3.1. GENERAL
The 2013 ICOLD Bulletin 165 on the Selection of Materials for Concrete Dams (ICOLD/CIGB, 2013) addresses the primary selection requirements for materials for all mass concrete dam types, including roller-compacted concrete (RCC) dams. Only issues of particular importance in respect of current RCC practice are further addressed and clarified in this Bulletin.
3.2. CEMENTITIOUS MATERIALS
3.2.1. General
RCC can be manufactured with any of the basic types of cement or, more typically, a combination of cement and a supplementary cementitious material. The great majority of RCC mixtures for dams contain supplementary cementitious materials, with the most common type being a low-lime flyash (ASTM, Standard Specification C618), although an increasing trend in the use of natural pozzolans is apparent.
Physical and chemical consistency and reliability of delivery are particularly important aspects to be considered in selecting the sources of cementitious materials for an RCC dam. At the high percentage supplementary cementitious materials contents often used in RCC, the strength development characteristics of the mix can vary significantly from one particular cement/supplementary cementitious materials/chemical admixture (retarder) blend to another. Consequently, it is usually necessary to perform laboratory testing to optimise the cementitious materials types, sources and blend combinations for RCC mix design before construction is initiated. This should be done by the design engineer and not the contractor, to ensure that the concrete performance meets the structural design criteria.
3.2.2. Cement
While lower-heat cements have been used successfully for RCC dams, some preference exists for using an Ordinary Portland Cement (ASTM C150 Type 1, or EN 197 CEM 1), (ASTM Standard Specification C150 & BS EN 197-1 2011) due to the more restrictive and consistent composition of this cement type.
3.2.3. Supplementary cementitious materials
Supplementary cementitious materials (SCM) may be cementitious and react with the hydration products of the cement to form strong compounds or they may be inert fillers designed to increase the amount of total paste in the concrete mix. These admixtures may be included for reasons of economy and/or to enhance the fresh and hardened properties of the concrete.
Pozzolans are man-made or natural materials, which though not cementitious in themselves, contain constituents (e.g. amorphous quartz, aluminium and calcium silicates), which will combine with lime at ordinary temperatures in the presence of water to form compounds possessing cementitious properties. The lime, calcium hydroxide, results from hydration of cement.
Supplementary cementitious materials, which are often less expensive than cement, are beneficial in reducing the necessary cement content and consequently usually reducing cost and
hydration heat, while increasing the paste volume of the fresh RCC and giving rise to a slower strength development.
For example, in Chinese practice, flyash is almost always used as a pozzolan in RCC and the normal approach is to select the best quality material available. Should excessive free carbon content (loss on ignition, LoI) result in an inferior flyash quality, treatment methods are available to reduce the LoI to an acceptable level. In Japanese practice, fly ash is almost always used as a pozzolan in RCD concrete.
3.3. AGGREGATES
3.3.1. General
Supporting heavy earthmoving trucks during placement and compaction, RCC is necessarily less workable than an equivalent CVC. Accordingly, an RCC mix generally contains a lower paste volume and more aggregate than a CVC mix. In addition, aggregate fines (< 75 microns) are often used to enhance the total paste volume of RCC mixes when the cementitious paste required for concrete strength is inadequate to fill all voids within the compacted aggregate structure. Fines contents of 15%
and higher are not uncommon in the fine aggregates for RCC mixes. As a consequence of the typically low paste volumes, more stringent aggregate specifications than typical for CVC are often applied to ensure a minimum possible compacted aggregate void content in modern RCC. Increased aggregate particle shape and grading requirements are similarly beneficial in enhancing RCC workability without an otherwise necessary increase in total paste content.
In the process of consolidation, the application of external energy in the case of RCC causes the aggregates to be compacted until all excess paste is brought to the surface, resulting in a densely packed aggregate skeletal structure, in which a high level of friction exists between the composite granular materials.
To allow mixing with rapid, compulsory mixers and to limit segregation during transportation and handling, the maximum aggregate size typically applied for RCC is limited to 40 to 60 mm, although this is sometimes increased to 75 mm in the interior of the dam where workability and segregation are of lesser importance. To increase consistency and to reduce segregation, a higher fine aggregate content than for CVC is commonly applied for RCC mixes. The maximum aggregate size used in RCD concrete in Japan is either 80, or 150 mm, with the latter figure accounting for approximately 30 to 40% of all cases.
Aggregates that may once have been considered unsuitable for use in concrete have been successfully used for a number of RCC dams. In such instances, the design of the dam structure must obviously accommodate the specific reduction in performance of the aggregates used and lower quality aggregates are typically applied in interior zones, where they can be encapsulated within higher quality concrete, especially in severe, or moderately-severe climates (Oliverson & Richardson, 1984). While poor aggregate particle shape is particularly unfavourable in the case of higher workability RCC, RCC mixes in general are impacted in a quite different way to CVC mixes by the presence of elongated particles, due to the higher energy applied during compaction.
