SCC APPLICATION ON THE BRIDGE COLUMNS OF NORTH ROUND/CV350 CROSSROAD IN VALENCIA (SPAIN)

Pedro Serna and Jonhson Rigueira

ICITECH – Instituto de Ciencia y Tecnología del Hormigón, Universidad Politécnica de Valencia, Spain

Abstract

This paper shows an application of SCC in Valencia (Spain), which consisted in building six columns of twelve metres in height at the north round / CV350 Crossroad Bridge. To avoid concreting joints and to minimise the construction time, constructors wanted to cast each column in one day. This fact, in addition to the high density of reinforcement, made impossible the concrete vibration by traditional methods.

Two important aspects were also considered: temperatures were higher than 25ºC with peaks up to 35ºC, and pressure limits on the formwork were never to be surpassed. The proposed solution was a 550–600 mm slump-flow SCC, with a grain size distribution similar to traditional concrete.

This paper describes the SCC features, the concrete mix design, prior tests, production, delivery and quality control.

1. INTRODUCTION SCC was proposed for the construction of the six columns of the bridge at the north round /

CV350 crossroad at Valencia. Due to the short time available to begin the construction and the expected rates of construction (one column per week), the use of SCC was considered the best solution. In addition, only one formwork was available. Therefore, concrete had to be placed continuously to have the column finished in one day. The high level of reinforcement did not allow a good access neither to vibrate the concrete nor to verify its consolidation. Moreover, the heat generated due to the high concrete volume and weather conditions (July at Valencia) had to be considered. Concrete pressures on the formwork were also analysed.

2. GENERAL SPECIFICATIONS The columns had 3.91m2 of cross section and 12.3m in height, fig 1. The spacing between

the vertical reinforcement bars (32 mm diameter) was only 70 mm, and this spacing was even more reduced where reinforcement overlapped (Fig 2). The horizontal reinforcement blocked the workers access to vibrate the concrete and verify the concrete placement using traditional methods. The concrete strength required was 30 MPa.

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5th International RILEM Symposium on Self-Compacting Concrete3-5 September 2007, Ghent, Belgium

Fig 1. Column cross section Fig 2. Reinforced bars

3. MIX DESIGN SCC dosage is characterised by a high cement and powder content, small aggregate size

and high sand/gravel ratio. These characteristics improve the passing ability and placement [1-3]. However, the concrete properties as hydration heat, shrinkage and delayed strains can be affected.

Due to the short time available to start the construction, it was decided to act on a traditional 30 MPa concrete dosage with slump higher than 18 cm. From this reference dosage it was intended to prepare a SCC dosage using a similar aggregate size distribution avoiding important increases in terms of paste. Table 1 shows both, reference and proposed dosages.

A slump-flow diameter between 550 and 600 was proposed. The pumping energy and concrete self-weight were considered additional sources of consolidation energy. In order to avoid segregation risks and reach a satisfactory surface quality, it was decided to slightly increase the powder amount by changing the dry sand / washed sand ratio. A 6/12mm gravel and a strong superplasticizer were also included in the SCC dosage proposed.

Table 1. Reference and proposed SCC dosages

Dosages kg/m3 Material

Reference Proposed SCC

Cement CEM /II B-V 42,5 R 370 370

Total water (w/c = 0,48) 177 177

Dry sand 0/4 (powder – 12%) 140 300

Washed sand 0/4 (powder – 3%) 850 690

Gravel 6/12mm --- 270

Gravel 12/20mm 870 600

Total powder < 125μm 436 508

w/powder (ratio in volume) 1.2 1,0

Polifuncional Pozzolith 607 N 3,6 3,6

Superplasticizer 2,5 (Rheobuild 1000) Variable (Glenium C355)

0,75 0,15 1,45 0,15 0,75

0,15

1

,20

0,15

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5th International RILEM Symposium on Self-Compacting Concrete3-5 September 2007, Ghent, Belgium

In order to check the concrete behaviour in fresh state, previous trials were made, in situ, simulating the column conditions. These trials allowed the fitting of admixture amounts, reaching expected slump-flow diameter and a satisfactory cohesive aspect. This dosage flowed easily through the reinforced bars until 7 m from the pouring point. Only few honey-combs in the layer between different batches poured one hour later, were observed

Specimen cast for control without vibration presented a deficient surface and different aspect from the element. To verify the concrete quality in the element, cores were taken out from unfavourable zones. These cores showed no inner discontinuity in concrete elements.

