Factors of importance in relation to RMC behaviour and performance

4.1.1 Seepage

RMC dams seep, rather than leak, before sealing themselves and becoming watertight. In this process of sealing, efflorescence and calcite are deposited in layers on the downstream face and the white streaking effect detracts from the finish of the wall. However, as the seepage diminishes, the calcite deposits dry, weather and discolour to a greenish brown, depending on the dampness and extent of attached organic growth. When particularly pure, aggressive water is impounded, self-sealing, may not occur as effectively and careful consideration should be given to appropriate section dimensions and indeed even the appropriateness of RMC in such environments.

4.1.2 Cracking

General observation indicates that RMC does not crack, or does not exhibit drying shrinkage and thermal cracking in the same manner as conventional concrete. While this property is not widely understood and cannot currently be satisfactorily fully justified on engineering principles, reliance in design is placed on the no-cracking performance of RMC. In an effort to derive an explanation for this behaviour, several hypotheses can be evaluated.

These hypotheses include the following:

• Mortar drying shrinkage is of little influence, as continuous contact between rock particles will prevent related shrinkage of the body as a whole,

• Differential stresses established during hydration and cooling are minimal, as structural elements are thin and the pace of construction is slow,

• Micro-cracking may well occur within the mortar matrix in RMC, but the large random and interlocking stone particles prevent related continuity causing single, large cracks,

• On a macro scale, the large stone particles and related interlocking produces a material of relatively high tensile strength.

Experience has shown that high mortar content RMC can crack and will undoubtedly exhibit less favourable behaviour than will a high rock content matrix.6 _{Whilst conventional concretes are proportioned to ensure that all}

surfaces of the coarse aggregate particles are covered in mortar and paste, high rock content RMC comprises a matrix of large rock particles in contact and surrounded and in-filled by mortar. In compression, it is likely that the rock properties will play a dominant role, whilst in tension the mortar and the interlocking of rock particles are likely to be more significant.

It is believed that a sensible evaluation of cracking in RMC, on the basis of the current level of material knowledge, might be summarised as follows:

South African experience has confirmed the general behaviour patterns of RMC that form the basis of the empirical design approach applied for dams in Zimbabwe. This section presents the particular performance characteristics exhibited by RMC in dams, with a spe- cific focus on the aspects in which the behaviour of RMC differs from that of conventional mass concrete.

Performance characteristics of RMC

6_{This type of cracking is normally related to drying}

shrinkage and thus the water requirement of sand. High cement content mortar may cause larger cracks at larger intervals than a leaner mortar which will have smaller width cracks at closer intervals.

Due to its general proportions, concrete for dams does not usually exhibit drying shrinkage in the same manner as smaller concrete members. Observed

tallest cantilevers, but as is the case with other dam types, this cracking is not visible as it is only present well beneath the water level,

• Without sophisticated instrumentation and related measurement, the observance of structural deformation is not really possible, particularly on the small scale of the dams in question, and it is suggested that fairly large deformations allow the re-distribution of stresses reasonably effectively. The use of materials of a lower elastic modulus is obviously significant in this regard.

4.1.3 Research into the structural behaviour of RMC under load

Structural and materials research into RMC in Southern Africa is currently rather sparse and has to date largely been pursued by Rankine(1,2,3 and 4)_{. While the}

experimentation undertaken has addressed dams peripherally, the central focus has been the application of RMC in the construction of small bridges. Furthermore, the scale of the components comprising the RMC mix, with rock particles as large as 60 kg, makes testing rather difficult and expensive, requiring specialist, large-scale equipment. Within the scale of the industry related to RMC and the works constructed in this material, it is difficult to motivate the necessary, but expensive, research required to develop a comprehensive understanding of the material.

The most recent research, however, has focussed specifically on the isolation of the mechanical properties of RMC, with particular emphasis on the relevant aspects for dam engineering. Fine and Rankine(1,2 and 4)_{have used 500 mm cubes}

to test, evaluate and compare the properties of RMC, primarily in terms of crushing strength and deformation modulus. In this work it was demonstrated that the deformation modulus of RMC is largely determined by the elastic modulus, shape and orientation of the rock inclusions. With rock moduli varying from 24 to 120GPa, a range of RMC deformation modulus of 5 to 71GPa was demonstrated for a constant mortar deformation modulus of 18GPa. The particle orientation of large rock inclusions was demonstrated to be of significant impact on the modulus for RMC comprising all rock types, with particularly detrimental effects being observed for low moduli, elongated particles orientated parallel to the loading direction. A further important aspect that was determined through testing and modelling was the respective role of particle size and inter-particle contact within the mix. If the rock inclusions are in

intimate contact, the overall modulus will be higher for a large number of small particles than for a few large particles, with the latter scenario developing significantly higher local stresses in transferring stress through the matrix. If, however, all particles are surrounded by mortar, the stiffness modulus is

Cracking of RMC can, however, occur and care should be taken to avoid typ- ical situations that can induce high tensile stresses, such as:

• large rock particles randomly inserted into a high cement content, and/or wet mortar,

• an excessively high mortar / rock ratio, with negligible rock contact and interlocking of particles,

• construction at the warmest time of the year, particularly in an extreme climate,

• straight sections of wall with a significant length to height ratio.

Avoid inducing high tensile stresses

The findings of the RMC research completed to date have been presented in a largely factual format, with very little analysis and review of associated causative mechanisms, or related structural implications. It is, however, pos-

Key aspects may be summarised as follows:

Particle orientation and mortar separation

In dam engineering, placement of RMC in horizontal layers tends to ensure that any particle to particle contact is predominantly vertical and that the longer dimension of particles is orientated vertically. This suggests that RMC in dams is likely to exhibit a higher deformation modulus in the vertical direction than in the horizontal direction. In an arch dam this can be seen to be of some advantage, as cantilevers become stiffer, creating lower overall displacement and arch action allows more yielding, which lowers overall maximum stresses. In the arch structures typically constructed in RMC, the largest compressive stresses are furthermore orientated in a predominantly horizontal direction, suiting the better performance axis of the RMC. While no related testing has yet been completed, it is quite likely that such particle orientation will also give rise to greater tensile strength in a vertical direction, where this is required.

The improved performance of RMC when the rock particles are evenly distributed and evenly separated by mortar of 7 mm to 20 mm thickness tends to reflect the importance of mortar workability and construction technique on the final structural performance characteristics and these must be given careful consideration in the compilation of construction specifications.

Structural mechanisms and Poisson’s ratio

The research of Fine and Rankine(4)_{suggested the possible existence of a number}

of structural mechanisms within the 500 mm test cubes. These are illustrated in Figure 2 and each of the mechanisms postulated serve to suggest the possible modes of cube failure and explain why some cubes might exhibit particularly high poisson’s ratios.

The mechanisms suggested are likely to be particularly influenced by scale effects. When the size of the stone particles is smaller in relation to the member under stress, it is very likely that the influence of the identified mechanisms will be significantly reduced. Furthermore, some measure of three dimensional restraint will also assist in distributing stresses more evenly through any member that is relatively massive in relation to its constituent components.