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Development of pfa use in precast concrete manhole units
CEMENT and CONCRETE RESEARCH. Vol. 22, pp. 35-46, 1992. Printed in the USA. 0008-8846D2 $5.00+.00. Copyright © 1991 Pergamon Press plc.
DEVELOPMENT OF PFA USE IN PRECAST CONCRETE MANHOLE UNITS R K Dhir and M R Jones Concrete Technology Unit Department of Chill Engineering, University of Dundee, Dundee, DD1 4HN, Scotland, UK (Communicated by C.D. Pomeroy) (Received June 19, 1991)
ABSTRACT A series of full-scale factory trials of precast concrete manhole units using pfa are discussed. The trials demonstrated that the use of pfa was successful in this process. However, the link between rheology of zero slump concrete and surface finish was unclear. Unit production cost was important and the advantage of pfa concrete is shown.
Introduction Precast concrete manhole units, due to their low cost and simplicity of installation, are the most widely used material for pipeline and sewer entry. Of particular importance for such units is therefore economy, finish and durability. The production of these units is normally closely specified by a national standard, eg. BS 5911 (1~. Consequently the scope for variations in concrete parameters such as cement content, water / cement ratio etc. is limited. Economies can therefore only be gained by the introduction of a less expensive cementitious material such as pulverized fuel-ash (pfa) to partially replace Portland cement. Other benefits may accrue such as increased resistance to sulphate attack (2.2) and reduced permeation properties (4~ Following laboratory investigations and full-scale factory trials into the use of pfa concrete for the manufacture of precast railway sleeper units ~5'8), it was decided to extend the transfer of this technology to precast manhole units. Two factories, agreed to collaborate with these feasibility studies, one in the North of England (Works A) and another in Scotland (Works B). All trials were accommodated within the normal production cycles. Surface finish was of particular importance as it was suggested that this was related to water absorption and permeability, although more realistically only the aesthetic aspects of the units were affected. Chamber rings made from OPC concrete tended to suffer from surface pits, apparently formed by the bridging of coarse aggregate, accompanied by a local deficiency of fine material. It was anticipated that the introduction of pfa could ameliorate this problem, as noted during the laboratory development work, as well as providing economic benefits. 35
36
R.K. Dhir and M.R. Jones
Vol. 22, No. 1
This paper describes the results of research and development work, undertaken over one year, to establish the utilisation of pfa in the production of manhole units.
Production of Manhole Chamber Rings At Works A, chamber rings were cast vertically using a "Prinzing" semi-automatic machine. Dry (zero slump) concrete was mixed in a fully automatic concrete batching plant and deposited into a fixed mould by a circular tray. Prior to this an annular base ring is placed at the bottom of the mould and the reinforcing steel and step irons introduced; the number of reinforcing rings being dependant upon the depth of the manhole unit. Sizes produced varied from 900 to 1350 mm dia. and 250 to 1000 mm deep. A hydraulically operated disc was then lowered onto the concrete, compacting it by a combination of compression and high frequency vibration applied throughout the height of the units by means of a "Tornado" vibrator. At the end of the cycle, (around 3 mins), the outside of the mould was raised and the "green" chamber ring removed to an initial storage area. The top lips of the units were then manually brushed to give a final finish, after which they were stored indoors for 24 hrs at a nominal 16-20 °C air temperature. The units were then removed to an outside storage area. Thus, there was a requirement for the unit to possess sufficient strength to stand up and be moved immediately after demoulding, without distortion. Moreover, after the initial period of covered storage, the units were expected to be sufficiently strong and durable to withstand extensive handling without the benefit of any active moist curing. The major difference in the production process at Works B was that the machinery was manually controlled. The mould was placed around a fixed core and concrete poured into the annulus which was then compacted by a combination of core vibration and direct compression of the top surface. The completed unit was then lifted and placed temporarily on a platform where the mould was removed and the unit hand finished, both inside and out. Step irons were installed at this stage using an air hammer. The units were then stored at 15-20 °C for 18-24 hrs, after which they were transferred to an outdoor storage yard. Again no moist curing was provided.
Works A An initial visit was made to the Works to establish the type of materials and the normal concrete mix proportions used. The pfa used throughout the trials, as well as the laboratory development work, complied with BS 3892 : Part 1 (7~.Details of the physical and chemical properties of the Portland cements and pfa used are given in Tables 1 and 2. The sand used was a grade C, and the coarse aggregate was in single sizes of 14mm and 6mm.AII aggregates complied with BS 882 (8),and the gradings are given in Table 3. As a consequence of the dryness of the normal mix, i.e. zero slump (w/c ratio of 0.36), it was not possible to proportion a pfa mix with conventional pfa mix design methods (6,9) and therefore it was decided to modify the existing normal concrete mix proportions partly based on previous experience 16)and partly by trial and error. The evolution of the trial mix proportions are given in Figure 1.
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PRECAST CONCRETE, FLY ASH, MANHOLE UNITS
37
TABLE 1 Properties of cements used at Works A and B WORKS A
WORKS B
PROPERTY OPC
SRPC
OPC
RHPC
21.1 5.0 3.0 64.3 2.4 2.6 0.9
20.9 3.5 4.6 65.1 2.3 2.3 1.1
21.2 5.3 3.3 64.4 2.3 2.5 1.2
20.6 4.6 3.1 64.5 2.3 2.7 1.0
54.5 18.1 7.7 9.1
67.9 7.7 1.4 13.8
53.2 20.1 8.3 9.8
60.2 12.9 8.1 8.9
360 3,13
400 3.12
392 3.13
446 3.14
145 200
95 140
95 145
85 130
Comp~ssive St~ngth (Nmm 2) 3 days 25 7 days 34 28 days 46
27 36 47
23 30 44
26 34 45
Chemical Composition (%) SiO2 AI203 Fe203 CaO MgO SO3 L.O.I.
