Slope Layer Method for the construction of RCC Dams

USE OF THE SLOPED LAYER METHOD FOR BONDED JOINTS ON TANNUR RCC DAM, JORDAN

ANCOLD 2003 Conference on Dams


Tony Qiu1 and Brian A. Forbes2

ABSTRACT The RCC design review and construction phase services of the 60m high Tannur Dam in Jordan was carried out by GHD, Australia.

The 220,000m3 of RCC was placed during February-December 2000; the change to the sloped layer method was made once the dam reached 15m height. It produced a 50% increase in placing rate and a considerable saving in costs.

The use of the method is the first known use outside of China, where it was developed during the construction of the 130m high Jiangya Dam in 1997-8. The sloping of the 300mm thick layers of RCC across the dam from bank to bank at grades between 5-8% ensures subsequent layers of RCC can be placed within the initial set time of the lower layer and hence the RCC is monolithic across the lift joint.

This paper briefly describes the project in Jordan and then gives specific details of the use of the sloped layer method. Typical results from the quality control testing during placement and subsequent coring and testing of the lift joints are also provided. The benefits of its use in adverse climatic conditions, such as extreme heat or rainfall and the ways it can be integrated with forming the upstream-downstream slope are also discussed.

The sloped layer method is a significant advancement, particularly for large structures, where lift joint cohesion, tensile resistance and RCC placing rates are vitally important.

1 Introduction Tannur Dam is a 60m high, 250m long Roller Compacted Concrete (RCC) gravity dam with a reservoir capacity of 17 million cubic metres. It is located in Jordan on the Wadi Al Hasa about 30km east of the Dead Sea and 50km south of the town of Karak. The dam provides irrigation water to the Southern Ghors Irrigation Scheme and industrial water for the local area.

The owner of the dam is the Jordan Valley Authority of the Ministry of Water and Irrigation, Hashimite Kingdom of Jordan. The construction contract was awarded in February 1999. RCC placing began in January 2000 and was completed in December 2000. Construction of the dam was completed in 2001. It is the first RCC dam in Middle East.

Design review and modifications, construction phase services and contract administration for the Tannur Dam were carried out by a joint venture of Mott MacDonald, UK and GHD, Australia in association with the Consulting Engineering Centre (CEC) of Jordan. GHD were responsible for RCC aspects of the project.

The main construction contractor was the joint venture of BEC Freres and Camperon Bermard of France, with Al-Jafar, the locally based sub-contractor.

2 Project Description The location, typical section and layout of the dam are shown in Figures 1, 2 and 3 respectively. The dam is located in a highly seismic area near the Dead Sea Fault and founded on interbedded marls and limestone of only moderate strength.

Total RCC volume 220,000m3 was placed during January and December 2000. RCC mixes began with a 125kg/m3 OPC and 75kg/m3 pozzolan blend; adjustments were made as construction proceeded and representative results became available. The last 50 percent of the RCC to be

1 Senior Dams Engineer, GHD 2 Manager, Dams Engineering, GHD



Figure 1 – Locality plan

Figure 3 – Tannur Dam layout

placed used a 120/50kg/m3 blend. Daratard P2 retarder was used in the RCC mix, and a dose rate of 1 l/m3 was used throughout the construction based on earlier laboratory RCC trial studies. The

retarder extended the workable life and reduced the free water content of the mix for the same workability, with the lower water/cement (W/C) ratio giving a higher cementitious efficiency.

Figure 2 – Typical Dam cross-section



RCC was transported from the mixing plant to the dam wall via a system of Rotec conveyors and discharged with a "swinger". The conveyors were relocated progressively as the dam wall rose. The RCC was transported from the swinger to the dumping point by three-25 tonne Volvo A25C 6WD dump trucks. They were fitted with smooth profile tyres which helped to minimise damage to fresh RCC surfaces. Chutes were welded to the sides of the tray to prevent spillage as the swinger was moved from the full truck to the empty one alongside.

Grout enriched RCC (GE-RCC) was used in all the upstream face, downstream steps including the 180m wide stepped spillway section, gallery walls, waterstop encasement and the interface between the RCC and limestone rock abutments.

