Concrete Mixture: Air Voids & Water Quality

• Concrete mixture consists of paste and aggregates.
• A typical concrete mix includes 10-15% cement, 60-75% aggregate, and 15-20% water

Air Voids

• Concrete mixtures have two types of air voids: entrapped (coarse) and entrained (fine).
• Large air voids lead to poorly consolidated concrete that has less strength, also known as honeycombing
• Adding air-entrained admixture preserves smaller voids.
• Air-entrained admixture improves workability and durability.
• It reduces water content, bleeding, and segregation.
• Entrained air makes up 5-8% of concrete mixes.

The Importance of Water Quality in Concrete

• Water quality is an important factor in concrete.
• Drinkable water that has no taste or odor can be used for mixing.
• Excessive impurities in the water can affect setting time, strength, corrosion of reinforcement, durability, and appearance (staining).

Properties of Fresh Concrete

• Workability
• Segregation
• Bleeding
• Plastic Shrinkage

Workability of Fresh Concrete

• Workability can be defined as the ease of handling, transporting, and placing concrete.
• Higher water-cement ratio relates to good workability.
• Water/Cement Ratio
• Aggregate Size and Shape
• Admixtures
• Temperature affect workability.
• Ample cementitious material and water = increased consolidation and better finish.
• Too little water for hydration= poor strength development and difficult mix.
• Excess water increases workability and placement ease.
• It can increase segregation leading to a negative effect on the concrete's finishing and strength.
• Common water/cement ratios are 0.4 to 0.6.

Aggregate Size and Shape

• Aggregates make up 60-75% of total concrete volume.
• Elongated, angular, and flaky aggregates have large surface areas that decrease workability.
• Rounded aggregates have small surface areas that lack angularity, this affects their cement paste bond strength.
• Crushed aggregates with proper portions create better bonding with cement paste in addition to better workability.

Effect of Admixtures on Workability

• Superplasticizers reduce water content but allow flowable concrete mixtures.
• Superplasticizers reduce water content by about 30%, and plasticizers reduce it by 15%.
• Set-retarding admixtures slow the rate of hydration, this allows the concrete to be workable for longer.
• Temperature: Increased temperature equates to hardened concrete sooner

Workability Measurement

• The Slump test is the most widely used and low-cost measurement method of workability.
• It measures the concrete's resistance to flow.

Segregation in Fresh Concrete

• Segregation can be defined as the separation of the components from a heterogeneous mixture, this causes the mixture to no longer appear uniform
• Segregation can be caused by differences in particle size of the concrete mix, high water/cement ratios and excessive vibration.
• Segregation can take the form of the separation of course aggregate or cement paste(grout) from the mix.

Preventative Measures of Segregation

• Aggregates properly graded.
• Concrete mix design properly performed.
• Concrete handled, cared for and compacted properly.
• Height at which concrete is placed carefully considered (concrete column)

Bleeding in Fresh Concrete

• Bleeding can be defined as the development of water at the top or surface of the concrete once it is freshly placed
• It is a form of segregation, and predominant when there is high water content, badly portioned, or badly mixed.
• Aggregate goes down, while cement particles and water rises to the top. This is known as Laitance (surface evidence).
• However, intercepted water by aggregates creates water voids and decrease the bond strength between aggregates and paste (no surface evidence).
• Microcracking is possible.

Preventative Measures of Bleeding

• Maintain the appropriate water/cement ratio.
• Add air entrainment to the mix.
• Use finer cements and add more to the mix.
• Add chemical admixtures to reduce the need for water (workability).
• Add sand.

Plastic Shrinkage in Fresh Concrete

• Plastic shrinkage occurs from the loss of water by evaporation after placing and before hardening of concrete. Plastic shrinkage cracking results from high air temperature, low humidity, sun exposure, and high concrete temperature.
• Strength and durability of concrete are unaffected.

Preventative Measures

• Reduce the time between placing and starting to cure by working effectively.
• Use fog sprays to maintain high humidity and low air temperature.
• Use sunshades for concrete.
• Place concrete early in the morning or late in the afternoon.
• Dampen absorptive dry aggregates.

Properties of Hardened Concrete

• Strength (compressive and tensile)
• Degree of compaction
• Permeability
• Curing
• Durability
• Abrasion Resistance
• Dry Shrinkage
• Concrete Creep

Strength of Hardened Concrete

• Strength is known as the concrete's resistance to rupture.
• Compression, tension, and flexural.
• Strength depends on cement type, aggregate, bond, water/cement ratio, curing temperature, and size of specimen.

