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SURFACE PREPARATION AND SURFACE TREATMENT OF CONCRETE
SURFACE PREPARATION AND SURFACE TREATMENT OF CONCRETE I. Gilmour Scottish Divisional Manager, Errut Products Ltd., U.K.
ABSTRACT Some of the problems of surface preparation and surface treatment are dealt with. The need for improved techniques for laying and finishing concrete floor slabs and the method of meeting specifications and tolerances are described. Texturing of road and airport pavements is also described. The final section deals with new and simple methods to carry out efficient remedial work on concrete slabs if and when this becomes necessary. INTRODUCTION Many failures of concrete floor slabs have resulted largely because of their poor designs and specifications. Although the situation has now been greatly improved, largely due to the efforts of Transport and Road Research Laboratory (1), Cement and Concrete Association (2,3,4), British Standards Institution (5) and British Ready Mixed Concrete Association (6) defective concrete slabs and their occasional failures are still one of the primary causes of complaint in the construction industry. Such defects and failures are often caused by inadequate attention being given to laying and finishing of concrete. In addition, indiscriminate use of toppings often results in disappointing floor performance since they are one of the prime causes of cracking and failures. An alternative to toppings is to specify correct surface texture at the design stage, which can then be produced on the concrete floor by using good quality concrete and new finishing techniques. However, in circumstances where the concrete needs to receive some special topping it is necessary for the concrete surface to be planed, ground, or scabbled to receive such toppings in order that a good bond is made between the concrete and the topping. This paper describes briefly some of the equipment which has been successfully used for the preparation and finishing of concrete slabs, after concrete has been delivered to site. With the best will in the world some defects are still unavoidable and the paper describes some of the machines which have been employed for remedial work. PREPARATION OF CONCRETE SLAB Concrete technology has advanced considerably in recent years but, unfortunately, concrete laying and finishing remains a semi-skilled (or unskilled on many sites) occupation. This causes a number of problems on site, including: 1.
Tolerances do not meet the specification
381
382
I. Gilmour
2. 3.
Finishes are poor and not acceptable Toppings (if used) crack or curl.
It should be appreciated that remedial work on badly prepared slabs can be expensive and time consuming, and efforts should be made to minimise such work by using the correct preparation procedures when laying the concrete. Irrespective of the final finish or surface texture desired, the preparation of the concrete should follow the same pattern, i.e. the concrete should be evenly spread, properly compacted and the surface smoothed and/or sealed prior to the commencement of the finishing operation. Proper compaction is essential since any addition of air voids in concrete will reduce the strength (1% air voids produce approx. 5% strength reduction) and lead to porous surfaces. In order to ensure an evenly compacted and level surface over the slab length and across the width, a twin-beam vibrating screeder/compactor, Fig. 1, can be used. The twin-beam screeder will compact a slab up to 150 mm thickness at one pass, leaving a reasonably smooth finish and strike off to the correct level. However, the use of a vibrating poker is recommended to compact the edges and joints. Slabs over 150 mm thickness will require the vibrating poker to be used all over the slab before using the twin-beam screeder. When a smooth surface texture is required, the finishing operation can be made easier by passing an Easy Float (or bull float), Fig. 2, over the surface to even out any small irregularities and help smooth out any minor waves left by the compacting beam. To obtain best results, the float should be applied soon after compaction, each pass should overlap the previous by about 50 mm only, to keep ridging to an absolute minimum. In the author1s opinion, hand trowelling should now be used to tidy up all edges and joints and to ensure that no high or low areas exist. With the floor slab correctly prepared in this manner, it can either be left as a tamped surface or finished to a predetermined design specification suitable for the loading and wear to which it will be put. DIRECT FINISHING OF CONCRETE FLOORS During compaction of concrete, some water tends to rise to the surface and consequently wet finishing will generally result in the formation of a weak layer at the top. The presence of this layer will affect adversely the performance (durability, wear resistance etc.) of the hardened surface. For this reason, as well as to obtain the desired surface texture, certain finishing procedures are applied after laying the concrete. The most common finishing techniques are: (a)
Direct finishing of the concrete slab;
(b)
Leaving the base slab in a rough condition and then putting on a topping or levelling screed;
(c)
Applying toppings to meet special conditions such as heavy abrasive conditions, chemical attack on the surface, or particularly, abrasive resistant surfaces.