Where there is a choice of available materials, the materials with the best combination of physical properties should be selected. RCC placing rates are generally high and large aggregate stockpiles are often advantageous in allowing reduced aggregate crushing plant capacity, or to create an adequate buffer in case of plant breakdowns, etc. In such circumstances, the development of an appropriate quantity of stockpiled aggregate prior to the start of RCC placement must be assured and is often specified by the design engineer. For large RCC dams without seasonal shutdowns, the aggregate crushing and stockpiling operations should at least match the average RCC production rate.
For good quality concrete, aggregates, in terms of materials composition, quality and gradations, must be consistent over the duration of construction, which requires that a stringent quality control programme is maintained.
3.3.2. Coarse aggregates
The most important factor to consider when selecting the source, shape and grading of a coarse aggregate is the avoidance of segregation, which can substantially compromise the in-situ performance of compacted RCC. Continuous aggregate gradations and the use of crushed, rather than natural, rounded coarse aggregates have been demonstrated to be advantageous in reducing segregation. In high-workability RCC, the maximum combined flakiness and elongation of coarse aggregate particles, when specified to (BS 812 Part 105), is often limited to 25%, and sometimes as low as 20%.
Using a smaller maximum aggregate size in RCC, the associated beneficial reduction, or elimination of segregation must be balanced against the consequential increase in aggregate production costs.
3.3.3. Fine aggregates
The grading of fine aggregates strongly influences the paste requirement and compactibility of RCC (Japanese Ministry of Construction, 1981 and Hollingworth & Druyts,1986). An increase in the fine aggregate content has been found to reduce the tendency of RCC to segregate during handling and a higher percentage of non-plastic fines (<75 microns) within the fine aggregate grading is commonly perceived as beneficial in increasing paste for all RCC types. With aggregate fines contributing to the total paste content, impermeable RCC can be designed for lower water and cementitious materials contents, particularly when well-graded aggregates, with good particle shape, allow a low aggregate compacted void content.
Quarry crusher fines are particularly beneficial to RCC mixtures. Natural fines are more susceptible to fluctuations in water demand, due to plasticity variations.
In order to reduce the required cementitious paste in a high-workability RCC, the compacted void content of the fine aggregates is generally limited to between 30 and 32%, with even lower values offering additional benefit.
The addition of non-plastic aggregate fines beyond that required to achieve sufficient total paste to fill the aggregate voids can decrease workability and increase water demand, with a consequential reduction in RCC strength. The use of plastic fines can substantially change the properties of both the fresh and mature RCC.
3.3.4. Overall grading
In general, lower cementitious material content RCC mixes usually include higher fines contents than higher cementitious content RCC mixes. Fine aggregate contents generally vary between 30% and 45% across all RCC types, with lower cementitious material RCC mixes tending towards higher fine aggregate contents.
The number of grading bands into which RCC aggregates are separated for batching depends on the level of grading control desired. While cost benefits are obviously developed through minimising the number of aggregate grading bands to be handled and batched, the broader the grading stored in a single stockpile the more likely segregation will develop during aggregate handling. The favoured balance in this regard is a total of four aggregate grading bands, with five generally selected when fine aggregates must be blended from two sources and three selected when a smaller maximum size aggregate is used. Effective control of undersize and oversize particles in each size group must be maintained.
3.4. ADMIXTURES
One of the most significant developments in RCC technology since the publication of ICOLD Bulletin 126 in 2003 has been the increasingly widespread use of set retarder admixtures in RCC for dams. With the initial setting time of RCC mixes often retarded to between 20 and 24 hours and final set to approximately 30 hours, better bonding between placement layers has been achieved through the realisation of as much as 90% hot joints, which typically (but not always) involves placing successive RCC layers before the initial set of the receiving layer beneath (sometimes termed “fresh” on “fresh”).
In RCD dams, successive placement of RCD concrete layers based on the concept of hot joints mentioned above is not adopted (Japanese Ministry of Construction, 1981).
Different set retarders react differently with different cementitious materials and cementitious materials blends. It is consequently essential to identify the optimal retarder product and the proper dosages before placement of RCC for the dam is initiated. Furthermore, it is noted that laboratory (ASTM C403) set time measurements almost always over-estimates the retardation achieved in the field in moderate or warm environments, and can under-estimates the retardation in cold environments. In situ initial and final set testing is accordingly essential to optimise the retarder dosages to be applied under the range of climate and day/night conditions to be experienced at the dam site.
In addition, water-reducing and air-entraining admixtures (used in all RCD) have been successfully used in RCC dams, with air-entrained RCC providing improved freeze-thaw durability and improved workability.
