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Pointing with NHL – Guide

Re-Pointing Masonry Walls: Brick, Blocks and Natural Stone.

Understanding mortars.

Before deciding to re-point a basic understanding of the function of mortars is required.  Mortar is used for jointing individual units in a mass of masonry. The final structure must have certain characteristics to function satisfactorily. It must carry the load for which it was designed, it must be durable and it must give protection against wind, rain and frost.
Mortar should develop sufficient strength and at such a rate as to be capable of withstanding the stresses to which it will be subjected during the construction of the building and subsequently when the structure is fully loaded. It should not however set and harden so quickly that it becomes inflexible at any stage and cannot accommodate slight movement. There is normally no requirement for significant structural strength in the mortar of traditional masonry buildings, particularly in re-pointing work.

Mortar should be permeable in itself, both so that the quantity of free water on the face of the building is reduced, thus reducing the possibility of wind-driven water penetration and so that moisture evaporation is not concentrated in the masonry, which may then be vulnerable to accelerated breakdown in the vicinity of the joints. Mortar should bond firmly to the units so that a tight joint is obtained through which rain will find it difficult to penetrate.
Mortar should be workable, so that the material may be applied easily and to ensure that the vertical as well as the horizontal joints can be adequately filled. Historic masonry buildings rely on their mass and the interlocking of individual units for their stability and the mortar in a masonry building serves in the main to provide a bedding for often very irregular components, filling the voids and maintaining the wind and watertight integrity of the building.

Choosing an appropriate re-pointing mortar.

Analysis of original bedding material is usually a good first step in determining the correct materials to use in the repair of historic masonry. Mortar can and does change with time, it is better that the analysis is carried out by a suitably experienced person or laboratory.

It will not always be the case that simple matching will be sufficient. The exposure and condition of the building today may well be different from its original construction. Ruins and monuments in severe disrepair (i.e: lack of roofs, copings to wall heads etc…) might well require a repair mortar different from the original as the new mortar will have to cope with weather and exposure conditions for which the original mortar was not designed.

Most important of all: the new mortar should be compatible with the old. Do not introduce mortars containing potentially damaging elements or that can constitute an impermeable barrier obstructing vapor exchange and retaining moisture. The consequences in a relatively short time could be disastrous and, in most cases, irreversible.

Resistance to salts
Soluble salts (sulphates, nitrates, chlorides) can be present in walls, they can be in the original mortar, in bricks
and stones (often as a result of previous repair work), in ground water or from airborne pollutants. All St Astier NHL limes are resistant to salts.  They do not contain the reactive components such as high levels of aluminium, potassium and sodium oxides and gypsum. Existing salts will therefore be allowed to migrate out of the structure without affecting the soundness of the mortar and, in time, be washed off.


Protection and good working practice.
For a
ll lime mortar work, best practice requires proper curing and aftercare against the effects of drying winds, strong sunlight, rain and frost. Lime mortar may require slightly longer curing times but the methods and principles are the same.

Where scaffolding is in place, fine mesh debris netting securely fixed to the outside of the scaffold gives basic protection to the working area slowing down strong wind while allowing good natural light for the works. In regards to external protection, the work should be covered with hessian sheets, polythene or both, depending upon conditions. Polythene should never come in contact with the work.  To avoid rapid drying and consequent high shrinkage, especially in hot or windy weather conditions keep all work damp by repeatedly applying a fine mist of clean potable water, if necessary several times a day, until the mortar has hardened.  


