ACI key terms

The mass of water in a mixture divided by the total mass of cement plus supplementary cementitious materials. Lowering it generally increases strength, reduces permeability, and improves durability.

A material such as fly ash, slag cement, silica fume, or natural pozzolan used in addition to portland cement to react chemically or contribute to binder performance. SCMs affect setting, strength gain, and durability.

A siliceous or siliceous-aluminous material that, in finely divided form and in the presence of moisture, reacts with calcium hydroxide to form cementitious compounds. It does not harden appreciably on its own.

A finely divided residue from coal combustion that acts as a pozzolan. It can improve workability, reduce heat of hydration, and increase long-term strength, but may slow early strength gain.

A hydraulic material made from granulated blast-furnace slag, used as an SCM. It can improve sulfate resistance and reduce permeability while often lowering early strength compared with plain portland cement.

An extremely fine byproduct of silicon or ferrosilicon production used as a highly reactive pozzolan. It increases strength and reduces permeability but raises water demand and requires careful handling.

The intentional introduction of tiny air bubbles into concrete using an admixture, improving resistance to freezing and thawing and often workability. Air content is specified and verified by testing.

The total volume of air voids in a concrete mixture, expressed as a percentage of volume. It is measured by pressure, volumetric, or gravimetric methods and checked against specification limits.

The ease with which fresh concrete can be mixed, placed, consolidated, and finished without segregation. It is influenced by water content, aggregate grading, admixtures, and temperature.

A measure of the consistency of fresh concrete obtained by the standard slump test. It indicates relative workability but does not directly measure strength or durability.

A test for highly flowable or self-consolidating concrete that measures how far a sample spreads after a slump cone is lifted. It assesses filling ability and flow without vibration.

Highly flowable concrete that spreads under its own weight, fills forms, and consolidates without mechanical vibration. It requires careful mixture proportioning to resist segregation and maintain stability.

The separation of coarse aggregate or paste from the rest of the fresh concrete mixture. It can result from excessive water, poor grading, overhandling, or dropping concrete from too great a height.

The upward migration of mixing water to the surface of freshly placed concrete. Excessive bleeding can weaken the surface layer, delay finishing, and reduce durability.

The heat released as cement reacts with water. In massive placements, it can cause high internal temperatures and thermal cracking unless controlled by mixture design or cooling measures.

Concrete placed in a volume large enough that measures are needed to control heat generation and temperature differentials. These measures reduce the risk of thermal cracking.

Cracking caused by temperature differences or restraint of contraction as concrete cools. It often occurs in thick sections where internal heat dissipates more slowly than surface heat.

Chemical deterioration of concrete caused by sulfates reacting with hydration products, leading to expansion and cracking. Resistant mixtures use low-permeability concrete and sulfate-resistant cementitious materials.

The ability of concrete to allow fluids to pass through it. Low permeability is important for durability because it limits ingress of water and aggressive chemicals.

The ability of concrete to withstand repeated cycles of freezing and thawing without excessive deterioration. It depends on adequate air entrainment, low permeability, and proper curing.

Maintaining adequate moisture and temperature in concrete after placement so cementitious materials can hydrate. Proper curing improves strength, durability, and surface quality.

A visible or weak plane formed when fresh concrete is placed against concrete that has already stiffened. It can reduce structural integrity and allow water penetration.

The process of removing entrapped air and compacting fresh concrete into forms, usually by vibration. Proper consolidation reduces voids and improves strength and durability.

A mechanical device used to consolidate fresh concrete. Internal vibrators are inserted vertically at regular intervals and withdrawn slowly to avoid leaving voids.

Temporary or permanent molds that shape and support fresh concrete until it can support itself. Formwork must be strong, tight, and properly treated to prevent defects.

The removal of formwork after concrete has gained sufficient strength. Timing depends on structural requirements, temperature, and the type of member.

The operations that give a concrete surface its final texture and appearance, such as screeding, floating, and troweling. Timing is critical to avoid defects.

The process of leveling freshly placed concrete to the desired elevation using a straightedge or screed. It removes excess concrete and prepares the surface for floating.

A finishing operation that embeds aggregate, removes high spots, and opens the surface for troweling. Floating is done after bleed water has disappeared.

A finishing operation that produces a smooth, dense surface using a trowel. Overworking or troweling while bleed water is present can cause blistering or delamination.

A liquid applied to a concrete surface to form a membrane that limits moisture loss. It is used when wet curing or other methods are not practical.

The loss of water from the surface of fresh concrete to the air. Rapid evaporation can cause plastic shrinkage cracking before the concrete has gained strength.

Cracks that form in fresh concrete when surface moisture evaporates faster than water can migrate from below. They typically appear as short, parallel cracks.

The facility where concrete ingredients are measured and combined. Accurate batching of cement, water, aggregates, and admixtures is essential for consistent concrete quality.

Equipment that blends concrete ingredients into a uniform mixture. Mixing time and efficiency affect uniformity, strength, and workability.

A vehicle that mixes and delivers concrete in a rotating drum. Drum revolutions and mixing time after adding water affect uniformity and must be controlled.

A standard test in which concrete cylinders are cast, cured, and tested in compression to estimate strength. Proper sampling, handling, and curing are essential for valid results.

The maximum stress a concrete specimen can carry in compression before failure. It is commonly determined from cylinder or core tests and is a key acceptance criterion.

The planned activities and tests used to verify that materials, batching, placement, and curing meet specified requirements. It includes sampling, testing, and documentation.

Safety gear such as hard hats, gloves, eye protection, and respiratory protection worn to reduce injury risk. Appropriate PPE is required for tasks involving concrete, chemicals, and equipment.

Inhalation of respirable crystalline silica dust during cutting, grinding, or drilling concrete. It can cause serious lung disease and requires dust controls and respiratory protection.

A document that defines the required materials, methods, testing, and acceptance criteria for a concrete project. It governs the contractor's work and the inspector's verification.

A relative measure of a concrete's resistance to deterioration under repeated freeze-thaw cycles in laboratory testing. Higher values indicate better performance.