Posts Tagged mixing
As I sit and write this blog I realize I am writing about style and approach to a specific task that many of you have been doing for many years. So I will start with this disclaimer – I will share my experiences with you on many successful repointing projects that involved lime putty mortar and most required no washing after wards except an occasional vinegar quick rinse to remove the white film from red brick units. I have made mistakes along the way and worked over the years to attempt to figure out what is the best practice approach to this task of mortar replacement called repointing.
Repointing, unlike tuckpointing, requires the full preparation of the joints to a depth of 2 to 2-1/2 times the width as discussed in yesterdays post. The American version of tuckpointing is only a skim coating of cement-based mortar over the top of existing mortar joints without the removal process (Chicago). The British version of tuckpointing is where a mortar joint is actually made to the same color as the units and a grapevine line is established in the center of the joint and tucked with a different color and mixture of mortar – true tuckpointing. This true tuckpointing is remarkably difficult to learn but in the end it makes brickwork appear straighter and rubble stonework look like it was laid up in ashlar units.
Mixing lime putty mortar is straight forward. You mix the putty into the sand placing the sand in the wheelbarrow first. Create a hole in the sand for the putty and place the putty into the hole and start to twist and knead the material together. What is interesting is that you do not need additional water during the mixing process; there is enough water in the lime putty to give you a good brown-sugar consistency for repointing. If you are laying brick you can add a small amount of water to create a spreadable mortar. A good repointing mortar should not be able to be spread with a trowel, if it is, than you have too much water in the mixture. If this occurs all is not lost, simply set the material aside on a dry sheet of plywood and allow the excess water to run off (be absorbed into the wood) until you get the desired consistency. This may take several days.
Pre-soak the wall with water. Be sure to get the water between the units and back into the existing original mortar joints. I have had projects where the water pre-soaking process never made it back into the joints – the sprayer was held too far away from the wall surface and while the unit faces received water the joints remained dry – especially the tighter joints. To ensure that you are getting the water back into the joints run the sprayers right on the walls and into the joints filling them with water. Allow the water to absorb into the wall materials and become Saturated Surface Dry (SSD) – no standing water to the touch, no drips, no glistening or shining if viewed from an angle – usually takes 8 to 10 minutes depending on the rate of absorption. You are now ready to start placing the mortar material.
It is most important that your repointing tools fit between the masonry units to enable you to compress the mortar back to the original material. If your repointing tools are too wide you will stain the face of the masonry as you attempt to get the material back into the joint and you will not get the necessary compression required for a good job. You may need to alter your tools by using a bench grinder to create thinner repointing tools. Most big box stores only carry 3/8 inch size even though ¼ inch sizes are available. Work from a hawk. If you are right-handed, work from right to left, on a slight angle, leading with the tip of the repointing tool. Overfill the mortar joints past the surface of the masonry units at least ¼ inch. Allow to dry and become thumbprint hard. Scrap away the excess mortar using a margin trowel and follow the contours of the masonry edges if required. Match the mortar joint profile of the original work. Stipple brush finish the joints by beating the faces with a churn brush – opening up the surfaces to expose the aggregate and create a texture that will encourage prompt evaporation of water and rapid carbonation as the mortar cures.
Protect the mortar for the first 24 hours after placement from wind driven rain and direct sunlight. Keep the material moist – spraying the entire wall with water three times per day for the first three days after application. The material carbonates as it goes through wetting and drying cycles – a minimum of at least nine (9) cycles. Do not allow the mortar to dry out too quickly. Repointing into hot or warm masonry units (south elevation) during summer months is not recommended. The masonry units simply will draw out the water from the mortar to quickly as the temperature rises reducing the chances for proper curing and carbonation to take place.
Repointing tools are available at: http://ushg.macusa.net/heritagedetail1.1.php?Current_Name=Restoration%20Pointing%20Iron
As the market increasingly becomes aware of the use of building lime for historic masonry restoration there will always be challenges in making sure everyone understands the decisions they are making, why, and most importantly, the materials they are working with. Take lime for instance, everyone seems to believe that going back to the old mixes of yesteryear is a better choice than that found in Isle 14 at the local Home Depot when it comes to mortar selection for historic masonry structures. But just knowing about a subject and really understanding a subject are two entirely different things. The product of lime is pretty basic. You have lime putty, purchased wet (Philadelphia cream cheese consistency) in a bucket or barrel, and you have dry hydrate lime purchased in a 50lbs. (fifty pound) bags (fluffy and very light weight).
Common cement/lime mortar mix formulations in the restoration industry center around 1:1:6; 1:2:9; and 1:3:12 (Type N, O, and K respectively- ASTM C270-10, proportion specification). The second numeral reflects the amount of lime to be added to the formulation to create the desired mortar and thus the characteristics of that mortar. Generally, a mortar with more lime will tend to have better workability, higher flexural bond and more autogenious healing properties than a mortar with less. If its compressive strength your after than 1:1:6 is your answer, if you are looking for the flexibility to accommodate for future movement than you will likely be happy with a 1:2:9 or 1:3:12 formula. And then of course there is the historic straight 1:2.5 lime-sand mortar almost always made with lime putty and not dry hydrate lime, let me explain one of the reasons why.
Lime, like portland cement, is measured as a dry powder when mixing individual ingredients at the job site. Small batches of mortar are mixed from opened bags using a coffee can or some other used drinking cup (seven-eleven big gulp cup works good) up to a five gallon bucket depending of the project needs. But here’s the real scoop – Dry hydrate lime experiences a significant volumetric loss when converted to a wet paste during mixing. Let me say that again, Dry hydrate lime experiences a significant volumetric loss when converted to a wet paste during mixing. Volume changes that occur when dry hydrated lime is converted to a wet paste can cause sizable errors in proportioning mortar formulations; the most likely error is over-sanding.
A given amount of hydrated lime occupies far more volume as a dry powder than it does after mixing with water. Thus, when lime is measured as a dry powder, less is actually put into the mixture than is used if the lime is measured as putty. When wetted, dry hydrate lime will typically contract, on average, to 75% of the original dry volume. Using a nominal 1:2:9 mixture (Type O) cement/lime/sand, the variation caused by wet verses dry measure of the lime results in a 1:1.5:9 mixture. This ratio exceeds the allowable sand content in ASTM C270 of 2.5 to 3 times the binder, and is actually 3.6 times the cement plus lime; thus an unintended over-sanded mixture results. To avoid this problem an additional amount of dry hydrate lime (25%) must be added to all formulations during the proportioning stage, or just use lime putty. Note: Portland cement does not experience this volumetric loss when converted to a wet paste during mixing.
Phillips, Morgan, A Source of Confusion about Mortar Formulas, APT Bulletin 1993 http://www.jstor.org/pss/1504465