Frequently asked Questions:

Q. What is expansive cement?

A: Expansive or Shrinkage compensating cement is used in a concrete mix that is designed to reduce or eliminate cracks and curling due to dry shrinkage.

Expansive cement is used to make shrinkage-compensating concrete that is used:

(1) to compensate for volume decrease due to drying shrinkage,

(2) to induce tensile stress in reinforcement, and

(3) to stabilize long-term dimensions of post-tensioned concrete structures.

One of the major advantages of using expansive cement is in the control and reduction of drying-shrinkage cracks. In recent years, shrinkage-compensating concrete has been of particular interest in bridge deck construction, where crack development must be minimized.



 

Q. Can I achieve a 'maintenance free' floor slab?

A: No. A floor slab provides an operational platform for equipment and this very operation creates wear and tear that must be addressed. It is of paramount importance that the floor slab is subjected to an adequate housekeeping and maintenance regime in order to provide the on-going serviceability for which the floor was originally designed. This is the responsibility of the user, although maintenance contracts can be placed with the installer or technical advisor.

To accomplish this, an industrial concrete floor slab should be subject to regular maintenance inspections by the floor user (or appointed representative) and any identified issues dealt with promptly in order to avoid unnecessary, exponential deterioration of the concrete surface and particularly of joints. Continued trafficking by mechanical handling equipment will inevitably take its toll on the concrete but a “stitch in time” maintenance philosophy will pay great dividends.

 

Q. What is curling?

A: Curling is the warping of a concrete panel bounded by joints, saw cuts or a crack, where the edges are left higher than the centre. The process is complex and the subject of ongoing research both in the UK and USA but is primarily caused by the differential shrinkage of concrete, and can occur at corners and edges of panels. As the top surface dries and shrinks at a different rate to the bottom, the floor curls upwards, creating a ‘toast’ effect.

Whilst curling is quite common in concrete floors, it often does not cause any undue concern or disruption to the daily running of a warehouse. If good construction practice and joint detailing is carried out and attention paid to the mix design, then the practical significance of curling will usually require no further treatment.

Where induced contraction joints (or cracks) have curled significantly and present an issue regarding serviceability, the accepted method of treatment is to reinstate aggregate interlock by resin injection and then grout the void under the slab and grind smooth the surface restoring the surface regularity across the joint (or crack).

 

Q. What is delamination?

A: Delamination is a process whereby a thin layer (2-4mm) of a floor surface becomes detached, lifts slightly, and then breaks down rapidly when trafficked. The formation of a number of fine cracks are a tell-tale sign that delamination has occurred, and can be verified by tapping the surface with a metal object – the resulting hollow sound will confirm delamination of that particular localized patch.

The greatest cause of delamination (other than inexperience) is inadequate and inconsistent weather protection during (and for a 7 day period, after) the concreting and finishing operation. An enclosed environment is essential to ensure that cross winds do not travel across the fresh concrete at casting stage, and hence adversely affect the volume of bleed water available and required to allow the material to be worked monolithically into the base concrete.

Delamination can be repaired by squaring off the affected area, inducing shallow saw-cuts, removing the lifted surface, then filling the patch with a suitable cement or resin-based mortar system. Where small affected areas are away from frequent traffic and have yet to deteriorate, a low viscosity epoxy
resin can be injected between the lid and the base concrete to create the necessary bond.

 

Q. What is crazing and is it a serious problem?

A: Crazing is a pattern of fine and shallow cracks that develop over the initial few weeks and months after slab completion. These fine cracks are caused by differential contraction during drying shrinkage, giving a reduction in volume and creating tensile forces between the surface layer and the underlying concrete. This difference in the rate of contraction with depth creates the surface tension effect that leads to crazing.

Surface crazing of power trowelled concrete slabs is a very common phenomenon, affecting the majority of industrial floors, and can often be the price a client pays for wanting a “shiny” surface. Crazing is an aesthetic problem only and will usually have no negative implications on the structural integrity or abrasion characteristics of the slab, and any surface delamination that may be exhibited will be totally unrelated. Crazing is a normal occurrence, therefore no structural or serviceability issues are associated. These should be left untreated, but monitored on a regular basis for the foreseeable future in case of deterioration.

 

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