Hi all, What are your preferences for rendering on celcon blocks? Internal and external. I was always led to believe weaker mixes, IE 5 to 1 backing rather than 4 to 1. followed by a 5 to 1 top coat or slightly weaker. First time ever for me but top coat has cracked slighty and is apparently dusty. good drying time given between coats with good application of top coat. 6 months - a year later and boom this happens, any ideas. waterproof in backing correct, about 3/4 plastic cup full followed by a bit less in top coat, plastering sand as well.
Render on Breeze Blocks????
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johniosaif
Private Member
What I do following my own problems is to scud the breeze block with s/c with SBR , then apply 5/1/1 the next day sand cement lime, then a week later apply 6/1/1, stop it from over drying if its warm , damp it down over a few days etc. water proof does make the mix stronger so more prone to cracking so the scud with sbr helps with the grip, mesh is often used now embedded in the first coat and fibres too. this is for external, for internal its 6/1/1 floated in and devil floated..
jamesthefirst
Private Member
Wonder if webber rendaid would be goid for this?
johniosaif
Private Member
External Rendering
Externally rendered Celcon blockwork as an outer leaf of a cavity wall - or as a solid wall where exposure conditions allow - are recommended constructions. The choice of block type and render specification should reflect the exposure conditions and whether the cavity contains insulant or not (see TSD/45, page 3 of this data sheet). Work should comply with the relevant Codes[1], [2], [3] and the following is a summary of their principal recommendations.
BLOCKWORK – for all types of rendering Movement Joints: Clause 5.4.2[1] advises that for all masonry work, movement joints may need to be provided. For an outer leaf or solid wall of rendered blockwork bed joint reinforcement should be provided at openings (clause 5.4.5[1] and Figure 4[2] and the need for any movement joints considered at the design stage. Specific recommendations will depend on the circumstances, and each situation should be assessed on its own merits. See the Celcon Book of Blocks (at the UK) for general advice on Accommodation of Movement. Mortar: For ‘traditional’ (10mm thick) joints overstrong mortars should be avoided. Recommended mixes are given in the Celcon Book of Blocks. Generally, lime based mixes are preferable as these give a better bond than can be obtained with air-entrained mortars of similar compressive strength. In external situations, lime based mortars can also give higher resistance to rain penetration (clause 5.5.4.2.2[1]). Where a non-lime mix is to be used the appropriate proportions by volume are given in Table 14[1] Site Practice: All blocks (as with other materials) should be protected. Keeping the blocks dry will ensure minimal drying out movement.
RENDERS – a) ‘Monocouche’ and other proprietary systems
From our own involvement with several (e.g. Alsecco, Sto, Weber-sbd) we know they are familiar with the application of their products onto Celcon blocks. Depending on the supplier, some use only approved applicators whilst some will also supply their materials for others to apply and offer a site advice service plus Data and Specification sheets.
RENDERS – b) traditional sand:cement mixes Application: Refer to the table on page 3 as to whether the Rendering should be applied DIRECT to the blockwork or via a CARRIER system (in which case the guidance below will not apply).
Mix: Renders should not be stronger than the backgrounds onto which they are applied (clause 6.2.2[2]). A 1:1:6 (cement:lime:sand) mix is recommended for use on aircrete blocks. In sheltered or moderate exposure conditions a 1:2:9 mix may be used on Solar blockwork. Equivalent mixes using masonry cement and sand or Ordinary Portland Cement and sand with a plasticiser are given in our Technical Manual. See also clause 6.17[2] and the mortar specification code[3].
Each successive coat should be weaker / thinner than the previous one (clause 6.18.1[2]).
Aggregates: The use of a properly graded sand is important. Traditionally, the coarsest and sharpest sand which can be conveniently handled should be used for undercoats. Advice on ordering rendering sands is given in PD 6682-3:2003.
Thickness of Coats: As already noted, successive coats should be weaker / thinner than the previous one. See clause 6.18[2], Table 5[2] and under “Undercoats” below.
Preparation: The background should be adequately prepared. Unless appropriate precautions are taken, work should be carried out only under suitable weather conditions (clause 7.4.1[2] ).
