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Effects of three durability enhancing products on some physical properties of near surface concrete
Consrrurrion and Building Materials, Vol. 9, No. 5, pp. 261-272, 1995
Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 09504618/95 $lO.OO+O.OO
0950-0618(95)00020-8
Effects of three durability enhancing products on some physical properties of near surface concrete 6.Nolan,
F?A. M. Basheer and A. E. Long
Department of Civil Engineering, Belfast, BT7 lNN, UK Received
25 May 1994; revised
Queen‘s
6 February
University
of Belfast,
1995; accepted
Stranmillis
2 1 February
Road,
1995
This paper investigates the influence of three fundamentally different durability enhancing products, viz. microsilica, controlled permeability formwork and silane, on some of the physical properties of near surface concrete. Microsilica (silica fume) is a pozzolan, controlled permeability formwork (CPF) is used to provide a free draining surface to a concrete form, while silane is a surface treatment applied to hardened concrete to reduce the ingress of water. Comparisons are made between the products when used individually and used in conjunction with each other, with a view to assessing whether the use of combinations of products may be desirable to improve the durability of concrete in certain circumstances. The effect of these materials on various durability parameters, such as freeze-thaw deterioration, carbonation resistance and chloride ingress, is considered in terms of their effect on permeation properties and surface strength. The results indicated that a combination of silane and CPF produces concrete with very low air permeability and sorptivity values. The influence of microsilica was more pronounced in increasing the surface strength of concrete. Keywords: concrete durability; controlled permeability formwork; concrete durability enhancement
The durability of a structure is the property which shows whether or not the structure will remain useful for its full design life even though it may not be subjected to loads sufficient to destroy it. The long-term durability of reinforced concrete depends on the ability of the near surface concrete to protect the reinforcing steel from detrimental substances found in its environment. Once initiated, reinforcement corrosion can quickly propagate, impairing a structure’s utility and ultimately leading to collapse. With the considerable costs involved in refurbishment works the case for construction of durable reinforced concrete structures scarcely needs to be emphasized. Furthermore, it is a fundamental obligation of the construction industry as a whole, not only to provide a product which fulfils its functions adequately but that it also does so for at least a specified minimum period of time. Given a temperate climate and moderate exposure conditions durable concrete can be achieved by giving due consideration to the constituents, compaction, cover and curing, the ‘four C’s”. In more severe exposure conditions or in cases where proper curing may prove to be impractical, good concrete practice may need to be augmented by durability enhancing products. Silane has been used over the past 10 years to provide
Construction
near surface concrete with an outermost layer resistant to water absorption. Microsilica, ‘a .pozzolan, has also been used to improve concrete durability as it reduces bleedwater in the concrete. Bleedwater in normal concrete during compaction tends to move both upwards and also towards the formwork where it stays, causing the formation of a more porous and permeable near surface concrete. Microsilica concrete has less bleedwater than normal concrete and so the near surface concrete is less permeable and hence more durable’. Controlled permeability formwork (CPF), a relatively recent development, allows excess air and bleedwater from the setting concrete to drain out of the formwork and ensures a more dense near-surface concrete of reduced permeability3s4. Research’ has indicated that a concrete which is low in permeation properties lasts longer without exhibiting signs of distress and deterioration. Therefore, the permeation properties have been used principally for the comparison of the effectiveness of the three products. a
Product profiles As stated above, methods of enhancing concrete durability are diverse and the three products chosen for this investigation all use very different principles of operation.
and Building
Materials
1995 Volume
9 Number
5
267
Effects of three durability
enhancing
products:
E. Nolan et al.
Silune
100% monomeric isobutyl alkyl alkoxy silane was used in this investigation. The low viscosity colourless liquid is applied to hardened concrete to reduce and, if possible, prevent the ingress of water and soluble salts. The size of silane molecules is similar to that of water and so it penetrates into capillary cavities (50 A to 1000 A) in concrete, where sorption occurs. When applied, the silane adheres to the walls of the pores and reacts with the adsorbed water to form a hydrophobic layer. The depth of penetration is dependent on the pore structure of the near surface concrete and its moisture state. Since silane is a non-film forming application the air permeability of the near surface concrete is not significantly altered by its use. Silane treatments are not life long and need to be reapplied periodically. Silane is widely used on bridge structures by local authorities in Europe, USA and many other countries. Manufacturers of silane generally recommend a second application four hours after the first application.
theconcrete
and
water tendtowards the excess
ocr and
shutter
face
Formwork
’ Freshly placed concrete
I Excws water
concrete
a,r and drolns wt
of formwork
Excess mr and water reaches
Microsilicu
1s drolned
Microsilica, also known as condensed silica fume (CSF) consists of discrete microspheres of silicon dioxide of diameter 0.1 to 0.2 pm. Compared to the cement, microsilica has a much greater surface area and this provides a larger surface for reaction during the hydration process (Figure I). This, when added to a mix, aids the formation of a stronger and more durable concrete’. It fills the space between the cement and aggregate particles and acts as centres for the formation of a secondary calcium silicate hydrate crystal growth. The increased density of the mix reduces the bleed water when concrete is being placed and results in a less porous near surface concrete. The resulting near surface concrete is less permeable as less escaping water leaves fewer interconnecting pore systems of smaller size.
