Scielo RSS <![CDATA[Journal of the South African Institution of Civil Engineering]]> vol. 56 num. 2 lang. en <![CDATA[SciELO Logo]]> <![CDATA[<b>A CPM-based scheduling method for construction projects with fuzzy sets and fuzzy operations</b>]]> ABSTRACT The Critical Path Method (CPM), which is used to schedule construction activities that depend on one another through network relationships, is deterministic with regard to the duration assigned to the execution of the activities and the results produced in certain values. Unfortunately, construction activities are performed under uncertain conditions. Project risks cause variations in activity duration, and in turn the entire network is affected by uncertainty. In this context, activity duration can be represented by fuzzy sets, and CPM network calculations can be performed by fuzzy operations through a method developed in this study. Since the duration of activities is represented by fuzzy sets, and network calculations can be performed by fuzzy operations, the activity early/late start/finish times and the project completion time are calculated as fuzzy sets by the proposed method. An example CPM application with fuzzy sets is also presented in the paper. The findings show that CPM is applicable with fuzzy sets, and the developed method operates well for modelling the uncertainty in CPM calculations. <![CDATA[<b>Uncertainty evaluation with fuzzy schedule risk analysis model in activity networks of construction projects</b>]]> ABSTRACT Construction projects are prone to uncertainty due to various risk factors, such as unexpected weather conditions and soil properties. Depending on this, the actual duration of activities frequently deviates from the estimated duration time in either favourable or adverse directions. For this reason, evaluation of uncertainty is required to make the correct decisions when managing construction project network schedules. In this regard, this paper presents a new computer-aided schedule risk analysis model - the Fuzzy Schedule Risk Analysis Model - to evaluate uncertain construction activity networks when activity duration and risk factors are correlated. The proposed model utilises Monte Carlo Simulation and a fuzzified Critical Path Method procedure conducted by fuzzy sets and fuzzy operations. The paper also includes an example application of the model to a housing project. The findings of this application show that the model operates well and produces realistic results in capturing correlation indirectly between activity durations and risk factors regarding the extent of uncertainty inherent in the schedule. <![CDATA[<b>Effect of corrosion products on bond strength and flexural behaviour of reinforced concrete slabs</b>]]> ABSTRACT New performance-based design codes are currently being developed, where the design life of reinforced concrete structures will be estimated by taking into account not only the time to initiation of reinforcement corrosion, but also the time it would take for the extent of corrosion to reach a level where the structure is no longer fit for purpose. It is therefore important to establish what level of corrosion, if any, can be permitted before the structural behaviour of the reinforced concrete member is affected. In this paper the effect of corrosion products on the bond strength and flexural behaviour of reinforced concrete slabs was investigated. Pull-out tests confirmed that low levels of corrosion (less than 2% loss in steel cross-sectional area) can result in improved bond between the reinforcing bars and the concrete. At higher corrosion levels the empirical bond decay functions proposed by various researchers accurately predict the bond strength. The flexural behaviour of the slabs is affected by the reduced bond between the steel and the concrete, and this manifests during the load tests in a reduction in the number of cracks but an increase in crack width, with increased corrosion levels. At high corrosion levels (more than 8% loss in steel cross-sectional area) the flexural behaviour of the slabs is affected to such an extent that brittle failure occurs. <![CDATA[<b>Evaluation of rock types for concrete aggregate suitability for the construction of a gravimeter vault and access road at the Matjiesfontein Geodesy Observatory site near Matjiesfontein, South Africa</b>]]> ABSTRACT Often the use of local rock material for construction purposes is overlooked in engineering, which could reduce costs in terms of procurement and transport, as well as being more environmentally friendly by minimising the introduction of foreign materials. The rock materials at and around the site of a rock vault for the Matjiesfontein Geodesy and Earth Observation Observatory (MGO) were tested to determine if local materials may be used for construction purposes. In order to do this, strength and durability tests were conducted according to several South African National Standards (SANS). Slake durability was also tested where the potential of certain rock types, such as tillite of the Dwyka formation and shale of the Karoo Supergroup, typically slake/disintegrate into long angular fragments. This angular fragmentation during crushing of rock into aggregate for concrete also affects the workability of concrete in a fresh state. Concrete design using crushed local rock material was conducted to analyse performance and to establish a mix design that would be sustainable throughout the lifetime of the project. Tillite of the Dwyka formation was found to satisfy all test prerequisites best with minimal slaking due to the arid conditions at Matjiesfontein. Quartzite (Table Mountain Group) is very durable, but further testing for alkali-silicate reactivity is needed. All concrete cube tests accomplished the 40 MPa cube stren,gth, which was the design requirement, but problems arose in the workability of the concrete when river sand from nearby non-perennial rivers was used as fine aggregate in the concrete. This relates to too many particles of the same size within