3 Development and Introduction of The Sloped Layer Method

The Sloped Layer Method (SLM) of placing RCC was first developed and adopted in Jiangya Dam in China (Forbes, 1999). A number of projects in China, such as Mianhuatan and Dachaoshan dams, adopted the sloped layer method after successful application of SLM in Jiangya Dam.

A detailed development and the procedure of the Sloped Layer Method has been described by Forbes (2002).

The interest in RCC dams is driven by economic considerations and also where speed of construction is an important element. However this often compromises quality and water tightness, as shown by many RCC dams. A significant difference between a conventionally placed concrete dam and a roller compacted concrete dam are the number of horizontal construction joints. Horizontal joints are inevitable in RCC dams because of the layered method of construction. Each layer is the thickness of material compacted. In order to achieve required compacted densities by roller compaction, RCC layer thickness is typically maintained at 0.3m, whilst for internally vibrated conventional concrete, layers are typically 1.5m, i.e. 5 times more layer joints occur with RCC.

The performance of an RCC dam will almost entirely be dictated by the performance of the

horizontal joints between layers. The adhesion between layers of RCC is produced by two mechanisms, cementitious bond and penetration of aggregate from the new layer into the surface of the previously placed layer. Layer joints always exhibit lower tensile strength, shear strength and impermeability than the RCC. This is due to ageing of lift surface concrete, tendency for segregation of the overlying RCC at the joint and the potential for lower rolled density at depth.

In order to achieve homogeneous, monolithic RCC across a layer joint the overlying layer must be placed within the initial set time of the lower layer. It is impossible to achieve the time frame required to place a horizontal layer from one abutment across the other in major dams.

The Sloped Layer Method is used to construct lifts of multiple layers. RCC is placed in layers approximately 300mm thick for a total lift thickness of 3 to 4m. At Tannur Dam 1.2m lifts were used. With the Sloped Layer Method, each layer is placed at an inclination of approximately 1:10 to 1:20 instead of the typical horizontal orientation. The length of the slope depends on the plant capacity and production rate with typical slope lengths of 20 to 40m. A sloped layer is placed over the full width of the placement and the layers progress the full length of the placement. The primary goal of this method is to minimise the exposure of fresh RCC lift until it is covered with the next sloped layer. Bedding mortar is placed on the mature RCC lift surface prior to placing the next lift. The Sloped Layer Method is shown in Figure 4.

Placing a sloped layer generally involves commencing at the downstream face and moving across to the upstream face (or visa versa), working on the full height of the layer and compacting up-down slope. At lower elevations, where the sloped layer is wide enough, the placement area can be roughly divided into sub areas, placing commencing on the downstream (or upstream) third and progressing through the central and upstream thirds with the placing – spreading – compacting operations occurring accordingly.

Using this method the final clean up and surface preparation of the lower lift, including application of bedding mortar, is restricted to a narrow strip along the toe of the sloped layer. The width of the strip varies with the slope.



Figure 4 – Sloped Layer Method

In addition to improving lift joint quality, the sloped layer method significantly reduces the associated works such as surface preparation, curing and protection of RCC surface. Formwork can be lifted earlier and utilised more effectively.

The sloped layer method has been shown to work well with the use of GE-RCC for facing the upstream and downstream faces of the dam and connection of the RCC to the rock abutments. In particular RCC progress is not delayed whilst awaiting delivery and placing of conventional concrete facing.

The upstream crossfall of 2-3% adopted in placing the traditional ‘horizontal’ RCC layers for drainage purposes, can be retained for the sloped layer methods.

4 Application of Sloped Layer Method in Tannur Dam

The construction of Tannur Dam commenced in the traditional ‘horizontal’ RCC lift method (which included a mild upstream crossfall) until a height of 15m was reached. The original placement system was a series of 15m wide strips, placed from downstream to upstream in 6m sections. The 15m was in excess of the specified 10m limit but was permitted so as to better match the size of the bulldozer, haulage plant and other equipment. The entire strip was compacted before commencing the next adjoining strip, including the advancing edge which joins to the next strip if

this edge could not be covered within one hour. This ensured that all RCC was compacted while still fresh. The advancing edge of the strip was treated as a fresh joint with removal of any segregated RCC and treatment with bedding mortar before covering with RCC. The position of these fresh construction joints normal to the dam axis varied from layer to layer to avoid any possibility of a weakness in the dam.

