EFFECTS OF COARSE AGGREGATE CHARACTERISTICS ON THE PROPERTIES OF CONCRETE

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                                            Chapter - 3

      EFFECTS OF COARSE AGGREGATE CHARACTERISTICS ON
                        THE PROPERTIES OF CONCRETE

3.1    General

       An ingredient, which comprises three-fourth of its product and has a number of own
physical, mechanical and chemical attributes, must have significant impact upon the later.
In fact, a single feature of such an constituent, e.g. gradation of aggregate can have multi-
facet consequences for concrete. Similarly, it is also possible that different features of the
aggregate support or cancel out the effects of one another when combined to form
concrete.

       The effects on concrete properties can be observed in all stages of mixing, placing,
hardened state and service life. The aggregate characteristics of grading, shape, texture
etc influences workability, finishability, bleeding, pumpability, and segregation in fresh state,
while the same properties          affects density, strength, stiffness, shrinkage, creep,
permeability, and durability in hardened state      [Lafrenz, 1997]. The long term effects of
durability such as alkali-silica reaction and alkali-carbonate reaction are particularly more
important as the durability of concrete depends on potential susceptibility of aggregate for
these reactions.
       The effects of various features of coarse aggregate upon concrete are described in
the succeeding paragraphs.

3.2    Effects of physical/mechanical properties
       3.2.1 Particle Size and Gradation. Particle size distribution and grading is one of
       the most complex properties of aggregate and it influences the concrete properties
       in a multifarious manner and at all stages. Its effects on some properties of concrete
       are discussed below:-
              3.2.1.1    Workability.

                         (1)    In fresh concrete, particle size distribution and grading
                                significantly affects mixing, pumpsbility, placement, packing
                                density, segregation, and void content of concrete. Many
                                authors claim that uniformly distributed aggregate produce
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                  better workability than gap-graded mixtures although; higher
                  slumps have been achieved with gap-graded materials. It is
                  also believed that bleeding is better controlled with well
                  graded aggregate.
          (2)     Shilstone and Galloway have carried out extensive work on
                  contents of fine vs coarse aggregate and their effects on
                  properties of concrete in plastic state. If the aggregate is too
                  coarse, it will cause bleeding, segregation and harshness.
                  Contrarily, too fine aggregate will raise the demand for
                  water. A smooth grading curve, having neither deficiency
                  nor excess of any one particle size, generally produces
                  mixtures with fewer voids between particles.
          (3)     Because cement costs more than aggregate and the cement
                  paste requirement for concrete increases with increasing
                  void content of the combined aggregates, it is desirable to
                  keep the void content as low as possible. If there is not
                  enough fine aggregate to fill the voids between coarse
                  aggregate particles, the space must be filled with cement
                  paste. Such under-sanded mixtures also tend to be harsh
                  and difficult to finish. On the other hand, aggregate
                  combinations with excessive amounts of fine aggregate or
                  excessively       fine   sands   may   produce   uneconomical
                  concretes because of the larger surface area of finer
                  particles, which requires additional cement [ACI Education
                  Bulletin E-07].
3.2.1.2   Strength. The effect of grading on strength is a bit controversial.
Some authors believe that the strength of fully compacted concrete with a
given water/cement ratio is largely independent of the aggregate grading.
Hence, a given strength can be achieved with both well-graded and poorly
graded aggregate. Other studies, however, indicate that increased strength of
concrete can be achieved with well-graded mixtures only [Quiroga, 2004]. It is
also believed that since grading has a direct bearing upon density of concrete
which in turn is a function of the compressive strength of hardened concrete,
it follows that grading does effects the strength as well.
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3.2.1.3     Absorption, Porosity and Permeability. Grading determines the
amount of cement paste which controls other properties like absorption,
porosity and permeability of concrete. Aggregate with high void contents
requires more paste (cement, sand and water) for a given level of workability,
whereas, uniformly distributed mixtures generally lead to higher packing,
resulting in concrete with higher density and less permeability. [Quiroga,
2004].
3.2.1.4     Creep and Shrinkage. Uniformly distributed mixtures require less
paste, thus decreasing bleeding, creep, and shrinkage. Although excess of
coarse aggregate results in reduced drying shrinkage, it is likely to increase
the amount of micro cracks within the paste.
3.2.1.5     Modulus of Elasticity. Investigation by Chun-Qing Li and Jian-Jun
Zheng shows that the two most important factors affecting the elastic modulus
of concrete are the elastic modulus of aggregate and the water-cement ratio
(w/c). They have concluded that the elastic modulus of concrete increases
with increase in the aggregate volume fraction, the elastic modulus of
aggregate, the elastic modulus of interfacial transition zone, and the
maximum aggregate diameter.
3.2.1.6     Durability. Within limits, lesser the paste at a constant w/c ratio,
the more durable the concrete is [Shilstone, 1994]. Hence, minimizing the
aggregates voids content should be one of the objectives of optimization of
concrete mixtures. Mixture proportioning methods should encourage concrete
optimization and consequently aggregate optimization [Quiroga, 2004]. But if
ASR or ACR is suspected, it will be directly influenced by the amount and
particle size distribution of the aggregate.
3.2.1.7     From the above discussion it is evident that no ideal grading can be
advocated and, therefore, comprise is the outcome.                    At times, only the
construction method may overrule grading above other factors, e.g. the fine
content required for pumping the concrete.