Of these, the direct finishing method is becoming more and more the accepted method since it eliminates the toppings, thereby speeding up the job, reducing the possibility of the screed curling and cutting costs by avoiding a fwet trade1 finishing operation.
Surface Preparation and Surface Treatment
Fig. 1. Twin-beam vibrating screeder/compactor.
Fig. 2. Easy float.
383
384
I. Gilmour
Direct finishing of concrete slabs can be achieved by: 1. 2. 3.
Hand trowelling Power trowelling Early age power grinding.
Although hand trowelling is an excellent method it requires highly skilled men, who are becoming increasingly difficult to get. Thus, power trowelling and early age power grinding systems are being frequently adopted for direct finishing of concrete slabs.
Power Trowelling
Method
The power trowel shown in taken through a clutch to carrying detachable steel finish. It is normal to
Fig. 3, is a power operated tool with the drive a vertical shaft on to which are attached four arms blades, which can be fitted for various degrees of use two types of blades. The first one, the float
Fig. 3. Power trowel.
blades, are used for the initial floating operation to regulate and close the surface. The commencing time for the floating operation is very critical, and depends on surface stiffness and its moisture conditions. In the author1s opinion floating could start satisfactorily when the concrete is hard enough to take the weight of a man, leaving a 3 mm footprint on the surface. The floating operation produces a matt finished surface, which is then allowed to stand until it is hard enough to receive the final finish (power trowelling) using the second set of blades, finish blades. Since no mortar is removed during trowelling, timing is therefore critical if a hard-wearing surface is to be obtained. The correct time for starting with power trowelling is when
Surface Preparation and Surface Treatment
385
a hand can be placed on the surface without the concrete sticking to the hand; this time varies with temperature. In order to produce a good, hard-wearing surface, it may be necessary to have two or three successive trowellings with the blades tilted at greater angles. Where early access to the surface is needed, the process prior to floating and trowelling can be considerably speeded up by first vacuum dewatering the slab. The surface achieved by this method is very hard, with an almost glass-like texture. The appearance is excellent and the technique gives a very good cosmetic treatment to the floor. There are, however, many disadvantages of this operation which are normally caused by: (a)
the operator not being skilled enough to operate the machine,
(b)
going on the concrete too early, which brings too much fat to the surface making it very weak,
(c)
finishing the concrete in the early hours of the morning with no supervision, leaving a bad unlevel finish that can only be made right by remedial work, and
(d)
starting time being very critical and often resulting in considerable overtime.
Early Age Power Grinding
Technique
This relatively new technique was first introduced in Denmark by the author's company and has now gained a wide reputation in Britain as well as in many other parts of the world. The system was developed to eliminate some of the disadvantages associated with the use of the power trowelling method. Preparatory work is very similar to that for power trowelling. After placing, the concrete is compacted and levelled with the twin-beam screeder/compactor (Proto Screed). The surface is then given a pass with an Easy Float fitted with a wide blade. The concrete is then allowed to harden for normally between 2 and 7 days depending on curing conditions before the grinder, ERTGrind (Fig. 4 ) , is used. Grinding time is not critical as in power trowelling, nor is the machine difficult to use; some contractors have successfully ground floors to a good finish 4 to 6 weeks after the concrete was placed. The machine itself is either diesel or electrically driven. The drive is through a centrifugal clutch by Vee belts to an oil immersed gearbox which, in turn, drives two counter rotating discs. Three grinding blocks, normally 10 grit, are attached to each disc. The grinding head revolves at around 200 rev/min, the optimum grinding speed for concrete. At this speed, very little dust is created into the atmosphere during grinding. As the concrete is still green, the dust formed by grinding is moist and remains on the surface, where it can be easily swept away or vacuumed off. The surface achieved by using this method is a smooth, sandpaper-like texture, which gives a reasonably non-slip concrete floor. It is hard wearing and, due to the fact that the laitence is removed during grinding, is virtually dust free. Tolerances of within +_ 2 mm in 2 m (or 8 mm in 8 m) can be achieved. The major advantages of the early power grinding technique are that overtime is eliminated, concrete can be laid all day if necessary and unskilled operators can be used for the finishing process. Carpets and floor tiles 2 mm thick can be laid directly on the finished surface.