One problem, which arose from the use of trucks as opposed to a full conveyor system, was the close out of each layer at the loading point. This area became contaminated from spillage of RCC during loading operations and from the tyres of the equipment. The adopted system was to clean this area every second lift and to fill it with two RCC layers placed in quick succession directly from the swinger.

This horizontal layer approach involved very extensive lift joint cleanup operations and attendant high costs in bedding mortar.

To overcome these problems and at the suggestion of GHD, the Contractor introduced the Slope Layer Method of placing from Layer 51 and continued through to the final layers at the non overflow crest and spillway weir where the width had reduced down to 9m. No particular difficulties were encountered constructing down to this width. The method changed placing from the traditional 0.3m thick horizontal lifts to 1.2m lifts constructed with 4 layers each 0.3m thick placed on a slope such that successive sloped layers



could be placed within the initial set time of the RCC.

The slope layer approach as developed in China uses a 3m high sloped layer as standard. At Tannur Dam, a 1.2m high slope layer was chosen to fit in with height of the downstream steps.

Photo 1 - RCC placement showing sloped layer construction view to upstream

Placing commenced from the left abutment working across to the right abutment where the Rotec “swinger” delivery system discharged the RCC into the dumpers. Layer slopes commenced with a slope of 1 on 12.5 and, as the dam rose and the width between upstream and downstream faces reduced with height of dam, the slope was flattened to 1 on 20. Actual time between successive lifts was controlled to about 45 minutes.

Photo 2 – Mortar spreading and feather edge treatment view to upstream

A 6mm thick mortar bedding mix was spread over the full width of each 1.2m cold lift surface in order to ensure adequate bond was achieved between the old lift and new layer surfaces. The

quantity of bedding mix required was greatly reduced between the slope layers leading to considerable cost savings to the owner.

Cold joints were cleaned using high pressure air/water jetting once the RCC had gained sufficient strength. Air/water jetting was assisted by rotary brooming. Old joints were first treated by scraping with a blade attached to an excavator bucket. The layers and 1.2m lift surfaces were inclined at 1.5% from downstream to upstream to assist in removal of debris and surplus water.

The Sloped Layer Method minimises the area of fresh RCC exposed to ambient conditions, i.e. heat gain is reduced when the RCC temperature at time of placing is lower than ambient and, in the case of rainfall occurring, a smaller area of fresh RCC is exposed to damage or required to be ‘roller sealed’ to limit damage; the method therefore permits tighter control to be exercised during adverse ambient conditions. An added benefit was the fast rate at which RCC layers were being covered by the next layer, mostly this was 30-45 minutes. Heat gain from the atmosphere, which was about 32-36°C during RCC placement, was limited. This would not have been the case with horizontal layers which would remained exposed for 24 hours or more. The thermocouple measurements for the dam at the end of construction are presented in Figure 5, which correlate well with the results from the thermal-structural study (Malkawi 2003) even though the RCC construction continued into the hot summer.

Figure 5 – Temperature contours at the end of RCC construction



To overcome aggregate being crushed under the heavy steel roller in the thinning toe of each sloping layer, a solution developed at Tannur Dam was to thicken this ‘feather’ edge to form a 150mm horizontal ‘foot’ which was then compacted with the smaller Dynapac roller in an upstream downstream direction, prior to compaction of the balance of the layer by the 16 tonne Bomag rollers operating parallel to the dam axis.

The introduction of the Sloped Layer Method led to an immediate increase in production rates of around 50% percent, mainly because of the reduced clean-up requirements and lift joint treatment, and the ability to lift the forms earlier.

Rapid construction, which is the key to the economics of RCC, results in the quality control of the concrete constituents and the production facilities to be a most important factor in ensuring concrete quality. Once the RCC has been produced and compacted into the dam, it is expensive, and also often unrealistic, to remove the deficient material. Quality control after production should provide final verification of the concrete properties. Quality control of the fresh concrete must emphasise test methods that can give a quick indication of concrete quality and identify any minor adjustments required to maintain the concrete within the specification.