Compressive Strength

• Compressive strength is mostly useful and an important property.
• It is the capacity to resist axially directed pushing forces.
• It is important to determine whether the concrete mixture has enough strength requirements for the job specification.

Tensile Strength

• Tensile strength is the ability to resist stretching or breakage when pulled apart.
• Lower than the compressive strength at about 10%.
• Difficult to perform and results are not reliable.
• Split cylinder test

Flexural Strength

• Flexural strength measures concrete's tensile strength.
• Measures an unreinforced concrete slab's or beam's resistance to failure in bending.
• Modulus of rupture measures the flexural strength.
• Concrete is elastic at low stress levels but begins to decrease at higher stresses.
• The elastic modulus of concrete is measured as the stiffness of the concrete
• The measurement of the concrete's stiffness is a valuable indicator of its strength.

Degree of Compaction in Hardened Concrete

• Compaction accomplished by ramming or vibration.
• Vibration needs to be applied uniformly so that all parts achieve maximum compaction.
• Too dry mix can't be worked by hand, conversely, too wet mix can't be vibrated (segregation).
• Compaction improves strength and bond with reinforcement.
• It also increases abrasion resistance, decreases permeability, and minimizes shrinkage.

Permeability of Hardened Concrete

• Permeability is the rate of low of fluid in a porous solid.
• It measures how much air, water, and other substances can enter the concrete mix.
• Permeability affects durability since it controls the rate that moisture enters the concrete, and the movement of water.
• The entrance of water and air may lead to corrosion (reinforced concrete); this prevents the concrete from withstanding frost action.
• Curing and the water/cement ratio affect permeability.

Curing of Hardened Concrete

• Curing maintains the humidity and temperature of freshly placed concrete during a definite period. This occurs following placing and finishing, this ensures hydration of cement and proper hardening of concrete.
• Water curing
• Application of heat
• Membrane curing

Water Curing

• The best method of curing.
• Immersion, Ponding, Spraying/fogging, and wet covering.
• Wet covering best suited for horizontal / vertical, sloping surfaces.
• Ponding is the best method, and suited for curing horizontal surfaces such as floors or pavements. It also has a large water requirement.
• Immersion means that samples are immersed in water.

Steam Curing

• Steam is sprayed over the concrete surface.

Application of Heat

• Increased temperature, which enhances the hydration process, leading to increased strength.
• Acceleration of the hydration process requires moisture. The compressive strength is tested after curing.
• Researched indicate that concrete gains about 99% strength when cured for 28 days.
• Curing for 7 and 14 days are tested and assessed.
• ACI states minimum curing time is when concrete hits 70% strength, or 7 days.

Durability of Hardened Concrete

• This is the ability to resist deterioration due to weather, chemical attack, or abrasion.
• Factors affecting durability: freezing and thawing, mechanical elements (abrasion), along with chemical reaction of concrete.
• Permeability highly contributes to durability.

Abrasion of Hardened Concrete

• This refers to the ability of hardened concrete to resist wearing away by abrasive forces.
• The higher the concrete surface strength, the better the abrasion resistance.
• Vehicular traffic is one of the most damaging forms of abrasion.
• Adequate water/cement ratio.
• Well graded aggregates.
• Proper curing procedure
• Adding a fly ash as a binder: enhances workability, strength, durability, and is cost effective.
• Increasing the binder content.
• Proper joint-filling procedures to reduce edge-spalling.

Dry Shrinkage of Hardened Concrete

• This involves cracking happening in hardened concrete due to loss of excess moisture.
• Mixing of concrete.
• The amount of water added.
• Higher surrounding temperature.

Dry Shrinkage Control

• Control by adding joints (pre-cut cracks/Control Joints).
• Reduce water content (admixtures)
• Improve curing.
• Have good understanding of the surrounding, particularly how dry it is

Concrete Creep of Hardened Concrete

• Creep involves deformation of concrete by applying constant stress (compressive loading) over a long period.
• Deformation typically happens in the direction to where the force is applied
• The aggregate
• The magnitude of the loading
• The age and strength of concrete at time of loading
• The total amount of time the concrete is stressed.
• Water/cement ratios

An overview of concrete mixtures, including the role of entrapped and entrained air voids and the importance of water quality. Air-entrained admixtures enhance workability and durability by preserving smaller air voids. The quality of water significantly affects setting time, strength, and durability of concrete.