Scholarly article on topic 'Investigating the possibility of constructing low cost roller compacted concrete dam'

Investigating the possibility of constructing low cost roller compacted concrete dam Academic research paper on "Civil engineering"

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Alexandria Engineering Journal
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{"Roller Compacted Concrete Dams “RCCDs”" / "Local material" / "Fly ash" / "Construction stages" / "Monitoring system" / "Thermal measurements"}

Abstract of research paper on Civil engineering, author of scientific article — Mohamed I. Abu-Khashaba, I. Adam, A. El-Ashaal

Abstract This research was set with the objective of investigating the possibility of constructing Roller Compacted-Concrete Dam, RCCD, using local material to reduce its cost. Trial laboratory concrete mixtures were conducted to define RCCD proportions in stage-I. Twelve mixtures with fly ash, FA, of cement replacement percentages (0%, 50%, and 60%) designed with water/cementitious-materials ratio, w/cm of 1.0 and 0.9. Cm -content of 1.50kN/m3 and 1.20kN/m3 was also examined. In stage-II, RCCD scale model was constructed based on the laboratory results. Descriptions of RCCD construction stages, dam monitoring system instrumentations and temperature measurements analysis were conducted. Results clarified how FA interacts with Portland cement and showed its effect on concrete properties especially strength development ratio. FA long-term reaction refines the pore concrete structure to reduce water ingress and control its seepage. FA reduces the thermal stresses by reducing the concrete hydration heat and reduces temperature-related cracking due to low early young’s modulus and finally leads to durability improvement that minimizes the dam construction cost. Results indicated that RCCD could be effectively implemented with site local Egyptian materials. However, more further field measurements and a RCCD prototype are required to be examined analytically and verified with in situ data to evaluate that technique.

Academic research paper on topic "Investigating the possibility of constructing low cost roller compacted concrete dam"

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Alexandria Engineering Journal (2014) 53, 131-142


Alexandria University Alexandria Engineering Journal


Investigating the possibility of constructing low cost roller compacted concrete dam

Mohamed I. Abu-Khashaba *, I. Adam, A. El-Ashaal

Construction Research Institute, National Water Research Center, Delta Barrages, ElKalyobia 13621, Egypt

Received 16 June 2013; revised 24 November 2013; accepted 27 November 2013 Available online 22 December 2013


Roller Compacted Concrete Dams "RCCDs''; Local material; Fly ash;

Construction stages; Monitoring system; Thermal measurements

Abstract This research was set with the objective of investigating the possibility of constructing Roller Compacted-Concrete Dam, RCCD, using local material to reduce its cost. Trial laboratory concrete mixtures were conducted to define RCCD proportions in stage-I. Twelve mixtures with fly ash, FA, of cement replacement percentages (0%, 50%, and 60%) designed with water/cementi-tious-materials ratio, w/cm of 1.0 and 0.9. Cm-content of 1.50 kN/m3 and 1.20 kN/m3 was also examined. In stage-II, RCCD scale model was constructed based on the laboratory results. Descriptions of RCCD construction stages, dam monitoring system instrumentations and temperature measurements analysis were conducted.

Results clarified how FA interacts with Portland cement and showed its effect on concrete properties especially strength development ratio. FA long-term reaction refines the pore concrete structure to reduce water ingress and control its seepage. FA reduces the thermal stresses by reducing the concrete hydration heat and reduces temperature-related cracking due to low early young's modulus and finally leads to durability improvement that minimizes the dam construction cost. Results indicated that RCCD could be effectively implemented with site local Egyptian materials. However, more further field measurements and a RCCD prototype are required to be examined analytically and verified with in situ data to evaluate that technique.

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1. Introduction

* Corresponding author. Tel.: +20 1061104316; fax: +20 242188508. E-mail address: (M.I. Abu-Khashaba). Peer review under responsibility of Faculty of Engineering, Alexandria

Roller compacted concrete, RCC, or rolled concrete is a special blend of concrete that has essentially the same ingredients as conventional concrete but in different ratios and increasingly with partial substitution of fly ash for Portland cement [1]. RCC is a mix of cement/fly ash, water, sand, coarse aggregate and common additives, but contains much less water. The produced mix is drier and has no-slump. RCC is placed in a manner similar to paving. The material is delivered by dump trucks or conveyors, spread by small bulldozers or specially


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modified asphalt pavers and then compacted by vibratory rollers.

