SURFACE PREPARATION

The production of a high quality, durable, industrial floor is completely dependent on the type and condition of the surface to which it is applied.

Therefore, before the start of all contracts, thorough preparation is essential, having first assessed surface strength, flatness, surface profile and any possible contamination. These factors will all influence the ultimate quality and serviceability of the finished floor.

Any short cuts taken at this stage of the contract WILL NOT SAVE MONEY especially when compared with the cost of repairing or replacing a failed floor.

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Strength

Concrete subfloors meant to receive high performance coatings and overlay systems should have sufficient compressive and tensile strength to withstand dynamic loadings. They should also have adequate surface strength.

Generally, concrete floor slabs in these situations have been designed to have a minimum compressive strength of 25 MPa. There are a number of non-destructive test methods available for evaluating the strength of existing concrete floors. A Schmidt hammer is probably the best known and most readily available device for this application, but they do not give definitive values and should only be used to compare readings taken from the same slab. A minimum of two readings per 10m² should be taken, recorded on a grid and analysed. Where surface strengths appear doubtful, direct tensile strength can be measured using the "pull off" method. eg Elcometer. These values can be used to predict compressive as well as giving a good indication as to the expected bond/adhesive strength of the coating or overlay. Subfloors should have a
tensile strength value in excess of 0.75 N.

Captive Blast Cleaning

A metallic abrasive (steel shot) is used in shotblasting machines to scour the concrete surface. Shot is propelled by a rotating wheel, impacts on the concrete surface and rebounds into a recovery unit. A pneumatically driven vacuum system collects dust and shot, separates the two materials, recycles the re-usable shot and removes the dust through a filter system.

Manual, self-propelled and ride-on models are available to suit most situations.

Good control of dust is one of the major advantages of shotblasting. The machines have been used in without problems.

Since no water is used the surface is immediately ready for the application of systems that need a dry surface. Shotblasting is typically used for cleaning and scarifying to depths up to 3mm.

In preparation for epoxy or polyurethane coatings, fine shot is used to produce a very light etching (brushblast) of the surface. A brushblast breaks the laitance on the concrete surface and opens pores, to improve the adhesion of the coating.

When a topping or overlay system is to be placed, an aggressive blast exposes sand particles. A deeper exposed aggregate blast leaves protruding coarse particles to provide an excellent mechanical key.

Three factors control the surface profile obtained:

a) Size of abrasive
b) Amount of abrasive
c) Rate of travel

In general 50m² per hour can be prepared using a mid range machine.

Scabblers

Scabblers use compressed air to hammer piston mounted bits into the concrete surface, and provide a rougher surface than is achieved by shotblasting.

For horizontal surfaces push and self-propelled machines are available. The self-propelled machines offer some of the same features as shotblasting - mobility, speed, optional vacuum and filter systems.

Scabblers can remove up to 8mm of concrete surface in one pass, but production rates obviously depend on the condition and strength of the existing floor. Self-propelled scabblers cannot remove
concrete immediately adjacent to walls unless a special attachment is fitted. Needle guns of hand grinders with vacuum attachments are available for this application. Scabbling operations are dusty, unless a vacuum attachment is used. Alternatively, wetting the surface to be scabbled will help to control dust. Scabblers are also noisy and produce vibration. Skilled operation of scabblers is necessary. If improperly used, the result can be a cleaned but damaged concrete surface with loosened aggregate articles.

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Scarifiers

Machines for scarifying concrete can be known as planers, scarifiers, milling machines or rotary cutters. Unlike scabblers in which bits move vertically to hammer the concrete surface, planers apply a rotating circular cutting wheel to precisely control the depth of removal. These machines have different styles of inter-changeable cutter assemblies that are used for cleaning, grinding and light or heavy planing. Planers are normally available in push type or self-propelled models, powered by compressed air, petrol or electricity. A fine adjustment is used control the cutting wheel height and in some models cutters can easily changed when a different surface texture is required.

Planing machines generate a lot of dust. Wetting the surface will help to control this but a more effective means is an optional vacuum dust collector. Like scabblers planers are noisy and create vibrations.

High Pressure Water Blasting

High pressure water blasters are sometimes used to prepare concrete surfaces in specific environments . These units have been used in applications ranging from laitance removal to hydrodemolition of concrete. The depth of removal is controlled by adjusting water
pressure, speed of nozzle as it passes over the surface, and the speed of the machine itself. A recently developed system incorporates a vacuum that sucks up waste water and debris to
dispose of a selected site. This system lessens the obvious problem of excessive water and disposal of watery waster.

Advantages of high pressure water blasting include:
 

a)	No dust created.
b)	Noise is minimal.
c)	No mechanical vibrations.
d)	Machines selectively remove deteriorated concrete leaving good concrete intact.

Summary

It is not possible to set rigid rules on the selection of surface preparation techniques, since each contract has its own requirements.

Having made a selection therefore , it is in the interest of the applicator and client to conduct a preparation trial, so that standards for future preparation can be agreed, and the adhesion of the floor finish can be assured.