4. ASPECTS RELATED TO THE HYDRATION HEAT Daily average temperatures of 25ºC with peaks of 35ºC were expected. In order to avoid

dangerous temperatures inside the concrete, it was decided to use a type II-cement and a water reducer admixture with features to delay the hardening of concrete.

Concrete temperature was controlled with thermal sensors placed inside the columns. During the cast, concrete temperature varied between 33ºC and 40ºC and reached temperatures from 70 to 74ºC between 14 and 48 hours after the end of the placement.

5. CONCRETE PRESSURE Studies on self-compacting concrete usually state that pressures on the formwork are

slightly higher than in traditional concretes [4-9]. However, the SCC dosage proposed with a maximum diameter of 20mm and a slump-flow diameter less than 600mm puts it at the limit between traditional and self-compacting concretes. In addition, steel bars, mainly placed in the borders with a distance between bars of 70mm, act restraining the pressure on the formwork.

In order to reduce the pressure on the formwork a cast with 1.5 metres (a truck of 6m3) per hour was decided. In this way it was possible to take advantage of the concrete thixotropy. The criteria proposed by ACI 347/78, DIN 28218 and STBTP were used to evaluate the concrete pressure considering as unfavourable condition a placement speed of 1.5 m/h, 25ºC temperature, 280mm slump and the use of a concrete retarder (Table 2). A good safety level was obtained on the evaluation, as the formwork supplier guaranteed admissible pressures of 60kN/m2.

Average transportation time of 60 minutes from the concrete plant was considered. To guarantee the safety, control and protection measures on job site were introduced

including formwork strengthening and concrete setting process control by formwork drilling.

6. RECEPTION, ACCEPTANCE CRITERIA AND CASTING A checklist containing the following entries: hour of depart from plant, arrival at job site,

pouring start and finish, test results and any other incidence, was filled for each truck. Trucks should arrive 15 min before cast in order to check the concrete. A sample of about 40 litres was taken after a first free discharge of 50 litres. Using this sample, slump-flow test was carried out and “d” value was measured. Concrete would be automatically accepted if 470 < d < 600 and no segregation was detected (Fig. 3-5). Otherwise, a new dose of superplasticizer or viscosity agent could be added to the concrete.

If a new admixture dose was required, an additional 10 min mixing and a new slump-flow test was carried out. A second admixture dose could also be accepted but never a third one.

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5th International RILEM Symposium on Self-Compacting Concrete3-5 September 2007, Ghent, Belgium

After concrete acceptance, the pumping started. The pump hose was positioned about 20 cm under the concrete previously poured (Fig. 6-9) Table 2. Estimated pressures over the formwork

Standard ACI 347 / 78 DIN 28218 STBTP

Maximum pressure 3,5 t / m2 4,2 t / m2 5,7 t / m2

Fig 3. d = 450 Fig 4. 470 < d < 600 Fig 5. d = 620

Fig. 6. Column overview

Fig. 7. First batches

Fig. 8. Last batch

Fig. 9. Column final aspect

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5th International RILEM Symposium on Self-Compacting Concrete3-5 September 2007, Ghent, Belgium

Each truck discharge should take no less than 20 min. A second sample was taken in the middle of the discharge. Using this sample a new slump-flow test was made. Five cylindrical specimens compacted by hitting 25 times after dropping the concrete were cast.

In order to guarantee the quality of the surface and the reinforcement cover, 4 surface vibrators were placed at the poured concrete level. They work during 20 seconds in the middle of each discharge. Vibrators were moved up after each 6m3. Before finishing the cast, another 20 second-vibration was applied on the column top.

7. QUALITY CONTROL New admixture doses were required for many trucks during the cast of the first column and

part of the second. In general, the test made at the middle of the discharge showed a more stable behaviour. At the second column the aggregates constancy properties and sand humidity quality control were improved. After these changes, the number of new admixture dose was strongly reduced.

The pumping system generated a concrete flow over the column section that covered the reinforcement bars and kept a surface almost horizontal during the cast.

