Mid-range water-reducing admixtures (MRWRAs) provide many benefits, but occasionally produce an unwanted effect: unintentionally entrained air.
MRWRAs have several features that make them particularly suitable for many of today's ready-mixed concrete applications. They include achieving the 5- to 8-inch slumps many contractors prefer.
However, it's well known that the lignosulfonates (lignins) found in many of these products entrain air, but it isn't as clear why air entrainment is reported only sporadically when MRWRAs are used.
The unwanted air may cause two possible problems.
In machine-finished flatwork, high air contents may lead to finishing problems. Because most interior flatwork isn't exposed to freezing and thawing, entrained air isn't needed for durability. If the air content increases unexpectedly because of the MRWRA, blistering or surface delaminations are possible effects.
For placements that require air-entrained concrete, MRWRA may increase the air content and require a reduced air-entraining-admixture (AEA) dosage. This can cause two problems. If air entrained by the MRWRA is unstable, control of air content will be difficult since air measured at the concrete plant may not be present when the truck arrives at the jobsite. And if the air-void system is stable but the bubbles produced by the MRWRA don't have the proper size and spacing, concrete durability may be compromised.
The article includes tips producers can use to avoid air problems from MRWRAs.
[keywords: mid-range, MRWRA, air, entrain]