I'm confused. I am a small-sized subcontractor that contracts for work in both concrete and masonry. Most inspectors don't let us add more water to concrete. For masonry mortar, however, some inspectors let us retemper with water and some don't. I know that adding extra water to concrete lowers its strength. Doesn't extra water also lower the strength of mortar?
You are correct about concrete. Adding extra water onsite increases the water-cement ratio and decreases strength. A good rule of thumb to remember is the addition of 1 gallon of water for every bag of cement decreases the compressive strength by 1000 psi. Masonry mortar and concrete are similar materials but are used quite differently. However, because of their similarities, sometimes specifications for concrete are incorrectly applied to masonry mortars. Mortars differ from concrete in method of placement, workability, and structural use. Concrete is used as a single structural element; mortar is used to bind other structural units together. Consequently, mortar bond strength is more important than its compressive strength. It's the bond of the mortar to the masonry units that controls the failure of most masonry. If water in the mortar mix evaporates or is absorbed by the masonry units, the mortar-unit bond is reduced. Highest bond strengths are obtained with moist mixes having good workability. Retempering, the addition of mixing water to compensate for water loss, is an acceptable practice for masonry construction. There are, however, some requirements for this practice. American Society for Testing and Materials (ASTM) standards require that all mortar be used within 2 1/2 hours but permits retempering as frequently as needed within this time period. Tests have shown that the decrease in compressive strength due to additional water is minimal if retempering occurs within 2 1/2 hours. Mortar that is not used within 2 1/2 hours or that has begun to set should be discarded. Also, the retempering should not be accomplished by simply splashing water on the mortar surface. This practice tends to wash the fines from the mortar. Instead, form a basin in the mortar, add the water, and then thoroughly mix. Some inspectors probably rely on their concrete experience to assess masonry construction. Refusing to allow retempering results in mortars that have maximum compressive strength, but less than optimum bond strength.