Water and Admixtures in Concrete

Water and Admixtures

• Water is essential in concrete, and admixtures are used in some concretes.

Water for Mixing Concrete

• Water makes fresh concrete workable and reacts with cement to form strength-giving compounds.
• Concrete-making water should be fit to drink, so municipal water sources are usually suitable.
• Doubtful water suitability can be tested by making mortar cubes: they must set normally and achieve 90% of the 28-day strength of cubes made with known suitable water.
• Seawater can be used in unreinforced concrete where salt content doesn't matter, and aggregates resist alkali attack, but seawater chlorides cause reinforcing steel to rust.

Water Requirement

• Fresh concrete's water requirement refers to the water content in liters per cubic meter needed to achieve specific consistence.
• Water requirement affects hardened concrete's strength due to its dependence on the water-to-cement (W/C) ratio.
• The water requirement impacts material cost and dimensional stability.
• Water requirement is affected by the properties and content of the mix materials, and the consistence of the concrete.
• Sand quality significantly impacts water requirements.
• "Average" water requirement is about 210 l/m³ for concrete:
  • 19-mm crushed stone.
  • Average quality sand.
• CEM I type cement and no admixture.
• 75 mm slump.

Admixtures

• Admixtures are substances other than cement, extenders, water, and aggregates added in small amounts to modify concrete properties during mixing.
• Admixtures are not a substitute for good materials, mix design, and workmanship.

Using Admixtures

• Admixtures used should be sourced from reputable specialist manufacturers.
• Admixture use is appropriate in properly designed mixes that are accurately batched by mass, mechanically mixed and manufactured to high-quality control standards.
• Admixtures' impact on fresh and hardened concrete needs assessment with preliminary tests, using trial mixes, on concrete with specific materials.
• Admixtures require proper storage and handling to prevent deterioration.
• They must be protected from frost, stored out of sunlight, and away from heat sources, and should follow a first-in-first-out usage.

Dosing

• Admixture amounts must be accurate and per the manufacturer's recommendations.
• Dispensing equipment needs proper maintenance and calibration.
• Addition during mixing ensures even dispersion in concrete:
  • added with the last portion of water.
  • as per manufacturer's recommendation.

Admixture Types

• Admixtures are classified by their effect on concrete.

Plasticizers

• Plasticizers make cement paste and concrete more workable by physically dispersing cement particles.
• Plasticizers help lower material costs by decreasing cement.
• Actual cost saving per cubic metre equals value of cement saved, minus cost of admixture, minus cost of additional aggregate to make up yield.
• Accurate plasticizer dosing is crucial.
• Underdosing can decrease strength.
• Overdosing can cause severe retardation in setting.

Superplasticizers

• The effect of superplasticizers is about three times greater than plasticizers.
• Superplasticizers are used to make high workability or "self-levelling" concrete with normal water content.
• They are essential in high-strength concrete made at normal workability.
• Superplasticizers can enable water reductions of 15 to 25%.
• Overdosing can result in segregation.
• Some have a limited effective period of approximately 30 minutes, after which slump decreases.

Air Entrainers

• Air entrainers introduce small air bubbles (0.05 to 0.25 mm) uniformly into fresh concrete.
• This entrained air is distinct from entrapped air from incomplete compaction.
• Entrained air is not removed by vibration.
• In fresh concrete, air bubbles behave like frictionless sand particles.
• They increase workability and reduce water requirements.
• They improve cohesiveness, reduce bleeding, and increase yield.

Accelerators

• Accelerators are chemical substances that increase the rate of cement hydration.
• Two types are available.
• Strength-gain accelerators for concrete are for use where need high early strengths.
• Setting-time accelerators for shotcrete reduce rebound and allows rapid build-up, and must never be used in concrete.
• Strength-gain accelerators include calcium chloride, but chloride mixes should not be used for reinforced concrete.
• Typical dosages causes.
• Slightly reduced compressive strength.
• Significantly reduced flexural strength.
• Increased shrinkage and creep.
• Promote efflorescence.

Retarders

• Retarders are chemicals that delay the setting of concrete.
• Retarders work by physically inhibiting cement hydration.
• Retardation is used if concrete has to be transported for a long time or to prevent cold joints in structures.
• Depending on time lag, retardation may increase bleeding, which leads to strength loss, higher permeability, and settlement cracking that revibrating concrete can reverse.
• Overdosing of retarders can cause excessive retardation.