Bond strength between mortar and masonry units is the most important physical property of masonry. Low bond strength causes every problem that can happen to masonry--cracks, leaks, stains, weathering, and structural failure. HOW DO YOU ACHIEVE ADEQUATE BOND STRENGTH? Bond strength is determined by the physical properties of the mortar and units and by the care with which they are assembled. Important properties of the mortar are: composition, air content, flow, water retentivity, sand gradation, shrinkage, and age. Important properties of masonry units are: initial rate of absorption (IRA), surface condition, cores, and frogs. Important aspects of workmanship are: wetting brick; batching, mixing, and retempering mortar; the time lapse between spreading mortar and laying brick; the solidity of mortar joints; and curing. The following techniques have been proven to increase bond strength. MATERIALS Use portland cement-lime mortar without air entrainment rather than masonry cement mortar. Don't let the air content of the mortar exceed 12%. Increase the cement-lime ratio of the mortar up to about 2. Use mortar with a high water retentivity in summer and a low water retentivity in winter. WORKMANSHIP Inspect workmanship. Batch mortar accurately. Use finer sands within the gradation limits specified in ASTM C 144. Use sufficient water to increase mortar flow up to about 145%. Lay brick within 1 minute of spreading mortar and sooner in warm, cold, or windy weather. Wet high-suction (high-IRA) brick before laying.