Figure 2
Controlled
permeability
Figure 1
268
Comparison
Construction
between
Microsilica Concrete
hydration
and Building
I
reactions
Materials
1995 Volume
and
away
(CPF)
While being widely used in Scandinavian countries, microsilica is normally used only in special circumstances in other parts of the world, mainly due to its cost. Levels of microsilica addition vary widely in different circumstances. As a percentage of the cement content the addition of 7.5% microsilica is commonly used in Iceland to reduce expansion due to ASR while 15-20% microsilica has been shown to be required in Canadian pavements3 to provide a durable concrete with siliceous limestone aggregates and a cement high in alkalis. Generally a 5% dosage is considered to be adequate to enhance various durability related properties of concrete. Controlled permeability
Conventional concrete
formwork-liner
the shutter
jbrmwork
The concept of CPF has long been accepted; however, the difficulties of producing a practical and cost effective solution have hindered progress. Work has been done in Japan on a system4 which requires drainholes in the formwork; however, a new formwork liner has arrived on the UK market’. This fabric when fixed to formwork allows excess air and water, which would normally form blow-holes on the concrete’s surface, to drain away (Figure 2). As the concrete is being vibrated the excess water (that which is in excess of the 0.3 w/c ratio required for hydration) tends to migrate upwards and out towards the formwork. The pore size of the surface of the textile adjacent to the concrete is approximately 0.07 mm and is suitable for the drainage of water and air while not allowing cement particles through. The free draining surface of the formwork means that the w/c ratio at the surface is reduced considerably, which, in turn, leads to a surface region with enhanced physical properties. 9 Number
5
Effects ‘Pull-off’ force applied and recorded at failure
of three durability
enhancing products: g. Nolan et al.
Table 1 Experimental variables Variables Water cement ratio Mixes Formwork type
t
50 mm d lometer
Levels
Treatment
Near surface
Figure 3
Non-CPF
concre
0.55 0.65 Normal 5% microsilica Garudaform filmfaced CPF plywood None 100% silane - two coats
faces
CPF faces
Limpet ‘pull-off test
/
Test methods Auto&m
uir permeubility
Half of the specimen treated with s~lane on,day 32
test
The permeability of the near surface concrete was measured with the Autoclam permeability system7. In this test a 50 mm diameter circular area of concrete surface is isolated by means of a bonded or clamped ring. The measurement head of the apparatus is fixed onto this ring and the pressure inside the ring is recorded and displayed on a controller unit. For the air permeability test a pressure of 500 mbar (50.66 kPa) is introduced inside the test ring. The pressure decay is monitored with time and an air permeability index is determined from the rate of air flow through the ‘covercrete’. An average of three tests is reported in each case. Auto&m
sorptivity
test
The Autoclam instrument is set up in a similar manner for a sorptivity test7 except that a piston which is a part of the measurement head is reset and the chamber is filled with water and pressurized to a nominal pressure of 20 mbar (2.03 kPa). This nominal pressure is maintained by movement of the piston during the test as water is absorbed into the concrete. The quantity of water entering the concrete is obtained by multiplying the travel of the piston by the internal area of the piston chamber. The pressure of the water in the chamber and the travel of the piston are displayed and recorded with time for the 15 minute duration of the test. This system has been shown to give comparable but more consistent and accurate results than ISAT’. The results are reported in terms of an index in units of M3/,lmin. Strength
,I
//
I_
Figure 4
300mm
_I
Specimen details
Druined rvuter measurement
During the casting of test specimens the water draining from the moulds through the formwork-liner material was collected and measured accurately to indicate the effectiveness of the liner in removing the bleed water. SEM
study
A scanning electron examine:
microscope
@EM)
was used to
1. used samples of CPF liner 2. concrete samples taken from the surface of each specimen. The used samples of CPF liner were examined on both faces of the CPF liner fabric - that facing the concrete and that facing the formwork. The concrete samples were cut from the specimens and examined so that the structure of the ‘covercrete’ could be examined.
Experimental variables
tests
The compressive strength of standard 100 mm cubes was taken as an estimate of the strength of the internal concrete (heartcrete). The Limpet ‘pull-off test’ was employed to give a direct measurement of the tensile strength of the near surface concrete. In this test a disc was fixed to the hardened surface of the concrete with an epoxy resin adhesive with a tensile strength greater than that of the concrete (Figure 3). The force required to ‘pull-off the disc along with the part of the concrete bonded to it was recorded and from this force and the area of the disc the tensile strength of the near surface concrete was calculated. Construction
Tuble 1 shows the variables used in this experimental programme. In order to incorporate these variables, test specimens as shown in Figure 4 were cast. The silane was applied at the age of 32 days.