Quality control of the RCC at Tannur Dam included the following field and laboratory testing:

• Loaded VeBe time and maximum (2 minute) VeBe density

• Single probe nuclear densimeter density and moisture content testing of the 300mm layers measured at 150 and 250mm depths.

• Verification of nuclear densities and moisture contents by sand replacement density methods (one per 300mm layer)

• RCC washed gradings, monitoring in particular the minus 0.075mm fraction, which contained the aggregate fines plus the added cementitious material, and minus 4.75mm sand fraction, which largely determines the sensitivity of the RCC to segregation.

• RCC test specimen manufacture and testing for compressive, indirect and direct tensile strengths at all the usual standard-cured ages

up to 180 days as well as the 24 hour hot-cured (60°C water bath) compressive strength which provided early confirmation that the required long term RCC strengths would, or would not, be achieved.

• GE-RCC and bedding mortar test specimen manufacture and testing for compressive strength.

• RCC temperature as placed and long term (by installed thermocouple system) temperature history.

• Slump testing of bedding mortar.

A computer program which GHD developed, manipulated all the recorded data, prepared averages, maintained graphical presentations and updated statistical results for continuous monitoring and regular reporting purposes.

Table 1 gives the statistical summary of the RCC density (also as a proportion of the Theoretical Air Free density) and moisture content and the coefficients of variation as measured by nuclear and VeBe methods.

Table 2 below gives the RCC and GE-RCC statistical summaries of the specimen compressive strengths whilst Table 3 gives those for the tensile strengths and elastic modulus.

The average cube compressive strength of the RCC for the entire dam (all mixes) was 25.7MPa at 90 days with a coefficient of variation of 10.6% and a standard deviation of 2.7MPa. The average cube compressive strength of the RCC for the last 50% was 24.8MPa at 90 days with a coefficient of variation of 9.8% and a standard deviation of 2.4MPa. All results exceeded the specified characteristic strength of 20MPa.

The indirect tensile strength of 150x300mm cylinder test samples averaged 2.24MPa while the direct tensile strength averaged 1.42MPa at 90 days. The tensile strength therefore is some 7.5% of the equivalent cylinder compressive strength.

The static elastic modulus averaged 23.1GPa with a standard deviation of 3.8GPa compared with the design assumption of 18GPa.

Some limited 150mm diameter coring of RCC and GE-RCC has been undertaken. Strength testing has been carried out, which confirms the mortared lift joints (1.2m intervals) are giving direct tensile strengths which exceed 1MPa, and are similar to



that of the RCC core itself. Shear testing confirms that the lift joint cohesion exceeds 1MPa, and the angle of friction is 40 -45°.

Comparing the test results tabulated above with the consistency standards in the draft ICOLD Bulletin “State-of-the-Art of RCC Dams” (ICOLD, 2002), as shown in Table 4, confirms that the standards achieved at Tannur Dam can be classified as being in the “Excellent” to “Good” ICOLD ranges.

The use of the sloped layer method of construction in Tannur Dam assisted in speeding up construction and reducing costs. However, a certain number of constraints must be taken into account when using this method, these include the following:

• Some problems were encountered with the vertical 3.0m high upstream RMD (Rapid Metal Developments Ltd) forms that had been designed expecting the usual ‘horizontal’

RCC placing method. Additional anchors had to be placed into the RCC to hold the forms in place when loaded with 1.2m of ‘live’ GE-RCC and RCC during compaction by the 16 tonne vibrating roller. Individual sloped layers were placed in 30-40 minutes, well within the retarded initial set time of the RCC, the 1.2m lifts are thus essentially monolithic and without evidence of joints between the 300mm layers.

• Joint treatment is limited to one cold joint treatment every 1.2m. Such treatment is a little more difficult than usual insofar as the surface of the layers are more uneven. Retaining the traditional crossfall for drainage of wash water is a definite advantage.

• “Feather edge” treatment is required.

• Survey control using lasers was more difficult and the layer profiles were painted on the forms to assist.

