"Floating" Ultra-Thin Hybrid Reinforced High Performance Concrete overlay with a thickness of only 22 - 25 mm after 21 years intensive usage.

Introduction

When the first Ultra High Performance Concrete was developed in 1978 by Hans Henrik Bache and commercialized in 1983 by Densit A/S, the first challenge was to find applications for this “super” concrete. One of the first and also one of the most complex applications for Ultra High Performance Concrete are toppings (0.8 – 3.0 cm thickness) on industrial floors.

The function of an industrial floor or industrial pavement is of crucial importance for a company. Surface damage, cracks, broken joint edges, dust, etc. can result in an uncomfortable use and damage of transportation equipment. In the food industry and with the so-called fluid tight constructions not only the requirements of the user are of importance but as well the requirements from the authorities. In the rehabilitation or renovation from industrial floors or industrial pavements the factor time is often the most important key factor. Shutdown time must be minimized since every stagnation in the complex process of production, transport and storage will have immediately great financial consequences for the company involved. The choice for a resurfacing system for the clients’ industrial floors will be mainly based on two factors: the necessary shut down time and the expected service life of the new topping or overlay. The application of thin toppings and overlays is still difficult. Failure or success is depending on factors as quality of the sub base, bonding, placing conditions, and curing and last but not least the properties (including workability, set time, method of placing, finishability, etc.) of the High Performance Concrete or Ultra High Performance Concrete. In rehabilitation projects other factors are also of main importance especially the local circumstances and, of course, the sub base with regard to strength, quality, cracks, pollution, etc. Many times, the contractor is confronted with these problems after demolishing the existing topping and then, under pressure of the time schedule and planning, a good technical solution must be found. This requires a lot of technical and practical “improvisation” talent since the interests and financial consequences can be large for both end-user and contractor. Between the contractor and the other parties involved, such as the client, the consultant and suppliers, there must thus be a good collaboration to solve the flooring problems under time- and financial pressure.

Many floors are not sound, they have cracks or are contaminated with oil, chemicals, etc., or do have an existing topping made from polymer, bitumen, ceramics, etc. or are even under dimensioned for the actual loads. The only good way is to clean the sub base, to remove weak areas, to repair or placing new concrete, etc. This can be a huge problem for companies who are working 24 /7, don’t want to have dust, noise etc., who cannot accept a thick overlay and/or when they need a new working surface very fast because of limited maximum shut down time. Because of the unique properties of High Performance Concrete and Ultra High Performance Concrete it is possible to make a solution for this kind of problems. A flooring system that can be laid without joints in places where the quality of the existing base is not satisfactory.

Ultra-Thin Hybrid Reinforced High Performance Concrete overlay or topping

The Ultra-Thin Hybrid Reinforced Ultra High Performance Concrete overlay is originally developed in 1984 and first used in the cargo hold of the Danish vessel Star Skandia as an 50 mm overlay on 100 mm thick polystyrene foam panels. The Ultra-Thin Hybrid Reinforced Ultra High Performance Concrete overlay is in fact a Compact Reinforced Composite (CRC Hans Henrik Bache 1986) and is using the principles as for constructions as used in the security industry (vaults, walls, shelters, etc.) and later panels, balconies, stairs, bridges, etc. Many researchers are reporting the effects of high amounts of fibres in combination with high amounts of conventional reinforcement in HPC or UHPC (as thus first introduced by Bache). Composites like SIFCON, RPC and Ductal which are consisting of a HPC or UHPC with the addition of a high volume of fibres are not technically attractive since they are not designed to place in large thin horizontal overlays which will result in difficulties with casting and finishing.

Later the Ultra-Thin Hybrid Reinforced Ultra High Performance Concrete overlay is used for the rehabilitation of a multi-storey car park in Austria and many other deteriorated constructions and structures like industrial floors and industrial pavements but also bridges.

For the use in civil engineering, accentuate emphasize to industrial floors, pavements and concrete renovation, there are 3 main disadvantages from the original DSP (Densified Systems containing homogeneously arranged Ultra-Fine Particles) concept i.e. the workability, the finishability and the chemical shrinkage, all related, direct or indirect, to the extreme low water/ binder ratio and the high amount of micro-silica containing binder. Especially in cases where it is important to get full benefit of the properties of an Ultra High Performance Concrete like in ultra-thin overlays in combination with high volumes of traditional steel bars and steel fibres the workability, compactibility and finishability is of main importance for the final result and the qualities of the hardened matrix. 