In dam construction, roller compacted concrete began its initial development with the construction of the Alpa Gera Dam near Sondrio in North Italy between 1961 and 1964. Concrete was laid in a similar form and method but not rolled [2-4]. RCC had been touted in engineering journals during the 1970s as a revolutionary material suitable for, among other things, dam construction [5]. Initially and generally, RCC was used for backfill, sub-base and concrete pavement construction, but increasingly it has been used to build concrete gravity dams. The low cement content and use of fly ash cause less heat of hydration while curing corresponding to that of conventional mass concrete. RCC has cost benefits over conventional mass concrete in dams. This includes higher rates of concrete placement, lower material costs and lower costs attributed to less post-cooling and formwork. For dam applications, RCC sections are built lift-by-lift in successive horizontal layers resulting in a downstream slope that resembles a concrete staircase. Once a layer is placed and is initially hardened, it can immediately support the earth-moving equipment to place the next layer. After RCC is deposited on the lift surface, small dozers typically spread it in one-foot-thick (300 mm) layers [6]. The first RCC dam built in the USA was Willow Creek Dam in Oregon on a tributary of Columbia River. US Army Corps of Engineers initiated such construction between November 1981 and February 1983 [5,7]. Construction progressed well under budget (estimated $50 million, actual $35 million).

On initial filling, leakage occurred between the compacted layers within the dam body was unusually high. This condition was treated by traditional remedial grouting at a further cost of $2 million which initially reduced the leakage by nearly 75%. Over the years, seepage has decreased to less than 10% of its initial flow. Concerning the dam's long-term safety, it is indirectly related to its RCC construction. Within few years of construction, problems were noted with stratification of the reservoir water caused by upstream pollution and anoxic decomposition which produced hydrogen sulfide gas which could in turn give rise to sulfuric acid, and thus accelerate damage to the concrete. The controversy itself, as well as its handling continued for some years. In 2004 an aeration plant was installed to address the root cause in the reservoir, as had been suggested 18 years earlier [5]. In the quarter century since Willow Creek Dam, considerable studies have yielded numerous improvements in the concrete mix designs, dam designs and construction methods for RCC dams. By 2008, about 350 RCC dams existed world-wide [8]. Currently the highest dam of this type is Longtan Dam, at 216 m, with Dia-mer-Bhasha Dam planned at 272 m. RCC techniques reduced the cost of conventional concrete dam construction and were used in massive concrete structures with the advantage of limited construction period and cement content [9,10]. Since the RCC placement rate is much faster and the cost of placement is lower than the cost of conventional concrete, the cost may be reduced by one-half to possibly one-third the cost of conventional concrete [11]. From the overall design criteria, the soils approach to RCC mixture proportioning considers RCC as cement-enriched aggregate and the mix design is based on moisture-density relationships. There is sufficient paste in the RCC mix to fill all the voids in the well graded aggregate for the concrete approach, making no-slump and a fully compacted con-

crete mixture. This approach has a moist consistency than that of the soil approach when the same aggregate type is used [12]. Since there is no ''one procedure'' which is best and fits all cases, therefore, design needs, materials availability and planned placement procedures are the governing factors for proportioning RCC mixes [13].

Nowadays, several completed RCC dams all over the world, are being constructed in all types of climate. The size of RCC dams has significantly increased where some of the largest dams in the world are now being constructed implementing RCC technology [14]. Considering the RCC development, its application in developed countries, especially the Arab countries, is still limited [10]. Since one of the purposes of constructing RCCD is to control floods and redirect them to certain specific passage-way to be optimally used for agriculture and domestic uses, therefore its construction is importantly needed. This importance increases especially when using available local materials in RCCD construction in Egypt, especially Sinai.

This research was initiated with the objective of investigating the possibility of constructing roller compacted concrete dams using local material in order to reduce the dam construction cost. The investigation phases after reviewing the literature reported above are presented in this paper under the following headlines:

• Executing experimental work.

• Experimental results and analysis.

• Scale model of the RCCD.

• Analyzing and presenting RCCD thermal results.

It is necessary to mention that the experimental work and the RCCD model construction and different measurements were executed at Construction Research Institute-National Water Research Center-Ministry of Water Resources and Irrigation, CRI-NWRC-MWRI.

