ended December 31, 2015 compared to December 31, 2014, is due primarily to a 2% decrease in sales volumes, which adversely impacted revenues by approximately $2.1 million, partially offset by an increase in average net sales prices that positively impacted revenues by approximately $0.4 million. The decrease in sales volumes was related to lower sales volumes in the gypsum wallboard industry, as noted above.
Operating earnings decreased to $22.1 million for the nine months ended December 31, 2015, compared to $24.6 million for the nine months ended December 31, 2014. The decrease in operating earnings is primarily due to increased operating costs and decreased sales volumes, which negatively impacted operating earnings by approximately $2.3 million and $0.4 million, respectively, partially offset by increased sales volumes, which positively impacted operating earnings by $0.4 million. The increase in operating costs is primarily related to increased recycled fiber and annual maintenance costs, which negatively impacted operating earnings by approximately $1.6 million and $1.3 million, respectively, partially offset by lower natural gas costs, which positively impacted operating earnings by approximately $1.2 million. The increase in operating costs was the primary reason operating margin decreased to 20% during the nine months ended December 31, 2015, compared to 22% during the nine months ended December 31, 2014.
Concrete and Aggregates Operations. Concrete and aggregates revenues increased 17% to $31.8 million for the three months ended December 31, 2015, compared to $27.1 million for the three months ended December 31, 2014. The primary reason for the increase in revenue for the third quarter of fiscal 2016, compared to the third quarter of fiscal 2015, was the 5% and 13% increase in average net sales prices for concrete and aggregates, respectively, which positively impacted revenues by approximately $2.1 million. Additionally, sales volumes for our concrete and aggregates businesses increased 8% and 16%, respectively, during the three months ended December 31, 2015, compared to December 31, 2014, positively impacting revenue by approximately $2.6 million. Increases in sales volumes were primarily in our greater Kansas City market.
Operating earnings decreased 7% to approximately $1.5 million for the three months ended December 31, 2015, compared to $1.6 million for the three months ended December 31, 2014. Operating earnings during the three months ended December 31, 2015, compared to the three months ended December 31, 2014, were adversely impacted by increased operating costs, which decreased operating earnings by approximately $2.3 million, partially offset by increased average net sales prices, which positively impacted operating earnings by approximately $2.1 million. The increase in operating costs during the three months ended December 31, 2015, compared to the three months ended December 31, 2014, was primarily related to purchased materials and maintenance, which adversely impacted operating earnings by approximately $1.4 million and $0.6 million, respectively.
Concrete and aggregates revenues increased 14% to $97.0 million for the nine months ended December 31, 2015, compared to $85.2 million for the nine months ended December 31, 2014. The primary reason for the increase in revenue for the nine months ended December 31, 2015, compared to the nine months ended December 31, 2014, was the 7% and 10% increase in average net sales prices for concrete and aggregates, respectively, which positively impacted revenues by approximately $6.6 million. The increase in sales volumes by our concrete business was partially offset by lower sales volumes in our aggregates during the nine months ended December 31, 2015, compared to December 31, 2014, and positively impacted revenues by approximately $5.2 million.
Operating earnings increased 23% to approximately $7.3 million for the nine months ended December 31, 2015, compared to $6.0 million for the nine months ended December 31, 2014. Operating earnings were positively impacted by increased average net sales prices and sales volumes, which positively impacted operating earnings by approximately $6.6 million and $0.4 million, respectively, partially offset by increased operating costs of approximately $5.7 million during the nine months ended December 31, 2015, compared to the nine months ended December 31, 2014. The increase in operating costs during the nine months ended December 31, 2015, compared to the nine months ended December 31, 2014, was primarily related to purchased materials and maintenance, which adversely impacted operating earnings by approximately $4.5 million and $0.7 million, respectively.