floor problems Flashcards
key factors of good hardcore?
be chemically inert be easy to compact
not be affected by water
contain no soil, delirious material
concrete floor slabs - points to watch out for?
excess water in the mix
exposure during curing to frost
inadequate thickness
punctured DPM
DPM welted / bonded correctly at junctions
not levelling properly (requiring a levelling screed)
what is sulphate attack?
sulphates can be found in the sub soil, hardcore and / or brickwork
needs the presence of sulphates, cement and water to create a chemical reaction leading to tri-calcium sulfone alumniate
process is slow
concrete expands and cracks leading to uplift and can expand laterally affecting surrounding walls
how can sulphate attack be avoided?
what shouldd occur if sulphate attack has occurred?
hardcore should be clean - avoid colliery waste, gypsum plaster and other industrial wastes
beware of certain clay sub soils
always. ensure DPM and DPC overlap correctly
if discovered remedy is to replace floor
what is radon?
a naturally occurring radioactive gas that causes lung cancer
colourless and odourless it is given off by radioactive decay of uranium
uranium is found in small quantities in all soils and rocks mainly granite and limestone varies from place to place
where are high concentrations of radon found in buildings?
high concentrations are found in ground floors of buildings because they are usually at slightly lower pressure than the surrounding atmosphere
this allows radon from sub soil underneath buildings to enter through cracks and gap in the floor
how is radon prevented?
prevention involves sealing lowest floor and providing natural or assisted underfloor ventilation
what is passive approach in relation to controlling radon?
passive involves sealing house from underground gas at floor level integrated to walls
what is active approach in relation to controlling radon?
ground depressurisation system beneath a ground bearing slab
In relation to ground floors in domestic properties which are the most commonly constructed?
in-situ ground bearing slab
in situ suspended slab
pre cast concrete (beam/block)
timber suspended
how is a in-situ ground bearing slab achieved?
top soil and any contaminants removed
hardcore based is placed between walls
hardcore should not exceed 600mm thickness, is blinded with fine aggregate to prevent damage to 0.3mm thick polythene damp proof membrane.
concrete should be at least 100mm thick
a DPM is required above and below the slab and should be continuous with DPC in walls
floor should contain layer of insulation
why should the DPM used in an in-situ ground bearing slab be continuous with the DPC in the walls?
this is to prevent moisture ingress into the building
how is a in-situ suspended slab achieved?
concrete must be a minimum of 100mm thick. the floors reinforced with sheets of steel mesh are onto the inner leaf of clockwork
concrete usually supported by inert granular or sub soil infill
how is precast concrete (beam/block) achieved?
concrete beams are laid on the leaf go the walls and infilled with lightweight concrete beams to provide the structural floor
insulation is then provided above the floor and protected with a screed
a DPM and sub floor drain provided to the floor if the ground level under floor is below that of the surrounding ground to prevent water collecting under floor construction.
a ventilated space of at least 150mm between the ground covering and underside of floor must be provided
floor should be cross ventilated on opposite external walls
how is suspended timber floor achieved?
ground under floor must be covered to prevent growth and the passage of moisture or vapour resistant covering
moisture ingress is prevented by covering the ground with either 100mm of concrete laid on hardcore or 50mm concrete / fine aggregate laid on 0.3mm sealed polythene membrane
space between the ground covering and underside of floor ventilated on opposing sides via air bricks.
DPC 150mm above ground level
150mm gap between concrete/hardcore and floor joist
what are the sizes of structural timbers for suspended floors and why?
how should they be fixed?
timbers used in floor construction must be adequate to carry their own self weight and to other loads from the building such as partitions, floor boards, occupants etc.
minimum end bearing of at least 40mm and be either build in and sealed abasing air leakage or fixed with joist hangers
why should floor joists not be built into party walls?
shrinkage of timber or masonry can allow air paths to form what will allow the transition of sound from one dwelling to another
why should ground sub strata be checked before construction?
to ensure it has adequate bearing strength to support the floor and is free of possible conditions that may affect its integrity
what characteristics should be looked for in the ground sub strata before construction?
swelling, shrinkage, slippage or subsidence that can be caused by geological conditions or facts such as moisture take up tree roots and peat shrinkage caused by water table changes
what range of contaminate can cause problems on sites?
chemical works
asbestos works
gas works
methane gas production
how does approved document C suggest an approach to contaminants which can cause issue on sites?
hazard identification hazard assessment
risk estimation
risk evalution
what remedial measures are there to contaminants in the sub strata on site?
treatment
containment
removal
what is hardcore and why is it used?
hardcore is a filler to make up levels and prevents the concrete from being contaminated by the sub soil below
helps spread load evenly across the ground
helps reduce risk of rising damp
what is a key characteristic of good hardcore?
has open porous structure which limits capillary action
chemically inert
easy to compact and. should not be affected by water
what is the use of a steel mesh in concrete?
prevents the slab from cracking due to uneven loading or minor settlement
what types of damp proof membranes are there?
hot applied liquid membranes
cold applied liquid membranes
sheet materials
how do damp proof membranes need to be applied in order to be affected?
needs to be continuous and form good joint with DPC in external walls.
where can sheet material DPM’s be laid?
where can cold/hot applied liquid materials be laid?
sheet materials can be laid above and below the slab
cold/hot liquid materials can only be used above the slab
what is the benefit of DPM’s below the slab?
how should they be protected?
relatively cheap and will protect concrete from chemicals below
thinner floor screeds can be used because it will partially bond onto slab.
laid on sand layer to protect from any sharp stones in the hardcore
what is a con of DPM’s above the slab?
what is benefit of DPM’s above the floor slab
do not protect slab from harmful chemicals in the hardcore or ground
can offer the very best in terms of damp proofing
where are concrete ground bearing slab not suitable and instead suspended pre cast floor is more appropriate?
where there are aggressive chemicals in the ground
where water tables are high
on sloping ground where uneven depth of hardcore may lead to differential settlement on ground likely to heave
where hardcore is likely to be more than 600mm thick
where and how does sulphate attack occur?
occurs when sulphates in hardcore or possibly ground below migrate in solution to the underside of the slab, react with a by product of the cement in the slab and form tricalcium aluminate.
Resulting crystal expands as it forms the slap lifts and domes leaving a gap between hardcore and slab.
how long does the sulphate process take to visibly manifest its self?
slow process and can take up to several years
what can reduce the risk of sulphate attack in ground floors? why?
where DPM’s are effective because considerable amounts of moisture are required to complete the chemical reaction
which hardcores are susceptible to sulphate attack?
colliery shale
old bricks containing gypsum plaster and variety of industrial wastes.
which sub soils are susceptible to sulphate attack?
London clay
Oxford clay
Keuper Marl
how can sulphate attack be avoided?
careful choice of hardcore
use of sulphate resisting cement in the slab
isolating the slab from the hardcore with a sheet of polythene and good sub soil drainage
what is the only effective solution if sulphate attack occurs?
break up the slab and renew the hardcore
what is radon?
a colourless which is radioactive
how can occupants be protected from radon gas?
a house should be sealed to prevent the gas from rising through the sub soil and entering the building
which agency is concerned with radon and what do they do?
health protection agency
publishes guidance showing which areas are most at risk, how the gas can be detected and measured and appropriate steps for safeguarding against it