Materials All mixes were designed using a combination and 12.5 mm maximum size basalt coarse along with a class M sand as defined by B.S. The 20 mm and 12.5 mm aggregates were equal amounts to make up the total coarse and Building
Materials
of 20 mm aggregate 882:1983. added in aggregate
1995 Volume 9 Number 5
269
Effects
of three durability
enhancing
products:
g. Nolan
et al.
content of the mix. Ordinary Portland cement was used as the primary binder in all mixes. The mix proportions were 1 cement: 1.65 sand:3 coarse aggregate. Microsilica was added as a percentage of the cement content and was mixed fully with an equal amount of water (not additional to the calculated water content) before its addition to the mix. It is important to note that microsilica was added as a quality enhancer (an addition to the cementitious materials in the mix) and not as a cement replacement.
5Yl
Specimen preparation The specimens (Figure 4) were compacted using a poker vibrator as it was found that CPF required this method of compaction to force water through it. Unlike site conditions, extreme care was taken to compact all the specimens in the same manner and for a controlled period of time. This procedure was adopted to ensure an accurate comparison between specimens. Overall it was intended to provide a plausible representation of a cast-in-situ member. The specimens were removed from the mould after 24 h and exposed to the following conditioning regime: Air storage at 20°C and a relative humidity (RH) of 55% ? 5% until day 21. From day 21 until day 27 the specimens were kept in a controlled environment of 40°C and RH of 20% t 2%. The latter conditioning was employed in order to reduce the moisture in the concrete’s near surface pores and capillaries. Moisture in the capillary pores near to the surface has the effect of reducing the perceived permeation properties of the near surface concrete. After a cooling period of 1 day, Autoclam testing was carried out at the age of 28 days. The application of silane in two coats at a rate of 0.3 1itres/m2 was carried out on day 32 and these treated surfaces were not tested until day 60, thereby giving the chemical reaction time to take place. Up until day 60, the specimens were kept in a stable laboratory environment of 20°C and 50% k 5% RH. The curing regime adopted for this investigation sought to provide a near surface concrete at day 28 which would have a capillary pore system with reduced
CPF
Figure 5
270
Drained
CPF+ MS
water measurement
Construction
CPF
CPF+ MS
results
and Building
Materials
1995 Volume
Figure 6
Tensile strength
of covercrete
Air permeability
indices
results
moisture content. The presence of large amounts of moisture in the capillary pores of a concrete reduces the sensitivity of air permeability and sorptivity readings significantly. It was also necessary to have a reasonably dry surface concrete in order to apply the silane surface treatment successfully. Results and discussion The drained water is presented in terms of litres passing through a square metre of CPF (Figure 5). The reduction in bleedwater being drained through the formwork liner out of the specimen mould, due to the addition of microsilica to the mix, is caused by a reduction in the quantity of bleedwater due to the inclusion of microsilica in the mix. (A quantity of water is used in the formation of the secondary crystalline structure associated with microsilica concrete.) The SEM study of the CPF after use showed that drainage pores in the fabric were blocked by microsilica when it was used in the mix, whereas when used with ordinary concrete the formwork-liner was relatively clear from any blockage. This may also have reduced the drainage capacity of the CPF when used with microsilica. The true significance of the clogging effect compared to the reduction in the bleedwater due to the inclusion of microsilica into the mix was not extracted from the results of the investigation. 9 Number 5
Effects of three durability enhancing products: f?. Nolan et al.