386
I. Gilmour
Fig. 4. Errut power grinder. For road and runway pavements, the choice of surface finish is between brushed texture or grooved texture, although, with the exception of high speed pavements, the brushed texture is perfectly satisfactory. For high speed roads (motorways or runways) a transverse grooved texture (Fig. 5) means that the texture will have a much longer life, will improve skid-resistance, increase braking force coefficient, reduce the risk of aquaplaning and markedly cut down obscuration due to spray. The Transport and Road Research Laboratory are at present studying this method to find the correct groove pattern to keep surface noise to a minimum. Although the technique is not used presently with plastic concrete in noise sensitive areas it is allowed in non-noise sensitive areas. The grooving technique is, however, permitted by the Department of Environment for remedial work in both noise sensitive and nonnoise sensitive areas. SCREEDS AND TOPPINGS Toppings are generally applied in the following situations: (i) where the abrasive in-service conditions require high strength concrete, which if used throughout the full depth of the slab would become uneconomic and (ii) the site conditions are such that it is not possible to direct-finish the slab. It is important that, for every application, the toppings are used in conjunction with a suitable grade of concrete as well as their constituent materials and mix proportions satisfying the relevant specifications (2). Toppings can be laid in two distinctly different ways. In the first method, monolithic construction, the topping is placed within not more than 3 hours of the structural concrete being placed and compacted and it becomes part of the slab. In
Surface Preparation and Surface Treatment
387
Fig. 5. Plastic concrete grooving machine.
the second method, bonded construction, which is commonly used, the topping is added near the end of the contract, well after the concrete slab has been prepared. In this case satisfactory performance of toppings depend on their being fully bonded to the slab. In order to facilitate a strong bond between the base concrete and the topping, the weak surface laitance (usually 1 to 3 mm thick) formed on the top of the slab must be removed and coarse aggregate exposed since this weakness can cause separation of the topping from its base, causing floors to fail, or the top finished surface of the screed to crack, curl or break up. The use of bending agents or admixtures cannot be effective where concrete surfaces are not prepared thoroughly. The preparation of slab surface can be carried out economically by using heavy duty floor scabblers, Fig. 6, or, when vibration has to be kept to a minimum (as in the case of suspended floors), by using lightweight scabblers, which are smaller but similar to the heavy duty ones. The use of these tools produce a rough exposed aggregate surface finish with all the laitance removed, an extremely good bond is achieved and failures minimised. To achieve a fully compacted topping the use of a twin-beam vibrating screeder is strongly recommended. Finishing is normally by hand or power trowelling, or by early age grinding, and is carried out in a similar manner to that used for direct finishing. Where vacuum dewatering is employed with monolithic construction it may be desirable to dewater the base concrete before the topping is applied and dewatered. Another problem associated with laying toppings is that of mixing, transportation and placing. Since toppings are normally applied near the end of the project access is sometimes difficult and, with multi-storey construction, it
I. Gilmour
388
Fig. 6. Floor scabblers»
has been necessary to utilise hoists, wheelbarrows etc. Recent advances in pumping screed material have seen the introduction of special screed pneumatic conveyors which can now be used for the normal sand and cement, and granolithic materials as well as for lightweight concrete mixes. These new techniques have enabled contracts to be completed with about half the labour in less time and give considerable savings in cost. REMEDIAL WORK In most cases, the faults will only be discovered after the concrete has hardened. The contractor is then faced with two choices: 1.
To remove the whole bay of concrete and re-lay it, at considerable cost in time and money, or
2.
To use some form of machine to repair the fault or damage at a more economical cost.
From practical experience, a list has been drawn up showing the machine type and the remedial work done, Table 1. Only the most common applications have been listed since the range of remedial work is extensive, each problem being different in size and form. For the purpose of this paper, the individual remedial applications are relatively unimportant, but it is important to know that machines are available to rectify, repair and re-texture existing concrete surfaces. The use of these machines have frequently reduced the cost and time taken to complete the work
Surface Preparation and Surface Treatment
TABLE 1 MACHINE TYPE
389
Some Examples of Remedial Work APPLICATIONS
PRM GRINDER
Removal of trowel marks; Repairing rain and frost damage; Levelling raised joints; Smoothing high spots; Removing droppings of plaster, etc.; Removing compacted oil or grease; Laitance removal. Generally, the grinder will remove up to 1-2 mm of surface material only; for heavier removal, other machines should be considered.