3.5. REFERENCES
AMERICAN SOCIETY FOR TESTING AND MATERIALS. “Flyash and raw or calcined natural material admixtures for use as a mineral admixture in Portland cement concrete”. Standard Specification C618, ASTM, Philadelphia, USA.
AMERICAN SOCIETY FOR TESTING AND MATERIALS. “Portland cement. Standard Specification C150”. ASTM Philadelphia, USA.
BRITISH STANDARDS INSTITUTION. “Testing aggregates – Part 105: Methods for determination of particle shape”. BS 812-105. 1990. London, UK.
BRITISH STANDARDS INSTITUTION. “Cement. Composition, specifications and conformity criteria for common cements”. BS EN 197-1. 2011. London, UK.
HOLLINGWORTH, F. and DRUYTS, F.H.W.M. “Rollcrete: some applications to dams in South Africa”.
Water Power and Dam Construction. London, January 1986.
ICOLD / CIGB. “Selection of Materials for Concrete Dams”. Bulletin No 165, ICOLD / CIGB, Paris, 2013.
JAPANESE MINISTRY OF CONSTRUCTION. “Design and Construction Manual for RCD concrete”.
Technology Centre for National Land Development, Tokyo, 1981.
OLIVERSON, J.E. and RICHARDSON, A.T. “Upper Stillwater Dam: design and construction concepts”.
Concrete International. ACI, Chicago, May 1984.
4. MIXTURE PROPORTIONS
4.1. GENERAL
RCC is generally designed as a “strength concrete” and as such is subject to the same design approach as all concrete, in terms of maximum allowable water/cement ratio and minimum allowable cementitious materials content to meet durability and strength requirements. As a dam concrete, however, RCC is rarely designed for compressive strength, although this is typically the parameter used for hardened concrete quality control, and permeability, tensile and shear strength between layers and elastic modulus are realistically the most important target properties for the hardened concrete.
The approach to establishing the cementitious materials content of each RCC type varies quite significantly, sometimes depending on the local availability of particular materials and the related proportioning of the cementitious materials in the RCC mix development process is accordingly not addressed in detail in this chapter.
RCC mix composition has traditionally been divided into three categories based on the content of cementitious materials (Portland cement and supplementary cementitious materials), with 100 kg/m3 representing the transition between low-cementitious (LCRCC) and medium-cementitious (MCRCC) and 150 kg/m3 representing the limit above which RCC is categorised as high-cementitious (HCRCC).
As discussed in Section 1.8, the RCD method, as used in Japan, is a medium-cementitious RCC approach.
With regard to dam design, two primary approaches are presently considered, depending whether the RCC mass forms the impermeable structure or whether there is a supplementary upstream impermeable element of some form. The mixture proportions of the RCC and other mixes used in the dam body will differ depending upon which approach is followed.
Logically, high-cementitious RCC mixes (HCRCC) have been designed as an impermeable structure and low-cementitious (LCRCC) with the requirement for a separate upstream impermeable element. However, over recent years there has been an increasing trend towards designing MCRCC, and in some cases LCRCC, for impermeability. Designing LCRCC for impermeability requires the inclusion of bedding mixes between layers.
Medium-cementitious RCC mixes (MCRCC) and RCD mixes have been frequently designed with an upstream impermeable element, even though the RCD method (Japanese practice) does typically produce good in-situ impermeability. Recently, MCRCC mixes have been designed for impermeability through the incorporation of increased quantities of non-plastic fines in the fine aggregate, although this approach requires considerable expertise in the design of the RCC. Given this expertise, this opens the possibility, in certain circumstances, of creating an impermeable structure with a MCRCC mix, or, a combination of HCRCC and MCRCC mixes in the same dam.
Probably the most important development in RCC mix design in recent years is the introduction of high-workability, super-retarded RCC. This is a variation of a HCRCC mix that combines very low VeBe times (8 to 12 seconds, or even less) with a very long initial setting time (circa 20 to 24 hours).
The design features of these mixes and their implications in the design and construction of RCC dams are described in the following sections, as well as in other chapters of this document.
Typically, average air void contents of 1% are applicable for modern high-workability RCC mixes (see Section 4.10.5). Testing of other RCC types, with different aggregates and cementitious materials contents has demonstrated that strength is essentially unaffected for compacted densities above 96%
of the theoretical air-free density (t.a.f.d.) (López & Schrader, 2012).
4.2. RCC CONSISTENCY – LOADED VEBE TEST
RCC is a mass concrete with zero slump consistency. The Loaded VeBe procedure has been used successfully to date to measure the consistency of most RCC mixes. However, some early mixes, with very low workability, were found to fall outside of the working range of the test.
The vibration time defined as the Loaded VeBe time of the mix should be established accurately.