Figure 8
Sorptivity
indices - 0.55 w/c
Figure 9
Sorptivity
indices - 0.65 w/c
Another part of the SEM study concentrated upon the structure of the near surface concrete. This showed that the formwork-liner and microsilica systems both tended to produce a more compact near surface concrete which could be expected to provide extra protection for the steel in a reinforced concrete structure. The surface strength test results (Figure 6) show clearly the benefit of using both CPF and microsilica to increase the strength of the near surface concrete and as expected the lower water cement ratio mixes gave higher strengths. The formwork-liner was responsible for a greater increase in surface strength than microsilica. By combining microsilica and CPF a noteworthy strength increase was realized. Silane has been shown to have no apparent influence on the tensile strength of the ‘coverCrete”‘, and hence is not reported in Figure 6. Near surface concrete formed using CPF gave a substantially larger reduction in air permeability than that formed using microsilica (Figure 7). When the products were combined the resultant near surface concrete was found to have caused a similar reduction in air perConstruction
meability to that of CPF alone. The reduction in air permeability due to CPF was seen to be greater in specimens of 0.65 water cement ratio. As air permeability is related to carbonatior?, in cases where carbonation is expected to be the limiting factor, therefore, there appears to be no benefit in combining microsilica and CPF. Silane did not have any significant effect on the air permeability of the near surface concrete and hence is not reported in Figure 7. The water cement ratio was shown to effect the air permeability index of the near surface concrete on non CPF faces whereas the CPF faces had consistently lower, and insignificantly different, air permeability indices. All three products were expected to reduce the near surface concrete water absorption characteristic and as can be seen from Figures 8 and 9, they did so with differing degrees of effectiveness. An interesting observation from this data is the apparent failure of the microsilica to reduce sorptivity by any great amount especially in the higher water cement ratio specimen, contrary to expectations. The combination of microsilica and silane was also unsuccessful in achieving a further reduction in sorptivity. The reason for this may be that the smaller pore size associated with microsilica concrete may have restricted penetration of the silane and hence did not promote a significant reduction in sorptivity. This idea is supported by the fact that the addition of microsilica to the mix was seen to reduce significantly the air permeability index for the concrete, indicating a possible reduction in the capillary pore size. The microsilica and CPF also did not combine together to give a significant reduction in sorptivity over the use of CPF alone and this may be attributed to the ‘clogging effect’ and the reduction in bleedwater outlined earlier. CPF and silane gave the highest reduction in sorptivity witnessed in the experiment, however, the difference between this combination and silane alone was not significant. The effectiveness of both CPF and silane seems to be better at ‘the high water cement ratio. When all three systems were combined the resultant near surface concrete sorptivity was much the same as for silane alone and silane + CPF. This finding indicates that in cases where sorptivity action is likely to prove the critical transport mechanism for the aggressive substance (e.g. deicing salts present in meltwater in contact with concrete bridgeworks leading to chloride ingress) that the most cost effective treatment, to retard the onset of reinforcement corrosion, is silane alone.
Conclusions Based on the results reported in this paper, the following conclusions have been drawn: 1. Microsilica was seen to impair CPF by clogging the fabric and of bleedwater draining through 2. CPF was seen to be the most decreasing the air permeability concrete. and Building
Materials
1995 Volume
the efficiency of the reducing the amount the fabric. effective product at of the near surface
9 Number 5
271
Effects of three durability
enhancing
products:
E. Nolan et al.
Silane is the most effective of the products tested in reducing the near surface concrete sorptivity. However the use of CPF also resulted in a substantial reduction in the sorptivity of the near surface concrete. Silane + CPF combined produced near surface concrete with the lowest permeation characteristics in this experiment, with CPF being mostly responsible for the reduction in air permeability and silane being mainly responsible for the reduction in sorptivity. Cost effectiveness dictates that in all but extreme cases combining products may not prove worthwhile. Microsilica and CPF combined to give a significant increase in the tensile strength of the near surface concrete. The use of both microsilica and CPF individually resulted in a significant increase in the tensile strength of the near surface concrete. The products tested in this experimental programme performed equally well at both low and high water/cement ratios.
Acknowledgements
Construction
References 1 2 3
4 5 6 7
8
9
The CPF liner used in this experiment was supplied by DuPont (Nemours, Luxembourg). Microsilica for this work was donated by Larsen Assoc, Belfast. The experimental work was funded by the Department of Civil
272
Engineering, Queen’s University, Belfast. Mr A. Sha’at provided considerable help in the execution of the experimental programme. These contributions are gratefully acknowledged.
and Building
Materials
1995 Volume
IO
Somerville, G. On the trail of concrete durability. NAIL.Civ. Engr October 1985 Male, P. Properties of microsilica concrete. Concr. 1989. 2,3, (8) Moranville-Regourd, M. Durability of high performance concretes: Alkali - Aggregate reaction & carbonation. In High performance concrete: ,from material to structure E & F N Spon, London, 1992, pp. 225-233 Marossezeky, M. et al. Textile form method to improve concrete durability. Concr Int., 1993, 15. (1 I), pp. 3742 Basheer, M. Clam permeability tests for assessing the durability of concrete. PhD Thesis, Queen’s University ofllelfaast, May 1991 DuPont launches new liner for longer life concrete. Const. Maint Rep, 1992, 5, (2) Basheer P.A.M.. Long A.E. and Montgomery, F.R. The Autoclam for measuring the surface absorption and permeability of concrete on site. In Proc. CANMET/A.C I. Int. Con/ on dances in concrete technology, ed. V. M. Malhotra Athen\, Greece. 1992, pp. 21 l-221 Montgomery, F.R.. Basheer. P.A.M., Long, A.E. A comparison between the Autoclam permeability system and the initial surface absorption test. In Proc. 5th Int. Conf: on Structurul fhult.s oncl repair, 3, July 1993, pp. 71-77 Long. A.E. and Murray, A.M. The pull-off partially destructive test for concrete. In-situ/non-destructive testing of concrete. In A. C.I. SP-82. Detroit, 1984, pp. 327-350 Sha’at A’A, Long, A.E., Montgomery, F.R. and Basheer. P.A.M. The influence of controlled permeability formwork liner on the quality of the cover concrete. In A. C. I. SP-139, 1992. pp. 9lllO5
