The Effects of Self Compacting Concrete on Formwork

Transcription

1 Session: # The Effects of Self Compacting Concrete on Formwork Presented by: Vaughan Burgess Form-Scaff

2 OVERVIEW Formwork Concrete pressure (results from tests) Concrete pressure considerations Formwork incidents Formwork removal Formwork on support work Pro s and cons Conclusions

3 WHAT IS FORMWORK? Formwork is used to support and form (mould) the shape of fresh concrete It remains in position until the fresh concrete becomes stiff enough to maintain its own shape

4 Formwork is usually made from steel, timber, fibre glass, plastic or a combination of these materials

5 The formwork may be supported on support work

6 The facing of the formwork can be lined to produce various textured or patterned concrete finishes.

7 The patterned finish may be glued or screwed to the formwork facing.

8 Examples of various textured or patterned concrete finishes. Reduced pressure

9 The facing of the formwork can be lined to produce various textured or patterned concrete finishes.

10 It can be made to any reasonable shape to accommodate the aesthetic and practical needs of the architects and engineers. FORMWORK - SPECIALS

11 CONCRETE PRESSURE (results from tests) (Khayat, Bonen, Shah and Taylor) compiled a summary of some tests/experiments. Correctly understanding and interpreting the results agreements and disputes. Listed, are some of the many statements/findings from the numerous SCC tests that have been conducted over a period of many years :

12 Formwork test results (overview) : The maximum pressure of SCC depends on the speed of casting and the rate of the continuous pressure decrease in the SCC already cast. The placing method very strongly influences the rising formwork pressure. SCC pumped from the base can locally surpass hydrostatic pressure.

13 Formwork tests results (overview) : Formwork should be designed to withstand the full hydrostatic head of fluid concrete. ACI 347R-7 document stipulates, when working with SCC, the presumed lateral pressure should be equal to hydrostatic pressure. The actual pressure of SCC varies from full hydrostatic pressure to 60% of full hydrostatic pressure.

14 Formwork tests results (overview) : Initial low shear stress of the plastic SCC results in high pressure. Concrete mixtures containing fly ash develop higher lateral pressure. An increase in slump results in greater lateral pressure.

15 Formwork tests results (overview) : Irrespective of the composition and workability of the mixture, lateral pressure increases as the casting rate increases. Thixotropy (materials gel at rest) has been identified as a main factor that affects the initial lateral pressure and its rate of decline. High degree of thixotropy = higher rate of pressure drop There is no consensus on the rate of decline of lateral pressure.

16 Formwork tests results (overview) : Change in the w/c has a significant effect on SCC. Test using a SCC with a slump flow of 550mm and a w/c of 0.46 results in lower thixotropy and greater initial pressure than a w/c of 0.4. An increase of lateral pressure may result from the use of more cement. Increasing the w/c or/and superplasticiser content increases the lateral pressure.

17 Formwork tests results (overview) : If the content of the fines is increased, the ability of the course aggregate to carry loads decreases and the lateral pressure increases. A lower age SCC and higher flowability (slump flow) generates higher pressures. The higher the initial temperature of the concrete/ambient temperature the lower the lateral pressure and the faster the rate of hydration etc etc.

18 CONCRETE PRESSURE CONSIDERATIONS Temperature : Concrete Ambient (Roby) Concrete : lower temp = higher pressures Form face Steel vs timber

19 Form face steel vs timber : FORMWORK SURFACE MEAN PRESSURE (kpa) Populus nigra (wet) 22.8 Populus nigra 20.9 Pinus silvestris (wet) 23.7 Pinus silvestris 19.9 Reduced pressure if : Timber Not wet Lined Rough Plywood (wet) 24.6 Plywood 21.5 Steel 26.2 (Arslan)

20 Method of placing : - pumped T&B, bucket tremie (impact) Concrete bucket Wall formwork

21 Method of placing : - pumped T&B, bucket tremie (impact) Wall formwork

22 Method of placing : - pumped T&B, bucket tremie (impact) B Wall formwork A

23 Formwork shape

24 Formwork shape

25 Form size, concrete thickness and rate of placement : The effect of concrete pressure on formwork using SCC relative to the thickness of concrete, rate of placement and the presence of reinforcement has been tested. (Khayat, Assaad and Mesbah) Tests (slump flow = 650mm) : Two columns were tested using pressure sensors : 200mm Ø x 2100 high (12.6 mins & 5.0 mins) 920mm Ø x 3600 high (21.6 mins and 8.6 mins).

26 Form size, concrete thickness and rate of placement : Results : A varying rate of rise had limited influence due to the short casting period. The 200mm Ø column exerted slightly less pressure immediately after placing. This is possibly due to the arching effect.

27 Reinforcement : The presence of reinforcing bars may slightly reduce the concrete pressure on formwork since the reinforcement can partially support the concrete weight. Reduction of pressure due to thickness & rebar = negligible.

28 Compaction : Concrete compaction method & duration Pressure greatest during compaction..needle Ø, frequency, mix. The increase of the diameter of the head & the radius of action increases the workability. The vibration reduces the friction between the particles in the concrete mix and allows the concrete to flow.

29 Concrete mix - variable proportions of : Aggregate type, size, texture, shape, source Sand Water - w/c Cement - type Extenders - fly ash, silica fume, slag fillers Admixtures - retarders, accelerators, air entrainers, plasticizers, VMA (types) and, Lapsed time between batching and placing Strict control measures in place and implemented = reliability of the mix.

