Simple criteria for evaluating sulphate attack in concrete

This paper attempts to analyse results from the standard test methods employed for sulphate attack and evaluates their correlations, consistency and contrasts, as well as physical observations. Data from expansions and mass change of 25 x25 x 285 mm mortar prisms and 75 x 75 x 285 mm concrete prisms were used. Mortar mixtures consisted of 1: 2.25: 0.5 cement to sand to water while concrete mixtures were of water-cementitious ratio (w/cm) of 0.45, 0.50, 0.65. Mixtures were made using CEM I 42.5N with or without 30, 50, 70% ground granulated blast furnace slag (GGBS) and stored in sodium sulphate solutions of 28 g/L and 50 g/L as SO4. Results show that ASTM C 1012 mortar expansion criteria of 0.10% corresponds to 1.2% mass gain. Similarly, concrete prism expansion criteria of 0.05% is equivalent to 0.75% mass gain. It is proposed that in the absence of expansion monitoring, the use of mass gain criteria of 1.2% mass in mortar prisms or 0.75% in concrete prisms may be sufficient for evaluating sulphate attack

Cronbach’s alpha reliability coefficient in engineering assessments – a preliminary study on possibilities and precautions

Abstract: This paper attempts to apply the Cronbach’s alpha to engineering studies. There is hardly any available literature or research on application of this method to engineering course assessments. Alpha coefficient is commonly used in psychometric tests, as a measure of estimating internal consistency. The data used in this preliminary study consisted of five modules taught over five years by different instructors..

Potential South African standard sand for cement mortar testing and research

Mortars are used extensively in accelerated methods of testing the physical properties of cementitious materials. Mortar strength is invariably used to determine cement strength grade of cements and for general evaluation of material behavior, as typically employed in research and testing. Presently, the South African (SA) cement industry imports standard from Europe for cement testing purposes. This study was conducted to formulate South African standard sand that could be used to conduct mortar strength tests for compliance to specifications and for general research purposes. Two formulations of local South African silica sands, referenced as SA513S and SA413S, were developed to suit standard gradings. The formulated South African standard sands (SASS) where tested in comparison with European standard sand (ENSS) of EN196-1 and American graded standard sand (GSS) of ASTM C- 778. Following the formulation and grading of SASS, mortar mixtures of 1:3:0.5 cement to sand to water were cast in 50 mm cubes and tested for compressive strength for ages up to 90 days. Two types of cements were employed in the tests, the ordinary Portland cement CEM 1 42.5N and pozzolanic cement CEM V 32.5N. Comparison of the 28-day compressive strength results between the four SS types showed an excellent agreement, giving standard deviations of 1.96 and 2.51 for CEM I and CEM V. The flow results for the standard sand types varied, with EN196-1 giving the highest mortar flow followed by GSS sand and SA413S. The GSS and SA513S gave precisely the same flow results

Proposed method of evaluating the eligibility criteria for supplementary assessments

Abstract: During assessments, the results of an examination or test are assigned into a category of pass or fail. In engineering studies and most other disciplines, the pass/fail category is determined from grading of assessments, typically requiring allocation of a quantitative mark achieved by a candidate, in a given module or course program. Institutions have well-defined policies on mark criteria used to determine the pass/fail category. However, quite a challenging issue arises from the category of students whose performance falls within the pass/fail borderline, typically 40 to 51% mark category..

Correlation between formative and summative assessment results in engineering studies

Abstract: This paper attempts to study the relationship between results of formative and summative assessments. Generally, there is limited research work available in this subject, most especially relating to engineering education. The study is conducted using data of modules lectured to undergraduate civil engineering students over a period of 10 years. It consists of data sets for 409 students under nine (9) assessment events of various modules..

On capacity and quality issues in engineering studies across Sub-Saharan Africa : a review

Abstract: This paper highlights the challenges of 21st century engineering studies and practice relating to infrastructure growth in Sub-Saharan Africa (SSA). Tertiary education in SSA is experiencing explosive growth of 8.6% driven by the higher education institutions (HEI) of which the private sector accounts for 22% of enrolments. The unintended consequences have been the escalation of capacity deficits and quality problems in engineering studies. Capacity deficit for the SSA has also been exacerbated by brain drain and severely small pool of highly qualified academics. The causes of these issues are discussed while considering University as the centrepiece for development of appropriate remedial measures..