2. Executing experimental work

Twelve concrete mixes with different fly ash replacement percentages divided into two groups of six (6) mixes were designed (i.e., with w/cm of 1.0 and 0.9) [15].

2.1. Used materials

Commercially available Egyptian Ordinary Portland Cement (OPC), complying with the Egyptian Standard Specifications (ESS), was used. Most RCC projects have used Class-F pozzo-lanic fly ash (FA), due primarily to the effect of its spherical particles on workability. Use of FA Class-F in RCC serves three purposes:

• partially replaces cement to reduce heat of hydration generation,

• reduces cement quantities, and

• acts as a mineral addition to the mixture to provide fines to improve workability.

The used aggregates were chosen from a quarry located on Cairo-Suez road. Overall aggregate is composed of four components, namely, fine and three fractions of coarse aggregate

(size#1, 2, and 6). The physical properties of both fine and coarse aggregate fractions are listed in Table l whereas the overall grading curve of the aggregate is shown in Fig. l compared to the typical mass RCC grading mentioned in ACI 207.5R [13]. Tap drinking water was used to mix all concrete mixes.

2.2. Fly ash specifications and properties

As mentioned earlier; fly ash (FA) complying with both BS 3892 and ASTM C618 Class-F; was selected to be the only pozzolanic material to be added to RCC mixes. The physical properties and chemical composition of fly ash, as provided by the manufacturer, are presented in Table 2.

2.3. Mixture proportioning

According to ACI 207.5R, there are three different mixture design procedures:

• proportioning RCC to meet specified limits of consistency,

• relying on trial mixture tests to select the most economical aggregate-cement like materials combination, and

• proportioning RCC using soils compaction concepts.

Among the common three procedures, the trial mix tests were adopted in the current study as one suitable for all types of aggregates. The evaluation tests are similar to that of conventional concrete. Concrete mixes were designed to be of no-slump using absolute volume method.

Two groups of concrete mixes were designed with w/cm materials ratio of 1.0 and 0.9. Each group contains six mixes with different fly ash replacement percentages as presented in Table 3. The lower specific gravity of fly ash compared with that of Portland cement implies that when replacement is based on mass, a larger volume of fly ash is required in replacement of the equivalent volume of cement removed [16,17]. Due to low paste content and the difficulties being faced in casting, handling, and compaction during lab work, mixes No. 10-12 containing Cm of 1.20 kN/m3 and w/cm = 0.9 were discarded.

2.4. Casting and compaction procedures

The various components of the mixture are weighed. A dry blend of the cement and FA was first prepared manually in a container outside the mixer. The coarse aggregates sequenced, sizes 6, 2 then 1 were placed in the mixer followed by Cm and sand. These components were dry-flipped in the mixer for 90 s. The amount of water was added to be distributed during the

Table 1 Aggregates physical properties.

Aggregate type % Unit weight (kN/m3) Specific gravity

Fine 43 18.2 2.71

Coarse Size 1 10 14.4 2.65

Size 2 22 14.4 2.59

Size 6 25 14.1 2.53

continuous mixing of different components for a period of 30 s. Afterward, the mixing continued for an additional 90 s. Mixing was stopped and the sticky concrete was removed from the walls using an iron bar [18,19]. Finally, the mixing process was resumed for another 90 s to reach a total mixing time to 5 min ''300 s.'' After mixing, concrete was released from the mixer and remixed manually to assure the homogeneity. Then, slump and unit weight tests were carried out on fresh concrete. Concrete was placed in oiled steel molds for the preparation of testing samples. Filling was in three layers where each layer was internally vibrated due to the relatively low paste content of RCC mixes. All molds for all examined mixtures were finally vibrated on a shaking table until good compaction is reached (casting surface shows slight traces of bleeding) and the settlement of the final surface was reached. The vibrating compaction generated a better distribution of particle orientation angles than the normal compaction. Therefore, vibrating compaction gives higher values of shear strength, cohesion and angle of friction of the samples compared with the normal compaction. Immediately after casting, the top surface of the specimens was covered with a plastic sheet to reduce the rate of water evaporation until de-molding. Samples were extracted from the molds after 24 h and all the specimens were put in the water curing basin until testing.

