Optimum superplasticiser dosage and aggregate proportions for SCC

An experimental study has been carried out to investigate the effect of superplasticiser dosage on the performance of self-consolidating concrete (SCC). Seven concrete mixes with water/cement (w/c) ratios ranging from 0-35 to 0-45 and fine/total (F/T) aggregate ratios ranging from 0-40 to 0-60 were designed and 42 batches of concrete containing different dosages of superplasticiser were produced. The workability, passing and filling abilities, and segregation stability of the concrete produced were measured using the slump flow, U-box, and sieve segregation tests respectively. For each concrete mix designed, the superplasticiser dosages for maximum slump flow without segregation and maximum filling height were determined. It was found that the maximum performance of the concrete mix and the respective required superplasticiser dosage are dependent on the w/c and F/T aggregate ratios. In general, increasing the F/T aggregate ratio would improve the maximum performance but would also increase the superplasticiser dosage needed. Lastly, the robustness of each concrete mix was evaluated quantita-tively as the range of superplasticiser dosage or slump flow satisfying all requirements for SCC Based on these results, the optimum superplasticiser dosage for maximum robustness of each concrete mix was determined.published_or_final_versio

Adding steel fibres to improve shock vibration resistance of concrete

A newly developed shock vibration test method was employed to study the effects of shock vibration on steel-fibrereinforced concrete so as to explore the possibility of improving the shock vibration resistance of concrete. In total, 21 batches of concrete with steel fibre contents ranging from 0 to 4% were cast and subjected to the shock vibration test at ages of 12 h, 1 day and 7 days. The results revealed that the effectiveness of adding steel fibres to alleviate the short-term damage caused by shock vibration (in terms of immediate reduction in ultrasonic pulse velocity) was quite low, especially for shock vibration applied at an early age. However, the effectiveness of adding steel fibres to mitigate the long-term damage caused by shock vibration (in terms of reduction in 28-day direct tensile strength) turned out to be much higher for shock vibration applied at age within 1 day than at later age. One probable reason is that, even after vibration damage had been caused, the continuing development of the steel-concrete bond while the concrete was still young could restore part of the reduced tensile strength. Finally, a new set of shock vibration control limits for steel fibre reinforced concrete was established.published_or_final_versio

Chapter Development of Sustainable High-Strength Self-Consolidating Concrete Utilising Fly Ash, Shale Ash and Microsilica

With high flowability and passing ability, self-consolidating concrete (SCC) does not require compaction during casting and can improve constructability. The favourable properties of SCC have enabled its widespread adoption in many parts of the world. However, there are two major issues associated with the SCC mixes commonly used in practice. First, the cement content is usually at the high side. Since the production of cement involves calcination at high temperature and is an energy-intensive process, the high cement content imparts high embodied energy and carbon footprint to the SCC mixes. Besides, the exothermic reaction of cement hydration would cause high heat generation and early thermal cracking problem that would impair structural integrity and necessitate repair. Second, the strength is usually limited to around grade 60, which is considered as medium strength in nowadays achievable norm. With a view to develop sustainable high-strength self-consolidating concrete (HS-SCC), experimental research utilising fly ash (FA), shale ash (SA), and microsilica (MS) in the production of SCC has been conducted, as reported herein

Grade 80-100 self-consolidating concrete for Hong Kong

Compared to normal concrete mixes, high-strength concrete mixes are more cohesive and sticky, and would in general require a larger slump for proper consolidation. On the other hand, apart from high strength, high workability is also desired because it would allow faster and quieter concreting operation and reduce the risk of incomplete consolidation. However, it is not easy to achieve both high strength, which demands a low water/binder ratio, and high workability, which demands a high water content, without increasing the binder paste volume, which may lead to thermal and shrinkage cracking problems. In this study, the feasibility of producing high-strength, self-consolidating concrete with a binder paste volume of not more than 35% in Hong Kong was investigated by trial concrete mixing. A new polycarboxylate-based superplasticiser was used. In total, 18 trial mixes have been cast. All of them have slump higher than 200 mm and 28-day cube strength ranging from 80 to 117 M Pa. Some of them are suitable for making grade 80-100, self-consolidating concrete in Hong Kong.link_to_subscribed_fulltex