Industrial floors and industrial pavements can be resurfaced with an Ultra-Thin Hybrid Reinforced Ultra High Performance Concrete overlay as a renovation method even when the surface is unacceptable or deteriorated. When the concrete sub base is cracked due too high loads, wrong design, poor quality, etc. there are usually not too many possibilities for rehabilitation and/or strengthening what will result, in most cases, in demolishing of the existing concrete and the application of a new floor or pavement. The Ultra-Thin Hybrid Reinforced High Performance Concrete Overlay and the Ultra-Thin Hybrid Reinforced Ultra High Performance Concrete Overlay are offering new solutions for problems with industrial floors and pavements without demolishing the existing floor and this will result in large financial savings and shorter shut down times in the industry. The Ultra-Thin Hybrid Reinforced High Performance Concrete Overlay and the Ultra-Thin Hybrid Reinforced Ultra High Performance Concrete Overlay are successfully applied as a thin (22 – 50 mm) bonded, semi-bonded and non-bonded topping or overlay on sub bases varying from polluted and cracked concrete to asphalt, ceramics and steel.    

To develop an Ultra-Thin Hybrid Reinforced Ultra High Performance Concrete overlay which can be placed under different conditions with traditional equipment and skilled labour it is, most of all, important to understand the complicated process and the difficulties of floor laying since this is the most important key to success of any system. From the point of view of the floor layer the following items are important: The laying of the concrete must be fast and easy enough to be able to place a large area without spending too much time on details; Compaction must be easy and without bleeding and/or segregation in the concrete mix; Floating the surface must be as fast as possible after compacting; It must be easy to reach a smooth and pore free surface. For the engineer and the end-user the final quality and properties are important like for instance: the strength, the wear resistance, the chemical resistance and the esthetical aspects.

The Ultra-Thin Hybrid Reinforced Ultra High Performance Concrete overlay consists of a special pre-blended micro silica containing HPC or UHPC, acrylic fibres, steel fibres and one or more layers of reinforcement . The HPC or UHPC can be mixed at the building site or in a batching plant and can be transported with dumpers or truck mixers. The flow and workability are, despite the large amount of aggregates and fibres, made so that the material is easy to compact with the use of a laser screed, a double vibration screed or slipformpaver. Immediately after compacting it is possible to float the overlay with a mechanical finishing machine (power float) with a closed disc. The set time of the HPC is equal of that from traditional concrete and thus also depending on temperature and humidity. During several hours after casting it is possible to finish the HPC with mechanical finishing machines to obtain the desired surface structure. After finishing the Ultra-Thin Hybrid Reinforced High Performance Concrete overlay, it must be protected against further evaporation of the mixing water. After curing for approx. 24 hours of the HPC, a very high-quality overlay with a very high bending tensile-, compressive- and impact strength is ready for use. Due the mesh reinforcement and steel fibres the Ultra-Thin Hybrid Reinforced Ultra High Performance Concrete overlay is able to absorb a certain amount of movement from the base without transmitted damaging cracking to the surface. 

The technical explanation for the success of the Ultra-Thin Hybrid Reinforced Ultra High Performance Concrete overlay is that the High Performance Concrete is containing just enough mixing water and a good composition of fine and larger aggregates what makes that the HPC or UHPC is relatively easy to place and to compact through which the reinforcement is completely encapsulate in the matrix. Due to the long process of finishing during several hours the HPC or UHPC is even more compacted and air is pressed out, the longer the finishing process the better thus the result. Also, this finishing process is of positive influence on the chemical shrinkage of the High Performance Concrete despite the fact that a large part of the mixing water will evaporate during placing and finishing. By compressing the High Performance Concrete during several hours with mechanical finishing machines the chemical shrinkage is partly compensated. 

