Archive for October, 2011
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
Building materials crack for different reasons. In some cases it could be a material failure of the individual masonry unit and not associated with the wall assembly. But in most historic masonry loadbearing walls the cracks have something to do with thermal change and or ground movement or possibly a water related issue dealing with the foundation. Either way cracks must be repaired to prevent water from penetrating the structure and causing further damage.
There are many different types of crack repair products on the market today. Basically they break down into two main categories 1) ridged repair or stitching the masonry units back together – for example, epoxies would be in this category. You would use epoxy material when you are repairing an individual masonry unit like a piece of terra cotta damaged from a wall anchor or manufacturing defect.
The second category, and more common, is that of a movement crack, or a dynamic crack, that changes its width and dimension during the change of seasons and under different weathering cycles and building movement. These cracks are generally continuous and follow a distinct pattern from the ground level through to the top of the wall and often through the entire width of the masonry transferring on the inside of the wall surfaces. A crack like this would need to be repaired with something other than epoxy.
Because experience has taught us that if we inject a movement crack with epoxy – another crack will likely appear 6 to 8 inches on either side of the original one – as the building still moves in this location. A better choice of materials like a dispersed hydrated lime (DHL) injection material, or a flexible injection grout, one that accommodates movement would be the best material to specify in these areas. These materials seal off water infiltration while at the same time allow the movement to occur naturally.
It is also helpful to establish how much movement you have on elevations by the use of crack gauge monitors. These monitors allow for the periodic measurement and evaluation of the crack at different times of the weathering cycles. It is best to know as much there is to known about your building problems before material specifications are finalized to be sure you have the right material for the right treatment.
Training in Application: http://speweikpreservation.com/2011/05/22/training-dhl-dispersed-lime-injection/
Most people don’t realize that lime particles are said to be some 500 times smaller than portland cement particles. This might explain, in part, why some lime mortar applications for repointing get so messy with mortar on the faces of the stonework – and yes, very difficult to remove especially the longer the lime mortar sits on the stone. The key in delivering a clean project, one that needs minimal cleaning (just with a small brush and water at the edges of the stone) is water content of the repointing mortar when being applied.
The consistency should be like that of stiff compacted brown sugar like you find in the kitchen cabinet. Yep, just like that. The feeling, the moisture content, and the compaction power. Mortar made to this consistency will not stick to the surfaces of the stonework and cannot be dragged along the top causing a stain from the hawk being pressed against the wall. You get a cleaner wall and the chances of shrinkage cracks are reduced as the mortar cures.
But moisture is important when repointing a wall. Bond strength is delivered during the application when a thoroughly soaked wall (with water) is allowed to partially dry-out and become surface-saturated-dry or (SSD). The SSD condition gives the dry mortar a bond potential with the advancement of each masons pressurized push against the material to the back of the joint. This is why it is so important that the masons repointing tool be sized to fit within the joint to allow for this compression.
I have seen a trend in recent years to rely on washing down the repointed walls with a light solution of an acid-based cleaner to remove the mortar stains from the stone surfaces. Problem is that the cleaner also cleans the mortar and dissolves the binder paste from the surfaces. While some in the industry call this aging the mortar, because it exposes the aggregate and gives the appearance of an old mortar joint. Well, you would get that appearance anyway if you waited 10 years as the lime paste naturally wears off the surface of the aggregate particles.
So, in fact, what you are doing in washing the wall down is giving the customer a used wall – a clean wall, but a wall that has been exposed to accelerated weathering is how I look at it. I figure you rip off at least 10-15 years of life cycle performance from the face of a mortar joint by washing it down with an acid-based cleaner. Seems the evidence is clear that the lime mortars do not withstand a cleaning as well as portland cement-based mortar mixtures. What makes things much worse is that lime mortar is very absorbent to water by its natural ability to transfer water in and out through evaporation which often causes the cleaners to penetrate deeper into the joint surface weakening the material even further.
The story I tell in my masonry seminars is a fun one to illustrate the point. It would be like selling someone a brand new set of tires for their truck and make them pay full price for them, but just as they are ready to drive away, you tell them, “Let me use your tires for say 20,000 miles first, then I will give them back to you” – essentially selling them used tires for the price of brand new ones. Don’t sell used lime mortar.
I often get asked this time of year, “How late in the construction season can we work with lime-sand mortars?” Well the quick and fast answer is 45 days before the first hard frost. Which means you should have your projects wrapping up by the end of October at the latest just to be safe. The reason for this safe period (as suggested by lime mortar manufacturers) is because of the way lime-sand mortars initially cure, by carbonation – absorbing CO2 back into the material through wetting and drying cycles. Most specifications call for a minimum of nine (9) wetting and drying cycles of misting the walls down with generous amounts of water after installation and allowing them to dry out naturally – drawing in the CO2 from the evaporation of the water. Obviously, this wet-curing process can become problematic during freezing temperatures.
The Brick Industry Association, March 1992, Technical Note 1 states, “Mortar which freezes is not as weather-resistant or as watertight as a mortar that has not been frozen. Furthermore, significant reductions in compressive and bond strength may occur. Mortar having a water content over 6 to 8 percent of the total volume will experience disruptive expansive forces if frozen due to the increase in volume of water when it is converted to ice. Thus, the bond between the unit and the mortar may be damaged or destroyed.”
