The Effect of Cracking on the Dynamic Modulus of Reinforced Concrete Beams
Structural engineers continue to refine their understanding of material and structural behaviour, which has had consequences on the dynamic behaviour of modern structures in the following ways:
- Structures have becoming increasingly slender
- A reduction in material results in a lower member mass, and therefore a lower natural frequency
- Structures may experience vibration above the acceptable limit as a consequence of people walking or other excitations.
To create a good design taking into account these issues, requires better understanding of the dynamic modulus of a reinforced concrete slab.
Aims and objectives
- To determine the relationship between the dynamic modulus and the extent of cracking. This will be achieved by building and testing a number of reinforced concrete (RC) beams of varying natural frequencies and subjected to a number of different static loading conditions
- The group will determine the relationship between amplitude of vibration and how this relates to cracking
- Investigations will be made to determine the properties of the RC beam, by altering the percentage of reinforcement, grade of concrete and type of aggregate used. Oasys GSA software (industry standard software, used for projects such as the Welsh National Assembly, the Olympic Aquatic Centre in Beijing and The Pinnacle in London) will be used to monitor the effects.
Design of beams, concrete and rig
- Three cantilever beam lengths will be tested – 3.0m, 2.5m and 2.0m. Each cantilever length will comprise three specimens totalling 9 beams. The beams cross-section are kept constant at 300mm wide by 100mm deep.
- The aim is to design a concrete mix and reinforcement typical of that used in industry.
- The concrete was mixed and cast in the engineering laboratory using reusable wooden moulds and will be cured for a minimum of 28 days prior to testing. Once cured, the beams will be craned into the testing rig.
- Two rigs have been constructed, using steel meccano, to allow the beams to be tested simultaneously which reduces the testing time required.
Theoretical and computational predictions
Predictions regarding the development of cracks, displacement and frequencies have been achieved through utilising Excel, MATLAB and Oasys GSA.
It is more difficult to make predictions on the extent of cracking in a concrete beam. The more extensivly a beam cracks, can lead to a lower second moment of area. As the amount of cracking cannot be accurately predicted, the behaviour of the beam is difficult to anticipate with certainty. The project group uses these experiments to come to a conclusion.
Quantifying the extent of cracking is necessary to understand the changes in second moment of area. Together with experimental dynamic testing, the beam will be used to determine the effects of cracking on the dynamic modulus of a concrete beam.
Illustrations above show Oasys GSA beam shape predictions