Reproduced from my Atkins blog of 30.06.2011.
Isn’t it great when something that has been quietly bugging you for years finally gets sorted out in your head?
Back in 1989 I worked on site in Newcastle using tunnel formwork to pour hotel bedrooms - the Copthorne Closegate Hotel, now you ask. The keys to the rapid reuse of this expensively hired formwork were heating of the newly poured slabs and walls using propane burners and ‘match cured’ cubes resting on top of the slab. These were crushed twelve hours after the pour and proved that we had the strength we needed for depropping.
Looking back it was all rather basic and not helped by a particularly dopey contractor (who went bust later in the project, so I’m probably safe to say that). But at least we thought it was conservative. However it has always nagged at me that there must be a more scientific way accurately establish the rate of strength gain on site.
Christer Isgren from OBTconcrete gave a lunchtime presentation to the Building Structures team in Epsom last week and spoke about their START system for monitoring the early strength gain of insitu concrete. They work closely with RC frame contractors who are looking to accelerate their programmes through rapid turn-around of formwork or early prestressing.
This is probably all old hat for the nuclear and dam boys, used to big pours, who need to control early age temperatures and cracking, but it looked like rocket science to this building engineer!
But the great thing is that, at heart, it is blindingly simple once you look past all the temperature sensors and wiring around the pour:
1. It is all based around knowing exactly how your concrete performs at 20degC. They crush lots of cubes at the start of the job for every type of concrete used – cement supplier, cement replacements, additives.....all must match.
2. ‘Setting’ is actually a chemical reaction and it relies on water. Thus when the concrete is at zero the reaction stops as the water is frozen.
3. The brilliantly simple bit is that the speed of ‘setting’ is basically linearly proportional to the temperature above zero. So if the concrete were at 10degC for four days it would have the same strength as a two day cube. If it had been at 40degC it would be the same as an eight day cube.
Of course it is more complicated than that actually, but that is enough for me to know what is going on.
The START system monitors the temperature of the concrete both on the surface and inside. It accurately measures that early period (perhaps several days) when the heat of hydration pops the temperatures up to 40degC or more. During this period the insitu concrete’s strength is actually ahead of the cube in its warm bath, and it is this period that START allows the contractor to exploit.
Christer’s background is from Sweden and Skanska, and it was interesting to hear how they had to ‘relearn’ many things in the UK due to our regular use of PFA and GGBS. He also discussed the important differences between, and impacts of, the many retarders, accelerators, etc. out there on the market.
Of course, as a building design consultant all this is ‘interesting’ but will be the responsibility of the contractor to sort out, not me.
However, I have stopped that 22 year old doubt nagging at me. No longer wilI I feel the need to I ask for the whole building to have a warm bath for 28 days.
Isn’t it great when something that has been quietly bugging you for years finally gets sorted out in your head?
Back in 1989 I worked on site in Newcastle using tunnel formwork to pour hotel bedrooms - the Copthorne Closegate Hotel, now you ask. The keys to the rapid reuse of this expensively hired formwork were heating of the newly poured slabs and walls using propane burners and ‘match cured’ cubes resting on top of the slab. These were crushed twelve hours after the pour and proved that we had the strength we needed for depropping.
Looking back it was all rather basic and not helped by a particularly dopey contractor (who went bust later in the project, so I’m probably safe to say that). But at least we thought it was conservative. However it has always nagged at me that there must be a more scientific way accurately establish the rate of strength gain on site.
Christer Isgren from OBTconcrete gave a lunchtime presentation to the Building Structures team in Epsom last week and spoke about their START system for monitoring the early strength gain of insitu concrete. They work closely with RC frame contractors who are looking to accelerate their programmes through rapid turn-around of formwork or early prestressing.
This is probably all old hat for the nuclear and dam boys, used to big pours, who need to control early age temperatures and cracking, but it looked like rocket science to this building engineer!
But the great thing is that, at heart, it is blindingly simple once you look past all the temperature sensors and wiring around the pour:
1. It is all based around knowing exactly how your concrete performs at 20degC. They crush lots of cubes at the start of the job for every type of concrete used – cement supplier, cement replacements, additives.....all must match.
2. ‘Setting’ is actually a chemical reaction and it relies on water. Thus when the concrete is at zero the reaction stops as the water is frozen.
3. The brilliantly simple bit is that the speed of ‘setting’ is basically linearly proportional to the temperature above zero. So if the concrete were at 10degC for four days it would have the same strength as a two day cube. If it had been at 40degC it would be the same as an eight day cube.
Of course it is more complicated than that actually, but that is enough for me to know what is going on.
The START system monitors the temperature of the concrete both on the surface and inside. It accurately measures that early period (perhaps several days) when the heat of hydration pops the temperatures up to 40degC or more. During this period the insitu concrete’s strength is actually ahead of the cube in its warm bath, and it is this period that START allows the contractor to exploit.
Christer’s background is from Sweden and Skanska, and it was interesting to hear how they had to ‘relearn’ many things in the UK due to our regular use of PFA and GGBS. He also discussed the important differences between, and impacts of, the many retarders, accelerators, etc. out there on the market.
Of course, as a building design consultant all this is ‘interesting’ but will be the responsibility of the contractor to sort out, not me.
However, I have stopped that 22 year old doubt nagging at me. No longer wilI I feel the need to I ask for the whole building to have a warm bath for 28 days.