Photo 3 - Tannur Dam 1.2m high downstream form showing survey mark

Photo 4 - Downstream view showing the sloped layer construction in progress on the final RCC lifts in the spillway area

Photo 5 – Upstream view of GE-RCC face andthe 5m high, 180m long conventional concreteogee spillway crest nearing the end ofconstruction



Table 1 - Summary of RCC Density and Moisture

Entire Dam (all mixes) Nuclear Test VeBe Test Moisture Content

(%) Density(kg/m3)

% of TAF(%)

Moisture Content

(%)

Density (kg/m3)

VeBe Time

(Seconds) Average 7.9 2425 100.5 8.3 2402 20 Maximum 10.5 2540 104.9 9.2 2435 28 Minimum 6.1 2309 96.3 7.4 2380 12 Number of Tests 3546 3546 3546 480 521 1487 Standard Deviation 0.6 19.6 0.7 0.4 8.7 2.2 Coefficient of Variation (%)

7.0 0.8 0.7 4.4 0.4 11.1

Last 50% of Dam (with mix of 120 kg/m3 cement and 50 kg/m3 pozzolan) Nuclear Test VeBe Test

Moisture Content (%)

Density(kg/m3)

% of TAF(%)

Moisture Content

(%)

Density (kg/m3)

VeBe Time

(Seconds) Average 8.1 2422 101 8.6 2398 18 Maximum 10.5 2538 105 9.2 2416 22 Minimum 6.4 2309 98 7.8 2380 14 Number of Tests 2439 2439 2439 249 258 815 Standard Deviation 0.5 20.4 0.7 0.3 7.1 0.9 Coefficient of Variation (%)

6.6 0.8 0.7 3.7 0.3 4.7

Table 2 - Summary of Compressive Strengths

Entire Dam (all mixes) 150mm Cube Specimen

RCC Compressive Strength GE-RCC Compressive Strength

Age Hot 24 hr 7 day 28 day 90 day 180 day Hot 24 hr 28 day 90 day Average (MPa) 11.9 14.8 21.2 25.7 27.7 10.7 20.0 24.6 Number of Tests 335 331 344 340 120 325 335 336 Standard Deviation 1.7 1.9 2.6 2.7 3.0 1.8 2.8 3.1 Coefficient of Variation (%)

14.5 12.9 12.2 10.6 10.7 16.9 14.0 12.5

Last 50% of the Dam ( with mix of 120 kg/m3 cement and 50 kg/m3 Pozzolan)

150mm Cube Specimen

RCC Compressive Strength GE-RCC Compressive Strength

Age Hot 24 hr 7 day 28 day 90 day 180 day Hot 24 hr 28 day 90 day Average (MPa) 11.6 14.9 20.9 24.8 27.2 10.6 19.6 23.5 Number of Tests 142 142 146 142 36 135 143 138 Standard Deviation 1.4 2.0 2.5 2.4 2.9 1.7 2.4 2.3 Coefficient of Variation (%)

12.4 13.1 11.8 9.8 10.8 16.3 12.3 10.0

Note to convert: 150mm diameter cylinder strength = 80% cube strength (approximately).



Table 3 - Summary of RCC Tensile Strengths and Elastic Modulus

Entire Dam Last 50% of Dam 150mm Cylinder Specimens. All 90-100day Age

Indirect (MPa)

Direct (MPa)

Modulus (GPa)

Indirect (MPa)

Direct (MPa)

Modulus (GPa)

Average 2.24 1.42 23.08 2.24 1.38 21.91 Number of Tests 118 112 68 64 54 24 Standard Deviation 0.31 0.23 3.76 0.33 0.15 3.93 Coefficient of Variation (%) 13.97 16.14 16.32 14.65 10.61 17.93

Table 4 - Consistency of RCC Quality (ICOLD)

Ranges of Coefficients of Variation (%) Assessed Performance Excellent Good Average Poor Very poor

Strength of Manufactured Test Specimens Compressive <10 10 to 15 15 to 20 20 to 25 >25 Indirect Tensile <12.5 12.5 to 17.5 17.5 to 22.5 22.5 to 27.5 >27.5 Direct Tensile <15 15 to 20 20 to 25 25 to 30 >30 Density Loaded VeBe <0.75 0.75 to 1.25 1.25 to 1.75 1.75 to 2.25 >2.25 Nuclear Densimeter <0.75 0.75 to 1.25 1.25 to 1.75 1.75 to 2.25 >2.25

Extract from Table 6.2 ICOLD (2002) (Values for cores not shown above).

