TOLERANCES AND FINISH SPECIFICATIONS

The tolerances specified for the surfaces of slabs or other flatwork should be appropriate to their final use. Achieving tight tolerances increases costs. It may be necessary, for example, to have tight tolerances in warehouses with high-racking bays but it would be unnecessarily expensive to have very tight tolerances for the loading dock areas where the delivery trucks are received.

NZS 3109 specifies floor tolerances to ensure that the structural behaviour is not impaired (it does not specify tolerances for the serviceability or usefulness of the floor.

Floors generally have to meet two independent tolerance criteria. One deals with the desired elevation and the other the 'flatness' of the floor.

Chapter 10 Finishing Concrete Flatwork

Guide to Concrete Construction 10.9

The 'elevation tolerance' gives the permitted variation of the slab surface from a fixed external reference point or datum.

NZS 3114 specifies in more details the ‘flatness tolerances’. This Standard sets out 11 different types of what it defines as ‘unformed’ finishes for slabs. These are designated U1 through to U11. The more common finishes used are described: U1 Screeded

U2 Floated

U3 Trowelled – manual or machine steel trowelling

U4 Machine screeded

U5 Shallow texture – bristle broom U6 Deep texture – wire broom.

Tolerances are set fro the finishes which are considered normally achievable for the type of work. However, it may be necessary to have tighter tolerances in the aisles of premises using high- stacking forklift trucks. It has to be understood, however, that as the floor tolerances are reduced, then so the cost of producing the floor will be increased. An abrupt deviation of 3 mm is acceptable, except where floors are to receive thin sheet tiles such as vinyl or carpet.

The ‘flatness tolerance’ is most usually expressed in terms of a permitted deviation under a straightedge of specified length, usually 3 m.

Two common grades of ‘flatness’ in decreasing order of precision are:

ƒ

±3 mm under a 3 m straightedge

ƒ

±6 mm under a 3 m straightedge

A tolerance of ±3 mm under a 3 m straightedge should be specified only for very high quality floors where flatness is a major functional requirement, for example in a warehouse where turret-type, high- stacking forklift trucks are used. The exact requirements for the project and the floor areas

over which the tolerance applies should be clearly defined.

An American system based on ASTM E1155 to assess the accuracy of the floor profile in terms of an Fr number which is based on a flatness over 600 mm and a levelness over 3 m, it available for use where additional accuracy of floor profiles is necessary. Because of the accuracy of measurement equipment needed and the amount of data required the method should only be considered where extra-ordinary accuracy of flatness is required usually when using a sophisticated automatic storage and retrieval system of high level racking.

For pavements requiring very high tolerances (e.g. warehouses in which loosely stacked items are moved on forklift pallets), reference should be made to the CCANZ’s TM 26 and TM 38 Concrete Ground Floors and Pavements for Industrial and Commercial Use10.2, Parts 1 and 2 respectively and C&CAA’s Industrial Floors and Paverments10.3.

REFERENCES

10.1

Bowman, R. An Introductory Guide to the Slip Resistance of Pedestrian Surface Materials (HB 197) CSIRO and Standards Australia, 1999.

10.2

Concrete Ground Floors and Pavements for Industrial and Commercial Use: Part 1 (TM26) Cement & Concrete Association of New Zealand, 1999. Concrete Ground Floors and Pavements for Industrial and Commercial Use: Part 2 (TM 38) Cement & Concrete Association of New Zealand, 2001.

10.3

Industrial Floors and Pavements – Guidelines for design construction and specification (T48) Cement and Concrete Association of Australia, 1999.

ACKNOWLEDGEMENT

Figure 10.3, 10.4, 10.5 and 10.6 – Binding Concrete Limited, Hamilton, on behalf of the New Zealand Master Concrete Placers’ Association.

Chapter 11 Curing

Guide to Concrete Construction 11.1

Chapter 11

Curing

This chapter discusses the curing of concrete, providing: first an outline of the hydration of cement, to highlight the fundamental importance of keeping concrete moist during its early life; then describing the effect of curing, or its lack, on the properties of concrete; and finally the methods which may be used to cure concrete under the wide variety of conditions met on building and construction sites. In this Guide, 'water curing' describes any method of curing designed to keep concrete moist during its early life by preventing the loss of moisture from it.