Compound Composition (%) C3S C2S C3A C4AF Physical Properties Fineness(m2kg -1) Specific Gravity Setting Times (rains) Initial Final
The trials were carried out in two phases. Phase 1, was aimed at demonstrating that adequate strength and the specified maximum water absorption could be achieved with pfa concrete. Chamber rings of 1350mm dia. x 1000mm depth were cast using the first nine mixes, all of which behaved similarly to the normal concrete mix during casting and initial handling. Units from each mix were tested for proof load and water absorption; all complied with the requirements of BS 5911 <1~.The proof load test consisted of loading the generatrix of the units in a large, hydraulically operated compression machine. BS 5911 requires various proof loads to be achieved
38
R,K. Dhir and M.R. Jones
Vol. 22, No. 1
TABLE 2 Main properties of ashes used at both works. PROPERTY
WORKS A
WORKS B
45urn Sieve Residue (%)
7.5
8.5
L.O.I. (%)
2.2
2.5
Water Requirement* (%)
90
91
Pozzolanic Activity Index* (%)
99
88
* w.r.t. OPC depending on the size of the unit being tested. The test had the additional advantage of identifying any misalignment in the outer edge of the unit, which may cause point loading to occur. Such units tended to fail the proof load test even though the concrete was of equivalent strength to units passing the test. The water absorption test consisted of removing a 25 mm diameter core from the wall of a unit, drying the core at 100°C for 3 days and after cooling, measuring the uptake of water on immersion over a period of 30 minutes and 24 hours. In both cases only a random sample of the units are tested. The sampling rate is again specified in BS 5911 and typically consisted of testing one in ten units produced. The water absorption results are summarised in Table 4. TABLE 3 Aggregate gradings, Works A. BS SIEVE SIZE (mm)
MASS PASSING (%) Sand
20.00 14.00 10.00 6.30 5.00
100
2.36 1.18
88 76
0.60 0.30
47 14
0.15
3
100
6mm
14mm
-
100
-
90 24 3 1
100 63 32 1
Vol. 22, No. 1
PRECAST CONCRETE, FLY ASH, MANHOLE UNITS
39
1000
500
400
- 800
O~
'~-. =.
~ ~ O P C - ,lp....~_
. .o.,
-.4. -
- o~
.-e.- --e-
-e--
-e-
--
-
-.e-
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(Yl)
e----e
300 --
-
600 )-
>¢/) Z
~
O
F (Y2)
Z
g
o
-400
IZ 200 o Q. 0 G. X
WATER (Y1)
p~ o a. 0 efx
=E
,.O.- O. -O" . -(~ . . e . -o. ~.
100
o
,0- ~- "ok.
/
/
PFA (Y1) ~ -
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,O--
..O-
-200
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-.(D
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Y1
•
.
.
.
.
.
Y2 • 0
I 5
NORMAL MIX
I 10
I 15
I 20
MIX NUMBER
FIGURE 1. Evolution of mix proportions developed at Works A. Contrary to expectation, in one respect the overall surface finish obtained with the pfa mixes was inferior to that obtained with the normal concrete mix, with the exception of Mix A9. This was caused by a greater incidence of bridged aggregate pits, although the surface between the pits was often superior to that obtained with the normal concrete mix. The end spigot of the units, which were hand finished, were of particularly fine finish and in addition this was demonstrably easier to produce. Phase 2, aimed at improving the overall surface finish and culminated in Mix A16, which gave a surface finish equivalent to that of the normal concrete mix. A total of 93 units were cast, in sizes of 1050mm dia. x 750mm depth, 1050mm dia. x 500mm depth, and 1200mm dia. x 1000mm depth. Each mix was proof load and water absorption tested. All complied with BS specifications, except 3 units which gave 30 minute absorption results just above the permitted limit. However, the normal mix absorption results for those days were also high. The 24 hr results for both concretes complied with BS 5911 (1) The significant factor appears to be that Mix A16 had an identical particle grading to the normal concrete mix, but required a lower water content. This can be attributed to the lower water demand associated with the use of pfa Ilo) Having established a mix (A16) capable of producing manhole units with a surface finish
40
R.K. Dhir and M.R. Jones
Vol. 22, No. 1
TABLE 4 Water absorption test results, Works A. WATER ABSORPTION (%) SAMPLE REFERENCE 30 minutes
24 hours
Phase 1
Pfa concrete
Mean Range
2.3 1.9-2.9
4.3 3.8-4.5
Normal concrete
Mean Range
2.4 2.1-2.7
4.4 4.2-4.6
Pfa concrete
Mean Range
2.1 1.3-3.8
4.2 3.9-6.5
Normal concrete
Mean Range
2.2 2.1-3.6
4.8 4.7-5.6
Phase 2
equivalent to that of the normal concrete mix, the production of small depth units (<500mm) was undertaken. These were reported to be more variable in surface finish, an effect that can probably be attributed to the fact that the units were manufactured further from the centre of vibration of the "Tornado" machine. A total of 8 separate mixes were cast, producing a total of 37 units of both 750mm and 250mm depth (1200mm dia.). For the latter, SRPC was used in place of OPC, this being standard Works practice for this size ring. The surface finish was satisfactory but there was a slightly higher incidence of bridged aggregate pits. One unit from each mix was proof load tested, and two of the eight (one 750mm, one 250mm deep) failed the BS load criterion. This was attributed to point loading due to incomplete compaction and misalignment during handling in the green state, since earlier units produced from this mix had easily exceeded the proof load. Works B
As for Works A, an initial appraisal of the materials and normal mix proportions was made (Tables 1 and 2). Most units were made with OPC, but during the period of potentially cold weather (November-April), RHPC was utilised, with SRPC used to special order. The aggregates used were 14mm graded and a mixture of grades C and F sands (Table 5), all complying with BS 882. The trials were divided into three phases.