2.5. Test types and specimens configuration

Specimens were prepared and tested according to ASTM standards. Compressive strength was measured at ages of 3, 7, 28, 90 and 180 days after casting on standard cube specimens (150 x 150 x 150 mm). Split and flexural strength were investigated at 28 and 90 days on cylinders (150 x 300 mm) and prismatic specimens (100 x 100 x 500 mm), respectively. Static modulus of elasticity (Es) test was carried out according to ASTM C469 standards on cylindrical specimens of (150 x 300 mm) as well. Es and water absorption were only measured at 28 days.

A Lower Limit —♦—Aggregate mixture □ Upper Limit _

0.001 0.01 0.1 1

Grain Size (mm)

Figure 1 Aggregate gradation used for RCC mixtures.

Table 2 Fly ash physical properties and its chemical composition.

Physical property Value BS 3892 limits ASTM C618 limits

Retained on sieve 45 im (%) 10.10 12 max 34 max

Soundness (mm) 1.00 10 0.8%

Density (kN/m3) 22.4 20.0 -

Moisture content (%) 0.02 0.5 max 3.0% max

Chemical compound % by wt. BS 3892 ASTM C618

Silicon dioxide, SiO2 61.30 - Total 70% min

Aluminum oxide, Al2O3 29.30 -

Ferric oxide, Fe2O3 4.60 -

Calcium oxide, CaO 1.11 10% max 10% max

Magnesium oxide, MgO 0.59 - -

Titanium dioxide, TiO2 1.78 - -

Manganese, Mn2O3 0.03 - -

Sulfur trioxide, SO3 0.14 2% max 5% max

Potassium oxide, K2O 1.00 - -

Sodium oxide, Na2O 0.22 - -

Loss on ignition, LOI 0.80 7% max 6.0% max

Table 3 Concrete mix proportions.

Mix no. w/cm Weight per unit volume (kN/m3)

Cm W Coarse agg. S

C FA Size 6 Size 2 Size 1

1 1.0 1.50 0 1.50 5.28 4.64 2.11 9.08

2 0.75 0.75 1.50 5.22 4.59 2.09 8.97

3 0.60 0.90 1.50 5.20 4.58 2.08 8.95

4 1.20 0 1.20 5.54 4.87 2.21 9.52

5 0.60 0.60 1.20 5.49 4.83 2.19 9.44

6 0.50 0.70 1.20 5.48 4.82 2.19 9.42

7 0.9 1.50 0 1.35 5.38 4.73 2.15 9.24

8 0.75 0.75 1.35 5.31 4.67 2.13 9.14

9 0.60 0.90 1.35 5.30 4.66 2.12 9.12

10 1.20 0 1.08 5.62 4.94 2.25 9.66

11 0.60 0.60 1.08 5.57 4.90 2.23 9.57

12 0.50 0.70 1.08 5.56 4.89 2.22 9.56

3. Experimental results and analysis

3.1. Fresh concrete properties

The properties of fresh concrete are measured in terms of slump and unit weight. The slump measurements confirmed that all mixes have zero-slump. The unit weight results proved that using fly ash, as a partial replacement of cement, has no significant effect on the unit weight of fresh concrete for both examined w/cm ratios listed in Table 4.

3.2. Hardened concrete properties

The compressive strength, fc is inversely proportional to the FA quantity Fig. 2. Increasing the FA replacement ratio decreases fc of concrete. For all investigated w/cm ratios, Cm-content and age of testing, the obtained results verified that fc values for concrete mixtures containing FA are always smaller than that corresponding to concrete without FA. The ratio of 90-day compressive strength to the 28-day one (fc-90/fc-28) is illustrated in Table 4 to measure strength development rate.

The fc-90 values, for concrete mixtures without FA ranged from 105% to 120% of fc-28. The corresponding ratio for RCC mixtures with FA ranged from 122% to 151%. This observation matches the findings by Lane and Best [20]. As expected, increasing the FA replacement ratio increases the (fc-90/fc-28) ratio, which may indicate that FA acts as strength development retarder which may be attributed to its long-term pozzolanic reaction.

Tensile strength in terms of split and flexural strength was measured. Results clarified that concrete mixtures with FA had smaller split tensile Fig. 3, and flexural strength values than the corresponding values of mixtures without FA for all examined w/cm ratios, Cm-content, and testing ages adopted in the current study. Increasing the FA percent decreases both split tensile and flexural strengths of concrete. The examined tensile strength is directly proportional to the total cement like materials content.