the sand. <![CDATA[<b>Resistance model uncertainty factors for ultimate limit state design of pile foundations</b>]]> ABSTRACT The current limit state design procedure for pile foundations presented in geotechnical codes (e.g. SANS 10160-5, EN 1997-1) stipulates that, when the pile capacity is determined using an analytical approach, such as the static analysis using engineering properties of the soil as determined from laboratory or in-situ field testing, the prescribed partial resistance factors (γr) need to be corrected by a partial factor for the uncertainty in the resistance model (γR, d) or the resistance model uncertainty factor. The international position is to derive model uncertainty factors for both actions and resistances from available experimental data. Accordingly this paper makes use of a local pile load test database to derive the appropriate γR, d values for ultimate limit state design of pile foundations. The analysis indicates that γR, d values of 1.3 and 1.5 for piles in cohesive and non-cohesive materials respectively are appropriate. Alternatively, a single value of γR, d = 1.4 for all piles in all soils can be adopted with different γr values for the two distinctive pile classes of piles in cohesive and non-cohesive soils. <![CDATA[<b>Buckling of short cold-formed lipped channels in compression</b>]]> ABSTRACT This paper presents an experimental investigation of short cold-formed lipped channel columns compressed between pinned ends. The short columns are subjected to pure axial compressive loading. Twelve column specimens are tested and the columns are categorised into three groups, depending on the length and thickness. The buckling modes of failure that occurred include local buckling and distortional buckling. A comparison of the experimental results with the loads predicted by the South African standard for the design of cold-formed steelwork (SANS 10162-2) shows that the code is not conservative enough to cater for these columns. <![CDATA[<b>The issue of personal safety on dolomite: A probability-based evaluation with respect to two- and three-storey residential units</b>]]> ABSTRACT While single-storey (single-house) residential developments were considered in a previous paper, two- and three-storey (multiple house) residential developments, which result in significantly higher densities of land occupation, are considered in this paper. The overall probability of injury for the occupants of two- and three-storey dwelling units is defined as co-dependent probabilities of sinkhole occurrence, coincidence of the sinkhole with a dwelling unit, structural collapse of the dwelling unit, occupancy of the dwelling unit, occupants in residence when the sinkhole occurs and fatal injury as a result of the event. The probability of sinkhole occurrence is determined by the associated infiltration regime for residential development, and the geological ground profile. The probability of coincidence between a sinkhole and a dwelling unit is treated in terms of overlapping geometric shapes. The probabilities for the remaining events are subjectively assigned by engineering judgement. The resulting overall probability of injury enables the number of dwelling units and the associated population densities for each of the Inherent Hazard Classes to be determined. It is found that the allowable population densities for two- and three-storey residential units amount to 890, 425, 170, 125, 40, 0, 0 and 0 people per hectare respectively for the eight Inherent Hazard Classes of dolomite land. This corresponds in principle with the allowable population densities for single-storey dwelling houses of 800, 400, 150, 100, 30, 0, 0 and 0 people per hectare respectively for the eight Inherent Hazard Classes. <![CDATA[<b>The issue of personal safety on dolomite: A probability-based evaluation with respect to transient passage in a city centre</b>]]> ABSTRACT For the past fifty years empirical knowledge guided the development of rules regarding population density on dolomite land. The insatiable demand for land, the improvement in transportation infrastructure and the associated need for improving the functionality of towns require that these rules on the risk of personal injury and damage to assets are revisited from a more scientific perspective. Probability theory provides a basis for decision-making in this regard. SANS 1936:2012 defines development densities for different types of land usage, including non-residential improvement, high- and low-rise buildings and single-storey dwelling houses. The paper is devoted to public safety along the roads, thoroughfares and open spaces outside buildings in a heavily populated city centre as a study in "transient density" on dolomite land. People are transported through the city in a range of vehicles. Some people walk through the city and some appear in particular locations as dispersed groups. The densities at which people appear differ during peak hours, other times of the day, and at night. The overall probability of fatal injury is determined by the mutually dependent probabilities of sinkhole occurrence, appearance of the sinkhole in a particular location, appearance of the sinkhole at a particular time, coincidence with the vehicle, people being unaware of the sinkhole, people falling into the sinkhole, people not being protected by the vehicle and the relative number of fatal injuries Sinkholes are invariably caused by water-bearing services that tend to leak at isolated locations, as a result of which only one sinkhole occurs at a time in a particular stretch of land. In developed land the leaky service and the sinkhole are generally repaired soon after the sinkhole has occurred, which precludes the recurrence of sinkholes in that area for a very long period of time. The probability of sinkhole occurrence can therefore be evaluated on the basis of the binomial distribution. The infiltration regime that determines the sinkhole return period for this purpose is based on the water and wastewater