Permeation properties of recycled concretes containing various types of recovered aggregates

Abstract: This paper reports an experimental study conducted to asses durability characteristics of concretes made using different types of recycled aggregates. Different types of recycled coarse aggregates comprising natural dolomite (DOL), crushed concrete (RCA), recycled rubble (RUBL) and brick (BRIC) were studied using concrete mixtures of water-cement ratios (w/c) = 0.60 and 0.45 containing 100% of the recycled stone. The same type of crushed natural sand was used in all the mixtures. In addition to compressive strength and workability, water sorptivity and oxygen permeability (K) properties of the concretes were measured. It is found that the recycled aggregates showed higher porosity than the control crusher dolomite aggregate. Oxygen permeability results of the recycled aggregate concretes were of the same order of magnitude, giving about K = 2.0 x 10-10 m/s. Recycled brick aggregates produced concretes of consistently higher permeability and water sorptivity than the others. The oxygen permeability values for the various recycled aggregate concretes increased in the order kDOL < KRCA < KRUBL < KBRIC. Permeability of brick aggregate concrete was four times higher than that of control dolomite concrete. The relatively adverse influence of recycled brick on concrete properties is attributed to its highly porous characteristics

Effect of superplasticizers on concrete durability indexes

Use of durability index (DI) is one of the several approaches that have been used or advanced through various test methods for the purpose of evaluating the durability of concrete mixtures. In South Africa, use of DI's has developed through testing and research over the past years. A consortium of three tests of oxygen permeability index (OPI), water sorptivity and chloride conductivity are employed. This paper presents an investigation conducted to determine the effect that superplasticizers may have on durability indexes of concrete. Concrete mixtures of strengths ranging from 30 to 50 MPa were cast using two types of commercially available superplasticizers, herein referred to as GL and RSP. The admixtures were of different chemical bases with RSP being a sulphonated naphthalene superplasticizer while GL was a polycarboxylate ether superplasticizer. The tests conducted were compressive strength, oxygen permeability and water sorptivity. It was found that use of GL increased the compressive strength of concrete by about 10 MPa over the strengths of mixes made using RSP. Correspondingly, GL had an adverse effect on durability performance, as it decreased the OPI and increased the sorptivity indexes. The results implied the possible coarsening of the pore structure of the hardened concrete which could have resulted from the use of GL. Results also showed that the influence of chemical admixtures on hardened concrete properties and durability characteristics of high strength concretes may be less significant for high strength concretes

Application of a new natural carbonation prediction (NCP) model to evaluation of durability design factors - strength, cover, and cement type

Abstract : In this paper, durability design parameters that are typically specified in structural design codes, were evaluated for their impacts on service lifespan. These parameters comprising concrete strength, cover depth, and cement type, were used as input variables into the new natural carbonation prediction (NCP) model. The resulting changes on the predicted lifespans were then evaluated. It was found that 10 MPa increment in concrete strength increases the structures lifespan geometrically 2.5 to 5.0 times. Similarly, 10 mm increment in cover extends the service lifespan 2.0 to 2.5 times. Also CEM II concretes with 10 MPa higher strength over that of CEM I concretes, gave the same carbonation resistance as the latter. Understanding of the impacts of durability design parameters aided by practical service life models, contributes towards a rational approach for service life design of concrete structures

Effect of surface treatments on abrasion and permeation properties of clay concretes

The resistance of concrete surfaces to various types of wear can be a defining performance characteristic in certain applications. Some concrete structures are required to be abrasion-resistant; among them are dams, canals, roads, floors, footpaths, parking lots, and paths in parks. The work reported in this paper is based on a laboratory study of the concrete-surface treatments (CSTs) consisting of dry shake and screeding. Four control concrete mixtures were prepared with ordinary Portland cement (OPC), having a cement content (CC) of 350 kg/m3 at water-cementitious ratio (w/cc) = 0.70, 0.75; and 280 kg/m3 CC of w/cc = 0.80, 0.85. Other concrete mixtures were then prepared by substituting OPC in the control mixtures with 10, 20, 30, and 40% local raw clay. Compressive strength, abrasion resistance, and air permeability of the surface-treated concretes were measured at the ages of six months, while drying shrinkage was monitored for up to year. Drying shrinkage of concrete reduced with decrease in w/cc ratio and increased with increase in raw clay content. Among all the mixtures, the control concrete of w/cm ratio = 0.70 and the claycement concrete containing w/cm=0.80 at 70% OPC/30% raw clay showed the least and highest shrinkage increments respectively. Abrasion resistance of concrete, measured by the rotating-cutter method, increased with increasing clay content and decreasing compressive strength. However, abrasion performance results of clay-cement concrete mixtures with CSTs were generally higher. Air permeability of the concretes was measured and its relationship with the depth of abrasion of the concrete was examine