Jim Nickolas will then review our first quarter 2020 financial results and liquidity position, and then Ward will provide some closing comments.
We're making an exception and as highlighted on our supplemental information slides 13 and 14, we are sharing preliminary April shipment and pricing trends to provide greater transparency as to what we're now seeing in a COVID-19-impacted month.
April revenues for the Magnesia Specialties business are $10 million lower than the comparable prior year period.
Prior to the COVID-19 outbreak, we were highly confident in realizing the full announced increase of $8 per ton implemented April 1, given overwhelming market support in a tight Texas cement environment.
Florida DOT, for example, has accelerated over $2 billion of critical transportation projects to leverage construction efficiencies resulting from lower vehicle traffic, including closing additional travel lanes and performing more daytime hours work.
The first recommendation is the federal backstop of nearly $50 billion in immediate flexible funding to offset the estimated 30% loss in state transportation revenues over the next 18 months.
In addition to strengthening our balance sheet through a timely $500 million bond offering in early March, we've cut nonessential costs, reduced capital spending for discretionary projects and implemented hiring restrictions.
First, Martin Marietta has a much stronger geographic and competitive position today compared with any previous downturn in our more than 25 years as a public company.
Today, we continue to generate record profitability on aggregate shipment levels much lower than our peak volumes in 2005, and with a geographic footprint that we've not only considerably expanded but also improved.
We now have leading positions in 90% of our markets, up from 65% a decade ago, which supports favorable pricing trends, economies of scale and cost flexibility.
In brief, Martin Marietta has the right strategies, priorities, experience and teams to responsibly manage us through these challenging times.
On a consolidated basis, total revenues increased 2% to $958 million, a first quarter record.
Selling, general and administrative expenses improved 10 basis points as a percent of total revenues.
Diluted earnings per share was $0.41 and adjusted earnings before interest, taxes, depreciation, depletion and amortization, or adjusted EBITDA, decreased 6% to $149 million.
The Building Materials business achieved record first quarter revenues.
Key takeaways include the following: aggregates product gross profit decreased $5 million, largely due to our quarterly update of inventory standard costs.
Our per ton production costs have been trending downward over the last 12 months as cost control measures and operating efficiencies have their desired effect.
But in this quarter, it also resulted in a $4 million expense as we reflected the lower cost in our updated inventory valuation.
In contrast, during the first quarter of 2019, we recorded an $11 million inventory valuation adjustment to reflect the higher production costs experienced during the weather-impacted year in 2018.
While this cumulative $15 million year-over-year inventory standard variance reduced quarterly product gross margin by 260 basis points, lower unit production costs will provide future benefits.
Cement product gross margin expanded 1,170 basis points driven by improved cost absorption from higher production levels as well as increased shipments to the San Antonio, Austin and Houston markets.
Gross profit for the ready mixed concrete business declined $9 million.
For the Magnesia Specialties business, product revenues decreased 13%, consistent with expectations as chemicals customers continue to reduce inventory levels for reasons unrelated to COVID-19.
Notably, product gross margin improved 500 basis points despite lower revenues, driven by ongoing cost control measures and lower energy costs.
In addition to the aggregates inventory standard adjustments discussed earlier, these items also included $6 million of other nonoperating expenses to finance third-party railroad maintenance in exchange for a federal income tax benefit of approximately $7 million, which drove the low-income tax rate for the first quarter 2020.
Lastly, we incurred a noncash expense of $2 million to implement a new paid time-off policy for our employees.
As you will see on slide five, first quarter 2020 earnings before income tax, absent items affecting comparability, improved $15 million.
Before the extent of the economic disruption became better understood, we repurchased 211,000 shares during the first quarter.
We now estimate full year capital expenditures will be $325 million to $350 million, down from our original guidance of $425 million to $475 million.
We've strengthened the company's balance sheet and cash position with our timely bond offering in early March, issuing $500 million of 10-year senior notes at a 2.5% coupon.
Proceeds will be used to repay the $300 million of floating rate notes that mature later this month, with the bulk of the remaining cash preserved on the balance sheet.
Net cash, combined with the nearly $760 million available on our existing revolving facilities, provided total liquidity of approximately $880 million at the end of the quarter.
Additionally, at 2.3 times net debt to the consolidated adjusted EBITDA, we remain well within our target leverage ratio of two to 2.5 times at the end of the first quarter.
In regards to our 2020 full year guidance, we will reinstate earnings guidance once we have sufficient visibility to do so.

Summary:
Jim Nickolas will then review our first quarter 2020 financial results and liquidity position, and then Ward will provide some closing comments.
In brief, Martin Marietta has the right strategies, priorities, experience and teams to responsibly manage us through these challenging times.
Diluted earnings per share was $0.41 and adjusted earnings before interest, taxes, depreciation, depletion and amortization, or adjusted EBITDA, decreased 6% to $149 million.
The Building Materials business achieved record first quarter revenues.
In regards to our 2020 full year guidance, we will reinstate earnings guidance once we have sufficient visibility to do so.