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PRECAST CONCRETE, FLY ASH, M A N H O L E UNITS
41
TABLE 5 Grading of aggregates, Works B. BS SIEVE SIZE (mm)
MASS PASSING (%) Sand F
Sand C
6mm
14mm
20.00
100
14.00
92
10.00
100
6.30
91
5.00 2.36 1.18
100
98
10
98
81
1
93
61
0.60
81
40
0.30 0.15
40
10
11
1
0.07
2
1
1
28
1
Phase 1, a total of 10 separate mixes were cast, producing a total 23 manhole units of 900mm dia. x 900mm depth. A summary of these mixes are given in Figure 2. Initially Mixes B1-B3 were tried and all three appeared similar to the normal mix in the mixer. However, on placing, units made from Mix B1 were dryer than normal, whilst those made from Mixes B2 and B3 were wet. The surface finish was satisfactory, but not as good as normal. No handling problems were encountered. Following on from this and developing from Mix B3, Mixes B4-B6 were designed. Units produced from these mixes handled without problems, but despite some improvement in surface finish, this was still not comparable to the normal concrete finish. The best surface appearance occurred on units produced from Mix B6, which had the highest grade C/F sand ratio. Based on this, Mixes B7 - B9 were designed with a gradually increasing C/F sand ratio and increased water and cement contents to compensate for the increased fines content. Units produced from these mixes showed an improvement in surface finish, but they were still not equivalent to the normal finish. One final mix was then designed, B10, in which 6mm aggregate was introduced as a partial replacement for the 14mm aggregate. Units produced from this mix had a comparable surface appearance to the normal concrete units. All units produced from pfa concrete were proof load tested and satisfied the BS requirements. The water absorption test was carried out on one sample from each mix, Table 6. Two of the samples did not comply with the 30 min BS requirement, but all
42
R.K. Dhir and M.R. Jones
Vol. 22, No. 1
500
1000
i|
SAND (Y2) f
1
-800
E ,J¢
P 300 >-
~
0~ Z
g
OPC (Y1)
-600 ~-, t'q >Z
6ram AGG (Y2)
_o
I-, IZ
I,IX
200.
-4O0 0
o na. X
0
z
x
n
100 -
.O~
"'O'"
,0 . . 0 . ,
, o - "e~
~O
"'e"B--
" ° ' " ~ - ~" - ' O - - O - - - , O . - - ~ PFA (Y1)
- 200
/ Y1 =
t
Y2= 0
NORMAL MIX
I
I
5
10
15
I
'0
2O
MIX NUMBER
FIGURE 2. Evolution of mix proportions developed at Works B.
passed the 24 hr requirement. No cause for this was identified but a similar numberof random failures also occurred with the normal mix. It is possible that this was caused by any one of a series of factors, eg a slightly wetter mix, exceptional drying conditions during early storage of the units or exceptionally low temperatures in the outdoor storage yard for the units. However, BS 5911 permits a number of random failures to occur which allows units from the same mix to be used. Only when failures reach a statistically unacceptable level is the production process modified. In the case of the failures noted in this study the number was statistically acceptable. Phase 2, aimed at producing further improvements to the surface appearance but without the use of 6mm aggregate as this was not used in general production. Additionally pfa concrete was used for the manufacture of two smaller sized units. Mixes B11 and B12 were designed with a replacement-addition, and Mixes B13 and B14 were designed to study the effect of direct cement replacement with pfa on surface appearance. For Mixes B12 and B14, 6mm aggregates were also used for comparison. All four mixes were used to produce 900mm dia. x 900mm depth units. The surface finish of units made from Mixes B11, B12 and B14 were not satisfactory. However, the units
Vol. 22, No. 1
PRECAST CONCRETE, FLY ASH, MANHOLE UNITS
43
TABLE 6 Water absorption test results, Works B. WATER ABSORPTION (%) SAMPLE REFERENCE 30 mins
24 hr
Phase 1 Pfa concrete
Mean Range
3.2 2.7-3.8
6.0 5.9-6.4
Normal concrete
Mean Range
2.2 2.0-2.4
5.6 5.0-6.0
Pfa concrete
Mean Range
3.3 2.9-3.4
5.8 5.5-6.1
Normal concrete
Mean Range
2.8 2.8-2.9
5.8 5.7-5.8
Pfa concrete
Mean Range
3.2 2.6-3.5
6.2 5.8-6.4
Normal concrete
Mean Range
3.1 3.0-3.1
6.4 6.3-6.4
Phase 2
Phase 3
made from Mix B13, had a good surface appearance comparable to that obtained with the normal concrete mix, without the use of 6mm aggregates. Mixes B13 and B15 were then used to produce 900mm dia. x 450mm units. Nine units were cast, all of which had a consistently good finish, equivalent to that of normal concrete units. Mix B15 was then used to produce 'ogee' pipes of 900mm dia. x 1000mm. However, the surface finish of these units was not satisfactory. Mix B16 was, therefore, introduced with an increased grade C/F sand ratio. Units produced using this mix had a significantly improved surface appearance. All pfa concrete units were proof load tested and satisfied the BS requirements. Water absorption tests were carried out on a total of 13 pfa concrete samples and complied with BS requirements (Table 6). Phase 3, further investigated the influence of the grade C/F sand ratio on surface appearance. Three further Mixes B17 - B19 were designed, based on Mix B16, with a progressively increasing the grade C/F sand ratio. Mixes B16 and B17 were each cast twice, producing 7 manhole units of 1200mm dia. x 750mm depth. Mixes B18 and B19 were also cast twice, producing 8 units of 1200ram dia. x 600mm depth. The surface finish
44
R.K. Dhir and M.R. Jones
Vol. 22, No. 1
of all these units was similar to that of the normal concrete units, indicating that further increasing the C/F sand ratio did not give a concomitant improvement in surface finish. All pfa concrete units were proof load tested and complied with the BS requirements. Water absorption tests were carried out on four samples of pfa concrete, representative of each mix used, and two samples of normal concrete. All complied with BS the requirements (Table 6). As with the trials at Works A, it must be considered significant that the optimum mix, Mix B16, in terms of surface finish had an identical particle size grading to the normal concrete mix.