Note: Grading of aggregate will not be discussed any further keeping in view the objectives of
      this study. Since crushed aggregate from existing sources is being investigated,
      gradation can be ordered at will for any work at hand.
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3.2.2 Maximum Size and Nominal Maximum Size
      3.2.2.1    Maximum size of aggregate influences concrete properties like
      workability, strength, shrinkage, and permeability [Quiroga, 2004].
      3.2.2.2    Large size of aggregate means small surface area which in turn will
      result in lower w/c ratio and thus higher strength.
      3.2.2.3    It has, however, been reported that the gain in strength due low w/c
      ratio is affected by weaker bond of large size particles. Therefore, with usual
      proportions of concrete, aggregate with maximum size greater than 1 or 1 ½
      is of little use [Neville. p.174].
3.2.3 Particle Shape & Surface Texture.
      3.2.3.1    Shape and Texture of aggregate effects the behavior of both fresh
      and hardened concrete and determines the demand for FA.
      3.2.3.2    An excess of poorly shaped particles increases water demand
      resulting in reduced strength of concrete. Also concrete made with poorly
      shaped aggregate requires more cement paste because of the higher voids
      content. Consequently, savings in cement paste of about 4 to 5 percent can
      be expected when cubical aggregate is used [Hudson, 1999].
      3.2.3.3    Shape of aggregate also effects the packing and thereby the
      density. More number of coarse particles can be packed when these are
      relatively cubical or spherical in shape [Hudson, 1999].
      3.2.3.4    Flaky, elongated, angular, and rough particles have high voids and
      require more FA to fill voids in order to provide workable concrete, thus
      increasing the demand for water [Quiroga, 2004].
      3.2.3.5    Flaky and elongated particles tend to produce harsh mixtures and
      affect finishability.    According to Shilstone [1990], flaky and elongated
      particles, principally those of intermediate sizes {between 3/8 in. (9.5mm),
      and No. 8 (2.36 mm)}, can affect the mobility of mixtures and contribute to
      harshness.
      3.2.3.6    Flaky or flat particles also tend to orient themselves in on direction
      which results in anisotropic concrete impairing its strength and durability.
      3.2.3.7    Texture affects bond between particles and mortar affecting
      strength and shrinkage. Rough particles tend to provide stronger bond than
      smooth particles. As a result, rough particles tend to produce higher strengths
      [Kaplan, 1959] and tend to decrease shrinkage.
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3.2.3.8    Although rough surfaces tend to increase the demand for water for
given workability, thus decreasing strength and increasing bleeding, workable
concrete can be made with well-graded, cubical crushed aggregate.
Satisfactory concrete has been obtained with aggregates having surface
texture from very smooth to very rough (except that the amount of “glassy”
pieces in slag coarse aggregate is limited for some specifications) [Quiroga,
2004].
3.2.3.9    There is also an agreement amongst various researchers that since
shape and texture influence micro-cracking in the transition zone of concrete,
they affect the stress-strain curve of concrete hence its modulus of elasticity.
3.2.3.10 The effect of shape on strength is also not without controversy.
Although there is an agreement that concrete with the same strength level
have been made with aggregates of different shapes at given cement content,
some authors state that well-shaped aggregates tend to produce higher
strengths than poorly shaped aggregates [Quiroga, 2004].
3.2.3.11 Considering all of the factors that have an effect on concrete
strength, the following appear to be most important [ACI Education Bulletin
E1-07]:-
           (1)    The surface area available for bond to the cement paste.
                  Here, the shape and texture of the largest particles is most
                  important.
           (2)    The surface texture of the largest pieces, which affect the