Re-pointing:
Before any re-pointing work is undertaken a survey of the building should be carried out by the supervising officer and the contractor to determine the precise areas to be re-pointed, and the nature and style of finish to be achieved.  
Often much of the old lime mortar raked out is sound and could, with advantage, have been left in place.  Today’s builder expects mortar to be strong, hard, dense and cement rich.  Strength is perceived to be a prerequisite and soft lime mortars are often removed in the belief that the softness is a sign of failure.  In other instances, entire elevations are re-pointed to provide a uniform color, rather than re-pointing defective joints with a suitable and compatible mortar.
It is essential that all pointing is carried out to match previously approved samples. This will remove any tendency for artistic licence on the part of the builder. The finish achieved on mortar joints can have a dramatic effect on the performance and visual appearance of the completed work, although this is often not immediately realised, sometimes only being condemned after the scaffold has been taken down and the full visual impact becomes apparent. Preparation:
Starting from the top of the building, defective joints should be carefully cut out with appropriate tools and the joint thoroughly cleaned out, ensuring the inside upper and lower edges of the masonry are properly scraped clean of old mortar. In brickwork the joint should be raked out to 11/2 times the width of the joint or a minimum 3/4 of an inch. Ensure the back of the joint is square. The joints can be cleaned with a vacuum, low pressure compressed air and/or rinsed out with a garden hose to remove
all loose materials: this is important as the mortar will adhere to dust which is left in the joints and deplete the bond. This is normally done form the top of the building down as the old lime in the mortar can stain the masonry if it is not properly washed of, working from the top down avoids unnecessary wetting of previously finished work. Prior to any re-pointing, controlling the absorption rate of the background is essential.Application:
Mortar should be plastic and workable but as stiff as possible. It should be pushed into the back of the joints in layers, avoiding large volumes of deep filling at all times. On rubble elevations, pinning stones should be used to fill wide and deep joints in the same style as the original build. This will reduce the volume of mortar required and will assist the process of setting and final full carbonation. A good yardstick is to keep the joint thickness to no more than a “finger” thick, if the joints are wider than this they should be pinned with compatible matching masonry.Finishing:
In natural stone masonry, to ensure good compaction and adhesion within the joint, the mortar can be tamped firmly back with a stiff bristle brush as it starts to firm up. The timing of this is critical. If it is carried out too soon after placing, fines in the mix will be drawn to the surface and will form a dense skin, inhibiting the proper curing of the mortar. Once the surface of the mortar is firm (usually the next day) lightly scraping the surface to expose the aggregate can improve the appearance of the mortar and make the joints less visible. This process should not be undertaken before the surface has stiffened or mortar will be smeared onto the face of the stone. A “well filled” joint is close to or flush with the surrounding masonry or to the weathered edge. Recessed joints define the masonry components and detract from the appearance of the wall, becoming a feature in themselves. Historically the common practice was to fully flush point and line out rubblework. Brickwork has a number of specific joint finishes too numerous to go into in this general guide, but the principles of timing  the finishing of the joint still apply.
The fines in the mix will determine the finished color, therefore a wide range of natural colors is achievable without pigmentation. The whiteness of St Astier limes ensures the best color reproduction of the chosen aggregate.

Re-pointing dense impervious masonry.

Some masonries, such as granite, basalt etc. and dense impervious bricks require special considerationDue to their very nature these materials have little, if any, moisture absorption and therefore moisture is transferred to the joints.

In these circumstances the choice of mortar and method of application and finishing is very important.

The joints are more vulnerable to the effects of wetting during placing and immediately afterwards until a full set and carbonation has taken place. Using St. Astier NHL mortars will ensure setting without having to rely completely on carbonation. The stiffest mix possible should be used, avoiding free water in the joint cavity and consequent de-bonding effect. The vapor permeability
of NHL mortars will ensure moisture evaporation.

Joints should be filled to flush, never recessed. Recessed joints will leave ledges for the accumulation of water that will keep the mortar joint wet for longer periods and accelerate the decay process. while feebly hydraulic limes were often used for the building of walls with impervious masonry the construction period usually left sufficient time for the joints to set up and cure before exposure to rain.  Re-pointing is a much quicker process and more hydraulic materials are almost always a better option.

Joints should be raked back to approximately 1″, thoroughly cleaned, including the top and bottom faces of the beds, ready for the new mortar. Pinning stones should not be removed, but if they are loose, they should be removed and put back during the re-pointing. Where a wall has previously been re-pointed and the pinning stones have been lost, suitable replacements should be used. The walls should be well washed to remove any dust and loose friable material making sure that the entire elevation is cleaned down to prevent staining on the walls. Impervious masonry should be dry when the work commences, however the original backing mortar should be kept damp.