Discussion A total of 37 different pfa concrete mixes were tried in a series of full scale factory production trials, producing a total of 212 manhole rings, of various sizes. The mixes were designed by altering the existing normal concrete mixes at each factory. Production of units which complied with the BS requirements for strength and water absorption, from these mixes was easily achieved. It is clear from this that the replacement of Portland cement with pfa can be successfully carried out even with a very low batch water content and no active moist curing. Although the critical immediate handling criterion is probably achieved by compaction alone, the fact that the water absorption and strength requirements are also achieved demonstrates the effect pfa has on Portland cement hydration as pozzolanic activity at this time will be very limited. The major defect occurring in the surfaces of units was the occurrence of pits. Although, in general the pfa mixes gave a much better surface finish between such defects, no improvement could be made to reducing their incidence. At both Works the optimum pfa concrete mix in terms of surface finish had an identical overall particle size grading to the normal concrete mix. However, the amount of material <2.36mm was different for each Works. This would tend to indicate that the differences in production processes are affecting the particle size distribution necessary to obtain adequate surface quality, as shown in Table 7. The frequency of vibration was higher at Works A than at Works B, as was the compaction pressure. This suggests that the surface finish is dependant upon a combination of vibration frequency, compaction pressure and particle size distribution. A partial corroboration of this was the observation that the surface finish of shorter units was more variable, which can probably be attributed to such units being asymmetric to the vertical centre of vibration. Based on 1989 material prices and that haulage distances were low with both factories, the use of pfa resulted in a reduction in cost of about 24% per cubic metre of concrete at Works A and around 12% at Works B. This reduction is based mainly on the reduction in total cement costs by the partial replacement of Portland cement with pfa. There are environment considerations in the use of pfa, particularly in high volume production centred around a single outlet close to a pfa source. These figures do not, however, include any increased costs associated with, for example, storage requirements and handling of an additional mix constituent.
Vol. 22, No. 1
PRECAST CONCRETE, FLY ASH, MANHOLE UNITS
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TABLE 7 Overall grading of optimum mixes. MASS PASSING (%) BS SIEVE SIZE (mm) 20.00 14.00 10.00 5.00 2.36 1.18 0.60 0.30
Works A
Works B
Normal
Pfa
Normal
Pfa
97.5 80.6 60.4 49.5 44.4 33.5 20.8
97.5 81.0 60.5 49.5 44.4 33.5 20.8
97.1 74.0 63.7 58.9 52.7 44.7 27.8
97.0 74.7 64.8 59.7 53.0 44.6 27.5
Despite the initial difficulties, the trials demonstrated that pfa concrete can be used successfully in the production of manhole chambers manufactured under precast factory conditions without the additional benefit of accelerated curing. Further research and development, however, is needed to achieve an understanding of the relationship of. the rheology of zero slump concrete compacted under high pressure and frequency vibration and surface finish, as some aspects of the behaviour of this material, particularly when modified by the use of pfa, remain unresolved.
Conclusions . A total of six full-scale factory production trials have been carried out to assess whether pfa could be used in zero-slump concrete. Having successfully completed a total of 37 separate mixes it could be concluded that pfa had at least equal performance to conventional OPC concrete. . There was no active curing applied to the manhole units and this did not prejudice the performance of the pfa concrete units when compared to the normal concrete units. A minor number of random failures occurred with the standard water absorption tests with both the OPC and pfa concrete mixes due to exceptional deviations from normal factory practice. . Zero-slump concrete was found to be extremely difficult to work with and prone to surface defects in the form of pit formed from bridging coarse aggregates and a local deficiency of fines. Although the general finish of the pfa units was excellent, the occurrence of these pits could not be reduced given the factors that could be controlled at the works.
46
R.K. Dhir and M.R. Jones
Vol. 22, No. 1
. Even though the surface finish problem could not be resolved fully, it appeared that this was mainly affected by the frequency of compaction of the proprietary production machines.
Acknowledgement The authors wish to acknowledge the assistance of Mr Ionnis Koutsokos with the experimental work.
