Precast Concrete

Materials, Manufacture, Properties and Usage, Second Edition

Maurice Levitt

• Publisher: CRC Press
• Year: 2014
• Topic: Technology & Engineering
• eBook ISBN:
  9781498718301
• Subtopic: Civil Engineering
• Index: Technology & Engineering

This general treatise on precast concrete reflects Maurice Levitt's extensive experience in the construction industry and as a researcher and consultant. It gives detailed coverage of the subject from the material's properties through its manufacture and quality control, and on to specialist topics such as accelerated curing and use in hot and cold

1 Ingredients

1.1 Cements

1.2 Aggregates

1.3 Water

1.4 Admixtures

1.5 Additives

1.6 Pigments

The first edition¹ of this book was a little off balance in comparing the amount of text on additives, admixtures and pigments with the main ingredients of cements and aggregates. The reason for this was that in 1982 there were a large number of publications on cements and aggregates which were referred to by a large readership as being applicable to all concrete production, including precast. However, this assumption cannot be made as the precast industry generally has more stringent demands on raw material requirements compared to in situ concrete. In view of the changes that have taken place in the precast concrete industry over more than two decades, coupled with stricter regimentation in the use of Standards and system approaches, what is considered to be a better balance has been adopted in this edition.

The Notes and references section is deliberately brief so that the reader can pick up the main points in the current text. This edition is in stand-alone form and the reader should not require to refer to the first edition.

In situ concrete producers, users and specifiers now are subject to the European Standard² and Code.³ These documents do not apply to the vast majority of precast products as they are generally covered by specific Eurostandards. The Standard for concrete is supported by a complementary two-part British Standard⁴ giving national guidance. The other matter to be borne in mind with virtually all Eurocodes is that they are written with a mixture of ‘shoulds’ and ‘shalls’ compared to the current or superseded recommended ‘shoulds’ of BS Codes.

1.1 Cements

To list all the types of cements covered in the various parts of the Eurostandards for cements⁵ would take an inordinate amount of space. As only a few of these Standards are likely to be used in the precast concrete industry, only those cements will receive primary attention.

There are several types of cement covered in these Standards even including masonry cement. The Portland cements listed contain OPC varying from 100 per cent down to 5 per cent clinker content with CAC aluminous cement being the only one having 100 per cent clinker. Although CAC (originally known as high alumina cement, HAC, Ciment Fondu, Lightning Cement) received an unjustified⁶ bad name in the 1960s, it is still considered to have useful potential in the precast concrete industry.

Cement designations are complex and many sub-classifications are used. A rapid-hardening Ordinary Portland Cement, (RHPC) could be designated by:

• CEM I/A-mac.52.5N
• where:
• ‘CEM I’ is the main (95–100 per cent clinker) cement type
• ‘A’ confirms a high clinker content and could be omitted
• ‘mac’ is an inorganic ‘minor additional constituent’ at 0–5 per cent
• ‘52.5’ is the 28-day characteristic cube strength which is specified to be 52.5MPa minimum. If the ‘52.5’ had been ‘42.5’, the 28-day characteristic cube strength would have to lie between 42.5MPa and 62.5MPa
• ‘N’ is a normal rate gain of strength. If the ‘N’ had been ‘R’, standing for rapid strength gain, there would be no limits for the characteristic strength.

The characteristic strength is the figure below which not more than 5 per cent of the cube results would be expected to fall. In the Standard, the concrete strength covers both cube and cylinder testing, with the cylinder strength given first. Thus, a characteristic 25MPa cube strength would be specified as ‘C20/25’ as the cylinder’s 2/1 slenderness ratio would result in a lower crushing load for the identical concrete in the 1/1 slenderness ratio cube.

White cement, commonly used for cast stone and architectural concrete manufacture, could be categorised as ‘CEM I 42.5N’ but would need the word ‘White’ somewhere in the description. The description could be made more complex in the case of a white Portland cement containing, say, 36 per cent GGBS where ‘White CEM III/A-S 32.5N’ could apply. The sub-class ‘S’ means that slag is the second constituent and the ‘32.5’ limiting the characteristic strength to lie between 32.5 and 52.5 MPa at 28 days. The purchaser does not need to learn all the cement classifications by heart but should have a basic knowledge of those classes relevant to the work. This is necessary so that the most appropriate cement for the application be ordered and that the product, the delivery note and the corresponding cement test certificate all tally.