But what if your schedule backs you up against old man winter and you have no choice but to work into November? Don’t lose heart. We carried out some testing 8 years ago on a project in Chicago [Lime Mortars, Two Recent Case Studies, Ed A. Gerns and Joshua Freedland] to find out how late in the season you could repoint a building using lime-sand mortars. Trials were conducted at various times in the fall and early winter at approximately six week intervals. The last installation occurred 48 hours before the first frost on November 23, 2003. Observations during installation, following initial curing, and periodically through the winter and following year were noted. The high-calcium lime putty and sand mortar showed no signs of shrinkage cracks, the bond between the mortar and brick units was well-adhered, and no erosion or cyclic freeze-thaw damage was observed. We were working with butter joint brickwork of 1/8 inch mortar joints.
To supplement the insitu testing, limited concurrent laboratory testing was conducted to evaluate the depth of carbonation and the impact of freezing temperatures had on the depth and rate of carbonation of the lime. Two inch mortar cubes were made from the same mortar formulation (1:2.5) – no additional water was added. The mortar cubes (eight sets) each went under freezing temperatures once for a four-hour duration at various times after initial mixing. The exposure to freezing temperatures (10F) was established at 24 hrs; 48hrs; 72hrs; 96hrs; 1 week and 2 weeks. The mortar cubes were then broken at various times and the depth of carbonation was measured using a phenolthalein solution as an indicator.
After 2 weeks, the depth of carbonation suggested that mortar cubes that were exposed to a freezing temperature for a limited duration during the first week had less depth of carbonation than the cubes that did not experience any freezing temperatures. After three weeks, however, this difference was no longer observable. All mortar cubes seemed to equalize after three weeks. Interesting. The success of this trial may be a result of the forgiving nature of lime-sand mortars, the low water content of the repointing mortar, and the narrow joint width of this particular project.
This paper was presented at the 2005 International Building Lime Symposium in Orlando, Florida. Proceedings are available on CD (ISBN 0-9767621-0-2). The CD includes 39 papers by authors from 10 countries. Also, included are several important historical documents related to building lime–some as old as 1920.
The conference proceedings are available for $25 at: http://www.lime.org/documents/publications/free_downloads/summary-ibls-2005.pdf
I attended a social networking gathering at my wife’s college last week. The university has an architecture program with an emphasis on sustainability. I was introduced to one of the students a “MArch Candidate” it said on his business card. I mentioned I was involved with historic preservation of traditional architecture and his eyes lit up.
His young energy filled his mouth with words of interest, but I truly knew he had not a clue what I did for a living. We exchanged pleasantries and he mentioned sustainability as one of his schools focal points, and his as well. I discussed my specialty in Division 4, Historic Masonry, and followed it with the statement that preserving architecture is the ultimate sustainability.
When you stop and think about it for a moment, it really is something.
The brick are already fired, no need for fuel or manpower to extract the clay from the earth – already done! Same goes for the stone – already quarried to size, and transported to the site – lifted into place. The mortar ingredients have already been located, sifted, fired, mixed, and installed between the units.
All the collective energy and labor effort in Division 4 has been paid for and is waiting to provide more life-cycle performance to the next generation – for perhaps another hundred years. With the right preservation plan for its reuse you have sustainability at its finest.
Unlike new masonry construction, restoration masonry requires matching to existing surfaces. Whether it’s a brick, stone, or mortar, samples must be submitted and often tested to determine the original material components. So what is the best way to specify a match? Well, first let’s talk about the way it is generally done now in the restoration business. An architect writes a specification that includes the details of matching the masonry; for instance, “match the brick in color, texture, size, and physical characteristics to that of the original historic brick”….nothing wrong with that, right?
Well consider the brick match needing to be located (research, calling around to suppliers, submittal of samples) after the contract has been awarded and the construction schedule is starting. The pressure to find a good and acceptable brick match is now the responsibility of the contractor who is thinking about mobilizing the site, balancing manpower to get the project done on time, and the overall responsibilities for the entire project. Question: Is this the best time and the right person to be carrying out the important responsibility of finding a successful brick match?
The same goes for the stone or mortar match as well. Question: Is placing these decisions on the back of the contractor at the start of the project in the best interest of the project? Under this pressure mistakes can be made and searching for the best most appropriate match compromises are often made (“that’s the best we can get, or, they don’t make that any longer”). So what might be a better strategy? A relatively new movement is occurring in the architectural design world in the restoration business.
Architects are working with building owners directly and sometimes with consultants to assist them in matching historic masonry materials – prior to bid….during the design development stage of the project (often 1-year in advance of the bidding process). The brick, stone and mortar testing work are accomplished and often times pre-approved in a pre-construction test panel installed by a local mason contractor or preservation consultant. This strategy helps to eliminate delays in the construction phase of the project and it gives more time, without the pressure, to find the best available match on the market.
So the next time you are considering specifying replacement masonry materials on a historic restoration project consider this new approach to an age old problem. It takes a little more planning on your part, and yes, the owners need to pay up front for some pre-construction test panels, installed into the actual masonry for evaluation. But in the end, the surprises related to change orders are often minimized and the team approach to getting the project done on-time and under budget becomes a reality-not just a dream. And, its money the owners will be spending anyway with the contractor after the bid award….. something to consider.