Generally, it can be noticed that Es values Fig. 4, of concrete without FA are higher than those of concrete containing FA. Similar findings were reported by Lane and Best [20].

Through its pozzolanic activity, fly ash chemically combines with calcium hydroxide, CH, liberated by cement

Table 4 Effect of fly ash on unit weight, absorption and strength development ratio.

Mix no. w/cm Cm (kN/m3) FA (kN/m3) Unit weight (kN/m3) Absorption (%) /c-9o//c-28 (%)

1 1.0 1.50 0 23.7 7.42 105

2 0.75 23.6 7.20 140

3 0.90 24.0 8.19 151

4 1.20 0 21.5 6.28 120

5 0.60 21.4 6.22 136

6 0.70 21.4 6.59 124

7 0.9 1.50 0 21.8 6.70 106

8 0.75 22.6 6.45 122

9 0.90 22.4 6.95 129

X -%

x/ i . .__- X ■ A M

Jy 1IX 1 J Mi) 2 o m MIX 3

A M x M 1IX 4 IX 7 MI MI X 5 X 8 ♦ + MIX MIX 6 9

0 14 28 42 56 70 84 98 112 126 140 154 168 182 196

Age, days

Figure 2 /c of the studied mixtures.

Figure 3 Splitting strength of studied mixtures.

hydrating, and water to produce C-S-H, thus reducing the risk of leaching calcium hydroxide [15]. The long-term reaction of fly ash refines the concrete pore structure reducing water ingress [21]. The results of mixtures containing FA as 50% cement replacement ratio verify this result. Despite this finding, water absorption values of concrete containing FA of 60% are slightly higher than that of concrete without FA Table 4. The reason may be due to the relatively low cement content, and consequently, there is a reduced amount of CH for pozzo-lanic reaction of fly ash to take place. Water absorption was

^ 1 r\

FA = 75 : cm = 150 w/cm = 0.9

Figure 4 Static elastic modulus of concrete.

measured at 28 days while the greatest influence of fly ash on permeability occurs within the first 2-20 months [13].

4. Scale model of RCCD

4.1. Quality Control tests

The Quality Control, QC, tests were carried out on fine and coarse aggregate to determine their physical, mechanical properties, chemical composition and the global grading curve for the combined aggregate with respect to the upper and lower boundaries of the grading listed in ACI 207.5R [13]. The grading curve for the combined aggregate is verified as was previously shown in Fig. 1.

4.2. Design o/ RCC mixture

The mixture must be proportioned and designed to provide the strength, durability, and impermeability necessary to meet all design requirements for stability and performance [22,23]. Various procedures for mixture proportioning have been developed at CRI material laboratory [15]. Based on experimental results, the proportions of the examined mixture No. 8 having 0.75 kN FA as a partial replacement (50%) of cement content and w/cm of 0.90 are selected for RCCD model construction. This selection was based on a reasonable FA content required to control temperature variations inside the dam; the lower absorption percent (6.45%) compared with that of the corresponding mixture having w/cm of 1.0 (7.20%) and the

corresponding mixture without FA (6.70%); early strength development ratio (122%) compared with the corresponding one having w/cm of 1.0 (140%) and its reasonable strength

4.3. Pre-construction stage

Before constructing RCCD model, some of trial slabs Figs. 5 and 6, are cast in situ to confirm slump and strength and define the compaction process and its efficiency provided by the soil compactor.

4.4. Consideration for lift thickness and number of passes selection

The benefits of thicker or thinner lifts should be optimized. Thicker lifts mean fewer lift joints and fewer potential seepage paths. Thinner lifts allow joints to be covered sooner with better bond potential. Thinner lifts are generally more suited to smaller jobs and thicker lifts more suitable for larger jobs [22-26]. Maneuverability, compactive force, drum size, frequency, amplitude, operating speed and required maintenance are all parameters to be considered in the selection of the specified lift thickness.

4.5. Lift thickness and minimum number of passes

The minimum number of passes for a given vibrating roller machine to achieve specified compaction depends primarily on the RCC mixture, lift thickness, and type of a compactor machine. Experience shows that lift thickness is governed more by spreading efficiency than by compaction requirements.