17 Oct 2007 REN/1195
Page 1 of 3RENDERS – b) traditional sand:cement mixes (cont’d)
Preparation (cont’d) Joints: Where mortar joints have not been recessed during construction, raking out will provide a
mechanical key thus improving the render bond. For Thin-Joint constructions, where recessed joints are not practicable, an external-use grade bonding agent or Spatterdash render coat may first be applied to the wall.
Surface condition: The wall should be cleaned of any dust, loose particles and contamination which may have occurred during construction (clause 7.4.1[2] ). In extreme cases of bad site storage or wet conditions where fungi or algae may have formed on the blocks, the surface must first be treated with a fungicide, applied in accordance with the manufacturer’s instructions.
Movement joints: Any joints present in the wall should be continued through the render finish. Proprietary types of external-grade stop bead incorporating a cover strip are available for this purpose (Figure 5[2]).
Before work is commenced, particularly in warm weather, it should be ensured that the wall is not too dry (clause 6.2.3[2] and Table 4[2]). If workability of the render is impaired, the wall should be wetted as described in our Book of Blocks (see also clause 7.4.2[2]).
Undercoats (clause 6.18.4[2]) should be combed or scratched (clause 7.8.3.(b)[2]) not only to provide a key for the following coat but also to reduce the ‘sheet’ strength[4]. The previous[5] advice of "not less than 8mm nor more than 12mm thick" should be heeded as anything thicker than this can give rise to a risk of de-bonding due to 'sheet' strength[4]. Newly applied rendering should be kept damp for the first few days (clause 7.8.2[2]) and the next coat should not be applied until the undercoat has adequately cured (clause 7.8.3(c)[2]).
Final Coat: Where a proprietary type of finish is used, it should be applied strictly in accordance with the manufacturer’s instructions. For traditional finishes (floated, scraped, textured, etc.) guidance is given in clause 6.18.5[2]. It should be noted that: “Cement-rich and/or lime-rich steel-trowelled finishes are particularly liable to craze...leaner mixes with a scraped, textured or other rough finish are highly resistant to this defect” (clause 6.19[2]). Dry dash and roughcast finishes require a strong render mix. Such finishes are not suitable on Solar blockwork but they may be used on Standard and Hi- Seven provided a 1:1:6 (or equivalent) mix is used. Most finishes (i.e. texturing, painting etc.) should not be applied until the top coat has dried out, but dry dash and roughcast is part of the top coat and should be applied whilst the top coat is in a suitable state to receive the aggregate to allow adequate embedment.
Bonding and Waterproofing agents: Providing the work and materials comply as above, the use of a bonding agent should not be necessary other than as noted above under ‘Joints’. The use of unsuitable waterproofing additives can have an adverse effect on the bond of the render coats (clause 5.1.3[2]).
Externally rendered Celcon blockwork as an outer leaf of a cavity wall - or as a solid wall where exposure conditions allow - are recommended constructions. The choice of block type and render specification should reflect the exposure conditions and whether the cavity contains insulant or not (see TSD/45, page 3 of this data sheet). Work should comply with the relevant Codes[1], [2], [3] and the following is a summary of their principal recommendations.
BLOCKWORK – for all types of rendering Movement Joints: Clause 5.4.2[1] advises that for all masonry work, movement joints may need to be provided. For an outer leaf or solid wall of rendered blockwork bed joint reinforcement should be provided at openings (clause 5.4.5[1] and Figure 4[2] and the need for any movement joints considered at the design stage. Specific recommendations will depend on the circumstances, and each situation should be assessed on its own merits. See the Celcon Book of Blocks (at the UK) for general advice on Accommodation of Movement. Mortar: For ‘traditional’ (10mm thick) joints overstrong mortars should be avoided. Recommended mixes are given in the Celcon Book of Blocks. Generally, lime based mixes are preferable as these give a better bond than can be obtained with air-entrained mortars of similar compressive strength. In external situations, lime based mortars can also give higher resistance to rain penetration (clause 5.5.4.2.2[1]). Where a non-lime mix is to be used the appropriate proportions by volume are given in Table 14[1] Site Practice: All blocks (as with other materials) should be protected. Keeping the blocks dry will ensure minimal drying out movement.
RENDERS – a) ‘Monocouche’ and other proprietary systems
From our own involvement with several (e.g. Alsecco, Sto, Weber-sbd) we know they are familiar with the application of their products onto Celcon blocks. Depending on the supplier, some use only approved applicators whilst some will also supply their materials for others to apply and offer a site advice service plus Data and Specification sheets.
