Analyzing and matching samples of mortar are the first steps in repairing cracks in masonry. Though this step is often overlooked, it is ill advised.
When repairing masonry cracks, it is important that the new mortar bond to the old mortar existing in the mortar joints. When the new mortar fails to bond, it will tend to crumble or pop loose and fall out of the wall. In such, the repairs do not last, and can cause additional damage to the existing bricks.
Damage to the bricks occurs when the replacement mortar is harder than the original mortar and the bricks. As it pops loose, it can spall or chip off the faces of bricks. This can become a real problem if the brick are discontinued or historic and antique bricks, and you are unable to locate replacement bricks.
When the new mortar is softer than the original cement, it tends to crumble under the pressure of the brick wall. This could result in the whole wall falling down depending on the area repaired. Cracked or falling archways and cracks above garage door openings are the most likely areas for this to happen.
And we have all seen the most commonly recognized form of mismatched mortar repairs. You know, the giant lightning bolt running down the side of brick walls. This occurs when the color of replacement mortar doesn’t match the existing mortar, and causes what I call the lightning bolt effect.
All of these issues are avoided easily by properly matching the mortar. The crushing pressure of cement and the formula of mortar composition determine how the mortar bonds and performs in between the bricks, and the composition effects the color of mortar and how pigments react in the cement. For this reason, matching the composition and crushing pressure of the mortar will resolve the first two issues and make it easier to get a perfect color match for the replacement mortar.
A common misconception about matching mortar color is that it cannot be done. This is because the mason doesn’t know how or is unwilling to take the time required for testing samples and do it right. Or worse, they may tell you how they can match it on site and show you how close they can match the color by adjusting pigments and portions of premixed mortar bags.
When premixed type n bags of masonry cement are re-proportioned by mixing small amounts of different colors from each bag, the formula that makes them a certain mortar type and crushing pressure are changed, and they can have no reasonable idea of the new composition created because the materials within the bags or buckets separate by weight during transport and vibration. Because they are guessing at the composition at this point, you are likely to have one of the first two issues of the mortar not bonding because it is too hard or soft. Analysis of mortar composition is the only way to ensure the quality of masonry repairs and that repair mortars last.
Mortar analysis of hardened cement mortars generally takes about one or two weeks to get results back from the testing lab depending of how busy they are at the time, and some repair companies have mobile testing equipment that they bring on site to determine the mortar type and formula. Lab testing of mortar for matching is generally more reliable and accurate for color matches because they have all the time needed to match the color, and onsite matching is limited to the time allowed for completing repairs less the preparation, installation, and clean up.
Like I said, mortar analysis can take about a week give or take a few days. Some mortar testing labs take payments online, but most require a check/money order sent with samples for testing.
The amount of mortar does matter, and where the mortar comes from will be important. Mortar analysis labs need enough mortar for all of the tests requested. Generally a half a cup is more than enough to complete all tests and repeat some analyses as needed for accuracy. Color matches require surface mortar to get the most accurate results.
Soft mortars tend to crumble. This is OK. There are generally some larger pieces to test compressive strength and then the rest of the testing is performed with crushed mortar anyway. Historic mortars are the most common to crumble.
When collecting mortar samples for analysis, consider where they are being used. Interior samples are generally better for testing, but not always. If you have questions about this, you can contact the testing lab and consult their advise.
Historic mortar composition analysis will be the same, but sometimes different mortars were used for bedding mortar to lay the bricks or stone than were used to point the mortar joints. You may need to test both the bedding mortar and the pointing mortar.
When you get the mortar testing report and results back, you can order or make masonry pointing mortar for tuck pointing the cracks, and begin restoring the masonry.