Tests were performed in test fills prior to the early stages of dam construction to determine the minimum number of passes required for full compaction using the design mixture and the planned lift thickness. Three to six passes of a double-drum vibratory roller will achieve the desired density for RCC lifts in the range of 150-450 mm thick [22]. Mehta and Monteiro [23] reported that the thickness of the lifts normally ranges from 150 to 900 mm. In the US a lift thickness of 300 mm is often adopted. This allows compaction in a timely manner with appropriate equipment since over compaction reduces RCC density and should be avoided which is proved in Fig. 7. However, soil compactor was used as an appropriate vibrating machine for providing compaction for the investigated scale model of RCCD. A number of slabs with the same mix proportions were cast and examined to determine the optimum thickness of lift and the required number of passes provided by soil compactor machine to allow satisfactory compaction Figs. 5 and 6. Different lifts' thicknesses, 200, 250, 350 mm and number of passes, 4, 6, 8, 10, for every thickness

20 cm Layer

£ 1.90

-Fresh density -Dry density

2 4 6 8 10

Number of paths

Figure 7 Results of sand cone test.

and Es.

(a) Verification of no slump (b) Compacting slabs using a soil compactor Figure 5 Casting trial slabs using RCC at RCCD model site.

were examined for this purpose. Sand cone test was conducted to define the dry concrete density after compaction Fig. 6. As a result Fig. 7, a 200 mm lift thickness and 6 passes are required to construct the RCC dam model. Some core samples were extracted from the trial slabs Fig. 8a and b.

4.6. RCC dam cross section

The dam model was hydraulically designed to determine its dimensions taking into consideration the different situations that will face the scaled model and the available space for testing. The RCC dam with a height of 3600 mm, base width of 3500 mm, top width of 500 mm and 1:1 back slope was selected for execution of the dam model Fig. 9a.

4.7. Instrumentation

Instruments were installed at selected locations throughout the dam and its foundation to monitor the structure's behavior

during construction and the subsequent operation stages. The number, type, and location of gauges installed during construction become a concern that may hinder the rapid placement, thereby increasing construction costs. A designer, with a clear understanding of the project's purpose, can design an inexpensive, functional instrument package to provide immediate and long-range dam and foundation behavioral data [22]. Embedded gauges can determine temperature, strains, and hydrostatic pore pressure, and measure cracks. Twelve thermocouples were installed in the RCCD during construction in a predetermined grid to provide continuous temperature data during and after construction. Fig. 9a shows thermocouples that were distributed as follows: 5 devices in the bottom third of the dam, 5 devices along the top third of the dam, 1 thermocouple in the middle height of the dam and the last one embedded at the dam crest. Piezometers were also installed to measure the water depth (dam permeability) along the dam bottom lift and distributed along the dam width. To measure the bond strength between the different dam lifts, 2 electrical

(a) Drilling cmcme (b) Extracted core samples

core machine

Figure 8 Extracting cores from the trial slabs using drilling machine.

' Lift No. (15)

Lift No. (12)

Lift No. (8)

Lift No. (4)

Ft ' '• ■ -R_ - ■ ■' .. _fl ' / J ' R

T^-1.00 T 1.00 T 1.00-



THERMOCOUPLE (All RCCD dim. in m) (a) Thermo-couple

Figure 9 Instrumentations used for RCCD monitoring system


(b) Piezometers, strain gauges, cell pressure

strain gauges were bonded on two steel bars; one horizontal and one inclined. They were installed at the interface between layers 11 and 12. Four electrical strain gauges were bonded to the exterior front concrete surfaces and a pressure cell was cinstalled upstream the dam to measure the water upstream pressure that the dam will face. Fig. 9b illustrates the instrumentation layout. Piezometers, strain gauges and pressure cell readings were not in a fashion to be included in the current paper and may be presented later. Instruments associated with RCC dams are read at prescribed time intervals for an extended period of time after construction to establish the RCCD behavior history. Immediate attention will be required to determine whether the readings are correct.

