Ask Your Mates Open Forum

Reinforced concrete over buried unreinforced concrete - Reflective cracking or not?

  • 1.  Reinforced concrete over buried unreinforced concrete - Reflective cracking or not?

    Posted 19 January 2020 22:15
    There is a concrete renewal project in a cleared reserve where width and height does not appear to be a construction restriction.
    The old concrete path is badly cracked but is low enough that a new path complete with a layer of bedding material could be constructed right over the top of the old path and therefore avoiding costs of removal.

    Q: Would there still be reflective cracking considering that
     a. A construction bed could effectively smooth out the existing cracks in the old unreinforced footpath
     b. The new footpath would be steel reinforced?

    Alternatively is it viable to construct a new path beside the existing path and bury the old cracked path in soil/turf? Again the landscape is accommodating for this.

    Thanks.

    ------------------------------
    Troy McGrath
    Asset Engineer
    Maitland City Council
    troy.mcgrath@maitland.nsw.gov.au
    0249349781
    ------------------------------
    BlogPageSpacerBlank


  • 2.  RE: Reinforced concrete over buried unreinforced concrete - Reflective cracking or not?

    Posted 20 January 2020 21:00

    Given that you have indicated that there is enough height to do a full reconstruction over the top of the existing footpath for myself I would recommend following the existing. If there are any significant variations in the existing concrete surface these can be ground off and if desired a geotextile mat placed over the top of the existing concrete before the bedding material. The advantages of following the existing concrete path in terms of drainage, existing signs, familiarity for users etc make it a worthwhile approach. 


    BlogPageSpacerBlank


  • 3.  RE: Reinforced concrete over buried unreinforced concrete - Reflective cracking or not?

    Posted 21 January 2020 21:13
    You may want to investigate the "crack and seat" method with a new overlay of reinforced concrete. Admittedly this method has been used for rehabilitation of larger concrete roads but may be of use for a smaller footpath project.

    https://www.ipwea.org/blogs/intouch/2018/10/03/a-solution-for-ageing-concrete-roads
    BlogPageSpacerBlank


  • 4.  RE: Reinforced concrete over buried unreinforced concrete - Reflective cracking or not?

    Posted 09 February 2020 17:22
    ​Interesting read. Qld Main Roads did something like the 'crack and seat' approach on the old Ipswich Rd in the late 1980s. Ipswich Road was a concrete road build by the Yanks during the second world war and was well overdue for rehab. There we mounted a Demag diesel pile driver on a front end loader. The pile driver sat on a vetrical steel plate that acted as a cutting edge (with a timber cushion between the pile driver and steel plate). The plate was placed transversly on each concrete slab and the pile driver fired for about three blows to crack the slab at metre intervals. This process was repeated for the full length of the concrete road and then an airport roller (80T I think) run over the broken slabs to seat them. We then buried the lot under asphalt.

    For an unreinforced footpath, you might get away with running a roller over the top to seat the old concrete and then build the new path over it.

    ------------------------------
    Dereck Sanderson
    ------------------------------

    BlogPageSpacerBlank


  • 5.  RE: Reinforced concrete over buried unreinforced concrete - Reflective cracking or not?

    Posted 05 February 2020 22:34
    New footpath is reinforced?

    Do you mean reinforced with structural reinforcement to provide added bending capacity to bridge over the old footpath?

    OR

    Shrinkage cracking reinforcement? Noting shrinkage crack reo is not there to increase strength but to control shrinkage cracks. Practically it must add some strength of course, but notionally is not there for structural purposes. You balance out the cost of either removing old path completely (expensive) versus doing a bit of prep work on it and adding SL82 mesh in the footpath instead of SL62 or SL72. Could also look at a path a little thicker than standard 75mm thick - xtra concrete doesn't cost that much it is the labour and disposal (of waste) that are drivers of cost.

    In relation to reflective cracking - is this not more of an issue where you have an object with relatively high Elastic Modulus (concrete) embedded in say a flexible pavement with lower Elastic Modulus. When loads are applied the flexible pavement strains more (moves where concrete doesn't) and the stress field around the edge of the concrete causes a crack to reflect up through the flexible pavement.

    I do not think you have this scenario. You are putting a stiff relatively high Modulus material over a subgrade. As long as you ensure the subgrade is prepared reasonably by filling in voids and adding increasing reinforcement mesh size to make the overlay footpath stronger so it can bridge across any weak areas, I think you will be fine. You could argue that an existing footpath is a good subgrade as it is unlikely to displace too much, particularly if it has already settled. Might just form a deeper slab and fill voids as you pour.

    I guess it is a cost balancing exercise, but it seems logical to place footpath over old one. Just prepare it properly as you would for any concrete slab.

    ------------------------------
    Matthew Holt
    Construction & Maintenance Engineer
    Northern Beaches Council (NSW)
    t 02 9942 2843 m 0466 926 193

    northernbeaches.nsw.gov.au
    ------------------------------

    BlogPageSpacerBlank


  • 6.  RE: Reinforced concrete over buried unreinforced concrete - Reflective cracking or not?

    Posted 08 June 2020 00:23
    Working for a similar construction bussiness here in the States, this forum certainly helped me in various decision making requirements when it comes to Asset Management Software and other requirements.
    BlogPageSpacerBlank


  • 7.  RE: Reinforced concrete over buried unreinforced concrete - Reflective cracking or not?

    Posted 08 June 2020 00:24
    I agree to following the existing.
    Being from a construction bussiness, I also realise the importance of Asset Management Software required in the field with a tons of assets to manage among employees.
    BlogPageSpacerBlank