CROSS Safety Report
Gain in strength of mortar slower than concrete
This report is over 2 years old
Overview
A masonry wall settled and cracked after four courses of Engineering grade B bricks were substituted (not by the designer) for the original in-situ concrete padstones on cost and handling grounds.
Key Learning Outcomes
For the construction team:
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Any alterations to structural elements should be approved by the designer prior to works being carried out on site
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Quality control and competent supervision on site can help to ensure that the structure is built in accordance with the design
Full Report
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On a four storey load-bearing masonry residential scheme, steel beams were used where the walls did not align. To support these 600mm long padstones were required but four courses of Engineering grade B bricks were substituted (not by the designer) for the original in-situ concrete padstones on cost and handling grounds.
The programme was such that the next floor of precast units was installed after a week, causing the brickwork to settle and crack. Whilst multiple unit brick padstones may provide an attractive handling and cost solution, it needs to be noted that the gain in strength and stiffness will be nearer the 28 days and that such padstones do not have the inherent robustness of an in-situ concrete component.
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Feedback
This issue can be addressed with the use of a rapid hardening cement. Not to confuse hardening and setting, the two are independent and can be designed separately.
Final hardness/strength is often taken at 28 days, but continues indefinitely at a slowing rate. It is quicker at higher temperatures. Many modern cements gain substantial strength in much less time. Match testing can be done when strength on loading is important. Strength is a function of water/cement ratio for mortar as for concrete.
There are numerous specialised cements/mortars, and Contractors accustomed to using them. Any of the larger firms' Intranets should refer, and it is impressive what can be found on the web. Useful information can be found on the Sustainable Concrete website: http://www.sustainableconcrete.org.uk/main.asp?page=140.
Expert Panel Comments
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It is common practice for engineering bricks to be used at the top of walls to carry high concentrated load positions. Any wet placed material like masonry mortars or indeed in situ concrete need a period of curing to establish sufficient strength for loading. The full strength gain is normally expected to occur at 28 days, but loading of upper construction can take place earlier as usually only dead load is being added.
At high load positions under beams, in-situ or precast concrete spreaders are better options. They do need to be of the right proportions to distribute the concentrated loading and any bedding mortars need to be specified correctly with time given for them to reach a suitable strength to carry construction loads from above.
Substitution of a component on site without reference to the designer is an unacceptable practice. This report illustrates the often significant difference between formal procedures i.e. no changes are made to the permanent works without a competent designer agreeing to them, and custom and practice on many sites where significant changes are made on a unilateral basis.