Project Royal Flora Holland 1997

The Aalsmeer Flower Auction (Royal Flora Holland) building is the industrial building with the largest footprint in the world (740 x 700 m or 518,000 m2). However, the total complex of the Royal Flora Holland Aalsmeer (near Amsterdam and Schiphol) is even much larger. The transport of flowers is done by trucks, which are using many doors to come in and out of the building. Internal transport is done by special tractors with a mass of several hundred until more than 2.000 kg with 1 up to more than 30 coupled flower lorries with a mass from minimum 400 kg each on relatively small hard wheels. Due the absence of enough cold storage space, a former internal parking deck with a floor surface from 9.200 m2 was in 1995 rebuild in a cold store for flowers. The construction was original dimensioned for passenger cars of employees and visitors, which had consequences for the total acceptable weight of the insulated floors. A choice was made for foam glass isolation with a hydraulic bound bitumen emulsion topping. The building-up from the total construction was thus as follows:

Double T beams with a length of 23 m1 and an arch from 20 mm; A levelling layer from a hydraulic bound bitumen emulsion topping with a thickness of approx. 20 mm; An insulation layer of 80 mm foam glass and a topping from 30 mm hydraulic bound bitumen emulsion topping.  

Damage

Already during the placing of the last part of the floors in the cold store, damage occurred in the first cold store room due the very intense and heavy traffic during loading and unloading of the cold store. The Association Royal Flora Holland Aalsmeer asked ABT-Consultants for Building Techniques and Netherlands Pavement Consultants (NPC) to research the damage and to work out a repair proposal. During a period of two years (?) different proposals were worked out and different test areas were placed but without any satisfactory results. The damage of the insulated floors continued and could only be repaired temporary and therefore a good and durable repair was even more necessary for the owners of the building since their partners/ shareholders started to complain seriously since lorries were falling resulting in large replacing costs of flowers.

“Floating” Ultra-Thin Hybrid Reinforced High Performance Concrete overlay

In the spring of 1996 I visited ABT together with the trading company Eurofloor. I was consulted by ABT for a possible solution for this problem because of my expertise with the rehabilitation of industrial floors and UHPC. The Ultra-Thin Hybrid Reinforced High Performance Concrete overlay was suggested in a thickness of 20 - 22 mm (maximum acceptable weight) in combination with a welded mesh reinforcement of ? 6 mm #50 mm. Two trials areas from 25 m2 each were placed, one on top of the existing hydraulic bound bitumen emulsion topping and one on the foam glass directly after removing the existing hydraulic bound bitumen emulsion topping. The test areas were placed in front of two doors between two cold storage rooms with thus the maximum dynamic loads and maximum load cycles. At the same time tests were performed in the laboratory from TNO Delft and fatigue test were made in the laboratory from NPC.

The fatigue tests in NPC were done in a complete simulation of the actual floor construction in combination with the Ultra-Thin Hybrid Reinforced High Performance Concrete overlay and earlier alternatives. For the tests a special machine was used to simulate the wheel load stresses on the total construction and the effects of these load stresses on the different layers. The Ultra-Thin Hybrid Reinforced High Performance Concrete overlay was tested in two thicknesses, 20 and 30 mm, and with the reinforcement placed in the favourable- and unfavourable position. Even with extreme loads, which resulted in a considerable bending of several cm, and after cracking of the high performance concrete matrix the bended sample came back in the original zero position after unloading and almost without any visible cracks. The flexural strength of the 20 mm thick samples was 85 – 90 MPa.

After a successful testing period of 9 months from the two-test areas made, the end-user told ABT that this was the only test area that actually worked and gave the order for the placement of the overlay in the first cold store with a total floor surface from 2.500 m2. Decided was to place the Ultra-Thin Hybrid Reinforced High Performance Concrete overlay on top of the hydraulic bound bitumen emulsion topping, this to meet the requirements to control the amounts of waste material and to shorten the necessary shutdown period. Also, from a technical and practical perspective this was the best option, there was a maximum friction between the hydraulic bound bitumen emulsion topping and the Ultra-Thin Hybrid Reinforced High Performance Concrete overlay and the hydraulic bound bitumen emulsion topping gave a relatively good “working surface” less sensitive for damage than the foam glass isolation panels.   