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
BRITISH STANDARDS INSTITUTION. BS 5911, London (1989) HARRISON, T.A. and SPOONER, D.C. Interim Technical Note 10, Cement and Concrete Association (1986) BUILDING RESEARCH ESTABLISHMENT. Digest No. 250 (1981) R K DHIR et al. Concrete 20, 12 (1986) R K DHIR et al. Concrete 19, 6 (1985) R K DHIR et al. Concrete 20, 1 (1986) BRITISH STANDARDS INSTITUTION. BS 3892, London (1982) BRITISH STANDARDS INSTITUTION. BS 882, London (1983) J G L MUNDAY. ACI Special Publication, SP-79, 1 (1983). D RAVlNA and P K MEHTA Cem Conc Res 16, 6 (1986)
DEVELOPMENT OF PFA USE IN PRECAST CONCRETE MANHOLE UNITS R K Dhir and M R Jones Concrete Technology Unit Department of Chill Engineering, University of Dundee, Dundee, DD1 4HN, Scotland, UK (Communicated by C.D. Pomeroy) (Received June 19, 1991)
ABSTRACT A series of full-scale factory trials of precast concrete manhole units using pfa are discussed. The trials demonstrated that the use of pfa was successful in this process. However, the link between rheology of zero slump concrete and surface finish was unclear. Unit production cost was important and the advantage of pfa concrete is shown.
Introduction Precast concrete manhole units, due to their low cost and simplicity of installation, are the most widely used material for pipeline and sewer entry. Of particular importance for such units is therefore economy, finish and durability. The production of these units is normally closely specified by a national standard, eg. BS 5911 (1~. Consequently the scope for variations in concrete parameters such as cement content, water / cement ratio etc. is limited. Economies can therefore only be gained by the introduction of a less expensive cementitious material such as pulverized fuel-ash (pfa) to partially replace Portland cement. Other benefits may accrue such as increased resistance to sulphate attack (2.2) and reduced permeation properties (4~ Following laboratory investigations and full-scale factory trials into the use of pfa concrete for the manufacture of precast railway sleeper units ~5'8), it was decided to extend the transfer of this technology to precast manhole units. Two factories, agreed to collaborate with these feasibility studies, one in the North of England (Works A) and another in Scotland (Works B). All trials were accommodated within the normal production cycles. Surface finish was of particular importance as it was suggested that this was related to water absorption and permeability, although more realistically only the aesthetic aspects of the units were affected. Chamber rings made from OPC concrete tended to suffer from surface pits, apparently formed by the bridging of coarse aggregate, accompanied by a local deficiency of fine material. It was anticipated that the introduction of pfa could ameliorate this problem, as noted during the laboratory development work, as well as providing economic benefits. 35
36
R.K. Dhir and M.R. Jones
Vol. 22, No. 1
This paper describes the results of research and development work, undertaken over one year, to establish the utilisation of pfa in the production of manhole units.
Production of Manhole Chamber Rings At Works A, chamber rings were cast vertically using a "Prinzing" semi-automatic machine. Dry (zero slump) concrete was mixed in a fully automatic concrete batching plant and deposited into a fixed mould by a circular tray. Prior to this an annular base ring is placed at the bottom of the mould and the reinforcing steel and step irons introduced; the number of reinforcing rings being dependant upon the depth of the manhole unit. Sizes produced varied from 900 to 1350 mm dia. and 250 to 1000 mm deep. A hydraulically operated disc was then lowered onto the concrete, compacting it by a combination of compression and high frequency vibration applied throughout the height of the units by means of a "Tornado" vibrator. At the end of the cycle, (around 3 mins), the outside of the mould was raised and the "green" chamber ring removed to an initial storage area. The top lips of the units were then manually brushed to give a final finish, after which they were stored indoors for 24 hrs at a nominal 16-20 °C air temperature. The units were then removed to an outside storage area. Thus, there was a requirement for the unit to possess sufficient strength to stand up and be moved immediately after demoulding, without distortion. Moreover, after the initial period of covered storage, the units were expected to be sufficiently strong and durable to withstand extensive handling without the benefit of any active moist curing. The major difference in the production process at Works B was that the machinery was manually controlled. The mould was placed around a fixed core and concrete poured into the annulus which was then compacted by a combination of core vibration and direct compression of the top surface. The completed unit was then lifted and placed temporarily on a platform where the mould was removed and the unit hand finished, both inside and out. Step irons were installed at this stage using an air hammer. The units were then stored at 15-20 °C for 18-24 hrs, after which they were transferred to an outdoor storage yard. Again no moist curing was provided.
Works A An initial visit was made to the Works to establish the type of materials and the normal concrete mix proportions used. The pfa used throughout the trials, as well as the laboratory development work, complied with BS 3892 : Part 1 (7~.Details of the physical and chemical properties of the Portland cements and pfa used are given in Tables 1 and 2. The sand used was a grade C, and the coarse aggregate was in single sizes of 14mm and 6mm.AII aggregates complied with BS 882 (8),and the gradings are given in Table 3. As a consequence of the dryness of the normal mix, i.e. zero slump (w/c ratio of 0.36), it was not possible to proportion a pfa mix with conventional pfa mix design methods (6,9) and therefore it was decided to modify the existing normal concrete mix proportions partly based on previous experience 16)and partly by trial and error. The evolution of the trial mix proportions are given in Figure 1.
Vol. 22, No. 1
PRECAST CONCRETE, FLY ASH, MANHOLE UNITS
37
TABLE 1 Properties of cements used at Works A and B WORKS A
WORKS B
PROPERTY OPC
SRPC
OPC
RHPC
21.1 5.0 3.0 64.3 2.4 2.6 0.9
20.9 3.5 4.6 65.1 2.3 2.3 1.1
21.2 5.3 3.3 64.4 2.3 2.5 1.2
20.6 4.6 3.1 64.5 2.3 2.7 1.0
54.5 18.1 7.7 9.1
67.9 7.7 1.4 13.8
53.2 20.1 8.3 9.8
60.2 12.9 8.1 8.9
360 3,13
400 3.12
392 3.13
446 3.14
145 200
95 140
95 145
85 130
Comp~ssive St~ngth (Nmm 2) 3 days 25 7 days 34 28 days 46
27 36 47
23 30 44
26 34 45
Chemical Composition (%) SiO2 AI203 Fe203 CaO MgO SO3 L.O.I.
