The article describes the laboratory evaluation of mixtures of sand modified with wood fly ash (WFA) and additionally stabilized with different amounts of cement. Laboratory research includes determining the California Bearing Ratio (CBR), compressive and indirect tensile strengths of the mixtures, and the resistance of mixtures to freezing/thawing cycles. The aim of the research is to determine if WFA, an alternative material, can improve sand bearing capacity and contribute to strength development while reducing necessary cement amounts and satisfying the technical regulation for use in pavement base courses. The test results obtained show that WFA has a considerable stabilization effect on the sand mixture and improves its load bearing capacity. By adding a small quantity of the cement, the hydraulic reaction in the stabilized mixture is more intense and results in greater strengths and an improved resistance to freezing. The test results show that, by replacement of part of the sand with WFA (in the quantity of 30%), greater strengths can be achieved in relation to the mixture of only sand and cement. Additionally, the content of cement necessary for the stabilization of sand (usually 8–12%) is considerably reduced, which enables cost savings in the construction of pavement structures.

Stabilized mixes that are used in pavement structures are composed of aggregate bound with hydraulic binders (cement, lime) or bitumen. The most commonly used for the construction of base layers are mixes stabilized with cement. A long-standing construction practice for pavement structures was based on the use of quality granular materials for the construction of base layers. However, when designing the pavement structure and selecting materials, economy, sustainability, and environmental impact, in addition to their mechanical properties, should also be considered. Clear requirements and guidelines for sustainable development have imposed the need to explore the possibility of using non-standard materials in construction. Wood ash, which is formed as a residue from the combustion of biomass in the production of electricity and heat, is one of the newer and, in Croatia, less researched alternative materials that can be applied in construction. The paper describes compressive strength tests of mixtures of sand from the Drava River and cyclone wood ash stabilized with various contents of cement. The obtained results showed that with wood fly ash (in a content of 30 % mass.) in the stabilization mixture of sand, values of compressive strengths can be achieved within the required limits necessary for the construction of base layers of the pavement structure stabilized by a hydraulic binder.

Different types of natural aggregates (gravel, crushed stone, sand) are used in the construction of load-bearing layers of pavement structures, the granulometric composition of which has a great influence on the compaction and load-bearing capacity of the layer. Special care should be taken when compacting single-grain materials, such as sand, as difficulties can be expected with these. For this reason, it is useful to modify the granulometric composition of the sand by adding new material of finer fractions. The bearing capacity of mixtures for the production of unbound load-bearing layers of a pavement structure is tested by determining the California Bearing Ratio (CBR), the value of which depends on the dry bulk density achieved and the granulometric composition of the mixture. The bearing capacity of the mixture is influenced by the grain shape and grain surface characteristics of the stone material. This paper describes the testing of mixtures intended for the performance of load-bearing layers of pavement structures, consisting of Drava sand and wood ash (WA) of different fractions (bottom wood ash (WBA) and fly (cyclonic) wood ash (WFA)) and different proportions. The CBR index was determined after immersion of the samples for 4 days under water, during which time swelling was monitored. The results showed that wood fly ash (WFA) has a greater impact on the bearing capacity of the sand mixture and, using it, lower linear swelling results are achieved as well as higher CBR results. Different types of natural aggregates (gravel, crushed stone, sand) are used in the construction of load-bearing layers of pavement structures, the granulometric composition of which has a great influence on the compaction and load-bearing capacity of the layer. Special care should be taken when compacting single-grain materials, such as sand, as difficulties can be expected with these. For this reason, it is useful to modify the granulometric composition of the sand by adding new material of finer fractions. The bearing capacity of mixtures for the production of unbound load-bearing layers of a pavement structure is tested by determining the California Bearing Ratio (CBR), the value of which depends on the dry bulk density achieved and the granulometric composition of the mixture. The bearing capacity of the mixture is influenced by the grain shape and grain surface characteristics of the stone material. This paper describes the testing of mixtures intended for the performance of load-bearing layers of pavement structures, consisting of Drava sand and wood ash (WA) of different fractions (bottom wood ash (WBA) and fly (cyclonic) wood ash (WFA)) and different proportions. The CBR index was determined after immersion of the samples for 4 days under water, during which time swelling was monitored. The results showed that wood fly ash (WFA) has a greater impact on the bearing capacity of the sand mixture and, using it, lower linear swelling results are achieved as well as higher CBR results.