Chapter 3 : Concrete
properties
Workability
It is important that the fresh concrete can be properly compacted, transported, placed and finished without segregation. These factors affect the strength, volume, stability and durability of the concrete when it hardens.
Workability is the amount of useful internal work needed to produce full compaction. This work is need to overcome the internal friction between the individual particles in the concrete, also additional energy is need to overcome the friction between concrete and formworks or steel bars.
The strength of concrete depends mainly of the volume of voids in the compacted mass, therefore it's important to achieve the maximum density possible; this requires sufficient workability to virtually full compaction could be possible.
Voids in hardened concrete are either entrapped air (depends on the grading of fine particles) or spaces left by excess water, which depends on the w/c ratio. The fact that the air bubbles are much easier to remove in the fresh state of concrete but the water bubbles are not easy to remove, therefore, there must be an optimum water content for the mix at which sum of volumes of air volumes and water space will be a minimum and the density will be a maximum.
The most important factor affecting the workability is the amount of the water in the mix that exceeds what is needed for hydration. The best way to describe the water content in the concrete is the water to cement ratio (w/c) expressed as the weight of free water divided by the weight of cement where free water is the water available for reaction including the moisture on the aggregates and excluding the water absorbed by the aggregates.
Workability depends of many
factors: water content, aggregate type and grading, aggregate/cement ratio,
presence of admixtures, and the fineness of cement.
Water content of the mix: This is the single most important fact or governing workability of concrete. A group of particles requires a certain amount of water. Water is absorbed on the particle surface, in the volumes between particles, and provides "lubrication" to help the particles move past one another more easily. Therefore, finer particles, necessary for plastic behavior, require more water. Some side effects of increased water are loss of strength and possible segregation.
Aggregates mix proportions: Increasing the proportion of aggregates relative to the cement will decrease the workability of the concrete. Also, any additional fines will require more cement in the mix. An "over sanded" mix will be permeable and less economical. A concrete deficient of fines will be difficult to finish and prone to segregation.
Aggregate properties: The ratio of coarse/fine aggregate is not the only factor affecting workability. The gradation and particle size of sands are important. Shape and texture of aggregate will also affect workability. Spherical shaped particles will not have the interaction problems associated with more angular particles. Also, spherical shapes have a low surface/volume ratio, therefore, less cement will be required to coat each particle and more will be available to contribute to the workability of the concrete. Aggregate which is porous will absorb more water leaving less to provide workability. It is important to distinguish between total water content, which includes absorbed water, and free water which is available for improving workability
Time and temperature: In general, increasing temperature will cause an increase in the rate of hydration and evaporation. Both of these effects lead to a loss of workability
Loss of Workability: Workability will decrease with time due to several factors; continued slow hydration of C3S and C3A during dormant period, loss of water through evaporation and absorption, increased particle interaction due to the formation of hydration products on the particle surface. Loss of workability is measured as "slump loss" with time
Cement characteristics: Cement characteristics are less important than aggregate properties in determining workability. However, the increased fineness of rapid-hardening cements will result in rapid hydration and increased water requirements, both of which reduce workability.
Admixtures: In general,
air-entraining, water-reducing, and set-retarding admixtures will all improve
workability. However, some chemical admixtures will react differently with
cements and aggregates and may result in reduced workability
Bleeding and segregation
Segregation refers to a separation of the components of fresh concrete, resulting in a non-uniform mix. This can be seen as a separation of coarse aggregate from the mortar, caused from either the settling of heavy aggregate to the bottom or the separation of the aggregate from the mix due to improper placement. Some factors that increase segregation are:
1. Larger maximum particle size (25mm) and proportion of the larger particles.
2. High specific gravity of coarse aggregate.
3. Decrease in the amount of fine particles.
4. Particle shape and texture.
5. Water/cement ratio.
Good handling and placement techniques are most important in prevention of segregation.
Bleeding is defined as the appearance of water on the surface of concrete after it has consolidated but before it is set. Since mixing water is the lightest component of the concrete, this is a special form of segregation. Bleeding is generally the result of aggregates settling into the mix and releasing their mixing water. Some bleeding is normal for good concrete.