RENDERS – b) traditional sand:cement mixes Application: Refer to the table on page 3 as to whether the Rendering should be applied DIRECT to the blockwork or via a CARRIER system (in which case the guidance below will not apply).
Mix: Renders should not be stronger than the backgrounds onto which they are applied (clause 6.2.2[2]). A 1:1:6 (cement:lime:sand) mix is recommended for use on aircrete blocks. In sheltered or moderate exposure conditions a 1:2:9 mix may be used on Solar blockwork. Equivalent mixes using masonry cement and sand or Ordinary Portland Cement and sand with a plasticiser are given in our Technical Manual. See also clause 6.17[2] and the mortar specification code[3].
Each successive coat should be weaker / thinner than the previous one (clause 6.18.1[2]).
Aggregates: The use of a properly graded sand is important. Traditionally, the coarsest and sharpest sand which can be conveniently handled should be used for undercoats. Advice on ordering rendering sands is given in PD 6682-3:2003.
Thickness of Coats: As already noted, successive coats should be weaker / thinner than the previous one. See clause 6.18[2], Table 5[2] and under “Undercoats” below.
Preparation: The background should be adequately prepared. Unless appropriate precautions are taken, work should be carried out only under suitable weather conditions (clause 7.4.1[2] ).
17 Oct 2007 REN/1195
Page 1 of 3RENDERS – b) traditional sand:cement mixes (cont’d)
Preparation (cont’d) Joints: Where mortar joints have not been recessed during construction, raking out will provide a
mechanical key thus improving the render bond. For Thin-Joint constructions, where recessed joints are not practicable, an external-use grade bonding agent or Spatterdash render coat may first be applied to the wall.
Surface condition: The wall should be cleaned of any dust, loose particles and contamination which may have occurred during construction (clause 7.4.1[2] ). In extreme cases of bad site storage or wet conditions where fungi or algae may have formed on the blocks, the surface must first be treated with a fungicide, applied in accordance with the manufacturer’s instructions.
Movement joints: Any joints present in the wall should be continued through the render finish. Proprietary types of external-grade stop bead incorporating a cover strip are available for this purpose (Figure 5[2]).
Before work is commenced, particularly in warm weather, it should be ensured that the wall is not too dry (clause 6.2.3[2] and Table 4[2]). If workability of the render is impaired, the wall should be wetted as described in our Book of Blocks (see also clause 7.4.2[2]).
Undercoats (clause 6.18.4[2]) should be combed or scratched (clause 7.8.3.(b)[2]) not only to provide a key for the following coat but also to reduce the ‘sheet’ strength[4]. The previous[5] advice of "not less than 8mm nor more than 12mm thick" should be heeded as anything thicker than this can give rise to a risk of de-bonding due to 'sheet' strength[4]. Newly applied rendering should be kept damp for the first few days (clause 7.8.2[2]) and the next coat should not be applied until the undercoat has adequately cured (clause 7.8.3(c)[2]).
Final Coat: Where a proprietary type of finish is used, it should be applied strictly in accordance with the manufacturer’s instructions. For traditional finishes (floated, scraped, textured, etc.) guidance is given in clause 6.18.5[2]. It should be noted that: “Cement-rich and/or lime-rich steel-trowelled finishes are particularly liable to craze...leaner mixes with a scraped, textured or other rough finish are highly resistant to this defect” (clause 6.19[2]). Dry dash and roughcast finishes require a strong render mix. Such finishes are not suitable on Solar blockwork but they may be used on Standard and Hi- Seven provided a 1:1:6 (or equivalent) mix is used. Most finishes (i.e. texturing, painting etc.) should not be applied until the top coat has dried out, but dry dash and roughcast is part of the top coat and should be applied whilst the top coat is in a suitable state to receive the aggregate to allow adequate embedment.
Bonding and Waterproofing agents: Providing the work and materials comply as above, the use of a bonding agent should not be necessary other than as noted above under ‘Joints’. The use of unsuitable waterproofing additives can have an adverse effect on the bond of the render coats (clause 5.1.3[2]).
superspread
Well-Known Member
Blimey John I thought I was reading war and peace:-0
johniosaif
Private Member
its stuff I had to find out due to an arse of an agent, its always good to be able to quote chapter and verse, some like the trades to be 'cap tippers to the gentry' still
imo 3 problems, celcon blocks on external, waterproofe additive weakens the mix, plastering sand ,i would have used sharpe sand on render. i would not have taking the work on if i had seen celcon blocks on external!