After taking off the formwork, columns showed surfaces with a very good quality. Each column was considered as a lot at which a resistance control was analysed from four

of the trucks. In order to complete the control, 3 cores of 500 mm in length by 75 mm of diameter were taken. From each one, 2 specimens of 150 mm in length were taken from the superficial and the bottom part of the sample. The first core corresponded to a concrete with a very dry consistence (d = 450 mm) and with no reference resistance. The second core corresponded to a concrete with a good strength, and the third from a concrete with a low strength. The results from the control on the specimens and cores are shown in Table 3.

It can be noticed that the ranges obtained are small and always under the values that correspond to a good premix concrete plant.

The values obtained from the cores and the ones from the corresponding batches are in very good agreement. Even the core from column 2 which had a 440 mm flow shows a good finished surface and an adequate strength.

Table 3. Compression strength

Compression strength at 28 days

Column nº

1 2 3 4 5 6

Minimum 33.7 31.8 35.2 30.6 28.1 30.5

Average 37.2 36.5 36.7 36.1 32.5 30.7

Range 0.17 0.18 0.08 0.26 0.17 0.01

Estimated 32.7 30.8 34.1 29.7 27.3 29.6

Core - 37.7 34.9 - 28.8 -

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8. CONCLUSIONS SCC has been used to build six bridge columns. Dosages criteria intended to reach self-

compactability keeping a granular structure similar to traditional concretes. A slow cast program has been defined in order to reduce formwork pressures. A stable, 540 ± 60 mm slump-flow diameter concrete was used with very good flowability

and allow an easy pumping. Hose pump position 20 cm under the concrete mass has guaranteed a horizontal concrete surface during the cast.

Strength quality control was made by producing specimens dropping the concrete and hitting 25 times. This system had shown to be similar to pumping.

These control level allowed to keep good and constant concrete properties. The experience showed to be possible, reaching the conditions to fit a SCC dosage adapted

to the needs of the structure to be built. Moreover, it was possible to place the SCC at the job site, with the qualities required.

ACKNOWLEDGEMENTS Authors would like to thank Ing. Francisco Zamarvide from “Conselleria d’Infraestructures

i Transport, Generalitat Valenciana” (regional administration), technical consulting “GPS Obra Civil y Medio Ambiente”, constructor industry “UTE – Ronda Norte – Dragados / Torrescámara”, concrete supplier “HOLCIM” and quality control laboratory “GIA S.L.”, for their support and efforts during this work to guarantee an optimum result.

REFERENCES [1] Slarendahl, Å. and Petersson, Ö., ‘Self-Compacting Concrete: State of the Art’ report of RILEM

Technical Committee 174 SCC. France 2000 (RILEM Publications S.A.R.L. 2000. ISBN: 2-912143-23-3)

[2] Rigueira, J., ‘Caracterización Reológica y Mecánica de Hormigones Autocompactables. Influencia de la Composición.’ Diploma de Estudios Avanzados (Universidad Politécnica de Valencia, España, September, 2004 (in Spanish).

[3] EFNARC. Specification and Guidelines for Self-Compacting Concrete. http://www.efnarc.org, May 2005

[4] Vanhove, Y., Djelal, C., and Magnin, A., ‘Prediction of the lateral pressure exerted by self-compacting concrete on formwork’, Magazine of Concrete Research, 56 (1) (2004) 55-62.

[5] Brameshuber, W. and Uebachs, S., ‘Investigations on the formework pressure using self-compacting concrete’, Proceedings of 3rd International Symposium on Self-Compacting Concrete, Reykjavik, August, 2003 (RILEM publications, 2003) 281-287.

[6] Billberg, P. ‘Form pressure generated by self-compacting concrete’, Proceedings of 3rd International Symposium on Self-Compacting Concrete, Reykjavik, August, 2003 (RILEM publications, 2003) 271-280.

[7] Leemann, A. and Hoffmann, C., ’Pressure of self-compacting concrete on the formwork’, Proceedings of 3rd International Symposium on Self-Compacting Concrete, Reykjavik, August, 2003 (RILEM publications, 2003) 288-295.

[8] ACI 347/78 ‘Pressure on Formwork’, ACI manual of concrete practice. Part 2, 2000. [9] DIN 18218, ‘Frishbeton auf lotrechte pressure of concrete on vertical formwork’, Berlin, 1980.

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