4.8. Concrete placement and spreading

Firstly, concrete placement was conducted using the gutter Fig. 10a. Then, for concrete homogeneity, concrete was remixed to overcome segregation problems Fig. 10b. A preferred technique of RCC placement was to push an advancing face of each lift progressing from one abutment to the other [22]. The lift extends from the downstream to the upstream face. Soil compactor Fig. 5b, had proven to be best for spreading RCC uniformly in the model. By careful spreading, a soil compactor may remix RCC and minimize the segregation that occurred during dumping. Careful attention should be given to assure that remixing is occurring and that the soil compactor

(a): Placement of concrete by gutter

(b): Remixing the concrete manually

(c): Layering dam into 200 mm lifts-thickness

(d): Concrete placement into one of dam-lifts

(e): No formwork removal for the previous casted lift

(f): Strengthen formwork during lift placement

(g): Plastic covering for (h): Installing piezometers, (i): RCCD after construction casted lifts strain gauges, cell pressure completion in its final shape

Figure 10 Photos describing different RCCD model construction stages.

is not simply hiding segregated material. Each RCC mix will have its own characteristic behavior for compaction depending on temperature, humidity, wind, plasticity of the aggregate fines, overall gradation and the maximum aggregate size [2224]. Fig. 10c shows that RCCD construction was in 200 mm s lifts, and Fig. 10d-f describe how the formwork was strengthened during the concrete placement process. There was no formwork removal for the previously casted lift until it was completely covered by the next two subsequent lifts. Moreover, the formwork of the upstream face of the dam was not removed until the full construction completion of the dam. The time period between lifts construction ranged from 16 to 18 h as the concrete was placed on daily bases.

4.9. RCCD curing

After RCC has been placed and compacted, the lift surface must be cured and protected from weather. The surface must be moist so that water does not escape. It should be protected from temperature extremes until it gains sufficient strength. During placing, a brief and very light rain or mist can be tolerated provided that equipment does not track mud onto the RCC or begin to add moisture into the surface damaging the compacted RCCD. As soon as damage is evident or the roller begins to pick up material on the drum, concrete placement should be halted. When conveyors are used for delivery and little or no vehicular traffic is required on the RCC, construction can continue into slightly more moist weather [22]. This requires a slight decrease in the amount of mixture water used because of the higher humidity and lack of surface drying. During construction, the compacted surfaces of RCC lifts should be maintained in a damp condition without water ponding. The surface may be covered with plastic or other means to prevent moisture loss Fig. 10g.

Each RCCD should be evaluated for its exposure conditions and material properties. The hydrated heat generated by the RCC mass and the continuous placing sequence can allow very cold weather placement [26,27]. The RCCD scale

Figure 12 Thermal distribution along the dam at air temperature of 15 °C (17/1).

Figure 11 Thermal distribution along the dam at air temperature of 18.6 °C (16/1).

Figure 13 Thermal distribution along the dam at air temperature of 15.2 °C (18/1).

model was constructed in winter, at the end of December, and completed on January 15. Fig. 10h shows photos of some instrumentation installed in the dam such as piezometers, electrical strain gauges, and the pressure cell. Fig. 10i shows photos of the RCC dam after construction completion.

5. Analyzing and presenting RCCD thermal results

The principal changes in volume associated with massive concrete placement are from the temperature change during the dam life. Drying shrinkage is limited to the exposed surfaces of the mass. Autogeneous changes in volume are normally inconsequential. They are primarily dependent on the quality and quantity of the cement like materials used but may also

0 0.5 1 1.5 2

1.5 2 2.5 3

Figure 14 (a) Thermal distribution along the dam at air temperature of 30.33 °C (29/1 day). (b) Thermal distribution along the dam at air temperature of 23.2 °C (29/1 night).

Figure 15 (a) Thermal distribution along the dam at air temperature temperature of 11.31 °C (2/2 night).

be influenced by aggregates, [22]. A disadvantage of RCCD is that it may have a poor resistance to tension and may crack from the thermal strains along the major axis of the dam following the dam cooling. Cracking of mass will occur when restraint in volume exceeds the strain capacity of the concrete [27-29]. Thermal strains are the major cause of wide cracks that can affect the overall performance of dams. These thermal cracks are mainly vertical and perpendicular to dam main axis. The structural consequences of these cracks may not be significant for a gravity straight dam. However, they can be very undesirable for gravity/arch dams where part of the load is transferred horizontally. Cracks could lead to an unsafe gravity section due to an interruption of arch action load path. By decreasing the monolithic action of the structure, cracks can

of 19.54 °C (2/2 day). (b) Thermal distribution along the dam at air

reduce the ability of the dam to resist settlement, and vibrations, earthquakes, live loads and waves. Cracks may increase seepage volume through the dam. This seeped water may contain chemicals that weaken the concrete strength [28]. The principal factors affecting cracking are the internal peak temperature reached after placement, the average annual ambient temperature to which the mass will eventually cool, creep, the modulus of elasticity, and the degree of restraint action at the crack location. These cracks appear during the first or second winter season and generally initiate at exposed surfaces adjacent to the foundation where restraint is the greatest. Cracks propagate inwards and upwards with the continued cooling of the mass. If volume change is sufficiently large, cracking will penetrate the full dam thickness and become a source of

leakage. Figs. 11-15 illustrate the measured temperatures and its distribution inside the RCCD for different external climate temperatures.