INTRODUCTION 11.2

Relevant New Zealand and Australian Standards

11.1 HYDRATION OF CEMENT 11.2

11.1.1 General

11.1.2 Effect of Temperature

11.2 EFFECT OF DURATION OF CURING

ON PROPERTIES OF CONCRETE 11.3 11.2.1 Effect on Strength 11.2.2 Effect on Durability 11.3 CURING METHODS 11.4 11.3.1 General 11.3.2 Impermeable-Membrane Curing 11.3.3 Water Curing 11.3.4 Accelerated Curing 11.4 SELECTING A METHOD OF CURING 11.8 SUMMARY 11.9

Chapter 11 Curing

INTRODUCTION

Curing is the process or operation that controls the loss of moisture from concrete after it has been placed in position, or in the manufacture of concrete products, thereby providing time for the hydration of the cement to occur. Since the hydration of cement does take time (days, and even weeks, rather than hours) curing must be undertaken for some specified period of time if the concrete is to achieve its potential strength and durability. Curing may also encompass the control of temperature since this affects the rate at which cement hydrates. The curing period will depend on the properties required of the concrete, the purpose for which it is to be used, and the ambient conditions, i.e. the temperature and relative humidity of the surrounding atmosphere.

Since curing is designed primarily to keep the concrete moist by preventing the loss of moisture from the concrete while it is gaining strength, it may be done in two ways:

ƒ

by preventing an excessive loss of moisture from the concrete for some period of time, e.g. by leaving formwork in place, covering the concrete with an impermeable membrane after the formwork has been removed, or by a combination of such methods; or

ƒ

by continuously wetting the surface thereby preventing the loss of moisture from it. Ponding or spraying the surface with water are methods typically employed to this end.

In the manufacture of concrete products, the temperature of the concrete may be raised to accelerate the rate of strength gain. Very importantly, the concrete must be kept moist during such treatment. Curing the concrete in saturated steam, or curing it with high-pressure steam in a suitable container, i.e. autoclaving it, are used to cure concrete at elevated temperatures. Other methods that are or have been employed include the use of flue or exhaust gases, the use of heated formwork, and electrical curing, but these are beyond the scope of this Guide. Some of these methods are used overseas in colder climates and reference can be made to overseas literature for information on them.

Relevant New Zealand Standards

NZS 3101 Concrete structures

NZS 3109 Concrete construction

Relevant Australian Standards

AS 3600 Concrete structures

AS 3799 Liquid membrane-forming curing compounds for concrete

11.1

HYDRATION OF CEMENT

11.1.1 General

When water is mixed with portland or blended cement a series of chemical reactions commence, which proceed rapidly at first, but then more slowly, for just about as long as moisture is present. These reactions result in new chemical compounds being formed which cause the cement paste first to stiffen, then to harden and gain strength.

We need not concern ourselves in this Chapter with the details of these reactions (for further information see Chapter 2 Hydraulic Cements), except to note the following:

ƒ

The different compounds in the cement react with water at different rates. Those which are responsible for the early stiffening of the paste and its early strength react quite rapidly but then contribute little to subsequent strength gain.

ƒ

Those compounds which contribute most to the strength of the paste and, hence, to the strength of the concrete, react more slowly. As a result, they require the paste to be kept moist until the concrete gains its strength.

ƒ

Allowing the paste to dry out causes the chemical reactions to cease (for all practical purposes). Whilst rewetting the paste causes the reactions to recommence, their effect on the subsequent strength and other desirable properties of the paste may be permanently impaired or reduced.

Figure 11.1 (page 11.3) provides a schematic

representation of the chemical reactions which take place when water is mixed with cement, and the time-dependent nature of these reactions.

11.1.2 Effect of Temperature

The temperature of the cement paste can have a quite marked effect on the rate at which it hydrates. The temperature may also affect the nature of the new compounds formed and, hence, have a permanent effect on the long-term strength and durability of the concrete.

Thus, lower temperatures reduce the rate at which hydration occurs whereas high temperatures increase the rate of hydration, hence rate of strength gain, but reduce the potential strength of the concrete achieved at later ages.

Chapter 11 Curing

Guide to Concrete Construction 11.3

Figure 11.1 Schematic representation of chemical reactions when cement and water are mixed

Figure 11.2 Effect of curing temperature on the rate of strength gain of concrete

Figure 11.3 Effect of duration of water curing on strength of concrete

Figure 11.2 illustrates the effect of curing

temperature on the rate of strength gain of concrete. The figure is illustrative only as the mag- nitude of temperature effects is very much related to the composition and fineness of the cement. For practical purposes, however, it may be noted that, provided the temperature of the concrete is maintained within the normal range of ambient temperatures encountered in temperate zones of New Zealand and Australia, no significant harm will result to the strength of the concrete. For concrete operations outside this range, i.e. in very hot or very cold weather, special precautions may be necessary. These are discussed in Chapter 12 Hot- and Cold-Weather Concreting.

11.2 EFFECT OF DURATION OF

In document NZ Guide to Concrete Construction (Page 116-121)