Historic preservation of masonry and analysis of mortars
When restoring historic masonry, it is important to identify the composition of the original mortar used in the construction of the masonry structure. While this may seem like a suggestion and an attempt to sell you an idea, there are many reasons why you should analyze the mortar and determine the composition of the original mortar used.
Ancient or historic masonry units were made by hand and tend to be softer or more brittle than the fired clay bricks used in construction today, and the stone will have different varying factors. Depending on the type of stone, it will have different crushing pressures and will require a softer mortar than rock. This also applies to handmade bricks. The mortar must be smoother than the masonry unit being used to avoid damage to the bricks or stones. This part carries over to today’s mortars, but is often overlooked by today’s builders.
Each type of mortar is based on a variable range of composition or compressive strength. An example of this is n-type masonry cement (most commonly used today) which has a compressive strength of 750 psi and is generally manufactured using one part Portland cement mixed with one to one and a quarter. of lime and about two and a quarter to three parts of sand by volume, and a lime mortar has a compressive strength of about 75 psi.
The different types of mortar are lime mortar, historical lime mortar (containing historical lime), type k mortar, type o mortar, type n mortar, type s mortar and type m mortar. And the crushing pressure of these types of mortar is 75 psi for lime mortar, 150 psi for type k, 350 psi for type o, 750 psi for type n, 1800 psi for type s and 2800 psi for type m mortar.
As you can see, there is a big difference between each type of mortar and it will certainly affect the results of any masonry repair.
Historic mortars tend to range from lime mortar to type o, in part due to the cost of Portland cement, but also to protect masonry units. The lime used often looses in place and tends to have large chunks. This is known as a historic file. These large pieces make the difference between historic lime and today’s S / SA lime, which are screened to ensure smaller particles. The only real effect these pieces have on the mortar is visibility. They change the appearance of the color and texture of the mortar.
As it was often necessary to add strength to the mortar, but not exceeding the compressive strength of the masonry units, the fibers were often mixed with the mortar. The most used fiber was horse hair. Horse hair and different herbs were also used to make the bricks and hold them together.
To preserve the historic masonry, a lot of attention must be paid to detail. To do this, you must first collect the available details about the mortar and masonry units. This requires testing the hardened mortar and / or the bricks or stones.
Through the analysis of the mortar, the composition of the mortar, the compressive strength of the mortar, the type of lime used, if fibers were used, and the type, color and size of the sand particles used in the old mortar are identified. .
These are the details required to make a matching mortar for re-targeting.
There are now some sophisticated and often expensive test methods used to analyze mortar samples that are simply not very reliable. Some of these tests take beautiful photographs of the mortar sample that identify different elements of the mortar by color. This image is then visually looked at and a best estimate of the proportions is determined. This method has many limitations, the worst of which is the understanding, experience and knowledge of the technician testing the mortar samples. If they don’t understand masonry, how it’s used, and how it’s built, they can’t interpret the data accurately.
A good example of this is calcium, and this carries over into many of the mortar test methods and the results are analyzed only by chemists. Across the industry, emphasis is placed on the amount of calcium in mortar. This has very little to do with the composition of the mortar and little to no change in the way the mortar behaves.
You will find calcium in sand usually in small amounts, but some sands are higher in calcium than others. In this use calcium is important. However, the use of calcium added to the mortar tells us more about the temperature and weather conditions during construction time.
As lime mortars cure, the lime acquires and adheres to the CO2, causing it to cure and harden. Portland cement releases CO2 as it cures. This makes them work well together. However, in cold climates, Portland’s chemical reaction can slow down or stop. When this happens, the mortar cures more slowly and may not allow Portland to add strength from its chemical reaction, causing the mortar to cure a lime mortar. Calcium can be added to mortar to increase the chemical reactions of Portland and lime and cause more heat. With the added heat and the Portland retarding the CO2 quickly, there is plenty of CO2 for the line to adhere to and the mortar will cure even in colder temperatures.
As you can see, mortar analysis is necessary to identify the composition of the mortar, but the results should be interpreted by a person with knowledge and experience in masonry. This does not mean that you can rely solely on the knowledge and experience of any bricklayer. You need the precise test data to obtain the composition of the masonry preservation mortar. The understanding and use of those data can only be interpreted by a person with knowledge of how they work, and recommendations for variation should be made with this understanding.
When all of these mortar considerations, its composition and performance tests, and its use have been met, you will have a good starting point toward the preservation and restoration of historic masonry. And the most important fact in restoring a historic building or restoring a historic mortar is to remember that while you may be the owner of a historic building, you are simply the caretaker of the property. This structure was here before you were born, and if properly cared for, it will last long after you are gone.
So for people involved in restoring history, remember that the historic building is the real customer and not necessarily the person who signs the check.
To put that in a more understandable and less offensive term, consider this. When you visit the doctor, your insurance is paying the bill, but the professional (the doctor) should consider you as the patient or client and your needs first.
Do not damage, destroy or alter the history just to receive a paycheck. Take the time and do it right.
