Yahya Ghasemi Print III pdf

2. AGGREGATES IN CONCRETE
2.1. Purpose and role of aggregates
Understanding the role of aggregates in concrete is fundamental to the production of good 
concrete as aggregates have their greatest influence on the performance of fresh concrete 
(Alexander and Mindess, 2010). 
The aggregates form 60 to 80 per cent of the volume of concrete. Despite the amount of 
aggregates used in production of concrete, their influence is sometimes overlooked and the 
aggregates are considered only as fillers. However, it is essential to consider and study all of 
constitutes in concrete mixtures, including aggregates, in order to obtain a comprehensive 
understanding of the behaviour of the final product. As a result of such considerations, it is 
clear that the role of aggregates is crucial to ensure satisfactory performance of mixtures since 
they govern the volumetric stability of concrete. They also may influence the moisture related 
deformations (e.g. shrinkage) as shown in Figure 2.1.
Figure 2.1. Effect of addition of aggregate to a cement paste on reduction of shrinkage (Powers, 
1971)(from Addis and Owens, 2001)

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In addition, aggregates have an important effect on concrete strength by providing rigidity to 
the material which governs resistance to applied loads and undesired deformations.
The original w/c ratio law for concrete strength introduced by Abram (1918) was limited to 
normal structural concrete with maximum aggregate size of 38 mm and w/c ratio greater than 
0.4. The simplified approach proposed by Abram led to common inaccurate assumption that 
properties of aggregates have no effect on the compressive strength of concrete causing the 
aggregate to be considered solely as inter fillers. While Abram’s law was defined for a special 
scenario, at present it is known that other factors than w/c ratio also play a role in estimating 
the compressive strength of concrete. This is especially true in case of high strength concrete 
(HSC) in which, due to the dense microstructure and strong transition zone, the mechanical 
properties of the aggregates become more important (Alexander and Mindess, 2005). In 
addition, as an example, a study conducted by Walker and Bloem (1960) on the effect of 
maximum size of aggregates on concrete strength independently of w/c ratio suggests that the 
strength decreases as aggregate size increases over the full range of w/c, the typical results are 
shown in Figure 2.2. However, generally the discussed relationship has limited significance as 
aggregates with the same size but from different sources can lead to broader strength 
differences comparing to the influence of size.
Figure 2.2. Water/Cement ratio vs. strength for maximum size of aggregates. (Walker and Bloem, 
1960)
The mentioned effects should be separated from the characteristics of aggregate that influence 
the water requirement and consequently strength of a mix such as shape, surface texture, 
grading, and maximum size of aggregate. For example, regarding surface texture, concretes 
made with smoother gravels suffer from crack initiation at lower compressive stresses 
comparing to concretes made with coarser textured aggregates (Alexander and Mindess, 
2005). 

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Aggregates in concrete