It also emerged that the ICE was working with the government to beef up the maths and physics components.
Teaching unions expressed unease about how the new qualification would be funded and how its IT system would function.
They also demanded new guidance on career advice for pupils considering the diploma and sought information on how students from non-urban areas would reach the specialist schools set up to teach the diploma.
The ICE acknowledged that some of the academic content of the qualification was lacking, and that the profile of the diploma was too low.
ICE director of membership David Lloyd-Roach said: "[The Institution] is currently working with [government skills body] ConstructionSkills to ensure the maths and physics content is up to scratch and also working with universities, employers and teachers to secure support for and understanding of these qualifications."
Association of Head Teachers general secretary Mick Brookes said: "Inevitably with systemic change of this magnitude, there are bound to be doubts and uncertainties."
One teacher working on the new diplomas said: "I am currently writing the curriculum. There's an awful lot of work to be done – it's a new way of working, which always makes things difficult."
He added that many teachers were worried about the implementation of the new courses, but admitted there was enthusiasm from pupils. This tallies with research conducted by the National Union of Teachers (NUT).
"Even in the schools introducing the diplomas the majority of staff are unfamiliar with them," said acting general secretary Christine Blower.
"Neither have the majority received clear, unambiguous information about the diplomas. Although teachers directly involved in teaching the diplomas consider that training for them has been appropriate, 43% believe that they have not received sufficient training."
The latest concerns surfaced after the Civil Engineering Contractors' Association (CECA) again called for the government to release figures for the number of students confirmed to study the new diploma in construction (News last week).
Lloyd-Roach said that the diplomas could be a success "if delivered correctly".
"We believe these diplomas could encourage a wider range of school children into the civil engineering and construction industries and help produce a new generation of highly qualified professionals," he said.
ICE Council member for London Angus McAvoy said he was urging pupils to opt for the diploma. He said the maths content would be more attractive to universities (see comments here).