Compound Composition (%) C3S C2S C3A C4AF Physical Properties Fineness(m2kg -1) Specific Gravity Setting Times (rains) Initial Final
The trials were carried out in two phases. Phase 1, was aimed at demonstrating that adequate strength and the specified maximum water absorption could be achieved with pfa concrete. Chamber rings of 1350mm dia. x 1000mm depth were cast using the first nine mixes, all of which behaved similarly to the normal concrete mix during casting and initial handling. Units from each mix were tested for proof load and water absorption; all complied with the requirements of BS 5911 <1~.The proof load test consisted of loading the generatrix of the units in a large, hydraulically operated compression machine. BS 5911 requires various proof loads to be achieved
38
R,K. Dhir and M.R. Jones
Vol. 22, No. 1
TABLE 2 Main properties of ashes used at both works. PROPERTY
WORKS A
WORKS B
45urn Sieve Residue (%)
7.5
8.5
L.O.I. (%)
2.2
2.5
Water Requirement* (%)
90
91
Pozzolanic Activity Index* (%)
99
88
* w.r.t. OPC depending on the size of the unit being tested. The test had the additional advantage of identifying any misalignment in the outer edge of the unit, which may cause point loading to occur. Such units tended to fail the proof load test even though the concrete was of equivalent strength to units passing the test. The water absorption test consisted of removing a 25 mm diameter core from the wall of a unit, drying the core at 100°C for 3 days and after cooling, measuring the uptake of water on immersion over a period of 30 minutes and 24 hours. In both cases only a random sample of the units are tested. The sampling rate is again specified in BS 5911 and typically consisted of testing one in ten units produced. The water absorption results are summarised in Table 4. TABLE 3 Aggregate gradings, Works A. BS SIEVE SIZE (mm)
MASS PASSING (%) Sand
20.00 14.00 10.00 6.30 5.00
100
2.36 1.18
88 76
0.60 0.30
47 14
0.15
3
100
6mm
14mm
-
100
-
90 24 3 1
100 63 32 1
Vol. 22, No. 1
PRECAST CONCRETE, FLY ASH, MANHOLE UNITS
39
1000
500
400
- 800
O~
'~-. =.
~ ~ O P C - ,lp....~_
. .o.,
-.4. -
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-
600 )-
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MIX NUMBER
FIGURE 1. Evolution of mix proportions developed at Works A. Contrary to expectation, in one respect the overall surface finish obtained with the pfa mixes was inferior to that obtained with the normal concrete mix, with the exception of Mix A9. This was caused by a greater incidence of bridged aggregate pits, although the surface between the pits was often superior to that obtained with the normal concrete mix. The end spigot of the units, which were hand finished, were of particularly fine finish and in addition this was demonstrably easier to produce. Phase 2, aimed at improving the overall surface finish and culminated in Mix A16, which gave a surface finish equivalent to that of the normal concrete mix. A total of 93 units were cast, in sizes of 1050mm dia. x 750mm depth, 1050mm dia. x 500mm depth, and 1200mm dia. x 1000mm depth. Each mix was proof load and water absorption tested. All complied with BS specifications, except 3 units which gave 30 minute absorption results just above the permitted limit. However, the normal mix absorption results for those days were also high. The 24 hr results for both concretes complied with BS 5911 (1) The significant factor appears to be that Mix A16 had an identical particle grading to the normal concrete mix, but required a lower water content. This can be attributed to the lower water demand associated with the use of pfa Ilo) Having established a mix (A16) capable of producing manhole units with a surface finish
40
R.K. Dhir and M.R. Jones
Vol. 22, No. 1
TABLE 4 Water absorption test results, Works A. WATER ABSORPTION (%) SAMPLE REFERENCE 30 minutes
24 hours
Phase 1
Pfa concrete
Mean Range
2.3 1.9-2.9
4.3 3.8-4.5
Normal concrete
Mean Range
2.4 2.1-2.7
4.4 4.2-4.6
Pfa concrete
Mean Range
2.1 1.3-3.8
4.2 3.9-6.5
Normal concrete
Mean Range
2.2 2.1-3.6
4.8 4.7-5.6
Phase 2
equivalent to that of the normal concrete mix, the production of small depth units (<500mm) was undertaken. These were reported to be more variable in surface finish, an effect that can probably be attributed to the fact that the units were manufactured further from the centre of vibration of the "Tornado" machine. A total of 8 separate mixes were cast, producing a total of 37 units of both 750mm and 250mm depth (1200mm dia.). For the latter, SRPC was used in place of OPC, this being standard Works practice for this size ring. The surface finish was satisfactory but there was a slightly higher incidence of bridged aggregate pits. One unit from each mix was proof load tested, and two of the eight (one 750mm, one 250mm deep) failed the BS load criterion. This was attributed to point loading due to incomplete compaction and misalignment during handling in the green state, since earlier units produced from this mix had easily exceeded the proof load. Works B
As for Works A, an initial appraisal of the materials and normal mix proportions was made (Tables 1 and 2). Most units were made with OPC, but during the period of potentially cold weather (November-April), RHPC was utilised, with SRPC used to special order. The aggregates used were 14mm graded and a mixture of grades C and F sands (Table 5), all complying with BS 882. The trials were divided into three phases.