reticulation infrastructure, stormwater control measures, landscaping and irrigation provisions, occurrence of impermeable pavements and dewatering protocols characteristic of a business district in a city centre. It is shown that the probability of potential fatal injury during peak time is larger than an internationally prescribed threshold value for Inherent Hazard Classes 6, 7 and 8 for minibus taxis, buses and pedestrians at road intersections for sinkholes 10-20 m in diameter. These unacceptable cases may be resolved by marginally changing the values for some of the input probabilities that may be somewhat conservative. Alternatively, the adopted threshold level for tolerable risk could be relaxed from "As Low As Reasonably Practical" to "Slight", which may more accurately represent the fait accompli sense of risk in the brownfields situation in Centurion City. A further way to view the unacceptable cases is that they are largely compatible with the prescribed land usages in SANS 1936:2012, in that precautionary measures corresponding to area designations D3 + FP1, D3 + DL1 or D4 are required for all but Inherent Hazard Class 1. These requirements are fully justified for Inherent Hazard Classes 6, 7 and 8, may be somewhat conservative for Inherent Hazard Classes 4 and 5, and are quite likely too conservative for Inherent Hazard Classes 2 and 3 in the open spaces in a city centre environment. A fourth way of dealing with the unacceptable cases in a greenfields situation is to implement engineering designs to pavement structures that would mitigate the hazard. <![CDATA[<b>The issue of personal safety on dolomite: Update of a probability-based evaluation with respect to single-storey residential houses</b>]]> ABSTRACT Sinkholes may be treated as a chance phenomenon of which the likelihood of occurrence is a key parameter in determining their effect on public safety in the development of dolomite land. However, determining the likelihood of the occurrence of sinkholes remains a complex if not evasive problem in view of the sparsely populated datasets available at present. Three approaches to determining the likelihood of sinkhole occurrence that were developed over a period of time are reviewed and comparatively evaluated. The geological time-based return periods postulated for a residential infiltration regime in the third approach is applied to determine the maximum allowable population densities for a single-storey residential house development as an update of a previous paper on the problem. The maximum population densities based on this updated approach were found to be one and a half to three times larger than those previously considered to be allowable for Inherent Hazard Classes 1, 2 and 3. This relaxation on the historically established empirical rules on population density is considered by the authors to better represent observations in recent years. <![CDATA[<b>The diversification strategies of large South African contractors into southern Africa</b>]]> ABSTRACT This article presents the results of a study into the current diversification strategies of large South African contractors and the reasons for choosing southern Africa as a diversification strategy. It investigated the major risks involved in doing business in southern Africa, which of the southern African countries are more favourable to diversify into and the impact of diversification on performance. It also attempted to identify the major competitors in southern Africa and the source of their competitive advantage. The study consists of a literature review and utilised the data from the annual reports for the period 2007 to 2011 of nine selected large South African contractors, as well as relevant publicised news articles. All the data used in this study is already in the public domain. The research found that the countries of southern Africa present significant opportunities for South African construction companies. Southern Africa forms a definite part of the diversification strategies of large South African contractors; they are, however, mostly moderately diversified, and the 'big' players are highly diversified both operationally and geographically. The extent of diversification has an impact on performance, as undiversified is clearly associated with underperformance, and moderately diversified within a specialised field with outperformance. The research was inconclusive on the identities of the major competitors in southern Africa and the source of their competitive advantage. It was, however, found through the literature study that Chinese contractors are the major competitive force in southern Africa and in the rest of Africa, and their governmental support systems on policy and financial backing are the source of their competitive advantage. <![CDATA[<b>Mechanical properties of self-compacting concrete with binary and ternary cementitious blends of metakaolin and fly ash</b>]]> The main advantage of using ternary-blended concrete is to eliminate the drawbacks of the particular supplementary cementitious materials through combining with other superior quality material in order to reduce the overall cost of concrete production. It is also useful to enhance the structural properties of concrete. In this paper, fresh state and mechanical properties of self-compacting concrete (SCC) with binary and ternary cementitious blends of metakaolin (MK) and fly ash (FA) were evaluated and their inter-relationships discussed. For this purpose, different mixtures were prepared with different amounts of MK and FA by substituting ordinary Portland cement (OPC) by 5% to 40% of MK or FA. As a result of increasing the percentage of MK, FA and MK+FA, the mechanical properties of SCC improved considerably. It was found that the specimen incorporating the ternary blend of cement with 15% MK and 15% FA showed better workability and mechanical properties than that of the normal SCC specimen without MK or FA. This blend was proved to be the optimum combination for achieving maximum synergic effect.