The present invention relates generally to a foam producing apparatus and method, and is particularly concerned with such an apparatus usable for entraining air bubbles in concrete to produce various types of lightweight concrete structures having a high density of air voids, such as concrete wall panels.
It has been known for some time that lighter weight concrete structures can be made by entraining air bubbles in the concrete mixture at some point prior to placing the concrete, so that the concrete hardens to leave air voids throughout the structure. This type of lightweight concrete is generally known in the field as cellular concrete. Cellular or foamed concrete may be made by pre-forming a foam and then adding the foam to a slurry of cement, aggregate and water in a mixing device. Another known technique is to add a gas-forming agent to the slurry, causing the mix to swell as gas bubbles are formed. The reduced density of cellular concrete, and thus the reduced weight, reduces transportation and handling costs, and also reduces dead load imposed on a structure constructed from such concrete. It also has better heat insulation, freeze and thaw resistance, reduced water permeability and sound absorption properties than conventional concrete.
One foam generating device for producing a foam suitable for injection into a concrete slurry is described in U.S. Pat. No. 4,789,244 of Dunton et al. However, this and other known foam generating devices have been found to be subject to several disadvantages when attempting to entrain air bubbles into concrete mixtures. If the foam is injected into concrete in a concrete mixer, the bubbles will often have insufficient strength to withstand mixing for several hours during transportation to a construction site. If the bubbles break, water contained in the bubbles will mix in with the concrete, altering the concrete consistency and producing undesirable mix properties, such as high slump. Additionally, the known foam generating devices typically produce foams which contain too much water. Thus, the prior art foams, when mixed with concrete, produce bubbles which break too easily, either during mixing or working of the concrete as it is transported, or during placing and finishing of the concrete. Thus, previous attempts to produce lighter weight concrete structures have been unsuccessful or have produced concrete with a relatively low air void concentration, and which tends to be too wet and have too high a slump.
One prior art machine included an atomization chamber having a through flow of pressurized air and plural inlets for a mixture of water and foaming agent under pressure, and a mixing chamber connected to the outlet of the atomization chamber, the mixing chamber being filled with steel wool which acts as an agitating medium. This produces a foam which may be mixed with concrete, but still is subject to some of the disadvantages outlined above.
Thus, up to now, it has not been possible to produce a foam which is sufficiently stable to withstand the effects of mixing of concrete for extended periods of several hours or more during transportation and subsequent agitation of the concrete as it is placed and finished, without collapse of a substantial portion of the bubbles mixed with the concrete.