1,824 research outputs found
Estimating soil plasticity properties from pedological data
Abstract: A number of pedological soil classification systems have been developed worldwide. These include an internationally accepted system and various national systems, some of which have been incorporated into databases which include maps. Such information is used primarily for agricultural purposes. Various physical and chemical soil properties are used for classifying soils according to these pedological systems. This paper proposes an approach, based on a research project, which may be used to statistically significantly determine the plasticity characteristics of soils from the physical and chemical properties that are used to pedologically classify soils by systems, such as the South African Binomial System. These plasticity characteristics may be used to establish the engineering soil classification groups which may, in turn, be used as a means of rapidly determining the general suitability of areas for proposed developments, particularly during the preliminary stages of transportation route locations and township developments, with a resultant saving of time and money
The effect of site practices on the integrity of large diameter bored piles
In South Africa, concrete in large diameter bored piles is generally placed by discharging a high flow concrete mix directly from the truck mixer and allowing the concrete to fall freely to the base of the pile hole. While certain site practices have been used by piling contractors for years, many engineers are not convinced of their acceptability. This paper discusses the results of an investigation which assessed the effect of site practices on the integrity of cast in-situ bored piles. Such practices include the method of concrete placement and the amount of water and/or loose spoil in the pile hole at the time of casting. The results of this investigation dispel the myth that the free fall placement of concrete in clean, dry pile holes has a detrimental effect on the degree of compaction and compressive strength of the concrete
The influence of aggregate stiffness on the creep of concrete
Abstract: Creep is the time dependant increase in strain of a solid body under sustained stress. In concrete, the negative effects of creep are often responsible for excessive defl ection at service loads which can result in cracking, creep buckling of long columns and loss of prestressing force. While it is conceptually easy to appreciate that the stiffness of the aggregate in concrete will infl uence the magnitude of creep, the extent of this effect across the range of commonly used aggregates in South Africa has not been assessed. This paper discusses the results of an investigation that was aimed at quantifying the infl uence of aggregate stiffness on the measured creep behaviour of plain concrete. The experimental programme included measurements of total creep on concrete specimens of two different strength grades for each of three different but commonly used South African aggregate types (quartzite, granite and andesite). In addition, elastic modulus tests were conducted on cores of the aggregate types assessed. The test results revealed that no clear correlation exists between the creep of concrete and the stiffness..
An assessment of the accuracy of nine design models for predicting creep in concrete
Abstract: Creep of concrete is a complex phenomenon that has proven difficult to model. Nevertheless, for many reinforced and prestressed concrete applications, a reasonably accurate prediction of the magnitude and rate of creep strain is an important requirement of the design process. Although laboratory tests may be undertaken to determine the deformation properties of materials, these are time consuming, often expensive and generally not a practical option. In addition, this is not often an option at the design stage of a project when decisions about the actual concrete to be used have not yet been taken. National design codes therefore rely on empirical prediction models to estimate the magnitude and development of the creep strain. This paper considers the suitability of nine ‘design code type’ creep prediction models when compared with the actual strains measured on a range of concretes under laboratory control conditions. The concretes tested incorporate three aggregate types and two strength grades for each aggregate type. The results are compared with the predictions of creep using models contained in BS 8110 (1985), SABS 0100 (1992), SABS 0100 (1992) modified, ACI 209 (1992), AS 3600 (1988), CEB-FIP (1970, 1978 & 1990), the RILEM Model B3 (1995) methods. The results indicate that the CEB-FIP (1970) and BS 8110 (1985) methods provide suitably accurate predictions over all the concretes tested. These methods yielded overall coefficients of variation of approximately 18 % and 24 %, respectively. The least accurate method was the CEB-FIP (1978) which yielded a coefficient of variation of approximately 96 %. The results of this investigation led to recommending the BS 8110 (1985) model for South African conditions
The influence of slag fineness on the workability of cementitious pastes