5 Advantages and Constraints of the Sloped Layer Method

The advantages resulting from the slope layer approach in Tannur Dam are: -

• Provided for homogeneous, monolithic RCC across the joints of the 0.3m layers and reduced the number of horizontal lift joints.

• Cleanup operations were restricted to the horizontal surface on the top of each 1.2m high slope layer. As this was generally a cold joint with hardened concrete, cleanup could be facilitated by mechanical rotary brooming and high-pressure air/water jetting without causing excessive damage to the surface.

• Bedding mortar requirements were reduced by around 60% with the associated cost saving. Bedding mortar was only used on joints where

the underlying layer was over 2 hours old (1 hour for a strip 4m from the upstream face).

• The sloping joints were generally very fresh being less than 1 hour old. Because of the retarder, the underlying RCC had not achieved its initial set. Bonding quality between successive sloped layers is extremely good.

• Green cutting and layer surface preparation of GE-RCC facing was not required. The material was so fresh that the poker vibrators penetrated into the previously placed layer of facing.

• Formwork could be left on longer for curing or thermal protection purposes. The required quantity of formwork on site could be reduced by up to 40-50%.

• Closeout of the layer under the Swinger was easier.



• Plant could generally be confined to the area in front of the advancing slope layer.

• Curing operations were facilitated by reducing the total surface area of RCC required to be cured. Reduced the area of exposed young RCC that could be damaged by rainfall or affected by hot conditions.

• Easy compliance with time/temperature criteria. Reduced the potential for the cooler RCC to gain heat from hot ambient conditions.

• Overall production rates were increased by around 30-50%.

The disadvantages of the system were: -

• Survey control of the advancing layers was more difficult.

• The more rapid rate of rise of the fresh concrete put greater loading on the formwork. The formwork and their anchors therefore need to be sized accordingly.

• “Feather edges” require special treatment.

6 Future Application of the Sloped Layer Method

The development of placing multiple layers on a slope to achieve a much thicker lift achieves an increased lift joint bond quality and production rate. As shown on the previous projects in China (Forbes 2002), complete lengths of 8 to 10m of unbroken RCC core were recovered from vertical cores extracted from RCC dams where the Sloped Layer Method was used, thus confirming a monolithic mass of the RCC is achievable.

The method has already raised significant interest from other projects in construction around the world and is expected to be adopted for many future RCC dams, especially the larger structures, when cohesion and tensile resistance between layers of RCC is an important issue.

Photo 6 – Tannur Dam downstream view in April 2002, reservoir partly filled

7 Acknowledgements The authors would like to acknowledge the support given by GHD Pty Ltd in undertaking the preparation of this paper.

8 References Forbes, B.A. (2002). “The RCC Sloped Layer Construction Method.” Middle East RCC Conference, Jordan, 2002.

Forbes, B.A., Iskander, M.M. and Malkawi, A.I.H. (2001). “High RCC Standards achieved at Jordan’s Tannur dam.” International Journal on Hydropower and Dams, June, Volume 8, Issue 3, 2001.

Forbes, B.A., Lichen, Y., Guojin, T. and Kangning, Y. (1999). “Jiangya Dam, China: some interesting techniques developed for high quality RCC construction.” Proc. Of the International Symposium on Roller Compacted Concrete Dams, Chengdu, China, April 1999.

ICOLD, Committee on Concrete Dams. Bulletin on “State-of-the-Art of Roller Compacted Concrete Dams” (September 2002).

Malkawi, A.I.H. Mutasher, S.A. and Qiu T. (scheduled for publication in November 2003), “Thermal-Structural Modeling and Temperature Control of an RCC Gravity Dam”. Journal of Constructed Facilities, American Society of Civil Engineering, November 2003.

Slope Layer Method for the construction of RCC Dams

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