The most common defects allowing moisture into the cavity are separations (breaches) between the mortar and brick, which develop from poor workmanship or moisture incompatibility between the brick and mortar, and contraction cracks in the mortar—very little moisture gets through the actual mortar or brick unit. ‘Slushing’ mortar, the practice of throwing mortar onto the brick with the edge of the trowel, should not be permitted, as it results in leaky joints.
Tooling mortar joints improves the water resistance of the brick masonry. Striking the mortar joint with a jointer tool presses the mortar into the brick and produces a glaze on the surface of the mortar that is more water-repellent than mortar that has not been struck. Mortar is ready for tooling when it is fingerprint-hard.
Type N mortar and workability
Aside from cushioning and locking the masonry units together, the most important function of mortar in exterior face brick is to make the joint weather-resistant. The mortar type, moisture compatibility between the mortar and the brick, and workability (i.e. ease with which the mason can get the mortar to spread and stick to the brick), along with the craftsmanship of the mason, are essential to reducing water penetration of the masonry joint.
Workability, perhaps the most critical factor for moisture resistance, is improved by using the lowest-strength mortar consistent with structural requirements. Thus, Type N mortar (i.e. the lowest-strength commonly used mortar at 17 MPa [750 psi]) is recommended for face brickwork except where prohibited by building codes.
Other mortars include Types M, S, and O. Type O is the weakest mortar, and is rarely used in brick masonry. Type M (16.5 MPa [2400 psi]), on the other hand, is the strongest in compressive strength and is used mostly for below-grade applications. Type S (12.4 MPa [1800 psi]) is recommended for below-grade applications and interior and exterior structural-bearing masonry.
Although it is the weakest of the three common mortar types, Type N mortar can easily support brick masonry more than a few storeys high. It is weaker in compression than Type S due to its manufactured properties and mixing proportions (higher sand and water content), but these proportions and other manufactured properties improve workability, making Type N mortar stickier and thus easier for the mason to spread across the entire joint. How much water penetrates the joint into the cavity behind the brick has a direct relationship to how completely the mason butters the head joints.
The bond between brick and mortar
The two characteristics of the bond between brick and mortar—extent of bond and strength of bond—are, for somewhat different reasons, both related to the compatibility of the moisture content of the brick and the water retention quality of the mortar.
The extent of bond, the critical factor in making weather-resistant joints, refers to how completely the mortar is spread in intimate contact with the brick, across the entire surface of the joint—a result of the moisture properties of the brick and mortar, skill of the mason, and mortar workability. There is no field test for determining mortar has the right workability, but an experienced mason feels when it is right and can make field adjustments to the mortar to improve workability.
The strength of bond is not explored in this article, but it involves the force required to break the mortar and brick apart. This is related to structural engineering considerations, such as compressive strength and modulus of elasticity.
Initial rate of absorption and extent of bond
When brick and mortar are brought together, a tug of war begins between the suction of the brick (trying to draw water out of the mortar) and the water retention qualities of the mortar (trying to keep its water to itself). The objective is to keep the water and cement together so they can be absorbed as paste, and to let the brick absorb as much of that paste as possible. That makes a good extent of bond. Water retention of mortar is a manufactured quality, and can vary between manufacturers and types of mortar—whether it is Portland cement/lime mortar, masonry cement, or mortar cement.
Suction properties of brick, measured as the initial rate of absorption (IRA), produce the bond between brick and mortar when a certain amount of cement/lime paste is absorbed into the pores of the brick. However, if the brick is too dry (i.e. an IRA above 30 g of water per minute per 0.02 m2 [30 si] of surface area), the brick suction prematurely draws too much water from the mortar, leaving a dry paste behind. If it is too wet (i.e. an IRA below 30 g), the brick has too little suction—reducing the extent of bond and weakening compressive strength. An experienced mason can feel if the mortar is not sticking properly to the brick because of mismatched moisture properties and make adjustments by rewetting brick or retempering mortar that has gotten too dry on the board.
|TEN COLD-WEATHER REQUIREMENTS|
|Certain requirements apply when using masonry during cold weather.