Setting and hardening times are terms that are applicable to cements and not concretes. The setting characteristics of the product are not the only properties relevant to the ability for early demoulding and handling. An equally important property for many of the machine-intensive processes is ‘green strength’, a property of the ability of retention of the moulded shape at a few seconds to a few minutes old. Probably only the fineness of the cement relates to green strength, most of this being a function of aggregate grading, cement fineness, water/cement ratio, free, W/C(F) and compaction efficacy.

1.1.1 Cement problems

The list of problems that follows the general preamble on cements refers to problems either encountered by the author or possible future ones. Both the in situ and precast concrete industries now have access to a far larger range of cements than used to be available. This, for most of the Portland cements, is largely due to the use of what may be termed inert extenders that are added to the clinker prior to grinding.

Generally speaking, the cements available fall into two fairly distinct groups:

1. Diluted clinkers containing a small percentage of materials, such as limestone, resulting in properties not significantly different from the undiluted version. With carbon emissions from industry needing to be minimised to help inhibit the impending global warming and 1T of cement manufacture said to produce 1T of carbon (carbon dioxide), users should condone clinker dilution. On the other hand, a degree of wariness should be exercised as other extenders may be used without full assessment (cement tests and applicability to the precast product in question).
2. Cements containing significant quantities of additives such as PFA, granulated ground blastfurnace slag (GGBS) and microsilica (MS) which are used either for the specific properties of the additive and/or for properties that arise from its reaction with the cement component.

The following is a list of problems that can arise:

1. The finer the cement, the faster the setting and hardening times and this makes the higher characteristic strength cements more attractive to the concrete industry, especially the precast side. The main downside to such selection is an enhanced rate of exotherm leading to too rapid a curing rate, excess thermal gradient and cracking. In addition, some machine-intensive processes do not operate well with very fine cements. One process very sensitive to the fineness was spinning, not thought to be practised in the UK at present. The process was used for pipes and lighting columns and manufacturers used to order a special cement categorised as ‘coarse ground’ even though its specific surface complied with the then limit for Ordinary Portland.
2. Disputes often arise because of the cement and, if the precaster has not kept a representative sample, there is no proof as to where the blame lies one way or the other. Every delivery of cement should be appropriately sampled and stored in airtight containers labelled with all the relevant information relating to the delivery. The label should also identify what part(s) of the production that cement delivery relates to.
3. Reliance is often placed upon a sulfate-resisting cement being enough so that attention is not paid to the main properties that affect sulfate-resistance such as mix design, compaction and curing. Blaming the cement for poor performance of the concrete can be misplaced.
4. While the word ‘misplaced’ is fresh in the mind, the reader is referred to the text Concrete Materials,⁷ where in, the 1960s and 1970s the misplaced blame culture against aluminous cement was taken to task. It was estimated there that the three main precasters involved in production of pretensioned prestressed units in the years 1946–1975 manufactured about 15 million individual units and these went into about 60,000 constructions. A housing estate was defined as a single construction. About 1000 of the constructions (not estates) were examined by consulting engineers and four of these were found wanting. This 4 out of the total of 60,000 was small and most of the troubles were based on insufficient column bearings for beams and conversion leading to increased permeability and access of aggressive chemicals (e.g. chloride stabiliser from woodwool slabs) leaching through to the prestressing wires.
5. If the cement is not stored properly, this can lead to caking if moisture has access or the cement is stored too long. This is more of a problem with sack storage rather than with silos as, with silos, the newest cement delivery has to wait for the lower and older cement to be used first. Bag storage needs to be rotated so ...

Table of contents

• Cover
• Half Title
• Series Page
• Title Page
• Copyright Page
• Dedication Page
• Table of Contents
• Preface to the second edition
• Acknowledgements
• List of abbreviations
• Introduction
• 1 Ingredients
• 2 Reinforcement, prestressing, hardware
• 3 Moulds
• 4 Production control
• 5 Labour-intensive processes
• 6 Machine-intensive processes
• 7 Accelerated curing
• 8 Hot and cold climates
• 9 Properties and performance
• 10 Standards, testing and quality
• 11 Finishing, repairing and jointing
• Notes and references
• Index