Vol. 22, No. l
PRECAST CONCRETE, FLY ASH, M A N H O L E UNITS
41
TABLE 5 Grading of aggregates, Works B. BS SIEVE SIZE (mm)
MASS PASSING (%) Sand F
Sand C
6mm
14mm
20.00
100
14.00
92
10.00
100
6.30
91
5.00 2.36 1.18
100
98
10
98
81
1
93
61
0.60
81
40
0.30 0.15
40
10
11
1
0.07
2
1
1
28
1
Phase 1, a total of 10 separate mixes were cast, producing a total 23 manhole units of 900mm dia. x 900mm depth. A summary of these mixes are given in Figure 2. Initially Mixes B1-B3 were tried and all three appeared similar to the normal mix in the mixer. However, on placing, units made from Mix B1 were dryer than normal, whilst those made from Mixes B2 and B3 were wet. The surface finish was satisfactory, but not as good as normal. No handling problems were encountered. Following on from this and developing from Mix B3, Mixes B4-B6 were designed. Units produced from these mixes handled without problems, but despite some improvement in surface finish, this was still not comparable to the normal concrete finish. The best surface appearance occurred on units produced from Mix B6, which had the highest grade C/F sand ratio. Based on this, Mixes B7 - B9 were designed with a gradually increasing C/F sand ratio and increased water and cement contents to compensate for the increased fines content. Units produced from these mixes showed an improvement in surface finish, but they were still not equivalent to the normal finish. One final mix was then designed, B10, in which 6mm aggregate was introduced as a partial replacement for the 14mm aggregate. Units produced from this mix had a comparable surface appearance to the normal concrete units. All units produced from pfa concrete were proof load tested and satisfied the BS requirements. The water absorption test was carried out on one sample from each mix, Table 6. Two of the samples did not comply with the 30 min BS requirement, but all
42
R.K. Dhir and M.R. Jones
Vol. 22, No. 1
500
1000
i|
SAND (Y2) f
1
-800
E ,J¢
P 300 >-
~
0~ Z
g
OPC (Y1)
-600 ~-, t'q >Z
6ram AGG (Y2)
_o
I-, IZ
I,IX
200.
-4O0 0
o na. X
0
z
x
n
100 -
.O~
"'O'"
,0 . . 0 . ,
, o - "e~
~O
"'e"B--
" ° ' " ~ - ~" - ' O - - O - - - , O . - - ~ PFA (Y1)
- 200
/ Y1 =
t
Y2= 0
NORMAL MIX
I
I
5
10
15
I
'0
2O
MIX NUMBER
FIGURE 2. Evolution of mix proportions developed at Works B.
passed the 24 hr requirement. No cause for this was identified but a similar numberof random failures also occurred with the normal mix. It is possible that this was caused by any one of a series of factors, eg a slightly wetter mix, exceptional drying conditions during early storage of the units or exceptionally low temperatures in the outdoor storage yard for the units. However, BS 5911 permits a number of random failures to occur which allows units from the same mix to be used. Only when failures reach a statistically unacceptable level is the production process modified. In the case of the failures noted in this study the number was statistically acceptable. Phase 2, aimed at producing further improvements to the surface appearance but without the use of 6mm aggregate as this was not used in general production. Additionally pfa concrete was used for the manufacture of two smaller sized units. Mixes B11 and B12 were designed with a replacement-addition, and Mixes B13 and B14 were designed to study the effect of direct cement replacement with pfa on surface appearance. For Mixes B12 and B14, 6mm aggregates were also used for comparison. All four mixes were used to produce 900mm dia. x 900mm depth units. The surface finish of units made from Mixes B11, B12 and B14 were not satisfactory. However, the units
Vol. 22, No. 1
PRECAST CONCRETE, FLY ASH, MANHOLE UNITS
43
TABLE 6 Water absorption test results, Works B. WATER ABSORPTION (%) SAMPLE REFERENCE 30 mins
24 hr
Phase 1 Pfa concrete
Mean Range
3.2 2.7-3.8
6.0 5.9-6.4
Normal concrete
Mean Range
2.2 2.0-2.4
5.6 5.0-6.0
Pfa concrete
Mean Range
3.3 2.9-3.4
5.8 5.5-6.1
Normal concrete
Mean Range
2.8 2.8-2.9
5.8 5.7-5.8
Pfa concrete
Mean Range
3.2 2.6-3.5
6.2 5.8-6.4
Normal concrete
Mean Range
3.1 3.0-3.1
6.4 6.3-6.4
Phase 2
Phase 3
made from Mix B13, had a good surface appearance comparable to that obtained with the normal concrete mix, without the use of 6mm aggregates. Mixes B13 and B15 were then used to produce 900mm dia. x 450mm units. Nine units were cast, all of which had a consistently good finish, equivalent to that of normal concrete units. Mix B15 was then used to produce 'ogee' pipes of 900mm dia. x 1000mm. However, the surface finish of these units was not satisfactory. Mix B16 was, therefore, introduced with an increased grade C/F sand ratio. Units produced using this mix had a significantly improved surface appearance. All pfa concrete units were proof load tested and satisfied the BS requirements. Water absorption tests were carried out on a total of 13 pfa concrete samples and complied with BS requirements (Table 6). Phase 3, further investigated the influence of the grade C/F sand ratio on surface appearance. Three further Mixes B17 - B19 were designed, based on Mix B16, with a progressively increasing the grade C/F sand ratio. Mixes B16 and B17 were each cast twice, producing 7 manhole units of 1200mm dia. x 750mm depth. Mixes B18 and B19 were also cast twice, producing 8 units of 1200ram dia. x 600mm depth. The surface finish
44
R.K. Dhir and M.R. Jones
Vol. 22, No. 1
of all these units was similar to that of the normal concrete units, indicating that further increasing the C/F sand ratio did not give a concomitant improvement in surface finish. All pfa concrete units were proof load tested and complied with the BS requirements. Water absorption tests were carried out on four samples of pfa concrete, representative of each mix used, and two samples of normal concrete. All complied with BS the requirements (Table 6). As with the trials at Works A, it must be considered significant that the optimum mix, Mix B16, in terms of surface finish had an identical particle size grading to the normal concrete mix.