9 Number
5
Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 09504618/95 $lO.OO+O.OO
0950-0618(95)00020-8
Effects of three durability enhancing products on some physical properties of near surface concrete 6.Nolan,
F?A. M. Basheer and A. E. Long
Department of Civil Engineering, Belfast, BT7 lNN, UK Received
25 May 1994; revised
Queen‘s
6 February
University
of Belfast,
1995; accepted
Stranmillis
2 1 February
Road,
1995
This paper investigates the influence of three fundamentally different durability enhancing products, viz. microsilica, controlled permeability formwork and silane, on some of the physical properties of near surface concrete. Microsilica (silica fume) is a pozzolan, controlled permeability formwork (CPF) is used to provide a free draining surface to a concrete form, while silane is a surface treatment applied to hardened concrete to reduce the ingress of water. Comparisons are made between the products when used individually and used in conjunction with each other, with a view to assessing whether the use of combinations of products may be desirable to improve the durability of concrete in certain circumstances. The effect of these materials on various durability parameters, such as freeze-thaw deterioration, carbonation resistance and chloride ingress, is considered in terms of their effect on permeation properties and surface strength. The results indicated that a combination of silane and CPF produces concrete with very low air permeability and sorptivity values. The influence of microsilica was more pronounced in increasing the surface strength of concrete. Keywords: concrete durability; controlled permeability formwork; concrete durability enhancement
The durability of a structure is the property which shows whether or not the structure will remain useful for its full design life even though it may not be subjected to loads sufficient to destroy it. The long-term durability of reinforced concrete depends on the ability of the near surface concrete to protect the reinforcing steel from detrimental substances found in its environment. Once initiated, reinforcement corrosion can quickly propagate, impairing a structure’s utility and ultimately leading to collapse. With the considerable costs involved in refurbishment works the case for construction of durable reinforced concrete structures scarcely needs to be emphasized. Furthermore, it is a fundamental obligation of the construction industry as a whole, not only to provide a product which fulfils its functions adequately but that it also does so for at least a specified minimum period of time. Given a temperate climate and moderate exposure conditions durable concrete can be achieved by giving due consideration to the constituents, compaction, cover and curing, the ‘four C’s”. In more severe exposure conditions or in cases where proper curing may prove to be impractical, good concrete practice may need to be augmented by durability enhancing products. Silane has been used over the past 10 years to provide
Construction
near surface concrete with an outermost layer resistant to water absorption. Microsilica, ‘a .pozzolan, has also been used to improve concrete durability as it reduces bleedwater in the concrete. Bleedwater in normal concrete during compaction tends to move both upwards and also towards the formwork where it stays, causing the formation of a more porous and permeable near surface concrete. Microsilica concrete has less bleedwater than normal concrete and so the near surface concrete is less permeable and hence more durable’. Controlled permeability formwork (CPF), a relatively recent development, allows excess air and bleedwater from the setting concrete to drain out of the formwork and ensures a more dense near-surface concrete of reduced permeability3s4. Research’ has indicated that a concrete which is low in permeation properties lasts longer without exhibiting signs of distress and deterioration. Therefore, the permeation properties have been used principally for the comparison of the effectiveness of the three products. a
Product profiles As stated above, methods of enhancing concrete durability are diverse and the three products chosen for this investigation all use very different principles of operation.
and Building
Materials
1995 Volume
9 Number
5
267
Effects of three durability
enhancing
products:
E. Nolan et al.
Silune
100% monomeric isobutyl alkyl alkoxy silane was used in this investigation. The low viscosity colourless liquid is applied to hardened concrete to reduce and, if possible, prevent the ingress of water and soluble salts. The size of silane molecules is similar to that of water and so it penetrates into capillary cavities (50 A to 1000 A) in concrete, where sorption occurs. When applied, the silane adheres to the walls of the pores and reacts with the adsorbed water to form a hydrophobic layer. The depth of penetration is dependent on the pore structure of the near surface concrete and its moisture state. Since silane is a non-film forming application the air permeability of the near surface concrete is not significantly altered by its use. Silane treatments are not life long and need to be reapplied periodically. Silane is widely used on bridge structures by local authorities in Europe, USA and many other countries. Manufacturers of silane generally recommend a second application four hours after the first application.
theconcrete
and
water tendtowards the excess
ocr and
shutter
face
Formwork
’ Freshly placed concrete
I Excws water
concrete
a,r and drolns wt
of formwork
Excess mr and water reaches
Microsilicu
1s drolned
Microsilica, also known as condensed silica fume (CSF) consists of discrete microspheres of silicon dioxide of diameter 0.1 to 0.2 pm. Compared to the cement, microsilica has a much greater surface area and this provides a larger surface for reaction during the hydration process (Figure I). This, when added to a mix, aids the formation of a stronger and more durable concrete’. It fills the space between the cement and aggregate particles and acts as centres for the formation of a secondary calcium silicate hydrate crystal growth. The increased density of the mix reduces the bleed water when concrete is being placed and results in a less porous near surface concrete. The resulting near surface concrete is less permeable as less escaping water leaves fewer interconnecting pore systems of smaller size.