Abstract: In South Africa the fineness of Ground Granulated Blastfurnace Slag (GGBS) has remained the same for nearly forty years. In 1999, producers of GGBS were considering the use of relatively finer ground GGBS with a view to improving the strength performance of concrete. The concomitant increase in compressive strength of concrete with an increase in GGBS fineness was confirmed in an extensive investigation carried out by Slagment (Pty) Ltd (Slagment, 1999). However, the effect of relative fineness of South African GGBS on the workability of concrete mixes had not been assessed. This paper discusses the results of an investigation that was aimed at quantifying the influence of finer ground GGBS on the workability of concrete, based on the work of Page (2001). Traditionally, GGBS was ground to a fineness of 3 600 cm2/g, GGBS (3600), and it was proposed to increase its fineness to 5000 cm2/g, GGBS (5 000). The experimental programme included four binder types, namely, CEM I 42,5 alone or in combination with either GGBS (3 600) or GGBS (5 000) or Fly Ash (FA). To eliminate the effects of aggregates on workability, only binder pastes were tested. The results revealed that, as expected, the binder containing the Fly Ash was the most workable. In general, the finer slag, GGBS (5 000), exhibited a higher viscosity and hence lower workability than the GGBS (3 600), due to its relatively higher surface area
CAST microbulk micromegas in the Canfranc Underground Laboratory
During the last taking data campaigns of the CAST experiment, the micromegas
detectors have achieved background levels of keVcms between 2 and 9 keV. This performance has
been possible thanks to the introduction of the microbulk technology, the
implementation of a shielding and the development of discrimination algorithms.
It has motivated new studies towards a deeper understanding of CAST detectors
background. One of the working lines includes the construction of a replica of
the set-up used in CAST by micromegas detectors and its installation in the
Canfranc Underground Laboratory. Thanks to the comparison between the
performance of the detectors underground and at surface, shielding upgrades,
etc, different contributions to the detectors background have been evaluated.
In particular, an upper limit keVcms
for the intrinsic background of the detector has been obtained. This work means
a first evaluation of the potential of the newest micromegas technology in an
underground laboratory, the most suitable environment for Rare Event Searches.Comment: 6 pages, 8 figures. To appear in the proceedings of the 2nd
International Conference on Technology and Instrumentation for Particle
Physics (TIPP 2011
The effects of placement conditions on the quality of concrete in large-diameter bored piles
Abstract: In South Africa, concrete in large-diameter bored piles is generally placed by discharging a highflow concrete mix directly from the truck mixer and allowing the concrete to fall freely to the base of the pile hole. While certain site practices have been used by piling contractors for years, many engineers are not convinced of their acceptability. In order to assess the effects of free-fall concrete placement, a series of tests were undertaken in which the properties of concrete placed in this manner were compared with the properties of conventionally placed concrete. The tests included an assessment of the effect of water and spoil in the pile hole at the time of casting, as well as poor placement techniques. The results of this investigation indicate that casting of concrete in 50 mm and 400 mm of water in the bottom of the pile hole significantly reduced the compressive strength by approximately 50% and 80%, respectively. Furthermore, the effect of spoil at the bottom of the pile hole was dependent on the amount of water present. Finally, a separate investigation, at a bridge site, indicated the free-fall placement technique to be at least as effective as the tremie technique
X-ray detection with Micromegas with background levels below 10 keVcms
Micromegas detectors are an optimum technological choice for the detection of
low energy x-rays. The low background techniques applied to these detectors
yielded remarkable background reductions over the years, being the CAST
experiment beneficiary of these developments. In this document we report on the
latest upgrades towards further background reductions and better understanding
of the detectors' response. The upgrades encompass the readout electronics, a
new detector design and the implementation of a more efficient cosmic muon veto
system. Background levels below 10keVcms have been
obtained at sea level for the first time, demonstrating the feasibility of the
expectations posed by IAXO, the next generation axion helioscope. Some results
obtained with a set of measurements conducted in the x-ray beam of the CAST
Detector Laboratory will be also presented and discussed
The Micromegas detector of the CAST experiment
A low background Micromegas detector has been operating in the CAST
experiment at CERN for the search of solar axions during the first phase of the
experiment (2002-2004). The detector, made out of low radioactivity materials,
operated efficiently and achieved a very low level of background rejection (5 x
10^-5 counts/keV/cm^2/s) without shielding.Comment: 13 pages, 12 figures and images, submitted to New Journal o
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