Heat loss compensation in semi-adiabatic curing test of concrete

To alleviate early thermal cracking of concrete, a maximum limit is often imposed on the adiabatic temperature rise, which has to be measured by an adiabatic curing test using a specially designed curing chamber. In practice, for convenience and simplicity, the curing tests conducted on site are not truly adiabatic and thus should be regarded as semi-adiabatic. In a semi-adiabatic curing test, due to heat loss, the measured temperature rise is lower than .the adiabatic temperature rise by an error dependent on the specimen size and the heat insulation provided. Herein, to minimize such error, a heat loss compensation method of estimating the heat loss from the spatial and time variations of the measured temperature and compensating for the heat loss, accordingly, is proposed. Ten laboratory-made concrete mixtures were tested to verify the applicability of the method. In addition, two batches of ready mixed concrete, each cast into 0.5 and 1.0 m (1.64 and 3.28 ft) cubes, were tested to demonstrate how a semi-adiabatic curing test could become independent of specimen size and much less affected by imperfect insulation after heat loss compensation. With the proposed heat loss compensation applied, the errors in the adiabatic temperature rise were all within ±1.3 °C (±2.3 °F). Copyright © 2008, American Concrete Institute. All rights reserved,.link_to_subscribed_fulltex

Adiabatic temperature rise of incompletely hydrated cement concrete

The temperature rise of concrete during hardening is intimately related to the mix proportion, among which the cement content is a major factor. However, high-strength concrete mixes are often proportioned with low water contents which leads to incomplete hydration of cement contained therein. Hence, the conventional rule of determining concrete temperature rise solely based on the cement content may not yield accurate estimations. An experimental program has been launched to investigate the coupled effects of cement and water contents on the adiabatic temperature rise of concrete. Eighteen concrete mixes were tested with a newly developed semi-adiabatic curing test method and their adiabatic temperature rise obtained by applying heat loss compensation to the test results. The results revealed that, when the water/cement ratio is lower than 0.36, both cement and water contents have effects on the adiabatic temperature rise of concrete. Prediction formula and design chart of adiabatic temperature rise, which are accurate to ±1.3° compared with the test results, are developed. Furthermore, prediction formula of the degree of hydration of concrete is recommended.link_to_subscribed_fulltex

Performance criteria for self-consolidating concrete

Self-consolidating concrete (SCC), also called self-compacting concrete, was first developed almost two decades ago. It does not require compaction to achieve good consolidation and is becoming more and more commonly used in many parts of the world. However, SCC is not the same as high-workability concrete; a SCC is a high-workability concrete, but a high-workability concrete is not necessarily a SCC. For a high-workability concrete to be qualified as a SCC, it has to be able to flow through narrow gaps and fill up far-reaching corners in the mould under its own weight, pass through closely spaced reinforcing bars, and remain homogeneous after flowing or dropping a certain distance or through obstacles. In other words, a SCC should have, apart from high workability, also high filling ability, passing ability and segregation stability. This paper introduces some new test methods for measuring the performance of SCC, recommends the performance criteria that may be adopted for different grades of SCC, and presents a laboratory study on the production of SCC using the locally available materials.link_to_subscribed_fulltex

Effects of cement and water contents on adiabatic temperature rise of concrete

The temperature rise of concrete during curing is dependent on the mixture composition and proportions, among which the cement content should be the major factor. If the water content is relatively low, however, the cement would not fully hydrate and the water content should also have significant effects. This paper reports an experimental program aiming to investigate the combined effects of cement and water contents by measuring the adiabatic temperature rise of concrete mixtures with different mixture proportions. A newly developed semi-adiabatic curing test method with heat loss compensation applied was employed to measure the adiabatic temperature rise of the concrete mixtures. The results reveal that, for a given paste volume, the adiabatic temperature rise would be highest at a water-cement ratio (w/c) of 0.36; therefore, the adiabatic temperature rise of a higher strength concrete is not necessarily higher. Furthermore, the degree of hydration would gradually decrease as the w/c decreases. Based on these results, design charts and formulas for predicting the adiabatic temperature rise, heat generation, and degree of hydration of concrete mixtures with w/c ranging from 0.28 to 0.48 have been developed. Copyright © 2009, American Concrete Institute. All rights reserved.link_to_subscribed_fulltex

Rheology of mortar and its influences on performance of self-consolidating concrete

A two-part experimental program is presented in this paper. In the first part, four self-consolidating mortar mixes were designed with different compositions of cementitious materials including cement, pulverized fuel ash and condensed silica fume. For each SCM mix composition, the superplasticizer dosage was varied and a total of 30 batches of mortar were produced. For every batch of mortar, the rheology was determined by a rheometer and the workability was measured by the mini slump flow test and the mini V-funnel test. From the test results, the saturation SP dosage of each SCM mix was determined. In the second part, four self-consolidating concrete mixes were produced, each comprising a SCM mix with saturation SP dosage and a fixed coarse aggregate content. The workability, filling and passing abilities and segregation stability were measured by the slump flow, U-box and sieve segregation tests, respectively. It was found that the performance of all the SCC mixes was satisfactory. The test results suggested that using a SCM mix with saturation SP dosage as the mortar phase can produce SCC with high performance and therefore is a good starting point to optimize the performance of SCC mixes.link_to_subscribed_fulltex