                  bond strength per unit of surface area. The mineralogy and
                  crystal structure of these largest pieces affects bond
                  strength per unit area as well.
           (3)    The relative rigidity of the aggregate particles compared with
                  the surrounding paste or mortar. The closer the deformation
                  characteristics of the aggregate are to that of the
                  surrounding media, the lower are the stresses developed at
                  particle surfaces.
           (4)    Maximum size of the aggregate. For a given w/c, as the size
                  of the larger particles is increased, the likelihood of bond
                  failure between paste and aggregate increases because
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                         stresses at the interface are higher than those for smaller
                         particles.
3.2.4 Specific Gravity
      3.2.4.1     The specific gravity of an aggregate is used in mixture
      proportioning calculations to find the absolute volume that a given mass of
      material will occupy in the mixture.
      3.2.4.2     Specific gravity is also useful in calculating other aggregate
      parameters such as void content, moisture content, identification and
      separation of deleterious particles etc.
      3.2.4.3     Relative density is not necessarily related to aggregate behavior.
      However, it has been found that some aggregates compounds of shale,
      sandstone, and chert that have low specific gravity may display poor
      performance, particularly in concrete exposed to accelerated freeze-thaw
      conditions.
      3.2.4.4     Changes in the aggregate specific gravity also cause the concrete
      density to change. This may be undesirable if a minimum density is specified,
      for example, in heavyweight concrete for nuclear-radiation shielding.
      3.2.4.5     While the specific gravity of an aggregate is not a measure of
      aggregate quality, a variation in the specific gravity may indicate a change in
      the aggregate characteristics.
      3.2.4.6     In a given concrete mixture, substituting one aggregate with
      another of a different specific gravity will cause the volume of concrete to
      change for the same batch mass. Because concrete is often sold by volume,
      this change means that either the purchaser is receiving less concrete than
      ordered or the producer is supplying more concrete than purchased.
3.2.5 Bulk Density & Void Ratio
      3.2.5.1     Since the bulk density of an aggregate is a function of grading,
      specific gravity, shape, angularity and surface texture of particles [ACI
      Education Bulletin E1-07], it will influence those properties of concrete that
      are affected by these factors.
      3.2.5.2     For a given specific gravity of CA, a higher bulk density means that
      there are fewer voids to be filled by FA and cement, hence economical
      concrete.
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3.2.6 Porosity & Absorption
      3.2.6.1     Since the aggregate represents a large volume of the concrete
      matrix, it is evident that porosity and absorption of aggregate will contribute to
      the overall porosity and absorption of hardened concrete.
      3.2.6.2     The types, sizes and amount of pores present in the aggregate
      determine the amount of water to be absorbed, passed or retained by the
      particle.
      3.2.6.3     Depending upon the aggregate moisture state, the absorption of
      water by aggregate during mixing will result in loss of workability. However,
      after about 15 minutes, the loss becomes smaller.
      3.2.6.4     There is also a possibility that after dry aggregate becomes coated
      with cement paste (particularly in rich mixes), some pores in the aggregate
      particle becomes filled or blocked such that there is no further absorption of
      water, though the particle has the capacity.
      3.2.6.5     Aggregate porosity may affect durability as freezing of water in
      pores of aggregate particles can cause surface popouts. However, some