Re-pointing ashlar masonry.

Ashlar masonry should not be re-pointed unless there is evidence of open
joints. It is very difficult to re-point ashlar masonry without causing
unsightly damage.

Protective tape should be applied to the joints of fine ashlar work before
mortar is pushed into place.  The vertical joints almost always require
greater amounts of filling than the bed joints, due to the lack of compaction
and filling when building. Loose mortar should be carefully raked out of
fine joints using a tool such as a hand-held hacksaw blade.

Mechanical removal of defective mortar can be particularly damaging and
is too risky to be used in most situations. Mechanical tools should not
be used on historic masonry except in very experienced hands and the use
of bolsters or quirks for cutting out mortar joints should be avoided.
Fine carving chisels or specially made tools should be used to remove hard
dense over pointing.

Ashlar joints are usually no more than 1/32″- 1/8″ wide. It has
become common practice to “pare point” or “ribbon point” these joints because
they are so difficult to fill. Should re-pointing be necessary, joints should
be carefully removed to a depth of 3/8″- 3/4″ and re-pointed using
a non-hydraulic or feebly hydraulic ashlar pointing mix available from St
Astier distributors (see Ashlar Joints). Where
the arises of the stone have become rounded or damaged from previous repairs
the weathered edge or a very slightly recessed joint produces a visually
more acceptable finish.

When re-pointing ashlar masonry the mortar should be brought out to the
edge of the masonry, taking care not to smear the face. Mortar is normally
inserted into fine joints by pressing it into place with a flexible blade
or spatula. The mortar needs to be inserted to an adequate depth and it
will be necessary to push it back into place with the thin edge of a blade
when working on very fine joints. Where possible the full depth of the joint
should be filled with mortar, however in some situations, where the joint
gets wider away from the face, it may be necessary to grout very deep joints
(see Grouting).

Pointing deep joints should be done in layers of 3/4″- 1″ at a
time, allowing the preceding layer to take up before applying the next.

Very badly worn or damaged edges may require surface repair (see stone
repair mortar sheet
)
and subsequent tuck pointing to reduce the
visual impact of the traditionally very white ashlar mortars, and new joint
lines struck and pointed.

Some re-pointing NHL mortar mixes

 

Joint typeJoint sizePre-mixed mortar
Lime
Sand
(sieve size)
Ratio
Lime  : Sand
Ashlar / Tuck joint1/32″- 3/16″Ashlar / Tuck pointing mix
or Ecologic F
NHL 2
#20 to #2002 : 1
High porosity masonry3/16″-3/8″Ecologic™ G
NHL 2
NHL 3.5
#18-10 to #2001 : 2 
1 : 2.5
3/8″-3/4″Ecologic™ G
NHL 2
NHL 3.5
#6-3 to #2001 : 2
1 : 2.5
Medium porosity masonry3/16″-3/8″Ecologic™ G
NHL 2  
NHL 3.5
NHL 5
#18-10 to #2001 : 2
1 : 2.5
1 : 2.5 or 3
3/8″-3/4″Ecologic™ G
NHL 2
NHL 3.5
NHL 5
#6-3 to #2001 : 2
1 : 2.5
1 : 2.5 or 3
Low porosity masonry3/16″-3/4″Ecologic™ G
NHL 3.5
NHL 5
#18-10 to #2001 : 2.5
1 : 2.5 or 3
3/8″-3/4″Ecologic™ G
NHL 3.5
NHL 5
#6-3 to #2001 : 2
1 : 2.5or 3
Floor tiles or stone slabs1/32″-3/16″Ecologic™ F
NHL 3.5
NHL 5
#18-10 to #2001 : 2
1 : 2 or 2.5
3/16″-3/8″Ecologic™ F or
G
NHL 5
#6-3 to #2001 : 2

 

This document is a guide only and is not intended to be a specification. Its purpose is to provide the reader with helpful information that may assist  in determining the correct choice of materials, methods of application and  determine the best working practice. The
guidelines refer to our experience with St. Astier NHL binders and some recommendations might not be applicable to other products.

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