Rigsby
TPF Special Forces
Are the cracks around the windows and doors? Expansion of the frames in the sun could cause the cracks if around the frames.
A bit dusty? Did it set with water in it? If it dried out early this would give you a weak top coat. If the waterproofer in the base coat worked well then the only way out for the water is into the air. Drying out before fully set will not let it fully cure.
I would always tell the customer Celcons and Thermalites are no good for rendering as they are weaker than the render.
A bit dusty? Did it set with water in it? If it dried out early this would give you a weak top coat. If the waterproofer in the base coat worked well then the only way out for the water is into the air. Drying out before fully set will not let it fully cure.
I would always tell the customer Celcons and Thermalites are no good for rendering as they are weaker than the render.
johniosaif
Private Member
Problematic but not impossible, it needs all the bells and whistles to prevent problems..
Putzmiester
Active Member
Weber rend aid every time mate and full mesh
johniosaif
Private Member
never have used rendaid,is it as good as an sbr cement scud, easier to apply I expect. I will mesh in future myself
D
Deleted member 14228
Guest
The problem with thermalite block as always been the suction ! you have to cure the suction pure and simple !! if you dont want problems with hollow spots do your prep right or you will have come backs !rend aid does the job and at very least mesh on stress points .
johniosaif
Private Member
an SBR scub would also cure suction and add more grip than rendaid?
D
Deleted member 14228
Guest
Arti
Well-Known Member
Trowel it on, put ya mesh on, trowel over the mesh then i use a roller to get a stipple...others might did it different
johniosaif
Private Member
Would you not bed the mesh in the scratch?
D
Deleted member 14228
Guest
Arti
Well-Known Member
You can if you want but im sure webers spec is mesh in the rendaid
johniosaif
Private Member
Thanks, but if you use sand/cement I thought that the mesh would be more use embedded, not a lot of experience with mesh, none with rendaid, I always scudded with SBR and cement...I would love a week at the new renders just to get the feel of that end of our work..
D
Deleted member 14228
Guest
lets cut to the chase use a 4to 1 mix of uni bond rolled on let it sink in !mix uni or sbr with fine sand ad cement to make a paste that you can roll on and bobs ur uncle the cheap way ....
most plasterers dont last the week! when they find out what is required to earn a weeks money!
you live in london ,go and have alook at the malbrough tower, on the broard water farm, estate tottenham, that was a bar steward
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Arti
Well-Known Member
You can just use fibres in both coats if you didnt fancy using mesh
johniosaif
Private Member
I think I would last a week ha ha, been at it since late eighties...**** going to the broad water farm estate...
heya rigsby, gonna be going to have a look this week to see what exactly has happened but from what ive been told it seems something may have killed top coat. suction generally in backing coat but applied and scratched in good time ( no dusting). Oh and there was fibres in backing. second coat went gradually through the day ( waiting mainly) so did not dry too fast. All looked perfect and like i said its taken over 6 months ( i think closer to a year though) for it to crack and go dusty. Whether the waterproof was no good, as u said possible expansion from windows and frames,bad sand, will have more of an idea when i see it. ( might have to cover the van when i pull up though,lol) Never used rendaid b4 so would like to know more info on application methods. With regards to using lime in a backing coat, again ive never done this, what benefits does it carry???? only ever used lime in top coats for paneling and patterning. Ive never had a base coat crack. sbr is a beauty but never used as a grout, will prob have 2 next time.generally use mesh on weak points and cracked blocks anyway. With regards to mesh, is that the netting sort or eml sheets????
cheers all
cheers all
what i mean john ,there is a lot of work to do to earn your money. broad water farm is type of contract that you would be working on.
that malbrough tower it would take so much time just getting your gear up there.
johniosaif
Private Member
To be honest, I do know what you meant, it is tough on those sites, I avoid them if at all possible, just doing refurbs all my working life, better money if you can keep busy..
johniosaif
Private Member
I never scudded inside,prefer hardwall myself, but internal sand/cement is not affected by the weather like outside..
johniosaif
Private Member
only use SBR outside, normal pva can add to the problems,,
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