CONCRETE PLANE
Reducing levels up to 3-4 mm; Repairing badly formed brush textures; Reducing very high joints prior to finishing with PRM Grinder; Retexturing smooth areas of concrete to a ribbed finish.
PNEUMATIC SCABBLER
Reducing levels up to 50 mm without damage to under surface; Repairing spalled areas of roads; Repairing cracked areas of roads prior to patching; Removal of faulty toppings; Preparing old surfaces to take new toppings.
ECS FLOOR SAW
Cutting out badly formed joints; Cleaning out saw or pre-formed joints prior to filling with pointing compound; Cutting out areas to be patched to ensure good butt joint.
GC1 CRACK CUTTERS
To follow random cracks on a concrete slab and open them up to re-sealing; can be used for stitching in mis-placed dowel bars or tie bars.
SPETINS
Re-texturing large concrete areas worn smooth by constant use; Grooving ramps where original texture worn smooth. These machines are being generally applied to new road construction where rain damage has prevented the contractor reaching texture specification, and the specification can be achieved quickly and at low cost.
by more than half. In some cases, by re-texturing instead of re-laying, savings of considerable capital expenditure can be made, particularly where the concrete is sound in all but surface texture. CONCLUSIONS Concrete is a very versatile material and, with the correct specifications, preparation and surface finishing a long-lasting, hard-wearing surface can be achieved. With the introduction of new techniques for preparation, finishing and remedial
390
I. Gilmour
work, better surfaces giving a longer life may be realised at lower costs. The further development of machines for re-texturing worn or smooth surfaces can increase the life of existing, heavily used concrete, thereby making these areas safer and more economical over a long term. REFERENCES 1. Department of Environment. A Guide to the Structural Design of Pavement for New Roads, Road Note No. 29, H.M.S.O., London (1970). Concrete Ground Floors, their Design, Construction and 2. R.C. Deacon. Finish, Cem. Concr. A s s o c , London (1974). 3. G. Barnbrook. Concrete Ground Floor Construction for the Man on Site, Cem. Concr. A s s o c , London (1974). 4. P.H. Perkins. Floors Construction and Finishes, Cem. Concr. A s s o c , London (1973). 5. British Standards Institution. The Structural Use of Concrete, C.P. 110, B.S.I., London (1972). 6. British Ready Mixed Concrete Association. Code for Ready Mixed Concrete, B.R.M.C.A., Shepperton (1975).
ABSTRACT Some of the problems of surface preparation and surface treatment are dealt with. The need for improved techniques for laying and finishing concrete floor slabs and the method of meeting specifications and tolerances are described. Texturing of road and airport pavements is also described. The final section deals with new and simple methods to carry out efficient remedial work on concrete slabs if and when this becomes necessary. INTRODUCTION Many failures of concrete floor slabs have resulted largely because of their poor designs and specifications. Although the situation has now been greatly improved, largely due to the efforts of Transport and Road Research Laboratory (1), Cement and Concrete Association (2,3,4), British Standards Institution (5) and British Ready Mixed Concrete Association (6) defective concrete slabs and their occasional failures are still one of the primary causes of complaint in the construction industry. Such defects and failures are often caused by inadequate attention being given to laying and finishing of concrete. In addition, indiscriminate use of toppings often results in disappointing floor performance since they are one of the prime causes of cracking and failures. An alternative to toppings is to specify correct surface texture at the design stage, which can then be produced on the concrete floor by using good quality concrete and new finishing techniques. However, in circumstances where the concrete needs to receive some special topping it is necessary for the concrete surface to be planed, ground, or scabbled to receive such toppings in order that a good bond is made between the concrete and the topping. This paper describes briefly some of the equipment which has been successfully used for the preparation and finishing of concrete slabs, after concrete has been delivered to site. With the best will in the world some defects are still unavoidable and the paper describes some of the machines which have been employed for remedial work. PREPARATION OF CONCRETE SLAB Concrete technology has advanced considerably in recent years but, unfortunately, concrete laying and finishing remains a semi-skilled (or unskilled on many sites) occupation. This causes a number of problems on site, including: 1.
Tolerances do not meet the specification
381
382
I. Gilmour
2. 3.
Finishes are poor and not acceptable Toppings (if used) crack or curl.