However, if bleeding becomes too localized, channels will form resulting in "craters". The upper layers will become too rich in cement with a high w/c ratio causing a weak, porous structure. Salt may crystallize on the surface, which will affect bonding with additional lifts of concrete. This formation should always be removed by brushing and washing the surface. Also, water pockets may form under large aggregates and reinforcing bars reducing the bond. Bleeding may be reduced by:
Strength of the concrete is the most important mechanical property. The aggregates in the concrete are considered to be a filler used to reduce the volume of the expensive cement paste. And because the aggregate is higher than the paste in strength, the strength of the concrete comes directly from the strength of the paste because the failure will happen in the paste.
The main factors affecting the strength are:
The porosity (the relative volume of voids in the cement paste) : As the capillary porosity decreases compressive strength increases. Also there is data to indicate that large pores may be more effective than small pores in reliving stress concentrations at crack tips.
The capillary porosity of a properly compacted concrete is determined by the w/c ratio. If concrete is not properly compacted it may contain voids which will contribute to its porosity. At low w/c ratios where full compaction is difficult to achieve, the relationship between w/c and strength is invalid.
Aggregate -- Second to w/c ratio, aggregate is an important factor affecting concrete strength. The most important properties of aggregate are shape and texture and the maximum aggregate size.
The most two important
criteria in the concrete strength is the compressive strength and the tensile
strength. The bond with the
aggregates is reflected in the
tensile strength while the strength of the aggregate itself influence the
compressive strength of concrete.
Texture affects both the bond and the stress level at micro-cracks. This type of behavior will affect the tensile strength but will not affect the compressive strength. Compressive strength depends on the strength of the aggregate itself.
Maximum aggregate size
affects tensile strength in several ways: larger particles reduce the specific
surface area of the aggregate which leads to a reduction in bond strength; also,
larger particles resist shrinkage better than paste by restraining volume
changes in the cement paste and therefore induce some internal stress which
will weaken the concrete.
Increasing the maximum size
of aggregates will decrease the volume of the paste and because aggretes is
usually stronger than paste the compressive strength will increase. Weak
aggregates should be prevented which can be looked at as empty voids.
Curing: If the internal relative humidity drops below 80%, hydration and strength gain will stop. The rate of strength gain is directly related to the amount of moist curing.
Cement: the chemical composition and fineness of cement affect the strength of concrete. Early strength comes from C3S and later strength from C2S. Cement that hydrates more slowly will have lower initial strength but higher ultimate strength. The degree of fineness also affects the strength; the rate of hydration increases with the increase of fineness.
Durability
Durability is an important property of concrete as the concrete should be able to withstand the conditions for which it has been designed for throughout the life of a structure.
Lack of durability can be
caused by external agents from the environment or internal agents from the
concrete itself. And causes could be physical (frost and thaw, thermal
differences between paste and aggregates), chemical (attack by sulphates,
acids, sea water and chloride) or mechanical (extreme loading and impact).
The main reason for the
durability problems in the concrete is the porosity and permeability. The
porosity is caused by the capillary pores in the paste while permeability is
caused by the inter connection system between pores in the paste. This effect
is clearly seen in mixes with a w/c ratio higher than 0.5 as after this ratio
the pore system becomes continues by forming channels. (Less than 0.5 is still
closer to the 0.3 ratio needed for hydration and thus not much capillary voids
are connected)
It’s important to achieve low
permeability as quickly as possible. A mix with a low water cement ratio is
advantageous because the stage at which capillary voids are disconnected is
achieved in a shorter time of moist curing that's why all standards and
specification suggests a maximum w/c ratio.
The permeability of concrete is
also related to the aggregates, aggregates with low permeability should be
used.
Air bubbles helps in the
increasing the durability in some cases because they act as a relief chambers
for the internal pressure caused by external and external attacks. The normal
concrete got up to 5% cir content which is not enough for the frost-thaw
resistance so air-entraining admixtures are used to increase the air content.
The entrapped air is usually irregular in shape and large in size (up to 2% of
the concrete volume) while the entrained air is spherical and smaller in size
the air bubbles should be evenly distributed with an average spacing not more than 0.2 mm usually from 5-8% will
do.
Stopping the curing process
will increase the permeability, as shrinkage cracks will occur especially
around large aggregates increasing permeability.