Placing of the Ultra-Thin Hybrid Reinforced High Performance Concrete overlay

After filling the many potholes and damages in the existing topping and foam glass with hydraulic bound bitumen emulsion, reinforcement rebar’s ? 5 mm and each 500 mm were placed over the whole surface to act as a distance keeper for the welded mesh reinforcement. The fixation of these rebar’s was done with Hilti nails which were shot under an angle from 60 degrees in flat steel strips and in some places with insulation fasteners. On top of these distance keepers a welded mesh reinforcement ? 6 mm #50 mm was placed and welded together to avoid any movements. Because of the total thickness from the overlay from only 22 – 25 mm is it very important that the welded mesh reinforcement is properly placed in each other with a dowel length from at least 50 mm.

Iron L – profiles where placed along the walls to avoid curling due to shrinkage and peel stresses. At the separation joints to the connected cold storage rooms a steel L-profile fixed to the sub base and was welded to the reinforcement. Welded mesh reinforcement was also connected to the many columns. The idea was to distribute the stresses as much as possible by a semi-bonding. Daily production joints where made by placing flat iron bars on the reinforcement, to avoid pollution from the next production area plastic sheets where taped under the flat iron bars. In the building was a dilatation joint and at that area ABT insisted to place a plastic foil under the Ultra-Thin Hybrid Reinforced High Performance Concrete overlay. The idea was that the floor construction was able to “move” under the Ultra-Thin Hybrid Reinforced High Performance Concrete overlay. However, in real life this didn’t work like that and an interruption was thus made between semi-bonding because of friction (concrete on hydraulic bound bitumen emulsion) and fixation and non-bonding and this resulted, as predicated by me, in very small cracks. However, the width of the cracks and perfect crack distribution above each rebar didn’t result in any damage after 21 years heavy use in a humid environment.  

The pre-blended dry materials for the steel fibre reinforced High Performance Concrete where supplied in big bags in units for ? m3 en was mixed in a mobile planetarium pan paddle mixer, which was placed outside the cold store. Transport from the mixer to the building site was done with a small dumper equipped with a movable storage bunker with some small vibrators and a screw pump to spread the steel fibre reinforced High Performance Concrete. The dumper could simply drive over the stiff reinforcement and place the steel fibre reinforced High Performance Concrete in front of the double vibration screed. After compacting the HPC, the surface could be floated immediately with troweling machines with a closed disc. After this and during the setting from the steel fiber reinforced High Performance Concrete, the surface from the overlay can be polished with troweling machines with steel blades during several hours to get the desired very smooth surface structure but in this particular case the end-user wanted to have a skid resistance surface structure. After finishing the surface, a curing compound was sprayed on the surface and the next morning plastic sheets where placed over the surface and kept wet for 3 days. After a testing period of 4 months the rest of the overlay in the cold store was placed including a trial area in a thickness from 30 mm with two layers welded mesh reinforcement ? 5 mm #50 mm directly placed on polystyrene isolation panels.

Situation after 1 month

After 1-month small cracks became visible in the area were a plastic separation foil was used and small shrinkage was visible at the walls and around the columns. At the locations where there is only a few mm cover on the reinforcement rebar’s the welded mesh reinforcement is visible but the surface is not more uneven than the surrounding surface. The surface texture is with a structure and seems to be skid resistance enough. 

Present situation 21 years later

Last week I had to make some photographs for an article I have to write about UHPC toppings for the magazine of the British Concrete Society and went, after 10 years, back to check “my” floor in the cold store of the Royal Flora Holland. Actually, the floor didn’t change at all and looked exactly the same as 20 and 10 years ago with small cracks visible in the area were a plastic separation foil was used and small shrinkage is visible at the walls and around the columns. At the locations where there is only a few mm cover on the reinforcement rebar’s the welded mesh reinforcement is visible but the surface is not more uneven than the surrounding surface. The surface texture is with a structure and seems to be skid resistance enough.

The surface and cracks are very good visible on the photographs with a small aperture and at a 1:1 size. The client and users are extremely satisfied since they have a perfect surface, maintenance free in the last 21 years and possible for another 21 years.

January 20 2018

Peter Buitelaar

[email protected]