It should be appreciated that remedial work on badly prepared slabs can be expensive and time consuming, and efforts should be made to minimise such work by using the correct preparation procedures when laying the concrete. Irrespective of the final finish or surface texture desired, the preparation of the concrete should follow the same pattern, i.e. the concrete should be evenly spread, properly compacted and the surface smoothed and/or sealed prior to the commencement of the finishing operation. Proper compaction is essential since any addition of air voids in concrete will reduce the strength (1% air voids produce approx. 5% strength reduction) and lead to porous surfaces. In order to ensure an evenly compacted and level surface over the slab length and across the width, a twin-beam vibrating screeder/compactor, Fig. 1, can be used. The twin-beam screeder will compact a slab up to 150 mm thickness at one pass, leaving a reasonably smooth finish and strike off to the correct level. However, the use of a vibrating poker is recommended to compact the edges and joints. Slabs over 150 mm thickness will require the vibrating poker to be used all over the slab before using the twin-beam screeder. When a smooth surface texture is required, the finishing operation can be made easier by passing an Easy Float (or bull float), Fig. 2, over the surface to even out any small irregularities and help smooth out any minor waves left by the compacting beam. To obtain best results, the float should be applied soon after compaction, each pass should overlap the previous by about 50 mm only, to keep ridging to an absolute minimum. In the author1s opinion, hand trowelling should now be used to tidy up all edges and joints and to ensure that no high or low areas exist. With the floor slab correctly prepared in this manner, it can either be left as a tamped surface or finished to a predetermined design specification suitable for the loading and wear to which it will be put. DIRECT FINISHING OF CONCRETE FLOORS During compaction of concrete, some water tends to rise to the surface and consequently wet finishing will generally result in the formation of a weak layer at the top. The presence of this layer will affect adversely the performance (durability, wear resistance etc.) of the hardened surface. For this reason, as well as to obtain the desired surface texture, certain finishing procedures are applied after laying the concrete. The most common finishing techniques are: (a)
Direct finishing of the concrete slab;
(b)
Leaving the base slab in a rough condition and then putting on a topping or levelling screed;
(c)
Applying toppings to meet special conditions such as heavy abrasive conditions, chemical attack on the surface, or particularly, abrasive resistant surfaces.
Of these, the direct finishing method is becoming more and more the accepted method since it eliminates the toppings, thereby speeding up the job, reducing the possibility of the screed curling and cutting costs by avoiding a fwet trade1 finishing operation.
Surface Preparation and Surface Treatment
Fig. 1. Twin-beam vibrating screeder/compactor.
Fig. 2. Easy float.
383
384
I. Gilmour
Direct finishing of concrete slabs can be achieved by: 1. 2. 3.
Hand trowelling Power trowelling Early age power grinding.
Although hand trowelling is an excellent method it requires highly skilled men, who are becoming increasingly difficult to get. Thus, power trowelling and early age power grinding systems are being frequently adopted for direct finishing of concrete slabs.
Power Trowelling
Method
The power trowel shown in taken through a clutch to carrying detachable steel finish. It is normal to
Fig. 3, is a power operated tool with the drive a vertical shaft on to which are attached four arms blades, which can be fitted for various degrees of use two types of blades. The first one, the float
Fig. 3. Power trowel.
blades, are used for the initial floating operation to regulate and close the surface. The commencing time for the floating operation is very critical, and depends on surface stiffness and its moisture conditions. In the author1s opinion floating could start satisfactorily when the concrete is hard enough to take the weight of a man, leaving a 3 mm footprint on the surface. The floating operation produces a matt finished surface, which is then allowed to stand until it is hard enough to receive the final finish (power trowelling) using the second set of blades, finish blades. Since no mortar is removed during trowelling, timing is therefore critical if a hard-wearing surface is to be obtained. The correct time for starting with power trowelling is when
Surface Preparation and Surface Treatment
385
a hand can be placed on the surface without the concrete sticking to the hand; this time varies with temperature. In order to produce a good, hard-wearing surface, it may be necessary to have two or three successive trowellings with the blades tilted at greater angles. Where early access to the surface is needed, the process prior to floating and trowelling can be considerably speeded up by first vacuum dewatering the slab. The surface achieved by this method is very hard, with an almost glass-like texture. The appearance is excellent and the technique gives a very good cosmetic treatment to the floor. There are, however, many disadvantages of this operation which are normally caused by: (a)
the operator not being skilled enough to operate the machine,
(b)
going on the concrete too early, which brings too much fat to the surface making it very weak,
(c)
finishing the concrete in the early hours of the morning with no supervision, leaving a bad unlevel finish that can only be made right by remedial work, and
(d)
starting time being very critical and often resulting in considerable overtime.
