SubscribeInditherm plc has conducted significant R & D to develop modern, efficient curing systems for the manufacture of precast concrete.
The systems developed use an innovative, flexible carbon polymer technology (CPT), adapted for mould heating systems and concrete curing blankets. When placed in direct contact with concrete or concrete moulds, the technology 'warms' the fresh concrete within, accelerating early age strength gain.
The development provides an alternative to traditional but wasteful practices used within the precast sector, such as steam systems, open flame burners and oil heaters. Compared to steam curing systems, using CPT will reduce health and safety hazards as well as vastly reducing costs and environmental impact associated with using non-renewable fossil fuels.
Traditional steam curing of precast concrete pipes
Existing heating systems used for accelerating concrete strength at early ages rarely achieve the optimum curing temperature or the rate of heating and cooling specified in national standards and codes of practices. By conducting heat directly into early age concrete at the specified rate and temperature, production can be maximised by optimising mould usage. The use of a reliable heating system also enhances the economics of the concrete mix design, reducing the use of additives high cement contents, which are often excessively high so as to generate internal hydrating heat at early ages.
Inditherm Conductive Polymer Technology
The conductive polymer technology (CPT) is a low resistance flexible fabric that provides uniform heat distribution without localised hot or cold spots. Typically, the elements operate at 24Vac to provide highly controlled temperature profiles up to 120°C. Compared with other heating systems the use of low voltage CPT provides significant energy savings over traditional heating technologies.
Inditherm Conductive Polymer is produced in flexible sheets that can be fitted around most shapes
The curing systems developed by Inditherm optimise the use of insulation to minimise heat loss through the outside of the mould and maximise heat transfer into the concrete. Unlike traditional steam curing systems, the performance of the thermal jackets can be tightly controlled to ±1°C and can be isolated when not in use. Also, the heat output is not affected by the position of a mould, whereas location of the mould to the steam lines within a precast concrete plant is critical to the heating performance.
Testing Programme
In tests, thermal curing jackets were manufactured to cure concrete cast in 100 mm and 400 mm concrete cube moulds. The 100mm moulds used throughout the testing programme were standard cast-iron moulds specified in BS 8110 and used to manufacture samples for compressive testing. The 400 mm mould was a bespoke fabrication for the testing programme and was manufactured using 12mm thick mild steel plates.
The thermal jackets were configured using CPT heating elements encased in insulated covers that provided an even temperature for transfer of heat into the concrete in the mould. To minimise heat loss from the top surface of the concrete, a non-heated cover was manufactured to fix to the thermal jacket. The curing jacket was designed to operate using a 24V supply, controlled via a portable electronic control / transformer unit. The portable transformer unit controller could be programmed to provide three temperature profiles to control the rate of heating and cooling of the thermal jacket, so that compressive strength testing of the thermally cured concrete could be conducted.
Three curing profiles were used throughout the testing programme:
- Profiles 1 and 2 raised the concrete to 50 °C at heating rates of 10 °C and 15 °C per hour respectively from the point of casting the concrete in the mould.
- Profile 3 heated the concrete to 60 °C at a heating rate of 10 °C per hour.
The three profiles were adopted to cure the concrete for a period of 18 hours, including a cooling segment of the profile, where the rate of cooling was the same as the rate of heating for each.
To determine the operating temperatures of the heating elements, the curing jackets, and the temperature profiles throughout the concrete being thermally cured, thermocouples were embedded in the concrete and a 9 channel Pico temperature sensing unit used to record the temperature at different locations.
In addition to monitoring the thermal profiles within the 400mm cubes, nine 100mm cores were taken from each cube, cut into three (top, centre, bottom), trimmed to 100mm in length and compressive tested three days after casting.
Test Results
The effect of the thermal curing jackets on short and long term strength development of concrete was determined by conducting a variety of tests on the 100mm cubes that had been stored in a controlled environment. A wide spread of results were obtained from the tests. The most significant was the strength development over time compared to concrete cured under ambient conditions and concrete baked in an oven a 70°C.
Strength development of concrete at different ages when cured using the thermal jacket compared to oven curing and no curing method.
The above results show that the early rate of strength development increased when using CPT thermal jackets to cure concrete, compared to samples cured in the control environment. These results clearly show that the target 28 day strength (42N/mm²) was achieved in less than 4 days.
At 92 days, there was also an increase in the compressive strengths for the samples cured using the CPT jackets, compared to samples cured in an oven at 70°C or at ambient temperature. It must be noted that the overall compressive strengths attained using the controlled curing process are higher and reached in a shorter time than both conventional methods.
Having taken core samples as described from 400mm cubes and tested them, there appeared to be a normal standard distribution of compressive strengths, indicating that the CPT blankets increased the strength development within the cubes, whilst not affecting the strength distribution compared to normally cured concrete.
Applying CPT
Inditherm’s CPT can be applied by using heating pads within the walls of concrete curing moulds.
Inditherm conductive polymer is installed within the precast mould walls
The benefits of using Inditherm CPT are significant:
- Health and safety are enhanced, with the use of a low voltage power supply and no need to connect and disconnect hot steam pipes.
- Elimination of the risk of steam pipes leaking or coming into contact with users causing personal injury.
- Significantly reduced energy costs. Energy consumption is typically only 25% of alternative technologies.
- There is no need for expensive plant room and associated piping, nor the need for pressure vessel approvals required to generate the steam.
- Concrete curing and strength development are enhanced, with more even temperature distribution throughout the moulds and precast elements.
Conclusions
The test results clearly show that using CPT thermal jackets will heat concrete evenly throughout, accelerating early age strength development and not affecting the long term strength development of the concrete. The testing also indicated that the CPT thermal jackets can be used to minimise the effect of temperature gradients throughout concrete elements caused by heat generated by hydration, thus reducing internal stresses.
Combined with the reduction in energy usage, maintenance costs and improving health & safety using CPT is the most effective method to deliver the best concrete in the shortest time.
For further information contact David Catley, Business Manager, Inditherm Construction on +44 (0)1709 761000 or at dcatley@indithermplc.com.