Figure 2
Controlled
permeability
Figure 1
268
Comparison
Construction
between
Microsilica Concrete
hydration
and Building
I
reactions
Materials
1995 Volume
and
away
(CPF)
While being widely used in Scandinavian countries, microsilica is normally used only in special circumstances in other parts of the world, mainly due to its cost. Levels of microsilica addition vary widely in different circumstances. As a percentage of the cement content the addition of 7.5% microsilica is commonly used in Iceland to reduce expansion due to ASR while 15-20% microsilica has been shown to be required in Canadian pavements3 to provide a durable concrete with siliceous limestone aggregates and a cement high in alkalis. Generally a 5% dosage is considered to be adequate to enhance various durability related properties of concrete. Controlled permeability
Conventional concrete
formwork-liner
the shutter
jbrmwork
The concept of CPF has long been accepted; however, the difficulties of producing a practical and cost effective solution have hindered progress. Work has been done in Japan on a system4 which requires drainholes in the formwork; however, a new formwork liner has arrived on the UK market’. This fabric when fixed to formwork allows excess air and water, which would normally form blow-holes on the concrete’s surface, to drain away (Figure 2). As the concrete is being vibrated the excess water (that which is in excess of the 0.3 w/c ratio required for hydration) tends to migrate upwards and out towards the formwork. The pore size of the surface of the textile adjacent to the concrete is approximately 0.07 mm and is suitable for the drainage of water and air while not allowing cement particles through. The free draining surface of the formwork means that the w/c ratio at the surface is reduced considerably, which, in turn, leads to a surface region with enhanced physical properties. 9 Number
5
Effects ‘Pull-off’ force applied and recorded at failure
of three durability
enhancing products: g. Nolan et al.
Table 1 Experimental variables Variables Water cement ratio Mixes Formwork type
t
50 mm d lometer
Levels
Treatment
Near surface
Figure 3
Non-CPF
concre
0.55 0.65 Normal 5% microsilica Garudaform filmfaced CPF plywood None 100% silane - two coats
faces
CPF faces
Limpet ‘pull-off test
/
Test methods Auto&m
uir permeubility
Half of the specimen treated with s~lane on,day 32
test
The permeability of the near surface concrete was measured with the Autoclam permeability system7. In this test a 50 mm diameter circular area of concrete surface is isolated by means of a bonded or clamped ring. The measurement head of the apparatus is fixed onto this ring and the pressure inside the ring is recorded and displayed on a controller unit. For the air permeability test a pressure of 500 mbar (50.66 kPa) is introduced inside the test ring. The pressure decay is monitored with time and an air permeability index is determined from the rate of air flow through the ‘covercrete’. An average of three tests is reported in each case. Auto&m
sorptivity
test
The Autoclam instrument is set up in a similar manner for a sorptivity test7 except that a piston which is a part of the measurement head is reset and the chamber is filled with water and pressurized to a nominal pressure of 20 mbar (2.03 kPa). This nominal pressure is maintained by movement of the piston during the test as water is absorbed into the concrete. The quantity of water entering the concrete is obtained by multiplying the travel of the piston by the internal area of the piston chamber. The pressure of the water in the chamber and the travel of the piston are displayed and recorded with time for the 15 minute duration of the test. This system has been shown to give comparable but more consistent and accurate results than ISAT’. The results are reported in terms of an index in units of M3/,lmin. Strength
,I
//
I_
Figure 4
300mm
_I
Specimen details
Druined rvuter measurement
During the casting of test specimens the water draining from the moulds through the formwork-liner material was collected and measured accurately to indicate the effectiveness of the liner in removing the bleed water. SEM
study
A scanning electron examine:
microscope
@EM)
was used to
1. used samples of CPF liner 2. concrete samples taken from the surface of each specimen. The used samples of CPF liner were examined on both faces of the CPF liner fabric - that facing the concrete and that facing the formwork. The concrete samples were cut from the specimens and examined so that the structure of the ‘covercrete’ could be examined.
Experimental variables
tests
The compressive strength of standard 100 mm cubes was taken as an estimate of the strength of the internal concrete (heartcrete). The Limpet ‘pull-off test’ was employed to give a direct measurement of the tensile strength of the near surface concrete. In this test a disc was fixed to the hardened surface of the concrete with an epoxy resin adhesive with a tensile strength greater than that of the concrete (Figure 3). The force required to ‘pull-off the disc along with the part of the concrete bonded to it was recorded and from this force and the area of the disc the tensile strength of the near surface concrete was calculated. Construction
Tuble 1 shows the variables used in this experimental programme. In order to incorporate these variables, test specimens as shown in Figure 4 were cast. The silane was applied at the age of 32 days.
