but should be rather used as a part of a mix design. In addition, it was found that the accuracy of calculating the specific surface area of particles based on their size distribution curve can be further improved by assuming angular platonic solids as uniform shape of aggregate instead of traditional approach of assuming spheres for aggregates’ shape.

1. INTRODUCTION

1.1. Background

Historical evidences indicate that cement was first used by ancient Macedonians around eighth century. The knowledge of making hydraulic cement was later on documented by French and British engineers in the 18^th century. Construction with cement and also usage of reinforcement in the structural design eventually led to making concrete the most used man made material. As the fundamental knowledge of making cement and concrete developed and was able to cover the basic questions about constitutes of concrete, researchers have been continuously working with the ways of optimizing mix design recipes. Optimization can be achieved by means of studying the ingredients of concrete mixes with the aim of maximizing the performance of concrete in both fresh and hardened state while keeping a low cost of production and limiting the pollutants released in the air due to cement production. As a result, several attempts have been made on formulating the mix design of concrete. Understanding the role of constitutes in fresh concrete is fundamental to the production of high quality concrete at fresh state, during hardening and as a hardened structural material. Fresh concrete can be characterized by several aspects among which workability is the most important one and is chiefly influenced by the water requirement, which in turn is a function of aggregates’ shape, grading, and fine content. As for the performance of the hardened concrete, the crucial factors are water to cement ratio which influences strength and permeability and cement characteristics and performance.

Among the components of concrete, aggregates have an important role especially in fresh stage as 60% to 80% of concrete volume is occupied by them. Moreover, increasing the amount of aggregates in volume of concrete corresponds to less usage of cement which has several beneficial effects, e.g. reduction in the cost of producing concrete, decrease in some of the durability problems of hardened concrete, reducing shrinkage and cracking, etc.

In addition, reduction in usage of cement leads to a decrease in pollution caused by its production. The cement industry produces about 5% of global man-made CO₂ emissions; the amount of CO₂ emitted by the cement industry can be as high as 900 kg of CO₂ for every 1000 kg of cement produced (Mahasenan et al., 2003). It should be noted that the cement industry worldwide and especially in Scandinavia and Europe takes its responsibility and strong efforts are taken to reduce the CO₂ emissions at production. Some companies (e.g. Cementa) have formulated a zero-vision (“Carbon capture newsletter”, 2014) and was able to reduce the CO₂ emissions per ton of cement to lower than 700 kg. Others companies are engaged in carbon capturing of emitted gas (“Meeting the challenge through a zero vision”, 2014) describing a Heidelberg Cement supported project. Also, concrete producers worldwide are now striving to reduce the amount of clinker and thus CO₂ by replacements such as fly ash, blast furnace slag, lime stone filler etc.

Currently, there are several models available for predicting the properties of concrete in both fresh and hardened states. Most of these models are based on the assumption that the properties of concrete in fresh state i.e. flow properties and workability are chiefly governed by the particle size distribution (PSD) and the particle packing (Glavind and Pedersen, 1999).

The packing density concept can be used as a part of concrete mix design with the aim of minimizing the inter-particle voids between the constituents of concrete in order to reduce the paste demand. Packing density is the ratio of the volume of solids to the bulk volume of the solid particles (Toufar et al., 1976; Quiroga et al., 2004). The date for one of the first articles on particle packing goes as far as 1892 (Feret, 1892), further research were conducted mainly concentrating on designing of an ideal aggregate size distribution curve (Fuller and Thompson, 1907; Andreasen and Andersen, 1930). In 1929 the first analytical packing model was designed to predict the void ratio of a mixture of two particle groups (Furnas, 1929). Since then, plenty of researches were conducted on the subject of packing resulting in development of several analytical models and computer-aided mix design software.

According to the above-mentioned models, particle packing can be increased by modifying particle size distribution (PSD) which in turn usually leads to increasing the share of fines. Packing theory assumes that adding fine particles to a particle structure helps fill up the voids in between the particles and leaving only minimum space for water. In this way, adding fine particles will reduce the water requirement (De Larrard, 1999; Kronlöf, 1994; Fennis, 2011). However, the packing of aggregate is dependent also on the shape of the aggregate particles, an effect that is more difficult to comprehend and it is indirectly accounted for by measuring the packing of mono- sized fractions.

