Though mortar does require regular maintenance in the form of repointing, repointing itself does not need to be difficult, though it may prove to be a rather tedious task!
The Principles of Repointing
The key to successful repointing is to consider how the water will act in relation to the mortar. So, for instance, you don’t want your mortar to leave ledges on top for water to pool up on. Also, don’t forget that the point of repointing is to seal the joints. Therefore, if the mortar is not completely bonded to the stone, the joint is not sound. If the joint is not sound, water can seep into the joint, breaking the mortar out. Therefore, there should be no cracks on the surface of the mortar joint, nor any cracks between the mortar and the stone.
Another point to consider in repointing is to clean out the dirt and derelict mortar that still remains into the joint. You will need to clean out old mortar to a depth about 2 to 2.5 times the width of the joint to ensure a good bond. However, for very deteriorated mortar joints, you ideally should clean back even further until reasonably sound mortar is reached. The point is after all to replace the bad mortar.
Repointing in Practice
The best type of mortar joint is concave, sloping back slightly into the joint. This ensures that the water cannot pool up on the mortar. Then, too, it is better aesthetically than a mass of mortar smeared on the stone, even if (and in this scenario it is rather doubtful) the mortar did bind well to the surface of the stone. Also, smeared mortar seems to trap an unnecessary amount of moisture into the masonry, which can sometimes cause problems. The concave joint, then, set ever so slightly back from the face of the stone is the best mortar joint.
As far as application is concerned, it is generally recognized that it is best to apply mortar in layers, allowing a previous layer to dry to thumbprint hardness before adding more mortar over it. It is generally recommended that the final 3/4″ of mortar in a joint should be done in 1/4″ -thick layers.
Ensuring a Good Bond
So what is the secret to ensuring that the mortar takes to the stone? The first secret is cleanliness. The area in the joints needs to be clean. This can be done by gently raking all the old crumbled mortar and dirt out of the joint.
The other key to success is moisture. The stone and mortar’s moisture content determines how well the binding works. Stone is generally a remarkably porous material. When the stone is dry, it can suck the moisture out of the new mortar in a disturbingly short amount of time. Making the mortar wetter can compensate somewhat, but overly wet mortar will be relatively weak and actually shrink as it dries, cracking the joint. Therefore, the stone itself should be well moistened ahead of time. Then, once moistened, the stone must be allowed to dry enough that there is no longer standing water on it; standing water in the joints tends to saturate the mortar where it contacts the stone, resulting in a crumbly, weak mortar that did not take well.
New mortar seems to take better if lightly pressed into the joint; this gentle pressure helps it to fill in all the minute nooks and crannies in the stone surface ensuring a good bond. The mortar also should be damp enough to work with and to ensure that it takes to the stone. With a little practice the ideal mix will become apparent.
One pitfall to be aware of is that, once mixed with water, the mortar will start to stiffen eventually becoming too stiff to use well. Avoid the temptation to add more water to the mix to loosen it. After all, the reason the mortar is stiffening is not so much that the water is evaporating out but that the mortar is setting up. Moistening it again will result in weaker mortar. The best bet is to discard the mortar when too stiff to use, and make small batches at a time to avoid this scenario altogether.
One final consideration is the curing. The mortar must not be allowed to freeze or dry out until it is set up. If the weather is very hot consider using plastic sheets to cover the work, as the sheets help keep the mortar from drying out. For the first day or two after new mortar is applied, consider moistening the work, especially if the weather is hot and dry but make sure that the mortar has set up enough first that it won’t be washed out and ruined by the water! This moistening helps ensure the mortar cures well and is strong.
And What About Those Interior Joints?
Mortar can be pressure injected into masonry joints. This is especially valuable as a means for replacing decayed interior mortar. This method is, obviously, a highly advanced one requiring special equipment. The three main considerations when pressure injecting mortar are as follows: First, do not use excessive pressure; too much pressure can start dislodging stones causing worse problems than dry mortar joints. Second, try not to make a tremendous mess. It may be helpful to coat the outside of the stones with paraffin wax ahead of time. The wax provides an easily removed barrier between the outside surfaces of the stones and the mortar. Finally, for really good joints, it is still preferable to hand work the final layers of the exterior mortar joints. All in all, however, pressure injecting mortar is a good option for major refurbishment of a stone arch bridge and can replace that otherwise inaccessible structural mortar located deep within the bridge.
Stone Arch Bridges 