This is especially important for mixes with high-workability, i.e. low Loaded VeBe times. These mixes usually have a specified range of consistency that is narrower than applicable for medium- and low- workability mixes.
The Loaded VeBe test uses two different total masses; 22.7 kg (ASTM C1170 Procedure A) for Loaded VeBe times greater than 20 seconds, and 12.5 kg (Procedure B) for Loaded VeBe times less than 30 seconds. It should be noted that the total mass includes that of the surcharge and the plunger.
The correct ASTM VeBe procedure reflects the time to achieve the development of a mortar ring around the full circumference of the acrylic plate. In view of the fact that lower paste and lower mortar-content RCC mixes do not necessarily provide consistent and reasonable times when tested using the correct VeBe procedure, however, different technicians sometimes report different times. In some instances, times are recorded for the appearance of paste beneath the entire area of the acrylic plate and in others for the appearance of paste beneath the majority of the area of the acrylic plate (allowing for small areas where nested stones prevent paste reaching the plate). In some cases, all three times are reported.
The trend in decreasing Loaded VeBe time over the past 30 years has resulted in a trend from Procedure A to B. Alternative procedures to ASTM Procedure B have been used specifically with high- workability mixes (i.e. Loaded Vebe times between 8-12 s). The modifications consist on (1) filling the mould completely instead of using a fixed weight of the concrete sample and (2) avoiding any pre- compaction of the sample prior to starting the vibration by placing the disc on the concrete surface at the same time than the vibration starts. The times cited in this chapter refer to the correct ASTM procedure and the alternative procedure for high-workability mixes.
4.3. TYPICAL CEMENTITIOUS MATERIALS MIXTURE PROPORTIONS
As mentioned in Section 1.8, when referring to the paste of an RCC mix it is important to distinguish between "Total Paste" and "Cementitious Paste". The difference between the two is the inclusion or not of the aggregate fines with a size of less than 75 microns that may, or may not, have some cementitious or pozzolanic benefit. The following table is an update of average mixture proportions of the cementitious paste of the various forms of RCC dams (Dunstan, 2015).
Table 4.1
Average mixture proportions (kg/m3) of the cementitious paste of the various RCC types
Material RCC type
LCRCC MCRCC RCD HCRCC
Portland Cement [C] 72 80 87 87
Supplementary cementitious
material [SCM] 9 37 35 108
Water [W] 122 116 96 111
Parameter
Cementitious materials [CM]=[C+SCM] 81 117 122 195
Water/ cementitious ratio [W/C]=[W/CM] 1.51 0.99 0.79 0.57
It can be seen from Table 4.1 that the average Portland Cement content and the water content of the different design philosophies does not vary significantly. The differences in the total cementitious contents are mainly due to the differences in the amount of supplementary cementitious materials.
However, caution is needed with this simplification because fines can have some degree of cementing benefit, in which case they would also be considered as a partial SCM. It may seem surprising that the average water content of an HCRCC mix can be lower than that of an LCRCC mix despite its much higher workability. This is a result of the fact that the higher content of supplementary cementitious material enhances workability and the higher attention paid to improve the quality of the coarse and fine aggregates.
4.4. DEVELOPMENTS IN MIX DESIGN
A common development for all types of RCC mixes and dam concepts is an increase in workability of the fresh concrete. In the past, the range of the workability, as measured using the Loaded VeBe test, generally ranged from 10 to more than 30 seconds across the spectrum from HCRCC to LCRCC. However, it is now usual to work with Loaded VeBe times in LCRCC mixes lower than 30 seconds, while typical times for HCRCC mixes lie between 8 to 15 seconds (8 to 12 seconds for high- workability RCC). This development allows greater segregation control, ensuring that all the aggregates are held within a matrix of paste whilst also improving the consolidation of the aggregate skeletal structure under compaction. In some cases, segregation of the RCC has caused relatively poor in-situ quality, i.e. low density, low strength, high permeability and low durability. This situation is typically more problematic in LCRCC than HCRCC.
Improved workability and reduced segregation are achieved through a reduction of the maximum size of aggregate, the use of crushed materials, improved aggregate shaping and grading and an increased volume of paste in the mix. The slight theoretical increase in the cementitious content typically necessary to maintain the same strength in a CVC with smaller MSA is minimal in RCC and usually more than offset by the benefits of reduced segregation, less wear on mixing equipment and better interfaces between layers. The use of a large proportion of fines (<75 microns) in the RCC aggregates compared with CVC aggregates is a common feature not only of LCRCC mixes but also now frequently in MCRCC and HCRCC. Non-plastic fines can play a major role in the reduction of the voids in the compacted fine aggregate of modern HCRCC and in the increased volume of total paste of HCRCC and MCRCC mixes.