Specification

The concrete is to have the compressive strength (f´_c ) as specified in the project documentation and should conform to the requirements of AS 3600, with regard to the materials for the concrete and the concrete manufacture.

Concrete Additives

Admixtures or concrete materials containing calcium chloride or other chloride slats must not be used. Chemical admixtures including plasticisers complying with Clause 19.1.1.7 "Chemical Admixtures" of AS 3600 may be used.

Preparation

Before concrete is placed, any accumulated debris, grease or any other substance will need to be removed to ensure a clean bonding surface to the BONDEK II sheeting. Any ponded rainwater lying on the sheeting may be removed by conventional methods such as blowing or sweeping.

Construction Joints

It is accepted building practice to provide construction joints where a concrete pour is to be discontinued. Such discontinuity may occur as a result of a planned or unplanned termination of a pour. A pour may be terminated at the end of a day’s work or as a result of bad weather or equipment failure. Where unplanned construction joints are to be made, the position must be on the approval of the design engineer.

In certain applications, the addition of water stops may be required, such as in roof and balcony slabs where protection from corrosion of reinforcement and sheeting is necessary.

Construction joints positioned transverse to the span of the BONDEK II sheeting are normally located where shear forces are a minimum such as the mid-third of a slab span and ideally over a line of propping (see Figure 21A). Construction joints positioned longitudinally are located in the pan (see Figure 21B).

Construction joints are formed with a vertical face using a variety of techniques. The easiest is to sandwich the continuous reinforcement between two boards.

Prior to recommencement of concreting, the construction joint will need to be prepared to receive the new concrete. Preparation method will depend upon the age and condition of the old concrete. Generally, proper cleaning is required to remove loose material, to roughen the surface and to expose the course aggregate.

Placing

The requirements for the handling and placing of the concrete are covered in Class 19.1.3 "Handling and Placing of Concrete" of AS 3600.

The concrete is placed between construction joints in a continuous operation so that new concrete is placed against plastic concrete to produce a monolithic mass. If the concreting has to be discontinued for any more than approximately one hour, depending on the temperature, a construction joint may be required.

Commence concreting close to one end and spread concrete uniformly, preferably over two or more spans. It is good construction practice to avoid excessive concrete heaping and heavy load concentrations. When concrete is transported by wheel barrows, the use of planks or boards is recommended.

During placing, the concrete will need to be thoroughly compacted, worked around ribs and reinforcement and into corners of the Edge Forms by using a vibrator compactor. Care will be required to ensure that the reinforcement remains correctly positioned so that the specified minimum concrete cover is achieved.

Unformed concrete surfaces are screeded and finished to achieve the specified surface texture, cover to reinforcement, depths, falls or other surface detailing.

Surfaces which will be exposed, such as edge Forms and exposed soffits, should be cleaned of concrete spills while still wet. This will reduce effort on subsequent work.

Curing

After placement, the concrete will need to be cured by accepted methods, for example, by keeping the slab moist for at least seven days by covering the surface with sand, building paper or polythene sheeting immediately after it has been moistened with a fine spray of water.

When curing in very hot or very cold weather, the requirements of Clause 19.1.5 "Curing and Protection of Concrete" of AS 3600 and acceptable building practice need to be observed.

Where curing compounds are used, then acceptable building practices need to be observed.

Until the concrete has cured, it is good construction practice to ensure protection from concentrated loads, such as barrows, and heavy trafficked passageways. Protection can be achieved by the use of planks or boards and roping off areas to ensure loadings are in defined areas.

Removing the Propping

The earliest times at which the slab may be initially loaded after the concrete has hardened and/or the propping removed are affected by various factors. It is usually necessary to perform calculations taking such factors into account when determining these times.

The average compressive strength of the slab concrete f _cm at the relevant age can be estimated either from control samples consisting of site-cured compressive cylinders (see Clause 19.6.2.8. of AS 3600), or by alternative methods such as given in Clause C.2.3 "Information to be provided in the Project Documentation" of AS 3610, Supplement No 2 Figure 22 is a reproduction of Fig. C 2.1 from this reference, and illustrates the typical compressive strength development of Portland cement concrete under different average ambient temperatures.

Prior to the concrete attaining a compressive strength f_cm of 20 MPa, it is desirable that the slab remains uncracked under flexural action, or at least that the degree of cracking or deflection is not greater than that which would occur for the slab under design loads.

Once the concrete has attained a compressive strength f_cm of at least 20 MPa, the slab may be designed as a composite slab for the appropriate loads in accordance with Lysaght publication BDII-2A by using f_cm in place of f´_c in the design equations.

In the absence of calculations, a ready reckoner may at least be used to estimate the earliest time at which the propping may be removed. For this purpose, reference may be made to Clause 19.6.2.5 "Removal of formwork supports from reinforced members not supporting structures above" of AS 3600. Table 1 summarises the requirements of this clause.