Materials All mixes were designed using a combination and 12.5 mm maximum size basalt coarse along with a class M sand as defined by B.S. The 20 mm and 12.5 mm aggregates were equal amounts to make up the total coarse and Building
Materials
of 20 mm aggregate 882:1983. added in aggregate
1995 Volume 9 Number 5
269
Effects
of three durability
enhancing
products:
g. Nolan
et al.
content of the mix. Ordinary Portland cement was used as the primary binder in all mixes. The mix proportions were 1 cement: 1.65 sand:3 coarse aggregate. Microsilica was added as a percentage of the cement content and was mixed fully with an equal amount of water (not additional to the calculated water content) before its addition to the mix. It is important to note that microsilica was added as a quality enhancer (an addition to the cementitious materials in the mix) and not as a cement replacement.
5Yl
Specimen preparation The specimens (Figure 4) were compacted using a poker vibrator as it was found that CPF required this method of compaction to force water through it. Unlike site conditions, extreme care was taken to compact all the specimens in the same manner and for a controlled period of time. This procedure was adopted to ensure an accurate comparison between specimens. Overall it was intended to provide a plausible representation of a cast-in-situ member. The specimens were removed from the mould after 24 h and exposed to the following conditioning regime: Air storage at 20°C and a relative humidity (RH) of 55% ? 5% until day 21. From day 21 until day 27 the specimens were kept in a controlled environment of 40°C and RH of 20% t 2%. The latter conditioning was employed in order to reduce the moisture in the concrete’s near surface pores and capillaries. Moisture in the capillary pores near to the surface has the effect of reducing the perceived permeation properties of the near surface concrete. After a cooling period of 1 day, Autoclam testing was carried out at the age of 28 days. The application of silane in two coats at a rate of 0.3 1itres/m2 was carried out on day 32 and these treated surfaces were not tested until day 60, thereby giving the chemical reaction time to take place. Up until day 60, the specimens were kept in a stable laboratory environment of 20°C and 50% k 5% RH. The curing regime adopted for this investigation sought to provide a near surface concrete at day 28 which would have a capillary pore system with reduced
CPF
Figure 5
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Drained
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1995 Volume
Figure 6
Tensile strength
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Air permeability
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moisture content. The presence of large amounts of moisture in the capillary pores of a concrete reduces the sensitivity of air permeability and sorptivity readings significantly. It was also necessary to have a reasonably dry surface concrete in order to apply the silane surface treatment successfully. Results and discussion The drained water is presented in terms of litres passing through a square metre of CPF (Figure 5). The reduction in bleedwater being drained through the formwork liner out of the specimen mould, due to the addition of microsilica to the mix, is caused by a reduction in the quantity of bleedwater due to the inclusion of microsilica in the mix. (A quantity of water is used in the formation of the secondary crystalline structure associated with microsilica concrete.) The SEM study of the CPF after use showed that drainage pores in the fabric were blocked by microsilica when it was used in the mix, whereas when used with ordinary concrete the formwork-liner was relatively clear from any blockage. This may also have reduced the drainage capacity of the CPF when used with microsilica. The true significance of the clogging effect compared to the reduction in the bleedwater due to the inclusion of microsilica into the mix was not extracted from the results of the investigation. 9 Number 5
Effects of three durability enhancing products: f?. Nolan et al.
