Air entrainment


Air entrainment

Air entrainment is the intentional creation of tiny air bubbles in concrete. The bubbles are introduced into the concrete by the addition to the mix of an air entraining agent, a surfactant (surface-active substance, a type of chemical that includes detergents). The air bubbles are created during mixing of the plastic (flowable, not hardened) concrete, and most of them survive to be part of the hardened concrete. The primary purpose of air entrainment is to increase the durability of the hardened concrete, especially in climates subject to freeze-thaw; the secondary purpose is to increase workability of the concrete while in a plastic state.

While hardened concrete appears solid, it is porous, having small capillaries resulting from the evaporation of water beyond that required for the hydration reaction. A water:cement ratio (w/c) of approximately 0.25 (this means 25 parts water for every 100 parts cement)is required for all the cement particles to hydrate. Water beyond that is surplus and is used to make the plastic concrete more workable or flowable. Most concrete has a w/c of 0.45 to 0.60, which means there is substantial excess water that will not react with cement. Eventually the excess water evaporates, leaving little pores in its place. Environmental water can later fill these voids. During freeze-thaw cycles, the water occupying those pores expands and creates stresses which lead to tiny cracks. These cracks allow more water into the concrete and the cracks enlarge. Eventually the concrete spalls - chunks break off. The failure of reinforced concrete is most often due to this cycle, which is accelerated by moisture reaching the reinforcing steel. Steel expands when it rusts, and these forces create even more cracks, letting in more water.

The air bubbles are typically 10 to 500 micrometres in diameter (0.0004 to 0.02 in) and are closely spaced. The air bubble can be compressed a little, and so the bubbles act to reduce or absorb stresses from freezing. Air entraining was introduced in the 1930s and most modern concrete, especially if subjected to freezing temperatures, is air-entrained. The bubbles contribute to workability by acting as a sort of lubricant for all the aggregates and large particles in a concrete mix.

In addition to entrained air, hardened concrete also contains entrapped air. These are larger bubbles, and are typically less evenly distributed than entrained air. Entrapped air is considered to not make a positive contribution to durability and is undesirable though not entirely avoidable.


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • air entrainment — oro vilkimas statusas T sritis fizika atitikmenys: angl. air entrainment vok. Luftmitführung, f rus. увлечение воздуха, n; унесение воздуха, n pranc. entraînement d’air, m …   Fizikos terminų žodynas

  • air entrainment — noun : the incorporation in concrete or mortar of minute air bubbles (as by addition of soap or grease) to increase resilience and resistance to wear and weathering …   Useful english dictionary

  • Entrainment — may refer to:* Air entrainment, the intentional creation of tiny air bubbles in concrete * Brainwave entrainment, the practice of entraining one s brainwaves to a desired frequency * Entrainment (biomusicology), the synchronization of organisms… …   Wikipedia

  • Entrainment (meteorology) — See entrainment for other types. Entrainment is a phenomenon of the atmosphere which occurs when a turbulent flow captures a non turbulent flow. It is typically used to refer to the capture of a wind flow of high moisture content, or in the case… …   Wikipedia

  • Entrainment (engineering) — See entrainment for other types. Entrainment as commonly used in various branches of engineering may be defined as the entrapment of one substance by another substance. [cite book|author=Perry, R.H. and Green, D.W. (Editors)|title=Perry s… …   Wikipedia

  • Air pollution dispersion terminology — describes the words and technical terms that have a special meaning to those who work in the field of air pollution dispersion modeling. Governmental environmental protection agencies (local, state, province and national) of many countries have… …   Wikipedia

  • Air parcel — An air parcel is an imaginary volume of air used by meteorologists to conceptualize the thermodynamic fluid motions of the atmosphere for use in weather forecasting. For mathematical simplicity, an air parcel is usually considered a rigid cube… …   Wikipedia

  • Entrainment (hydrodynamics) — See entrainment for other types. Entrainment is the movement of one fluid by another.One fluid moving in another can push or pull the other along with it. Eductors or eductor jet pumps are an excellent example. They are used onboard many ships to …   Wikipedia

  • Entrainment (physical geography) — In physical geography, entrainment is the process by which surface sediment is incorporated into a fluid flow (such as air or water) as part of the operation of erosion. [Thomas,D.S.G. and Goudie, A. (Eds.), The Dictionary of Physical Geography… …   Wikipedia

  • entraînement d’air — oro vilkimas statusas T sritis fizika atitikmenys: angl. air entrainment vok. Luftmitführung, f rus. увлечение воздуха, n; унесение воздуха, n pranc. entraînement d’air, m …   Fizikos terminų žodynas