The cast-in-situ foamed concrete is inherently susceptible to cracks due to its brittleness under external loads. In order to improve its mechanical properties, this paper investigated the effect of fibers on the mechanical properties and mechanism of cast-in-situ foamed concrete with designed dry densities in the range of 300–800 kg/m3. First, the influence of density on the mechanical properties of foamed concrete was studied, and the applicability of the splitting test, bending test, and

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... ct tensile test was analyzed. Secondly, alkali-resistant glass fiber (high elastic modulus) and polypropylene fiber (low elastic modulus) were selected to determine the optimal fiber length and content. Finally, failure mode analysis was carried out to explore the influence mechanism of fibers on the mechanical properties of foamed concrete. The results showed that the compression curve model of cast-in-situ foamed concrete was mainly divided into three stages: the elastic stage, the brittle stage, and the yield stage. There were still some problems in the application of direct tensile tests and splitting tests for foamed concrete materials, and only the bending test could be used to test the tensile strength of foamed concrete materials. For the foamed concrete with a designed dry density of 300–800 kg/m3, the compressive strength, elastic modulus, and tensile strength of foamed concrete increased upon increasing the casting density. The compressive strength, elastic modulus, and flexural strength of foamed concrete first increased and then decreased upon increasing the fiber content. The optimal fiber length was 6 mm, the optimal glass fiber content range was 0.2–0.6%, and the optimal polypropylene fiber content range was 0.4–0.8%. The fibers formed a bridge structure between the hardened products, which increased the adsorption force between the hardened products of foamed concrete, thus increasing its compressive strength and flexural strength.