Figure 8
Sorptivity
indices - 0.55 w/c
Figure 9
Sorptivity
indices - 0.65 w/c
Another part of the SEM study concentrated upon the structure of the near surface concrete. This showed that the formwork-liner and microsilica systems both tended to produce a more compact near surface concrete which could be expected to provide extra protection for the steel in a reinforced concrete structure. The surface strength test results (Figure 6) show clearly the benefit of using both CPF and microsilica to increase the strength of the near surface concrete and as expected the lower water cement ratio mixes gave higher strengths. The formwork-liner was responsible for a greater increase in surface strength than microsilica. By combining microsilica and CPF a noteworthy strength increase was realized. Silane has been shown to have no apparent influence on the tensile strength of the ‘coverCrete”‘, and hence is not reported in Figure 6. Near surface concrete formed using CPF gave a substantially larger reduction in air permeability than that formed using microsilica (Figure 7). When the products were combined the resultant near surface concrete was found to have caused a similar reduction in air perConstruction
meability to that of CPF alone. The reduction in air permeability due to CPF was seen to be greater in specimens of 0.65 water cement ratio. As air permeability is related to carbonatior?, in cases where carbonation is expected to be the limiting factor, therefore, there appears to be no benefit in combining microsilica and CPF. Silane did not have any significant effect on the air permeability of the near surface concrete and hence is not reported in Figure 7. The water cement ratio was shown to effect the air permeability index of the near surface concrete on non CPF faces whereas the CPF faces had consistently lower, and insignificantly different, air permeability indices. All three products were expected to reduce the near surface concrete water absorption characteristic and as can be seen from Figures 8 and 9, they did so with differing degrees of effectiveness. An interesting observation from this data is the apparent failure of the microsilica to reduce sorptivity by any great amount especially in the higher water cement ratio specimen, contrary to expectations. The combination of microsilica and silane was also unsuccessful in achieving a further reduction in sorptivity. The reason for this may be that the smaller pore size associated with microsilica concrete may have restricted penetration of the silane and hence did not promote a significant reduction in sorptivity. This idea is supported by the fact that the addition of microsilica to the mix was seen to reduce significantly the air permeability index for the concrete, indicating a possible reduction in the capillary pore size. The microsilica and CPF also did not combine together to give a significant reduction in sorptivity over the use of CPF alone and this may be attributed to the ‘clogging effect’ and the reduction in bleedwater outlined earlier. CPF and silane gave the highest reduction in sorptivity witnessed in the experiment, however, the difference between this combination and silane alone was not significant. The effectiveness of both CPF and silane seems to be better at ‘the high water cement ratio. When all three systems were combined the resultant near surface concrete sorptivity was much the same as for silane alone and silane + CPF. This finding indicates that in cases where sorptivity action is likely to prove the critical transport mechanism for the aggressive substance (e.g. deicing salts present in meltwater in contact with concrete bridgeworks leading to chloride ingress) that the most cost effective treatment, to retard the onset of reinforcement corrosion, is silane alone.
Conclusions Based on the results reported in this paper, the following conclusions have been drawn: 1. Microsilica was seen to impair CPF by clogging the fabric and of bleedwater draining through 2. CPF was seen to be the most decreasing the air permeability concrete. and Building
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the efficiency of the reducing the amount the fabric. effective product at of the near surface
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Silane is the most effective of the products tested in reducing the near surface concrete sorptivity. However the use of CPF also resulted in a substantial reduction in the sorptivity of the near surface concrete. Silane + CPF combined produced near surface concrete with the lowest permeation characteristics in this experiment, with CPF being mostly responsible for the reduction in air permeability and silane being mainly responsible for the reduction in sorptivity. Cost effectiveness dictates that in all but extreme cases combining products may not prove worthwhile. Microsilica and CPF combined to give a significant increase in the tensile strength of the near surface concrete. The use of both microsilica and CPF individually resulted in a significant increase in the tensile strength of the near surface concrete. The products tested in this experimental programme performed equally well at both low and high water/cement ratios.
Acknowledgements
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References 1 2 3
4 5 6 7
8
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The CPF liner used in this experiment was supplied by DuPont (Nemours, Luxembourg). Microsilica for this work was donated by Larsen Assoc, Belfast. The experimental work was funded by the Department of Civil
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Engineering, Queen’s University, Belfast. Mr A. Sha’at provided considerable help in the execution of the experimental programme. These contributions are gratefully acknowledged.
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Somerville, G. On the trail of concrete durability. NAIL.Civ. Engr October 1985 Male, P. Properties of microsilica concrete. Concr. 1989. 2,3, (8) Moranville-Regourd, M. Durability of high performance concretes: Alkali - Aggregate reaction & carbonation. In High performance concrete: ,from material to structure E & F N Spon, London, 1992, pp. 225-233 Marossezeky, M. et al. Textile form method to improve concrete durability. Concr Int., 1993, 15. (1 I), pp. 3742 Basheer, M. Clam permeability tests for assessing the durability of concrete. PhD Thesis, Queen’s University ofllelfaast, May 1991 DuPont launches new liner for longer life concrete. Const. Maint Rep, 1992, 5, (2) Basheer P.A.M.. Long A.E. and Montgomery, F.R. The Autoclam for measuring the surface absorption and permeability of concrete on site. In Proc. CANMET/A.C I. Int. Con/ on dances in concrete technology, ed. V. M. Malhotra Athen\, Greece. 1992, pp. 21 l-221 Montgomery, F.R.. Basheer. P.A.M., Long, A.E. A comparison between the Autoclam permeability system and the initial surface absorption test. In Proc. 5th Int. Conf: on Structurul fhult.s oncl repair, 3, July 1993, pp. 71-77 Long. A.E. and Murray, A.M. The pull-off partially destructive test for concrete. In-situ/non-destructive testing of concrete. In A. C.I. SP-82. Detroit, 1984, pp. 327-350 Sha’at A’A, Long, A.E., Montgomery, F.R. and Basheer. P.A.M. The influence of controlled permeability formwork liner on the quality of the cover concrete. In A. C. I. SP-139, 1992. pp. 9lllO5
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