Early Age Power Grinding
Technique
This relatively new technique was first introduced in Denmark by the author's company and has now gained a wide reputation in Britain as well as in many other parts of the world. The system was developed to eliminate some of the disadvantages associated with the use of the power trowelling method. Preparatory work is very similar to that for power trowelling. After placing, the concrete is compacted and levelled with the twin-beam screeder/compactor (Proto Screed). The surface is then given a pass with an Easy Float fitted with a wide blade. The concrete is then allowed to harden for normally between 2 and 7 days depending on curing conditions before the grinder, ERTGrind (Fig. 4 ) , is used. Grinding time is not critical as in power trowelling, nor is the machine difficult to use; some contractors have successfully ground floors to a good finish 4 to 6 weeks after the concrete was placed. The machine itself is either diesel or electrically driven. The drive is through a centrifugal clutch by Vee belts to an oil immersed gearbox which, in turn, drives two counter rotating discs. Three grinding blocks, normally 10 grit, are attached to each disc. The grinding head revolves at around 200 rev/min, the optimum grinding speed for concrete. At this speed, very little dust is created into the atmosphere during grinding. As the concrete is still green, the dust formed by grinding is moist and remains on the surface, where it can be easily swept away or vacuumed off. The surface achieved by using this method is a smooth, sandpaper-like texture, which gives a reasonably non-slip concrete floor. It is hard wearing and, due to the fact that the laitence is removed during grinding, is virtually dust free. Tolerances of within +_ 2 mm in 2 m (or 8 mm in 8 m) can be achieved. The major advantages of the early power grinding technique are that overtime is eliminated, concrete can be laid all day if necessary and unskilled operators can be used for the finishing process. Carpets and floor tiles 2 mm thick can be laid directly on the finished surface.
386
I. Gilmour
Fig. 4. Errut power grinder. For road and runway pavements, the choice of surface finish is between brushed texture or grooved texture, although, with the exception of high speed pavements, the brushed texture is perfectly satisfactory. For high speed roads (motorways or runways) a transverse grooved texture (Fig. 5) means that the texture will have a much longer life, will improve skid-resistance, increase braking force coefficient, reduce the risk of aquaplaning and markedly cut down obscuration due to spray. The Transport and Road Research Laboratory are at present studying this method to find the correct groove pattern to keep surface noise to a minimum. Although the technique is not used presently with plastic concrete in noise sensitive areas it is allowed in non-noise sensitive areas. The grooving technique is, however, permitted by the Department of Environment for remedial work in both noise sensitive and nonnoise sensitive areas. SCREEDS AND TOPPINGS Toppings are generally applied in the following situations: (i) where the abrasive in-service conditions require high strength concrete, which if used throughout the full depth of the slab would become uneconomic and (ii) the site conditions are such that it is not possible to direct-finish the slab. It is important that, for every application, the toppings are used in conjunction with a suitable grade of concrete as well as their constituent materials and mix proportions satisfying the relevant specifications (2). Toppings can be laid in two distinctly different ways. In the first method, monolithic construction, the topping is placed within not more than 3 hours of the structural concrete being placed and compacted and it becomes part of the slab. In
Surface Preparation and Surface Treatment
387
Fig. 5. Plastic concrete grooving machine.
the second method, bonded construction, which is commonly used, the topping is added near the end of the contract, well after the concrete slab has been prepared. In this case satisfactory performance of toppings depend on their being fully bonded to the slab. In order to facilitate a strong bond between the base concrete and the topping, the weak surface laitance (usually 1 to 3 mm thick) formed on the top of the slab must be removed and coarse aggregate exposed since this weakness can cause separation of the topping from its base, causing floors to fail, or the top finished surface of the screed to crack, curl or break up. The use of bending agents or admixtures cannot be effective where concrete surfaces are not prepared thoroughly. The preparation of slab surface can be carried out economically by using heavy duty floor scabblers, Fig. 6, or, when vibration has to be kept to a minimum (as in the case of suspended floors), by using lightweight scabblers, which are smaller but similar to the heavy duty ones. The use of these tools produce a rough exposed aggregate surface finish with all the laitance removed, an extremely good bond is achieved and failures minimised. To achieve a fully compacted topping the use of a twin-beam vibrating screeder is strongly recommended. Finishing is normally by hand or power trowelling, or by early age grinding, and is carried out in a similar manner to that used for direct finishing. Where vacuum dewatering is employed with monolithic construction it may be desirable to dewater the base concrete before the topping is applied and dewatered. Another problem associated with laying toppings is that of mixing, transportation and placing. Since toppings are normally applied near the end of the project access is sometimes difficult and, with multi-storey construction, it
I. Gilmour
388
Fig. 6. Floor scabblers»
has been necessary to utilise hoists, wheelbarrows etc. Recent advances in pumping screed material have seen the introduction of special screed pneumatic conveyors which can now be used for the normal sand and cement, and granolithic materials as well as for lightweight concrete mixes. These new techniques have enabled contracts to be completed with about half the labour in less time and give considerable savings in cost. REMEDIAL WORK In most cases, the faults will only be discovered after the concrete has hardened. The contractor is then faced with two choices: 1.