30 Rate of placing The pressure developed on the form face is a result of wet concrete being placed into the formwork. The faster the rate of rise (speed at which the concrete fills up the formwork) the higher the concrete pressure on the formwork face The stiffening of previously placed concrete reduces this pressure.

31 The pressure envelope for normal concrete according to CIRIA = 25kN/m 2. Wet concrete, relative to formwork design, has a density of approximately 24kN/m 3.

32 The pressure envelope for normal concrete according to CIRIA = 25kN/m 2. Wet concrete, relative to formwork design, has a density of approximately 24kN/m 3.

33 Rate of placing : Generally full hydrostatic pressure would be used for formwork design if the form is less than 1m in height. The pressure acts equally on opposite formwork faces and at 90 to the face. 1m h / hr NC 24kN/m 2 24kN/m 2 PRESSURE DIAGRAMS

34 Rate of placing : Maintaining 1mh/h limits the pressure of the form face. 2m 1m h / hr 1m NC NC 24kN/m 2 24kN/m 2 48kN/m 2 1m h/hr 24kN/m 2 1m

35 Rate of placing : 1m h/hr 2m 2m at 1m h/hr 1m h / hr 1m 1m h / hr NC SCC NC SCC 24kN/m 2 24kN/m 2 80% of Hydro? 48kN/m 2 24kN/m 2 ~ _ 48kN/m 2

36 FORMWORK INCIDENTS The site workers must be kept safe from incidents such as burst formwork. A burst form can lead to fatalities or serious injuries to workers.

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41 PRO S & CONS There are many advantages of using SCC in preference to NC.

42 SCC vs NC (formwork) SCC Pro s Vibrating equipment / labour is not required Free flowing - does not need compaction Confinement of space and congested rebar - the cost of formwork can be reduced

43 SCC vs NC (formwork) SCC Pro s Vibrating equipment / labour is not required Free flowing - does not need compaction Confinement of space and congested rebar - the cost of formwork can be reduced

44 SCC vs NC (formwork) SCC Pro s Vibrating equipment / labour is not required Free flowing - does not need compaction Confinement of space and congested rebar - the cost of formwork can be reduced Removes over and under vibration concerns Reduces heaping possibilities Improved off shutter finish on walls SCC Cons Pouring rate needs to be strictly controlled Concrete mix/admixtures must be maintained Slower formwork cycle time? Pressures could be as high as hydrostatic - increases the required strength of wall formwork and therefore the cost of formwork Slides, open formwork and sloping upper surfaces

45 Open formwork Depending on the variables / additives in the SCC, the flow of concrete varies. NC

46 Open formwork Depending on the variables / additives in the SCC, the flow of concrete varies. NC SCC

47 Sloping upper surfaces require a relatively stiff mix. NC

48 Sloping upper surfaces should be screeded. NC

49 SCC flows such that the upper surface remains level. SCC

50 When casting to a fall, the SCC should be contained in a form. SCC

51 FORMWORK SUSPENDED IN POSITION

52 CONCLUSIONS All the variable factors such as temperature, rate of placing, varying design mix (no specific composition), performance of the admixtures etc. makes it difficult to calculate and interpret the data and therefore a safe method of estimating the expected concrete pressure on the form face needs to be adopted. P = wr t - t h 2 2t 0 where t h = form height/r t 0 = t 400 (Gardner, Keller, Quattrociocchi & Charitou slump flow)

53 Conclusions P max = (1.0m Rt E )w > 30 kpa where R = rate of placement per hour t E = conc. setting time (1.13mm Vicat for 2,5mm) w = conc. mass (kn/m 3 ) (DIN setting time test)

54 Conclusions P max = wh{ h +0.6R 0.6T + 10D min 0.021PVƮ 0rest@15min@T=22C } ƒ MSA ƒ wp (static shear strength) where w = conc. mass (kn/m 3 ) h = height of placement R = rate of placement T = concrete temp. D = min. form dim. ƒ MSA = max agg size factor ƒ wp = factor for delay in lifts (Khayat & Omran on site shear strength)

55 Conclusions DIN : Based on concrete consistency class and concrete setting end : Concrete pressure (kn/m 2 ) F6 F5 Pour rate (m/h) F4 F3 F2 F1 (meva) Hydrostatic load height (m)

56 Conclusions A major problem is that these factors are unknown at tender/design stage lab/site test communication. Although the designer is expected to produce the most cost effective formwork solution, the designer is strongly advised to err on the side of caution when using SCC, even though the formwork cost could exceed 60% of the contract cost.

57 Conclusions It must remain the responsibility of the contractor to inform the temporary works designer of the : concrete mix (NC, General blend or SCC), expected temperatures on the construction site for the region and period, method of placing of the concrete and inform the experienced concreting team of the rate of placing, placing time limit/programme, concrete mix, compaction method if required etc.

58 Conclusions Designers + Suppliers + Teams + Conditions = SCC FORMWORK - SPECIALS

59 Conclusions It is important to resist the temptation, to cast the concrete as quickly as possible, as the only apparent limitation is the rate at which the concrete is delivered to site. The site workers must be kept safe from incidents such as burst formwork. A burst form can lead to fatalities or serious injuries.