Retentivity

Other mortar characteristics that influence general performance, such as aggregate grading, water retentivity, and flow, can be accurately measured by laboratory tests and are included in ASTM Standards. Water retentivity allows mortar to resist the suction of dry masonry units and maintain moisture for proper curing. It is the mortar's ability to retain its plasticity in contact with absorptive masonry so that the mason can carefully align and level the units without breaking the bond. Less retentive mixes will "bleed" moisture, creating a thin layer of water between mortar and masonry unit and substantially decreasing bond strength .  Highly absorptive clay units may be prewetted at the job site, but concrete products may not be moistened, thus requiring the mortar itself to resist water loss. Under laboratory conditions, water retention is measured by flow tests, and is expressed as the ratio of initial flow to flow after suction. The flow test is similar to a concrete slump test, but is performed on a "flow table" that is rapidly vibrated up and down for several seconds. Although they accurately reflect the properties of the mortar, laboratory values differ somewhat from field requirements. Construction mortars require initial flow values on the order of 130 to 150%. Laboratory mortars are required to have an initial flow of 100 to 115%. The amount of mixing water required to produce good workability, proper flow, and water retention are quickly and accurately adjusted by experienced masons. Results produced from assemblies prepared in the field reliably duplicate the standards set by laboratory researchers. Dry mixes lose water to the masonry units and will not cure properly. Excessively wet mixes cause units to float, and will decrease bond strength. The "proper" amount of mixing water is universally agreed upon as the maximum compatible with "workability," and workability is best judged by the mason. Retempering (the addition of mixing water to compensate for evapora tion) is acceptable practice in masonry construction. Since highest bond strengths are obtained with moist mixes having good flow values, a partially dehydrated mortar is less effective if the evaporated water is not replaced. Mortar normally begins to harden or set about 2.5 hours after initial mixing. After this point, retempering will decrease compressive strength by approximately 25%. ASTM standards require that all mortar be used within 2 .5 hours and permit retempering as frequently as needed within this time period. Tests have shown that the decrease in compressive strength is minimal if retempering occurs only 1 to 2 hours after mixing. Mortar that is not used within 2.5 hours or that has begun to set should be discarded.