      authors believe that the relationship between absorption and freeze-thaw
      behavior has not proven to be reliable. Nevertheless, absorption can be used
      as an initial indicator of soundness [Quiroga, 2004].
      3.2.6.6     Pores smaller than 4 µm are believed to affect      the durability of
      aggregate through freezing and thawing [Neville. p.128].
      3.2.6.7     Although there is no clear-cut relation between the strength of
      concrete and the water absorption of aggregate, the pores at the surface the
      particle affect the bond between the particle and the cement paste, and may
      thus result in reduction of the concrete strength [Neville. p. 129].
      3.2.6.8     It is also believed that aggregates with low absorption tend to
      reduce shrinkage and creep of the concrete.
3.2.7 Moisture Content
      3.2.7.1     The internal structure of an aggregate particle is made up of solid
      matter and voids that may or may not contain water.
      3.2.7.2     Porosity and absorption, discussed above, determines the moisture
      content of an aggregate which must be known so that the total water content
      of the concrete can be controlled and correct batch weights determined.
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      3.2.7.3   Depending on the aggregate moisture states, discussed in
      chapter-2, it will either absorb or contribute water to the concrete mix. The
      amount of water added to the concrete must be adjusted for the moisture
      conditions of the aggregates in order to accurately meet the water
      requirement of the mix design.
      3.2.7.4   Failure to make adjustment for moisture content and absorption of
      aggregate will alter the w/c ratio, thereby, changing other properties of
      concrete such as workability, strength etc.
      3.2.7.5   In addition, if the water content of the concrete mixture is not kept
      constant, the w/c ratio will vary from batch to batch causing other properties,
      such as the compressive strength and workability to vary from batch to batch.
      3.2.7.6   CA holds much less water compared to FA, has less variable
      moisture content and generally causes fewer problems.
3.2.8 Strength of Aggregate
      3.2.8.1   The strength of aggregate does not generally influence the strength
      of conventional concrete, where it is controlled by the paste or by the bond
      between paste and aggregate.
      3.2.8.2   Comparing concrete made with different aggregates, it has been
      observed that the effects of aggregate on the strength of concrete is
      qualitatively the same irrespective of the mix proportions, both for
      compressive as well as flexural strengths. It is also believed that besides
      mechanical strength of aggregate, its absorption and bond characteristics too,
      influence the concrete strength [Neville. p. 129].
      3.2.8.3   Aggregates with high absorption may have high shrinkage on
      drying. Quartz and feldspar aggregates, alongwith limestone, dolomite, and
      granite, are considered low shrinkage aggregates; while aggregates with
      sandstone, shale, slate, hornblende, and graywacke are often associated with
      high shrinkage in concrete [Kosmatka et. al].
      3.2.8.4   In addition, the strength of high-performance concrete depends not
      only on the CA strength but also on its mineralogy. De Larrard [1999],
      proposes the following equation to account for the aggregate characteristics
      on strength.
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                                             p. f cm
                                    fc 
                  
				
DOCUMENT INFO
Description: An ingredient, which comprises three-fourth of its product and has a number of own physical, mechanical and chemical attributes, must have significant impact upon the later. In fact, a single feature of such an constituent, e.g. gradation of aggregate can have multi-facet consequences for concrete. Similarly, it is also possible that different features of the aggregate support or cancel out the effects of one another when combined to form concrete.
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