Additionally, total backlog of $17 billion at the end of the quarter was also a record, which we believe reflects the benefits of our collaborative approach with the customers, favorable end market dynamics and continued advancement of our long-term growth strategies.
During the quarter, Hurricane Ida made landfall over Louisiana, which ultimately left 1.2 million customers across eight states without power, including one million outages in Louisiana alone.
Quanta deployed significant resources to support utility customers through electric power infrastructure who was damaged or destroyed by the hurricane, including more than 2,500 line workers and front-end support services and engineering staff.
For example, Blattner is currently constructing more than 30 utility-scale renewable energy projects across the country, and another Quanta company is currently working on the largest solar-powered battery storage projects in North America.
The initiative would leverage existing pipelines, requires a new pipeline and facility investment, which when fully constructed would be capable of transporting more than 20 million tons of carbon dioxide annually.
We believe Quanta's diversity, unique operating model and entrepreneurial mindset form the foundation that will allow us to continue to generate long-term value for our stakeholders.
Today, we announced record third quarter 2021 revenues to $3.4 billion.
Net income attributable to common stock was $174 million or $1.21 per diluted share.
And adjusted diluted earnings per share, a non-GAAP measure, was $1.48.
Our electric power revenues were $2.3 billion, a quarterly record and a 10% increase when compared to the third quarter of 2020.
Also contributing to the increase were revenues from acquired businesses of approximately $55 million.
Electric segment operating income margins in 3Q '21 were 12.4%, slightly lower than 12.7% in 3Q '20 but better than our initial expectations.
Operating margins benefited from record emergency restoration revenues of approximately $230 million, which typically present opportunities for higher margins than our normal base business activities due to higher utilization as well as overall solid execution across our electric operations.
Additionally, segment margins benefited from approximately $10 million of income associated with our LUMA joint venture.
Underground Utility and Infrastructure segment revenues were $1.02 billion for the quarter, 12% higher than 3Q '20 due primarily to increased revenue from gas distribution and industrial services.
Third quarter operating income margins for the segment were 6.7%, 170 basis points lower than 3Q '20, but generally in line with our expectations.
Our total backlog was $17 billion at the end of the third quarter, the fifth consecutive quarter we've posted record total backlog.
Additionally, 12-month backlog of $9.8 billion also represents a quarterly record.
However, the total backlog from Blattner and the other 4Q acquisitions is approximately $1.8 billion.
For the third quarter of 2021, we generated negative free cash flow, a non-GAAP measure, of $40 million compared to $70 million of positive free cash flow in 3Q '20.
Also, 3Q '20 benefited from the deferral of $41 million of payroll taxes in accordance with the CARES Act, 50% of which are due by December 31, 2021, with the remainder due by December 31, 2022.
Days sales outstanding, or DSO, measured 89 days for the third quarter of 2021, an increase of seven days compared to the third quarter of 2020 and an increase of six days compared to December 31, 2020.
Prior to the closing, in September 2021, we issued $1.5 billion aggregate principal amount of senior notes with a weighted average interest rate of 2.12%, receiving net proceeds of $1.48 billion.
Accordingly, as at quarter end, we had approximately $1.7 billion of cash.
Subsequent to the quarter, we amended our credit agreement to, among other things, provide a term loan facility of $750 million, which was fully drawn and combined with the net proceeds from the senior notes offering to fund a substantial majority of the cash consideration payable to the Blattner shareholders at closing.
That said, excluding the expected contributions from the recently acquired companies, we now expect full year revenues from our legacy operations to range between $12.15 billion and $12.35 billion.
Due to the strength of our consolidated performance for the first nine months of the year, we are increasing our expectations for the contribution of our legacy operations to adjusted EBITDA to range between $1.17 billion and $1.2 billion, with the midpoint of the range representing an increase over our previous guidance and 13% growth when compared to 2020's record adjusted EBITDA.
We continue to expect full year revenues to range between $8.7 billion and $8.8 billion for our legacy electric segment operations.
However, based on the strong performance through the first nine months of the year and continued confidence in our ability to execute on the opportunities across the segment, we've increased our full year margin range for the segment with 2021 operating margins expected to come in slightly above 11%.
Accordingly, we are reducing our full year expectations for the segment with revenues now expected to range between $3.45 billion and $3.55 billion while segment margins are now expected to range between 4.5% and 5%, which includes a $23.6 million provision for credit loss recognized in the second quarter, a nearly 70-basis-point negative impact on a full year basis.
With regard to the recently acquired companies operations I spoke of earlier, including Blattner, we expect post-closing revenue contributions for the year to range between $400 million and $500 million and adjusted EBITDA, a non-GAAP measure, ranging between $40 million and $60 million.
Accordingly, including the expected contributions from the recently acquired companies, we now expect our consolidated full year revenues to range between $12.55 billion and $12.85 billion and adjusted EBITDA, a non-GAAP measure, of between and $1.21 billion and $1.26 billion.
We currently estimate amortization expense for the full year will be between $149 million and $159 million, with $60 million to $70 million attributable to the recently acquired companies.
Stock compensation expense for the full year is now expected to be approximately $89 million, with approximately $2 million attributable to restricted stock units issued to employees of the acquired company.
Acquisition and integration costs are expected to be approximately $26 million for the fourth quarter, resulting in approximately $36 million for the year.
This includes approximately $10.5 million of expenses associated with change of control payments awarded to certain employees of Blattner by the selling shareholders, which require expense accounting as they have a one-year service period requirement.
Below the line, we expect interest expense for the year to be around $67 million, which includes approximately $16 million of incremental interest expense associated with debt financing used to fund the cash portion of the Blattner acquisition.
Additionally, we now expect our full year tax rate to be around 24%, reflecting a slight reduction from our prior expectations due primarily to a favorable shift in the mix of earnings between various taxing jurisdictions.
As a result, our expectation for full year diluted earnings per share attributable to common stock is now between $3.20 and $3.40.
And our increased expectation for adjusted diluted earnings per share attributable to common stock, a non-GAAP measure, is now between $4.62 and $4.87.
On a consolidated basis, we now expect free cash flow for the year to range between $350 million and $500 million.

Summary:
We believe Quanta's diversity, unique operating model and entrepreneurial mindset form the foundation that will allow us to continue to generate long-term value for our stakeholders.
Today, we announced record third quarter 2021 revenues to $3.4 billion.
Net income attributable to common stock was $174 million or $1.21 per diluted share.
And adjusted diluted earnings per share, a non-GAAP measure, was $1.48.
Prior to the closing, in September 2021, we issued $1.5 billion aggregate principal amount of senior notes with a weighted average interest rate of 2.12%, receiving net proceeds of $1.48 billion.
Accordingly, including the expected contributions from the recently acquired companies, we now expect our consolidated full year revenues to range between $12.55 billion and $12.85 billion and adjusted EBITDA, a non-GAAP measure, of between and $1.21 billion and $1.26 billion.
And our increased expectation for adjusted diluted earnings per share attributable to common stock, a non-GAAP measure, is now between $4.62 and $4.87.