Analyzing the thermal results Figs. 11-15, in the early ages after RCCD completion, the temperature inside the dam core are found to be (14 °C) smaller than that of the whole dam perimeter adjacent to the surrounding climate temperature (18.6 °C) Fig. 11. This may be attributed to the chemical process which is taking time to startup. In the following monitored temperatures Figs. 12 and 13, the temperatures values inside the dam core have risen from 15 °C to 20 °C representing the increasing rate of the heat of hydration. The same trend is noticed in Figs. 12 and 13, for the same climate temperatures of (15 °C). With dam age Fig. 14a and b, and representing day and night with average weather temperatures ranging from 30.33 °C at day to 23.2 °C at night, the measured temperatures inside the dam core were found less than the outside temperature. Finally after long time, temperatures reduced from 30.33 °C to 18 °C and from 23.2 °C to 19 °C at day and night, respectively. The obtained results were unexpected. Fig. 15a confirms this result since the monitored temperatures inside the dam were comparable with that of the outside temperatures ranging from 17.5 °C to 19.54 °C. However, at night temperatures inside the dam increased from 11.31 °C to 21.5 °C showing that the dam keeps its temperature at a constant rate during the day Fig. 15b. It also shows that the dam does not lose its heat quickly because of the slow chemical process inside due to the replacement of cement with the pozzolanic fly ash. In addition, the heat flow from the dam core to the outside temperature is restricted by the low values of the heat conduction parameters of the RCC.

All these temperatures findings may be due to one or all of these reasons:

• Using natural pozzolans fly ash in the massive concrete dam construction, it is possible to achieve a reduction in temperature rise without incurring the undesirable effects associated with very lean mixtures; i.e., harshness, bleeding, tendency to segregate, and tendency to increase permeability.

• Fly ash in the concrete reduces the thermal stresses due to the reduction in the heat of hydration in massive concrete structures.

• The early low RCC elasticity modulus may reduce temperature-related cracking.

6. Conclusions and recommendations

Material selection, mix proportioning design and testing, RCCD scale model construction, monitoring system and thermal measurements were introduced. In light of the obtained results of the experimental investigation and RCCD thermal behavior, the following conclusions were reached:

1. Based on lab trial mixes, RCCD scale model can be successfully constructed using local Egyptian materials available at sites in addition of using FA, as a partial replacement of cement. The purpose of using FA is due to its pozzolanic reaction, as well as to reduce cement usage required especially for mass structures.

2. The strength development rate of the investigated concrete mixtures (fc-9o/fc-2s) proved the long-term FA pozzolanic reaction. Test results stated that increasing the FA replacement ratio increases the tfc-900/fc-2g) ratio. The strength development rate, for concrete mixtures without FA was found to be in the range of 105120%, whereas the corresponding ratio for RCC mixtures containing FA ranged from 122% to 151%.

3. Using natural pozzolans FA in massive concrete dam construction, it is possible to achieve a temperature rise reduction without any undesirable effects such as bleeding, tendency to segregate and tendency to increase permeability. The long-term reaction of fly ash refines the pore structure of concrete to reduce water ingress and control its seepage.

4. Use of FA reduces the thermal stresses by reducing the concrete heat of hydration. Moreover, using FA and providing a maximum amount of aggregate and a minimum of cement while developing the required properties often results in low early modulus of elasticity of RCC that may reduce temperature-related cracking and finally leads to durability improvement and a reduction in the cost of concrete dam construction.

5. Results obtained through the current study are satisfactory and encouraging. Therefore, to implement RCC technology in dam construction, full scale RCC dams are required to be studied analytically and verified with in situ data. Furthermore, more measurements and further studies are still needed to be carried out to evaluate that technique using the low cost local available Egyptian materials.


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