Another approach to compiling a mix design model is based on excess paste/water layer theories first introduced by Kennedy (1940). A hypothesis by Brouwers and Radix (2005) states that the relative slump of a water-powder mixture becomes a function of the specific surface area (SSA) when sufficient water is present to flow. Based on the hypothesis, a thin layer of adsorbed water molecules around the particles is necessary to assure the flow characteristics of the hydrating system. It is reported that the thickness of this water layer is related to sensitivity of the mix to changes in the water content and also the specific surface area of the material used, as was later confirmed by Hunger (2010). Moreover, the layer theories assume that the water demand of a mixture depends on the specific surface area of the particles in that mixture. Increasing the surface area by adding small particles will increase the water requirement (Hunger and Brouwers, 2009; Maeyama et al., 1998; Midorikawa et al., 2009) which is in contrast to particle packing theory.

Both approaches (Particle packing and Water/paste layer theories) strongly depend on the shape of the aggregate in one way or another, that is especially more essential when it comes to water/paste layer theories which require specific surface area as an input for the model. While it is possible to directly measure specific surface area (SSA), the complexity of instrument required for the measurement imposes issues such as the availability of the testing instruments and the cost. It is also possible to estimate the SSA using the PSD data and the assumption that particles have ideal spherical shapes (McCabe et al., 1993).

Although currently there are several advanced concrete mix design models, they are rarely used by the concrete industry. One of the main reasons that these models are not used in practice is the complexity of the advanced models and the number of empirical input data that is required to use the models correctly. The input data for some of the models includes extensive chemical and physical tests on the ingredients of concrete. Moreover, some of the required data cannot be readily measured and/or in some cases there are no commonly accepted methods for conducting the measurement, as an example measuring the specific surface area of the particles. As a result of complexity and in cases lack of accuracy of the tests required for measuring the specific surface area, in most cases the value is calculated mathematically based on the size distribution curve and assumption of spherical shape for the particles. However, even in case of computation of specific surface area, the effect of square cube law is usually neglected.

The above mentioned issues emphasise the need for a comprehensive yet simple mix design model that can be both used in practice in the industry as well as further developments of mix design models. The thesis aims to lay a foundation for such a model by studying the role of aggregates as they form most of the concrete volume.

1.2. Research Objectives and Questions

According to above, the main objective of the project is to formulate an approach to mix design where the workability of the fresh concrete can be anticipated before actual mixing takes place. The mix design method may be based on principles of particle packing and water layer theory. Water demand, shape of aggregates and changes in size distribution curve, will be studied as the mentioned parameters can greatly influence the properties of concrete in fresh state.

The research is expected to answer the following questions:

• Is it possible to formulate a simple yet efficient mix design approach?
• Can optimizing concrete constitutes lead to a greener less pollutant production of concrete?
• What is the role and significance of water demand in workability of fresh concrete?
• What are the main parameters that results in aggregate from one quarry to have different performance properties comparing to another quarry? (shape, flakiness, size distribution curve, mineralogy etc.)

1.3. Limitations

The studies were restricted by various limitations which bounds the scope of this document: Firstly, the vast domain of interconnected parameters that affect the properties of fresh concrete makes it difficult to isolate and study each parameter without considering the possible effect of changes in other constitutes in concrete.

Secondly, it should be mentioned that all of the packing studies conducted were only based on loose packing method and does not include other packing methods such as hard packing and vibration compacted packing. Moreover, the used materials were taken from a limited number of Swedish quarries.

Thirdly, the complexity and the randomness in properties of aggregates such as shape, surface texture, size distribution curve, chemical components, etc. makes it almost impossible to simulate their flow in fresh concrete.

Fourth, this document only deals with the role of aggregates in mix design as a part of the overall research planned for completion of the PhD and does not include tests on concrete at this stage.

Finally, the complexity and the high cost of conducting accurate specific surface measurements limit the number of samples that can be tested.

1.4. Approach

Similar to general approach for scientific studies at Luleå University of technology, the presented research started by a literature review on available mix design models and the role and importance of concrete mix constitutes with the aim of understanding the current knowledge on the subject and also to understand the research gaps. The research questions were then formulated based on the revealed gaps and neglected aspects of the subject.

Available mix design approaches were divided in to three categories: a) Ideal size distribution curve, b) methods that are based on particle packing theory and c) the ones that are based on the concept of water demand and water/paste layer theories. The results from laboratory measurements of loose packing and calculations based on particle packing models (see Section 3.2) were presented and compared in Papers I and II. Papers III and IV deal with the principals of defining shape of particles by utilizing specific surface area which directly affects the water demand of mixtures. The last two papers were built on comparison of measured specific surface area by Blaine test to theoretically calculated specific surface area (3.3.2) based on size distribution curve and assumption of spherical shape for the particles.

COMPARATIVE ANALYSIS OF THE STRENGTH OF CONCRETE MADE FROM VARIOUS AGGREGATE