The Elgin-based group says the proceeds will help it to reinvest in larger infrastructure projects and continue its diversification into project management, consultancy and advisory services to the public sector.
The deal with Dalmore Capital, worth a total £73 million, will see investments in 21 projects pass to Cobalt Project Investments, a London-based fund managed by Dalmore.
They include 16 investments in the Elgin Infrastructure Fund, where 3i was minority investor alongside Robertson Capital projects (RCP), and a further five held by RCP Investments, a wholly-owned subsidiary which is being sold as part of the deal.
The RCP holdings were public sector projects developed by the company, which will continue to provide management services to Cobalt Projects. Robertson's facilities management division has an order book of around £1 billion covering the next two decades.
Bill Robertson, chief executive, said: "RCP, via these historic investments and established relationships, has been a key partner in the delivery of 63 schools with 42,000 pupils, 11 hospitals with more than 1100 primary care beds, 10 primary care resource centres, and 280 homes serving the armed forces. RCP has a further £500m secured pipeline within the public sector, with 10 identified projects requiring investment activity."
He said the portfolio sale was the third and largest undertaken by the company in its 15 years of activity in the sector. RCP had continued to grow over the last three years and had "an exceptionally strong forward order book at this time".
Mr Robertson added: "This well-timed sale has come at an important juncture for the business. We are currently in year three of a five-year plan aimed at enhancing the structure of the group and improving profitability in a changing and fluid market place."
The group's last accounts showed a near trebling of operating profit to £2.85m but a fall of around 80% in underlying pre-tax profits to £709,000, and a cutting of debt from £55m to £50m.
The capital projects arm last year led a consortium which secured the Scottish Futures Trust east central hub location with an estimated £500m pipeline of work, plus PFI hospital projects at Strathcathro in Angus and Murray Royal in Perth.
Mr Robertson is looking for greater incentives in Chancellor George Osborne's autumn statement for private sector companies to invest. He expects next year will see big public sector projects coming through, enabling the group to take on more skilled workers.
Michael Ryan, chief executive and partner of Dalmore Capital, said the portfolio being acquired comprised "21 operational phase assets throughout the UK".