Discussion A total of 37 different pfa concrete mixes were tried in a series of full scale factory production trials, producing a total of 212 manhole rings, of various sizes. The mixes were designed by altering the existing normal concrete mixes at each factory. Production of units which complied with the BS requirements for strength and water absorption, from these mixes was easily achieved. It is clear from this that the replacement of Portland cement with pfa can be successfully carried out even with a very low batch water content and no active moist curing. Although the critical immediate handling criterion is probably achieved by compaction alone, the fact that the water absorption and strength requirements are also achieved demonstrates the effect pfa has on Portland cement hydration as pozzolanic activity at this time will be very limited. The major defect occurring in the surfaces of units was the occurrence of pits. Although, in general the pfa mixes gave a much better surface finish between such defects, no improvement could be made to reducing their incidence. At both Works the optimum pfa concrete mix in terms of surface finish had an identical overall particle size grading to the normal concrete mix. However, the amount of material <2.36mm was different for each Works. This would tend to indicate that the differences in production processes are affecting the particle size distribution necessary to obtain adequate surface quality, as shown in Table 7. The frequency of vibration was higher at Works A than at Works B, as was the compaction pressure. This suggests that the surface finish is dependant upon a combination of vibration frequency, compaction pressure and particle size distribution. A partial corroboration of this was the observation that the surface finish of shorter units was more variable, which can probably be attributed to such units being asymmetric to the vertical centre of vibration. Based on 1989 material prices and that haulage distances were low with both factories, the use of pfa resulted in a reduction in cost of about 24% per cubic metre of concrete at Works A and around 12% at Works B. This reduction is based mainly on the reduction in total cement costs by the partial replacement of Portland cement with pfa. There are environment considerations in the use of pfa, particularly in high volume production centred around a single outlet close to a pfa source. These figures do not, however, include any increased costs associated with, for example, storage requirements and handling of an additional mix constituent.
Vol. 22, No. 1
PRECAST CONCRETE, FLY ASH, MANHOLE UNITS
45
TABLE 7 Overall grading of optimum mixes. MASS PASSING (%) BS SIEVE SIZE (mm) 20.00 14.00 10.00 5.00 2.36 1.18 0.60 0.30
Works A
Works B
Normal
Pfa
Normal
Pfa
97.5 80.6 60.4 49.5 44.4 33.5 20.8
97.5 81.0 60.5 49.5 44.4 33.5 20.8
97.1 74.0 63.7 58.9 52.7 44.7 27.8
97.0 74.7 64.8 59.7 53.0 44.6 27.5
Despite the initial difficulties, the trials demonstrated that pfa concrete can be used successfully in the production of manhole chambers manufactured under precast factory conditions without the additional benefit of accelerated curing. Further research and development, however, is needed to achieve an understanding of the relationship of. the rheology of zero slump concrete compacted under high pressure and frequency vibration and surface finish, as some aspects of the behaviour of this material, particularly when modified by the use of pfa, remain unresolved.
Conclusions . A total of six full-scale factory production trials have been carried out to assess whether pfa could be used in zero-slump concrete. Having successfully completed a total of 37 separate mixes it could be concluded that pfa had at least equal performance to conventional OPC concrete. . There was no active curing applied to the manhole units and this did not prejudice the performance of the pfa concrete units when compared to the normal concrete units. A minor number of random failures occurred with the standard water absorption tests with both the OPC and pfa concrete mixes due to exceptional deviations from normal factory practice. . Zero-slump concrete was found to be extremely difficult to work with and prone to surface defects in the form of pit formed from bridging coarse aggregates and a local deficiency of fines. Although the general finish of the pfa units was excellent, the occurrence of these pits could not be reduced given the factors that could be controlled at the works.
46
R.K. Dhir and M.R. Jones
Vol. 22, No. 1
. Even though the surface finish problem could not be resolved fully, it appeared that this was mainly affected by the frequency of compaction of the proprietary production machines.
Acknowledgement The authors wish to acknowledge the assistance of Mr Ionnis Koutsokos with the experimental work.
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
BRITISH STANDARDS INSTITUTION. BS 5911, London (1989) HARRISON, T.A. and SPOONER, D.C. Interim Technical Note 10, Cement and Concrete Association (1986) BUILDING RESEARCH ESTABLISHMENT. Digest No. 250 (1981) R K DHIR et al. Concrete 20, 12 (1986) R K DHIR et al. Concrete 19, 6 (1985) R K DHIR et al. Concrete 20, 1 (1986) BRITISH STANDARDS INSTITUTION. BS 3892, London (1982) BRITISH STANDARDS INSTITUTION. BS 882, London (1983) J G L MUNDAY. ACI Special Publication, SP-79, 1 (1983). D RAVlNA and P K MEHTA Cem Conc Res 16, 6 (1986)