Contents
1.1 Introduction
In the design and construction of slabs and footings it is important to consider the durability of the finished product and devise ways in which to ensure the strength for which the slab was designed is maintained. Construction techniques and standards exist to ensure that all work is carried out to a specific level of quality to save the hassle of repairing prior mistakes. As with all business time efficiency is of paramount importance and sometimes contractors can overlook crucial elements of the design and supervision. Of course second only to efficiency is cost and usually this is of a high priority.
Theoretically building can be planned in infinite detail and problems could usually be eliminated before construction commences, this is however not the case in practice. Builders on site often have to improvise and take unknown elements into consideration without consulting the designers. Experience in construction usually prevails and minor adjustments to the plan can be made with negligible consequence. This however can not be done in all circumstances and often attempts have costly results.
1.2 Theory
In order to minimize the possibility for these types of error, builders must adhere to a strict code (AS2870) in which all aspects of construction are outlined and processes are detailed. If mistakes occur and for any reason a structure’s integrity breached the builder responsible must ensure that they have performed all their work in accordance with the guidelines in order to disclaim responsibility.
The Building Services Authority (BSA) is responsible for the licensing of all builders in Queensland and is therefore required to hold a high degree of responsibility for their work. Slab and footing design is crucial to the integrity of a structure and therefore must be constructed under the most stringent supervision.
To reduce the frequency and eventually eradicate such error the system of Quality Control could be amended to include a more specific soil classification table and ideally a geotechnical report should be conducted including recommendations for stabilizing mediums such as sand and aggregate.
1.3 Background
In cases where foundations have been incorrectly laid, costly claims often follow. As a residence relies only on these foundations for support, any degree of movement can have disastrous results. Affects range from simply aesthetic to structural integrity failure. The serious nature of these mistakes usually requires repairs involving full or partial demolition and reconstruction.
Depending on the size, type and locality of a structure the foundations are exposed to a variety of elements both natural and manmade. The movement of foundations is the most commonly reported problem and also the most detrimental. Movement is attributed to uneven soil structure beneath typically of high percentage clay content.
2.0 Findings
In many reported cases most failures could have been prevented using better knowledge of the soil type and reactivity. Selecting the appropriate foundation design for specific soil types increases the resistance to movement. If cracking occurs due to excessive movement the depth and spacing of the footing beams or piles is usually responsible. In reactive clay soils the saturation of the soil is directly proportional to movement and the effects on the structure vary according to the positioning of the slab.
In all concrete structures cracking will usually occur and in most cases will not effect the ability of the member to support stresses. Because of the curing process shrinking and expanding will occur according to moisture but this is taken into account in the design process and doesn’t weaken a structure. However if cracking appears obvious and continues to occur Australian Standard AS2870 (1996: 57) gives the details of cracking and width significance.
Before visual signs of excessive movement become obvious there are indicators which if discovered early could save costly repairs from becoming necessary. Finishing items within a house such as windows and door jambs can often give insight into how stable a residence is. When installed these items are square and movement will cause them to warp. Excessive sticking or rough action may indicate settling of the foundations and if reported can be remedied using underpinning processes without further destruction occurring.
Indications of Potential Movement/Settling
Item Problem
Windows/Doors Sticky or Difficult opening or closing
Wall cladding Cracking along ceiling joinery
Cabinetry Joinery becoming uneven/ larger
As settling and natural fluctuations in moisture content is unavoidable other solutions must be discussed as to reduce the effect of these processes on structures. Because cost effectiveness is always a high priority a solution involving more material such as larger beams may not prove favorable but will reduce ongoing maintenance and repair expenses.
For the most part, greater supervision during the crucial stages of construction would remedy most problems. Adhering to the guidelines for reinforcing material like steel mesh as detailed in AS2870 (1996: 32 Table 3.2)
Where joining of mesh is required specifically in beams consistent high quality welds need to be performed or alternatively overlapping and the use of tie wire.