To remove the whole bay of concrete and re-lay it, at considerable cost in time and money, or
2.
To use some form of machine to repair the fault or damage at a more economical cost.
From practical experience, a list has been drawn up showing the machine type and the remedial work done, Table 1. Only the most common applications have been listed since the range of remedial work is extensive, each problem being different in size and form. For the purpose of this paper, the individual remedial applications are relatively unimportant, but it is important to know that machines are available to rectify, repair and re-texture existing concrete surfaces. The use of these machines have frequently reduced the cost and time taken to complete the work
Surface Preparation and Surface Treatment
TABLE 1 MACHINE TYPE
389
Some Examples of Remedial Work APPLICATIONS
PRM GRINDER
Removal of trowel marks; Repairing rain and frost damage; Levelling raised joints; Smoothing high spots; Removing droppings of plaster, etc.; Removing compacted oil or grease; Laitance removal. Generally, the grinder will remove up to 1-2 mm of surface material only; for heavier removal, other machines should be considered.
CONCRETE PLANE
Reducing levels up to 3-4 mm; Repairing badly formed brush textures; Reducing very high joints prior to finishing with PRM Grinder; Retexturing smooth areas of concrete to a ribbed finish.
PNEUMATIC SCABBLER
Reducing levels up to 50 mm without damage to under surface; Repairing spalled areas of roads; Repairing cracked areas of roads prior to patching; Removal of faulty toppings; Preparing old surfaces to take new toppings.
ECS FLOOR SAW
Cutting out badly formed joints; Cleaning out saw or pre-formed joints prior to filling with pointing compound; Cutting out areas to be patched to ensure good butt joint.
GC1 CRACK CUTTERS
To follow random cracks on a concrete slab and open them up to re-sealing; can be used for stitching in mis-placed dowel bars or tie bars.
SPETINS
Re-texturing large concrete areas worn smooth by constant use; Grooving ramps where original texture worn smooth. These machines are being generally applied to new road construction where rain damage has prevented the contractor reaching texture specification, and the specification can be achieved quickly and at low cost.
by more than half. In some cases, by re-texturing instead of re-laying, savings of considerable capital expenditure can be made, particularly where the concrete is sound in all but surface texture. CONCLUSIONS Concrete is a very versatile material and, with the correct specifications, preparation and surface finishing a long-lasting, hard-wearing surface can be achieved. With the introduction of new techniques for preparation, finishing and remedial
390
I. Gilmour
work, better surfaces giving a longer life may be realised at lower costs. The further development of machines for re-texturing worn or smooth surfaces can increase the life of existing, heavily used concrete, thereby making these areas safer and more economical over a long term. REFERENCES 1. Department of Environment. A Guide to the Structural Design of Pavement for New Roads, Road Note No. 29, H.M.S.O., London (1970). Concrete Ground Floors, their Design, Construction and 2. R.C. Deacon. Finish, Cem. Concr. A s s o c , London (1974). 3. G. Barnbrook. Concrete Ground Floor Construction for the Man on Site, Cem. Concr. A s s o c , London (1974). 4. P.H. Perkins. Floors Construction and Finishes, Cem. Concr. A s s o c , London (1973). 5. British Standards Institution. The Structural Use of Concrete, C.P. 110, B.S.I., London (1972). 6. British Ready Mixed Concrete Association. Code for Ready Mixed Concrete, B.R.M.C.A., Shepperton (1975).